1. 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:
Intermodal dispersion (also called modal dispersion) is the phenomenon that the group velocity of light propagating in multimode fiber (or other waveguides) depends not only on the optical frequency (chromatic dispersion) but also on the propagation mode involved.
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.
2. ______ 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.
3. 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.
4. _______ 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.
5. The main reason for _______ is also known as mode dispersion.
Intramodal dispersion
Intermodal dispersion
Waveguide dispersion
Material dispersion
Answer:2. Intermodal dispersion
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.
6. 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
Tmin = (12 × 1.5)/3 × 108
Tmin = 0.06μsec
7. The modal noise can be reduced by ________
Decreasing width of signal longitudinal mode
Increasing coherence time
Decreasing number of longitudinal modes
Using fiber with large numerical aperture
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.
8. 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.
9. A 4 km optical link consists of a 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
10. 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
11. 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
12. 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.
13. 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.
14. 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.
15. 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
