TSSM's

PADMABHOOSHAN VASANTDADA PATIL INSTITUTE OF TECHNOLOGY,

BAVDHAN, PUNE-21
DEPARTMENT OF ELECTRONICS & TELECOMMUNICATION ENGINEERING

MCQs with answers

Class :BE(E&Tc) Sub: BCS

Part-I Optical Fiber Communication

1. Multimode step index fiber has

a) Large core diameter & large numerical aperture
b) Large core diameter and small numerical aperture
c) Small core diameter and large numerical aperture
d) Small core diameter & small numerical aperture

Answer:a
Explanation: Multimode step-index fiber has large core diameter and large numerical
aperture. These parameters provides efficient coupling to inherent light sources such as
LED’s.

2. A typically structured glass multimode step index fiber shows as variation of attenuation
in range of
a) 1.2 to 90 dB km-1at wavelength 0.69μm
b) 3.2 to 30 dB km-1at wavelength 0.59μm
c) 2.6 to 50 dB km-1at wavelength 0.85μm
d) 1.6 to 60 dB km-1at wavelength 0.90μm

Answer: c
Explanation: A multimode step index fibers show an attenuation variation in range of 2.6 to
50dBkm-1. The wide variation in attenuation is due to the large differences both within and
between the two overall preparation methods i.e. melting and deposition.

3. A multimode step index fiber has a large core diameter of range

a) 100 to 300 μm
 b) 100 to 300 nm
c) 200 to 500 μm
d) 200 to 500 nm

Answer: a
Explanation: A multimode step index fiber has a core diameter range of 100 to 300μm. This
is to facilitate efficient coupling to inherent light sources.

4. Multimode step index fibers have a bandwidth of

a) 2 to 30 MHz km
b) 6 to 50 MHz km
c) 10 to 40 MHz km
d) 8 to 40 MHz km

Answer: b
Explanation: Multimode step index fibers have a bandwidth of 6 to 50 MHz km. These fibers
with this bandwidth are best suited for short -haul, limited bandwidth and relatively low-cost
application.

5. Multimode graded index fibers are manufactured from materials with

a) Lower purity
b) Higher purity than multimode step index fibers.
c) No impurity
d) Impurity as same as multimode step index fibers.
Answer: b
Explanation: Multimode graded index fibers have higher purity than multimode step
index fiber. To reduce fiber losses, these fibers have more impurity.
6. The performance characteristics of multimode graded index fibers are
a) Better than multimode step index fibers.
b) Same as multimode step index fibers.
c) Lesser than multimode step index fibers
d) Negligible
Answer: a
Explanation: Multimode graded index fibers use a constant grading factor. Performance
 characteristics of multimode graded index fibers are better than those of multimode step
index fibers due to index graded and lower attenuation.
7. Multimode graded index fibers have overall buffer jackets same as multimode step index
fibers but have core diameters
a) Larger than multimode step index fibers.
b) Smaller than multimode step index fibers.
c) Same as that of multimode step index fibers.
d) Smaller than single mode step index fibers.

Answer: b
Explanation: Multimode graded index fibers have smaller core diameter than multimode step
index fibers. A small core diameter helps the fiber gain greater rigidity to resist bending.

8. Multimode graded index fibers with wavelength of 0.85μm have numerical aperture of
0.29 have core/cladding diameter of
a) 62.5 μm/125 μm
b) 100μm/140 μm
c) 85 μm/ 125 μm
d) 50 μm/ 125μm

Answer: b
Explanation: Multimode graded index fibers with numerical aperture 0.29 having a core /
cladding diameter of 100μm/ 140μm. They provide high coupling frequency LED’s at a
wavelength of 0.85 μm and have low cost. They are also used for short distance application.

9. Multimode graded index fibers use incoherent source only. State whether the following
statement is true or false.
a) True
b) False

Answer: b
Explanation: Multimode graded index fibers are used for short haul and medium to high
bandwidth applications. Small haul applications require LEDs and low accuracy lasers. Thus
either incoherent or incoherent sources like LED’s or injection laser diode are used
10. In single mode fibers, the most beneficial index profile is
a) Step index
b) Graded index
c) Step and graded index
d) Coaxial cable

Answer: b
Explanation: In single mode fibers, graded index profile is more beneficial as compared to
step index. This is because graded index profile provides dispersion-modified-single mode
fibers.

