1. LASER2.OPTICAL FIBER

TOPICS:-

1. LASER

SUB TOPICS:-

• Definition of lasers• Characteristics of laser• Emission and absorption of radiation• Population Inversion• ND YAG laser• Application of laser

Definition:-

 A laser is a device which is built on the principles of quantum mechanics to create a beam of light where all of the photons are in a coherent state - usually with the same frequency and phase. (Most light sources emit incoherent light, where the phase varies randomly.) Among the other effects, this means that the light from a laser is often tightly focused and does not diverge much, resulting in the traditional laser beam.

Characteristics of laser:-Laser Light is Highly Coherent:- Wave trains for laser light may be several hundred

kilometers long. Interference fringes can be set up by combining two beams that have followed separate paths whose lengths differ by as much as this amount. The corresponding coherence length for light from a tungsten filament lamp or a gas discharge tube is typically considerably less than 1 m.

Laser Light is Highly Monochromatic :-

Tungsten light, spread over a continuous spectrum, gives us no basis for comparison. The light from selected lines in a gas discharge tube, however, can have wavelengths in the visible region that are precise to about 1 part in 106. The sharpness of definition of laser light can easily be a thousand times greater, or 1 part in 109.

Laser Light can be Sharply Focused:- This property is related to the parallelism of the laser beam. As for star light, the size of the focused spot for a laser beam is limited only by diffraction effects and not by the size of the source. Flux densities for focused laser light of 1015 W cm-2are readily achieved. An oxyacetylene flame, by contrast, has a flux density of only 103Wcm-2.

Brightness:-

The primary characteristic of laser radiation is that lasers have a higher brightness than any oilier light source. We define brightness as the power emitted per unit area per unit solid angle.

Emission and absorption of radiation:-

1. Absorption2. Spontaneous Emission3. Stimulated Emission

Therefore 3 process of light emission:

Absorption:-

E2

E2 E1

Spontaneous Emission E2

E1

‘’ Consider an atom (or molecule) of the material is existed initially in an excited state E2 No external radiation is required to initiate the emission. Since E2>E1, the atom will tend to spontaneously decay to the ground state E1, a photon of energy h =E2-E1 is released in a random direction as shown in (Fig. 1-ii). This process is called “spontaneous emission’’

Stimulated EmissionE2

E1

“stimulated emission” requires the presence of external radiation when an incident photon of energy h =E2-E1 passes by an atom in an excited state E2, it stimulates the atom to drop or decay to the lower state E1.

Population Inversion:-Therefore we must have a mechanism where N2 > N1This is called POPULATION INVERSIONPopulation inversion can be created by introducing a so call met stable centre

where electrons can piled up to achieve a situation where more N2 than N1The process of attaining a population inversion is called pumping and the

objective is to obtain a non-thermal equilibrium. It is not possible to achieve population inversion with a 2-state system. If the radiation flux is made very large the probability of stimulated emission

and absorption can be made far exceed the rate of spontaneous emission. But in 2-state system, the best we can get is N1 = N2. To create population inversion, a 3-state system is required. The system is pumped with radiation of energy E31 then atoms in state 3 relax

to state 2 non radioactively. The electrons from E2 will now jump to E1 to give out radiation.

3 states system

Population Inversion

ND YAG Laser:-

Nd YAG is a solid state laser four level laser. Nd stands for neodymium and YAG for Yttrium aluminium garnet (Y3Al5O12). It is developed by J.E. Geusic, H.M. Marcos and L.G. Van Vitert in 1964. The rod of Y3Al5O12  is doped 1% with triply ionized neodymium. Nd3+ ions will replace the Y3+ ions in the crystal. Maximum length of the rod is about 10 cm and diameter is 6-9 cm.

Active medium: Nd3+ ions act as active medium or active centers. YAG is just the host.

Pumping source: The pumping of Nd3+ ions to upper levels is done by krypton arc lamp. Xenon lamp can also be used as pumping source. Thus, the optical pumping is used to achieve population inversion.

Optical resonator system: The ends of the ND YAG rod are polished and silvered so as to act as the optical resonator system.

Application of laser:-• Medicine : Bloodless surgery, laser healing, surgical treatment, kidney

stone treatment, eye treatment, dentistry, etc.• Industry : Cutting, welding, material heat treatment, marking parts.• Defense : Marking targets, guiding munitions, missile defense,

electro-optical countermeasures (EOCM), alternative to radar.• Research : Spectroscopy, laser ablation, Laser annealing, laser

scattering, laser interferometer, LIDAR (Light Detection And Ranging).• Product development : Laser printers, CDs, barcode scanners,

thermometers, laser pointers, holograms, bubble grams.• Laser lighting displays : Laser light shows.• Laser skin procedures : such as acne treatment, cellulite reduction,

and hair removal.

2. FIBER OPTICS

Advantages of Optical Fiber

Thinner Less Expensive Higher Carrying

Capacity Less Signal

Degradation& Digital Signals

Light Signals Non-Flammable Light Weight

Advantages of fiber optics

Much Higher Bandwidth (Gbps) - Thousands of channels can be multiplexed together over one strand of fiber

Immunity to Noise - Immune to electromagnetic interference (EMI).

Safety - Doesn’t transmit electrical signals, making it safe in environments like a gas pipeline.

High Security - Impossible to “tap into.”

