Optical Communication

VIII Sem, ECE

Submitted By:-

Dimple Jhanwar

Lecturer

(ECE Department)

1

INDEX

S No. Name of Content Page No. I. Resume 5II Objective 8III Syllabus 9IV. Lecture Plan 10

Unit-I- OPTICAL FIBERS

1.1 Introduction1.2 Overview of fiber communication1.3 Ray propogation1.4 Total internal Reflection1.5 Modes in Optical fiber1.6 Transmission Characteristics of Optical Fiber Cables1.7 Propagation of Light along the fiber1.8 Ray Theory 1.9 Types of Optical Fibers :

1.9.1 Step index fibers :1.9.2 Graded index fibers

1.10 Attenuation in Optical Fibers1.10..1 Linear scattering losses1.10.2 Non linear scattering losses1.10.3 Material Absorption losses

1.11 . Bending Loss1.11.1 Microbend losses1.11.2 Macrobend losses

1.12. Dispersion1.12.1 Intramodal Dispersion1.12.2 Intermodal Dispersion

UNIT-II

OPTICAL FIBER SOURCES & CONNECTION

2.1 Introduction

2.2 Light Emitting Diode Structure

2.2.1 Dome LED

2.2.2 Planar LED

2.2.3 Basic Layer by Layer Structure

2.2.4 Basic Layer by Layer Structure

2.2.5 Homo- and Hetro-Junction

2.2.6 Edge Emitter

2

2.2.7 Edge Emitter

2.3 Characteristics of LED2.4 Quantum efficiency2.5 Fiber Alignment Technique2.6 Fiber Optical Splicing

2.6.1 Fusion Splicing2.6.2 Mechanical Splicing

2.7 Optical fiber connectorUNIT-IIIOPTICAL DETECTORS3.1 Introduction3.2 Optical Detector Properties3.3 Responsivity 3.4 Quantum efficiency3.5 PIN Photo Diodes

3.5.1 Response Time3.6 Avalanche Photo Diodes

3.7 Receiver Noise

3.7.1 Thermal Noise

3.7.2 Shot Noise

3.8 Photo Diode Materials

UNIT-IV

OPTICAL MEASUREMENT

4.1 Measurement of Attenuation4.1.1 Cut back Method

4.2 Measurement of Cut-off wavelength

4.3 Measurement of Dispersion

4.4 Measurement of Diameter

4.5 Measurement of Numerical Aperture

UNIT-V

LASER

5.1 Principles of Laser

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5.2 Rate Equations and Population Inversion

5.2.1 Two- Level system

5.2.2 Three – Level System

5.2.3 Four Level System

5.3 Optical feedback

5.4 Lasing threshold

5.5 Q-switching:

5.5.1 Principple5.5.2 Active Switching5.5.3 Passive Switching5.5.4 Applications

5.6 Mode Locking5.6.1 Laser active Mode Locking5.6.2 Mode Locking Theory5.6.3 Mode Locking Methods

5.7 Applications of LASER5.7.1 Holography5.7.2 Distance Measurement5.7.3 Velocity Measurement

V. Assignment-IVI. Assignment-IIVII. Assignment-III VIII Assignment-IVX Class-Test-IXI Class-Test-IIXII Class-Test-IIIXIV Class-Test-VXV Tutorial-IXVI Tutorial-IIXVII Tutorial-IIIXVIII Tutorial-IVXIX Tutorial-VXX I Mid Term PaperXXI II Mid Term Paper XXII Last Year PaperXXIII Evaluation sheet

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5

6

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Course Objective

1) This subject deals with the practical aspects of optical fiber communication by introducing optical fiber

2) This course aims to initiate an expose the newcomers to exciting area of optical communication. Technical concepts which are at the core of design, implementation and research will be discussed during this course in order that is conductive to understanding general concepts as well as latest development.

3)Basic optical networks and WDM will be studied. Students will do design calculation for a point-to-point optical fiber link and star networks.

4)To give the student understanding of working principle of optical fiber sourses(LEDs and LASERs) detectors(PIN, Avalance photodiodes) coupler,fiber connectors

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SYLLABUS

UNIT 1 : OPTICAL FIBERS- Introduction, Ray theory, Optical fibers: multimode, single mode, step

index,graded index, plastic & glass fibers.Transmission Characteristics of Optical Fibres -Introduction,

Attenuation, Material absorption loss, Fibrebend loss, Dispersion (intermodal & intramodal)

UNIT 2: OPTICAL FIBER SOURCES & CONNECTION - Light Emitting Diode - Structure,

Material,

Characteristics, Power & Efficiency.Fiber Alignment, Fiber splices, Fiber connectors, Expanded beam

connectors,

UNIT 3 : OPTICAL DETECTORS - Optical detection principles, quantum efficiency, responsivity,

PIN photo diode, Avalanche photo diodes, Noise in Detectors, Photo Diode Materials.

UNIT 4 : OPTICAL FIBER MEASUREMENTS - Measurements of Fiber Attenuation, Dispersion,

Refractive Index Profile, Cut off Wave Length, Numerical Aperture & Diometer.

UNIT 5 : LASER - Emission and absorption of radiation, Einstein relation, Absorption of radiation,

Population inversion, Optical feed back, Threshold condition. Population inversion and threshold

working of three level & four level laser. Basic idea of solid state, semiconductors, gas & liquid laser.

Basic concept of Q-switching & mode locking. Laser applications for measurement of distance,

Velocity, Holography.

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ARYA COLLEGE OF ENGINEERING & I.T.Lecture Plan

LECTURE PLAN

Name of Faculty: Dimple Jhanwar Subject: Optical Communication Branch: ECE Sem.: VIII

S. No. Unit No.

Date Name of Topic Proposed Lectures Required

Actual LectureTaken

Extra Activities

% of Syllabus Covered By the Unit

Reference Books

Remarks

1.

I. O

PT

ICA

L F

IBE

RS

Introduction, Ray theory 2

25%

1. Jhon M Senior

2. Gred Kaser

EasyTheoretical& Analytical

2. Optical fibers: multimode, single mode, step index,graded index, plastic & glass fibers.

1Numerical Problem

3. Transmission Characteristics of Optical Fibres - Introduction,Attenuation

1Assignmet -I

4. Material absorption loss, Fibre bend loss, Dispersion (intermodal & intramodal)

2

5. Class Test-I 1

10

6.

II. O

PT

ICA

L F

IBE

R S

OU

RC

ES

&

CO

NN

EC

TIO

N

Light Emitting Diode - Structure, Material,Characteristics, Power & Efficiency.

2

20%

1. Jhon M Senior

2. Navneet Gupta

EasyTheoretical& Analytical7. Fiber Alignment,

Fiber splices 2Numerical Problem

8. Fiber connectors, Expanded beam connectors 2

Assignment - II

9. Class Test-II 1

LECTURE PLAN

Name of Faculty: Dimple Jhanwar Subject: Optical Communication Branch: ECE Sem.: VIII

10.

III.

OP

TIC

AL

DE

TE

CT

OR

S

Optical detection principles, quantum efficiency, responsivity

1

15%

1. Jhon M Senior

2. Navneet Gupta

ModerateTheoretical& Analytical

11. PIN photo diode, Avalanche photo diodes 2

12. Noise in Detectors, Photo Diode Materials

1Assignment- III

13. Mid Term I

14.

IV.

OP

TIC

AL

FIB

ER

M

EA

SU

RE

ME

NT

S

Measurements of Fiber Attenuation,Dispersion,

2

15%

1. Jhon M Senior

2. Gred Kaser

ModerateTheoretical& Analytical15. Refractive Index

Profile ,Cut off Wave Length

2Numerical Problem

16. Numerical Aperture & Diometer 2

Assignment- IV

17. Class test III 1

11

18.

V

. LA

SE

R

Emission and absorption of radiation, Einstein relation

2

25%

1. Jhon M Senior

2. Gred Kaser

3. Navneet Gupta

Hard Theoretical& Analytical19.

Population inversion, Optical feed back, Threshold condition.

1

20.Threshold Working of three level & four level laser

2

21.Basic idea of solid state, semiconductors, gas & liquid laser

2

22.Basic concept of Q-switching & mode locking.

2

LECTURE PLAN

Name of Faculty: Dimple Jhanwar Subject: Optical Communication Branch: ECE Sem.: VIII

23. Laser applications for measurement of distance, Velocity,Holography

2Numerical Problem

24. Mid Term II

25. Total Lecture 36

Proposed Lecture: 36 Actual Lecture Taken: Signature of Faculty

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Unit-I

OPTICAL FIBERS

Introduction

An optical fiber (or fibre) is a glass or plastic fiber that carries light along its length. Fiber optics is the

overlap of applied science and engineering concerned with the design and application of optical fibers. Optical

fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at

higher bandwidths (data rates) than other forms of communications. Fibers are used instead of metal wires

because signals travel along them with less loss, and they are also immune to electromagnetic interference.

Fibers are also used for illumination, and are wrapped in bundles so they can be used to carry images, thus

allowing viewing in tight spaces. Specially designed fibers are used for a variety of other applications,

including sensors and fiber lasers.Light is kept in the core of the optical fiber by total internal reflection. This

causes the fiber to act as a waveguide. Fibers which support many propagation paths or transverse modes are

called multi-mode fibers (MMF), while those which can only support a single mode are called single-mode

fibers (SMF). Multi-mode fibers generally have a larger core diameter, and are used for short-distance

communication links and for applications where high power must be transmitted. Single-mode fibers are used

for most communication links longer than 550 metres (1,800 ft).Joining lengths of optical fiber is more

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complex than joining electrical wire or cable. The ends of the fibers must be carefully cleaved, and then spliced

together either mechanically or by fusing them together with an electric arc. Special connectors are used to

make removable connections.

GENERAL OVERVIEW OF OPTICAL FIBER COMMUNICATION SYSTEM :

Like all other communication system, the primary objective of optical fiber communication system also is to

transfer the signal containing information (voice, data, video) from the source to the destination. The source

provides information in the form of electrical signal to the transmitter. The electrical stage of the transmitter

drives an optical source to produce modulated light wave carrier. Semiconductor LASERs or LEDs are usually

used as optical source here. The information carrying light wave then passes through the transmission medium

i.e. optical fiber cables in this system. Now it reaches to the receiver stage where the optical detector

demodulates the optical carrier and gives an electrical output signal to the electrical stage. The common types

of optical detectors used are photodiodes (p-i-n, avalanche), phototransistors, photoconductors etc. Finally the

electrical stage gets the real information back and gives it to the concerned destination.

                 It is notable that the optical carrier may be modulated by either analog or digital information signal.

In digital optical fiber communication system the information is suitably encoded prior to the drive circuit

stage of optical source. Similarly at the receiver end a decoder is used after amplifier and equalizer stage.

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Principle of ray propagation :

This is the most interesting thing about optical fiber cables. Such an indispensable part of modern day

communication system works on an extremely simple property of light ray i.e. Total Internal Reflection. As we

all know that when light ray is passing from denser (refractive index is higher) dielectric medium to a rarer

(refractive index is lower) dielectric medium then from the point of incidence at the interface it bends away

from the normal. When the incidence angle is sufficiently high such that the angle of refraction is 90º then it is

called critical angle. Now if light ray falls at the interface of the two mediums at an angle greater than the

critical angle then the light ray gets reflected back to the originating medium with high efficiency (around

99.9%) i.e. total internal reflection occurs. With the help of innumerable total internal reflections light waves

are propagated along the fiber with low loss as shown in figure2. In this context, two parameters are very

crucial namely Acceptance Angle and Numerical Aperture

Acceptance angle is the maximum angle at which light may enter the fiber in order to be propagated and is

denoted by θa in figure3. The relationship between the acceptance angle and the refractive indices of the three

media involved-core, cladding and air, leads to the definition of Numerical Aperture which is given by – 

                        NA = (n1²-n2²)½ = n0 sin θa             where n0 is the refractive index of air.

The light ray shown in figure3 is known as a meridional ray as it passes through the axis of the fiber. However,

another category of ray exists which is transmitted without passing through the fiber axis and follows a helical

path through the fiber.

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Total internal reflection

When light traveling in a dense medium hits a boundary at a steep angle (larger than the "critical angle" for the

boundary), the light will be completely reflected. This effect is used in optical fibers to confine light in the

core. Light travels along the fiber bouncing back and forth off of the boundary. Because the light must strike

the boundary with an angle greater than the critical angle, only light that enters the fiber within a certain range

of angles can travel down the fiber without leaking out. This range of angles is called the acceptance cone of

the fiber. The size of this acceptance cone is a function of the refractive index difference between the fiber's

core and cladding.

In simpler terms, there is a maximum angle from the fiber axis at which light may enter the fiber so that it will

propagate, or travel, in the core of the fiber. The sine of this maximum angle is the numerical aperture (NA) of

the fiber. Fiber with a larger NA requires less precision to splice and work with than fiber with a smaller NA.

Single-mode fiber has a small NA.

Optical Fiber

Modes in optical fibers :

The electromagnetic wave theory must be taken into account for getting an improved model for propagation of

light through optical fibers. The optical waveguide can be considered to be either a planer guide or a

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cylindrical guide. Electromagnetic field comprises of a periodically varying electric field E and magnetic field

M which are oriented at right angle to each other. When the electric field is perpendicular to the direction of

propagation and hence Ez=0, but a corresponding magnetic field component is in the direction of propagation,

that mode is known as Transverse Electric (TE) mode. But when the reverse thing happens then it is termed as

Transverse Magnetic (TM) mode. Now when total field lies in the transverse plane, Transverse

electromagnetic (TEM) waves exist where both Ez and Hz are zero. The formation of modes in a planer

dielectric guide and the interference of plane waves are shown in figure4. Here the stable field distribution in

the x direction with only a periodic z dependence due to sinusoidally varying electric field in z direction is

known as a mode. In a cylindrical fiber transverse electric (TE) and transverse magnetic (TM) modes are

obtained which is bounded in two dimensions. Thus two integers (l & m) are necessary to specify the modes.

