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# Jawaharlal Nehru Technological University Anantapur 2009 B.E Communication Engineering Btech ece - Question Paper

Wednesday, 29 May 2013 04:25Web

optical communication

Code No: 37117    Set No. 1

IV B.Tech I Semester Supplimentary Examinations, May/Jun 2009 OPTICAL COMMUNICATIONS ( Common to Electronics & Communication Engineering and Electronics &

Telematics)

Time: 3 hours    Max Marks: 80

Answer any FIVE Questions All Questions carry equal marks

1.    (a) Explain the field distribution for lower order modes in a symmetrical wave

guide.

(b) Explain mode theory for circular wave guide.    [8+8]

2.    (a) Explain the following:

i.    Mode field diameter

ii.    Modal birefringence.

(b) Explain the effect of bending to losses? What is micro bending? How cant be reduced.

[8+8]

3.    (a) Calculate the insertion loss of the connector when the power in the fiber is

100 w and the output power after the connector is 80 w.

(b) A multimode graded index fiber exhibits the pulse broadening of 0.2 s over a distance of 15 Km. Estimate:

i.    Optimum bandwidth of fiber

ii.    Dispersion per unit length

iii.    Bandwidth length product.    [8+8]

4.    (a) Draw the schematic of an edge emitting LED and explain the reasons for such

construction.

(b) Discuss the major requirements of an optical source for use in optical communication systems.    [8+8]

5.    (a) Discuss the dependence of equilibrium numerical aperture on power coupling

from a source into a fiber.

(b) Estimate the losses encountered while coupling power from a source to a fiber due to mismatch in their numerical apertures and surface areas.    [6+10]

6.    (a) With the help of a suitable block diagram explain the functioning of every

element of a fiber optic receiver.

(b) Discuss briefly about various sources of noise in a PIN diode based fiber optic receiver.    [8+8]

(b) Estimate the maximum NRZ data rate allowed for transmission over an 8 Km optical fiber link operating at 850nm wavelength specified below: [10+6]

Source rise time    = 8ns

Inter-modal dispersion rise-time    = 5ns/Km

Intra modal pulse broadening    = 1ns/Km

Detector rise-time    = 6ns.

8. (a) Describe the operation of uni-directional and bi-directional wavelength division multiplexing de-multiplexing in optical communication links.

(b)    What is line coding? Explain the requirement of line coding in optical communication systems.

(c)    Does line coding contribute to improvement of eye patterns observed at an optical receiver? Justify your answer.    [5+6+5]

IV B.Tech I Semester Supplimentary Examinations, May/Jun 2009 OPTICAL COMMUNICATIONS ( Common to Electronics & Communication Engineering and Electronics &

Telematics)

Time: 3 hours    Max Marks: 80

Answer any FIVE Questions All Questions carry equal marks

k k k k k

1.    (a) Compute the V-number and no. of modes supported by a fiber with ni =

1.55, n2 = 1.50; core radius is 25 m and operating wavelength is 1300 nm.

(b) Compare single mode and multimode fibers.    [8+8]

2.    (a) Discuss different mechanisums causing scattering losses in SiO2 fiber as a func

tion of operating wavelengths.

(b) A K2O - SiO2 glass core optical fiber has an attenuation resulting from rayleigh scattering of 0.46 dB Km-1 at a wavelength of 1 m. The glass has an estimated fictive temperature of 758 K, isothermal compressibility of 8.4 x 10-11 m2N-1 and a photo elastic coefficient of 0.245. Determine the refractive index of glass core.    [8+8]

3.    (a) Write notes on broadening of pulse due to fiber dispersion.

(b) An LED operating at 850 nm has a spectral width of 45 nm. What is the pulse spreading in ns / km due to material dispersion?    [8+8]

4.    (a) Discuss about materials used for light sources of optical communication.

(b) FWHM power spectral width LEDs become wider at longer wavelengths. Justify this statement.    [8+8]

5.    Write short notes on the following:

(a)    Power coupling from a vertical Cavity Surface Emitting Laser (VCSEL) diode to a single mode fiber.

(b)    Radiation patterns in axial and vertical planes from surface emitting LED and edge emitting LED.    [8+8]

(c) Explain the effect of mode mixing factor, q, on modal dispersion induced rise time.    [8+4+4]

8. (a) Mention the mechanisms causing intramodal dispersion in optical fibers.

(b)    Differentiate between intermodal and intramodal dispersions. Which type of dispersion is measured in time domain using optical sampling oscilloscope?

(c)    With the help of suitable test set-up, describe a method for measurement of chromatic dispersion in optical fibers?    [4+4+8]

IV B.Tech I Semester Supplimentary Examinations, May/Jun 2009 OPTICAL COMMUNICATIONS ( Common to Electronics & Communication Engineering and Electronics &

Telematics)

Time: 3 hours    Max Marks: 80

Answer any FIVE Questions All Questions carry equal marks

1.    (a) Explain the evolution of fiber optic system.

