This set of Optical Communications Questions and Answers for Entrance exams focuses on “Practical Constraints of Coherent Transmission”.
1. Which technology development has helped the field of optical fiber communication?
a) Glass technology
b) Component technology
Explanation: Substantial developments in component technology have allowed the initial difficulties in the optical fiber communication to go away. The coherent factor experienced most of the difficulties.
2. __________ dictates the performance characteristics required from components and devices which are to be utilized in coherent optical fiber systems.
a) System considerations
b) Bluetooth technology
d) Practical constraints
Explanation: Practical constraints inhibit the development of coherent optical fiber communications. These constraints are derived from factors associated with the elements of the coherent optical fiber communication.
3. Coherent optical transmission is degraded by the ________ associated with the transmitter and local oscillator lasers.
a) Phase noise
b) White noise
Explanation: Phase noise is determined by the laser line width. The phase noise associated with both the transmitter and the mid-tier section severely degrades the coherent optical transmission as well as reception.
4. ___________ improves the spectral purity of the device output and noise current.
a) Power dissipation
b) Laser line width reduction
c) Laser line width injection
d) Phase noise
Explanation: Laser line width determines the level of phase noise and long term phase stability. The reduced phase noise is obtained using narrow-line width devices. This improves the spectral purity as well as reduces the noise current.
5. ____________ is the principal cause of line width broadening in the coherent devices.
a) Electromagnetic field
b) Power dissipation
c) Injection laser phase noise
d) Gaussian noise
Explanation: Injection laser phase noise affects the system performance. The system performance considerations include receiver noise, power loss and line width broadening.
6. Which technique was started for narrowing of injection laser line widths?
a) External resonator cavity
b) Long-hauled oscillator
Explanation: Many approaches evolved in time for laser line width problem. The one which sustained and showed effects was the use of external resonator cavity in the lasers.
7. The line width tolerance is wider for heterodyne receivers.
Explanation: The laser line width requirements depend on the modulation format, coherent detection mechanism which includes the use of heterodyne and homodyne receivers. The line width tolerance is wider for heterodyne receivers when employing FSK modulation.
8. ___________ is an alternative to reduce phase noise and line width requirements.
a) Homodyne detection
b) Heterodyne detection
c) FSK modulation
d) Phase diversity reception
Explanation: The more sensitive coherent transmission techniques are most affected by phase noise problem. A specially configured reception technique called as phase diversity reception technique is used to overcome phase noise problem.
9. ______________ is the progressive spatial separation between the two polarization modes as they propagate along the fiber.
a) Fiber birefringence
b) Fiber dispersion
c) Fiber separation
d) Fiber coupling
Explanation: In a perfectly formed fiber, both modes would travel together. But, in practice, the fiber contains random manufacturing irregularities. This result in a progressive spatial separation called as fiber birefringence.
10. How many compensator devices are required to provide full polarization-state control?
Explanation: At least two compensator devices are required to provide full polarization-state control. They can be placed in either the incoming signal path or the local oscillator output path.
11. Which technique was found to be providing an infinite range of polarization control?
a) Homodyne detection
b) Fiber squeezers
c) Heterodyne detection
d) Power dissipation
Explanation: Four fiber squeezers provide an infinite range of adjustment or endless polarization control. Stress was applied to the fiber in the local oscillator path using the squeezers which are angled at 45 degrees to each other.
12. What is the main drawback of the squeezer?
a) Damages the fiber
d) Signal degradation
Explanation: The squeezers are simple to configure. The main drawback of squeezer is that they tend to damage the fiber and could not be engineered into reliable transducers for practical systems.
13. The use of balanced receiver compensates the losses due to coupling optics.
Explanation: The losses due to coupling optics and the suppression of the excess noise in the local oscillator signal are eliminated by the use of balanced receiver. It is also called as balanced-mixer receiver.
14. ___________ is the phenomenon which occurs in the single carrier systems due to small refractive index changes induced by the optical power fluctuations.
a) SBS gain
b) Self-phase modulation
c) FSK modulation
Explanation: It occurs only in the single-carrier systems. It affects the phase of the transmitted signal.
Sanfoundry Global Education & Learning Series – Optical Communications.
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