Hybrid modes may also occur in the cylindrical fibers. These modes result from skew ray propagation and are

designated by HElm when H makes a larger contribution to the transverse field and EHlm when E makes

larger contribution to the transverse field.

Transmission Characteristics of Optical Fiber Cables:  

The transmission characteristics of optical fiber cables play a major role in determining the performance of the

entire communication system. Attenuation and bandwidth are the two most important transmission

characteristics when the suitability of optical fiber for communication is analysed. The various attenuation

mechanisms are linear scattering, non linear scattering, material absorption and fiber bends etc. The bandwidth

determines the number of bits of information transmitted in a given time period and is largely limited by signal

dispersion within the fiber.

PROPAGATION OF LIGHT ALONG A FIBER

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The concept of light propagation, the transmission of light along an optical fiber, can be described by two

theories. According to the first theory, light is described as a simple ray. This theory is the ray theory, or

geometrical optics, approach. The advantage of the ray approach is that you get a clearer picture of the

propagation of light along a fiber. The ray theory is used to approximate the light acceptance and guiding

properties of optical fibers. According to the second theory, light is described as an electromagnetic wave. This

theory is the mode theory, or wave representation, approach. The mode theory describes the behavior of light

within an optical fiber. The mode theory is useful in describing the optical fiber properties of absorption,

attenuation, and dispersion. These fiber properties are discussed later in this chapter.

Ray Theory

Two types of rays can propagate along an optical fiber. The first type is called meridional rays. Meridional

rays are rays that pass through the axis of the optical fiber. Meridional rays are used to illustrate the basic

transmission properties of optical fibers.

The second type is called skew rays. Skew rays are rays that travel through an optical fiber without passing

through its axis.

MERIDIONAL RAYS. - Meridional rays can be classified as bound or unbound rays. Bound rays remain in

the core and propagate along the axis of the fiber. Bound rays propagate through the fiber by total internal

reflection. Unbound rays are refracted out of the fiber core. Figure 2-10 shows a possible path taken by bound

and unbound rays in a step-index fiber. The core of the step-index fiber has an index of refraction n1. The

cladding of a step-index has an index of refraction n2, that is lower than n1. Figure 2-10 assumes the core-

cladding interface is perfect. However, imperfections at the core-cladding interface will cause part of the bound

rays to be refracted out of the core into the cladding. The light rays refracted into the cladding will eventually

escape from the fiber. In general, meridional rays follow the laws of reflection and refraction.

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It is known that bound rays propagate in fibers due to total internal reflection, but how do these light rays enter

the fiber? Rays that enter the fiber must intersect the core-cladding interface at an angle greater than the critical

angle (Θc). Only those rays that enter the fiber and strike the interface at these angles will propagate

along the fiber.

How a light ray is launched into a fiber is shown in figure 2-11. The incident ray I1 enters the fiber at the angle

Θa. I1 is refracted upon entering the fiber and is transmitted to the core-cladding interface. The ray then

strikes the core-cladding interface at the critical angle (Θ c). I1 is totally reflected back into the core and

continues to propagate along the fiber. The incident ray I2 enters the fiber at an angle greater than Θa.

Again, I2 is refracted upon entering the fiber and is transmitted to the core-cladding interface. I2 strikes the

core-cladding interface at an angle less than the critical angle (Θc). I2 is refracted into the cladding and is

eventually lost. The light ray incident on the fiber core must be within the acceptance cone defined by the

angle Θa.

Types of Optical Fibers :

According to the refractive index profile optical fibers can be divided into two categories namely Step index

fibers and Graded index fibers which are described below.     

1  Step index fibers :

If the refractive index profile of a fiber makes a step change at the core cladding interface then it is known as

step index fiber. A multimode step index fiber is shown in figure7(a), the core diameter of which is around

50µm. Some physical parameters like relative refractive index, index difference, core radius etc determines the

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maximum number of guided modes possible in a multimode fiber. A single mode fiber has a core diameter of

the order of 2 to 10µm and the propagation of light wave is shown in figure7(b). It has the distinct advantage of

low intermodal dispersion over multimode step index fiber. On the other hand multimode step index fibers

allow the use of spatially incoherent optical sources and low tolerance requirements on fiber connectors.

2 Graded index fibers :

The graded index fibers have decreasing core index n(r) with radial distance from a maximum value of n1 at

the axis to a constant value n2 beyond the core radius a in the cladding as shown in figure8. The graded index

fiber gives best results for multimode optical propagation for parabolic refractive index profile. Due to this

special kind of refractive index profile multimode graded index fibers exhibit less intermodal dispersion than

its counterpart i.e. multimode step index fibers

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Attenuation in Optical Fibers :

 Attenuation is defined as the loss of optical power over a set distance, a fiber with a lower attenuation, will

allow more power to reach to the receiver than a fiber with higher attenuation. Signal attenuation within optical

fiber is usually expressed in decibel per unit length (i.e. dB/km).

       Loss in decibel (dB) = 10 log₁₀(Pi/Po)

 where Pi and Po are the transmitted and output optical power respectively. Figure5 shows optical fiber

attenuation as a function of wavelength.

1. Linear scattering losses :

Through this mechanism a portion/total optical power within one propagating mode is transferred to another.

Now when the transfer takes place to a leaky or radiation mode then the result is attenuation. It can be divided

into two major categories namely Mie scattering and Rayleigh scattering.

(a) Mie Scattering :                        

Non perfect cylindrical structure of the fiber and imperfections like irregularities in the core-cladding interface,

diameter fluctuations, strains and bubbles may create linear scattering which is termed as Mie scattering.

(b) Rayleigh Scattering :

The dominant reason behind Rayleigh scattering is refractive index fluctuations due to density and

compositional variation in the core. It is the major intrinsic loss mechanism in the low impedance window.

Rayleigh scattering can be reduced to a large extent by using longest possible wavelength.

2. Non linear scattering losses :

Specially at high optical power levels scattering causes disproportionate attenuation, due to non linear

behaviour. Because of this non linear scattering the optical power from one mode is transferred in either the

forward or backward direction to the same, or other modes, at different frequencies. The two dominant types of

non linear scattering are :

a) Stimulated Brillouin Scattering and

b) Stimulated Raman Scattering.

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3. Material Absorption losses :

When there happens to be some defect in the material composition and the fabrication process of optical fiber,

there is dissipation of optical power in the form of heat in the waveguide. Here also there are two types of

absorption losses in the fiber such as intrinsic absorption and extrinsic absorption. When the absorption is

caused by interaction with one or more components of glass it is termed as intrinsic absorption whereas if it

Absorption is a major cause of signal loss in an optical fiber. Absorption is defined as the portion of

attenuation resulting from the conversion of optical power into another energy form, such as heat. Absorption

in optical fibers is explained by three factors:

Imperfections in the atomic structure of the fiber material

The intrinsic or basic fiber-material properties

The extrinsic (presence of impurities) fiber-material properties

Imperfections in the atomic structure induce absorption by the presence of missing molecules or oxygen

defects. Absorption is also induced by the diffusion of hydrogen molecules into the glass fiber. Since intrinsic

and extrinsic material properties are the main cause of absorption, they are discussed further.

(a) Intrinsic Absorption. - Intrinsic absorption is caused by basic fiber-material properties. If an optical fiber

were absolutely pure, with no imperfections or impurities, then all absorption would be intrinsic. Intrinsic

absorption sets the minimal level of absorption. In fiber optics, silica (pure glass) fibers are used

predominately. Silica fibers are used because of their low intrinsic material absorption at the wavelengths of

operation. In silica glass, the wavelengths of operation range from 700 nanometers (nm) to 1600 nm. Figure 2-

21 shows the level of attenuation at the wavelengths of operation. This wavelength of operation is between two

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intrinsic absorption regions. The first region is the ultraviolet region (below 400-nm wavelength). The second

region is the infrared region (above 2000-nm wavelength).

 

Intrinsic absorption in the ultraviolet region is caused by electronic absorption bands. Basically, absorption

occurs when a light particle (photon) interacts with an electron and excites it to a higher energy level. The tail

of the ultraviolet absorption band is shown in figure 2-21. The main cause of intrinsic absorption in the

infrared region is the characteristic vibration frequency of atomic bonds. In silica glass, absorption is caused by

the vibration of silicon-oxygen (Si-O) bonds. The interaction between the vibrating bond and the

electromagnetic field of the optical signal causes intrinsic absorption. Light energy is transferred from the

electromagnetic field to the bond. The tail of the infrared absorption band is shown in figure 2-21.

(b). Extrinsic Absorption. - Extrinsic absorption is caused by impurities introduced into the fiber material.

Trace metal impurities, such as iron, nickel, and chromium, are introduced into the fiber during fabrication.

Extrinsic absorption is caused by the electronic transition of these metal ions from one energy level to another.

Extrinsic absorption also occurs when hydroxyl ions (OH-) are introduced into the fiber. Water in silica glass

forms a silicon-hydroxyl (Si-OH) bond. This bond has a fundamental absorption at 2700 nm. However, the

harmonics or overtones of the fundamental absorption occur in the region of operation. These harmonics

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increase extrinsic absorption at 1383 nm, 1250 nm, and 950 nm. Figure 2-21 shows the presence of the three

OH- harmonics. The level of the OH- harmonic absorption is also indicated. These absorption peaks define

three regions or windows of preferred operation. The first window is centered at 850 nm. The second window

is centered at 1300 nm. The third window is centered at 1550 nm. Fiber optic systems operate at wavelengths

defined by one of these windows. The amount of water (OH-) impurities present in a fiber should be less than a

few parts per billion. Fiber attenuation caused by extrinsic absorption is affected by the level of impurities

(OH-) present in the fiber. If the amount of impurities in a fiber is reduced, then fiber attenuation is reduced.

3. BENDING LOSS. - Bending the fiber also causes attenuation. Bending loss is classified according to the

bend radius of curvature: microbend loss or macrobend loss.

Microbends are small microscopic bends of the fiber axis that occur mainly when a fiber is cabled.

Macrobends are bends having a large radius of curvature relative to the fiber diameter. Microbend and

macrobend losses are very important loss mechanisms. Fiber loss caused by microbending can still occur even

if the fiber is cabled correctly. During installation, if fibers are bent too sharply, macrobend losses will occur.

(a) Microbend losses are caused by small discontinuities or imperfections in the fiber. Uneven coating

applications and improper cabling procedures increase microbend loss. External forces are also a source of

microbends. An external force deforms the cabled jacket surrounding the fiber but causes only a small bend in

the fiber. Microbends change the path that propagating modes take, as shown in figure 2-23. Microbend loss

increases attenuation because low-order modes become coupled with high-order modes that are naturally lossy.

Figure 2-23. - Microbend loss. 

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(b) Macrobend losses are observed when a fiber bend's radius of curvature is large compared to the fiber

diameter. These bends become a great source of loss when the radius of curvature is less than several

centimeters. Light propagating at the inner side of the bend travels a shorter distance than that on the outer

side. To maintain the phase of the light wave, the mode phase velocity must increase. When the fiber bend is

less than some critical radius, the mode phase velocity must increase to a speed greater than the speed of light.

However, it is impossible to exceed the speed of light. This condition causes some of the light within the fiber

to be converted to high-order modes. These high-order modes are then lost or radiated out of the fiber. Fiber

sensitivity to bending losses can be reduced. If the refractive index of the core is increased, then fiber

sensitivity decreases. Sensitivity also decreases as the diameter of the overall fiber increases. However,

increases in the fiber core diameter increase fiber sensitivity. Fibers with larger core size propagate more

modes. These additional modes tend to be more lossy.

Dispersion

In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency,[1] or

alternatively when the group velocity depends on the frequency. Media having such a property are termed

dispersive media. Dispersion is sometimes called chromatic dispersion. There are generally two sources of

dispersion: material dispersion and waveguide dispersion. Material dispersion comes from a frequency-

dependent response of a material to waves.

(1) Intramodal Dispersion

Intramodal, or chromatic, dispersion depends primarily on fiber materials. There are two types of intramodal

dispersion. The first type is material dispersion. The second type is waveguide dispersion. Intramodal

dispersion occurs because different colors of light travel through different materials and different waveguide

structures at different speeds.

(a)Material dispersion occurs because the spreading of a light pulse is dependent on the wavelengths'

interaction with the refractive index of the fiber core. Different wavelengths travel at different speeds in the

fiber material. Different wavelengths of a light pulse that enter a fiber at one time exit the fiber at different

times. Material dispersion is a function of the source spectral width. The spectral width specifies the range of

wavelengths that can propagate in the fiber. Material dispersion is less at longer wavelengths.

(b)Waveguide dispersion occurs because the mode propagation constant (β) is a function of the size of

the fiber's core relative to the wavelength of operation. Waveguide dispersion also occurs because light

propagates differently in the core than in the cladding. In multimode fibers, waveguide dispersion and material

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dispersion are basically separate properties. Multimode waveguide dispersion is generally small compared to

material dispersion. Waveguide dispersion is usually neglected. However, in single mode fibers, material and

waveguide dispersion are interrelated. The total dispersion present in single mode fibers may be minimized by

trading material and waveguide properties depending on the wavelength of operation.

2. Intermodal Dispersion

Intermodal or modal dispersion causes the input light pulse to spread. The input light pulse is made up of a

group of modes. As the modes propagate along the fiber, light energy distributed among the modes is delayed

by different amounts. The pulse spreads because each mode propagates along the fiber at different speeds.

