MAHALAKSHMImahalakshmiengineeringcollege.com/pdf/ece/VIIsem/EC2402/UNIT III.pdfSUBJECT NAME : OPTICAL COMMUNICATION & NETWORKS . SUBJECT CODE : EC 2402 . UNIT – III: SOURCES AND

MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI - 621213

DEPARTMENT : ECE SUBJECT NAME : OPTICAL COMMUNICATION & NETWORKS SUBJECT CODE : EC 2402 UNIT – III: SOURCES AND DETECTORS

PART -A (2 Marks)

1. What are the three requirements of Laser action? 1. Absorption 2. Spontaneous emission

3. stimulated emission.

2. What are the three types of Laser diode structures? 1. Gain indexed guide 2. Positive indexed guide 3. Negative indexed guide

3. What are the fundamental structures of Index guided lasers? 1. buried hetero structure. 2. Selectively diffused construction 3. Varying thickness structure 4. Bent layer configuration.

4. What is the difference between LED and LASER diode? [AUC NOV 2013] LED LASER Incoherent Coherent No optical resonant cavity Optical energy from optical resonant

cavity Output radiation has broad spectral width

Highly monochromatic

No spatial and temporal coherence Spatial and temporal coherence 5. What is the significance of intrinsic layer in PIN diodes? [AUC NOV 2012]

• PIN photo detector structure consists of P and N regions separated by a very lightly n-doped intrinsic region. When the photodiode is reverse biased, the intrinsic region of diode is fully depleted of carriers.

6. what are the advantages of LED? [AUC MAY 2012] • Simpler fabrication • Low cost • Reliability • Generally less temperature dependence • Linearity • Simpler drive circuitry

EC 2402 OPTICAL COMMUNICATION & NETWORKS – IV/VII - V.SENTHAMIZH SELVAN ASST. PROF/ECE

7. what are the characteristics that a semiconductor laser diode should possess? [AUC NOV 2011]

• For fiber optical systems, semiconductor laser diodes are used as an

optical source. Because its output radiation is highly monochromatic and the light beam is very directional.

8. What are the properties of photo detector? [AUC NOV 2007]

• High response or sensitivity

• Minimum addition of noise

• Fast response speed

• Long operating life

• Reasonable cost

9. What is avalanche multiplication noise? [AUC NOV 2008]

The shot noise and bulk dark current noise in a photo detector are multiplied by avalanche gain mechanism. These multiplied noise are referred to avalanche multiplication noise.

10. What is dark current noise? [AUC April 2004,MAY2010]

• The dark current noise arises from electrons or holes which are thermally generated in the PN junction of the photo diode. This current continues to flow through the bias circuit of the device when no light is incident on the photodiode.

11. Define detector response time? [AUC MAY2013]

The response time of a photodiode together with its output circuit depends on following three factors.

• Transit time of a photo-carriers in the depletion region • The diffusion current of the photo-carriers generated outside the depletion

region. • The RC time constant of a photo diode and its associated circuit.

12. What are the merits of index guided lasers? [AUC MAY 2010]

• Index guided laser supporting only fundamental transverse mode and longitudinal mode is known as single mode laser. Such a device emits a single well collimated beam of light having bell shaped Gaussian intensity profile.

13. Define longitudinal modes. [AUC NOV 20O9]

• Longitudinal modes are associated with the length of the cavity and determine the typical spectrum of the emitted radiation.

14. Define lateral modes. [AUC NOV 2010] • These modes lie in the plane of the pn junction. They depend on the

sidewall preparation and the width of the cavity. • It determines the shape of the lateral profile of the laser beam.


PART (B)

1. Draw the schematic diagram of high radiance surface emitting LED and explain the working in detail? [AUC NOV 2011] Ans: High radiance is obtained by restricting the emission to a small active region within the

device. A well is etched in a substrate (GA As) to avoid the heavy absorption of the emitter

radiation and to accommodate the fiber. These structures have a low thermal impedance in the

active region and hence radiance emission into the fiber. Double hetero structures are used to

get increased efficiency and less optical absorption. The structure of a high radiance etched well

DH (Double Hetero structure) surface emitter which is also known as burros type LED is as

shown in figure (4.5).

