UNIT-IV

OPTICAL SOURCE, DETECTORS AND AMPLIFIERS

LED

Principle of action:

Fig: Light radiation by the pn junction of semiconductor

The forward bias voltage V causes the electrons and holes to enter the depletion region

and recombine.

In terms of energy band diagram, the external energy V excites electrons at conduction

band. From there they fall to the valence band and recombine with holes

Recombination results in the release of radiation in the visible (or) light part of the

spectrum

POPULAR SEMICONDUCTORS USED FOR LED FUBRICATION

Material Energy band Wavelength(nm)

Se 1.17 1067

Ge 0.775 1610

GaAs 1.424 876

InP 1.35 924

InGaAs 1.75-1.24 1664-1006

Al GaAs 1.42-1.92 879-650

InGaAsp 0.75-1.35 1664-924

Types of LED:

(i) Homostructure LED (or) Homojunction LED:

The n-type and p-type semiconductors are made from the same substrate. BY& adding

various dopans to make either n-type with excessive electrons (or) p-type with

excessive holes.

Both semiconductor have the same energy gap. The pn junction of such

semiconductors are known as “homo structure LED”

A typical wavelength of light emitted from the construction is 940 nm, and a typical

output power is 2MW at 100MA of forward element

Light emitted from Homo structure led spreads equally in all directions. Therefore,

only a small amount of light is coupled in to the fiber.

Homo junction devices are often called surface emitters.

Disadvantage

Because of the non-directionality of their light emission,which makes them a poor

choice as light source for optical fiber system.

(ii) Heterojunction LED:

In Hetero junction LED, both p-type and n-type semiconductor have the different

energy gap. The p-n junction of such semiconductors are called as “heterosturucture

LED”

This devices, continues the emitted light in to much smaller area.

With hetero junction devices, light is emitted from the edge of the material and are

often called edge emitters

Advantage:

The increase in current density generates a more brilliant light spot.

The smaller emitting area makes it easier to couple its emitted light in to a fiber.

The small effective area has a smaller capacitance, which allows the planer

heterojunction LED to be used at higher speed.

LED Structure:

The requirement of an LED to be used for fiber transmission are

(i) High radiance output

(ii) High quantum efficiency

(iii) Fast omission response time

(i) High radiance output

Radiance (or) brightness is a measure in watts of the optical power radiated in to a

unit solid angle Pe, unit area of emitting surface. High radiances are necessary to

couple sufficiently high optical power levels in to a fiber

(ii) High quantum efficiency

The quantum efficiency is related to fraction of injected electron hole pairs

that recombine relatively

(iii) Fast emission response time

It is the time delay between the application of a current pulse and the

emission of optical pulses. This time delay factor limit the band width of the

source.

The two basic configurations being used for fiber optics are

(i) Surface emitters

(ii) Edge emitters

Surface emitter LED (Suitable for multimode fiber):

The plane of the active light emitting region is oriented perpendicularly to the axis of

the fiber.

Here, a well is etched. Through the substrate of the device, in to which a fiber is then

cemented in order to accept the emitted light.

The circular active area in practical surface emitter is nominally 50 Um in diameter

and up to 2.5 Um thick. The emission pattern is essentially Iso tropic with a 120 half

power beam width.

This Iso tropic pattern from a surface emitter is called a lambertian pattern. In this

pattern source is equally bright when viewed from any direction. But the power

diminished as cos θ, where θ is the angle between viewing direction and the line

orthogonal to the radiating surface.

P=po, when ce=0, half power of the lambertian source is concentrated in a 120 cone.

Edge Emitter LED (suitable for single mode fiber)

Edge emitter LED consists of an active function region., which is the source of the

incoherent light and two guiding layers.

The guiding layers both have a refractive index which is lower than that of the active

region but higher than the index of the surround material. This structure forms a wave

guide channel that directs the optical radiation towards the fiber core.

To match the typical fiber core diameter (50-100hm)

(i) The contact stripes width should be 100-150hm

(ii) The length of the active region should be 100-150hm)

The emission pattern of edge emitter is more directional than that of the surface emitter.

Quantum efficiency and LED power:

Let excess densities of electrons is n

Let excess densities of holes is p

At equilibrium state, density ef electron is equal to density of holes

In general, the excess carrier density decays exponentially with time according the

relation

N=no exp(-t/c)____________(1)

No= Initial injected electron density (by biasing)

T= carrier life time (ranges from milliseconds to nanoseconds)

The excess carriers injected by the external source can recombine either radiatively (or)

nan-radiatevely

Due to radiatively recombination, energy is released is the form of photon hu, which

approximately equal to energy band gap.

