Answer:4. Using fiber with large numerical aperture
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.
Modal Noise can be reduced by the following the method
The use of a broad spectrum source in order to eliminate the modal interference effects. This may be achieved by either ( Increasing the width of the single longitudinal mode and hence decreasing its coherence time or ( by increasing the number of longitudinal modes and averaging out of the interference patterns
It is found that fibers with large numerical apertures support the transmission of a large number of modes giving a greater number of speckles, and hence reduce the modal noise-generating effect of individual speckles.
The use of single-mode fiber does not support the transmission of different modes and thus there is no intermodal interference.
The removal of disturbances along with the fiber. This has been investigated with regard to connector design in order to reduce the shift in speckle pattern induced by mechanical vibration and fiber misalignment.
43. Disturbance along the fiber such as vibrations, discontinuities, connectors, splices, source/detectors coupling result in __________
Modal noise
Inter-symbol interference
Infrared interference
Pulse broadening
Answer:1. Modal noise
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.
44. A 4 km optical link consists of multimode step-index fiber with core refractive index of 1.3 and a relative refractive index difference of 1%. Find the delay difference between the slowest and fastest modes at the fiber output.
0.173 μsec
0.152 μsec
0.96 μsec
0.121 μsec
Answer:1. 0.173 μsec
Explanation:
The delay difference is given by
δTs = Ln1Δ/c
Where
δTs = delay difference
n1 = core refractive index
Δ = Relative refractive index difference
c = velocity of light in a vacuum
δTs = (4 × 1.3 ×0.01)/(3 × 108)
δTs = 0.173 μsec
45. The modal noise occurs when uncorrected source frequency is?
δf>>1/δT
δf=1/δT
δf<<1/δT
Negligible
Answer:1. δf>>1/δT
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.
The modal noise is known as modal or speckle noise.
The speckle patterns are formed by the interference of the modes from a coherent source when the coherence time of the source is greater than the intermodal dispersion time δT within the fiber.
Modal noise is dependent on changes in frequency.
Frequency is inversely proportional to time.
The coherence time for a source with uncorrelated source frequency width δf is simply 1/δf. Hence, modal noise occurs when:
δf > 1/δT
46. A multimode step-index fiber has a core refractive index of 1.5 and relative refractive index difference of 1%. The length of the optical link is 6 km. Estimate the RMS pulse broadening due to intermodal dispersion on the link.
92.6 ns
86.7 ns
69.3 ns
68.32 ns
Answer:2.86.7 ns
Explanation:
The RMS pulse broadening due to intermodal dispersion is given by equation
$\sigma _{s}=\frac{Ln_{1}\Delta }{2\sqrt{3}c}$
Where
σs = RMS pulse broadening
L = length of optical link = 6 km
C = velocity of light in vacuum = 3 × 108
n1 = core refractive index = 1.5
Δ = Relative refractive index difference = 1% = 0.01
47. Practical pulse broadening value for graded index fiber lies in the range of __________
0.9 to 1.2 ns/km
0.2 to 1 ns/km
0.23 to 5 ns/km
0.45 to 8 ns/km
Answer:2. 0.2 to 1 ns/km
Explanation:
Pulse broadening is defined as the spreading of the light pulses as they travel down the fiber
The theoretical improvement factor of the graded-index fiber in relation to intermodal RMS pulse broadening is 1000.
All optical fiber sources have a finite spectral width, the profile shape must be altered to compensate for this dispersion mechanism.
The minimum overall dispersion for graded-index fiber is also limited by other intermodal dispersion mechanisms.
Thus pulse broadening values lie within the range of 0.2 to 1 ns/km with injection lasers and light-emitting diodes respectively.
Therefore, practical pulse broadening values for graded-index fibers lie in the range of 0.2 to 1 ns/km.
This gives bandwidth—length products of between 0.5 and 2.5 GHzkm when using lasers and optimum profile fiber.
48. The differential attenuation of modes reduces _______ pulse broadening on a multimode optical link.
Intramodal
Intermodal
Waveguide
Material
Answer:2. Intermodal
Explanation:
Intermodal dispersion may be reduced by propagation mechanisms within practical fibers.
For instance, there is differential attenuation of the various modes in a step-index fiber.
This is due to the greater field penetration of the higher-order modes into the cladding of the waveguide.
These slower modes, therefore, exhibit larger losses at any core-cladding irregularities which tends to concentrate the transmitted optical power into the faster lower order modes.
Thus the differential attenuation of modes reduces intermodal pulse broadening on a multimode optical link.
49. Intermodal dispersion in multimode fibers is minimized with the use of _______
Step Index Fiber
Single-mode graded index fiber
Multimode graded Index Fiber
All of the above
Answer:3. Multimode graded Index Fiber
Explanation:
Intermodal dispersion in multimode fibers is minimized with the use of graded-index fibers. Hence, multimode graded-index fibers show substantial bandwidth improvement over multimode step-index fibers.
Graded-index profile optical fiber cables exhibit far less intermodal dispersion than exhibited by the multimode step-index profile optical fiber cables mainly due to the nature of their refractive index profiles.
Different group velocities of the propagating modes get normalized with respect to the index grading.
It has a large core diameter (>30 pm).
Its bandwidth is greater than multimode step-index fiber bandwidth but less than single-mode step-index fiber bandwidth.
Graded index fibers accept less light.
50. Estimate RMS pulse broadening per km due to intermodal dispersion for multimode step-index fiber where the length of fiber is 4 km and pulse broadening per km is 80.6 ns.
18.23 ns/km
20.15 ns/km
26.93 ns/km
10.23 ns/km
Answer:2. 20.15 ns/km
Explanation:
The RMS pulse broadening per km due to intermodal dispersion for multimode step index fiber is given by
