SET-1

1. a) Discuss briefly the advantages of optical fiber communication.

ANS: Advantages of Optical Fibers Communication:

1. Information bandwidth is more.

2. Optical fibers are small in size and light weighted.

3. Optical fibers are more immune to ambient electrical noise, electromagnetic interference.

4. Cross talk and internal noise are eliminated in optical fibers.

5. There is no risk of short circuit in optical fibers.

6. Optical fibers can be used for wide range of temperature.

7. A single fiber can be used to send many signals of different wavelengths using

Wavelengths Division Multiplexing (WDM).

8. Optical fibers are generally glass which is made up of sand and hence they are

cheaper than copper cables.

9. Optical fibers are having less transmission loss and hence less number of repeaters are used.

10. Optical fibers are more reliable and easy to maintain.

b) Derive the Numerical Aperture of a Step Index Fiber (SIF) from Snell’s law

2. a) What are different fiber materials used in optical communication? Explain briefly.

ANS: Fiber Materials

Most of the fibers are made up of glass consisting of either Silica (SiO2) or Silicate. High-loss glass fibers are used for short -transmission distances and low -loss glass fibers are used for long distance applications. Plastic fibers are less used because of their higher attenuation than glass fibers.

The glass fibers are made from oxides. The most common oxide is silica whose refractive index is 1.458 at 850 nm. To get different index fibers, the

dopants such as GeO2, P2O5 are added to silica. GeO2 and P2O3 increase the

refractive index whereas fluorine or B203 decreases the refractive index. Few fiber compositions are given below as follows,

GeO2 – SiO2 Core: SiO2 Cladding

P2Q5 – SiO2, Core: SiO2 Cladding

The principle raw material for silica is sand. The glass composed of pure silica is referred to as silica glass, nitrous silica or fused silica. Some desirable properties of silica are

(i) Resistance to deformation at temperature as high as 1000°C.

(ii) High resistance to breakage from thermal shock.

(iii) Good chemical durability.

(iv) High transparency in both the visible and infrared regions.

b) Explain the core and cladding losses in optical fiber.

ANS:

3. a) Explain the material dispersion in optical wave guides.

Material dispersion is also called as chromatic dispersion. Material dispersion exists due to change in index of refraction for different wavelengths. A light ray contains components of various wavelengths centered at wavelength λ10. The time delay is different for different wavelength components.

This results in time dispersion of pulse at the receiving end of fiber.The material dispersion for unit length (L = 1) is given by

where, c = Light velocity λ = Center wavelength

Negative sign shows that the upper sideband signal (lowest wavelength) arrives before thelower sideband (highest wavelength).

A plot of material dispersion and wavelength is shown in Fig. 2.6.3

b) Glass fiber exhibits material dispersion given by λ2(d2n1/d λ2) of 0.025 Determine material dispersion parameter at a wavelength of 0.85μm and Estimate rms pulse broadening/km for good LED source with an rms spectral width of 20nm at this wavelength.

4. a) What is splicing? Explain different types of fiber splicing techniques.

Fiber SplicesA permanent or semi permanent connection between two individual optical

fibers is known as fiber splice. And the process of joining two fibers is called as splicing. Typically, a splice is used outside the buildings and connectors are used to join the cables within the buildings. Splices offer lower attenuation and lower back reflection than connectors and are less expensive.Types of SplicingThere are two main types of splicingi) Fusion splicing. ii) Mechanical splicing / V grooveFusion Splicing

Fusion splicing involves butting two cleaned fiber end faces and heating them until they melt together or fuse. Fusion splicing is normally done with a fusion splicer that controls the alignment of the two fibers to keep losses as low as 0.05 dB. Fiber ends are first pre aligned and butted together under a microscope with micromanipulators. The butted joint is heated with electric arc or laser pulse to melt the fiber ends so can be bonded together. Fig. 4.2.1 shows fusion splicing of optical fibers.

Mechanical Splicing / V GrooveMechanical splices join two fibers together by clamping them with a structure

or by epoxying the fibers together. Mechanical splices may have a slightly higher loss and back reflection. These can be reduced by inserting index matching gel. V groove mechanical splicing provides a temporary joint i.e fibers can be disassembled if required. The fiber ends are butted together in a V – shaped groove as shown in Fig. 4.2.2.

4 b) Give comparison between LED and LASER diode considering the different parameters.

5 Write a note oni) Power launching versus wavelengthii) Equilibrium Numerical Aperture

i) Power launching versus wavelength

Launching optical power from source into fiber needs the following considerations:

(i) Numerical Aperture (ii) Core Size (iii) Refractive index profile (iv) Core cladding index difference to the fiber(v) Radiance (vi) Angular power distribution of the optical source

A measure of the amount of optical power emitted from a source that can be coupled into a fiber usually given by the coupling efficiency is defined as

PF/PS

Where PF is the power coupled into the fiber and Ps is the power emitted from the light source.

The launching or coupling efficiency depends on the type of fiber that is attached to the source and on the coupling process, many source suppliers offer devices with short length of optical fiber (lm or less) attached in an optimum power configuration.

This section of fiber is generally referred to as fly lead devices. These fly lead sources reduce many power-launching problems and make the coupling easier.The effects to e considered are:(i) Fiber misalignment

(ii) Different core sizes (iii) Numerical apertures(iv) Core refractive index(v) The need for clean and smooth fiber end faces that are perpendicular to the fiber axis.

While considering the source to fiber power coupling efficiency, the radiance (spatial distribution of optical power) is important rather than the total output power.

ii) Equilibrium Numerical Aperture

Generally, the source is perfectly coupled into a system fiber by supplying a light source with short fiber fly lead. This fly lead should be connected to a system fiber with identical NA and core diameter. At this junction, around 0.1 to 1 dB optical power is lost. An excess power loss will occur in the system fiber in addition to the coupling loss, which is due to the non-propagating modes scattering out of the fiber as the launched modes come to an equilibrium condition. This loss has a severe effect on surface-emitting LED's (i.e., the power is launched into all modes of the fiber) but, the fiber coupled lasers (i.e., the power is launched into fewer non-propagating fiber modes) are less prove to this effect.

Because of the variation in effect of excess power loss on different type of fibers, it can be analyzed carefully in any system design. Figure shows the plot of excess power loss in terms of the fiber numerical aperture. The optical power in the fiber after the launched modes have come to equilibrium is,

Where,

P50 = Power expected in the fiber at 50m point based up on the launch NA = Equilibrium numerical aperture

The power coupled into the fiber, when the light emitting area of the LED is less than

the cross-sectional area of the fiber-core is given by,

PLED = P50 (NA)2

Where NA=NAin

5. a) Give the comparison of various optical detectors.