Due to non-radiative recombination, released energy is transformed is to another

camer in the form of kinetic energy.

When there is a constant current flow in the LED, an equilibrium condition is

established.

Let j/qd is externally supplied rate of carries

Where j=current density a/cm2

Q=charge of electrom

D=thickness of recombination region

Let n/Ʈc is thermal generation rate.

Then the rate equation for carrier recombination in an LED can be written as

dn/dt= (j/qd)-(n/Ʈ) ________________(2)

At equilibrium condition dn/dc=0

0= j/qd – n/Ʈ j/qd= n/Ʈ => n= jƮ/9d____________(3)

Internal quantum efficiency(η int):

η int= no. of radiated photon/ no. of injected charge carrier

If Rr is radioactive recombination rate, Rnr is non-radiative recombination rate, then η int is

given by

η int= Rr/Rr+Rm___________4

Let Tr is the radioactive recombination life time and it is given by

Tr= n/Rr= Rr=n/Tr__________5

Tnr is the non-radioactive recombination life time and it is given by

Tnr= n/Rnr => Rnr=n/tnr__________6

ηint= (n/tr)/((n/tr)+(n/tnr)) = (1/tr)/(1/tr+1/tnr)

Total recombination time is t then

1/t= 1/tr+1/tnr__________7

η int= 1/tr/1/t= t/tr_________8

In homostructure LED, Rr & Rnr are similar in magnitude ηint is 50%

But in double Hetero simutune LED, Rr & Rnr are different in magnitude. ηint is 60-

80%

LED power

If the current Injected in to the LED is I, then the total no. of recombination for second

is

Rr+Rnr=I/q_____________9

w.k. that ηint = Rr/Rr+Rnr(equation4)

ηint= Rr/I/q (i.e). Rr=ηint I/q_____________10

Rr= radioative recombination rate i.e. it represent total no. of Photons generated per second and

that each photon has an energy hu

Optical power generated internally to the LED is

P int= Rr.hu__________11

= ηint I/q.hu

= ηint I/q h.c/λ

P int= ηint Ihc/qλ_______12

External Quantum efficiency

Ext= No. of Photons escaping from a semi conductor/ No. of charge carrier Injected

Fig:

The external quantum efficiency can be calculated from the expression

η ext=1/4Πʃ T(ϕ) 2Π (sinϕ) dϕ

T(ϕ) is Fresnel transmission co-efficient Δϕ=sin^-1(n2/n1)

If ϕ=0 , then T(0) = 4n1n2/n1+n2

From the Fig

n1=R.I of semiconductor material

n2= R.I of axis =1

Consider n1as n

T(0)=4n/(n+1)2

ηext=1/n(n+1)2

Optical power emitted from LED

P=ηext Pint= Pint/ n(n+1)^2 =ηint Ihc/ qλn(n+1)^2

Laser diode:

The term laser, actually is an acronym for the phrase Light Amplication by

Stimulated Emission of Radiation. Thus laser is a source whose radiation has high

intensity, high coherence, high monochromacity and high directionality.

A laser diode consists of an active medium to produce optical amplification and

optical reason to provide the necessary optical feed back.

Laser action means the amplification of light by stimulated emission of radiation. To

get laser action

(i) The stimulated emission is necessary

(ii) There should be population inversion of atom

(iii) There should be stimulation photon

Absorption and emission of radiation

(a) Absorption

By this absorption process, an atom in level E, absorbs photon of frequency (E2-E1)/h and

goes to upper energy level E2.

The rate of absorption depends upon the no. of atoms present in the level E1 and density of

photons present in the system.

(b) Spontaneous emission

An atom in the energy level E2 can make transition to lower energy level E1

Spontaneously, and emitting a photon whose energy is equal to (E2-E1) (or) energy band gap

Eg.

The rate of emission depends on the no. of atoms present in the higher energy level (or)

executed level. The higher level atom can undergo different transitions and finally it can

reach the ground level E1 (results more spectral width)

Characteristics:

(1) It produces poly Chromatic

(2) It‟s intensity is always small

(3) It is not colerent->(generated photon has random phase & frequencies

(4) It has large divergence

(5) IT takes place without getting any external aid.

(c) Stimulated emission

An atom in an excited level (or) high energy level E2 can make a transistion to lower energy

level E1 by an external photon of energy (E1-E1). The stimulating (or) inducing

photon and emitted photon are in same phase with same energy and they travel in the same

direction.

The photon emitted in this process has same energy (i.e. the same wave length) as

incident photon, and is in phase with it. Also their amplitudes add to produce brighter level.

