This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “Polarization”.
1. For many applications which involve optical fiber transmission, an intensity modulation optical source is not required. State whether the given statement is true or false.
Explanation: In many optical fibers transmission, the cylindrical fibers used generally do not maintain polarization state of light input source not more than a few meters. So for this reason, optical sources intensity modulation is required.
2. The optical source used for detection of optical signal is
a) IR sensors
c) Zener diodes
Explanation: Optical signal is generally detected by photodiodes because photodiode is generally insensitive to optical polarization or phase of light with the fiber.
3. An optical fiber behaves as a birefringence medium due to differences in
a) Effective R-I and core geometry
b) Core-cladding symmetry
c) Transmission/propagation time of waves
d) Refractive indices of glass and silica
Explanation: In an optical fiber with ideal optically circulatory symmetric core, both polarization modes propagate with same velocities. These fibers have variations in internal and external stress; fiber bending and so exhibit some birefringence.
4. The beat length in a single mode optical fiber is 8 cm, when light from a laser with a peak wavelength 0.6μm is launched into it. Estimate the modal birefringence.
Explanation: Modal birefringence can be obtained by-
BF= λ/LB= 0.8×10-6/ 0.08
λ= peak wavelength
LB= beat length.
5. Beat length of a single mode optical fiber is 0.6cm. Calculate the difference between propagation constants for the orthogonal modes
Explanation: The difference between the propagation constant for two orthogonal modes can be obtained by:
βx – βy= 2Π/LB= 2×3.14/ 0.06
βx & βy are propagation constants for slow & fast modes resp.
LB = beat length.
6. A polarization maintaining fiber operates at a wavelength 1.2μm and have a modal birefringence of 1.8*10-3. Calculate the period of perturbation.
a) 0.7 seconds
b) 0.6 seconds
c) 0.23 seconds
d) 0.5 seconds
Explanation: The period of perturbation is given by-
T= λ/BF Where λ is operating wavelength, BF = Birefringence, T= period of perturbation.
7. When two components are equally excited at the fiber input, then for polarization maintaining fibers δΓgshould be around
b) 1 ns/km
Explanation: The differential group delay δΓg is related to polarization mode dispersion (PMD) of fiber. This linear relationship to fiber length however applies only to short fiber-lengths in which birefringence are uniform.
8. Polarization modal noise can _________ the performance of communication system.
Explanation: Polarization modal noise is generally of larger amplitude than modal noise. It is obtained within multimode fibers and so it degrades the performance of communication system and prevents transmission of analog signals.
Sanfoundry Global Education & Learning Series – Optical Communications.
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