6 b) Write a note on Analog receiver.

Fiber optic transmission also supports analog links i.e. voice channels. The performance of analog receiver is measured in terms of S/N ratio (ratio of mean square signal current to mean square noise current).

The current generated at optical receiver by analog optical signal is given as

For a photodiode detector mean noise current is sum of i) Mean square quantum noise current. ii) Equivalent resistance thermal noise current.

iii) Dark noise current. iv) Surface leakage noise current.

where,

Ip is primary photocurrent.ID is primary dark current.IL is surface leakage current.F (M) is photodiode noise factor.B is effective noise BW.Req is equivalent resistance of photo detector and amplifier.Ft is noise figure of baseband amplifier.

Signal – to – noise ratio (S/N ratio) is given as

This limits the sensitivity of analog receiver.

7. Derive an expression for the total system rise time budget in terms oftransmitter fiber and receiver rise time.

Rise time gives important information for initial system design. Rise-time budget analysis determines the dispersion limitation of an optical fiber link.

Total rise time of a fiber link is the root-sum- square of rise time of each contributor to the pulse rise time degradation.

The link components must be switched fast enough and the fiber dispersion must be low enough to meet the bandwidth requirements of the application adequate bandwidth for a system can be assured by developing a rise time

budget. As the light sources and detectors has a finite response time to inputs. The

device does not turn-on or turn-off instantaneously. Rise time and fall time determines the overall response time and hence the resulting bandwidth.

Connectors, couplers and splices do not affect system speed, they need not be accounted in rise time budget but they appear in the link power budget. Four basic elements that contributes to the rise-time are,

- Transmitter rise-time (ttx) - Group Velocity Dispersion (GVD) rise time (tGVD) - Modal dispersion rise time of fiber (tmod) - Receiver rise time (trx)

------ (1)

Rise time due to modal dispersion is given as

------ (2)

where, BM is bandwidth (MHz) L is length of fiber (km)q is a parameter ranging between 0.5 and 1. B0 is bandwidth of 1 km length fiber

Rise time due to group velocity dispersion is

------ (3)where, D is dispersion [ns/(nm.km)] Σλ is half-power spectral width of source L is length of fiber

Receiver front end rise-time in nanoseconds is

------ (4)where, Brx is 3 dB – bW of receiver (MHz).

Equation (1) can be written as

8. Define Line coding with example. Explain NRZ codes.

Signal encoding uses a set of rules for arranging the signal symbols in a particular pattern. This process is called channel or line coding.

One of the principal functions of a line code is to introduce redundancy into the data stream for the purpose of minimizing errors resulting from channel interference effects. Depending on the amount of redundancy introduced, any degree of error-free transmission of digital data can be achieved provided that the data rate that includes this redundancy is less than the channel capacity.

The three basic types of two-level binary line codes that can be used for optical fiber transmission links are the Non-Return-to-Zero (NRZ) format, Return-to-Zero (RZ) format, and the Phase-Encoded (PE) format. In NRZ codes a transmitted data bit occupies a full bit period. For RZ formats the pulse width is less than a full bit period. In PE format both full-width and half-width data bits are present.

The simplest NRZ code is NRZ-level (NRZ-L) is shown in figure 8-7. For a serial data stream an on-off signal represents a 1 by a pulse of current or light filling an entire bit period, whereas for a 0 no pulse is transmitted.

The minimum bandwidth is needed with NRZ coding, but the average power input to the receiver is dependent on the data pattern. For example, a high level of received power occurring in a long string of consecutive 1 bits can result in baseline wander effect, as shown in fig 8-8. This effect results from the accumulation of pulse trails that arise from the low-frequency characteristics of the ac coupling filter in the receiver.

In addition, a long string of NRZ ones and zeroes contains no timing information, since there are no level transitions. Thus, unless timing clocks are extremely stable, a long string of N identical bits could be misinterpreted as N-1 or N+1 bits.

SET-2

1 a) Explain the Ray theory of the optical fiber with the help of a neat sketch. The speed of light depends upon the material or medium through which it is moving.

In free space light travels at its maximum possible speed i.e. 3 x 108 m/s or 186 x 103 miles/sec. When light travels through a material it exhibits certain behavior explained by laws of reflection, refraction.Reflection

The law of reflection states that, when a light ray is incident upon a reflective surface at some incident angle Φ1 from imaginary perpendicular normal, the ray will be reflected from the surface at some angle Φ2 from normal which is equal to the angle of incidence. The below figure shows law of reflection.

Refraction Refraction occurs when light ray passes from one medium to another i.e. the light

ray changes its direction at interface. Refraction occurs whenever density of medium changes. E.g. refraction occurs at air and water interface, the straw in a glass of water will appear as it is bent. The refraction can also observed at air and glass interface.

When wave passes through less dense medium to denser medium, the wave is refracted (bent) towards the normal. The below figure shows the refraction phenomena.

The refraction (bending) takes place because light travels at different speed in different mediums. The speed of light in free space is higher than in water or glass.

Refractive Index

The amount of refraction or bending that occurs at the interface of two materials of different densities is usually expressed as refractive index of two materials. Refractive index is also known as index of refraction and is denoted by n.

Based on material density, the refractive index is expressed as the ratio of the velocity of light in free space to the velocity of light of the dielectric material (substance).

The refractive index for vacuum and air is 1.0 for water it is 1.3 and for glass refractive index is 1.5.

Snell’s Law

Snell‘s law states how light ray reacts when it meets the interface of two media having different indexes of refraction.

Let the two medias have refractive indexes n1 and n2 where n1 > n2.

Φ1 and Φ2 be the angles of incidence and angle of refraction respectively. Then according to Snell‘s law, a relationship exists between the refractive index of both materials given by,

A refractive index model for Snell‘s law is shown in figure below:

Critical Angle

When the angle of incidence (Φ1) is progressively increased, there will be progressive increase of refractive angle (Φ2). At some condition (Φ1) the refractive angle (Φ2) becomes 90o to the normal. When this happens the refracted light ray travels along the interface. The angle of incidence (Φ1) at the point at which the refractive angle (Φ1) becomes 90 degree is called the critical angle. It is denoted by Φc.