Characteristics:

(i) It produces mono chromatic radiation

(ii) It is intenoity is very high

(iii) IT is coherent

(iv) It has high directionality

(v) If takes place with the aid from an external photon having the same energy of

emitted photon.

Semiconduction injection laser diode (ILD):

Stimulated emission by the recombination of the injected carrier is encouraged in the

semiconductor injection laser (often called injection laser diode) by the provision of optical

cavity in the crystal structure in order to provide the feedback of photons.

Injection laser diode has several advantages other semi conductor surces (eg . LED)

(i) High radiance due to the amplifying effect of stimulated emission

(ii) Narrow line width of the order of 1 nm (or) less which is useful in minimizing the

effect of material dispassion

(iii) Modulation capabilities extend up to gigalength range

(iv) Relatively temporal coherence whoch ius considered essential to allow hetenodyne

detection in high capacity system

(v) Good spatial coherence this allows efficient coupling of optical power in to the fiber

even for fibers with low NA.

Structure of GaAS homo-junction Injection laser diode with Fabry-Perot

cavity:

Homo-junction laser means that a P-N junction formed by a single crystalline

material such that the basic material has been the same on both sides of the function.

For example in GaAS laser, both the P-layer and n-layer are made up of GaAS only

Principle of operation:

The crystal mirror act as light reflection mirrors. The photons generated in the pn

junction will be reflected by the mirrors

Since the fabryt-perot cavity is fairly long, the laser will osculate simultaneously in

several frequencies (happened in left side facet)

When these resonant frequencies are transmitted through the right hand facet, they add in

phase. This results in a greatly increased amplitude(or) brighter light beam, with a broad

spectrum.

Draw back:

1. Threshold current density is very large(400 A/mm2)

2. Only pulse made output is obtained

3. Laser output has large beam divergence

4. Coherence and stability are very poor

5. Electromagnetic field continement is poor.

Structure of Double heterojunction injection laser diode:

Hetero junction mkeans that the material one side of the function differs from that on

other side othe function. For example, heterofunction is formed between GaAS and leaAlAS.

Mostly the heterojunction laser diodes are used as optical sources in the optical source in

the optical fiber communication because they have so many advantages

(1) Threshold current density is small (10A/mm2)

(2) Continous wave operation can also be possible

(3) Due to efficient waveguide structure, the beam divergence is small, high coherence and

monochronaticity are obtained

(4) High output power

(5) Highly stable with longer life

Hetero junction laser divided into two types

1. Edge emitting laser injection(gives laser output through mirror and)

2. Surface emitting injection laser (gives laser output through surface of the diode)

Edge emitting injection lasers are divided in to two types

1. Gain guided lasers

2. Index guide laser

Gain guided laser diode:

A marrow metallic stripe runs along the length of the diode. The refractive index of

active area is greater than the refractive index of n-doped and p-doped region for

providing waveguide structure in the case of gain guided laser.

The structure of aluminium galliam arsinide (AlGaAs) oxide stripe DH laser is as

shown in the figure

Optical light confinement method of Grain guided laser diode as shown in the figure.

In the above structure, a narrow electrode stripe (less than 8 um wide) runs along the

length of the diode. The injection of electrons and holes in to the device alters the

refractive index of the active layer, directly below the stripe. The profile of these

injected carriers creates a weak, complex waveguide that continues the light laterally.

This type of device is commonly reformed to as gain guided laser.

Although these lasers can emit optical powers exceeding 100 MW, they have strong

instabilities and can have highly astigmatic, two peaked beams. So these structure are

not used in practice.

Index-guided laser diode:

These are most stable structures. Here the di electric waveguide structures are

fabricated in the lateral direction

The variations in the real refractive index of the canaes materials in these structures

control the lateral modes in the laser/ Thus these device are caller index guded laser.

Index guided lasers can have either +ve index (or) –ve index wave contining

structure. In +ve index waveguide fig(a), the central gegion has a high refractive

index than outer regions. Thus all of the guided light is reflected at dielectric

boundary (active region) just as it is at the core-claddding interface in an optical fiber.

By proper choice of the change in refractive index and width of the high index region,

one can make a device that supports only fundamental latter mode.

In a –ve index guidefig(b), the central region of the active layer has a lower

repractive index than the outer refions.

At the dielectric boundaries, part of the light is reflected and rest is refracted in to the

surrounding material and thus cost.

This radiation loss appears in the far dield radiation pattern as narrow side lobes to

main beam.

The +ve index laser is more popular than the gain guided laser and –ve index laser

structure.

