Pune University BE Electronics Optical and Microwave
Communication Question Papers
B.E. (Electronics) OPTICAL AND MICROWAVE COMMUNICATION (2008 Pattern)
(Sem. – II) (Elective – III)
Time :3 Hours] [Max. Marks :100
Instructions to the candidates:-
1) Answer QI or Q2, Q3 or Q4, Q5 or Q6 from Section I and Q7 or Q8, Q9 or QIO, QII or Q12 from Section II.
2) Answers to the two sections should be written in separate books.
3) Neat diagrams must be drawn wherever necessary.
4) Pigures to the right indicate full marks.
5) Assume suitable data, if necessary.
SECTION – I
QIA a) Explain in brief advantages and disadvantages of optical fiber cable over coaxial cable. 
b) What is LASER? Mention Population Inversion condition? Compare LASER and LED. 
c) A multimode step index fiber with a core diameter of 80(-i m and a relative index difference of 1.5% is operating at a wavelength of 0.85 m. If the core refractive index is 1.48, estimate: [R]
i) The normalized frequency for the fiber;
ii) The number of guided modes.
Q2) a) Explain how light is propagated within a fiber. Define the following terms with respect to an optical fiber: 
i) Acceptance cone.
ii) Numerical Aperature.
b) Define fiber splicing? Explain different types of splicing. 
c) A photodiode has a quantum efficiency of 65% when photons of energy 1.5 x 10 19 J are incident upon it. 
i) At what wavelength is the photodiode operating?
ii) Calculate the incident optical power required to obtain a photocurrent of 2.5 |i A when the photodiode is operating as described above.
Q3) a) What is dispersion? Explain intermodal dispersion and Intramodal dispersion? 
b) Explain the basis structure of an STS – NSONET frame. Also explain how it is different from STS – I. 
Q4) a) Describe the concept of wavelength Division Multiplexing and state the key system features of WDM. 
b) What are the three classes of Optical Amplifiers and explain their applications. 
Q5) a) Explain working of Laser Welding with the help of neat diagram. Also state its advantages. 
b) Explain how Laser can be used for measurement of current. 
c) Write short note on – LASER applications in Medicine. 
Q6) a) Describe the working principle of Michelson Interferometer. 
b) Explain how Laser can be used in trimming of resistor. 
c) Write short note on – Pulse – Echo Technique. 
SECTION – II
Q7) a) What are waveguides? Explain the following terms as applied to the waveguides: 
i) Cutoff wavelength.
ii) Guide wavelength.
iii) Phase velocity and Group Velocity.
iv) Characteristics wave Impedance.
b) Explain Directional Coupler. Define: 
i) Coupling coefficient.
iii) Insertion loss.
iv) Isolation loss.
Q8) a) Explain the working of E plane and H plane Tec. 
b) Write short notes on the following: 
i) Microwave Isolator.
ii) S Matrix and its properties.
c) Determine the cutoff wavelength for the dominant mode in a rectangular waveguide of breadth lOcms. For a 2.5 GHz signal propagated in this waveguide in the dominant mode; calculate the guide wavelength, the group and the phase velocities. 
Q9) a) Explain working of Two Cavity Klystron Amplifier. 
b) An X – band pulsed cylindrical magnetron has Vo = 30 KV, Io = 80 A, Bo = O.Ol Wb/sq.m a = 4cm, b = 8 cm. 
i) Cyclotron Angular Frequency.
ii) Cut-off Voltage.
iii) Cut-off Magnetic Flux Density.
c) Explain how mode jumping is avoided in Magnetron. 
QIO) a) A reflex Klystron operates under the following conditions: 
Vo = 500V, Rsh = 20k^, Fr = 8GHz L = lmm is the spacing between repeller and cavity. The tube is oscillating at Fr at the peak on n = 2 mode 3 or l mode.
4 Assume that the transit time through the gap and through beam loading effect can be neglected.
i) Find the value of repeller voltage VR.
ii) Find the dc current necessary to give microwave gap of voltage of 200V.
iii) Calculate the electronic efficiency.
b) What is Slow Wave Structures? What are their types? Which one is most practical? How does it aid the performance of Travelling Wave Tube? 
Describe how Tunnel Diode can be used as an amplifier and Oscillator.
What is Varacter Diode? Give its construction, working principle and explain any one application. 
Explain various modes of operation of Gunn Diode. Explain LSA mode and give limitation of this mode. 
Explain power frequency limitations of a microwave bijunction transistor.