omw unit-iv pdf

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UNIT – IV

OPTICAL SOURCES &DETECTORS

The principal light sources used in fibre optic communication applications are hetero

junction- Structured semiconductor laser diodes (also called injection laser diodes ILD) or

light emitting diodes(LEDs).

Requirements of optical sources (or) properties of optical sources (or) characteristics of

optical sources:

Properties of optical sources are important while configuring optical communication system

i) Wavelength:

Operating wavelength must be chosen such that it gives low loss and low

dispersion is optical fibers.

Wavelength around 0.85um, 1.3um & 1.6um gives low attenuation. The

wavelength that can be generated by various semiconductor photoemissive

materials are shown in following figure

ii) Modulation:

Direct modulation must be possible or it must be easy to couple for an external

modulation.

iii) Reliability:

Long life, good stability of operation and good reproductivity of output

characteristics are necessary. Life time of 10hrs must be the minimum requirement.

iv) Output power:

Minimum optical power required from the source determined from transmission

loss of the fiber, minimum detectable power (Pmin). For α=45db, and Pmin=-

45dbm, the required source output power is more than 1mw.

v) Power efficiency:

Let Pd=dc input power required to generate output power Po.

The device efficiency ηd is given by,

ηd =(Po/Pd)*100%

Power efficiency of > 50% should be minimum.

vi) Spectral width:

The spectral can strongly affect the magnitude of the transmission bandwidth.if

the spectral width of the is decreased then the bandwidth of the system is

increased. Spectral width is given by,

Δλ=Δ10/LΔf

vii) Focussing effect:

Longer the coherent length Lc, smaller the size of the focussed spot produced by the

lens.





*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 areoften 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 thesource.

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



diminished as cosθ, 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 therelation

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(ηint):

ηint= 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

ηint= 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

ηext=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/ qλ n(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 trunstion 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 lowerenergy 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 thesemiconductor 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 allowhetenodyne 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 homojunction Injection laser diode with Fabry-Perot

cavity:

Homojunction 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 morrors. The photons generated in the pn

junction will be reflected by the mirrors Since the fabryt-perot cavity is fairly long, the laser will osullare 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 obtained3. 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 lasers2. 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.



Photodetectors:

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 proportionalto wavelength

The longest wavelength at which the photodetector absorb light signal is

calledwet of wavelength (λ C)

λ C=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





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

Thge deplection layer widens with increasing bias until a certain voltage is reachers. (

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 deplection 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 intensic region. Because of

high electric field across the intrinsic region, generated photo c arrier kick more electrons

from valance band to conduction band, there by ineating 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

OPTICAL TRANSMITTER

P.NO: 401-fiberoptic comm.. tech by Mynbaev

The transmitter is the unit of the fiber optic communication system responsible for

converting an lectrical information signal in to an optical one. The major component of atransmitter-a light source in the form of an LED (or) a laserdide

Fig: Functional block diagram of a transmitter



(i) Data conversion unit

This unit performs three major function

(a) Encoding:

This means representing date(binary numbers in a physical format(pulses). This is

necessary because data are transmitted indifferent line code S. Manchester encoding

technique is commonly used.

(b) Parallel to serial conversion:

Date enter in parallel format but a laser diode can be driven only by serial pulses of

modulation amount. Thus parallel to serial convertor (multiplexer) is used to convert data in

to serial format.

(c) Reshaping the electrical format of data

This is performed by data conversion unit which uses either comparator (or) buffer for data

conversion purpose

(ii) Laser driver

Data prepared for light transmission p[ass in to a laser driver. We need this circuit

because a laser diode is a current driver device rather than voltage driver device, while the

power supply is always a voltage source.

The function of laser driver is to convert outside voltage in to the current needed to drive the

laser.

(iii) Modulator circuit

Intensity Modulation is take place. I.e. intensity of laser diode in varied with respect of

the amp. Of the information signal.

Modulation is controlled by simple changing the during cement form the bias level to

maximum.

(iv) Controlling and monitoring circuits

The control signal transmitter disable allows the user to shut down the transmitter whilekeeping the module in stand by mode.

The photocurrent produced by the rear-facet photodetector (PD) allows the user to know

whether the laser diode is operation

Also the signal from this PD allows the user to stabilize the laser O/P wavelength to a few

picometers (5 PM=0.0055nm)

Output monitoring signal used to trouble shoot the transmitter

The signal from the temperature mo0nitor permits the user to monitor the transmitters

temperature over the entire ambient temperature range.(say 25 degree celcious) The alarm processor provides alterts for abnormal operating condition



Coupling optics

It is used to efficiently couple the light from thansmitter to fiber. Also its protect the active

area of a light source from base returned light.

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

Fig: Functional block diagram of optical receiver (digital transmission)

(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‟ssensitivity

(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 butreshapes 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

List the characteristics of Photo detectors (or) Comparison of Photo detectors

PIN Photodiodes (Si, Go, InGaAS photodiodes)

Parameter Symbol Unit Si Ge InGaAS

Wavelength

range

Nm 400-

1100

800-

1650

1100-1700

Responsitivity R A/W 0.4-0.6 0.4-0.5 0.75-0.95

Dark current Id Na 1-10 50-500 0.5-2.0

Rise time Tr nS 0.5-1 0.1-0.5 0.05-0.5

Bandwidth B C71+Z 0.3-0.7 0.5-3 1-2

Dias voltage Vb V 5 5-10 5

Si, Ge and InGaAS avalanche Photodiodes:



Parameter Symbol Unit Si Ge ingaAS

Wavelength

range

Nm 400-110 800-1650 1100-1700

Avalanche

gain

M 20-400 50-200 10-40

Dark cement

Id nA 0.1-1 50-500 10-50

Rise time Tr Ns 0.1-2 0.5-0.8 0.1-0.5

Gain-band

width

m.b Ghz 100-400 2-10 20-250

Bias-voltage Vb V 150-400 20-40 20-30

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omw unit-iv pdf