The critical angle is defined as the minimum angle of incidence (Φ1) at which the ray strikes the interface of two media and causes an angle of refraction (Φ2) equal to 90 o. Figure below5 shows critical angle refraction

Hence at critical angle Φ1 =Φc and Φ2 = 90o .Using Snell‘s law: n1 sin Φ1 = n2 sinΦ2

Total Internal Reflection (TIR)

When the incident angle is increase beyond the critical angle, the light ray does not pass through the interface into the other medium. This gives the effect of mirror exist at the interface with no possibility of light escaping outside the medium. In this condition angle of reflection (Φ2) is equal to angle of incidence (Φ1). This action is called as Total Internal Reflection (TIR) of the beam. It is TIR that leads to the propagation of waves within fiber-cable medium. TIR can be observed only in materials in which the velocity of light is less than in air.

The two conditions necessary for TIR to occur are: 1. The refractive index of first medium must be greater than the refractive index of second one.

2. The angle of incidence must be greater than (or equal to) the critical angle.

Numerical Aperture (NA)

The numerical aperture (NA) of a fiber is a figure of merit which represents its light gathering capability. Larger the numerical aperture, the greater the amount of light accepted by fiber. The acceptance angle also determines how much light is able to be entering the fiber and hence there is relation between the numerical aperture and the cone of acceptance.

\

1 b) A silica glass fibers has a core refractive index of 1.5 and the cladding refractive

index of 1.45 calculatei) Critical angle for the core- cladding interfaceii) The NA of the fiber andiii) Percentage of light collected by the fiber.

2 a) Discuss the cutoff wave length for a step index fiber. Determine the cutoff wave

Length for a step index fiber to exhibit single mode operation when the core refractive index and radius are 1.46 and 4.5μm respectively with the relative index difference being 0.25%.

2 b) Describe the attenuation mechanism in an optical fiber.

Attenuation Attenuation is a measure of decay of signal strength or loss of light power that occurs

as light pulses propagate through the length of the fiber. In optical fibers the attenuation is mainly caused by two physical factors absorption

and scattering losses. Absorption is because of fiber material and scattering due to structural imperfection within the fiber. Nearly 90 % of total attenuation is caused by Rayleigh scattering only.

Micro bending of optical fiber also contributes to the attenuation of signal. The rate atwhich light is absorbed is dependent on the wavelength of the light and the characteristics of particular glass. Glass is a silicon compound; by adding different additional chemicals to the basic silicon dioxide the optical properties of the glass can be changed.

The Rayleigh scattering is wavelength dependent and reduces rapidly as the wavelength of the incident radiation increases.

The attenuation of fiber is governed by the materials from which it is fabricated, the manufacturing process and the refractive index profile chosen. Attenuation loss is measured in dB/km.

As attenuation leads to a loss of power along the fiber, the output power is significantly less than the couple‘s power. Let the couples optical power is p(0) i.e. at origin (z = 0). Then the power at distance z is given by,

where αp is fiber attenuation constant (per km).

This parameter is known as fiber loss or fiber attenuation.

3 a) Explain information capacity determination in an optical fiber.

A result of dispersion-induced signal distortion is that a light pulse will broaden as it travels along the fiber. This pulse broadening will eventually cause a pulse to overlap with neighboring pulses. After a certain amount of overlap has occurred, adjacent pulses can no longer be distinguished at the receiver and errors will occur. Thus the dispersive properties determine the limit of the information capacity of the fiber.

A measure of the information capacity of an optical waveguide is usually specified by the bandwidth-distance product in MHz-km. For a step-index fiber the various distortion effects tend to limit the bandwidth-distance product to about 20 MHz-km. In graded index fibers the radial refractive-index profile can be carefully selected so that the pulse broadening is minimized at a specific operating wavelength. This has led to bandwidth-distance product as high as 25 MHz-km. A comparison of the information capacities of various optical fibers with the capacities of typical coaxial cables used for UHF and VHF transmission is shown in the figure below. The curves are shown in terms of signal attenuation versus data rate. The flatness of the attenuation curves for the fibers extend up to the microwave spectrum.

b) Describe the connector return loss in an optical fiber.

4 a) Explain the following briefly i) Single mode fiber joint ii) Multimode fiber joint

4 b) With necessary expressions explain laser diode rate equations.

Lasing conditions and resonant FrequenciesThe electromagnetic wave propagating in longitudinal direction is expressed as

where,I(z) is optical field intensity.Ω is optical radian frequency.β is propagation constant.The fundamental expression for lasing in Fabry-Perot cavity is

Γ is optical field confinement factor or the fraction of optical power in the active layer.α is effective absorption coefficient of material.g is gain coefficient.h v is photon energy.z is distance traverses along the lasing cavity.Lasing (light amplification) occurs when gain of modes exceeds above optical loss duringone round trip through the cavity i.e. Z = 2L. If R1 and R2 are the mirror reflectivity‘s of the two ends of laser diode. Now the expression for lasing expressing is modified as,

The condition of lasing threshold is given as –i) For amplitude: I (2L) = I (0)ii) For phase: e-j2β L = 1iii) Optical gain at threshold = Total loss in the cavity.i.e. Γ gth = αt

Now the lasing expression is reduced to

Where,Α end is mirror loss in lasing cavity.An important condition for lasing to occur is that gain, g ≥ g th i.e. threshold gain.

5 Discuss the source output pattern of a fiber.

Consider the following figure 5.1.1, which shows a spherical coordinate system characterized by R, θ and φ with the normal to the emitting surface being the polar axis. The radiance may be a function of both θ and φ, and can also vary from point to point on the emitting surface. Surface emitting LEDs are characterized by their lambertian output pattern, which means the source is equally bright when viewed from any direction. The power delivered at an angle 'θ', measured relative to a normal to the emitting surface, varies as cosθ because the projected area of the emitting surface varies as cosθ with viewing direction. The emission pattern for a lambertian source thus follows the relationship.

B(θ,φ) =B0 cos θ

Figure 5.1.2, shows the radiation pattern for a lambertian source. The complexity of emission pattern is still increases, when we consider edge-emitting LEDs and laser diodes. In the planes parallel and normal to the emitting junction plane of the device. The radiances of these devices are given by, B(θ, 0°) and B(θ, 90°). Generally, these radiances can be approximated as,

Where,

T = Transverse power distribution coefficient

L = Lateral power distribution coefficient.

For edge emitter L=1 and T is significantly large value.

6 a) Explain the depletion layer photocurrent in a optical detector.Depletion Layer PhotocurrentConsider a reverse biased PIN photodiode.

