Category: Optical Communications Objective Questions

250+ TOP MCQs on Absorption and Answers

Optical Communications Multiple Choice Questions on “Absorption”.

1. The absorption of photons in a photodiode is dependent on __________
a) Absorption Coefficient α0
b) Properties of material
c) Charge carrier at junction
d) Amount of light
Answer: a
Explanation: Absorption in a photodiode is for producing carrier pans. Thus, photocurrent is dependent on absorption coefficient α 0of the light in semiconductor used to fabricate device.

2. The photocurrent in a photodiode is directly proportional to absorption coefficient.
a) True
b) False
Answer: a
Explanation: The absorption of photons produces carrier pairs. Thus, photocurrent is dependent on absorption coefficient and is given by
I = Po e(1-h)/hf(1-exp (-α rd))
Where r = Fresnel coefficient
D = width of absorption region.

3. The absorption coefficient of semiconductor materials is strongly dependent on __________
a) Properties of material
b) Wavelength
c) Amount of light
d) Amplitude
Answer: b
Explanation: In some common semiconductors, there is a variation in absorption curves for materials. It is found that they are each suitable for different wavelength and related applications. This is due to difference in band gap energies. Thus absorption coefficient depends on wavelength.

4. Direct absorption requires assistance of photon.
a) True
b) False
Answer: b
Explanation: Indirect absorption requires photon assistance resulting in conversation of energy and momentum. This makes transition probability less likely for indirect absorption than direct absorption where no photon is included.

5. In optical fiber communication, the only weakly absorbing material over wavelength band required is?
a) GaAs
b) Silicon
c) GaSb
d) Germanium
Answer: c
Explanation: The transition over wavelength band in silicon is due to indirect absorption mechanism. This makes silicon weakly absorbent over particular wavelength band.

6. The threshold for indirect absorption occurs at wavelength __________
a) 3.01 μm
b) 2.09 μm
c) 0.92 μm
d) 1.09 μm
Answer: d
Explanation: The band gap for silicon is 4.10 eV corresponding to threshold of 0.30 μm in ultraviolet. Thus it’s outside wavelength range is the one which is required.

7. The semiconductor material for which the lowest energy absorption takes place is?
a) GaAs
b) Silicon
c) GaSb
d) Germanium
Answer: d
Explanation: Germanium absorption is by indirect optical transition. The threshold for direct absorption is at 1.53μm. Below this, germanium becomes strongly absorbing to corresponding link.

8. The wavelength range of interest for Germanium is __________
a) 0.8 to 1.6 μm
b) 0.3 to 0.9 μm
c) 0.4 to 0.8 μm
d) 0.9 to 1.8 μm
Answer: a
Explanation: Germanium is used in fabrication of detectors over the whole wavelength range i.e. first and second generation 0.8 to 1.6 μm while specially taking into consideration that indirect absorption will occur up to a threshold of 1.85 μm.

9. A photodiode should be chosen with a ________________ less than photon energy.
a) Direct absorption
b) Band gap energy
c) Wavelength range
d) Absorption coefficient
Answer: d
Explanation: A photodiode selection must be made by choosing that diode having band gap energy less than photon energy corresponding to longest operating wavelength. This provides high absorption coefficient which ensures a good response and limits the thermally generated carriers to obtain low dark current with no incident light.

10. ________________ photodiodes have large dark currents.
a) GaAs
b) Silicon
c) GaSb
d) Germanium
Answer: c
Explanation: Germanium photodiodes provide narrow band gaps as compared to other semiconductor materials. This is main disadvantage with use of germanium photodiodes at shorter wavelength and thus they have large dark current.

11. For fabrication of semiconductor photodiodes, there is a drawback while considering _________________
a) GaAs
b) Silicon
c) GaSb
d) Germanium
Answer: d
Explanation: Due to drawback with germanium to be used as fabricating material, there
is an increased investigation of direct band gap III and V alloys for longer wavelength region.

12. _________________ materials are potentially superior to germanium.
a) GaAs
b) Silicon
c) GaSb
d) III – V alloys
Answer: d
Explanation: The band gap energies for III – V alloys materials can be tailored to required wavelength. This can be achieved by changing relative concentration of their constituents which results in low dark currents. Thus, III – V alloys are superior potentially to germanium.

