Answer QUESTION No.1 compulsorily 1X12=12M

1.

a) The Mode Field Diameter (MFD) is an important parameter related to the optical field. distribution in

the fiber. It has been shown that MFD provides useful information about. the cabling performances,

such as possible joint, macrobending, and microbending. losses.

b) The angle of incidence should be greater than critical angle of the denser medium then only total

Internal reflection (TIR) occurs.

c) i) wider bandwidth ii) Low transmission loss iii)signal security iv) small size and weight v) Long

distance communication vi) low cost.

d) i) Intrinsic absorption is mainly caused by the interaction of photons with molecular vibrations within

the glass ii) Extrinsic absorption is mainly caused by impurity atoms in the glass material.

e) Dispersion induced signal distortion is that a light pulse will broaden as it travels along the fiber.

This pulse broadening causes a pulse to overlap with neighbouring pulses.

f) A permanent joint formed in between two individual optical fibers in the field or factory is known

as a fiber splice.

g)

P-I-N photo diode Avalanche photodiode

Very low reverse bias is necessary High reverse bias voltage is required

No gain High Gain

Simple structure Fabrication is difficult due to complex structure

h) Radiance is a measure in watts of the optical power radiated into a unit solid angle per unit area of

the emitting surface.

i) Stimulated emission is the process by which an incoming photon of a specific frequency can interact

with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy

level.

j) Rise time budget is used to check the maximum bit rate at which system can work properly.

Tsys = 1.1 sqrt( T2TX + T2

RX + T2fiber)

k) The electric current in a photodetector circuit is composed of a superposition of the electrical pulses

associated with each photoelectron .The variation of this current is called shot noise

l) 1) It a non- destructive method because it requires only one end of fiber

2) To locate faults such as breaks.

Answer ONE Question from each unit

UNIT -I

2 ) A) Elements of Optical communication system:-

For analog communication diagram and explanation 3M

For digital communication diagram and explanation 3M

The shows a block schematic of the different elements in an optical fiber communication system.

The carrier is modulated using analog information signal. The variation of light emitting from the

optical source is a continuous signal. The information source provides an electrical signal to the

transmitter. The transmitter comprises electrical stage. The electrical stage (circuits) drives an optical

source. The optical source output is a light which is intensity modulated by the information. The optical

source converts the electrical signal into an optical signal. The source may be either semiconductor laser

or Light Emitting Diode (LED). The intensity modulated light signal is coupled to fiber. The fiber which

is made up of a glass acts as a channel between the transmitter and receiver.

.

Figure shows a schematic of a typical digital optic fiber link. The input is given as digital signal from

the information source and it is encoded for optical transmission in the encoder. The encoder, encodes or

modulates the digital signal as in the case of simple communication system where we are using a

message signal in which the signal is in analog form, but here the signal is in digital form which is

encoded i.e., modulated in the encoder. The laser drive circuit directly modulates the intensity of

semiconductor laser with the encoded digital signal. Hence a digital optical signal is launched into the

optical fiber cable. At the receiver we have to decode the digital optical signal for which we are using

another Avalanche Photo Diode (APD) as detector. The avalanche photo diode detector is followed by a

front-end amplifier and equalizer or filter to provide gain as well as linear signal processing and noise

bandwidth reductions. Then the signal is passed through the decoder to get original digital information

which is transmitted.

2 b) Mode field diameter of single mode fiber:-

Explanation of single mode fibers 3M

Explanation of mode field diameter 3M

The advantage of the propagation of a single mode within an optical fiber is that the signal

dispersion caused by the delay differences between different modes in a multimode fiber may be

Multimode step index fibers do not lend themselves to the propagation of a single mode due to the

difficulties of maintaining single-mode operation within the fiber when mode conversion (i.e. coupling)

to other guided modes takes place at both input mismatches and fiber imperfections. Hence, for the

transmission of a single mode the fiber must be designed to allow propagation of only one mode, while

all other modes are attenuated by leakage or absorption. Following the preceding discussion of

multimode fibers, this may be achieved through choice of a suitable normalized frequency for the fiber.

For single-mode operation, only the fundamental LP01 mode can exist. Hence the limit of single-mode

operation depends on the lower limit of guided propagation for the LP11 mode. The cut off normalized

frequency for the LP11 mode in step index fibers occurs at Vc = 2.405. Thus single-mode propagation of

the LP01 mode in step index fibers is possible over the range: 0 ≤ V < 2.405 .as there is no cut off for

the fundamental mode. It must be noted that there are in fact two modes with orthogonal polarization

over this range, and the term single-mode applies to propagation of light of a particular polarization.