The total current density through depletion layer is

Where,Jdr is drift current density due to carriers generated in depletion region.Jdiff is diffusion current density due to carriers generated outside depletion region.The drift current density is expressed as

where,A is photodiode area.φ0 is incident photon flux per unit area.Φ1 The diffusion current density is expressed as

where,Dp is hole diffusion coefficient.Pn is hole concentration in n-type material.Pn0 is equilibrium hole density.

Substituting in equation 3.2.7, total current density through reverse biased depletion layeris

6 b) Explain the characteristics of response time in a photo detector.

Response Time

Factors that determine the response time of a photodiode are

i) Transit time of photo carriers within the depletion region.

ii) Diffusion time of photo carriers outside the depletion region.

iii) RC time constant of diode and external circuit.

The transit time is given by

The diffusion process is slow and diffusion times are less than carrier drift time. By

considering the photodiode response time the effect of diffusion can be calculated.

Fig. 3.2.4 shows the response time of photodiode which is not fully depleted.

The detector behaves as a simple low pass RC filter having pass band of

where,

RT, is combination input resistance of load and amplifier.

CT is sum of photodiode and amplifier capacitance.

7 a) Discuss the component choices a system designer has when selecting components

for an optical fiber communication systems.The system designer has many choices when selecting components for an optical fiber

system. The major components choices areOptical Fiber Type and Parameters

Multimode or single mode, size, refractive index, attenuation, dispersion, mode coupling, strength, joints etc.

Source Type Laser or LED, optical power launched into the fiber, rise and fall time, stability etc.,

Transmitter Configuration Design for digital or analog, input impedance, supply voltage, dynamic range, feedback.

Detector Type and Characteristics p-n, p-i-n or avalanche photodiode, response time, active diameter, bias voltage, dark currentReceiver Configuration Preamplifier design, BER, SNR, range etc.Modulation and Coding Source intensity modulation, pulse frequency modulation, PWM and PPM transmission.Digital transmission or analog transmission such as biphase scheme and FM respectively.

These decisions will be taken depending on the system performance, ready availability of suitable components and cost.

b) Write a note on system design consideration.

The design of an optical fiber link involves many considerations regarding the fiber, source and photo detector operation and performance characteristics. Expected life time with performance and cost constrains were considered while designing a fiber link. The system considerations are choosing the transmitting wave length and components that operates in this region. The wavelength range 800-900 nm is chosen for shorter distances and 1300-1550 nm for longer distances with low attenuation and dispersion.

Next, we consider receiver, transmitter and implement them on a fiber to check whether the desired performance characteristics are obtainable or not. We first select a photo detector. In doing so, we should determine the minimum detectable optical power in order to satisfy Bit Error Rate at specified data rate. Cost and complexity are also considered here. The systems considerations with cheap and less complex one is preferred. P-i-n diodes are used as they are simple, high temperature tolerance, less bias voltages. APD's are used to detect low power levels which are not done by P-i-n.

Next, we choose the transmitting elements. While choosing LED or laser, we should consider data rate, dispersion, cost, and transmission distances. Laser provides the advantage of longer repeater less transmission distance and low dispersion. LED's offer low cost and simpler circuitry.

Next we have to choose an optical fiber that may be single mode or multimode with step or graded index core. The choice is made depending on dispersion tolerance and type of source. Edge emitting LED's along with single mode fiber launches sufficient power and support transmission data rates >500 Mbps. LED's can be used with multimode fibers.While choosing attenuation characteristics of fiber, we should consider cabling process loss, and fiber attenuation, connector loss, splice loss and losses due to atmospheric conditions also.

After considering a system satisfying above conditions, the "Link power Budget" and "Rise time budget" techniques are used to ensure the desired performance characteristics are met or not.

8 a) Explain different types of WDM.

Combining a number of wavelengths on a same fiber is known as WDM. 'N' independent optically formatted information streams, each transmitted at a different wavelength are combined with optical multiplexer and sent over the same fiber as shown in figure 8.3.1.

Each of the streams could be a different data rate. Each information stream maintains its individual data rate after being multiplexed with other streams and operates at its unique wavelength. The basic of WDM has to use multiple sources operating at slightly different wavelengths to transmit and WDM must be properly spaced to avoid inter channel interference.

b) Write short notes on eye pattern.Eye pattern is a simple and powerful technique in measuring the capacity and

performance of a digital transmission system. The measurements are in time domain and the waveform distortion can be seen on the CRO immediately. These eye patterns are formed by

superimposing the 2N possible combinations of N-bit long NRZ patterns.

As said, we need a variety of word patterns to measure the performance of a system using eye-pattern technique, these word patterns are provided by pseudo random bit generator. This pseudo random data (bit) pattern generator produces a random data signal that contains 1's and 0's in a random fashion providing uniform data rate. The random data from pseudo random data pattern generator is applied to the vertical input of CRO and the data rate triggers the horizontal sweep. This generates an eye-pattern. For example, consider 8 possible 3-bit NRZ patterns as shown in following figure 8.10.2.

An eye pattern is obtained by superimposing the above 8 patterns as shown in figure 8.10.3

The eye-width opening defines the Inter Symbol Interference (ISI) error free sampling rate of signal.

The best sampling rate is obtained when the height of eye is maximum.

One cannot recognize 1's and 0's if height of eye is reduced.

The eye height at a particular sampling period gives noise margin.

The system's sensitivity to time is determined by the closing rate of eye for a variation in sampling period.

One can also get rise and fall times of the system from the pattern.

Bit Error Rate (BER) is also estimated from the patterns and it can be reduced by inserting a small amount of redundancy into the transmitted pulse train.

SET-3

1 a) Explain what is meant by graded index optical fiber using simple ray theory concept. Indicate the major advantages of this type of fiber with regard to multimode propagation.

A dielectric waveguide that operates at optical frequencies is known as optical fiber. It is generally available in cylindrical form.Graded Index Fiber

Graded index fiber also contains single mode and multimode. The multimode graded index fiber is shown below,

In graded index fiber, the refractive index of the core is made to vary as a function of radial distance taken from the center of the fiber as shown in the equation below:

The dimension of its core is 50 to 100 μm and cladding is 125 to 140 μm.

Advantages of Graded Index Multimode Fibers:

Multimode graded index fibers exhibit far less intermodal dispersionthan multimode step index fibers due to their refractive index profile.

Multimode graded index fibers with parabolic or near-parabolic index profile cores have transmission bandwidths which may be orders of magnitude greater than multimode step index fiber bandwidths.

