SPPU Electronics and Telecom Engineering (Semester 8)
Optical Fiber Communication
June 2015
Total marks: --
Total time: --
(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) A typical relative refractive index difference for an optical fibre designed for long distance transmission is 1.3%. Estimate the NA and the solid acceptance angle in air for the fibre when the core index is 1.48. Further, calculate the critical angle at the core-cladding interface within the fibre. It may be assumed that the concepts of geometric optics for the fibre.
6 M
1 (b) Explain the characteristics and operating ranges of the four key optical fibre link components.
6 M
1 (c) Derive an expression for the acceptance angel for a skew ray which changes direction by an angle 2λ at each reflection in a step index fibre in terms of the fibre NA and λ. It may be assume that ray theory holds for the fibre.
6 M

2 (a) A single mode step index fibre has core and cladding refractive indices of 1.498 and 1.495 respectively. Determine the core diameter required for the fibre to permit its operation over the wavelength range 1.48 to 1.60μm. Calculate the new fibre core diameter to enable single -mode transmission at a wavelength of 1.30μm.
6 M
2 (b) A step index fibre in air has a numerical aperture of 0.16, a core refractive index of 1.45 and a core diameter of 60 μm. Determine the normalized frequency for the fibre when light at a wavelength of 0.9 μm. is transmitted. Further, estimate the number of guided modes propagating in the fibre.
6 M
2 (c) What requirement should be satisfied by the material chosen for fibre fabrication? According to the material used in fibre fabrication explain the types of fibre.
6 M

Answer any one question from Q3 and Q4
3 (a) Briefly describe linear scattering losses in optical fibres with regard to
i) Rayleigh scattering
ii) Mie scattering
8 M
3 (b) A 6 km optical link consists of multimode step index fibre with a core refractive index of 1.5 and a relative refractive index difference of 1% Estimate.
i) The delay difference between the slowest and fastest modes at the fibre output.
ii) The rms pulse broadening due to intermodal dispersion on the link;
iii) The maximum bit rate that may be obtained without substantial errors on the link assuming only intermodal dispersion;
iv) The bandwidth length product corresponding to (iii).
8 M

4 (a) Describe the phenomenon of modal noise in optical fibres and suggest how it may be avoided.
8 M
4 (b) A single mode step index fibre with a core refractive index of 1.49 has a critical bending radius of 10.4 mm when illuminate with light at a wavelength of 1.30 μm. If the cut off wavelength for the fibre is 1.15 μm calculate its relative refractive index difference.
8 M

Answer any one question from Q5 and Q6
5 (a) The radiative and non-radiative recombination lifetimes of the minority carriers in the active region of a double-heterojunction LED are 30ns and 80ns respectively. Determine the total carrier recombination life time and the power internally generated within the device when the peak emission wavelength is 0.87 μm at a drive current of 40 mA.
8 M
5 (b) i) State and explain the various advantages of LED in comparison with injection lasers.
ii) Explain the conditions necessary to attain lasing action in LASERs.
8 M

6 (a) A single mode fibre has a mode field diameter 4.95 μm, a core refractive index n1=1.47, a cladding refractive index n2=1.465 and a core diameter 2a=9 μm. Find.
i) Insertion losses of a fibre joint having a lateral offset of 1μm.
ii) Loss at a joint having an angular misalignment of 1° at a 1300-nm wavelength.
8 M
6 (b) Explain the various optical transmitter ? LED drive circuits for analog transmission.
8 M

Answer any one question from Q7 and Q8
7 (a) When 2.5×1011 photons each with a wavelength of 0.85 μm are incident on a photodiode, on average 1×1011 electrons are collected at the terminals of the device. Determine the following of the photodiode at 0.85 μm.
i) The quantum efficiency
ii) Responsivity
6 M
7 (b) Given that the following measurement were taken for an APD, calculate the multiplication factor for the device.
i) Received optical power at 1.35μm=0.2μW
ii) Corresponding output photocurrent = 4.9 μA (After avalanche gain)
iii) Quantum efficiency at 1.35 μm =40%
6 M
7 (c) Explain the requirements that must be satisfied by detectors for performance and compatibility.
6 M

8 (a) An analog optical fibre communication system requires an SNR of 40 dB at the detectors with a post-detection bandwidth of 30 MHz. Calculate the minimum optical power required at the detector if it is operating at a wavelength of 0.9 μm with a quantum efficiency of 70%. State any assumption made.
10 M
8 (b) Draw and explain the block schematic of the front end of an optical receiver showing the various sources of noise, and draw the equivalent circuit of it.
8 M

Answer any one question from Q9 and Q10
9 (a) A 1550-nm single mode digital fibre optic link needs to operate at 622Mbps over 80 km without amplifiers. A single-mode InGaAsP laser launches an average optical power of 13dBm into the fibre. The fibre has a loss of 0.35dB/km and there is a splice with a loss of 0.1dB every kilometer. The coupling loss at the receiver is 0.5dB, and the receiver uses InGaAs APD with a sensitivity of -39dBm. Excess-noise penalties are predicted to be 1.5dB. Setup an optical power budget for this link and find the system margin. Represent link loss budget graphically.
10 M
9 (b) Explain the RF over Fiber with reference to analog link.
6 M

10 (a) 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
Fibre: Intermodal 6 ns/km
(Pulse broadening) intramodal 1 ns/km
Detector (p-i-n photodiode) 5 ns

From system rise time considerations, estimate the maximum bit rate that may be achieved on the link when using an NRZ format.
8 M
10 (b) Write a short note on Multichannel Modulation techniques.
8 M

Answer any one question from Q11 and Q12
11 (a) Explain the WDM with respect to the following points:
Overview of WDM
Operational principles of WDM
8 M
11 (b) Explain in detail the architecture of EDFA.
8 M

12 (a) Consider an EDFA being pumped at 980 nm with a 30 mW pump power. If the gain at 1550 nm is 20dB, then calculate:
i) Maximum input power
ii) Maximum output power
8 M
12 (b) Write a note on Applications and type of Optical Amplifier.
8 M

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