13. ____________ alloys such as InGaAsP and GaAsSb deposited on InP and GaSb substrate.
a) Ternary
b) Quaternary
c) Gain-guided
d) III – V alloys
Answer: a
Explanation: Ternary alloys are used to fabricate photodiodes for longer wavelength band. Thus, these alloys such as InGaAsP and GaAsSb are deposited on InP and GaSb substrates.

14. _________________ alloys can be fabricated in hetero-junction structures.
a) InGaSb
b) III – V alloys
c) InGaAsP
d) GaAsSb
Answer: b
Explanation: III – V alloys enhances the high speed operations of hetero-junction structures. Thus these structures can be fabricated with III-V alloys.

15. The alloys lattice matched to InP responds to wavelengths up to 1.7μm is?
a) InAsSb
b) III – V alloys
c) InGaSb
d) InGaAs
Answer: d
Explanation: Although there were difficulties in growth of IOnGaAs alloys, the problems are now reduced. These alloys lattice matched to InP responding to wavelength around 1.7 μmare widely utilized for fabrication of photodiodes operating around 1.7μm.

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250+ TOP MCQs on Fiber Couplers and Answers

Optical Communications Multiple Choice Questions on “Fiber Couplers”.

1. When considering source-to-fiber coupling efficiencies, the ________ is an important parameter than total output power.
a) Numerical aperture
b) Radiance of an optical source
c) Coupling efficiency
d) Angular power distribution

Answer: b
Explanation: Radiance is the optical power radiated into a unit solid angle per unit emitting surface area. Since this optical power is dependent on radiance, radiance is much important factor than optical power.

2. It is a device that distributes light from a main fiber into one or more branch fibers.
a) Optical fiber coupler
b) Optical fiber splice
c) Optical fiber connector
d) Optical isolator

Answer: a
Explanation: Nowadays, requirements to divide combined optical signals for applications are increasing. Optical fiber coupler is one such device that is used for dividing and combining optical signals. It is generally used in LANs, computer networks etc.

3. Optical fiber couplers are also called as ________________
a) Isolators
b) Circulators
c) Directional couplers
d) Attenuators

Answer: c
Explanation: Optical fiber couplers are passive devices. The power transfer in couplers takes place either through the fiber core cross-section by butt jointing the fibers or by using some form of imaging optics between the fibers. It distributes light from one fiber to many fibers and hence it is also called as a directional coupler.

4. How many types of multiport optical fiber couplers are available at present?
a) Two
b) One
c) Four
d) Three

Answer: d
Explanation: Multiport optical fiber couplers are subdivided into three types. These are three and four port couplers, star couplers and wavelength division multiplexing (WDM) couplers. These couplers distribute light among the branch fibers with no scattering loss.

5. The optical power coupled from one fiber to another is limited by ____________
a) Numerical apertures of fibers
b) Varying refractive index of fibers
c) Angular power distribution at source
d) Number of modes propagating in each fiber

Answer: d
Explanation: When two fibers are coupled to each other, the optical power is limited by number of modes propagating in each fiber. For example, when a fiber propagating with 500 modes is connected to a fiber that propagates only 400 modes, then at maximum, only 80% of power is coupled into the other fiber.

6. ________ couplers combine the different wavelength optical signal onto the fiber or separate the different wavelength optical signal output from the fiber.
a) 3-port
b) 2*2-star
c) WDM
d) Directional

Answer: c
Explanation: WDM coupler is abbreviated as wavelength division multiplexing coupler. It is a category of multiport optical fiber couplers. It is designed to permit a number of different peak wavelength optical signals to be transmitted in parallel on a single fiber.

7. How many fabrication techniques are used for 3 port fiber couplers?
a) One
b) Two
c) Three
d) Four

Answer: b
Explanation: There are two fabrication techniques available for three port couplers. First is a lateral offset method which relies on the overlapping of the fiber end faces and the other is the semi-transparent mirror method. Using these techniques, three port couplers with both multimode and single-mode fibers can be fabricated.

8. Which is the most common method for manufacturing couplers?
a) Wavelength division multiplexing
b) Lateral offset method
c) Semitransparent mirror method
d) Fused bi-conical taper (FBT) technique

Answer: d
Explanation: The FBT technique is basic and simple. In this technique, the fibers are generally twisted together and then spot fused under tension such that the fused section is elongated to form a bi-conical taper structure. A three port coupler can be obtained by removing one of the input fibers.