Multimode graded index fibers have the advantage of large core diameters (greater than 30 μm) coupled with bandwidths suitable for long distance communication.

1 b) A step index multimode fiber with a NA of 0.2 supports approximately 1000 modes

at 850nm wavelength. What is the diameter of its core? How many modes do the fiber supports at 1320nm and at 1550nm?

2 a) Explain mode field diameter (MFD) of a single mode fiber. Mode field diameter is\a primary parameter of single-mode fibers. It is obtained

from the mode field distribution of the fundamental mode. For step index and graded (near parabolic profile) single-mode fibers operating

near the cutoff wavelength λc, the field is well approximated by a Gaussian distribution.

In this case the MFD is generally taken as the distance between the opposite 1/e = 0.37 field amplitude points and the power 1/e2 = 0.135 points in relation to the corresponding values on the fiber axis.

The figure shows, the distribution of light in a single mode fiber.

In order to find the MFD for field intensity E2(r) must be calculated by using E2(r) MFD can be calculated as,

MFD = 2ω0

Where 2ω0 = spot size To avoid complexity, E(r) can be taken as,

E(r) = E (0) exp (r2/ ω02)

Where r= radius E (0) = field at (r=0)

By using this relation, we can write

MFD = (1/e2) width of optical power.

2 b) Write a note on i) Attenuation ii) Absorptioni) Attenuation

Refer to 2(b) of Set-2 Reg/Nov 2014.ii) Absorption

An absolutely pure silicate glass has little intrinsic absorption due to its, basic material structure in the near infrared region. However it does have two major intrinsic absorption mechanisms at optical wavelengths as illustrated in the following figure which shows a possible optical attenuation against wavelength characteristic

for absolutely pure glass (i.e., SiO2). There is a fundamental absorption edge, the peaks of which are centered in the

ultraviolet wavelength region. This is due to the stimulation of electrons transitions within the glass by higher energy excitation.

The tail of this peak may extend into the window region at the shorter wavelengths. Also in the infrared and far-infrared, normally at wavelengths above 7μm. Absorption bands from the interaction of photons with molecular variations within the glass occur.

These give absorption peaks which again extend into the window region. Hence, above 1.5μm, the tails of these largely far-infrared absorption peaks tend to increase the pure glass losses.

In practical optical fibers prepared by conventional melting techniques, a major source of signal attenuation is extrinsic (doped) absorption from transition metal element impurities. Certain impurities, namely Chromium and Copper, in their worst valence state can cause attenuation is excess of 1 dB/km in the near infrared region.

Transition element contamination may be reduced to acceptable levels i.e., one part is 1010 by glass refining techniques such as vapor -phase oxidation.

It may also be observed that the only significant absorption band in the region below a wavelength of 1urn is the second overtone at 0.95 am which causes attenuation of about 1 dB/km for one part per million (ppm) of hydroxyl.

At longer wavelengths the first overtone at 1.38 urn and its side band at 1.24 am are strong absorbers giving attenuation of about 2 dB/km ppm and 4 dB/km respectively.

Since most resonances sharply peaked, narrow window exist in the longer

wavelength region around 1.3 and 1.55μm which are essentially unaffected by

OH absorption, once the impurity, level has been reduced below one part in 107.

This situation is illustrated in. figure (b) which shows the attenuation spectrum of

an ultra-low-loss single mode fiber.

It may be observed that the lowest attenuation for this fiber occurs at a

wavelength of 1.55μm and is 0.2dB/km.

This approaching is the minimum possible attenuation of around 0.18 dB/km at

this wavelength.

3 a) Briefly explain intra modal and intermodal dispersion. Intermodal Dispersion

It is defined as the signal distortion that occurs as a result of different values of group delays for different modes of same wavelengths.

This results in pulse broadening because group delay leads to difference in time of travel for the zero order modes and the higher order modes.

Intramodal Dispersion

Intramodal Dispersion also known as Group Velocity dispersion or Chromatic

dispersion.

It is defined as the pulse spreading that occurs because of the changes in group

velocity as a function of wavelength occurring within an individual mode (i.e., single mode).

Since this distortion depends on wavelength, its effects on the signal distortion by

increasing with the spectral width of the optical source.

It is generally calculated as an r.m.s spectral width of a central wavelength. It has

two main causes

Material dispersion and Waveguide dispersion.

Material Dispersion

It is the pulse spreading that occurs when the different wavelengths follow the same path.

It causes a wavelength impendence of the group velocity of any given mode. The main cause of material dispersion is the variations in the refractive index of the core material as a function of wavelength.

Waveguide Dispersion It occurs because of the inability of the single mode fiber to confine the total

optical power in its core. Practically it is formed that the single mode fiber can confine up to a maximum

of 80% of optical power in its core. So the remaining 20% of light travelling in the cladding travels faster than the light in the core and giving rise to a dispersion called waveguide dispersion.

It depends on the fiber design.

3 b) Write a note on i) Group delay ii) Wave guide dispersion i) Group Delay

Consider a fiber cable carrying optical signal equally with various modes and

each mode contains all the spectral components in the wavelength band.

All the spectral components travel independently and they observe different time

delay and group delay in the direction of propagation.

The velocity at which the energy in a pulse travels along the fiber is known as

group velocity. Group velocity is given by,

Thus different frequency components in a signal will travel at different group

velocities and so will arrive at their destination at different times, for digital

modulation of carrier.

This results in dispersion of pulse, which affects the maximum rate of

modulation. Let the difference in propagation times for two side bands is δτ.

where,

Then,

ii) Wave Guide Dispersion

Waveguide dispersion is caused by the difference in the index of refraction between the core and cladding, resulting in a ‘drag’ effect between the core and cladding portions of the power.

Waveguide dispersion is significant only in fibers carrying fewer than 5-10 modes. Since multimode optical fibers carry hundreds of modes, they will not have observable waveguide dispersion.

The group delay (τwg) arising due to waveguide dispersion.

Where, b = Normalized propagation constant k = 2π / λ (group velocity) Normalized frequency V,

The second term

is waveguide dispersion and is mode dependent term..

4 Discuss the following i) External Quantum efficiency ii) LASER Diodes.(i) External Quantum Efficiency

The external quantum efficiency is defined as the ratio of photons emitted from LED to the number of photons generated internally. It is given by equation

The optical output power emitted from LED is given as

(ii) LASER Diodes

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,

Isotopes such as n-n or p-p type

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.

5 a) Discuss laser diode-to-fiber coupling.

