1. 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
Explanation: The modes of laser are separated by a frequency internal δf and this separation is given by-
δf = c/2nL
Where
c = velocity of light
n = Refractive index
L = Length of the crystal.
2. Doppler broadening is a homogeneous broadening mechanism.
a) True
b) False
Explanation: Doppler broadening is a inhomogeneous broadening mechanism. In this broadening, the individual groups of atoms have different apparent resonance frequencies. Atomic collisions usually provide homogeneous broadening as each atom in collection has same resonant frequency and spectral spread.
3. 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
Explanation: The laser gain coefficient is equivalent to the threshold gain per unit length and is given by –
gth = α + 1/L ln (1/r)
Where
α = active cavity loss
L = Length of the cavity
r = reflectivity.
4. Longitudinal modes contribute only a single spot of light to the laser output.
a) True
b) False
Explanation: Laser emission includes the longitudinal modes and transverse modes. Transverse modes give rise to a pattern of spots at the output. Longitudinal modes give only a spot of light to the output.
5. 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
Explanation: The mode separation in terms of free space wavelength is given by-
δλ = λ2/c δf
Where
δf = frequency separation
λ = wavelength
c = velocity of light.
6. A perfect semiconductor crystal containing no impurities or lattice defects is called as __________
a) Intrinsic semiconductor
b) Extrinsic semiconductor
c) Excitation
d) Valence electron
Explanation: An intrinsic semiconductor is usually un-doped. It is a pure semiconductor. The number of charge carriers is determined by the semiconductor material properties and not by the impurities.
7. The energy-level occupation for a semiconductor in thermal equilibrium is described by the __________
a) Boltzmann distribution function
b) Probability distribution function
c) Fermi-Dirac distribution function
d) Cumulative distribution function
Explanation: For a semiconductor in thermal equilibrium, the probability P(E) that an electron gains sufficient thermal energy at an absolute temperature so as to occupy a particular energy level E, is given by the Fermi-Dirac distribution. It is given by-
P(E) = 1/(1+exp(E-EF/KT))
Where K = Boltzmann constant, T = absolute temperature, EF = Fermi energy level.
8. What is done to create an extrinsic semiconductor?
a) Refractive index is decreased
b) Doping the material with impurities
c) Increase the band-gap of the material
d) Stimulated emission
Explanation: An intrinsic semiconductor is a pure semiconductor. An extrinsic semiconductor is obtained by doping the material with impurity atoms. These impurity atoms create either free electrons or holes. Thus, extrinsic semiconductor is a doped semiconductor.
9. The majority of the carriers in a p-type semiconductor are __________
a) Holes
b) Electrons
c) Photons
d) Neutrons
Explanation: The impurities can be either donor impurities or acceptor impurities. When acceptor impurities are added, the excited electrons are raised from the valence band to the acceptor impurity levels leaving positive charge carriers in the valence band. Thus, p-type semiconductor is formed in which majority of the carriers are positive i.e. holes.
10. _________________ is used when the optical emission results from the application of electric field.
a) Radiation
b) Efficiency
c) Electro-luminescence
d) Magnetron oscillator
Explanation: Electro-luminescence is encouraged by selecting an appropriate semiconductor material. Direct band-gap semiconductors are used for this purpose. In band-to-band recombination, the energy is released with the creation of photon. This emission of light is known as electroluminescence.