# Optical Communications Questions and Answers – The Semiconductor Injection Laser

This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “The Semiconductor Injection Laser”.

1. Stimulated emission by recombination of injected carriers is encouraged in __________
a) Semiconductor injection laser
b) Gas laser
c) Chemist laser
d) Dye laser

Explanation: Stimulated emission by use of optical cavity in crystal structure is used in semiconductor injection diodes. This provides the feedback of protons which gives injection laser many important advantages over other sources like LED’s.

2. In semiconductor injection laser, narrow line bandwidth is of the order?
a) 1 nm or less
b) 4 nm
c) 5 nm
d) 3 nm

Explanation: A narrow line bandwidth of order 1 nm or less is used. This narrow bandwidth is useful in minimizing the effects of material dispersion.

3. Injection laser have a high threshold current density of __________
a) 104Acm-2 and more
b) 102Acm-2
c) 10-2Acm-2
d) 10-3Acm-2

Explanation: Injection laser have a high threshold current density of 104Acm-2due to lack of matter and in-efficient light sources. These high current densities are largely utilized in pulse mode in order to minimize junction and thus avert damage.

4. ηT is known as slope quantum efficiency.
a) True
b) False

Explanation: ηD is known as slope quantum efficiency. It gives a measure rate of optical output power change with current and thus it determines slope of output characteristics in the region. So, ηT is referred to as slope quantum efficiency.

5. The total efficiency of an injection laser with GaAs active region is 12%. The applied voltage is 3.6 V and band gap energy for GaAs is 2.34 eV. Determine external power efficiency.
a) 7.8 %
b) 10 %
c) 12 %
d) 6 %

Explanation: The total external power efficiency is defined as
η = ηT(Eq/V)*100
= 0.12 (2.34/3.6) *100
= 7.8 %.
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6. In a DH laser, the sides of cavity are formed by _______________
a) Cutting the edges of device
b) Roughening the edges of device
c) Softening the edges of device
d) Covering the sides with ceramics

Explanation: In a DH laser, the sides of cavity are formed by the roughening edges of the device. This is done so as to reduce the unwanted emission in these directions and limit the number of horizontal transversal modes.

7. A particular laser structure is designed so that the active region extends the edges of devices.
a) True
b) False

Explanation: Laser structures are particularly designed so that the active region does not extend beyond the edges. This is done to reduce problems like difficult heat sinking, lasing from multiple filament in wide active areas, unsuitable light output geometry for efficient coupling and also to reduce required threshold current.

8. Gain guided laser structure are __________
a) Chemical laser
b) Gas laser
c) DH injection laser
d) Quantum well laser

Explanation: DH injection lasers are known as gain guided laser structure. This is because the optical mode distribution along the junction plane is decided by optical gain.

9. Laser modes are generally separated by few __________
a) Tenths of micrometer
b) Tenths of nanometer
c) Tenths of Pico-meter
d) Tenths of millimeter

Explanation: The spacing in between modes is about a few tenths of nanometer. The spacing of the modes depends on optical cavity length where each one corresponds to an integral number of lengths.

10. The spectral width of emission from the single mode device is __________
a) Smaller than broadened transition line-width
b) Larger than broadened transition line-width
c) Equal the broadened transition line-width
d) Cannot be determined

Explanation: Single mode device has a smaller spectral width as compared to that of broadening transition line-width. This is because for a single-mode operation, the laser optical output must have only a single longitudinal and single transverse mode.

11. Single longitudinal mode operation is obtained by __________
a) Eliminating all transverse mode
b) Eliminating all longitudinal modes
c) Increasing the length of cavity
d) Reducing the length of cavity

Explanation: Single longitudinal mode operation is obtained by reducing the length L of cavity. Length must be reduced until the frequency separation of adjacent modes is given in the equation
δf = c/2nL is larger than gain curve. Then only single mode falling in transition line width will oscillate in laser cavity.

12. A correct DH structure will restrict the vertical width of waveguide region is?
a) 0.5μm.
b) 0.69 μm
c) 0.65 μm
d) Less than 0.4 μm

Explanation: The vertical width DH structure should be less than 0.4μm. This allows only fundamental transverse mode and removes any interference of higher order transverse modes on emitted longitudinal waves.

13. The external power efficiency of an injection laser with a GaAs is 13% having band gap energy of 1.64 eV. Determine external power efficiency.
a) 0.198
b) 0.283
c) 0.366
d) 0.467

Explanation: The external power efficiency of an injection laser is given by
ηep = ηT (Eq/V)*100
ηT = ηep/100 (v/Eg)
= 13/100 (2.5/1.64)
= 0.198.

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