(OR)

3)a) Given RI of core n1= 1.50

RI of cladding n2 = 1.47

i) Critical angle =sin-1(n2/n1)= 1.370 2M

ii)Numerical aperture NA = sqrt(n12 – n22) = 0.298 2M

iii) Acceptance angle = sin-1(NA)= 0.303 2M

3)b) Total Internal Reflection (TIR):- 3M

To consider the propagation of light within an optical fiber utilizing the ray theory model it is necessary

to take account of the refractive index of the dielectric medium. The refractive index of a medium is

defined as the ratio of the velocity of light in a vacuum to the velocity of light in the medium.

It may also be observed in Figure 2.2(a) that a small amount of light is reflected back into the

originating dielectric medium (partial internal reflection). As n1 is greater than n2, the angle of

refraction is always greater than the angle of incidence. Thus when the angle of refraction is 90° and the

refracted ray emerges parallel to the interface between the dielectrics, the angle of incidence must be

less than 90°. This is the limiting case of refraction and the angle of incidence is now known as the

critical angle φc,

Sin φc = n2/n1

At angles of incidence greater than the critical angle the light is reflected back into the originating

dielectric medium (total internal reflection) with high efficiency (around 99.9%). Hence, it may be

observed in Figure 2.2(c) that total internal reflection occurs at the interface between two dielectrics of

differing refractive indices when light is incident on the dielectric of lower index from the dielectric of

higher index,

Numerical aperture:-

It is a way of measuring light gathering ability of an optical fiber. It is used to measure source to

fiber power coupling efficiencies. A high NA indicates a high source to fiber coupling.

Calculation of NA 3M

UNIT-II

4)a) Bending losses:-

Explanation and diagram of microbending 3M

Explanation and diagram of Macrobending 3M

Depending Upon the radius of the bend there are two types of bends

i) Microbending – If the radius of curvature is a few micrometers

ii) Macrbending - If the radius of curvature is larger than fiber diameter

4)b) Optical Switches: 6M

In telecommunication, an optical switch is a switch that enables signals in optical fibers or integrated

optical circuits (IOCs) to be selectively switched from one circuit to another.

The need for Optical Switching

High bit rate transmission must be matched by switching capacity

Optical or Photonic switching can provide such capacity

What is Optical Switching?

Switching is the process by which the destination of a individual optical information signal is controlled

All-optical switches get their name from being able to carry light from their input to their output

ports in its native state – as pulses of light rather than changes in electrical voltage.

All-optical switching is independent on data rate and data protocol.

Results in a reduction in the network equipment, an increase in the switching speed, a decrease

in the operating power.

(OR)

5)a) Absorption:-

Material absorption is a loss mechanism related to the material composition and fabrication process for

the fiber.Due to this some of the transmitted optical power is dissipated in the form of heat.

The absorption of the light may be in two types

1.Intrinsic:caused by the interaction with one or more of the major components of the glass.

2.Extrinsic: caused by impurities within the glass.

Intrinsic absorption: 3M

Pure glass has little intrinsic absorption due to the basic material structure in the near infrared region. A

low intrinsic absorption window over the 0.8 – 1.7 μm. This window shows a possible optical

attenuation against wavelength characteristic for absolutely pure glass. This is due to the simulation of

electron transitions within the glass by higher energy excitations. Wavelengths above 7 μm,

fundamentals of absorption bands from interaction of photons with molecular vibrations within the glass

occur. Then the absorption is extended to this window The strong absorption bands are occur due to

oscillations of structural units B-O, Ge-O, Si-O, P-O within the glass.

The intrinsic absorption loss may be minimized by:

1.suitable choice of core- cladding materials.

2.glases such as depends of fluorides and chlorides instead of oxides

Extrinsic absorption 3M

The metal element impurities( Nickel, Iron Chromium) cause extrinsic absorption loss.

Some metal impurities are shown below, which causes extrinsic absorption.

• However, another major extrinsic loss mechanism is caused due to water(OH ion) dissolved in

the glass.

• These OH ions may create fundamental vibrations in the glass structure.

These vibrations may occur at wave lengths between 2.7 and 4.2 μm depending on group position in the

network.