Advantages of fiber optics

Less Loss - Repeaters can be spaced 75 miles apart (fibers can be made to have only 0.2 dB/km of attenuation)

Reliability - More resilient than copper in extreme environmental conditions.

Size - Lighter and more compact than copper.

Flexibility - Unlike impure, brittle glass, fiber is physically very flexible.

Total Internal Reflection in Fiber

Critical angle, θc

The minimum angle of incidence at which a

light ray ay strike the interface of two

media and result in an angle of refraction of

90° or greater.

Acceptance angle:-The maximum angle in which external light

rays may strike the air/glass interface and

still propagate down the fiber.

Acceptance angle:-

θin (max) = sin-1

Where,θin (max) – acceptance angle (degrees)n1 – refractive index of glass fiber core n2 – refractive index of quartz fiber cladding

Consider an optical fiber having a core  of refractive index n1 and cladding of refractive index n2. let the incident light makes an angle i with the core axis as shown in figure. Then the light gets refracted at an angle θ and fall on the core-cladding interface at an angle where,

 .............(1)

 

 

By Snell’s law at the point of entrance of light in to the optical fiber we get,

When light travels from core to cladding it moves from denser to rarer medium and so it may be totally reflected back to the core medium if θ‘ exceeds the critical angle θ'c. The critical angle is that angle of incidence in denser medium (n1) for which angle of refraction become 90°. Using Snell’s laws at core cladding interface, 

or

Therefore, for light to be propagated within the core of optical fiber as guided wave, the angle of incidence at core-cladding interface should be greater than θ'c. As i increases,  θ increases and so θ' decreases. Therefore, there is maximum value of angle of incidence beyond which, it does not propagate rather it is refracted in to cladding medium . This maximum value of i say im is called maximum angle of acceptance and  n0 sin im is termed as the numerical aperture (NA)

The significance of NA is that light entering in the cone of semi vertical angle im only propagate through the fiber. The higher the value of im or NA more is the light collected for propagation in the fiber. Numerical aperture is thus considered as a light gathering capacity of an optical fiber.Numerical Aperture is defined as the Sine of half  of the angle of fiber's light acceptance cone.  i.e. NA= Sin θa where θa, is called acceptance cone angle.

Numerical Aperture (NA)Used to describe the light-gathering or

light-collecting ability of an optical fiber.

In optics, the numerical aperture (NA) of

an optical system is a

dimensionless number that characterizes

the range of angles over which the system

can accept or emit light

Numerical Aperture (NA)

The numerical aperture in respect to a point P depends on the half-angle θ of the maximum cone of light that can enter or exit the lens.

MODE OF PROPAGATION

Two main

categories of

optical fiber used

in fiber optic

communications

are

multi-mode optical

fiber

and

single-mode optica

l fiber

.

SINGLE-MODE FIBERS

Single-mode fibers – used to transmit

one signal per fiber (used in telephone and

cable TV). They have small cores(9 microns

in diameter) and transmit infra-red light

from laser.

MULTI-MODE FIBERS

Multi-mode fibers – used to

transmit many signals per fiber

(used in computer networks). They

have larger cores(62.5 microns in

diameter) and transmit infra-red

light from LED.

MULTI-MODE FIBERS

Multimode fiber has a

larger core (≥ 50

micrometres), allowing

less precise, cheaper

transmitters and

receivers to connect

to it as well as

cheaper connectors.

MULTI-MODE FIBERS

However, multi-mode fiber

introduces multimode distortion

which often limits the bandwidth and

length of the link. Furthermore,

because of its higher dopant content,

multimode fiber is usually more

expensive and exhibits higher

attenuation.

Fiber Optic Types

multimode step-index fiber the reflective walls of the fiber move the

light pulses to the receiver multimode graded-index fiber

acts to refract the light toward the center of the fiber by variations in the density

single mode fiber the light is guided down the center of an

extremely narrow core

36

INDEX PROFILE

The index profile of an optical fiber is

a graphical representation of the

magnitude of the refractive index

across the fiber.

The refractive index is plotted on the

horizontal axis, and the radial

distance from the core axis is plotted

on the vertical axis.

STEP-INDEX

A step-index fiber has a central core

with a uniform refractive index. An

outside cladding that also has a

uniform refractive index surrounds

the core;

however, the refractive index of the

cladding is less than that of the

central core.

GRADED-INDEX

In graded-index fiber, the index of

refraction in the core decreases

continuously between the axis and

the cladding. This causes light rays

to bend smoothly as they approach

the cladding, rather than reflecting

abruptly from the core-cladding

boundary.

Application of optical fiber:- Communication -  Telephone transmission method uses fibre-optic cables. Optical

fibres transmit energy in the form of light pulses. The technology is similar to that of the coaxial cable, except that the optical fibres can handle tens of thousands of conversations simultaneously.

  Medical uses -  Optical fibres are well suited for medical use. They can be

made in extremely thin, flexible strands for insertion into the blood vessels, lungs, and other hollow parts of the body. Optical fibres are used in a number of instruments that enable doctors to view internal body parts without having to perform surgery.

  Simple uses -  The simplest application of optical fibres is the transmission of

light to locations otherwise hard to reach. Also, bundles of several thousand very thin fibres assembled precisely side by side and optically polished at their ends, can be used to transmit images.

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