This structure emits light in band of 0.8 to 0.9 um wavelength. The plane of the active light

emitting region is made perpendicular to the fiber axis. The fiber is cemented in a well matched

through the substrate of the fiber so that maximum emitted light is coupled to the fiber. Due to

large band gap conjoining area, the internal absorption is less and the reflection coefficient at

the back crystal face is high, hence forward radiance is good. The active area in circle is of

50μm in diameter and up to 2.5μm thick. The emission from this active area is isotropic with


120° half power beam width is used for practical purpose. Isotropic pattern from a surface

emitter is lambert Ian pattern.

The source is equally bright when viewed from any direction but power diminishes as cosΦ

where $ is the angle between viewing direction and to the normal to the surface. Power is down

to 50%, when Φ = 60°, so that the total half power beam width is 120°. The power coupled into a

multimode step index fiber may be estimated from the relationship.

PC =П(1-r)ARD (NA)2 ..(1)

Where, PC = Power coupled into fiber

r = Fresnel reflection coefficient

A = Emission area of source

RD = Radiance of the source

NA = Numerical aperture

Power coupled into the fiber depends on

(i) Distance and alignment between emission area and the fiber.

(ii) Medium between the emitting area and the fiber.

(iii) Emission pattern of SLED

Addition of Epoxy resin in the etched well reduces the refractive index mismatch and increases

the external power efficiency of the device. Hence the power coupled in the double hetero

structure surface emitters are more than Pc(optical power) that is given by equation (1), For

graded index fiber-direct coupling requires the source diameter of about one half the fiber core

diameter.

2. Draw the schematic of edge emitting double hetero junction LED and explain its working in detail? [AUC MAY 2012] Ans: Double Hetero junction Laser If a single p-n junction diode is fabricated from suitable single crystal semiconductor material it

exhibits photo emissive properties. It is known as 'homo junction' p-n diode. However the

emissive properties of a junction diode can be improved considerably by the use of 'hetero

junction'. A hetero junction is an interface between two adjoining crystal semiconductors having

different values of band gap energies. Devices are fabricated with hetero junction are said to

have hetero structures.

They are of two types,

(i) Isotopes such as n-n or p-p type

(ii) Anisotope such as p-n type.

The isotope p-p junction has a potential barrier within the structure. The structure is capable of

confining min carriers to small active region called cavity. It effectively reduces the diffusion


length of the carrier and thus the volume of the structure where radioactive recombination may

occur.

Figures show the schematic layer structure, energy band diagram and refractive index profile,

for a double hetero junction injection laser diode with biasing. The laser oscillations take place in

the central p-type GaAs region which is known as active layer. There is hetero junction at the

both sides of the active layer. A forward bias voltage is applied by connecting the positive

electrode of the power supply voltage to the P-side of the structure and negative electrode to the

n-side when a voltage which is almost equal to the band gap energy. The hetero junctions are

used to provide potential barrier in the injection laser. In this structure it is possible to obtain both

carrier and optical containment to the active layer.


The above figure represents the layer structure of a broad area DH. The GaAs layers acts as

active layer which is sandwiched between p-type AZGaAs and n-A/GaAs layer and these two

layers act as the confinement layers. Light is emitted from the central GaAs active layer through

the front and back side of the device.

In the case of the DH broad area laser structure, the optical confinement in the vertical direction

is achieved by the refractive index at the hetero junction interfaces between the active layer and

the containment layers, but the laser .action takes places across the whole width of the device.

In a broad area laser the sides of the cavity are formed by simple roughening the ends of the

device to reduce the unwanted emission and limit the horizontal transverse modes.

Stripe Geometry Laser In order to overcome the difficulties in a broad area laser structure, the stripe geometry laser

structure has developed and in this structure the active area does not enter upto the edges of

the device.

A common method is used to introduce the stripe geometry to the structure which provides the\

optical contaminant in the horizontal plane as shown in the figure above. The stripe geometry is

usually formed by creating a high resistance area on either side of the stripe by 'Proton

bombardment' technique or by oxide oscillation. The stripe usually acts as a guiding mechanism

which avoids all major difficulties encountered in the case of a broad area laser. The contact

stripe provides the balance of guiding single transverse mode operation in a direction parallel to

the junction plane, whereas broad area devices allow multiple mode operation in this horizontal

plane. The width of the stripe generally ranges from 2.0 to 65 mm and stripe laser find wide

application in fiber communications.