This extrinsic absorption loss may reduced using vapor phase oxidation method

5)b) Optical Connectors:-

Diagram &Explanation of cylindrical ferrule connectors 3M

Diagram & Explanation of expanded beam connectors 3M

Demountable fiber connectors are more difficult to achieve than optical fiber splices. This is

because they must maintain similar tolerance requirements to splices in order to couple light between

fibers efficiently, but they must accomplish it in a removable fashion. Also, the connector design must

allow for repeated connection and disconnection without problems of fiber alignment, which may lead

to degradation in the performance of the transmission line at the joint. Hence to operate satisfactorily the

demountable connector must provide reproducible accurate alignment of the optical fibers.

Hence optical fiber connectors may be considered in three major areas, which are:

(a) the fiber termination, which protects and locates the fiber ends;

(b) the fiber end alignment to provide optimum optical coupling;

(c) the outer shell, which maintains the connection and the fiber alignment, protects the fiber ends from

the environment and provides adequate strength at the joint.

Fiber connectors may be separated into two broad categories: butt-jointed connectors and expanded

beam connectors. Butt-jointed connectors rely upon alignment of the two prepared fiber ends in close

proximity (butted) to each other so that the fiber core axes coincide. Expanded beam connectors utilize

interposed optics at the joint (i.e. lenses) in order to expand the beam from the transmitting fiber end

before reducing it again to a size compatible with the receiving fiber end.

Cylindrical Ferrule Connectors

The basic ferrule connector (sometimes referred to as a concentric sleeve connector), which is perhaps

the simplest optical fiber connector design,. The two fibers to be connected are permanently bonded

(with epoxy resin) in metal plugs known as ferrules which have an accurately drilled central hole in their

end faces where the stripped (of buffer coating) fiber is located. Within the connector the two ferrules

are placed in an alignment sleeve which, using accurately machined components, allows the fiber ends

to be butt jointed. The ferrules are held in place via a retaining mechanism It is essential with this type

of connector that the fiber end faces are smooth and square(i.e. perpendicular to the fiber axis). This

may be achieved with varying success by:

(a) cleaving the fiber before insertion into the ferrule;

(b) inserting and bonding before cleaving the fiber close to the ferrule end face;

(c) using either (a) or (b) and polishing the fiber end face until it is flush with the end of the ferrule.

Expanded Beam Connectors

An alternative to connection via direct butt joints between optical fibers is offered by the principle of the

expanded beam. Fiber connection utilizing this principle is illustrated in above figure. which shows a

connector consisting of two lenses for collimating and refocusing the light from one fiber into the other.

UNIT-III

6)a) Quantum Efficiency and Power of LED:- 6M

The absence of optical amplification through stimulated emission in the LED tends to limit the

internal quantum efficiency (ratio of photons generated to injected electrons) of the device. Reliance on

spontaneous emission allows non radiative recombination to take place within the structure due to

crystalline imperfections and impurities giving, at best, an internal quantum efficiency of 50% for

simple homojunction devices. However, as with injection lasers, double-heterojunction (DH) structures

have been implemented which recombination lifetime measurements suggest give internal quantum

efficiencies of 60 to 80%.

The power generated internally by an LED may be determined by consideration of the excess

electrons and holes in the p- and n-type material respectively (i.e. the minority carriers) when it is

forward biased and carrier injection takes place at the device contacts (see Section 6.3.2). The excess

density of electrons Δn and holes Δp is equal since the injected carriers are created and recombined in

pairs such that charge neutrality is maintained within the structure. In extrinsic materials one carrier type

will have a much higher concentration than the other and hence in the p-type region, for example, the

hole concentration will be much greater than the electron concentration. Generally, the excess minority

carrier density decays exponentially with time t according to the relation:

Δn = Δn(0) exp(−t/τ)

6)b)Photodetector Response Time:-

Explanation of principle of photo detectors with any example 4M

Explaining response time parameter 2M

Photodiodes are preferred for photo detection in optical system. The photodiodes provide good

performance and compatibility with relatively low cost. These photodiodes are made from

semiconductors such as silicon, germanium and an increasing number of III-V alloys. Internal

photoemission process may take place in both intrinsic and extrinsic semiconductors. The intrinsic

absorption process is preferred as they have fast response coupled with efficient absorption of photons.

