Optical Fiber Communication Test

Department of Electronics and Communication Subject: Optical Fiber Communication (TEC-701) Branch: 4th Year Time: 90 min

Max Marks: 30

SESSIONAL TEST 1 1. Attempt any two out of three questions. (2*2=4Marks) a) Draw and explain block diagram of optical communication system. b) Write any two disadvantages and any two applications of optical fiber communication? c) Define the relative refractive index difference for an optical fiber and show how it may be related to the numerical aperture of optical fiber?

(2*3=6 2. Attempt any two out of three questions. a) . Describe with the help of simple ray diagrams:  Multimode step index fiber.  The single mode step index fiber. Write any one advantage of these two types of fiber for use as an optical channel. b)Define modes in a cylindrical waveguide. c) Explain the following terms:  Mode conversion  Mode field diameter

Marks)

(2*5=10 Marks) 3. Attempt any two out of three questions. a) Explain a relative refractive index difference. A step index fiber has a solid acceptance angle in air of 0.115 radians and a relative refractive index difference of 0.9%. Estimate the speed of light in the fiber core. b) A graded index fiber with a core axis refractive index of 1.5 has a characteristic index profile of 1.90, a relative refractive index difference of 1.3% and a core diameter of 40 µm. Estimate the number of guided modes propagating in the fiber when the transmitted light has a wavelength of 1.55 µm. and determine the cutoff value of the normalized frequency for single mode transmission in the fiber. c) A step index fiber in air has a numerical aperture of 0.16, a core refractive index of 1.45 and a core diameter of 60µm.Determine the normalized frequency for the fiber when light a wavelength of 0.9 µm is transmitted. Further, estimate the number of guided modes propagating in the fiber. (2*5=10 Marks) 4. Attempt any two out of three questions. a) A graded index fiber with a parabolic index profile supports the propagation of 742 guided modes. The fiber has a numerical aperture in air of 0.3 and a core diameter of 70µm. Determine the wavelength of the light propagating in the fiber. Further estimate the maximum diameter of the fiber which single-mode operation at the same wavelength. b)Define Numerical aperture. The relative refractive index difference between the core axis and the cladding of a graded index fiber is 0.7% when the refractive index at the core axis is 1.45. Estimate value for the numerical aperture of the fiber c)Explain cut-off wavelength. Determine the cutoff wavelength for a step index fiber to exhibit single-mode operation when the core refractive index and radius are 1.46 and 4.5µm, respectively, with the relative index difference being 0.25%.

Department of Electronics and Communication Subject: Optical Fiber Communication (TEC-701) Branch: 4th Year Time: 90 min SESSIONAL TEST 2

Max Marks: 30

Note: Attempt any two parts out of three in each question. 1. (2*2=4 Marks) a) The external power efficiency of an InGaAsP/lnP planar LED is 0.75% when the internally generated optical power is 30 mW. Determine the transmission factor for the InP-air interface if the drive current is 37 μA and the potential difference across the device is 1.6 V. The refractive index of InP may be taken as 3.46. b) A multimode step index fiber has core refractive index of 1.5 with a relative refractive index difference of 3% and an operating wavelength of .82 μm. Estimate the critical radius of curvature at which large bending losses occur. c) A GaAs injection laser has an optical cavity of length 250 μm and width 100 μm. At normal operating temperature the gain factor β is 21 x 10-3 A cm-3 and the loss coefficient a per cm is 10. Determine the threshold current density for the device. It may be assumed that the cleaved mirrors are uncoated and that the current is restricted to the optical cavity. The refractive index of GeAs may be taken as 3.6. 2. a) List all the types of losses in fibers and explain linear scattering losses in detail. b) Explain Dispersion mechanism. What are dispersion-flattened fibers? c) Distinguish between: (a) LASER and LED. (b) Characteristics of Laser.

(2*3=6Marks)

3. (2*5=10 Marks) a) Discuss the mechanism of optical feedback to provide oscillations and hence amplification within the laser. Also give the threshold condition for Laser Oscillation. b) Explain the working of semiconductor injection laser. What are its advantages and disadvantages? c) Explain Avalanche photodiode in detail. List three factors limiting the speed of response of a photodiode. 4. a) Define:

(2*5=10 Marks)

(a)Quantum efficiency of LED (d) Zero Dispersion wavelength (b) Responsivity (e)The beat length (c)NEP b) The threshold optical powers for stimulated Brillouin and Raman scattering in a long 8 µm core diameter single mode fiber are found to be 190 mW and 1.70 W, respectively, when using an injection laser source with a bandwidth of 1 GHz. Calculate the operating wavelength of the laser and the attenuation in decibel per kilometer of the fiber at this wavelength. c) A ruby LASER contains a crystal length 4 cm with a refractive index of 1.78. The peak emission wavelength from the device is 0.55micrometer. Determine the number of longitudinal modes and their frequency separation.

Department of Electronics and Communication Subject: Optical Fiber Communication (TEC-701) Branch: 4th Year Time: 60 min Marks: 20 RESESSIONAL TEST 2

Max

Note: Attempt any four out of five questions. All questions carry equal marks

1. Define the noise equivalent power (NEP) for a photodetector. Starting with photocurrent, derive an expression for the NEP of a photo diode in which the dark current noise dominates. 2. Explain briefly polarization maintaining fibers. Two polarization maintaining fibers operating at a wavelength of 1.3µm have beat lengths of 0.7mm and 80m.Determine the modal birefringence in each case and comment on the results. 3. Photons of energy 1.53 X 10-19 J are incident on a photodiode which has responsivity of 0.65A/W. If the optical power level is 10µW, then find the generated photocurrent. For the wavelength range 1300nm < λ < 1600nm, the quantum efficiency for InGaAs is around 90%. Find the responsivity in this wavelength range. 4. Explain semiconductor injection laser. Find its internal quantum efficiency and show how it is related to the differential quantum efficiency. 5. The threshold optical powers for stimulated Brillouin and Raman scattering in a long 8 µm core diameter single mode fiber are found to be 190 mW and 1.70 W, respectively, when using an injection laser source with a bandwidth of 1 GHz. Calculate the operating wavelength of the laser and the attenuation in decibel per kilometer of the fiber at this wavelength.