3. Obtain the expression for the 3dB modulation bandwidth of LED and discuss the importance of radiative recombination life time? [AUC NOV 2010] Ans: The expression for the 3 dB modulation bandwidth of LED in optical communication may

be obtained in either electrical and optical terms.If we consider the associated electrical circuitry

in an optical fiber communication system to use the electrical definition, where the electrical

signal power has dropped to half of its constant value due to the modulated portion of the optical

signal. Hence, this corresponds to the electrical 3 dB frequency at which the output electrical

power is reduced by 3 dB with respect to the input electrical power. We can also consider the

high frequency 3 dB point, when the optical source operates down to D.C. The expression for

the electrical bandwidth can be obtained from the ratio of the electrical output power to the

electrical input power in decibels and is given as


Therefore In optical regime the bandwidth is defined by the frequency at which the output

current has dropped to 0.5 of the input current to the system.

The Modulation bandwidth of LED is generally determined by three methods ,

They are

1. The doping level in the active layer,

2. Due to the injected carriers, the reduction in radiative lifetime.

3. The parasitic capacitance of the device.

If we assume that the parasitic capacitance is negligible, then the speed at which an LED can

be directly current modulated is fundamentally limited by the recombination lifetime of the

carriers,


.

4. Discuss the major requirements of an optical source for use in optical communication systems? [AUC NOV 2010] Ans: The development of efficient semiconductor optical sources along with low-loss optical

fibers, led to substantial improvements in fiber optic communications. Semiconductor optical

sources have the physical characteristics and performance properties necessary for successful

implementations of fiber optic systems. It is desirable that optical sources must be,

1. Compatible in size to low-loss optical fibers by having a small light emitting which are capable

of launching light into fiber.

2. Launch sufficient optical power into the optical fiber to overcome fiber attenuation and

connection losses allowing for signal detection at the receiver.

3. Emit light at wavelengths that minimize optical fiber loss and dispersion.

4. Optical sources should have a narrow spectral width to minimize dispersion.

5. Allow for direct modulation of optical output power.

Maintain stable operation in changing environmental conditions (such as temperature). Cost less

and be more reliable than electrical devices, permitting fiber optic communication systems to

compete with conventional systems. Semiconductor optical sources suitable for fiber optic

systems range from inexpensive Light Emitting Diodes (LEDs) to more expensive EC 2402 OPTICAL COMMUNICATION & NETWORKS – IV/VII - V.SENTHAMIZH SELVAN ASST. PROF/ECE

semiconductor lasers. Semiconductor LEDs and laser diodes are the principle light sources

used in fiber optics.

Semiconductor sources are designed to operate at wavelengths (i.e., 850 nm, 1300 nm and

1500 nm) that minimize optical fiber absorption and maximum system bandwidth. By designing

an optical source to operate at specific wavelengths, absorption from impurities in the optical

fiber such as hydroxyl ions (OH-) can be minimized. Maximizing system bandwidth involves

designing fibers and sources that minimize chromatic and inter modal dispersion at the intended

operational wavelength.

5. Compare the advantageous and disadvantageous of LED and Explain the key process involved in the LASER operation? [AUC NOV 2013] Ans: 1. Simple Fabrication: There are no mirror facets and is some structures no striped geometry

2. Cost: The simpler construction of LED leas to much reduced cost which is always likely to be

maintained.

3. Reliability The LED does not exhibit catastrophic degradation and has proved to be less sensitive to

gradual degradation than the injection laser.

4. Simpler Drive Circuitry This is due to lower drive currents and reduced temperature dependence which makes

temperature compensation circuits unnecessary.

5. Linearity Ideally, the LED has a linear light output against current characteristics unlike the injection laser.

Disadvantage An LED radiates rather dispersed light, which makes coupling this light into an optical fiber a

problem.

The key processes involved in laser action are as given below.

(i) Absorption.

(ii) Spontaneous emission.

(iii) Stimulated emission.


These three key processes are represented by 2-energy level diagrams. Where, E1= Energy of

ground state. E2 = Energy of excited state.