We know that the edge emitting laser diodes have the emission pattern that nominally

has a Full-Width at Half-Maximum (FWHM) of 30° - 50° in the perpendicular plane

to the active area junction and an FWHM of 5° to 10° in the parallel plane to the

junction.

Since the fiber acceptance angle is smaller than the angular output distribution of the

laser and since the fiber core is much greater than the laser emitted area, spherical (or)

cylindrical lenses (or) optical fiber tapers can also be used to improve the coupling

efficiency between edge emitting laser diodes and optical fibers.

This phenomenon also works well for Vertical Cavity Surface Emitting Lasers

(VCSELs). Here the outcome 35% of coupling efficiencies to multimode fibers for

mass-produced connections of laser arrays to parallel optical fibers are possible by

direct coupling from a single Vertical Cavity Surface Emitting Lasers (VCSELs)

source to a multimode fiber.

b) Explain equilibrium Numerical aperture. Refer to 5(ii) of Set-1, Reg/Nov 2014.

6 a) With a neat block diagram explain the digital signal transmission through an optical data link.

The optical fiber transmission link is shown below,

Here a two level binary signal is used for transmission purpose. The two levels are represented by 1 and 0 respectively.

Each level has finite time duration known as bit period Tb. The stream of l’s and 0's are transmitted using amplitude shift keying modulation technique.

In ASK the voltage has two levels which are V volts for binary 1 and 0 volts for binary 0. Corresponding to these voltage levels the optical source will produce pulses of optical power.

These pulses are coupled to an optical fiber and are transmitted. The signal gets attenuated for various reasons and therefore it is distorted. The below figure 6.2.2 shows a block diagram of an optical receiver.

At the receiver the distorted signal is coupled to a photo detector generally a pin

diode which produces an electric current which is equivalent to the incoming signal. The amplifier and filter removes noise and amplifies the signal. The decision making

device gives output binary 1 for a voltage V and 0 for a voltage 0 respectively. At the signal processing circuiting the signal is demodulated thus producing the

desired output.

6 b) Briefly discuss the possible sources of noise in optical fiber receivers.

Errors in detection mechanism can arise from various noises and disturbances associated with the signal detection system, as shown in figure.

The term noise is used customarily to describe unwanted components of an electric signal that tend to disturb the transmission and processing of the signal in a physical system, and over which we have incomplete control.

The noise sources can be either external to the system or internal to the system. The two most common noises are shot noise and thermal noise.

Shot noise arises in electronic devices since of the discrete nature of current flow in the device.

Thermal noise arises from the random motion of electrons in a conductor. The random arrival rate of signal photons produces a quantum or shot noise at the photo detector.

Since this noise depends on the signal level, it is of particular importance for pin receivers that have large optical levels and for avalanche photodiode receivers.

An additional photo detector noise comes from the dark current and leakage current. These are independent of photodiode illumination and can generally be made very small in relation to other noise currents by a judicious choice of components.

Thermal noises arising from the detector load resistor and from the amplifier electronics tend to dominate in application with low signal-to-noise ratio when a pin photodiode is used.

When an avalanche photodiode is used in low Optical signal level applications the optimum avalanche gain is determined by a design tradeoff between the thermal noise and gain-dependent quantum noise.

A further error source is attributed to inter symbol interference (ISI) which results front pulse spreading in the optical fiber.

7 a) Write a note on Wavelength Division Multiplexing (WDM). Refer to 8(a), Set-2, Reg/NOV 2014

b) Explain link power budget & rise time budget with examples.

(i) Link Power Budget The power budget determines, whether the fiber optic link meets the attenuation

requirements or amplifiers are needed to boost the power level. It also determines the power margin between the optical transmitter output and

the minimum receiver sensitivity needed to establish a specific Bit Error Rate (BER).

Figure shows the power loss model for a point-to-point link.

In addition to losses, a link power margin is provided in the analysis to allow for

component aging, temperature fluctuation and losses arising from components

that might be added in future.

The link loss budget considers the total optical power loss PT that allowed

between the light source and photo detector.

This loss is allocated to cable attenuation, connector loss, splicing loss and

system margin. The total power loss in the link is given by,

Where,

Ps = Optical power emerging from the end of

a fiber fly lead. PR = Receiver sensitivity

lc = Connector loss

αf = Fiber attenuation in dB/km

L = Transmission distance.

A margin of 6 to 8 dB is generally used for systems which do not have additional components incorporated into the link in future

(ii) Rise Time BudgetRefer to 7 of Set-1, Reg/NOV 2014

8 a) Explain any one method used for attenuation measurements in a optical system. Attenuation measurement

Experimental setup for Cutback Technique

Cutback technique is the common technique used for the measurement of

fiber attenuation. In this technique, the white focused light is mechanically chopped [cut into pieces] at a low frequency [say 200Hz] which enables lock-in amplifier to perform a phase-sensitive detection.

Then the light is passed through a monochromator which uses a prism to

select the required wavelength for the measurement of attenuation.

After filtering the light, it is focused onto the fiber by means of a microscopic objective lens.

A beam splitter is used before the fiber to provide light for vision and a reference signal is used compensate for output power fluctuations (changes).

A mode scrambler is also attached to fiber within the first meter.

Fiber is passed through S-shaped groove cut in the Teflon in a cladding mode stripper device through which radiation removes light launched into the fiber and then sends to index matched-glycerin.

A p-i-n (or) avalanche photodiode is used to detect the optical power at the receiving end.

At last the output from photo detector is fed to a lock-in amplifier and this output is recorded. The relationship optical attenuation per unit length for the fiber is

Where,

L1 = Original fiber length

L2 = Cutback fiber length

Po1 = Output optical power from original fiber

Po2 = Output optical power from cutback fiber.

b) Explain how noise margin timing jitter & rise time can be measured with the help of simplified Eye diagram. Refer to 8(b) of Set-2, Reg/NOV 2014.

SET-4

1 a) Explain the structure of single mode and multimode step index and graded index optical fibers with cross section and Ray path.

Single mode Step index Fiber

In single mode step index fiber has a central core that is sufficiently small so that there is essentially only one path for light ray through the cable. The light ray is propagated in the fiber through reflection. Typical core sizes are 2 to 15 μm.

Single mode fiber will permit only one mode to propagate and does not suffer from mode delay differences.

The disadvantage of this type of cable is that because of extremely small size interconnection of cables and interfacing with source is difficult. Another disadvantage of single mode fibers is that as the refractive index of glass decreases with optical wavelength, the light velocity will also be wavelength dependent. Thus the light from an optical transmitter will have definite spectral width.