Since modes travel in different directions, some modes travel longer distances. Modal dispersion occurs

because each mode travels a different distance over the same time span, as shown in figure 2-25. The modes of

a light pulse that enter the fiber at one time exit the fiber a different times. This condition causes the light pulse

to spread. As the length of the fiber increases, modal dispersion increases.

Figure 2-25. - Distance traveled by each mode over the same time span. 

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Modal dispersion is the dominant source of dispersion in multimode fibers. Modal dispersion does not exist in

single mode fibers. Single mode fibers propagate only the fundamental mode. Therefore, single mode fibers

exhibit the lowest amount of total dispersion. Single mode fibers also exhibit the highest possible bandwidth

Unit-II

OPTICAL FIBER SOURCES & CONNECTION

Light-emitting diode

A light-emitting diode (LED) is an electronic light source. LEDs are used as indicator lamps in many kinds of

electronics and increasingly for lighting. LEDs work by the effect of electroluminescence, discovered by

accident in 1907. The LED was introduced as a practical electronic component in 1962. [2] All early devices

emitted low-intensity red light, but modern LEDs are available across the visible, ultraviolet and infra red

wavelengths, with very high brightness.LEDs are based on the semiconductor diode. When the diode is

forward biased (switched on), electrons are able to recombine with holes and energy is released in the form of

light. This effect is called electroluminescence and the color of the light is determined by the energy gap of the

semiconductor. The LED is usually small in area (less than 1 mm2) with integrated optical components to

shape its radiation pattern and assist in reflection.[3]LEDs present many advantages over traditional light

sources including lower energy consumption, longer lifetime, improved robustness, smaller size and faster

switching. However, they are relatively expensive and require more precise current and heat management than

traditional light sources.Applications of LEDs are diverse. They are used as low-energy indicators but also for

replacements for traditional light sources in general lighting, automotive lighting and traffic signals. The

compact size of LEDs has allowed new text and video displays and sensors to be developed, while their high

switching rates are useful in communications technology.

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Light Emitting Diode Structure

LEDs are p-n junction devices constructed of gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP),

or gallium phosphide (GaP). Silicon and germanium are not suitable because those junctions produce heat and

no appreciable IR or visible light. The junction in an LED is forward biased and when electrons cross the

junction from the n- to the p-type material, the electron-hole recombination process produces some photons in

the IR or visible in a process called electroluminescence. An exposed semiconductor surface can then emit

light

1. Dome LED

A hemisphere of n-type Ga As is formed around p-type dome. The diameter of dome is chosen to maximize the

amount of internal emission reaching the surface within the critical angle. This device has high efficiency.

28

2. Plannar LED

3. Basic Layer by Layer Structure

29

4. Surface Emitter

In surface emitter the emitting area is defined by oxide isolation, with the metal contact area a circle of

diameter ~ 10mm-15 mm.The surface layer is kept as thin as possible (10-15 mm) to minimise reabsorbtion

5. Homo- and Hetro-Junction

Homojunction = a p-n junction made out of two differently doped semiconductors that are of the same

material (i.e having the same band gap).

Heterojunction = junction formed between two different band gaps semiconductors.

Heterostructure device = semiconductor device structure that has junctions between different bandgap

materials

Double Heterojunction LED

30

The double heterostructure is invariably used for optical sources for communication as seen in the figure in the

pervious slide. Heterostucture can be used to increase:Efficiency by carrier confinement (band gap

engineering)Efficiency by photon confinement (refractive index)The double heterostructure enables the source

radiation to be much better defined, but further, the optical power generated per unit volume is much greater as

well. If the central layer of a double heterostructure, the narrow band-gap region is made no more than 1mm

wide.

6.Edge Emitter

In edge emitter a double heterostructure band gap engineering is used to achieve carrier confinement and

recombination in an active layer but in addition layers of relatively low refractive index are included to

produce optical guide. A large fraction of the photons are therefore confined between two ‘plates’ of material

and emerge at the edge of the device as highly directional flux compatible with coupling to a fibre optic cable.

31

9.Electroluminescence in LEDs

32

When the applied forward voltage on the diode of the LED drives the electrons and holes into the active region

between the n-type and p-type material, the energy can be converted into infrared or visible photons. This

implies that the electron-hole pair drops into a more stable bound state, releasing energy on the order of

electron volts by emission of a photon. The red extreme of the visible spectrum, 700 nm, requires an energy

release of 1.77 eV to provide the quantum energy of the photon. At the other extreme, 400 nm in the violet, 3.1

eV is required.

Characteristics of LED

1. The energy of an emitted photon = to the size of the band gap

2. The energy of an emitted photon from LED is distributed appropriately according to the energy

distribution of electrons on the conduction band and holes in the valance band.

Quantum efficiency

• Internal quantum efficiency can of some LED approaches 100% but the external efficiencies are much lower.

This is due to reabsorption and TIR.

• III-V materials have small critical angles therefore the radiation emitted suffers from TIR

33

Fiber Alignment Technique

ALIGNMENT of an optical fiber within an optoelectronicmodule is a continuing challenge in photonics

packagingand often dominates module cost. Ultimately, passivealignment and packaging techniques would be

preferred fortheir simplicity. Passive systems utilizing silicon waferboardshave reported alignment accuracies

of 1–2 m [1], yet thisaccuracy is achieved by increasing process controls. Thesetight fabrication and assembly

tolerances complicate fabricationand increase the demands on pick-and-place machines,severely limiting

throughput. Relaxing placement tolerancesfrom the 1- m level to 20- m level could potentially

increasethroughput of a pick-and-place machine by an order of magnitude[2], but a mechanism for optimizing

alignment withinthe package is then required.

34

Fiber Optical Splicing

Two optical fiber splicing methods are available for permanent joining of two optical fibers. Both methods

provide much lower insertion loss compared to fiber connectors.

1. Fiber optic cable fusion splicing – Insertion loss < 0.1dB

2. Fiber mechanical splicing – Insertion loss < 0.5dB

Fiber optic cable fusion splicing

Fiber optic cable fusion splicing provides the lowest-loss connection. Special equipment called fusion splicer is

used to perform the fiber fusion splicing. The fusion splicer performs optical fiber fusion splicing in two steps.

1. Precisely align the two fibers

2. Generate a small electric arc to melt the fibers and weld them together

35

High precision fusion splicers are usually bulky and expensive. With proper training, a fiber splicing technician

can routinely achieve less than 0.1dB insertion loss splicing for both single mode and multimode fiber cables.

Fusion splicing

36

37

Mechanical Splicing

Optical fiber connector

An optical fiber connector terminates the end of an optical fiber, and enables quicker connection and

disconnection than splicing. The connectors mechanically couple and align the cores of fibers so that light can

pass. Most optical fiber connectors are spring-loaded: The fiber endfaces of the two connectors are pressed

together, resulting in a direct glass to glass or plastic to plastic, respectively, contact, avoiding any glass to air

38

or plastic to air interfaces, which would result in higher connector losses.

A variety of optical fiber connectors are available. The main differences among types of connectors are

dimensions and methods of mechanical coupling. Generally, organizations will standardize on one kind of

connector, depending on what equipment they commonly use, or per type of fiber (one for multimode, one for

singlemode). In datacom and telecom applications nowadays small form factor connectors (e.g. LC) and multi-

fiber connectors (e.g. MTP) are replacing the traditional connectors (e.g. SC), mainly to pack more connectors

on the overcrowded faceplate, and thus reducing the footprint of the systems.

39

UNIT-III

OPTICAL DETECTORS

Introduction

A transducer is a device that converts input energy of one form into output energy of another. An optical

detector is a transducer that converts an optical signal into an electrical signal. It does this by generating an

electrical current proportional to the intensity of incident optical radiation. The relationship between the input

optical radiation and the output electrical current is given by the detector responsivity. Responsivity is

discussed later in this chapter.

OPTICAL DETECTOR PROPERTIES

Fiber optic communications systems require that optical detectors meet specific performance and compatibility

requirements. Many of the requirements are similar to those of an optical source. Fiber optic systems require

that optical detectors:

Be compatible in size to low-loss optical fibers to allow for efficient coupling and easy

packaging.

Have a high sensitivity at the operating wavelength of the optical source.

40

Have a sufficiently short response time (sufficiently wide bandwidth) to handle the system's

data rate.

Contribute low amounts of noise to the system.

Maintain stable operation in changing environmental conditions, such as temperature.

Optical detectors that meet many of these requirements and are suitable for fiber optic systems are

semiconductor photodiodes. The principal optical detectors used in fiber optic systems include semiconductor

positive-intrinsic-negative (PIN) photodiodes and avalanche photodiodes (APDs).

SEMICONDUCTOR MATERIAL AND DEVICE PROPERTIES

The mechanism by which optical detectors convert optical power into electrical current requires knowledge of semiconductor material and device properties. As stated in chapter 6, providing a complete description of these properties is beyond the scope of this manual. In this chapter we only discuss the general properties of semiconductor PINs and APDs.

Semiconductor detectors are designed so that optical energy (photons) incident on the detector active area

produces a current. This current is called a photocurrent. The particular properties of the semiconductor are

determined by the materials used and the layering of the materials within the device. Silicon (Si), gallium

arsenide (GaAs), germanium (Ge), and indium phosphide (InP) are the most common semiconductor materials

used in optical detectors. In some cases aluminum (Al) and indium (In) are used as dopants in the base

semiconductor material.

Responsivity

Responsivity is the ratio of the optical detector's output photocurrent in amperes to the incident optical power

in watts. The responsivity of a detector is a function of the wavelength of the incident light and the efficiency

of the device in responding to that wavelength. For a particular material, only photons of certain wavelengths

will generate a photocurrent when they are absorbed. Additionally, the detector material absorbs some

wavelengths better than others. These two properties cause the wavelength dependence in the detector

responsivity. Responsivity is a useful parameter for characterizing detector performance because it relates the

photocurrent generated to the incident optical power.

Quantum efficiency

• Internal quantum efficiency can of some LED approaches 100% but the external efficiencies are much lower.

This is due to reabsorption and TIR.

• III-V materials have small critical angles therefore the radiation emitted suffers from TIR

41

PIN PHOTODIODES

A PIN photodiode is a semiconductor positive-negative (p-n) structure with an intrinsic region sandwiched

between the other two regions (see figure 7-2). It is normally operated by applying a reverse-bias voltage. The

magnitude of the reverse-bias voltage depends on the photodiode application, but typically is less than a few

volts. When no light is incident on the photodiode, a current is still produced. This current is called the dark

current.

The dark current is the leakage current that flows when a reverse bias is applied and no light is incident on the

photodiode. Dark current is dependent on temperature. While dark current may initially be low, it will increase

as the device temperature increases.

Figure - The basic structure of a PIN photodiode. 

42

Response Time

There are several factors that influence the response time of a photodiode and its output circuitry .

The most important of these are the thickness of the detector active area and the detector RC time constant. The detector thickness is related to the amount of time required for the electrons generated to flow out of the detector active area. This time is referred to as the electron transit time. The thicker the detector active area, the longer the transit time will be.

Figure - A schematic representation of a photodiode.  .

AVALANCHE PHOTODIODES

An avalanche photodiode (APD) is a photodiode that internally amplifies the photocurrent by an avalanche

process. In APDs, a large reverse-bias voltage, typically over 100 volts, is applied across the active region.

This voltage causes the electrons initially generated by the incident photons to accelerate as they move through

the APD active region. As these electrons collide with other electrons in the semiconductor material, they

cause a fraction of them to become part of the photocurrent. This process is known as avalanche

multiplication. Avalanche multiplication continues to occur until the electrons move out of the active area of

the APD.

43

Figure The basic structure of an APD. 

The gain of the APD can be changed by changing the reverse-bias voltage. A larger reverse-bias voltage results

in a larger gain. However, a larger reverse-bias voltage also results in increased noise levels. Excess noise

resulting from the avalanche multiplication process places a limit on the useful gain of the APD. The avalanche

process introduces excess noise because every photogenerated carrier does not undergo the same

multiplication. The noise properties of an APD are affected by the materials that the APD is made of. Typical

semiconductor materials used in the construction of low-noise APDs include silicon (Si), indium gallium

arsenide (InGaAs), and germanium (Ge). Trade-offs are made in APD design to optimize responsivity and

gain, dark current, response time, and linearity. This chapter does not attempt to discuss trade-offs in APD

design in more detail. Many aspects of the discussion provided on responsivity, dark current, and response

time provided in the PIN photodiodes section also relate to APDs. The response time of an APD and its output

circuitry depends on the same factors as PIN photodiodes. The only additional factor affecting the response

time of an APD is the additional time required to complete the process of avalanche multiplication. To learn

more about APD design trade-offs and performance parameters.

44

RECEIVER NOISE

Noise corrupts the transmitted signal in a fiber optic system. This means that noise sets a lower limit on the

amount of optical power required for proper receiver operation. There are many sources of noise in fiber optic

systems. They include the following:

Noise from the light source

Noise from the interaction of light with the optical fiber

Noise from the receiver itself

Because the intent of this chapter is to discuss optical detector and receiver properties, only noise associated

with the photodetection process is discussed. Receiver noise includes thermal noise, dark current noise, and

quantum noise. Noise is the main factor that limits receiver sensitivity.

Noise introduced by the receiver is either signal dependent or signal independent. Signal dependent noise

results from the random generation of electrons by the incident optical power. Signal independent noise is

independent of the incident optical power level.