6. Explain the structure and operation of injection laser diode. [AUC NOV 2011] INJECTION LASER DIODE: Stimulated emission by the recombination of the injected carriers is encouraged in the

semiconductor injection laser by the provision of an optical cavity in the crystal structure in order

to provide feedback of photons. The injection laser has many advantages over other

semiconductor sources. They are

i) High radiance due to amplifying effect of stimulated emission.

ii) Narrow line width of the order of 1nm or less.

iii) Modulation capability in the range of gigahertz.

iv) Relative temporal coherence which allows heterodyne detection.

v) Good spatial coherence, which allows the output to be focused by a lens into a spot

which has greater intensity than the dispersed unfocussed emission.

Early injection lasers had the form of Fabry Perot cavity often fabricated in gallium arsenide.

The basic structure of this homojunction device is shown below.

Fig: Schematic diagram of a GaAs homojunction injection laser with a Fabry Perot cavity. If a PN junction is formed by a single crystal semiconductor material, it is said to be

homojunction laser diode. Generally the homojunction lasers are not used in the

communication applications due to their higher divergence and high threshold current. Further

they produce high dispersion and have low optical confinement.

A heterojunction is an interface between two adjoining single crystal semiconductor

with different band gap energies. Improved carrier confinement and lower threshold current

densities were achieved using heterojunction structures.


Efficiency The differential external quantum efficiency ηD which is the ratio of the increase in

photon output rate for a given increase in the number of injected electrons. If Pe is the optical

power emitted from the device, I is the current, e is the charge of an electron and hf is the

photon energy, then

Where Eg is the band gap energy expressed in electron volts. ηD is sometimes referred to as the

slope quantum efficiency. For a continuous wave semiconductor laser it ranges between 40-

60%.

The internal quantum efficiency of the semiconductor laser η i, which is defined by

Its value ranges between 50- 100%. It is related to differential external quantum efficiency by the

expression

Where is the loss coefficient of the laser cavity, L is the length of the laser cavity and r1 and

r2 are the cleaved mirror reflectives.

Another parameter is the total efficiency ηr which is defined as

As the power emitted Pe changes linearly when the injection current I is greater than the

threshold current Ith, then


For high injection current (e.g. I= 5 Ith ) then ηT ≈ηD, where as for lower currents, I= 2 Ith the total

efficiency is lower and around 15- 25 %.

The external power efficiency of the device ηep in converting electrical input to optical output is

given by

Where P = IV is the dc electrical input power.

Using eqn 6.29 for the total efficiency, we find

7. Explain single mode laser structure. [AUC MAY 2009] Single mode laser structures: i) Buried Heterostructure (BH) Laser: The BH laser is obtained by etching a narrow metal stripe in DH semiconductor

material and effectively burying it in high resistivity, lattice matched n type material with an

appropriate bandgap and low refractive index. This rocess involves growth of double liquid

phase eitaxial (LPE) to give the structure as shown below.

Fig: schematic representation of the structure of GaAs/ Al Ga As buried heterostructure laser


These devices may have very small active regions to allow single mode operation. The small

active region also gives low threshold currents of 10 mA or less and good linearity without kinks.

Wide modulation bandwidths of 2 GHz have also been obtained. The major drawback is the

beam divergence in the junction plane (40-50°) from the small active region.

ii) Transverse Junction Stripe (TJS) layer: This structure is shown below which consists of a DH multilayer in n type

semiconductor material. The lateral waveguide is achieved by two consecutive zinc diffusions in

order that the structure is confined by p+ -n and p-n gallium arsenide homojunctions. Carrier

injection is obtained laterally across the homojunction in the central layer and the DH structure

confines the carriers to the central active region. The device has good characteristics for single

longitudinal and transverse mode operation with typical threshold currents of the order of 20 mA

giving continuous wave optical output power of around 3mW.

Fig: structure of a GaAs/ AlGaAs transverse junction stripe laser.

iii) Distributed Feedback (DFB) Lasers:

These devices consist of a complex structure which determines the wavelength

of the longitudinal mode emission. An optical grating is incorporated into the heterosructure

waveguide to provide periodic variations in refractive index along the direction of wave

propagation so that feedback of optical energy is obtained through Bragg scattering. The

corrugated grating may be applied over the whole active length of the device where it gives what

is known as distributed feedback and eliminates the need for end mirrors.