Multimode step Index Fiber

Multimode step index fiber is more widely used type. It is easy to manufacture. Its core diameter is 50 to 1000 μm i.e. large aperture and allows more light to enter the cable. The light rays are propagated down the core in zig-zag manner. There are many paths that a light ray may follow during the propagation.

The light ray is propagated using the principle of total internal reflection (TIR). Since the core index of refraction is higher than the cladding index of refraction, the light enters at less than critical angle is guided along the fiber.

Light rays passing through the fiber are continuously reflected off the glass cladding towards the centre of the core at different angles and lengths, limiting overall bandwidth.

The disadvantage of multimode step index fibers is that the different optical lengths caused by various angles at which light is propagated relative to the core, causes the transmission bandwidth to be fairly small. Because of these limitations, multimode step index fiber is typically only used in applications requiring distances of less than 1 km.

Multimode Graded Index Fiber

The core size of multimode graded index fiber cable is varying from 50 to 100 μm range. The light ray is propagated through the refraction.

The light ray enters the fiber at many different angles. As the light propagates across the core toward the center it is intersecting a less dense to more dense medium.

Therefore the light rays are being constantly being refracted and ray is bending continuously. This cable is mostly used for long distance communication.

Figure below shows the light trajectory in detail. It is seen that light rays running close to the fiber axis with shorter path length, will have a lower velocity because they pass through a region with a high refractive index.

1 b) With the help of neat sketch explain the phenomenon of mode coupling in fiber.

Waveguide perturbations such as deviations of the fiber axis from straightness, variations in the core diameter, irregularities at the core–cladding interface and refractive index variations may change the propagation characteristics of the fiber.

These will have the effect of coupling energy traveling in one mode to another depending on the specific perturbation.

It may be observed from the above figure that in both cases the ray no longer maintains the same angle with the axis.

In electromagnetic wave theory this corresponds to a change in the propagating mode for the light.

Thus individual modes do not normally propagate throughout the length of the fiber without large energy transfers to adjacent modes, even when the fiber is exceptionally good quality and is not strained or bent by its surroundings.

This mode conversion is known as mode coupling or mixing.

2 a) Differentiate between (i) Plastic fibers. (ii) Active and Chalgenide glass. (i) Plastic fibers

Plastic optical fibers are the fibers which are made up of plastic material. The core of this fiber is made up of Polymethylmethacrylate (PMMA) or Perflourmated Polymer (PFP).

Plastic optical fibers offer more attenuation than glass fiber and is used for short distance applications. These fibers are tough and durable due to the presence of plastic material.

The modulus of this plastic material is two orders of magnitude lower than that of silica and even a 1 mm diameter graded index plastic optical fiber can be installed in conventional fiber cable routes.

The diameter of the core of these fibers is 10-20 times larger than that of glass fiber which reduces the connector losses without sacrificing coupling efficiencies. So we can use inexpensive connectors, splices and transceivers made up of plastic injection-molding technology.

Graded index plastic optical fiber is in great demand in customer premises to

deliver high-speed services due to its high bandwidth

(ii) Active and Chalgenide glass. Active glass fibers are formed by adding erbium and neodymium to the glass

fibers. The above material performs amplification and attenuation

Chalgenide Glass Fibers

Chalgenide glass fibers are discovered in order to make use of the nonlinear properties of glass fibers.

It contains either "S", "Se" or "Te", because they are highly nonlinear and it also contains one element from “Cl”, "Br”, “Cd”,”Ba” or”Si”.

The mostly used Chalgenide glass is AS2- S3, AS40S58Se2 is used to make the core and AS2S3 is used to make the cladding material of the glass fiber. The insertion loss is around 1 dB/m.

b) Explain the following parameters of optical fiber. (i) Absorption (ii) Scattering loss (i) Absorption Refer to 2(b) of Set-3, Reg/NOV 2014.

(ii)Scattering loss

Rayleigh Scattering Losses

Scattering losses exists in optical fibers because of microscopic variations in the material density and composition. As glass is composed by randomly connected network of molecules and several oxides (e.g. SiO2, GeO2 and P2O5), these are the major cause of compositional structure fluctuation.

These two effects results to variation in refractive index and Rayleigh type scattering of light.

Rayleigh scattering of light is due to small localized changes in the refractive index of the core and cladding material. There are two causes during the manufacturing of fiber.

The first is due to slight fluctuation in mixing of ingredients. The random changes because of this are impossible to eliminate completely.

The other cause is slight change in density as the silica cools and solidifies. When light ray strikes such zones it gets scattered in all directions.

The amount of scatter depends on the size of the discontinuity compared with the wavelength of the light so the shortest wavelength (highest frequency) suffers most scattering. Fig. 2.3.1 shows graphically the relationship between wavelength and Rayleigh scattering loss.

Scattering loss for single component glass is given by,

where, n = Refractive index kB = Boltzmann‘s constant βT = Isothermal compressibility of material Tf = Temperature at which density fluctuations are frozen into the glass as it solidifies (Fictive temperature)

Mie Scattering Losses

Linear scattering also occurs at in homogeneities and these arise from imperfections in the fiber‘s geometry, irregularities in the refractive index and the presence of bubbles etc. caused during manufacture.

Careful control of manufacturing process can reduce mie scattering to insignificant levels.

3 a) Explain signal distortion in fiber. The pulse gets distorted as it travels along the fiber lengths. Pulse spreading in fiber is

referred as dispersion.

Dispersion is caused by difference in the propagation times of light rays that takes different paths during the propagation.

The light pulses travelling down the fiber encounter dispersion effect because of this the pulse spreads out in time domain.

Dispersion limits the information bandwidth. The distortion effects can be analyzed by studying the group velocities in guided modes.

b) With the principal requirements of good connector design, explain briefly connector types. Fiber Optic Connectors:

Connectors are mechanisms or techniques used to join an optical fiber to another fiber or to a fiber optic component.

Different connectors with different characteristics, advantages and disadvantages and performance parameters are available. Suitable connector is chosen as per the requirement and cost.

Various fiber optic connectors from different manufacturers are available SMA 906, ST, Biconic, FC, D4, HMS-10, SC, FDDI, ESCON, EC/RACE.

Principles of good connector design

1.Low coupling loss.

2. Inter-changeability.

3. Ease of assembly.

4. Low environmental sensitivity.

5. Low cost.

6. Reliable operation.

7. Ease of connection.

Connector Types Connectors use variety of techniques for coupling such as screw on, bayonet-mount,

push-pull configurations, butt joint and expanded beam fiber connectors.