9. Couplers insertion loss is same as that of excess loss.
a) True
b) False

Answer: b
Explanation: Excess loss is defined as the ratio of input power to output power. The insertion loss is defined as the loss obtained for a particular port-to-port optical path. Thus, the insertion loss and excess loss are different in nature.

10. A four-port multimode fiber FBT coupler has 50 μW optical power launched into port 1. The measured output power at ports 2,3 and 4 are 0.003, 23.0 and 24.5 μW respectively. Determine the excess loss.
a) 0.22 dB
b) 0.33 dB
c) 0.45 dB
d) 0.12 dB

Answer: a
Explanation: Excess loss is a ratio of power input to power output of the fiber and it is given by Excess loss = 10log10 P1/(P3+P4)
WhereP1, P3, P4 = output power at ports 1,3 and 4 resp.

11. A four-port FBT coupler has 60μW optical power launched into port one. The output powers at ports 2, 3, 4 are 0.0025, 18, and 22 μW respectively. Find the split ratio?
a) 42%
b) 46%
c) 52%
d) 45%

Answer: d
Explanation: Split ratio indicates the percentage division of optical power between the outputs ports. It is given by
Split ratio = [P3/(P3+P4)]*100%
Where P3 and P4 are output powers at ports 3 and 4 respectively.

12. How many manufacturing methods are used for producing multimode fiber star couplers?
a) Two
b) One
c) Three
d) Five

Answer: a
Explanation: The manufacturing methods of star couplers are mixer-rod technique and FBT technique. In the mixer-rod method, a thin platelet of glass is employed, which mixes light from one fiber, dividing it among the outgoing fibers. FBT method involves twisting, heating and pulling of fiber.

13. Calculate the splitting loss if a 30×30 port multimode fiber star coupler has 1 mW of optical power launched into an input port.
a) 13 dB
b) 15 dB
c) 14.77 dB
d) 16.02 dB

Answer: c
Explanation: The splitting loss is related to the number of output ports N of a coupler. It is given by-
Splitting loss (Star coupler) = 10log10N (dB).

14. A _____________ coupler comprises a number of cascaded stages, each incorporating three or four-port FBT couplers to obtain a multiport output.
a) Star
b) Ladder
c) WDM
d) Three-port

Answer: a
Explanation: A star coupler can be realized by constructing a ladder coupler. It consists of many cascaded stages. If a three-port coupler is used, then a ladder coupler does not form symmetrical star coupler. It is a useful device to achieve a multiport output with low insertion loss.

15. 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

Answer: d
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).

250+ TOP MCQs on Linear & Non-Linear Scattering Losses and Answers

Optical Communications Multiple Choice Questions on “Linear & Non-Linear Scattering Losses”.

1. Rayleigh scattering and Mie scattering are the types of _____________
a) Linear scattering losses
b) Non-linear scattering losses
c) Fiber bends losses
d) Splicing losses
Answer: a
Explanation: Rayleigh scattering and Mie scattering both result from non-ideal physical properties of the fiber. These losses may be impossible to eradicate. Linear scattering mechanisms cause the transfer of optical power contained within one propagating mode to be transferred linearly into a different mode.

2. Dominant intrinsic loss mechanism in low absorption window between ultraviolet and infrared absorption tails is ___________
a) Mie scattering
b) Rayleigh scattering
c) Stimulated Raman scattering
d) Stimulated Brillouin scattering
Answer: b
Explanation: Rayleigh scattering results from non-ideal physical properties of fiber. It is a type of linear scattering loss and is difficult or impossible to eradicate. Hence, it is termed as dominant intrinsic mechanism.

3. Rayleigh scattering can be reduced by operating at smallest possible wavelengths.
a) True
b) False
Answer: b
Explanation: Rayleigh scattering results from inhomogeneity of a random nature occurring on a small level compared with the wavelength of light. The Rayleigh scattering is inversely proportional to the wavelength. Thus, as wavelength scattering reduces.

4. The scattering resulting from fiber imperfections like core-cladding RI differences, diameter fluctuations, strains, and bubbles is?
a) Rayleigh scattering
b) Mie scattering
c) Stimulated Brillouin scattering
d) Stimulated Raman scattering
Answer: b
Explanation: Linear scattering also occurs at inhomogeneity which are comparable in size with the guided wavelength. These results from non-perfect cylindrical structures of the waveguide and hence caused by fiber imperfections.