Thermal noise is the noise resulting from the random motion of electrons in a conducting medium. Thermal

noise arises from both the photodetector and the load resistor. Amplifier noise also contributes to thermal

noise. A reduction in thermal noise is possible by increasing the value of the load resistor. However, increasing

the value of the load resistor to reduce thermal noise reduces the receiver bandwidth. In APDs, the thermal

noise is unaffected by the internal carrier multiplication.

Shot noise is noise caused by current fluctuations because of the discrete nature of charge carriers. Dark

current and quantum noises are two types of noise that manifest themselves as shot noise. Dark current noise

results from dark current that continues to flow in the photodiode when there is no incident light. Dark current

noise is independent of the optical signal. In addition, the discrete nature of the photodetection process creates

a signal dependent shot noise called quantum noise. Quantum noise results from the random generation of

electrons by the incident optical radiation.

45

In APDs, the random nature of the avalanche process introduces an additional shot noise called excess noise.

For further information on the excess noise resulting from the avalanche process, refer to the avalanche

photodiode section.

PHOTODIODE MATERIAL

The material used to make a photodiode is critical to defining its properties, because only photons with

sufficient energy to excite electrons across the material's bandgap will produce significant photocurrents.

Materials commonly used to produce photodiodes include[2]:

Because of their greater bandgap, silicon-based photodiodes generate less noise than germanium-based

photodiodes, but germanium photodiodes must be used for wavelengths longer than approximately 1 µm.

Unwanted photodiodes

Since transistors and ICs are made of semiconductors, and contain P-N junctions, almost every active

component is potentially a photodiode. Many components, especially those sensitive to small currents, will

not work correctly if illuminated, due to the induced photocurrents. In most components this is not desired,

so they are placed in an opaque housing. Since housings are not completely opaque to X-rays or other high

energy radiation, these can still cause many ICs to malfunction due to induced photo-currents.

UNIT-IV

46

OPTICAL MEASUREMENT

MEASUREMENT OF ATTENUATION

Attenuation is the loss of optical power as light travels along the fiber. It is a result of absorption, scattering,

bending, and other loss mechanisms as described in chapter 3. Each loss mechanism contributes to the total

amount of fiber attenuation.

End users measure the total attenuation of a fiber at the operating wavelength (&lambda;). The total

attenuation (A) between an arbitrary point X and point Y located on the fiber is

Px is the power output at point X. P y is the power output at point Y. Point X is assumed to be closer to the

optical source than point Y. The total amount of attenuation will vary with changes in wavelength &lambda;.

The attenuation coefficient (&alpha;) or attenuation rate, is

L is the distance between points X and Y. &alpha; is a positive number because Px is always larger than Py.

The attenuation coefficient will also vary with changes in &lambda;.

CUTBACK METHOD. - In laboratory situations, end users perform the cutback method for measuring the

total attenuation of an optical fiber. The cutback method involves comparing the optical power transmitted

through a long piece of test fiber to the power present at the beginning of the fiber. The cutback method for

measuring multimode fiber attenuation is EIA/TIA-455-46. The cutback method for measuring single mode

fiber attenuation is EIA/TIA-455-78. The basic measurement process is the same for both of these procedures.

The test method requires that the test fiber of known length (L) be cut back to an approximate 2-m length. This

cut back causes the destruction of 2-m of fiber. This method requires access to both fiber ends. Each fiber end

should be properly prepared to make measurements. EIA/TIA-455-57 describes how to properly prepare fiber

ends for measurement purposes. The cutback method begins by measuring, with an optical power meter, the

output power P1 of the test fiber of known length (L) (figure 5-1, view A). Without disturbing the input light

47

conditions, the test fiber is cut back to an approximate 2-m length. The output power P2 of the shortened test

fiber is then measured (figure 5-1, view B). The fiber attenuation AT and the attenuation coefficient &alpha;

are then calculated.

Figure Cutback method for measuring fiber attenuation: A. Test measurement; B. Cut-back measurement

LAUNCH CONDITIONS. - Measurement personnel must pay attention to how optical power is launched

into the fiber when measuring fiber attenuation. Different distributions of launch power (launch conditions)

can result in different attenuation measurements. This is more of a problem with multimode fiber than single

mode fiber. For single mode fiber, optical power must be launched only into the fundamental mode. This is

accomplished using a mode filter on the fiber. For multimode fiber, the distribution of power among the modes

of the fiber must be controlled. This is accomplished by controlling the launch spot size and angular

distribution.

The launch spot size is the area of the fiber face illuminated by the light beam from the optical source.

The diameter of the spot depends on the size of the optical source and the properties of the optical

elements (lenses, and so on) between the source and the fiber end face. The angular distribution is the

angular extent of the light beam from the optical source incident on the fiber end face. The launch

angular distribution also depends on the size of the optical source and the properties of the optical

48

elements between the optical source and the fiber end face. Multimode optical fiber launch conditions

are typically characterized as being underfilled or overfilled. An underfilled launch concentrates most

of the optical power in the center of the fiber. An underfilled launch results when the launch spot size

and angular distribution are smaller than that of the fiber core. Underfilling the fiber excites mainly

low-order modes. Overfilling the fiber excites both low-order and high-order modes. An overfilled

launch condition occurs when the launch spot size and angular distribution are larger than that of the

fiber core. Incident light that falls outside the fiber core is lost. In addition, light that is incident at

angles greater than the angle of acceptance of the fiber core is lost.

Figure . - Overfilled launch condition. 

In attenuation measurements, cladding-mode strippers and mode filters eliminate the effects that high-order

modes have on attenuation results. A cladding-mode stripper is a device that removes any cladding mode

power from the fiber. Most cladding-mode strippers consist of a material with a refractive index greater than

that of the fiber cladding. For most fibers, the fiber coating acts as an excellent cladding-mode stripper.

A mode filter is a device that attenuates specific modes propagating in the core of an optical fiber. Mode

filters generally involve wrapping the test fiber around a mandrel. For multimode, tight bends tend to remove

high-order modes from the fiber. This type of mode filter is known as a mandrel wrap mode filter. For

multimode fibers, mode filters remove high-order propagating modes and are individually tailored and adjusted

49

for a specific fiber type. For single mode fibers, a mode filter is used to eliminate the second-order mode from

propagating along the fiber. The propagation of the second-order mode will affect attenuation measurements.

Fiber attenuation caused by the second-order mode depends on the operating wavelength, the fiber bend radius

and length. The two most common types of mode filters are free-form loops and mandrel wraps.. Mandrel

wraps for multimode fibers consist of several wraps (approximately 4 or 5) around a mandrel. A 20-mm

diameter mandrel is typically used for 62.5 &mu;m fiber. Mandrel wraps for single mode fibers consist of a

single wrap around a 30-mm diameter mandrel. Another common mode filter for single mode fibers is a 30-

mm diameter circular free-form loop. Additional information on multimode and single mode filters (and

launch conditions) is available in EIA/TIA-455-50 and EIA/TIA-455-77, respectively.

Figure - Types of mode filters: A. Free-form loop; B. Mandrel-wrap.

Launch conditions significantly affect the results of multimode fiber attenuation measurements. If the fiber is

underfilled, high-order-mode power loss has minimal effect on the measurement results. If too much power is

launched into high-order modes, the high-order-mode power loss will dominate the attenuation results.

Generally, fiber attenuation measurements are performed using an underfilled launch condition. Power in high-

order modes is eliminated by either controlling the input spot size and angular distribution or using mode

filters to remove high-order mode power.

MEASUREMENT OF CUT_OFF WAVE LENGTH

50

The wavelength at which a mode ceases to propagate is called the cutoff wavelength for that mode. However,

an optical fiber is always able to propagate at least one mode, the fundamental mode. The fundamental mode

can never be cut off. The cutoff wavelength of a single mode fiber is the wavelength above which the fiber

propagates only the fundamental mode.

Determining the cutoff wavelength of a single mode fiber involves finding the wavelength above which the

power transmitted through the fiber decreased abruptly. This power decrease occurs when the second-order

mode propagating in the fiber is cut off. The cutoff wavelength of single mode fibers depends on the fiber

length and bend conditions. The effects of length and bending are different on different fibers depending on

whether they are matched-clad or depressed-clad in design. The cutoff wavelength of matched-clad fibers is

more sensitive to bends than the cuttoff wavelength of depressed-clad fibers. The cutoff wavelength of

depressed-clad fibers is more sensitive to length than the cutoff wavelength of matched-clad fibers.

Cutoff wavelength may be measured on uncabled or cabled single mode fibers. A slightly different procedure

is used in each case, but the basic measurement process is the same. The test method for uncabled single mode

fiber cutoff wavelength is EIA/TIA-455-80. The test method for cabled single mode fiber cutoff wavelength is

EIA/TIA-455-170. The fiber cutoff wavelength (&lambda;cf) measured under EIA/TIA-455-80 will generally

be higher than the cable cutoff wavelength (&lambda;cc) measured under EIA/TIA-455-170. The difference is

due to the fiber bends introduced during the cable manufacturing process. Each test method describes the test

equipment (input optics, mode filters, and cladding-mode strippers) necessary for the test. Cutoff wavelength

measurements require an overfilled launch over the full range of test wavelengths. Since the procedures for

measuring the cutoff wavelength of uncabled and cabled single mode fibers are essentially the same, only the

test method for measuring the cutoff wavelength of uncabled fiber is discussed. Measuring the cutoff

wavelength involves comparing the transmitted power from the test fiber with that of a reference fiber at

different wavelengths. The reference fiber can be the same piece of single mode fiber with small bends

introduced or a piece of multimode fiber. If the same fiber with small bends is used as the reference fiber, the

technique is called the <emphasis type="b.GIF">bend-reference technique</emphasis>. If a piece of

multimode fiber is used as the reference fiber, the technique is called the multimode-reference technique. For

both techniques, the test fiber is loosely supported in a single-turn with a constant radius of 140 mm. Figure 5-

5 shows this single-turn configuration. The transmitted signal power P s (&lambda;) is then recorded while

scanning the wavelength range in increments of 10 nm or less. The launch and detection conditions are not

changed while scanning over the range of wavelengths. The wavelength range scanned encompasses the

expected cutoff wavelength.

51

The reference power measurement is then made. For the bend-reference technique, the launch and detection

conditions are not changed, but an additional bend is added to the test fiber. The test fiber is bent to a radius of

30 mm or less to suppress the second-order mode at all the scanned wavelengths. For the multimode-reference

technique, the single mode fiber is replaced with a 2-m length of multimode fiber. The transmitted signal

power P r(&lambda;) is recorded while scanning the same wavelength range in the same increments of 10 nm

or less. The attenuation A(&lambda;) at each wavelength is calculated as follows:

52

Fiber cutoff wavelength determined by the multimode-reference technique

MEASUREMENT OF DISPERSION

Chromatic, or intramodal, dispersion occurs in both single mode and multimode optical fibers. Chromatic

dispersion occurs because different colors of light travel through the fiber at different speeds. Since the

different colors of light have different velocities, some colors arrive at the fiber end before others. This delay

difference is called the differential group delay &tau;(&lambda;) per unit length. This differential group delay

leads to pulse broadening. Chromatic dispersion is measured using EIA/TIA-455-168 in the time domain.

Chromatic dispersion is also measured in the frequency domain using EIA/TIA-455-169 and EIA/TIA-455-

175. These methods measure the composite optical fiber material and waveguide dispersion. To understand the

contribution that material and waveguide dispersive mechanisms have on multimode and single mode fiber

dispersion, refer to chapter 2. In this chapter we limit the discussion on chromatic dispersion to the time

domain method described in EIA/TIA-455-168. The chromatic dispersion of multimode graded-index and

single mode fiber is obtained by measuring fiber group delays in the time domain. These measurements are

made using multiwavelength sources or multiple sources of different wavelengths. A multiwavelength source

53

could be a wavelength-selectable laser. The pulse delay for both a long test sample fiber and a short reference

fiber are measured over a range of wavelengths. The pulse delay for the reference fiber as a function of

wavelength is &tgr;in(&lambda;). The pulse delay for the test fiber as a function of wavelength is &tgr;

out(&lambda;). The group delay &tgr;(&lambda;). per unit length at each wavelength is

 

where Ls is the test sample fiber length in kilometers (km) and L ref is the reference sample length in

km.The fiber chromatic dispersion is defined as the derivative, or slope, of the fiber group delay curve

with respect to wavelength. Generally, the group delay as a function of wavelength is fit to a simple

mathematical function and the derivative calculated. The range of wavelengths over which meaningful

data is obtained depends on the wavelength range of optical source(s) used. The zero-dispersion

wavelength (&lambda;0) and the zero-dispersion slope (S0) are determined from the chromatic

dispersion curve.

MEASUREMENT OF DIAMETER

Core diameter is measured using EIA/TIA-455-58. The core diameter is defined from the refractive index

profile n(r) or the output near-field radiation pattern. Our discussion is limited to measuring the core diameter

directly from the output near-field radiation pattern obtained using EIA/TIA-455-43. The near-field power

distribution is defined as the emitted power per unit area (radiance) for each position in the plane of the

emitting surface. For this chapter, the emitting surface is the output area of a fiber-end face. Near-field power

distributions describe the emitted power per unit area in the near-field region. The near-field region is the

region close to the fiber-end face. In the near-field region, the distance between the fiber-end face and detector

is in the micrometers (&mu;m) range. EIA/TIA-455-43 describes the procedure for measuring the near-field

power distribution of optical waveguides. Output optics, such as lenses, magnify the fiber-end face and focus

the fiber's image on a movable detector. Figure 5-8 shows an example setup for measuring the near-field power

distribution. The image is scanned in a plane by the movable detector. The image may also be scanned by

using a detector array. Detector arrays of known element size and spacing may provide a display of the power

distribution on a video monitor. A record of the near-field power is kept as a function of scan position.