These photodiodes are sensitive, have adequate speed, negligible shunt, conductance, low

dark current, long term stability. Thus they are widely used. Avalanche photodiodes are also

widely employed in fiber communication system. They have very sophisticated structure.

Responsivity

The ratio of generated photocurrent to incident light power, typically expressed in A/W when used in

photo conductive mode. The responsivity may also be expressed as quantum efficiency or the ratio of

the number of photo generated carriers to incident photons and thus, a unit less quantity.

(OR)

7)a) PIN photo detector:- Diagram 3M + Explanation 3M

In order to allow operation at longer wavelengths where the light penetrates more deeply into the

semiconductor material, a wider depletion region is necessary. To achieve this the n-type material is

doped so lightly that it can be considered intrinsic, and to make a low resistance contact a highly doped

n-type (n+) layer is added. This creates a p–i–n (or PIN) structure, where all the absorption takes place

in the depletion region.

The width of the intrinsic region should be larger than the space charge width of a normal p-n

junction. The PIN photodiode operates with an applied reverse bias voltage and when the reverse bias is

applied, the space charge region must cover the intrinsic region completely. Electron hole pairs are

generated in the space charge region by photon absorption.

The switching speed of frequency response of photodiode is inversely proportional to the life

time. The switching speed can be enhanced by a small minority carrier lifetime. For the photo detector

applications where the speed of response is important, the depletion region width should be made as

large as possible for small minority carrier lifetime as a result the switch speed also increases. This can

be achieved PIN photodiode as the insertion of intrinsic region the space charge width larger.

The response time of a PIN photodiode is determined by three main factors.

Explain three factors

7)b) LASER mechanisms:-

Laser has two mechanisms for lasing action

i) Population Inversion (explanation of DH laser) 3M

ii) Optical feedback ( explanation of DFB laser) 2M

Characteristics 1M

DH Laser:-

DFB Laser:-

UNIT-IV

8)a) Rise Time Budget:- 6M

Rise time budget is used to check the maximum bit rate at which system can work properly. In

other words, we can say that it determine the dispersion limitation of an optical fiber link. Dispersion is

the broadening of pulse, which cause overlapping of pulses that is, intersymbol interference in addition

to limiting the maximum data rate.For digital systems, a complete rise time analysis-including the

source, fiber, and detector rise time is required.

Rise Time Budget

➢ Similarly, the slowest component in the system will ultimately control the system bandwidth

since the system response time cannot be faster than the response time of the slowest component.

➢ Each element of the link is fast enough to meet the given bit rate.

➢ The process is called link rise time budgeting procedure.

➢ Apart from the component losses, a certain amount of power margin Psm, called as system

margin, is required for unexpected losses.

Rise time budget:

In a system with N cascaded components, each of which has a rise time ti, the total rise time tsys of the

system is

where

➢ tt = transmitter rise time

➢ tmat = material dispersion rise time of the fibre

➢ tmod = modal dispersion (broadening in time) of the fiber

➢ tr = receiver rise time

Explain MODAL distortion and Material dispersion values also.

8)b) Various Error sources in Optical receiver:Noise names 5M + explanation 1M

(OR)

9)a)Preamplifier Types:-

There are three commonly used amplifier configurations

a) Low impedance amplifier diagram & explanation 2M

b) High impedance amplifier diagram & explanation 2M

c) Transimpedance amplifier diagram & explanation 2M

Low impedance amplifier:

CT= Cd + Ca

High impedance amplifier:

Trans impedance amplifier:

9)b) Wavelength Division Multiplexing:- Block diagrams 3M + Explanation 3M

In the basic concept of a WDM, monochromatic optical signals of wavelengths are generated by laser

diodes and sent through N fibers to a multiplexer. The MUX combines these input signals in to a

polychromatic output signals through the process of multiplexing. The large bandwidth capacity of

optical fiber is exploited through the multiplexing process. This multiplexed polychromatic signal is

launched in to a single optical fiber for transmission. At the destination a demultiplexer seperates the

polychromatic signal into original monochromatic wavelengths.

The DEMUX must be designed such that the centre wavelengths of channels should be same as the

original wavwlwngths and the channel spectral widths. Therefore to overlapping of the channels the

pass bands should be choosen to accommodate system tolerances.

Depending on application needs, different types of WDM systems are developed such as point to point

long distance transmission, local access networks, reconfigurable network etc. Each of these systems

needs different WDM components

Sk. Idrish

Assistant Professor

ECE Department

9866600354