Butt Joint Connectors

Fiber is epoxies into precision hole and ferrules arc used for each fiber. The fibers are secured in a precision alignment sleeve. Butt joints are used for single mode as well as for multimode fiber systems. Two commonly used butt-joint alignment designs are:

Straight-Sleeve.

Tapered-Sleeve/Bi conical.

In straight sleeve mechanism, the length of the sleeve and guided ferrules determines the end separation of two fibers. Fig. 3.1 shows straight sleeve alignment mechanism of fiber optic connectors

In tapered sleeve or bi conical connector mechanism, a tapered sleeve is used to accommodate tapered ferrules. The fiber end separations are determined by sleeve length and guide rings. Fig. 3.2 shows tapered sleeve fiber connectors

4 a) Explain briefly LED structures. Heterojunction

A heterojunction is an interface between two adjoining single crystal semiconductors with different band gap.

Heterojunction are of two types, Isotype (n-n or p-p) or Antistype (p-n).

Double Heterojunction (DH)

In order to achieve efficient confinement of emitted radiation double heterojunction are used in LED structure. A heterojunction is a junction formed by dissimilar semiconductors.

Double heterojunction (DH) is formed by two different semiconductors on each side of active region. Fig. 3.1.1 shows double heterojunction (DH) light emitter.

The crosshatched regions represent the energy levels of free charge. Recombination occurs only in active InGaAsP layer.

The two materials have different band gap energies and different refractive indices. The changes in band gap energies create potential barrier for both holes and electrons.

The free charges can recombine only in narrow, well defined active layer side. A double heterojunction (DH) structure will confine both hole and electrons to a

narrow active layer. Under forward bias, there will be a large number of carriers injected into active region where they are efficiently confined.

Carrier recombination occurs in small active region so leading to an efficient device.

Another advantage DH structure is that the active region has a higher refractive index than the materials on either side, hence light emission occurs in an optical waveguide, which serves to narrow the output beam.

4 b) Explain the principle of laser diode with a neat diagram. Injection Laser Diode (ILD)

The laser is a device which amplifies the light, hence the LASER is an acronym for light amplification by stimulated emission of radiation. The operation of the device may be described by the formation of an electromagnetic standing wave within a cavity (optical resonator) which provides an output of monochromatic highly coherent radiation.

Principle:

Material absorption light than emitting. Three different fundamental process occurs between the two energy states of an atom.

1) Absorption2) Spontaneous emission3) Stimulated emission.

Laser action is the result of three process absorption of energy packets (photons) spontaneous emission, and stimulated emission. (These processes are represented by the simple two-energy-level diagrams).

Where, E1 is the lower state energy level.E2 is the higher state energy level.

Quantum theory states that any atom exists only in certain discrete energy state, absorption or emission of light causes them to make a transition from one state to another. The frequency of the absorbed or emitted radiation f is related to the difference in energy E between the two states.

If E1 is lower state energy level. and E2 is higher state energy level.E = (E2 – E1) = h.f.

Where, h = 6.626 x 10-34 J/s (Plank‘s constant).

An atom is initially in the lower energy state, when the photon with energy (E2 – E1) is incident on the atom it will be excited into the higher energy state E2 through the absorption of the photon.

When the atom is initially in the higher energy state E2, it can make a transition to the lower energy state E1 providing the emission of a photon at a frequency corresponding to E = h.f. The emission process can occur in two ways.

A) By spontaneous emission in which the atom returns to the lower energy state in random manner.

B) By stimulated emission when a photon having equal energy to the difference between the two states (E2 – E1) interacts with the atom causing it to the lower state with the creation of the second photon.

Spontaneous emission gives incoherent radiation while stimulated emission gives coherent radiation. Hence the light associated with emitted photon is of same frequency of incident photon, and in same phase with same polarization.

It means that when an atom is stimulated to emit light energy by an incident wave, the liberated energy can add to the wave in constructive manner. The emitted light is bounced back and forth internally between two reflecting surface.

The bouncing back and forth of light wave cause their intensity to reinforce and build-up. The result in a high brilliance, single frequency light beam providing amplification.

5 Describe the power launching concepts in a laser diode.Refer to 5 (a) of Set-1, Reg/NOV 2014.

6 a) With necessary equations explain probability of error of a digital optical receiver.

b) Explain Depletion layer photo current of a photodetector. Refer to 6(b) of Set-2, Reg/NOV 2014

7 Explain overall fiber dispersion in multimode & single mode fibers. The core refractive index varies radially in case of graded index fibers, hence it

supports multimode propagation with a low intermodal delay distortion and high data rate over long distance is possible.

The higher order modes travelling in outer regions of the core, will travel faster than the lower order modes travelling in high refractive index region. If the index profile is carefully controlled, then the transit times of the individual modes

will be identical, so eliminating modal dispersion.

The r.m.s pulse broadening is given as :

The intermodal delay and pulse broadening are related by expression given by Personick

Where τg is group delay.From this the expression for intermodal pulse broadening is given as:

The intramodal pulse broadening is given as :

Where σλ is spectral width of optical source.Solving the expression gives:

8 a) With a neat diagram explain operational principles of WDM. Operation Principle of WDM

The key feature of WDM is that the discrete wavelengths form an orthogonal set of carriers that can be separated, routed and switched without interfering with each other as shown in the figure below.

The optical bandwidth can be expressed in terms of wavelength deviation “Δλ”.

Since the spectral width of a high quality source occupies only a narrow optical bandwidth. The two loss windows provide many additional operating regions. The light source, each emitting at different peak wavelengths is sufficiently spaced to avoid interference.

Fixed frequency spacing is selected because the operating mode of laser is locked, which means the frequency of the laser is fixed.

At the transmitter end, there are several modulated light sources which emit signals at different wavelength. A multiplexer is used to combine these signals into a spectrum of closed wavelength signals and mix them into a single fiber.

At the receiver, the demultiplexer separates the optical signal into appropriate detection channels for signal processing. A variety of active and passive devices are required to implement WDM networks.

The passive devices require no external control for their operation. The active devices can be controlled electronically. Hence they provide a degree of network flexibility.

b) Explain the following terms in brief. i) Time domain intermodal dispersion measurements. ii) Frequency domain intermodal dispersion measurements.ANS:

Time domain intermodal dispersion measurements.

Frequency domain intermodal dispersion measurements.