54

Figure:The measurement of the near-field power distribution

Near-field radiation pattern

55

The core diameter (D) is defined as the diameter at which the intensity is 2.5 percent of the maximum intensity

(see figure 5-9). The 2.5 percent points, or the 0.025 level, intersects the normalized curve at radial positions -a

and a. The core diameter is simply equal to 2a (D=2a).

MEASUREMENT OF NUMERICAL APERTURE

The numerical aperture (NA) is a measurement of the ability of an optical fiber to capture light. The NA can be

defined from the refractive index profile or the output far-field radiation pattern. Our discussion is limited to

measuring the NA from the output far-field radiation pattern. The NA of a multimode fiber having a near-

parabolic refractive index profile is measured the fiber NA is measured from the output far-field radiation

pattern. The far-field power distribution describes the emitted power per unit area in the far-field region. The

far-field region is the region far from the fiber-end face. The far-field power distribution describes the emitted

power per unit area as a function of angle &Theta; some distance away from the fiber-end face. The distance

between the fiber-end face and detector in the far-field region is in the centimeters (cm) range for multimode

fibers and millimeters (mm) range for single mode fibers. These procedures involve either an angular or spacial

scan. Figure illustrates an angular and spacial scan for measuring the far-field power distribution.

Field measurements differ from laboratory measurements because they measure the transmission properties of

installed fiber optic components. Laboratory measurements can only attempt to simulate the actual operating

conditions of installed components. Fiber optic component properties measured in the laboratory can change

after the installation of these components on board ship. End users must perform field measurements to

evaluate those properties most likely affected by the installation or repair of fiber optic components or systems.

The discussion on field measurements is limited to optical fiber and optical connection properties. Optical fiber

and optical connection field measurements evaluate only the transmission properties affected by component or

system installation or repair. Because optical fiber geometrical properties, such as core and cladding diameter

and numerical aperture, are not expected to change, there is no need to remeasure these properties. The optical

fiber properties that are likely to change include fiber attenuation (loss) and bandwidth. Bandwidth changes in

the field tend to be beneficial, so field bandwidth measurement is generally not performed. If field bandwidth

measurements are required, they are essentially the same as laboratory measurements so they will not be

56

repeated. The optical connection properties that are likely to change are connection insertion loss and

reflectance and return loss.

The installation and repair of fiber optic components on board ship can affect system operation. Microbends

introduced during installation can increase fiber attenuation. Modal redistribution at fiber joints can increase

fiber attenuation in the fiber after the joint. Fiber breaks or faults can prevent or severely disrupt system

operation. Poor fiber connections can also increase insertion loss and degrade transmitter and receiver

performance by increasing reflectance and return loss. End users should perform field measurements to verify

that component performance is within allowable limits so system performance is not adversely affected. There

are additional differences in measuring optical fiber and optical connection properties in the field than in the

laboratory. Field measurements require rugged, portable test equipment, unlike the sophisticated test

equipment used in the laboratory. Field test equipment must provide accurate measurements in extreme

environmental conditions. Since electrical power sources may not always be available in the field, test

equipment should allow battery operation. In addition, while both fiber ends are available for conducting

laboratory measurements, only one fiber end may be readily available for field measurements. Even if both

fiber ends are available for field measurements, the fiber ends are normally located some distance apart.

Therefore, field measurements may require two people.

The main field measurement technique involves optical time-domain reflectometry. An optical time-domain

reflectometer (OTDR) is recommended for conducting field measurements on installed optical fibers or links

of 50 meters or more in length. An OTDR requires access to only one fiber end. An OTDR measures the

attenuation of installed optical fibers as a function of length. It also identifies and evaluates optical connection

losses along a cable link and locates any fiber breaks or faults.

End users can also measure fiber attenuation and cable plant transmission loss using an optical power meter

and a stabilized light source. End users use this measurement technique when optical time-domain

reflectometry is not recommended. Measurements obtained with a stabilized light source and power meter are

more accurate than those obtained with an OTDR. Measuring fiber attenuation and transmission loss using a

power meter and light source requires access to both ends of the fiber or link. An optical loss test set (OLTS)

combines the power meter and source functions into one physical unit

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The backscattering method was invented by M. Barnoskim and M. Jensen in 1976 [1], in time when

technology of the optical fiber manufacturing was at early stages. The precise and reliable measurement of

local losses on the fiber was very important for further improvement of quality of fibers. In the paper cited

above the authors describe a new method for the loss distribution along the fiber. The basic idea of the

proposed method consisted in launching a rather short and high power optical impulse into the tested fiber and

a consequent detection of back scattered optical power as a response of the fiber to the test impulse. The

detected signal provides the detail picture about the local loss distribution or reflections along the fiber caused

by any of the attenuation mechanisms or some other nonhomogeneities on the fiber. An important feature of

the method is non-destructivity and the fact that the access to only input end of the fiber is needed.The

measurement of the time delay of the detected signal from the fiber end or from any perturbation on the fiber

allows to derive the information about the perturbation localization provided that the index of refraction in the

fiber core or group velocity of light propagation is known. In any point on the fiber the magnitude of the

backscattered optical power is proportional to the local transmitted optical power. Due to the nonzero losses

this power is gradually attenuated along the fiber and consequently also the backscattered power is also

attenuated. The measurement of the backscattered power as a function of time or position on the fiber gives the

information about the local distribution of the attenuation coefficient along the fiber. In this way one can

evaluate the space distribution and magnitude of various non-homogeneities along the fiber like optical

connectors, splicings, micro- and macro-bend losses and others measurand-perturbances. The comparison of

the losses closely before and after point of interest makes possible to evaluate insertion losses of the various

optical components on the fiber link.

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UNIT-V

LASER

Principles of Lasers — Spontaneous Emission, Stimulated Absorption and Emission

Atoms at higher energy levels tend to, or spontaneously jump to lower energy levels, in the same time they

give out the electromagnetic radiation having the energy equal to the difference between the two energy levels.

This is called spontaneous decay or spontaneous emission. The emission frequency is decided by:

hn = E2 - E1, E2>E1

All objects above absolute zero temperature have spontaneous emission. At thermal equilibrium, the number of

atoms at different energy levels obeys the Boltzmann population distribution equation:

N2= N1 exp[-(E2-E1)/kT]

Where N2 and N1 are numbers of atoms at energy state E2 and E1 repectively.

Since E2>E1, N2/N1 at thermal equilibrium will be less than one. This is the normal population distribution. If

N2>N1, we say population inversion exists.

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Spontaneous radiation is only one of the two forms of atomic relaxation or decay, the other is non-radiation

relaxation processes like collision, thermal dissipation, etc. So the total decay time of E2 to E1 is composed of

both radiative and non-radiative parts, T21=Trad,21+Tnr,21.

Stimulated transitions are essential for lasers to work. Let’s analyze the following experiment. We incident

broadband light on a collection of atoms, and use the grating spectrometer to measure the light passing through

the medium. We will find that the detected light energy distribution with wavelength has changed from a

relative smooth curve to a curve with discrete absorption lines, see the figure below.

This indicates that the atoms absorb the incident energy at certain frequencies. When the atoms at lower energy

levels absorb the incident energy with corresponding frequency, they jump to upper level states, this is called

Stimulated Absorption. This process reduces the lower level population and increases the upper level

population.

In the same time, under the action of the incident electromagnetic field with the corresponding frequency, the

atoms at upper level have the same possibility to jump to the corresponding lower levels, emitting

electromagnetic waves or photons with the same frequency, direction and phase with the incident waves. This

process is called Stimulated Emission. Stimulated emission reduces the upper level population, increases the

lower level population. Another very important thing is that it also coherently increases the incident EM wave,

it transfers the pumping or incident energy into light energy! One incident photon after stimulated emission

becomes two photons with same frequency, direction and phase. If this process can dominate over absorption

processes, the coherent light can be amplified, i.e., becomes more and more intense with time. Then the Laser

— Light Amplification by Stimulated Emission of Radiation occurs. Aren’t you excited?

But under normal conditions, we could not get laser. Why? The reason is: the lower level and upper level

atoms have the same chance to jump upward or downward, the upward transition absorbs the incident energy,

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downward transition amplifies the incident energy, so it is the population difference between the two levels

that decides whether there can exist a net amplification of the incident energy. At thermal equilibrium of

normal conditions, the upper level atom population is always less than the lower level atom population, so the

stimulating radiation can only attenuate when it interacts with the medium. We must first make the upper level

population larger than the lower level to generate laser light, i.e., we must create Population Inversion first.

Rate Equations and Population Inversion

Two-level Atomic System

Let’s first examine the two-level atomic system. Pumping process provides the incident radiation satisfying hn

= E2 - E1, E2>E1. Let’s define W12 as the possibility of atoms jumping from E1 to E2 because of stimulated

absorption, define W21 as the possibility of atoms jumping from E2 to E1 because of stimulated emission,

define w12 and w21 as the corresponding relaxation or decay rate (including both spontaneous radiation rate

and non-radiation decay rate). Then the rate equations for two-level atomic system are:

N is the total atom number, ”N=N1-N2 is the population difference. T1 is the population recovery time or

energy relaxation time of the system. We also define N10 and N20 as the atom population at thermal

equilibrium, ”N0=N10-N20 as the population difference at thermal equilibrium. Note also W12=W21. After

some operation, we get:

At steady state, i.e., when ”N doesn’t change with time, we get:

”N0 is bigger than zero, W12 is closely related with the incident signal and the stronger the incident signal, the

bigger the W12, W12>0. From the above equation we see, for a two-level atomic system, the incident signal

will make the population difference ”N approach zero when the signal is bigger enough, but no population

inversion occurs! The best it can go is at steady state or at saturation, the population difference becomes 0.1

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The conclusion is: to get population inversion, we must use atomic systems with more than two related energy

levels.

Three-level Laser System

Now let’s examine the three level laser system.

For the three-level laser system, E1 is the ground state, lasing is between E2 and E1. Ruby laser is a typical

three level laser system.

Supposing the pumping process produces a stimulated transition probability between E1 and E3,

W13=W31=Wp. Atoms at E3 have fast decay time T32, i.e., atoms at E3 decay to E2 in a very short time T32.

Atoms at E2 have a relatively slow transition time T21, i.e., atoms at E2 will take a longer time to change to E1

than atoms from E3 to E2. Also we have N=N1+N2+N3. So we have rate equations for three level laser

systems:

At steady state, the population difference between E2 and E1 is:

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From above we see, for population inversion to occur, i.e., for N2-N1>0, the pumping rate Wp must satisfy:

WpT21>1/(1-b ). If we assume the atoms at E3 decay to E2 immediately, thus b =0, then at steady state, we

have:

Conclusion: population inversion is possible for a three level atomic system. The condition is: T32<<T21 and

the pumping rate must be bigger than a positive threshold value. Because N1 is the ground level, N1 is always

very big in the beginning. Population inversion starts after half of the ground level atoms being pumped to the

E2 level. So three-level laser system is not very efficient, present lasers are usually four-level or more level

systems.

Now let’s see what advantages the four-level laser systems have over three-level systems

Four-level Laser System

Nd:YAG laser is a four level laser system. Look at the four level laser model below.

 

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We have E1, N1 at the ground level, pumping process raise the atoms from E1 to E4, pumping rate is

Wp=W14=W41. Atoms at E4 have fast decay to E3, decay time is T43. Lasing happens between E3 and E2,

the transmission time is Trad. Atoms at E2 then decay very fast to E1, the decay time is T21. We have the

relation: N=N1+N2+N3+N4. Then we have rate equations for four-level laser systems:

For simplification, we assume T43 is short enough that the pumped atoms to E4 immediately decay to E3, N4

is nearly 0. Also atoms at E2 decay so fast that we can say N2 is nearly zero. Let’s examine population

inversion between N3 and N2. We list the conclusion here:

Since b » 0 for a four level system, N3-N2 is readily bigger than zero. There is almost no threshold for Wp to

generate population conversion. The advantage of four-level laser system is very clear now.

Up to now we have discussed population inversion conditions by analyzing the pumping rate equations. We

have to make a statement here: we make some simplifications to make the material easily understood. The

actual detailed rate equations are far more complex than we see here, we suggest the interested readers refer

books on principles of lasers for further questions.

When we get population inversion, our next step is to analyze the amplification of incident waves.

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Optical feedback

Optical feedback is the optical equivalent of acoustic feedback. The feedback occurs when a loop exists

between an optical input, for example, a videocamera, and an optical output, for example, a television screen or

monitor. (A simple example of optical feedback is an image cast between mirrors.)In this GIF movie, and the

JPG still image examples (right), light from a candle is received by a videocamera, amplified and then sent by

cable to a monitor projecting electron beams on the inside of the monitor screen. The image on the monitor is

then captured by the videocamera again, and fed back to the monitor in a continuous loop.The original light

source, in this case from the candle, can then be extinguished, while the feedback loop continues. For each

loop the image is doubled and the image interferes with itself. The electronic loop moves with near light speed,

but as the resulting image is projected onto the phosphor dots on the inside of the screen by the electron beam,

the phosphor points take time to begin and stop glowing, and this creates a persistence which prevents the

patterns changing too fast, and thus they survive long enough to be perceived.The resulting images depend on

different camera and monitor settings, such as light amplification, contrast, distance, angle and physical

vibrations. Optical feedback can be combined with music, or other sound sources, to influence the image loop.

Lasing threshold

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The lasing threshold is the lowest excitation level at which a laser's output is dominated by stimulated

emission rather than by spontaneous emission. Below the threshold, the laser's output power rises slowly with

increasing excitation. Above threshold, the slope of power vs. excitation is orders of magnitude greater. The

linewidth of the laser's emission also becomes orders of magnitude smaller above the threshold than it is

below. Above the threshold, the laser is said to be lasing. The term "lasing" is a back formation from "laser,"

which is an acronym, not an agent noun.

