Code: A-20

AMIETE – ET (OLD SCHEME)

Code: AE24 Subject: OPTO ELECTRONICS AND

Time: 3 Hours OPTICAL COMMUNICATION

Max. Marks: 100

NOTE: There are 9 Questions in all.

· Question 1 is compulsory and carries 20 marks. Answer to Q.1 must be written in the space provided for it in the answer book supplied and nowhere else.

· Out of the remaining EIGHT Questions answer any FIVE Questions. Each question carries 16 marks.

· Any required data not explicitly given, may be suitably assumed and stated.

Q.1 Choose the correct or the best alternative in the following: (210)

a. Operating wavelength window 1550 nm is not yet in use with fiber optic system because

(A) loss in fiber is higher at 1300 nm

(B) suitable laser devices have not yet been obtained

(C) wavelength multiplexing is not possible at this wavelength

(D) Both (B) and (C)

b. Optical communication is feasible only in IR range because

(A) it offers minimum dispersion (B) it offers minimum attenuation

(C) Both (A) and (B) (D) none of these

c. A graded index profile in optical fiber cable provides

(A) less wave guide dispersion than SI profile

(B) less material dispersion than SI profile

(C) less attenuation in SI profile

(D) less modal dispersion than SI profile

d. Scattering Loss in optical fiber varies with wave length as

(A) (1/ λ²) (B) (1/ λ³)

(C) (1/ λ⁴) (D) none of these

e. If BW is the bandwidth of an optical fiber and NA is the numerical aperture then

(A) BW NA (B) BW (1/NA)

(C) BW (1/NA)² (D) BW (1/NA)³

f. Noise equivalent power of a photo detector is

(A) inversely proportional to dark current

(B) proportional to the square root of the dark current

(C) directly proportional to the dark current

(D) none of the above

g. Which of the following is not applicable for Laser?

(A) no tuning arrangement (B) higher emission efficiency

(C) narrow spectral width (D) provision for confinement

h. The configuration utilized by a line communication system capable of transmitting a data at the rate of 100 MB/s is

(A) coaxial cable system (B) open line system

(C) optical fiber system (D) twisted wire system

i. For extremely low transmission losses in optical fiber, operating wavelength is in the range of

(A) (0.1 to 1) μm (B) (0.01 to 2) μm

(C) (0.2 to 8) μm (D) (0.0001 to 2) μm

j. Square Law Device in optical source

(A) Linear variation in drive voltage results in a corresponding

linear change in the optical output power

(B) Linear variation in drive current results in a corresponding

linear change in the optical output power

(C) Non-linear variation in drive current results in a corresponding

linear change in the optical output power

(D) None of the above

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2 a. Give the advantages of optical communication systems over conventional copper systems. (8)

b. A step index optical fiber has a numerical aperture of 0.2 and a cladding refractive index of 1.59, determine:

(i) The acceptance angle of the fiber in water which has refractive index of 1.33.

(ii) The critical angle of core cladding interface.

(iii) Number of modes transmitted at a 1300 nm wavelength and 25 μm core radius. (3+3+2)

Q.3 a. Describe with the aid of simple ray diagram

(i) Single mode step index fiber

(ii) The multimode step index fiber

Compare the advantages and disadvantages of these two types of fibers for use as an optical channel. (10)

b. A typical relative index difference for an optical fiber designed for long distance transmission is 1%. Estimate the NA and the solid acceptance angle in air for the fiber when the core index is 1.50. Also find the critical angle at the core-cladding interface within the fiber. (6)

Q.4 a. Discuss absorption losses in optical fibers with relation to intrinsic and extrinsic absorption mechanism. (8)

b. A step index single mode fiber has a core index of 1.48 and Δ=1%. If the material dispersion at 1.55 μm for the fiber is 7 ps nm^-1 km^-1, what should be the radius of core so that the total dispersion at this wavelength is zero? (8)

Q.5 a. Why LEDs are commonly used with multimode fibers? Explain the hetrojunction structure used for LEDs. What are the parameters which influence the performance of LEDs? (8)

b. With the aid of suitable diagram, explain the principles of operation of the injection laser. (8)

Q.6 a. Explain the various types of losses when two fibers are joined together. (10)

b. A fusion splice is made for a broken multimode step index fiber. The splice shows a loss of 0.36 dB which is due to small air gap. Determine the refractive index of the fiber core. (6)

Q.7 a. How does the mechanism of optical feedback occur to provide oscillations and thereby the amplifications within a laser? Show that this provides a distinctive spectral output from the device. (8)

b. When 10¹² photons per second each with energy of 1.310^-19 J are incident on an ideal photodiode, find

(i) Wavelength of incident radiation

(ii) Output power

(iii) Output photocurrent if the device is an APD with a multiplication

factor of 18. (8)

Q.8 a. Explain the operation of reach through avalanche photodiode. (8)

b. A double hetrojunction InGaAsP LED emitting a peak wavelength of 1310 nm has radiative and non-radiative recombination times of 25 and 90 ns respectively. The drive current is 35 mA.

(i) Find the internal quantum efficiency and internal power level.

(ii) If the refractive index of the light source material is n= 3.5, find the power emitted from device. (8)

Q.9 a. Draw and explain the block diagram of four channel optical time division multiplexing. (8)

b. Discuss in brief:-

(i) Power budget analysis

(ii) Block codes (4+4)