Fig(a) fig(b)

Types of index guide laser

(1) Buried hetero structure

(2) a selectively diffused construction

(3) a varying thickness structure

(4) bent layer configuration

Buried hetero structure (BH) laser

Fig (a) Shost wave length (800-900nm) GA Al AS-buried heterostructure laser diodes

Fig (b) long wave length (1300-1600nm) InGaASP-Buried heterostructure laser diodes

To maked buried hetero

structure laser, one etches a narrow mesa stripe (1-2 Um wide) in double heterostructure

material.

The mesa is then embedded in high resistance lattice matched m-type material with

an appropriated bund gap and low repractive index (contining layer)

This material GaAlAS in 800-900nm laser with a GaAS active layer, and is Inp for

1300-1600nm laser with an In GA Asp Active layer

This configuration strongly traps generated light in a lateral waveguide.

Injection laser diode has several advantages other semi conductor surces (eg . LED)

(i) High radiance due to the amplifying effect of stimulated emission

(ii) Narrow line width of the order of 1 nm (or) less which is useful in minimizing the

effect of material dispassion

(iii) Modulation capabilities extend up to gigalength range

(iv) Relatively temporal coherence whoch ius considered essential to allow

hetenodyne detection in high capacity system

(v) Good spatial coherence this allows efficient coupling of optical power in to the

fiber even for fibers with low NA.

PIN

The first element of the receiver is a photodector. The photodetector senses the light signal

falling on it and covens the variation of optical power to a correspondingly varying electric

current

Performance requirements for Photodetector

(1) A high sensitivity to the emission wavgelength range of the received signal

(2) High quantum efficiency

(3) A minimum addition of noise to the sigtnal

(4) A fast response speed to handle the desired data rate

(5) Be sensitive to temperature variations

(6) Be compatible with the physical dimensan of the fiber

(7) Have ak reasonable cost

(8) Have a long operating life time

PIN photodiode & Avalanche Photodiode (APD) satisfy the above set of requirements

Operation of a PIN Photodiode

The device structure consists of P and N semiconductor region separated by a veryt

lightly n-doped intrinsic (i) region. In normal operatrion a reverse beas voltage is

applies access the devuice so that no free electrons on holes exist in the intrinsic

region

Electrons in the semiconductor materials are allowed to reside in only two specific

band (i.e. valance band & conduction band). These bands are separated by a

forbidden gap region called an energy gap.

Suppose an Incident Photon comes along that has an energy greater than or equal to

the abdgap energy of the semiconductor material.

This photon can give up its energy and excite an electron from the valance band to

conduction band. This process which occurs in the intrinsic region, generates free

electron hole pairs. These charge carriers are known as photocarriers. Silnce theyt are

generated by a photon.

The electric field access the device causes the photocarriers tot b swept out of the int

rinsic region, thereby giving rise to ta current thow in an external circuit. This current

flow is known as photo current.

An incident photon is able to boost an electron to the conduction band only if it has

an energy that is greater than (or) equal to energy band gap. Energy is inversely

proportional to wavelength

The longest wavelength at which the photodetector absorb light signal is calledwet

of wavelength (λC)

λC=Hc/Eg = 1.,240/Eg Where H=6.6256*10-34 T C=3*105 m/s

A photodetector has a certain wavelength range overwhich

Generated Photocarriers are immediately collected it may used by the external limit

before they recombine.

Analysis of PIN diode:

(i) Quantum efficiency

η= no. of electrons-hole pairs generated/No. of incident photons =

(Ip/q)/(Po/hu)

Q= electron charge

Po= incident optical power

Ip= photocurrent

V= Light frequery

h= planks constant Quantum efficiency ranging from 30to 95 %

(ii) Responsivity specifies the Photocurrent generated per unit optical power

R= generated Photo current/ incident optical power

=Ip/Po =ηq/hu

Responsivity of 900 nm silicon is 0.65

Responsivity of 1300 nm germanium is 0.45 A/W

Responsivity of 1550nm InGaAS isw 1.0A/w

Responsivity depends upon the function of wave length and photodetecor material

(iii) Speed of response.

Photodiodes needs to have fast response speed in order to properly interpret light

data rate signals. The detector response speed is measured in terms of

(i) Rise (time takes the output signal to rise 10% to 90% of its peak value when an

input to photodiode turned on instantaneously)

(ii) Fall time (Time takes the O/P signal to fall 90% to10% of its value)

(iv) Bandwidth

The 3-dB bw is defines the receiver bandwidth, which is the range of frequencies that a

receives can reproduce the signal. If rise time & fall time are equal, then the 3dB BW is

BW1(MHZ)= 350/rise time, (NS) nanoseconds

Avalanche Photodiodes

Avalanche Multiplication Process:

An avalanche photodiode internally multiplies the photocurrent before it enters the

input circuitry of the following amplification. The multiplication effect is achieved by

applying a very high electric field across the photodiode.

