# Optical Communications Questions and Answers – The Optical Transmitter Circuit

This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “The Optical Transmitter Circuit”.

1. _____________ must be operated in stimulated emission region.
a) Injection laser
b) LED’s
c) Detector

Explanation: Injection laser is a threshold device. In stimulated emission region, continuous optical output power levels are in the range of 1 to 10mW.

2. Coherent radiation is relatively __________
a) Parabolic
b) Elliptic
c) Directional
d) Rectangular

Explanation: Most of the light output is coupled into optical fibre. This is because of the isotropic distribution of narrow-line width, coherent radiation is directional.

3. _____________ are capable of launching powers between 0.5 and several mW.
a) LED’s
b) Injection laser
c) Attenuator
d) Reflector

Explanation: Coupling efficiency up to 30% may be obtained by placing a fiber close to laser mirror. These can approach 90% with suitable lens and optical coupling arrangements. So they can launch 0.5 to several mW of optical power into fiber.

4. LED’s display good linearity.
a) True
b) False

Explanation: LED’s appear to be suited to analog transmission. This is because of its output which is directly proportional to the drive current.

5. Which behaviour may prove as a limitation for injection lasers and LED’s?
a) Isotropic
c) Thermal
d) Photosensitive

Explanation: The thermal behaviour of the injection lasers and the LED’s limits their operation within the optical transmitter. The main problem is caused by the variation of injection laser threshold current.

6. Optical output power from an LED is directly proportional to the device junction temperature.
a) False
b) True

Explanation: Output power is dependent on the junction temperature in case of LED’s. Most LED’s exhibit a decrease in the optical output power following an increase in junction temperature.

7. _____________ from the LED is dependent on the effective minority carrier lifetime in the semiconductor material.
a) Spontaneous emission
b) Stimulated emission
c) Absorption
d) Diffusion

Explanation: The speed of the response of the LED is dictated by the respective emission mechanism. Spontaneous emission is related to the carrier lifetime and hence dictating the speed of response.

8. The _________ of the LED is twice that of the effective minority carrier lifetime.
a) Dwell time
b) Reflection scatters
c) Sensitivity
d) Rise time

Explanation: The response of the optical fiber source is specified in terms of the rise time. This rise time is reciprocally related to the device frequency response.

9. The finite spectral width of the optical source causes ___________
a) Depletion
b) Frequency burst
d) Efficient reflection

Explanation: The finite spectral width causes pulse broadening due to material dispersion on an optical fiber communication link. This results in a limitation on the bandwidth-length product.

10. The coherent emission from an injection laser has a line width of ________
a) 2 nm
b) 3nm
c) 8 nm
d) 1nm

Explanation: An optical source such as an injection laser is a narrow line width device as compared to the LED. It has a narrow line width of 1 nm or less.

11. Extinction ratio is denoted by symbol __________
a) ε
b) σ
c) β
d) ρ

Explanation: Extinction ratio is defined as the ratio of the optical energy emitted in the 0 bit period to that emitted during the 1 bit period. It is denoted by ε.

12. The use of low impedance driving circuit may increase _____________
a) Noise
b) Width
c) Intensity
d) Switching speed

Explanation: Pulse shaping is usually required to increase the switching speed. However, increased switching speed may be obtained from an LED without a speed-up element by use of a low-impedance driving circuit.

Sanfoundry Global Education & Learning Series – Optical Communications.

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