 MORE IN Optical Fiber Communication
SPPU Electronics and Telecom Engineering (Semester 8)
Optical Fiber Communication
December 2014
Total marks: --
Total time: --
INSTRUCTIONS
(1) Assume appropriate data and state your reasons
(2) Marks are given to the right of every question
(3) Draw neat diagrams wherever necessary

Answer any one question from Q1 and Q2
1 (a) Using ray theory derive an expression for the Numerical Aperture and the solid acceptance angle in terms of the physical parameters of a step index fibre and explain their importance in the propagation of optical signal through the fibre. Why do we need cladding?
8 M
1 (b) Calculate the number of modes propagating at 820nm wavelength in graded index fibre having parabolic refractive index profile. The core radius is 25 μm. The refractive index at the centre of core is 1.48 and cladding refractive index is 1.46.
8 M

2 (a) Under what conditions of the propagation constant 'β', mode remains guided in the core? Write an expression for the V number which is connected with the cutoff condition and determines the number of modes a fibre can support.
8 M
2 (b) A step index fibre has core refractive index 1.5 and Δ=1.3% with core diameter of 100μm. The operating wavelength is 850nm. Calculate assuming that the fibre is kept in air the
i) Numerical Aperture of fibre
ii) Critical angle
iii) Angle of incidence.
8 M

Answer any one question from Q3 and Q4
3 (a) What does the term 'The state of polarization' mean? Does it describes the fibre properties? Explain.
8 M
3 (b) The threshold optical powers for stimulated Brillouin and Raman scattering in a long 8 μm core diameter single-mode fibre are found to be 190mw and 1.70w respectively, when using an injection laser source with a bandwidth of 1GHz. Calculate the operating wavelength of the laser and the attenuation in decibels per kilometre of the fibre at this Wavelength.
8 M

4 (a) Describe the fibre structures utilized to provide.
i) Dispersion shifting &
ii) Dispersion flattening in single-mode fibres.
6 M
4 (b) A multimode fibre with near parabolic refractive index profile has a material dispersion of 30 psnm-1 km-1 when used with a good LED source of RMS spectral width of 25nm. The fibre has numerical aperture of 0.4 and core refractive index of 1.48 and cladding refractive index of 1.47. Estimate the total RMS pulse broadening per km within the fibre.
10 M

Answer any one question from Q5 and Q6
5 (a) Distinguish between spontaneous and stimulated emission. How is stimulated emission assured in Laser diode? Give comparison between LED and Laser.
8 M
5 (b) A step index fibre has a core refractive index of 1.47, a relative refractive index difference of 2% and a core diameter of 80μm. The fibre is jointed with a lateral offset of 2 μm an angular misalignment of the core axes of 3° and a small air gap (no longitudinal misalignment). Estimate the total insertion loss.
10 M

6 (a) Draw the schematic and energy band diagram of double heterojunction LED diode and explain the process involved in its operation as LED. Explain why it is more efficient in its action than a homojunction LED.
8 M
6 (b) The radiative and non-radiative recombination lifetime of minority carriers in active region of double heterojunction LED are 60 ns and 100ns respectively. Determine bulk recombination life time and power internally generated within the device when peak emission wavelength is 0.87μm at a drive current of 38mA.
10 M

Answer any one question from Q7 and Q8
7 (a) Explain what is 'P+π Pn+ structure of an avalanche photodiode? How the term 'reach through' is concerned with its operation? Which type of carriers are responsible for the avalanche action? How this avalanche effect causes a gain in the responsivity of the diode?
10 M
7 (b) The quantum efficiency of RAPD is 80% for the direction of radiation at wavelength of 0.9μm, when the incident power is 0.5μm, when the incident optical power is 0.5μw, the output current from the device is 11μA. Determine the multiplication factor or gain of the photodiode under this condition.
8 M

8 (a) Explain the different sources of noise in optical fibre receivers.
8 M
8 (b) State the types of receiver structures used in digital optical fibre system and explain each in brief.
10 M

Answer any one question from Q9 and Q10
9 (a) Explain the design procedure with graphical representation for link loss budget analysis.
8 M
9 (b) Components are chosen for a digital optical link of overall length of 6km. LED chosen is capable of launching -10dBm into a graded index fibre, which has an attenuation of 3dbkm-1. It requires splicing every kilometer with a loss of 0.5dB. per splice. The connector loss at the receiver is 1.5dB.The receiver requires mean optical power of -41dBm in order to give necessary BER of 10-10. It is also predicted that a safety margin of 6dB will be required. Write down the optical power budget for the system and hence determine its viability.
8 M

10 (a) Draw a neat block diagram representing basic elements of an analog link and hence explain the major noise contributors.
8 M
10 (b) An optical fibre 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.
Intermodal = 5ns/km
Intermodal = 1ns/km
Pin Detector = 6ns
Estimate the maximum bit rate that may be achieved on the link using NRZ format.
8 M

Answer any one question from Q11 and Q12
11 (a) Show the structure of an Erbium-doped fibre amplifier and explain the function of each component.
8 M
11 (b) Explain the principle of operation of Febry-Parot amplifier. State the advantages and drawbacks of SOA.
8 M

12 (a) Describe the concept of Wavelength Division Multiplexing and state the key system features of WDM.
8 M
12 (b) Describe and differentiate between active and passive WDM couplers.
8 M

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