MCQ on Transmission Characteristics of Optical Fibers

Optical Fiber / March 15, 2022

91. The beating between light at different frequencies or wavelengths in multichannel fiber transmission causes ________

  1. a) Attenuation
  2. b) Amplitude modulation of channels
  3. c) Phase modulation of channels
  4. d) Loss in transmission

Answer.3. Phase modulation of channels

Explanation:-

  • The beating between light at different frequencies or wavelengths in multichannel fiber transmission causes phase modulation of the channels and hence the generation of modulation sidebands at new frequencies which are termed four-wave mixing (FWM).
  • When three-wave components co-propagate at angular frequency w1, w2, w3, then a new wave is generated at frequency w4, which is w4 = w1 + w2 – w3
  • This frequency combination can be problematic for multichannel optical communications as they can become phase-matched if the channel wavelengths are close to the zero-dispersion wavelength.
  • FWM is therefore one of a broad class of harmonic mixing or harmonic generation processes in which two or more waves combine to generate waves at a different frequency that is the sum (or difference) of the signals that are mixed.

 

92. _________ semiconductor laser sources generally have broader bandwidths.

  1. a) Injection
  2. b) Pulsed
  3. c) Solid-state
  4. d) Silicon hybrid

Answer.2. 

Explanation:-

Pulsed lasers are lasers that emit light not in a continuous mode, but rather in the form of optical pulses (light flashes). Pulsed semiconductor lasers have broader bandwidths. Therefore, these sources prove to be inefficient pump sources. They prove inefficient, especially for the narrow gain spectrum.

 

93. ________ modulation causes modifications to the pulse spectrum.

  1. Pulse
  2. Phase
  3. Self-phase
  4. Self-pulse

Answer.3. Self-phase

Explanation:-

Self-phase modulation refers to the phenomenon in which a laser beam propagating in a medium interacts with the medium and imposes a phase modulation on itself. In the case of a pulsed laser input, the temporal variation of the laser intensity leads to an SPM in time.

Self-phase modulation can broaden the frequency spectrum of the pulse as the time-varying phase creates a time-varying frequency.

 

94. Self-phase modulation can be used for _____________

  1. a) Enhancing the core diameter
  2. b) Wavelength shifting
  3. c) Decreasing the attenuation
  4. d) Reducing the losses in the fiber

Answer.2. Wavelength shifting

Explanation:-

  • Self-phase modulation refers to the phenomenon in which a laser beam propagating in a medium interacts with the medium and imposes a phase modulation on itself. In the case of a pulsed laser input, the temporal variation of the laser intensity leads to an SPM in time.
  • Self-phase modulation can broaden the frequency spectrum of the pulse as the time-varying phase creates a time-varying frequency.
  •  Self-phase modulation is related to phase change. It imposes a positive frequency sweep on the pulse which in turn enables wavelength or frequency shifting.

 

95. ________ occurs when signal power reaches a level sufficient to generate tiny acoustic vibrations in the glass.

  1. Wavelength shifting
  2. Brillouin scattering
  3. Linear Scattering
  4. Non-linear Scattering

Answer.2. Wavelength shifting

Explanation:-

  • Stimulated Brillouin scattering occurs when signal power reaches a level sufficient to generate tiny acoustic vibrations in the glass. This can occur at powers as low as a few milliwatts in single-mode fiber.
  • Acoustic waves change the density of a material and thus alter its refractive index. The resulting refractive-index fluctuations can scatter light, called Brillouin scattering. Since the light wave being scattered itself generates the acoustic waves, the process is called stimulated Brillouin scattering. It can occur when only a single channel is transmitted.

 

96. The non-zero-dispersion-shifted fiber was introduced in the year ______.

  1. 2000
  2. 1990
  3. 1960
  4. 1940

Answer.1. 2000

Explanation:-

  • The non-zero dispersion-shifted fibers (NZ-DSF) become more prevalent in the marketplace in mid-1990 system designers are interested in ensuring compatibility with the existing fiber plant.
  • The non-zero-dispersion-shifted fiber was introduced in the mid-1990s to provide wavelength division multiplexing applications. In the year 2000, the dispersion profile for non-zero-dispersion-shifted fiber was introduced.
  • Nondispersion-shifted single-mode fiber, sometimes referred to as standard single-mode fiber, was the first single-mode optical fiber type in widespread use in telecommunications systems.
  • Nondispersion-shifted single-mode optical fiber normally has a simple step-index profile. It has a nominal zero-dispersion wavelength in the 1300 nm transmission window.

 

97. A multimode step-index fiber has a source of RMS spectral width of 60nm and the dispersion parameter for fiber is 150psnm-1km-1. Estimate RMS pulse broadening due to material dispersion.

  1. 12.5ns km-1
  2. 9.6ns km-1
  3. 9.0ns km-1
  4. 10.2ns km-1

Answer.3. 9.0ns km-1

Explanation:-

Pulse broadening due to material dispersion results from the different group velocities of the various spectral components launched into the fiber from the optical source.

Calculation

The RMS pulse broadening per km due to material dispersion is given by

σm = σλLM

Where

σλ = rms spectral width = 60 nm

L = length of fiber = 1 Km

M = dispersion parameter = 150psnm-1km-1

= 60 × 1 × 150

= 9.0nskm-1

 

98. The variant of non-zero-dispersion-shifted fiber is called as __________

  1. Dispersion flattened fiber
  2. Zero-dispersion fiber
  3. Positive-dispersion fiber
  4. Negative-dispersion fiber

Answer.4. Negative-dispersion fiber

Explanation:-

  • Nonzero-dispersion-shifted fiber (NZ-DSF) is sometimes simply called nonzero-dispersion fiber (NZDF) and a variant of this fiber type is negative-dispersion fiber (NDF).
  • Nondispersion-shifted single-mode fiber, sometimes referred to as standard single-mode fiber, was the first single-mode optical fiber type in widespread use in telecommunications systems.
  • Nondispersion-shifted single-mode optical fiber normally has a simple step-index profile. It has a nominal zero-dispersion wavelength in the 1300 nm transmission window.

 

99. The amount of dispersion in Nonzero Dispersion-shifted single-mode fiber is controlled by ________

  1. Managing Zero dispersion Wavelength
  2. Managing Characteristic dispersion slope
  3. Both 1 and 2
  4. None of the above

Answer.3. Both 1 and 2

Explanation:-

  • The amount of dispersion in Nonzero Dispersion-shifted single-mode fiber is controlled by managing the zero-dispersion wavelength and characteristic dispersion slope through the careful manipulation of the fiber’s complex index profile.
  • One characteristic of these fibers is that the light carrying region of the fiber is proportionally smaller than non-dispersion-shifted single-mode fibers. This results in a higher power density at a given source power level, which if severe enough can mitigate improvements in the fiber’s nonlinear performance.
  • The latest generation fibers are designed with a larger mode-field diameter to lower the transmitted power density, thereby reducing nonlinear effects.

 

100. A multimode fiber has RMS pulse broadening per km of 12ns/km and 28ns/km due to material dispersion and intermodal dispersion resp. Find the total RMS pulse broadening.

  1. 30.46ns/km
  2. 31.23ns/km
  3. 28.12ns/km
  4. 26.10ns/km

Answer.1. 30.46ns/km

Explanation:-

The overall dispersion in multimode fibers comprises both chromatic and intermodal terms. The total RMS pulse broadening σT is given by

σT = (σ2c + σ2n)½

where

σc is the intramodal or chromatic broadening

σn is the intermodal broadening caused by delay differences between the modes

calculation

σT = (122 + 282)½

σT = (144 + 784)½

σT = 30.46ns/km

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