Q-switching :

Q-switching:sometimes known as giant pulse formation, is a technique by which a laser can be made to

produce a pulsed output beam. The technique allows the production of light pulses with extremely high

(gigawatt) peak power, much higher than would be produced by the same laser if it were operating in a

continuous wave (constant output) mode. Compared to modelocking, another technique for pulse generation

with lasers, Q-switching leads to much lower pulse repetition rates, much higher pulse energies, and much

longer pulse durations. Both techniques are sometimes applied at once.

Principle of Q-switching

Q-switching is achieved by putting some type of variable attenuator inside the laser's optical resonator. When

the attenuator is functioning, light which leaves the gain medium does not return, and lasing cannot begin. This

attenuation inside the cavity corresponds to a decrease in the Q factor or quality factor of the optical resonator.

A high Q factor corresponds to low resonator losses per roundtrip, and vice versa. The variable attenuator is

commonly called a "Q-switch", when used for this purpose.

Initially the laser medium is pumped while the Q-switch is set to prevent feedback of light into the gain

medium (producing an optical resonator with low Q). This produces a population inversion, but laser operation

cannot yet occur since there is no feedback from the resonator. Since the rate of stimulated emission is

dependent on the amount of light entering the medium, the amount of energy stored in the gain medium

increases as the medium is pumped. Due to losses from spontaneous emission and other processes, after a

certain time the stored energy will reach some maximum level; the medium is said to be gain saturated. At this

point, the Q-switch device is quickly changed from low to high Q, allowing feedback and the process of optical

amplification by stimulated emission to begin. Because of the large amount of energy already stored in the gain

medium, the intensity of light in the laser resonator builds up very quickly; this also causes the energy stored in

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the medium to be depleted almost as quickly. The net result is a short pulse of light output from the laser,

known as a giant pulse, which may have a very high peak intensity.

There are two main types of Q-switching:

Active Q-switching

Here, the Q-switch is an externally-controlled variable attenuator. This may be a mechanical device such as a

shutter, chopper wheel or spinning mirror placed inside the cavity, or (more commonly) it may be some form

of modulator such as an acousto-optic device or an electro-optic device — a Pockels cell or Kerr cell. The

reduction of losses (increase of Q) is triggered by an external event, typically an electrical signal. The pulse

repetition rate can therefore be externally controlled.

Modulators generally allow a faster transition from low to high Q, and provide better control. An additional

advantage of modulators is that the rejected light may be coupled out of the cavity and can be used for

something else. Alternatively, when the modulator is in its low-Q state, an externally-generated beam can be

coupled into the cavity through the modulator. This can be used to "seed" the cavity with a beam that has

desired characteristics (such as transverse mode or wavelength). When the Q is raised, lasing builds up from

the initial seed, producing a Q-switched pulse that has characteristics inherited from the seed.

Passive Q-switching

In this case, the Q-switch is a saturable absorber, a material whose transmission increases when the intensity of

light exceeds some threshold. The material may be an ion-doped crystal like Cr:YAG, which is used for Q-

switching of Nd:YAG lasers, a bleachable dye, or a passive semiconductor device. Initially, the loss of the

absorber is high, but still low enough to permit some lasing once a large amount of energy is stored in the gain

medium. As the laser power increases, it saturates the absorber, i.e., rapidly reduces the resonator loss, so that

the power can increase even faster. Ideally, this brings the absorber into a state with low losses to allow

efficient extraction of the stored energy by the laser pulse. After the pulse, the absorber recovers to its high-

loss state before the gain recovers, so that the next pulse is delayed until the energy in the gain medium is fully

replenished. The pulse repetition rate can only indirectly be controlled, e.g. by varying the laser's pump power

and the amount of saturable absorber in the cavity. Direct control of the repetition rate can be achieved by

using a pulsed pump source as well as passive Q-switching.

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Applications

Q-switched lasers are often used in applications which demand high laser intensities in nanosecond pulses,

such metal cutting or pulsed holography. Nonlinear optics often takes advantage of the high peak powers of

these lasers, offering applications such as 3D optical data storage and 3D microfabrication. However, Q-

switched lasers can also be used for measurement purposes, such as for distance measurements ( range finding)

by measuring the time it takes for the pulse to get to some target and the reflected light to get back to the

sender.

Q-switched lasers are also used to remove tattoos. They are used to shatter tattoo pigment into particles that are

cleared by the body's lymphatic system. Full removal takes an average of eight treatments, spaced at least a

month apart, using different wavelengths for different colored inks.

Mode-locking

Mode-locking is a technique in optics by which a laser can be made to produce pulses of light of extremely

short duration, on the order of picoseconds (10-12s) or femtoseconds (10-15s).The basis of the technique is to

induce a fixed phase relationship between the modes of the laser's resonant cavity. The laser is then said to be

phase-locked or mode-locked. Interference between these modes causes the laser light to be produced as a train

of pulses. Depending on the properties of the laser, these pulses may be of extremely brief duration, as short as

a few femtoseconds.

Laser cavity modes

Although laser light is perhaps the purest form of light, it is not of a single, pure frequency or wavelength. All

lasers produce light over some natural bandwidth or range of frequencies. A laser's bandwidth of operation is

determined primarily by the gain medium that the laser is constructed from, and the range of frequencies that a

laser may operate over is known as the gain bandwidth. For example, a typical helium-neon (HeNe) gas laser

has a gain bandwidth of approximately 1.5 GHz (a wavelength range of about 0.002 nm at a central

wavelength of 633 nm), whereas a titanium-doped sapphire (Ti:Sapphire) solid-state laser has a bandwidth of

about 128 THz (a 300 nm wavelength range centred around 800 nm).The second factor that determines a

laser's emission frequencies is the optical cavity or resonant cavity of the laser. In the simplest case, this

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consists of two plane (flat) mirrors facing each other, surrounding the gain medium of the laser (this

arrangement is known as a Fabry-Perot cavity). Since light is a wave, when bouncing between the mirrors of

the cavity the light will constructively and destructively interfere with itself, leading to the formation of

standing waves between the mirrors.

These standing waves form a discrete set of frequencies, known as the longitudinal modes of the cavity. These

modes are the only frequencies of light which are self-regenerating and allowed to oscillate by the resonant

cavity; all other frequencies of light are suppressed by destructive interference. For a simple plane-mirror

cavity, the allowed modes are those for which the separation distance of the mirrors L is an exact multiple of

half the wavelength of the light λ, such that L = q λ/2, when q is an integer known as the mode order.

In practice, the separation distance of the mirrors L is usually much greater than the wavelength of light λ, so

the relevant values of q are large (around 105 to 106). Of more interest is the frequency separation between any

two adjacent modes q and q+1; this is given (for an empty linear resonator of length L) by Δν:

where c is the speed of light (≈3×108 m·s−1).

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Mode-locking theory

In a simple laser, each of these modes will oscillate independently, with no fixed relationship between each

other, in essence like a set of independent lasers all emitting light at slightly different frequencies. The

individual phase of the light waves in each mode is not fixed, and may vary randomly due to such things as

thermal changes in materials of the laser. In lasers with only a few oscillating modes, interference between the

modes can cause beating effects in the laser output, leading to random fluctuations in intensity; in lasers with

many thousands of modes, these interference effects tend to average to a near-constant output intensity, and the

laser operation is known as a c.w. or continuous wave.If instead of oscillating independently, each mode

operates with a fixed phase between it and the other modes, the laser output behaves quite differently. Instead

of a random or constant output intensity, the modes of the laser will periodically all constructively interfere

with one another, producing an intense burst or pulse of light. Such a laser is said to be mode-locked or phase-

locked. These pulses occur separated in time by τ = 2L/c, where τ is the time taken for the light to make exactly

one round trip of the laser cavity. This time corresponds to a frequency exactly equal to the mode spacing of

the laser, Δν = 1/τ.

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The duration of each pulse of light is determined by the number of modes which are oscillating in phase (in a

real laser, it is not necessarily true that all of the laser's modes will be phase-locked). If there are N modes

locked with a frequency separation Δν, the overall mode-locked bandwidth is NΔν, and the wider this

bandwidth, the shorter the pulse duration from the laser. In practice, the actual pulse duration is determined by

the shape of each pulse, which is in turn determined by the exact amplitude and phase relationship of each

longitudinal mode. For example, for a laser producing pulses with a Gaussian temporal shape, the minimum

possible pulse duration Δt is given byThe value 0.44 is known as the time-bandwidth product of the pulse, and

varies depending on the pulse shape. For ultrashort pulse lasers, a hyperbolic-secant-squared (sech2) pulse

shape is often assumed, giving a time-bandwidth product of 0.315.Using this equation, we can calculate the

minimum pulse duration which can be produced by a laser. For the HeNe laser with a 1.5 GHz bandwidth, the

shortest Gaussian pulse which can be produced would be around 300 picoseconds; for the 128 THz bandwidth

Ti:sapphire laser, this duration would be only 3.4 femtoseconds. These values represent the shortest possible

Gaussian pulses supported by the laser's bandwidth; in a real mode-locked laser, the actual pulse duration

depends on many other factors, such as the actual pulse shape, and the overall dispersion of the cavity.

Mode-locking methods

Methods for producing mode-locking in a laser may be classified as either active or passive. Active methods

typically involve using an external signal to induce a modulation of the intra-cavity light. Passive methods do

not use an external signal, but rely on placing some element into the laser cavity which causes self-modulation

of the light.

Active mode-locking

The most common active mode-locking technique places an electro-optic modulator into the laser cavity.

When driven with an electrical signal, this produces a sinusoidal phase modulation of the light in the cavity.

Considering this in the frequency domain, if a mode has optical frequency ν, and is phase-modulated at a

frequency f, the resulting signal has sidebands at optical frequencies ν-f and ν+f. If the modulator is driven at

the same frequency as the cavity-mode spacing Δν, then these sidebands correspond to the two cavity modes

adjacent to the original mode. Since the sidebands are driven in-phase, the central mode and the adjacent

modes will be phase-locked together. Further operation of the modulator on the sidebands produces phase-

locking of the ν-2f and ν+2f modes, and so on until all modes in the gain bandwidth are locked. As said above,

typical lasers are multi-mode and not seeded by a root mode. So multiple modes need to work out which phase

to use. In a passive cavity with this locking applied there is no way to dump the entropy given by the original

independent phases. This locking is better described as a coupling, leading to a complicated behavior and not

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clean pulses. The coupling is only dissipative because of the dissipative nature of the amplitude modulation.

Otherwise the phase modulation would not work.Related to this amplitude modulation (AM) active mode-

locking is frequency modulation (FM) mode-locking, which uses a standing wave modulator device based on

the acousto-optic effect. This device, when placed in a laser cavity and driven with an electrical signal, induces

a small, sinusoidally varying frequency shift in the light passing through it. If the frequency of modulation is

matched to the round-trip time of the cavity, then some light in the cavity sees repeated up-shifts in frequency,

and some repeated down-shifts. After many repetitions, the up-shifted and down-shifted light is swept out of

the gain bandwidth of the laser. The only light which is unaffected is that which passes through the modulator

when the induced frequency shift is zero, which forms a narrow pulse of light.

This process can also be considered in the time domain. The amplitude modulator acts as a weak shutter to the

light bouncing between the mirrors of the cavity, attenuating the light when it is "closed", and letting it through

when it is "open". If the modulation rate f is synchronised to the cavity round-trip time τ, then a single pulse of

light will bounce back and forth in the cavity. The actual strength of the modulation does not have to be large;

a modulator that attenuates 1% of the light when "closed" will mode-lock a laser, since the same part of the

light is repeatedly attenuated as it traverses the cavity.

The third method of active mode-locking is synchronous mode-locking, or synchronous pumping. In this, the

pump source (energy source) for the laser is itself modulated, effectively turning the laser on and off to

produce pulses. Typically, the pump source is itself another mode-locked laser. This technique requires

accurately matching the cavity lengths of the pump laser and the driven laser.

Passive mode-locking

Passive mode-locking techniques are those that do not require a signal external to the laser (such as the driving

signal of a modulator) to produce pulses. Rather, they use the light in the cavity to cause a change in some

intracavity element, which will then itself produce a change in the intracavity light. The most common type of

device which will do this is a saturable absorber.A saturable absorber is an optical device that exhibits an

intensity-dependent transmission. What this means is that the device behaves differently depending on the

intensity of the light passing through it. For passive mode-locking, ideally a saturable absorber will selectively

absorb low-intensity light, and transmit ligh

t which is of sufficiently high intensity.When placed in a laser cavity, a saturable absorber will attenuate low-

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intensity constant wave light (pulse wings). However, because of the somewhat random intensity fluctuations

experienced by an un-mode-locked laser, any random, intense spike will be transmitted preferenti

ally by the saturable absorber. As the light in the cavity oscillates, this process repeats, leading to the selective

amplification of the high-intensity spikes, and the absorption of the low-intensity light. After many round trips,

this leads to a train of pulses and mode-locking of the laser.Considering this in the frequency domain, if a

mode has optical frequency ν, and is amplitude-modulated at a frequency n f, the resulting signal has sidebands

at optical frequencies ν - n f and ν + n f and enables much stronger mode-locking for shorter pulses and more

stability than active mode-locking, but has startup problems.

