Stimulated Raman scattering (SRS) is an important nonlinear process that can turn optical fibers into broadband Raman amplifiers and tunable Raman lasers. It can also severely limit the performance of multichannel lightwave systems by transferring energy from one channel to the neighboring channels.
Stimulated Raman scattering (SRS) is similar to SBS except that a high-frequency optical phonon rather than an acoustic phonon is generated in the scattering process. Also, SRS can occur in both the forward and backward directions in optical fiber and may have an optical power threshold of up to three orders of magnitude higher than the Brillouin threshold in a particular fiber.
22. The scattering resulting from fiber imperfections like core-cladding RI differences, diameter fluctuations, strains, and bubbles is?
a) Rayleigh scattering
b) Mie scattering
c) Stimulated Brillouin scattering
d) Stimulated Raman scattering
Answer:2. Mie scattering
Explanation:
Mie scattering occurs when the size of the inhomogeneities within the optical fiber is comparable to the size of the guided wavelength.
When this scattering inhomogeneity size is larger than 1/10 of the operating wavelength, the scattered intensity which has an angular dependence can be very large.
These imperfections result from the imperfect cylindrical structure of the waveguide and may be caused by fiber imperfections such as irregularities in the core-cladding interface, the differences in the core-cladding refractive indices along the fiber length, strains, and bubbles, diameter fluctuations, etc.
23. High-frequency optical phonon is generated in ________
a) Rayleigh scattering
b) Mie scattering
c) Stimulated Brillouin scattering
d) Stimulated Raman scattering
Answer: 4. Stimulated Raman scattering
Explanation:
Stimulated Raman scattering (SRS) is an important nonlinear process that can turn optical fibers into broadband Raman amplifiers and tunable Raman lasers. It can also severely limit the performance of multichannel lightwave systems by transferring energy from one channel to the neighboring channels.
Stimulated Raman scattering (SRS) is similar to SBS except that a high-frequency optical phonon rather than an acoustic phonon is generated in the scattering process. Also, SRS can occur in both the forward and backward directions in optical fiber and may have an optical power threshold of up to three orders of magnitude higher than the Brillouin threshold in a particular fiber.
24. Mie scattering has in-homogeneities mainly in _________
a) Forward direction
b) Backward direction
c) All direction
d) Core-cladding interface
Answer:1. Forward direction
Explanation:
Mie scattering occurs when the size of the inhomogeneities within the optical fiber is comparable to the size of the guided wavelength.
Mie scattering occurs mainly in the forward direction.
When this scattering inhomogeneity size is larger than λ/10 of the operating wavelength, the scattered intensity which has an angular dependence can be very large.
These imperfections result from the imperfect cylindrical structure of the waveguide and may be caused by fiber imperfections such as irregularities in the core-cladding interface, the differences in the core-cladding refractive indices along the fiber length, strains, and bubbles, diameter fluctuations, etc.
25. The in-homogeneities in _______ can be reduced by the coating of fiber.
a) Rayleigh scattering
b) Mie scattering
c) Stimulated Brillouin scattering
d) Stimulated Raman scattering
Answer:2. Mie scattering
Explanation:
Mie scattering occurs when the size of the inhomogeneities within the optical fiber is comparable to the size of the guided wavelength.
Mie scattering occurs mainly in the forward direction.
When this scattering inhomogeneity size is larger than λ/10 of the operating wavelength, the scattered intensity which has an angular dependence can be very large.
Mie scattering homogeneity can be reduced by removing imperfections due to the glass manufacturing process. Coating of fiber increases relative refracting index difference.
These imperfections result from the imperfect cylindrical structure of the waveguide and may be caused by fiber imperfections such as irregularities in the core-cladding interface, the differences in the core-cladding refractive indices along the fiber length, strains, and bubbles, diameter fluctuations, etc.
26. Determine the threshold optical power in a long single-mode fiber with 0.5 dB km-1 for stimulated Brillouin scattering within the fiber at a wavelength of 1.3 nm. The fiber core diameter is 6 μm and laser source bandwidth is 600 MHz.
1.5 W
1.38 W
80.3 mW
65.2 mW
Answer:3. 80.3 mW
Explanation:
The threshold power is given by:
PB = (4.4 × 10-3) d2 λ2 αdB B Watts
PB = Threshold power
αdB = Fiber attenuation in decibels per kilometer
d = Core diameter in μm.
λ = Operating wavelength in nm
B = Source bandwidth in GHz
Calculation:
With αdB = 0.5, d = 6 μm, λ = 1.3 nm, and B = 600 MHz = 0.6 GHz, the threshold power will be:
PB = (4.4 × 10-3) (6)2 (1300)2 (0.5) (0.6) Watts
PB = 80.30 mW
27. Stimulated Brillouin scattering is mainly a ___________
a) Forward process
b) Backward process
c) Upward process
d) Downward process
Answer:2. Backward process
Explanation:
Stimulated Brillouin scattering (SBS) is similar to SRS in that energy is transferred from an optical pump beam to longer wavelengths through interaction with the glass medium, except that acoustic phonons are involved, and hence the frequency shift is small, about 11 GHz, and the bandwidth very small, typically 50 MHz.
The scattered light appears as upper and lower sidebands which are separated from the incident light by the modulation frequency. The resultant scattered wave propagates principally in the backward direction in single-mode fibers making SBS a mainly backward process.
28. Raman and Brillouin scattering are usually observed at _______
a) Low optical power densities
b) Medium optical power densities
c) High optical power densities
d) Threshold power densities
Answer:3. High optical power densities
Explanation:
Raman and Brillouin scattering are usually observed at high optical power densities in long single-mode fibers.
These scattering mechanisms in fact give optical gain but with a shift in frequency, thus contributing to attenuation for light transmission at a specific wavelength.
However, it may be noted that such non-linear phenomena can also be used to give optical amplification in the context of integrated optical techniques.
29. The _______ is a quantum of an elastic wave in a crystal lattice.
Phonon
Photon
Plasmon
Magnon
Answer: 1. Phonon
Explanation:
A phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids.
it is often referred to as a quasi-particle, it represents an excited state in the quantum mechanical quantization of the modes of vibrations of elastic structures of interacting particles.
Phonons play a major role in many of the physical properties of solids, including materials thermal and electrical conductivities.
30. A single-mode optical fiber has an attenuation of 0.3dB/km when operating at a wavelength of 1.1μm. The fiber core diameter is 4μm and the bandwidth is 500 MHz. Find threshold optical power for stimulated Brillouin scattering.
a) 11.20 mw
b) 12.77 mw
c) 13.08 mw
d) 12.12 mw
Answer: 2. 12.77 mw
Explanation:
The threshold optical power stimulated Brillouin scattering is given by-