An improved mesa stripe geometry DFB laser structure fabricated with the GaAs/AlGaAs

system which employs a separate confinement heterosructure is shown below


In this device the corrugations are separated from the active layer and formed in an

Al0.07Ga0.93.As layer on the p side of the junction. The n type Al0.3Ga0.7.As and ptype

Al0.17Ga0.83As layers confine the optical field to the p type GaAs active layer. However sufficient

optical power leaks across the thin p type Al0.17Ga0.83As buffered layer into the corrugated

region such that distributed feedback is obtained. The emission frequency from the structure is

determined by the corrugation period. Hence the DFB structure can provide superior longitudinal

mode discrimination over conventional Fabry Perot structures where single longitudinal mode

operation is dependent on the gain spectrum of the optical cavity.

When suitably fabricated and operating with a single longitudinal and transverse mode

such DFB lasers can give narrow emission line widths of less than 1nm in comparison with 1-2

nm for conventional DH injection lasers.

Another advantage of DFB laser over Fabry perot cavity design is that it exhibits reduced

wavelength sensitivity to changes in temperature and injection current. The emission from a

Fabry Perot injection laser follows the temperature dependence of the energy gap whereas the

lasing from the DFB structure follows the smaller temperature dependence of the refractive

index. The typical wavelength shift with temperature for a DFB laser is 0-0.05 nmK-1 while the

ordinary DH structure gives a typical shift of 0.2-0.5 nmK-1.


8. Distinguish between LED and ILD. [AUC MAY 2012]

Comparison of LED and ILD: S.No Parameters LED LASER 1 Principle of operation Spontaneous emission

Stimulated emission

2 Output beam Non coherent

Coherent

3 Spectral width Broad spectrum(20-100nm)

Much narrower (1-5nm)

4 Data rate Low

Very high

5 Transmission distance Smaller

Greater

6 Temperature sensitivity Less sensitive More temperature sensitive

7 Coupling efficiency Very low

High

8 Compatible fibers Multimode step index multimode GRIN

Single mode SI Multimode GRIN

9 Circuit complexity Simple

Complex

10. Life time 105 hours

104 hours

11 Cost Low

High

12 Output power Linearly proportional to drive current

Proportional to current above threshold

13 Current required Drive current 50 to 100 mA peak

Threshold current 5 to 40 mA

14 Wavelengths available 0.66 to 1.65 µm

0.78 to 1.65µm

15 Applications Moderate distance low data rate

Long distance high data rates


9. Explain detail about PIN photo detector. [AUC MAY 2008] PIN PHOTODETECTOR: The most common semiconductor photo detector is the pin photodiode shown below

Fig: pin photodiode with applied reverse bias

The device structure consists of p and n region separated by a very lightly n doped

intrinsic (i) region. In normal operation a sufficiently large reverse bias voltage is applied across

the device so that the intrinsic region is fully depleted of carriers. That is, the intrinsic n and p

carrier concentrations are negligibly small in comparison with the impurity concentration in this

region.

When an incident photon has an energy greater than or equal to the bandgap energy of

the semiconductor material, the photon can give up its energy and excite an electron from the

valence band to the conduction band. This process generates free electron from the valence

band to the conduction band. This process generates free electron hole pairs which are known

as photocarriers, since they are photon generated carriers as shown below.

Fig: Energy band diagram of a photodetector


The photodetector is normally designed so that these carriers are generated mainly in the

depletion region where most of the incident light is absorbed. The high electric field present in

the depletion region causes the carriers to separate and be collected across the reverse bias

junction. This gives rise to a current flow in an external circuit, with electron flowing for every

carrier pair generated. This current flow is known as photocurrent. On the average charge carriers move a distance Ln or Lp for electrons and holes

respectively. This distance is known as the diffusion length. The time it takes for an electron or

hole to recombine is known as carrier life time and is replaced by τm and τp respectively. The

life times and diffusion lengths are related by the expressions.

Where Dn and Dp are the electron and hole diffusion coefficients which are expressed in units of

centimeters squared per second. Optical radation is absorbed in the semiconductor material

according to the exponential law

Here αs(λ) is the absorption coefficient at a wavelength λ, P0 is the incident optical power level,

and P(x) is the optical power absorbed in a distance x.