Saturable absorbers are commonly liquid organic dyes, but they can also be made from doped crystals and

semiconductors. Semiconductor absorbers tend to exhibit very fast response times (~100 fs), which is one of

the factors that determines the final duration of the pulses in a passively mode-locked laser. In a colliding-pulse

mode-locked laser the absorber steepens the leading edge while the lasing medium steepens the trailing edge of

the pulse. There are also passive mode-locking schemes

that do not rely on materials that directly display an intensity dependent absorption. In these methods,

nonlinear optical effects in intra-cavity components are used to provide a method of selectively amplifying

high-intensity light in the cavity, and attenuation of low-intensity light. One of the most successful schemes is

called Kerr-lens mode-locking (KLM), also sometimes called "self mode-locking". This uses a nonlinear

optical process, the optical Kerr effect, which results in high-intensity light being focussed differently than

low-intensity light. By careful arrangement of an aperture in the laser cavity, this effect can be exploited to

produce the equivalent of an ultra-fast response time saturable absorber.

APPLICATION OF LASER

Holography (from the Greek, ὅλος-hólos whole + γραφή-grafē writing, drawing) is a technique that allows the

light scattered from an object to be recorded and later reconstructed so that it appears as if the object is in the

same position relative to the recording medium as it was when recorded. The image changes as the position

and orientation of the viewing system changes in exactly the same way as if the object were still present, thus

making the recorded image (hologram) appear three dimensional.In holography, some of the light scattered

from an object or a set of objects falls on the recording medium. A second light beam, known as the reference

beam, also illuminates the recording medium, so that interference occurs between the two beams. The resulting

light field is an apparently random pattern of varying intensity which is the hologram. It can be shown that if

the hologram is illuminated by the original reference beam, a light field is diffracted by the reference beam

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which is identical to the light field which was scattered by the object or objects. Thus, someone looking into

the hologram 'sees' the objects even though it may no longer be present. There are a variety of recording

materials which can be used, including photographic film.

Point sources

Holographic reconstruction process

A slightly more complicated hologram can be made using a point source of light as object beam and a plane

wave as reference beam to illuminate the photographic plate. An interference pattern is formed which in this

case is in the form of curves of decreasing separation with increasing distance from the centre.The

photographic plate is developed giving a complicated pattern which can be considered to be made up of a

diffraction pattern of varying spacing. When the plate is illuminated by the reference beam alone, it is

diffracted by the grating into different angles which depend on the local spacing of the pattern on the plate. It

can be shown that the net effect of this is to reconstruct the object beam, so that it appears that light is coming

from a point source behind the plate, even when the source has been removed. The light emerging from the

photographic plate is identical to the light that emerged from the point source that used to be there. An

observer looking into the plate from the other side will "see" a point source of light whether the original source

of light is there or not.This sort of hologram is effectively a concave lens, since it "converts" a plane wavefront

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into a divergent wavefront. It will also increase the divergence of any wave which is incident on it in exactly

the same way as a normal lens does. Its focal length is the distance between the point source and the plate

Distance Measurements with Lasers

The prevalent method to determine distance to an object or surface is to use laser pulses.

The primary difference between lidar and radar is lidar uses much shorter wavelengths of the electromagnetic

spectrum, typically in the ultraviolet, visible, or near infrared range. In general it is possible to image a feature

or object only about the same size as the wavelength, or larger. Thus lidar is highly sensitive to aerosols and

cloud particles and has many applications in atmospheric research and meteorology[1].

general there are two kinds of lidar detection schema: "incoherent" or direct energy detection (which is

principally an amplitude measurement) and Coherent detection (which is best for doppler, or phase sensitive

measurements). Coherent systems generally use Optical heterodyne detection which being more sensitive than

direct detection allows them to operate a much lower power but at the expense of more complex transceiver

requirements.

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In both coherent and incoherent LIDAR, there are two types of pulse models: micropulse lidar systems and

high energy systems. Micropulse systems have developed as a result of the ever increasing amount of

computer power available combined with advances in laser technology. They use considerably less energy in

the laser, typically on the order of one microjoule, and are often "eye-safe," meaning they can be used without

safety precautions. High-power systems are common in atmospheric research, where they are widely used for

measuring many atmospheric parameters: the height, layering and densities of clouds, cloud particle properties

(extinction coefficient, backscatter coefficient, depolarization), temperature, pressure, wind, humidity, trace

gas concentration (ozone, methane, nitrous oxide, etc.)[1].

There are several major components to a lidar system:

1. Laser — 600-1000 nm lasers are most common for non-scientific applications. They are inexpensive but since

they can be focused and easily absorbed by the eye the maximum power is limited by the need to make them

eye-safe. Eye-safety is often a requirement for most applications. A common alternative 1550 nm lasers are

eye-safe at much higher power levels since this wavelength is not focused by the eye, but the detector

technology is less advanced and so these wavelengths are generally used at longer ranges and lower accuracies.

They are also used for military applications as 1550 nm is not visible in night vision goggles unlike the shorter

1000 nm infrared laser. Airborne topographic mapping lidars generally use 1064 nm diode pumped YAG

lasers, while bathymetric systems generally use 532 nm frequency doubled diode pumped YAG lasers because

532 nm penetrates water with much less attenuation than does 1064 nm. Laser settings include the laser

repetition rate (which controls the data collection speed). Pulse length is generally an attribute of the laser

cavity length, the number of passes required through the gain material (YAG, YLF, etc.), and Q-switch speed.

Better target resolution is achieved with shorter pulses, provided the Lidar receiver detectors and electronics

have sufficient bandwidth[1].

2. Scanner and optics — How fast images can be developed is also affected by the speed at which it can be

scanned into the system. There are several options to scan the azimuth and elevation, including dual oscillating

plane mirrors, a combination with a polygon mirror, a dual axis scanner. Optic choices affect the angular

resolution and range that can be detected. A hole mirror or a beam splitter are options to collect a return signal.

3. Photodetector and receiver electronics — Two main photodetector technologies are used in lidars: solid state

photodetectors, such as silicon avalanche photodiodes, or photomultipliers. The sensitivity of the receiver is

another parameter that has to be balanced in a LIDAR design.

4. Position and navigation systems — Lidar sensors that are mounted on mobile platforms such as airplanes or

satellites require instrumentation to determine the absolute position and orientation of the sensor. Such devices

generally include a Global Positioning System receiver and an Inertial Measurement Unit (IMU).

76

Velocity Measurement using Laser Doppler Velocimeter

Introduction

Measurement of instantaneous point velocities in water is of very importance in two

types of studies in hydraulics viz. (i) boundary layer studies, in which point velocities are

measured close to a solid boundary and, (ii) turbulence studies, in which the random

fluctuations of velocity at a point are to be measured.

Only two types of instrument are used for the measurement of instantaneous point

velocities in such fields: (i) Hot film anemometer; and (ii) Laser Doppler velocimeter.

The Hot film anemometer represents a complicated technique of measurement, which

often gives erroneous results as the measurement is affected by changes in ambient

temperature, turbidity of water etc. For this reason, the simple and more reliable

77

technique represented by the Laser Doppler velocimeter is generally used.

The Laser Doppler velocimeter represents a unique no-probe technique of measuring

the instantaneous point velocities in a fluid flow. It was developed around the year 1968

as a result of research work carried out by the National Aeronautics and Space

Administration, U.S.A. The instrument in the Hydraulic and Water Resource Engineering

Laboratory IIT Madras was locally built in 1974 by Jayaraman and later on it was

modified in 1995 – 1997.

Principle of the Instrument

The instrument measures the velocity at a point in the fluid, flowing in a glass walled

conduit or channel, by detecting the Doppler shift in the frequency of the scattered light

originating from minute suspended particles in the flow that happen to cross the point of

measurement defined by two intersecting laser beams. The laser source is used as it

gives a narrow, intense and truly parallel (i.e. highly collimated) light beam of high

spectral purity (i.e. very low spectral bandwidth).

The salient features of this instrument are

(i) No physical probe is required, to obstruct the flow.

(ii) Excellent spatial resolution, (the measuring volume is of the order of 0.02 mm3

(iii) No transfer function is involved, the output voltage is linearly related to the Doppler

frequency which in turn, is linearly related to flow velocity.

(iv) Very fast response to fluctuating velocities, the typical frequency response going

upto 50 kHz for a Doppler frequency of 1 MHz.

(v) Can be used in both gas and liquid flows.

Theory of Measurement

Consider a light beam of wavelength μ crossing a flow stream at an angle β with

direction of flow (Figure). When illuminated by the light beam suspended particles in the

flow will scatter light in all directions. This scattered behaves as though it originated

from moving particles, and hence is Doppler-shifted with reference to the frequency ofthe incident light.

78

can be shown that if the scattered light is picked up at an angle θ with the direction of

the incident light beam, the Doppler shift fD is given by D ( )

f = v cosθ -1 cosβ - sinθ sinβ

μ

in which v is the flow velocity. It may be noted that when θ = 0, fD= 0 . For small values

of (i.e., less than 10 degrees), increases as increases.

Instrumental set-up

The instrument can be set up in either of two modes of measurements, namely, the

Reference beam mode and the interference fringe mode. Fig. 2 shows the set-up in the

reference beam mode.

79

The instrument has four components

(i) Laser source Helium-neon, (ii) Beam splitter; (iii) Light pick-up unit; and (iv) Signalprocessor. By means

of a Beam splitter the light from a 2 mill watt Helium-Neon laser issplit up into a strong (95% power)

scattering beam and a weak (5% power) referencebeam. The beam inclination θ could be raised using

adjustment scale (provided inbeam splitter) and also the intensity of reference beam could be attenuated by

means ofa lens in the beam splitterThe two beams, which are kept equally inclined to the flowdirection, are

made to intersect at the point of velocity measurement in the channel.When illuminated by the strong

scattered beam minute suspended particles in the flowmicrons. Too fine particles will lead to Brownian

motion, whereas too large particles willncrease the system noise by excessive masking of the light picked up.

Ordinary tapwater contains adequate fine suspended particles to provide enough scattering. Thepresence of

large particles, (which is not desirable) in water is indicated by theintermittent glittering of the laser beam.

The light scattered by suspended particles inDoppler-shifted. The magnitude of the shift depends on the

direction in which it is pickedup. On the other side of the channel, the light pick-up unit is so positioned, and

itselescope so focused, that it picks up the reference beam as well as scattered light inthe same direction

originating from the point of intersection of the two beams. The lightis focused on ta tiny electronic device

called P.I.N. photodiode, where optical mixing takes place and a weak electrical signal, (the Doppler signal)

emerges, having afrequency equal to the Doppler shift. The Light pick-up unit contains a built-ipreamplifier

to boost the Doppler signal to a more appropriate level for transmission tothe Signal processor unit. In the

Signal processor unit, the Doppler signal is first passedthrough one of a set of three sharp band-pass filters to

reduce the noise content of theDoppler signal. It is then further amplified. The frequency of the Doppler

signal is thenconverted to a proportional voltage by an f-V converter. The converter has fastresponse which

enables it to track closely the Doppler frequency.

80

Solid-state lasers

Solid-state laser materials are commonly made by "doping" a crystalline solid host with ions that

provide the required energy states. For example, the first working laser was a ruby laser, made from

ruby (chromium-doped corundum). The population inversion is actually maintained in the "dopant",

such as chromium or neodymium. Formally, the class of solid-state lasers includes also fiber laser, as

the active medium (fiber) is in the solid state. Practically, in the scientific literature, solid-state laser

usually means a laser with bulk active medium, while wave-guide lasers are caller fiber lasers.

"Semiconductor lasers" are also solid-state lasers, but in the customary laser terminology, "solid-state

laser" excludes semiconductor lasers, which have their own name.

Neodymium is a common "dopant" in various solid-state laser crystals, including yttrium orthovanadate

(Nd:YVO4), yttrium lithium fluoride (Nd:YLF) and yttrium aluminium garnet (Nd:YAG). All these

lasers can produce high powers in the infrared spectrum at 1064 nm. They are used for cutting, welding

and marking of metals and other materials, and also in spectroscopy and for pumping dye lasers. These

lasers are also commonly frequency doubled, tripled or quadrupled to produce 532 nm (green, visible),

355 nm (UV) and 266 nm (UV) light when those wavelengths are needed.Ytterbium, holmium,

thulium, and erbium are other common "dopants" in solid-state lasers. Ytterbium is used in crystals

such as Yb:YAG, Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF2, typically operating around

1020-1050 nm. They are potentially very efficient and high powered due to a small quantum defect.

Extremely high powers in ultrashort pulses can be achieved with Yb:YAG. Holmium-doped YAG

crystals emit at 2097 nm and form an efficient laser operating at infrared wavelengths strongly

absorbed by water-bearing tissues. The Ho-YAG is usually operated in a pulsed mode, and passed

through optical fiber surgical devices to resurface joints, remove rot from teeth, vaporize cancers, and

pulverize kidney and gall stones.

Titanium-doped sapphire (Ti:sapphire) produces a highly tunable infrared laser, commonly used for

spectroscopy as well as the most common ultrashort pulse laser.

Thermal limitations in solid-state lasers arise from unconverted pump power that manifests itself as

heat and phonon energy. This heat, when coupled with a high thermo-optic coefficient (dn/dT) can give

rise to thermal lensing as well as reduced quantum efficiency. These types of issues can be overcome

by another novel diode-pumped solid-state laser, the diode-pumped thin disk laser. The thermal

limitations in this laser type are mitigated by using a laser medium geometry in which the thickness is

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much smaller than the diameter of the pump beam. This allows for a more even thermal gradient in the

material. Thin disk lasers have been shown to produce up to kilowatt levels of power.[18

Semiconductor lasers

Semiconductor lasers are also solid-state lasers but have a different mode of laser

operation.Commercial laser diodes emit at wavelengths from 375 nm to 1800 nm, and wavelengths of

over 3 µm have been demonstrated. Low power laser diodes are used in laser printers and CD/DVD

players. More powerful laser diodes are frequently used to optically pump other lasers with high

efficiency. The highest power industrial laser diodes, with power up to 10 kW (70dBm), are used in

industry for cutting and welding. External-cavity semiconductor lasers have a semiconductor active

medium in a larger cavity. These devices can generate high power outputs with good beam quality,

wavelength-tunable narrow-linewidth radiation, or ultrashort laser pulses.

