31. _______ is not usually observed in multimode fibers.
a) Rayleigh scattering
b) Mie scattering
c) Stimulated Brillouin scattering
d) Stimulated Raman scattering
Answer: 4. Stimulated Raman scattering
Explanation:
Stimulated Brillouin scattering is not usually observed in multimode fibers because their relatively large core diameters make the threshold optical power levels extremely high.
Moreover, it should be noted that the threshold optical powers for both these scattering mechanisms may be increased by the suitable adjustment of the other parameters.
Operation at the longest possible wavelength is advantageous although this may be offset by the reduced fiber attenuation (from Rayleigh scattering and material absorption) normally obtained.
32. _______ Scattering is particularly applicable to colloidal mixtures and suspensions.
a) Rayleigh scattering
b) Mie scattering
c) Tyndall Scattering
d) Stimulated Raman scattering
Answer: 3. Tyndall Scattering
Explanation:
Tyndall scattering is similar to Mie scattering, without the restriction to the spherical geometry of the particles. It is particularly applicable to colloidal mixtures and suspensions.
33. ______ occurs from the interaction of photons with acoustic phonons in solids.
a) Rayleigh scattering
b) Mie scattering
c) Brillouin Scattering
d) Stimulated Raman scattering
Answer: 3. Brillouin Scattering
Explanation:
Brillouin scattering occurs from the interaction of photons with acoustic phonons in solids.
Brillouin scattering is caused by the nonlinearity of a medium. In glass fibers, Brillouin scattering shows modulation of the light by the thermal energy in the material.
34. _______ used to determine the chemical composition and molecular structure.
a) Raman scattering
b) Mie scattering
c) Brillouin Scattering
d) Stimulated Raman scattering
Answer: 1. Raman scattering
Explanation:
Raman scattering is another form of inelastic light scattering.
Raman scattering is used to determine the chemical composition and molecular structure.
Since most Raman lines are stronger than Brillouin lines and have higher energies, standard spectrometers using scanning monochromators may be used to measure them.
Raman spectrometers are standard equipment in many chemical laboratories.
Brillouin and Raman scattering is known as non-clastic scattering as wavelength changes due to such scattering. Both are used in laser operation.
35. _______ the inhomogeneities in Mie Scattering can be reduced by
Removing Imperfection
Fiber Coating
Increasing relative refractive Index
All of the above
Answer: 4. All of the above
Explanation:
Mie scattering occurs when the size of the inhomogeneities within the optical fiber is comparable to the size of the guided wavelength.
The inhomogeneities in Mie Scattering can be reduced by
(i) Removing imperfections due to the glass manufacturing process
(ii) Carefully controlled extrusion and coating of the fiber
(iii) increasing the fiber guidance by increasing the relative refractive index difference. By these means, it is possible to reduce Mie scattering to insignificant levels.
36. Intermodal dispersion occurring in a large amount in multimode step-index fiber results in ________
Propagation of the fiber
Propagating through the fiber
Pulse broadening at output
Attenuation of waves
Answer:3. Pulse broadening at output
Explanation:
Intramodal dispersion is pulse spreading that occurs within a single-mode. It is a result of the group velocity being a function of the wavelength λ. Since intramodal dispersion depends on the wavelength, its effect on signal distortion increases with the spectral width of the optical source.
Intermodal dispersion occurs in multimode fiber.
Multimode step-index fibers exhibit a large amount of intermodal dispersion, which gives the greatest pulse broadening of all dispersion types.
However, intermodal dispersion in multimode fibers may be reduced by adopting an optimum refractive index profile, which is provided by a near parabolic profile of most graded-index fibers.
37. ______ dispersion occurs in multimode fiber.
Intramodal dispersion
Intermodal dispersion
Both 1 and 2
None of the above
Answer:2. Intermodal dispersion
Explanation:
Intermodal dispersion occurs only in multimode fibers.
In multimode fibers, different modes travel at different group velocities due to the different path links used, and the output pulse width is dependent on the transmission times of the slowest and fastest modes.
Multimode step-index fibers exhibit a large amount of intermodal dispersion, which gives the greatest pulse broadening of all dispersion types.
However, intermodal dispersion in multimode fibers may be reduced by adopting an optimum refractive index profile, which is provided by a near parabolic profile of most graded-index fibers.
38. After Total Internal Reflection the Meridional ray ______
Makes an angle equal to acceptance angle with the axial ray
Makes an angle equal to critical angle with the axial ray
Travels parallel equal to critical angle with the axial ray
Makes an angle equal to critical angle with the axial ray
Answer:4.Makes an angle equal to critical angle with the axial ray
Explanation:
Meridional rays are rays that pass through the axis of the optical fiber. Meridional rays are used to illustrate the basic transmission properties of optical fibers.
The minimum angle supports total internal reflection for meridional ray. If the ray strikes the core-cladding interface at an angle less than the minimum angle, then they get refracted out of the core and they will be lost from the cladding.
After Total Internal Reflection, the Meridional ray makes an angle equal to the critical angle with the axial ray.
The Meridional ray travels along the axis of the fiber. When the ray is incident, makes an angle equal to the acceptance angle and thus it propagates through the fiber.
As the propagating ray gets refracted from the boundary, it makes an angle (i.e. critical angle) with the normal.
39. _______ transmission is more likely to be affected by modal noise.
Digital
Analog
Both 1 and 2
None of the above
Answer:2. Analog
Explanation:
Modal noise: Noise generated in an optical fiber system by the combination of mode-dependent optical losses and fluctuation in the distribution of optical energy among the guided modes or in the relative phases of the guided modes.
Analog transmission is also more susceptible to modal noise due to the higher optical power levels required at the receiver when quantum noise effects are considered. Therefore, it is important that modal noise is taken into account within the design considerations for these systems.
40. The main reason for _______ is also known as mode dispersion.
Intramodal dispersion
Intermodal dispersion
Waveguide dispersion
Material dispersion
Answer:2. Analog
Explanation:
The main reason for intermodal dispersion, also known as mode dispersion, is the difference in propagation delay between various propagation modes within a multimode fiber (hence, it is not applicable for single-mode fiber).
Different modes of a transmitted light pulse travel at different group velocities.
Different transmission times between the fastest and slowest modes of propagation yield in broadening of the transmitted optical pulse at the output of the fiber cable.
41. Consider a single mode fiber having core refractive index n1= 1.5. The fiber length is 12m. Find the time taken by the axial ray to travel along with the fiber.
1.00μsec
0.06μsec
0.90μsec
0.30μsec
Answer:2. 0.06μsec
Explanation:
The time taken for the axial ray to travel along a fiber of length L gives the minimum delay time Tm, and:
Tmin = Ln1/c
Where
L = length of the fiber
n1 = Refractive index of core
c = velocity of light in vacuum