The upper wavelength cutoff λc is determined by the band gap energy Eg of the material. It is

expressed in units of electron volts (eV), then λc is given in units of micrometers by

The cutoff wavelength is about 1.06 µm for Si and 1.6 µm for Ge. If the depletion region has a

width w, then the total power absorbed in the distance w is

If we take into account a reflectivity Rf at the entrance face of the photodiode then the primary

photocurrent Ip is given by

Where P0 is the optical power incident on the photodetector, q is the electron charge and hv is

the photon energy. The important characteristics of a photodetector are its quantum efficiency

and its response speed. These parameters depend on the material bandgap, the operating

wavelength and the doping and thickness of the p,I and regions of the device.

The quantum efficiency η is the number of electron hole carrier pairs generated per

incident photon of energy hv and is given by


Here Ip is the average photocurrent generated by a steady state average optical power P0

incident on the photodetector. The performance of a photodiode is often characterized by the

responsivity R. this is related to the quantum efficiency by

The responsivity falls off rapidly beyond the cutoff wavelength which is shown in following figure.

Fig: comparison of responsivity and quantum efficiency as a function of wavelength for

pin photodiode. 10. Explain detail about Avalanche photo diode. [AUC NOV 2008] AVALANCHE PHOTODIODES: Avalanche photodiodes (APDs) internally multiply the primary signal photocurrent before

it enters the input circuitry of the following amplifier. This increases receiver sensitivity, since the

photocurrent is multiplied before encountering the thermal noise associated with the receiver

circuit.

In order for a carrier multiplication to take place, the photogenerated carriers must

traverse a region where very high electricfield is present. In the high field region a

photogenerated electron or hole can gain enough energy so that it ionizes bound electrons in

the valence band upon colliding with them. This carrier multiplication mechanism is known as

impact ionization. The newly created carriers are also accelerated in the high electric field, thus gaining

enough energy to cause further impact ionization. This phenomenon is the avalanche effect.


A commonly used structure for achieving carrier multiplication with very little excess noise is the

reach through construction shown in the below figure.

Fig: Reach through avalanche photodiode structure

The reach through avalanche photodiode (RAPD) is composed of a high resistivity p type

material deposited as an epitaxial layer on a p+ substrate. A p-type diffusion or ion implant is

then made in the high resistivity material followed by the construction of an n+ layer. This

configuration is referred to as p+ πpn+ reach through structure. The π layer is basically an

intrinsic material that inadvertently has some p doping because of imperfect purification.

The term ‘reach through’ arises from the photodiode operation. When low reverse bias

voltage is applied most of the potential drop is across the pn junction. The depletion layer

widens with increasing bias until a certain voltage is reached at which peak electric field at the

pn junction about 5 to 10 percent below that is needed to cause avalanche breakdown. At this

point the depletion layer just “reaches through” to the nearly intrinsic π region.

The average number of electron hole pairs created by a carrier per unit distance

travelled is called the ionization rate. Most materials exhibit different electron ionization rates α and hole ionization rates β. experimentally obtained values of α and β for five different

semiconductor material are shown in following figure.


The ratio k= β/ α of two ionization rates is a measure of photodetector performance. The

multiplication M for all carriers generated in the photodiode is defined by

Where IM is the average value of the total multiplied output current and Ip is the primary

unmultiplied photocurrent.

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

Where Ro is the unity gain responsivity.

11. Comparison between PIN and AVALANCHE photo detector. [AUC MAY 2010]

Comparison of PIN and APD: S.No Parameters PIN APD 1 Sensitivity Less sensitive (0- 12 dB)

More sensitive (5-15 dB)

2 Biasing Low reverse biased voltage (5 to 10 V)

High reverse biased voltage (20- 400 volts)

3 Wavelength region 300- 1100 nm

400 -1000 nm

4 Gain No Internal gain

Internal gain

5 S/N ratio Poor

Better

6 Detector circuit Simple

More complex

7 Conversion efficiency 0.5 to 1 Amps/ watt

0.5 to 100 Amps/ watt

8 Cost Cheaper

More expensive


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MAHALAKSHMImahalakshmiengineeringcollege.com/pdf/ece/VIIsem/EC2402/UNIT III.pdfSUBJECT NAME : OPTICAL COMMUNICATION & NETWORKS . SUBJECT CODE : EC 2402 . UNIT – III: SOURCES AND