A 5.6 mm 'closed can' commercial laser diode, probably from a CD or DVD player.

Vertical cavity surface-emitting lasers (VCSELs) are semiconductor lasers whose emission direction is

perpendicular to the surface of the wafer. VCSEL devices typically have a more circular output beam

than conventional laser diodes, and potentially could be much cheaper to manufacture. As of 2005, only

850 nm VCSELs are widely available, with 1300 nm VCSELs beginning to be commercialized,[19] and

1550 nm devices an area of research. VECSELs are external-cavity VCSELs. Quantum cascade lasers

are semiconductor lasers that have an active transition between energy sub-bands of an electron in a

structure containing several quantum wells.The development of a silicon laser is important in the field

of optical computing. Silicon is the material of choice for integrated circuits, and so electronic and

silicon photonic components (such as optical interconnects) could be fabricated on the same chip.

Unfortunately, silicon is a difficult lasing material to deal with, since it has certain properties which

block lasing. However, recently teams have produced silicon lasers through methods such as fabricating

the lasing material from silicon and other semiconductor materials, such as indium(III) phosphide or

gallium(III) arsenide, materials which allow coherent light to be produced from silicon. These are

82

called hybrid silicon laser. Another type is a Raman laser, which takes advantage of Raman scattering

to produce a laser from materials such as silicon.

Gas lasers

Gas lasers using many gases have been built and used for many purposes.The helium-neon laser

(HeNe) emits at a variety of wavelengths and units operating at 633 nm are very common in education

because of its low cost.Carbon dioxide lasers can emit hundreds of kilowatts[14] at 9.6 µm and 10.6 µm,

and are often used in industry for cutting and welding. The efficiency of a CO2 laser is over

10%.Argon-ion lasers emit light in the range 351-528.7 nm. Depending on the optics and the laser tube

a different number of lines is usable but the most commonly used lines are 458 nm, 488 nm and

514.5 nm.A nitrogen transverse electrical discharge in gas at atmospheric pressure (TEA) laser is an

inexpensive gas laser producing UV light at 337.1 nm.[15]Metal ion lasers are gas lasers that generate

deep ultraviolet wavelengths. Helium-silver (HeAg) 224 nm and neon-copper (NeCu) 248 nm are two

examples. These lasers have particularly narrow oscillation linewidths of less than 3 GHz (0.5

picometers),[16] making them candidates for use in fluorescence suppressed Raman spectroscopy.

83

Holography

Holography (from the Greek, ὅλος-hólos whole + γραφή-grafē writing, drawing) is a technique that allows the

light scattered from an object to be recorded and later reconstructed so that it appears as if the object is in the

same position relative to the recording medium as it was when recorded. The image changes as the position

and orientation of the viewing system changes in exactly the same way as if the object were still present, thus

making the recorded image (hologram) appear three dimensional.

In holography, some of the light scattered from an object or a set of objects falls on the recording medium. A

second light beam, known as the reference beam, also illuminates the recording medium, so that interference

occurs between the two beams. The resulting light field is an apparently random pattern of varying intensity

which is the hologram. It can be shown that if the hologram is illuminated by the original reference beam, a

light field is diffracted by the reference beam which is identical to the light field which was scattered by the

object or objects. Thus, someone looking into the hologram 'sees' the objects even though it may no longer be

present. There are a variety of recording materials which can be used, including photographic film.

Point sources

Holographic reconstruction process

A slightly more complicated hologram can be made using a point source of light as object beam and a plane wave as reference beam to illuminate the photographic plate. An interference pattern is formed which in this case is in the form of curves of decreasing separation with increasing distance from the centre.

84

The photographic plate is developed giving a complicated pattern which can be considered to be made up of a diffraction pattern of varying spacing. When the plate is illuminated by the reference beam alone, it is diffracted by the grating into different angles which depend on the local spacing of the pattern on the plate. It can be shown that the net effect of this is to reconstruct the object beam, so that it appears that light is coming from a point source behind the plate, even when the source has been removed. The light emerging from the photographic plate is identical to the light that emerged from the point source that used to be there. An observer looking into the plate from the other side will "see" a point source of light whether the original source of light is there or not.

This sort of hologram is effectively a concave lens, since it "converts" a plane wavefront into a divergent wavefront. It will also increase the divergence of any wave which is incident on it in exactly the same way as a normal lens does. Its focal length is the distance between the point source and the plate

85

ARYA College of Engineering & ITVIII Sem ECE

Subject- Optical CommunicationAssignment-I

Q1. Explain Snell’s Law. What is total iinternal reflection?

Q2. Discuss the structure of an optical fiber. What are various types of fibers?

Q3. Differentiate betwwen meridonial and skew rays

Q4. Explain concept of wave optics and ray optics

86

ARYA College of Engineering & ITVIII Sem ECE

Subject- Optical Communication

Assignment-II

Q1. Explain the characteristics of LED.

Q2. Describe the various fiber alignment technique used in optical fibers.

Q3. Explain the LED edge emitter and surface emitter pattern

Q4. Define the term iinternal quantum efficiency.

87

ARYA College of Engineering & ITVIII Sem ECE

Subject- Optical CommunicationAssignment-III

Q1. What is an optical Detectors?

Q2 Define Quantum efficiency. Derive expression for respositivity.

Q3. Explain the impact ionization in Avalanche photodiodes

Q4. give the definition and explain the term Noise equivalent temperature

88

ARYA College of Engineering & ITVII Sem EIC

Subject- Fiber Optic InstrumentationAssignment-IV

Q1. Discuss with the help of suitable diagram the cut- back technique for total attenuationQ2. Explain under filled and overfilled conditionsQ3. Discuss the techniques used for measurement of Numerical Aperture.Q4. Explain Optical Time Domain Reflectometry.

89

ARYA College of Engineering & ITVII Sem EIC

Subject- Fiber Optic InstrumentationAssignment-V

Q1. What is LASER? Discuss anyone type of it in detail.

Q2. Explain various characteristics of a semiconductor LASER.

Q3. Explain the principle of operation of a quantum well laser diode

Q4. Explain the basic construction of a LASER diode.

90

ARYA College of Engineering & ITVIII Sem ECE

Subject- Optical CommunicationClass Test-I

MM-10 Time:1 hr Note: attempt any three question

Q1. Explain how glass fiber guides light from one end to other.

Q2. What is meant by acceptance angle for an optical fiber?

Q3. Derive an expression for NA.

Q4. Discuss basic law of ray optics.

91

ARYA College of Engineering & ITVIII Sem ECE

Subject- Optical CommunicationClass Test-II

MM-10 Time:1 hr Note: attempt any three question

Q1. Describe the mechanism of emission of light from one LED.

Q2. Define the term external Quantum efficiency and internal quantum efficiency.

Q3. What is LED? List out the main advantages of LED.

Q4. What is meant by a double hetero structure?

92

ARYA College of Engineering & ITVIII Sem ECE

Subject- Optical CommunicationClass Test-III

MM-10 Time:1 hr Note: attempt any three question

Q1. What is an optical detector? What are the required feature to be a detector?

Q2. Briefly explain the mechanism Avalanche photodiode/

Q3. Discuss the types of noise that comes in action with an optical detector.

Q4. Describe p-i-n Photodiode.

93

ARYA College of Engineering & ITVIII Sem ECE

Subject- Optical CommunicationClass Test-IV

MM-10 Time:1 hr Note: attempt any three question

Q1. What are optical splicing? Describe briefly the different types of optical splicing.

Q2 Explain the four basic components used in optical connectors.

Q3. Does underfilling a multimode optical fiber exite mainly high order or low order modes..?

Q4. In multimode fiber how do fiber joints increase finer attenuation following the joints.

94

ARYA College of Engineering & ITVIII Sem ECE

Subject- Optical CommunicationClass Test-V

MM-10 Time:1 hr Note: attempt any three question

Q1. With the aid of suitable diagram discuss the priciples of operation of the injection LASER.

Q2. Compare LED and LASER as optical sources.

Q3. How does a DFB laser diode operate?

Q4. Explain the charaterstics of emitted light by LASER diodes.

95

ARYA College of Engineering & ITVIII Sem ECE

Subject- Optical CommunicationTutorial Class- I

Q1. A silica ooptical fiber with a core diameter large enough to be considered by ray thoey analysis has a

refractive index of 1.5 and a cladding refractive index of 1.45. Deternmine the critical angle at the cor-cladding

interface.

Q2. A multiple step index fiber has a relative refractive index of 1 % and core refractive index of 1.5. The no.

of modes propagating a wavelength 1.3 m is 1100/ Estimate the diameter of fiber core

Q3. Calculate the refractive index of the core and cladding material of a fiber from – NA=0.22,

Step index profile=.012.

96

ARYA College of Engineering & ITVIII Sem ECE

Subject- Optical CommunicationTutorial Class- II

Q1. From the stand point of a pn junction how does light radiation occur in a semiconductor diode.

Q2. Explain the different constructions of index guided lasers

Q3. A laser beam has a power of 50MW., It has an aperture of .005 meter and wavelength of 7000 angstrom.

A beam is focuses with a lense of focal length 0.2m. Calculate the areal spread and intensity of the image.

97

ARYA College of Engineering & ITVIII Sem ECE

Subject- Optical CommunicationTutorial Class- III

Q1. Compute the responsitivity of a detector having a quantum efficiency of 1% at 0.8um.

Q2. Compute the current amplification ini a photo multipliers tube if the gain at each diode is 5 and there are 9

diodes.

Q3. estimate the minimum detectable power for a PIN diode, whose responsitivity is 0.5A/W, and whose dark

current is 1 nA

98

ARYA College of Engineering & ITVIII Sem ECE

Subject- Optical CommunicationTutorial Class- IV

Q1. The Fresnel reflection at a butt joint with an air gap in multimode SI fiber is 0.46dB.Determine the

refractive index of the fiber core.

Q2. An ideal four port directional coupler has a 4:1 splitting ratio. What refraction of the input power goes to

each of its port.

Q3. A laser diode feeding a glass fiber may be separated from it by a small air gap. Compute the return loss at

the air to fiber interface.

99

ARYA College of Engineering & ITVIII Sem ECE

Subject- Optical Communication Tutorial Class- V

Q1. A frersnel reflection at a butt joint with an air gap in a multimode SI0.46dB. Determine the RI of fiber

core.

Q2. A GiI fiber has a characteristics RI profile of 1.85 and a core diameter of 16um . Estimate the insertion

loss due to 5um lateral offset of an index matched fiber joint assuming the uniform illumination of all guided

modes.

Q3. Two mm SI fiber have NA 0.15 and 0.35 respectively and both have the same core RI, which is 1.45.

Estimate the insertion loss at a joint.

100

ARYA College of Engineering & ITVIII Sem ECE

Subject- Optical CommunicationSubject- Fiber Optic Instrumentation

MM-20 Time-1.30hr

Note: Attempt any four question.

All question carry equal marks

Q1 What is meant by acceptance angle? Show that it is related to NA.

Q2. Explain the fusion splicing. Discuss the advantages and drawbacks of it.

Q3. Draw the block diagram of OFC system and describe the function of each componenet.

Q4. What are intermodal and intramodal dispersion?

Q5. What is internal efficiency oa a LED? Derive it and describe construction of edge emitter LED.

101

ARYA College of Engineering & ITVIII Sem ECE

Subject- Optical Communication

Subject- Fiber Optic Instrumentation

MM-20 Time-1.30hr

Note: Attempt any four question.

All question carry equal marks

Q1. Explain the optical time domain reflectometry method used for measurement of attenuation.

Q2. What is Mach-Zehender interferomter ? How it is useful for optical phase modulation

Q3.Define quantum efficiency. And compute the respositivity of a detector having quantum

efficiency of 1% at 0.8micrometer.

Q4. Give a brief overview of Avalanche photodiode.

Q5. How does population inversion can be achieved in LASER? Also explain the threshold

condition of a LASER

102

Previous Year Questions

Q1. Explain detection process in pin diode as compare to avalanche photodiode.

Q2. What are the direct band and indirct band gap semiconductor? Give atleast two examples of

each. Which of these are suitable for fabricating LED’s and why?

Q3. Define quantum efficiency. And compute the respositivity of a detector having quantum

efficiency of 1% at 0.8micrometer

Q4. What do you understand by the term splicing? Explain the types of splices and steps

involved in splicing a fiber.

Q5. Expalin the mechanism associated with noise in OFC:

(a). Quantum Noise

(b) Shot Noise

Q6. What is Mach-Zehender interferomter ? How it is useful for optical phase modulation?

Q7. Give the techniques used for measurement of refractive index.

Q8. List two major types of optical couplers. Describe construction and working of simple coupler.

Q9. Explain the losses in optical fibers.

Q10. Explain the basic principle of photo detection in an optical system.

Q11What are stimulated emission and spontaneous emission?

Q12. Explain the principle of avalanche photodiode.

Q13. What are the advantages of stimulating LASER over LED for communication?

Q14. Explain the advantages of optical communication.

Q15. Define step index and graded index fiber.

Q16. Explain different types of losses in optical fiber.

Q17. What are the general requirement for a source in optical fiber communication? Discuss.

Q18. Define quantum efficiency.

Q19. Differentiate between single mode and multimode optical fiber.

Q20. Explain pin photo diode.

103

104