When photo generated electron encounters this high electric field, it can acquire

sufficient energy to kick more electrons from the valence to the conduction band,

thereby creating secondary electron-hole pairs.

These secondary pairs as longer accelerated to high energies and therefore can

generate even more electron-hole pairs. This increases receiver sensitivity since the

photo current is multiplies prior to encountering the electrical noise associated with

the receiver circuiting.

The process is called avalanche multiplication, and hence the device is called an

avalanche photodiode. This carrier multiplication mechanism is known as „Impact

conization‟

A commonly used structure for achieving carrier multiplication is the” reach through

construction” (RAP)

Reach trough APD is composed of a high resistivity p-type material, heavily doped

p+ substrate, heavily doped n+ layer and II layer (or) intrinsic layer

Operation:

The term „Reach through‟ arises from the photodiode operation. When a low reverse

bias voltage is applies, most of the potential drop is across the Pn+ function

The depletion layer widens with increasing bias until a certain voltage is reaches. (

That voltage is known as peak electric field) and this voltage is 10-15% below the

avalanche break down voltage

At this point, the depletion layer just “reaches through” to nearly intrinsic II region.

Now high electric field is across the depletion layer.

Light enters the device through the p+ region and is absorbed in the II-material (or)

intrinsic layer

Due to incident of light some carriers will be generated in the intrensic region.

Because of high electric field across the intrinsic region, generated photo carrier kick

more electrons from valance band to conduction band, there by igniting secondary

electro hole pairs. IN this way more no. of carriers generated and causes more photo

current. This increases the receiver sensitivity

The mean no. of electron-hole pairs created is a measure of the carrier multiplication.

This is called the gain and is designated by M.

M= Im/Ip

Where Im=average value of the total multiplied output element

Ip= Primary un multiplies photo current. (before carrier multiplication take place)

Performance of an APD is characterized by its responsivity RAPD, which is given by

Rapd= ηq/hu M=RoM

Where Ro= unity gain responsivity

Performance requirements for receiver.

(1) A high sensitivity to the emission wavgelength range of the received signal

(2) High quantum efficiency

(3) A minimum addition of noise to the sigtnal

(4) A fast response speed to handle the desired data rate

(5) Be sensitive to temperature variations

(6) Be compatible with the physical dimensan of the fiber

(7) Have ak reasonable cost

(8) Have a long operating life time

PIN photodiode & Avalanche Photodiode (APD) satisfy the above set of requirements

Optical receiver

An optical receiver converts an optical input signal in to an appropriately formatted electric

output. During this conversion process various noises and distortics will be introduced due to

imperfect component response. This can leads to error in the interpretation of received signal

(i) Photodetector(PD): It converts light in to photo current

(ii) Preamolifier: It converts the photocurrent in to voltage, amplifies the signal and present

in to a quantizer

(iii) Quantizer: A typical quantizer includes three components

(a) A noise filter: This improves the signal to noise ratio (or) receivers‟s sensitivity

(b) Amplifier/ limiter: Amplification (performed by amplifier) is necessary to attain a

signal with enough power to drive the decision circuit.

If the amplified signal is high enough the limiter circuit clips the signal (larger

the amplitude lesser the gain)

(c) Decision circuit: This unit determines the logical meaning of the received signal.

When the received signal above the threshold, the comparator output is high. This

means the decision is made that the received signal carriers logic high (or)

When the received signal below the threshold, the comparator output is low.

This means the decision is made that the received signal carries logic low (or) „o‟

(iv) Buffers:

A buffer transfers a logical signal from the input to output unchanged but

reshapes the electrical form of this signal. Typically, this is an emitter follower

circuit.

(v) Clock Recovery:

Clock Recovery extracts timing information from the data stream and helps

the decision circuit to generate clean and reshaped differential DATA and

NON-DATA outputs.

(vi) Signal Detector:

Signal detector is an essential alarm circuit. It monitors the level of the

incoming signal and generation a logic low signal when the SNR is not

sufficient.

(vii) Monitoring circuits:

Input monitoring circuit is used to monitor the voltage drop produced by

photo current flowing through a resistor, allows engineer to keep tabs on input

power.

The flag signal from a signal detector circuit watches for a possible SNR lost

situation