code no: r41041 optical communications€¦ · optical communications (electronics and...

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IV B.Tech I Semester Regular/Supplementary Examinations, Nov/Dec - 2014

OPTICAL COMMUNICATIONS

(Electronics and Communication Engineering)

Time: 3 hours Max. Marks: 75

Answer any FIVE Questions

All Questions carry equal marks

*****

1 a) Discuss briefly the advantages of optical fiber communication. [7]

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

[8]

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

briefly.

[8]

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

[7]

3 a) Explain the material dispersion in optical wave guides. [7]

b) Glass fiber exhibits material dispersion given by λ2(d

2n1/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 20

nm at this wavelength.

[8]

4 a) What is splicing? Explain different types of fiber splicing techniques. [7]

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

parameters.

[8]

5 a) Derive an equation for power coupling tutor in a the step index fiber. [7]

b) Write a note on

i) Power launching versus wavelength

ii) Equilibrium Numerical Aperture

[8]

6 a) Give the comparison of various optical detectors. [7]

b) Write a note on Analog receiver. [8]

Code No: R41041 Set No. 1 R10

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7 a) Following are the parameters of a point-to-point optical link :

i) Optical power launched : +3dBm

ii) Sensitivity of detector : -32 dBm

iii) Source/detector connector loss : 1 dB

iv) Length of optical cable : 60 km

v) Cable attenuation : 0.3 dB/km

vi) Jumper cable loss : 3 dB

vii) Connector loss at each fiber joint : 1 dB

(two at each transmitter and receiver end because of the jumper cables)

Compute the power margin of the link using spread sheet method.

[7]

b) Derive an expression for the total system rise time budget in terms of

transmitter fiber and receiver rise time.

[8]

8 a) Define Line coding ? with example Explain NRZ codes. [7]

b) A 2km length of multimode fiber is attached to apparatus for spectral loss

measurement. The measured output voltage from the photo receiver using the

full 2km fiber length is 2.1V at a wavelength of 0.85 µm, when the fiber is then

cut back to leave a 2m length, the output voltage increases to 10.7V. Determine

the attenuation per kilometer for the fiber at a wavelength of 0.85µm and

estimate the accuracy of the result.

[8]

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*****

1 a) Explain the Ray theory of the optical fiber with the help of a neat sketch. [8]

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

index of 1.45 calculate.

i) Critical angle for the core- cladding interface

ii) The NA of the fiber and

iii) Percentage of light collected by the fiber.

[7]

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 %.

[7]

b) Describe the attenuation mechanism in an optical fiber.

[8]

3 a) Explain information capacity determination in an optical fiber. [7]

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

[8]

4 a) Explain the following briefly

i) Single mode fiber joint ii) Multimode fiber joint

[7]

b) With necessary expressions explain laser diode rate equations.

[8]

5 a) Discuss the source output pattern of a fiber. [8]

b) Consider an LED that has a circular emitting area of radius 35µm and a

lambertian emission patter with 150 w/(cm2.sr) axial radiance at a given drive

current. Determine the optical powers coupled into two step-index fibers. One of

which has a core radius of 25µm with NA=0.20 and the other which has a core

radius of 50µm with NA= 0.20

[7]

6 a) Explain the depletion layer photocurrent in a optical detector. [8]

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

[7]

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

for an optical fiber communication systems.

[8]

b) Write a note on system design consideration.

[7]

8 a) Explain different types of WDM. [7]

b) Write a short notes on eye patter. [8]


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*****

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.

[7]

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

modes at an 850nm wavelength. What is the diameter of its core? How many

modes do the fiber supports at 1320nm and at 1550nm?

[8]

2 a) Explain mode field diameter (MFD) of a single mode fiber. [7]

b) Write a note on i) Attenuation ii) Absorption

[8]

3 a) Briefly explain intra modal and intermodal dispersion. [7]

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

[8]

4 a) A single mode fiber has the following parameters.

Normalized frequency (V)=2.40

Core refractive index n1=1.46

Core diameter (2a)=8µm

Numerical Aperture (NA) =0.1

Estimate the total insertion loss of a fiber joint with a lateral misalignment of

1µm and an angular misalignment of 1o (one degree).

[7]

b) Discuss the following i) External Quantum efficiency ii) LASER Diodes.

[8]

5 a) Discuss laser diode-to-fiber coupling. [7]

b) Explain equilibrium Numerical aperture.

[8]

6 a) With a neat block diagram explain the digital signal transmission through an

optical data link.

[8]

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

[7]

7 a) Write a note on Wavelength Division Multiplexing (WDM). [7]

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

[8]

8 a) Explain two methods used for attenuation measurements in a optical system. [8]

b) Explain how noise margin timing jetter & rise time can be measured with the

help of simplified Eye diagram.

[7]


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*****

1 a) Explain the structure of single mode and multimode step index and graded

index optical fibers with cross section and Ray path.

[8]

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

[7]

2 a) Differentiate between following

i) Glass fiber and plastic fibers.

ii) Active glass and chalgenide glass.

[8]

b) Explain the following parameters of optical fiber.

i) Absorption

ii) Scattering loss

[7]

3 a) Explain signal distortion in fiber. [7]

b) With the principal requirements of good connector design, explain briefly

connector types.

[8]

4 a) Explain briefly LED structures. [8]

b) Explain the principle of laser diode with a neat diagram.

[7]

5 a) With necessary equation explain power coupling calculation. [8]

b) Describe the power launching concepts in a laser diode. [7]


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

receiver.

[8]

b) Explain Depletion layer photo current of a photodetector.

[7]

7 a) An optical fiber system is to be designed to operate over an 8km length without

repeaters. The rise times of the chosen components are,

Source (LED) 8ns

Fiber Inter modal 5ns km-1

(Pulse broadening)

Intramodal 1ns km-1

detector (p-i-n photodiode) 6ns for system rise time

considerations. Estimate the maximum Bit rate that may be achieved on the link

when using an NRZ format.

[8]

b) Explain overall fiber dispersion in multimode & single mode fibers.

[7]

8 a) With a neat diagram explain operational principles of WDM. [8]

b) Explain the following terms in brief.

i) Time domain intermodal dispersion measurements.

ii) Frequency domain intermodal dispersion measurements.

[7]

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code no: r41041 optical communications€¦ · optical communications (electronics and...

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