# Optical Communications Questions and Answers – Narrow – Linewidth and Wavelength – Tunable Lasers

This set of Optical Communications Quiz focuses on “Narrow – Linewidth and Wavelength – Tunable Lasers”.

1. Which of these factors are critical in affecting the system performance in the case of coherent optical fiber transmission?
a) Laser line-width and stability
b) Refractive index and index difference
d) Frequency

Explanation: The system employing intensity modulation does not consider line-width and stability as the factors of utmost importance. In coherent optical source transmission, laser line-width and stability are critical factors. These factors affect the system performance and are in the range of 0.5-1 Megahertz.

2. _______________ occurs as a result of the change in lasing frequency with gain.
a) Frequency multiplication
b) Dispersion
c) Attenuation

Explanation: Line-width broadening is a fundamental consequence of spontaneous emission process. It is related to the fluctuations in the phase of the optical fields. These phase fluctuations are due to the phase noises associated with the spontaneous emission process.

3. Laser cavity length can be extended by ___________
a) Increasing the refractive index
b) Reducing frequency
c) Introduction of external feedback
d) Using GRIN-rod lenses

Explanation: the lasers having long external cavity are referred to as LEC lasers. The extension of the laser cavity length by introduction of external feedback can be achieved by using an external cavity with a wavelength dispersive element.

4. What is the purpose of wavelength dispersive element is LEC lasers?
a) Wavelength selectivity
b) Reduction of line-width
c) Frequency multiplication
d) Avalanche multiplication

Explanation: A wavelength dispersive element is a part of the laser cavity. It is required because the long resonator structure has very closely spaced longitudinal modes which necessitates additional wavelength selectivity.

5. An effective method to reduce the line-width is to make the cavity longer.
a) True
b) False

Explanation: As the laser power increases, the device line-width decreases. The output power f laser cannot be mode arbitrarily large. Thus, the line-width is reduced by making the cavity longer. Longer cavity also enables increased wavelength selectivity.
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6. Which devices are used to modulate the external cavity in order to achieve the higher switching speeds?
a) Electromagnetic
b) Acousto-optic
c) Dispersive

Explanation: The devices are tuned mechanically to extend the cavity of laser. The disadvantage of using mechanically tuned devices is low. Thus, electro-optic devices are used to modulate the external cavity in order to achieve higher switching speeds.

7. How many techniques are used to tune monolithic integrated devices (lasers)?
a) Five
b) One
c) Two
d) Three

Explanation: There are two techniques which can be employed to tune monolithic integrated devices. In the first method, the mode selectivity of a coupled cavity structure is used. Other method is used to a refractive index change in the device cavity provided by application of an electric field.

8. _________________ laser can be produced when a coupler section is introduced between the amplifier and phase sections of a structure.
a) SG-DBR
b) GCSR
c) Y 4-shifted
d) DSM

Explanation: DBR lasers are capable of wavelength tuning. Grating assisted co-directional coupler with sampled reflector (GCSR). Laser is capable of a tuning range greater than 40 nm. It consists of a co-directional coupler between the amplifier and the phase section.

9. The rare-earth-doped fiber lasers have spectral line-width in the range of _________________
a) 0.1 to 1 nm
b) 1.2 to 1.5 nm
c) 6 to 10 nm
d) 2 to 2.3 nm

Explanation: The rare-earth-doped fiber lasers have spectral line-width in the range of 0.1 to 1 nm. These line-widths are too long for high speed transmission is possible in this range.

10. The lasing line-width of Fox-smith resonator is ____________________
a) Less than 1 MHz
b) 1 MHz
c) 2 MHz
d) Greater than 3 MHz

Explanation: Fox-smith resonator employs a fused coupled fabricated from erbium-doped fiber. Narrower spectral line-width can be obtained using a resonator. It provides favorable line-widths than semiconductor laser.

11. What is the widest tuning range obtained in optical fiber laser structure?
a) 60 nm
b) 80 nm
c) More than 100 nm
d) 100 nm

Explanation: A tuning range greater than 100 nm by using an erbium-doped photonic crystal fiber. A wider tuning range greater than 100 nm is obtained at wavelength 1.55 nm.

12. How many techniques can be used to increase the injection cavity length?
a) One
b) Two
c) Three
d) Four

Explanation: Two techniques can be used to increase the injection laser cavity length. These are using laser chips and by extending a cavity with a passive medium such as air, glass etc.

13. The mechanism which results from a refractive index change in the passive waveguide layer is called as ___________
a) Absorption
b) Spontaneous emission
c) Monolithic inversion
d) Bragg wavelength control

Explanation: A wider wavelength tuning length is obtained by separating the Bragg region in the passive waveguide and by introducing a phase region within a waveguide control mechanism provides phase control. It takes place by some changes in a passive waveguide layer.

14. How many sections are included in a sampling grating distributed Bragg-reflector laser (SG-DBR)?
a) Four
b) Five
c) Three
d) Two

Explanation: In SG-DBR laser, five sections are longitudinally integrated together on a semiconductor substrate. These five sections include two diffraction Bragg grating sections, a gain, a phase and an amplifier section.

15. Fiber based lasers provide diffraction-limited power at higher levels than solid-state laser.
a) True
b) False

Explanation: In fiber lasers, the active gain medium is doped with rare earth elements. These lasers have active regions several kilometers long and thus provide high optical gain. Solid-state lasers, on the other hand, provide diffraction limited power at lower levels.

Sanfoundry Global Education & Learning Series – Optical Communications.

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