1. A number of three-port single-mode fiber couplers are used in the fabrication of a ladder coupler with 16 output ports. The three-port couplers each have an excess loss of 0.2 dB along with a splice loss of 0.1 dB at the interconnection of each stage. Determine the excess loss.
a) 1.9 dB
b) 1.4 dB
c) 0.9 dB
d) 1.1 dB
Explanation: The number of stages M within the ladder design is given by 2M=16. Hence M=4.
Thus, excess loss is given by-
Excess loss = (M×loss in each 3-port coupler) + (Number of splices×Loss in each stage)
Where number of splices = 3 (as the value of M is equal to 4).
2. 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
Explanation: An Optical source is an active component in an optical fiber communication system. It converts electrical energy into optical energy and allows the light output to be efficiently coupled into the Optical fiber.
3. How many types of sources of optical light are available?
a) One
b) Two
c) Three
d) Four
Explanation: Three main types of optical light sources are available. These are wideband sources, monochromatic incoherent sources. Ideally the optical source should be linear
4. 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
Explanation: In the given equation, difference in the energy E is directly proportional to the absorbed frequency (f) where h is used as a constant and is called as Planck’s constant. The value of h is measured in Joules/sec & is given by-
h = 6.626×10-34Js.
5. The radiation emission process (emission of a proton at frequency) can occur in __________ ways.
a) Two
b) Three
c) Four
d) One
Explanation: The emission process can occur in two ways. First is by spontaneous emission in which the atom returns to the lower energy state in a random manner. Second is by stimulated emission where the energy of a photon is equal to the energy difference and it interacts with the atom in the upper state causing it to return to the lower state along with the creation of a new photon.
6. Which process gives the laser its special properties as an optical source?
a) Dispersion
b) Stimulated absorption
c) Spontaneous emission
d) Stimulated emission
Explanation: In Stimulated emission, the photon produced is of the same energy to the one which cause it. Hence, the light associated with stimulated photon is in phase and has same polarization. Therefore, in contrast to spontaneous emission, coherent radiation is obtained. The coherent radiation phenomenon in laser provides amplification thereby making laser a better optical source than LED.
7. 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
Explanation: The ratio of the stimulated emission rate to the spontaneous emission rate is given by-
Stimulated emission rate/ Spontaneous emission rate = 1/exp (hf/KT)-1.
8. 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
Explanation: Under the conditions of thermal equilibrium, the lower energy level contains more atoms than the upper level. To achieve optical amplification, it is required to create a non-equilibrium distribution such that the population of upper energy level is more than the lower energy level. This process of excitation of atoms into the upper level is achieved by using an external energy source and is called as pumping.
9. __________________ 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
Explanation: Laser emits coherent radiation of one or more discrete wavelength. Lasers produce coherent light through a process called stimulated emission. Light amplification is obtained through stimulated emission. Continuation of this process creates avalanche multiplication.
10. 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
Explanation: The number of longitudinal modes is given by-
q = 2nL/λ
Where
q = Number of longitudinal modes
n = Refractive index
L = Length of the crystal
λ = Peak emission wavelength.