5. Mie scattering has in-homogeneities mainly in ___________
a) Forward direction
b) Backward direction
c) All direction
d) Core-cladding interface
Answer: a
Explanation: In Mie scattering, the scattering in-homogeneities size is greater thanλ/10. Also, the scattered intensity has an angular dependence which is very large. The in-homogeneities are mainly in the direction of guided wavelength i.e. in forward direction.

6. The in-homogeneities in Mie scattering can be reduced by coating of a fiber.
a) True
b) False
Answer: a
Explanation: Mie scattering is a type of linear scattering loss. It results from fluctuations in diameter, differences in core-cladding refractive index, and differences along the fiber length. Therefore, such in-homogeneities can be reduced by controlled extrusion and coating of the fiber.

7. Raman and Brillouin scattering are usually observed at ___________
a) Low optical power densities
b) Medium optical power densities
c) High optical power densities
d) Threshold power densities
Answer: c
Explanation: Raman and Brillouin scattering mechanism are non-linear. They provide optical gain but with a shift in frequency, thus contributing to attenuation for light transmission at a particular wavelength. They can be seen at high optical power densities.

8. The phonon is a quantum of an elastic wave in a crystal lattice.
a) True
b) False
Answer: a
Explanation: A phonon is an elastic arrangement of atoms or molecules in condensed matter. This matter maybe solids or liquids. A phonon is a discrete unit of vibrational mechanical energy given by hf joules;
Where h = Planck’s constant
f = frequency.

9. A single-mode optical fiber has an attenuation of 0.3dB/km when operating at wavelength of 1.1μm. The fiber core diameter is 4μm and bandwidth is 500 MHz. Find threshold optical power for stimulated Brillouin scattering.
a) 11.20 mw
b) 12.77 mw
c) 13.08 mw
d) 12.12 mw
Answer: b
Explanation: The threshold optical power stimulated Brillouin scattering is given by-
PB = 4.4*10-3d2λ2αdBv
Where, PB = threshold optical power
d = diameter of core
λ = wavelength
αdB = attenuation.

10. 0.4 dB/km, 1.4μm, 6μm, 550MHz. Find threshold optical power for stimulated Raman scattering.
a) 1.98 W
b) 1.20 W
c) 1.18 W
d) 0.96 W
Answer: c
Explanation: The threshold optical power stimulated Raman scattering is given by-
PR = 5.9*10-2d2λαdB
Where, PR = optical power for Raman scattering
d = diameter of core
λ = wavelength
αdB = attenuation.

11. Stimulated Brillouin scattering is mainly a ___________
a) Forward process
b) Backward process
c) Upward process
d) Downward process
Answer: b
Explanation: The incident photon in Stimulated Brillouin scattering reduces a phonon of acoustic frequency as well as scattered photon. This produces an optical frequency shift which varies with the scattering angle. This frequency shift is max. in backward direction reducing to zero in forward direction making Stimulated Brillouin scattering a backward process.

12. High frequency optical phonon is generated in stimulated Raman scattering.
a) False
b) True
Answer: b
Explanation: An acoustic proton is generated in Stimulated Brillouin scattering. Raman scattering may have an optical power threshold higher than Stimulated Brillouin scattering.

13. Stimulated Raman scattering occur in ___________
a) Forward direction
b) Backward direction
c) Upward direction
d) Forward and backward direction
Answer: d
Explanation: Stimulated Raman scattering is similar to Stimulated Brillouin scattering except that a high frequency phonon is generated in Stimulated Raman scattering. Stimulated Raman scattering can occur in forward and backward direction as it has optical power threshold higher than Stimulated Brillouin scattering.

14. Stimulated Raman scattering may have an optical power threshold of may be three orders of magnitude ___________
a) Lower than Brillouin threshold
b) Higher than Brillouin threshold
c) Same as Brillouin threshold
d) Higher than Rayleigh threshold
Answer: b
Explanation: Stimulated Raman scattering involves generation of high- frequency phonon. Stimulated Brillouin scattering on the other hand, involves the generation of an acoustic phonon in a scattering process.

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