(b) Discuss the advantages of optical fibers over copper cables.    [8+8]

2.    (a) Explain the following:

i.    Mode field diameter

ii.    Modal birefringence.

(b) Explain the effect of bending to losses? What is micro bending? How cant be reduced.

[8+8]

3.    (a) List out the differences between intra modal and inter modal dispersion.

(b) Explain about tapered sleeve fiber optic connectors.    [8+8]

4.    (a) Draw the schematic of surface emitting LED and discuss the characteristics

of such LED.

(b) Explain how the optical radiations are generated from fabry perot resonator cavity laser with neat schematics.    [8+8]

5.    (a) Describe all the factors giving rise to losses while coupling optical power be

tween any two devices of a fiber optic link.

(b)    What is a pig-tailed device? List out the advantages and disadvantages of pig-tailing either a fiber optic source or a fiber optic detector.

(c)    Write expressions for power coupling from an LED into a step index fiber for larger and smaller active area relative to the area of the fiber.

[5+5+6]

7.    (a) Describe a method to carryout rise time budget analysis for a fiber optic link

(b)    Explain the procedure to determine the maximum allowable RZ and NRZ data rates from rise time budget analysis.

(c)    Explain the effect of mode mixing factor, q, on modal dispersion induced rise time.    [8+4+4]

8.    (a) List the conditions under which cut-back method of measurement of fiber

attenuation yields more accurate values.

(b)    Suggest a non-destructive method for measurement of fiber attenuation. Mention the principle behind this method.

(c)    Output of a PIN detector preamplifier of an optical receiver for 1.6Km fiber is 2.26 Volts at 820nm wavelength. The output of PIN preamplifier increases to 9.06 Volts when this fiber is cutback to 4m length at the same wavelength. Compute the total attenuation and attenuation per unit length (dB/Km) of the cut-off fiber.    [5+5+6]

IV B.Tech I Semester Supplimentary Examinations, May/Jun 2009 OPTICAL COMMUNICATIONS ( Common to Electronics & Communication Engineering and Electronics &

Telematics)

Time: 3 hours    Max Marks: 80

Answer any FIVE Questions All Questions carry equal marks

1.    (a) Describe the basic block diagram of a optical communication system and ex

plain how it differs from conventional co-axial cable communication system.

(b) Determine the normalized frequency at 0.82 m for a step index fiber having 25 m core radius, n = 1.48 and n2 = 1.46. How many modes propagate in this fiber at 0.82 m? What percentage of optical fiber power flows in cladding?

[8+8]

2.    (a) Discuss different mechanisums causing scattering losses in SiO2 fiber as a func

tion of operating wavelengths.

(b) A K2O - SiO2 glass core optical fiber has an attenuation resulting from rayleigh scattering of 0.46 dB Km-1 at a wavelength of 1 m. The glass has an estimated fictive temperature of 758 K, isothermal compressibility of 8.4 x 10-11 m2N-1 and a photo elastic coefficient of 0.245. Determine the refractive index of glass core.    [8+8]

3.    Define and distinguish between the different types of signal distortion in optical fibers.    [16]

4.    (a) Two multimode step index fibers have NAs of 0.2 and 0.4 respectively and

both have the same core refractive index which is 1.48. Estimate the insertion loss at a joint in each fiber caused by a 50 angular misalignment of the fiber core axes. It may be assumed that the medium between the fibers in air.

(b) Explain the intrinsic coupling losses at fiber joint due to mismatch of core diameter, NA and refractive index profile difference.    [8+8]

5.    Write short notes on the following:

(a)    Power coupling from a vertical Cavity Surface Emitting Laser (VCSEL) diode to a single mode fiber.

(b)    Radiation patterns in axial and vertical planes from surface emitting LED and edge emitting LED.    [8+8]

6.    (a) Define sensitivity and Bit Error Rate (BER) with reference to a fiber optic

(b) Describe the dependency of BER on Signal-to-Noise Ratio at the output of a receiver.

(c) Express Noise due to photocurrent generation process as an equation. Describe the significance of such noise on overall SNR of the receiver.    [4+4+8]

7.    (a) Discuss the effect of mode mixing factor on modal dispersion for calculating

the maximum allowable transmission data rate in a fiber optic link.

(b) The rise times for various components of intensity modulated fiber optic link are listed below. Determine if these specifications support a 5 Km repeater-less fiber optic link with 6 MHz optical bandwidth:    [8+8]

Risetime of LED transmitter electronics = 10ns Inter modal dispersion induced    = 8ns/Km

Intra modal dispersion induced    = 2ns/Km

Risetime of Detector and receiver electronics = 3ns

8.    (a) Describe the principle of wavelength division Multiplexing for optical commu

(b)    Describe the method of implementation of wavelength division multiplexing in full duplex optical communication links.

(c)    Describe briefly the necessity and procedure of Line coding in optical links.

[5+6+5]

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