Optical Sources : Basic Concepts

Optical Communications

This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “Optical Sources : Laser Basics”.

1. A device which converts electrical energy in the form of a current into optical energy is called as ___________
a) Optical source
b) Optical coupler
c) Optical isolator
d) Circulator

2. How many types of sources of optical light are available?
a) One
b) Two
c) Three
d) Four

3. The frequency of the absorbed or emitted radiation is related to difference in energy E between the higher energy state E2 and the lower energy state E1. State what h stands for in the given equation?

E = E2 - E1 = hf

a) Gravitation constant
b) Planck’s constant
c) Permittivity
d) Attenuation constant

4. The radiation emission process (emission of a proton at frequency) can occur in __________ ways.
a) Two
b) Three
c) Four
d) One

5. Which process gives the laser its special properties as an optical source?
a) Dispersion
b) Stimulated absorption
c) Spontaneous emission
d) Stimulated emission

6. An incandescent lamp is operating at a temperature of 1000K at an operating frequency of 5.2×1014 Hz. Calculate the ratio of stimulated emission rate to spontaneous emission rate.
a) 3×10-13
b) 1.47×10-11
c) 2×10-12
d) 1.5×10-13

7. The lower energy level contains more atoms than upper level under the conditions of ________________
a) Isothermal packaging
b) Population inversion
c) Thermal equilibrium
d) Pumping

8. __________________ in the laser occurs when photon colliding with an excited atom causes the stimulated emission of a second photon.
a) Light amplification
b) Attenuation
c) Dispersion
d) Population inversion

9. A ruby laser has a crystal of length 3 cm with a refractive index of 1.60, wavelength 0.43 μm. Determine the number of longitudinal modes.
a) 1×102
b) 3×106
c) 2.9×105
d) 2.2×105

10. A semiconductor laser crystal of length 5 cm, refractive index 1.8 is used as an optical source. Determine the frequency separation of the modes.
a) 2.8 GHz
b) 1.2 GHz
c) 1.6 GHz
d) 2 GHz

11. Doppler broadening is a homogeneous broadening mechanism.
a) True
b) False

12. An injection laser has active cavity losses of 25 cm-1 and the reflectivity of each laser facet is 30%. Determine the laser gain coefficient for the cavity it has a length of 500μm.
a) 46 cm-1
b) 51 cm-1
c) 50 cm-1
d) 49.07 cm-1

13. Longitudinal modes contribute only a single spot of light to the laser output.
a) True
b) False

14. Considering the values given below, calculate the mode separation in terms of free space wavelength for a laser. (Frequency separation = 2GHz, Wavelength = 0.5 μm)
a) 1.4×10-11
b) 1.6×10-12
c) 1×10-12
d) 6×10-11

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