Monday, September 12, 2016



1.  (a)         
What are the advantages of optical fiber cables over the co-axial cables?    
Calculate NA, Acceptance angle and Angle of fiber with core and cladding refractive index values as n1=1.5 and n2=1.45 respectively.                                                                  
Explain the “mode theory” of optical fiber.   
2.  (a)         
What is meant by acceptance angle and acceptance cone for optical fiber? Show and explain it’s relationship to numerical aperture and refractive indices of core & cladding.
If a MMSI fiber of core radius 25 micrometer operating at 1300nm, has  core and cladding refractive index as 1.5 & 1.38 respectively; Calculate Numerical aperture, Normalized frequency, Solid acceptance angle and number of modes entering in the fiber
If the mean optical power in a 8 km long fiber at launching and output zone is 12 and 2 microwatt; determine (1) overall signal attenuation in dB and (2) when length is 10 km with the splices of 1dB attenuationeach at the interval of 1km.
3.  (a)         
What is dispersion in fiber? Explain intra-modal dispersion in details.
A SMSIfiber with core and cladding refractive indices as 1.447 and 1.442respectively is designed to operate at wavelength 1.3 micrometer. When core diameter is 7.2 micrometer; confirm that the fiber permits single mode transmission and estimate the range of wavelengths over which this will occur.           
4.  (a)         
Explain the “linear scattering losses” in fibers.
A MMSIF has NA as 0.3 and core refractive index as 1.45. The material dispersion parameter as 250 ps/ makes it totally dominating intra-modular dispersion mechanism. Estimate (1) total pulse broadening/km when the fiber is used with the LED source of RMS spectral width of 50 nm, and (2) corresponding bandwidth-length product.
5.  (a)         
Explain the three transmissions windows of optical communication along with the attenuation curves for SMF.
Explain bending losses in fiber.         
6.  (a)         
Explain material absorption losses in silica fiber.

  1. (a) Compare the surface and Edge emitting LED and explain one of them in details.                                                                                                             [10]
      (b)The radiative and non-radiative recombination lifetime of the minority carriers in the active region of a double heterojunction LED are 50 and 110 ns respectively. Determine the total carrier recombination life time and the power internally generated within the device when the peak emission is 0.87 µm at the derive current of 40mA.                                                                                [08]

2. (a) what do you understand by line coding? Why is it needed? Explain NRZ and RZ code in details.                                                                                       [10]
   (b)An InGaAsP Surface ELED has an activation energy of 0.9 eV with a constant of proportionality β0 = 1.85*107 h-1. Find out the operating life time of the LED at constant junction temp. of 200C, if assumed that the device is no longer useful when it’s optical output power goes down to 0.65 of it’s original value.                                                                                                                                   [08]

3. (a) Draw block schematic of the front end of an optical receiver showing and explaining various noise sources at different locations/stages.                              [08]
    (b)A given silicon APD has quantum efficiency of 65% at a wavelength of 900 nm. Suppose 0.5 µW of optical power produces a multiplied photocurrent of 10µA; find the avalanche gain.                                                                                    [08]         

4.(a) What are the criteria for photo diodes to be used as detectors in optical fiber communication? Draw the structure of APD and explain it’s working. Also list its advantages and drawbacks.                                                                        [08]
    (b)A photo diode used for an optical receiver has a quantum efficiency of 65% when operating at 850 nm. The dark current is 3.5 nA and the load resistance is 5 k-ohm. If the incident optical power and post detection BW are 300 nW and 6.5 MHz; compare the shot noise generated in photodiode with the thermal noise in the load resistor at room temperature.                                                                     [08]
5.(a) What are the key system requirements that are needed in analyzing a “point to point” link? Explain the point to point link design with reference to the choice of components and their associated characteristics.                                                  [08]
   (b) An analog optical fiber system employs an LED that emits 3dB mean optical power into air. However, a coupling loss of 17.5 is encountered when launching into a fiber cable. The fiber cable extended to 6 km without repeator exhibit a loss of 5 dB/km. it is spliced every 1.5 km with an average loss of 1.1dB/splice. In addition there is a connector loss at the receiver of 0.8 dB. The receiver has a sensitivity of -54dBm at the operating bandwidth of the system.  Assuming there is no dispersion-equalization penalty, perform an optical power budgeting for the system and establish a safety margin.                                                               [08]
6. (a) Explain basic elements of an analog link through block diagram, show major noise contributors and discuss working of analog transmission system.                [08]
(b) The 10 to 90% rise time for possible components to be used in D-IM analog optical fiber link are specified as: LED source-9 ns/km, fiber cable- 9 ns/km (intermodal) and 2 ns/km (intramodal), APD detector- 3 n sec. the desired link length without repeator is 5 km and the required BW is 6 MHz. Determine the combination for components give an adequate response.                                          [08]

Unit -V  - Satellites

1.      With the help of block diagram, explain typical tracking, telemetry system.
2.      With the help of block diagram, explain typical command and monitoring system.
3.      Explain the transponder arrangement and frequency plan (uplink and downlink) for any satellite. Also draw block diagram of single conversion transponder for 6/4 GHz band.
4.      What are different types of antennas used in satellite systems, explain importance of each.
5.      Explain the term reliability in connection with satellite communication.
6.      Explain satellite communication systems and their applications
7.      Write a note on power subsystems.
8.      Write note on Equipment lifetime and space qualification.

Unit VI – Introduction Satellite Link Design
1.      Explain basic transmission theory of satellite communication link design.
2.      Derive the equation of Gain as G = 4πηA/λ
3.      Derive the equation of Gain as G = 4πηA/λ2
4.      In relation to satellite communication, define noise temperature and derive the equation for carrier to noise ratio at the output of demodulator. 
5.      Explain system noise temperature and G/T ratio.
6.      Explain  design parameters and features of uplink systems.
7.      Explain  design parameters and features of downlink systems.
8.      Write a short note on satellite systems using small earth stations