11. The fibers mostly not used nowadays for optical fiber communication system are
a) Single mode fibers
b) Multimode step fibers
c) Coaxial cables
d) Multimode graded index fibers

Answer: a
Explanation: Single mode fibers are used to produce polarization maintaining fibers which
make them expensive. Also the alternative to them are multimode fibers which are complex
but accurate. So, single-mode fibers are not generally utilized in optical fiber communication.

12. Single mode fibers allow single mode propagation; the cladding diameter must be at least
a) Twice the core diameter
b) Thrice the core diameter
c) Five times the core diameter
d) Ten times the core diameter

Answer: d
Explanation: The cladding diameter in single mode fiber must be ten times the core diameter.
Larger ratios contribute to accurate propagation of light. These dimension ratios must be
there so as to avoid losses from the vanishing fields.

13. A fiber which is referred as non-dispersive shifted fiber is

a) Coaxial cables
 b) Standard single mode fibers
c) Standard multimode fibers
d) Non zero dispersion shifted fibers

Answer: b
Explanation: A standard single mode fiber having step index profile is known as non-
dispersion shifted fiber. As these fibers have a zero dispersion wavelength of 1.31μm and so
are preferred for single-wavelength transmission in O-band.

14. Standard single mode fibers (SSMF) are utilized mainly for operation in
a) C-band
b) L-band
c) O-band
d) C-band and L-band

Answer: c
Explanation: SSMFs are utilized for operation in O-band only. It shows high dispersion in
range of 16 to 20ps/nm/km in C-band and L-band. So SSMFs are used in O-band.

15. Fiber mostly suited in single-wavelength transmission in O-band is

a) Low-water-peak non dispersion-shifted fibers
b) Standard single mode fibers
c) Low minimized fibers
d) Non-zero-dispersion-shifted fibers

Answer: b
Explanation: Standard single mode fibers with a step index profile is called non dispersion
shifted fiber and it is particularly used for single wavelength transmission in O-band and as if
has a zero-dispersion wavelength at 1.31μm.

16. When optical fibers are to be installed in a working environment, the most important
parameter to be considered is
a) Transmission property of the fiber
b) Mechanical property of the fiber
c) Core cladding ratio of the fiber
d) Numerical aperture of the fiber
Answer: b
Explanation: Nowadays, optical fibers are used alternatively to electric transmission lines.
They are installed safely and maintained in all environments including underground areas.
This requires mechanical strengthening of fibers in order to ensure proper transmission.
17.It is not important to cover these optical fibers required for transmission. State whether the
given statement is true or false.
a) True
b) False
Answer: b
Explanation: Unprotected optical fibers have number of losses regarding its strength and
durability. Bare glass fibers are brittle and have small cross-section area that makes them
highly susceptible to damages while handling and maintenance. Thus, to improve tensile
strength, optical fibers should be covered by surrounding them with number of protective
layers.

17. Optical fibers for communication use are mostly fabricated from
a) Plastic
b) Silica or multicomponent glass
c) Ceramics
d) Copper
Answer: b
Explanation: Silica or a compound of glass are brittle and have almost perfect elasticity
until reaching their breaking point. Strength of these materials is high. Thus, optical
fibers are fabricated from these materials.
18. ____________ results from small lateral forces exerted on the fiber during the cabling
process.
a) Attenuation
b) Micro-bending
c) Dispersion
d) Stimulated Emission
Answer: b
Explanation: Optical fibers must be designed so that the transmission characteristics of the
fiber are maintained after the cabling process. The main problem which occurs in the cabling
process is the meandering of the axis of the fiber core on a microscopic scale within the cable
form. This phenomenon is called as micro-bending.
19. Microscopic meandering of the fiber core axis that is micro-bending is caused due to
a) Environmental effects
b) Rough edges of the fiber
c) Large diameter of core
d) Polarization

Answer: a
Explanation: Micro-bending can be generated at any stage during manufacturing process,
cable installation process or during service. This is mainly due to environmental effects,
mainly varying temperatures causing differential expansion or contraction.
20 How many forms of modal power distribution are considered?
a) One
b) Two
c) Three
d) Four
Answer: b
Explanation: Two forms of modal power distribution are considered. The first form is seen
when a fiber is excited by a diffuse Lambertian source, and is called as fully filled mode
distribution. The second form occurs when, due to mode coupling and attenuation, the
distribution of optical power becomes invariant with the distance of propagation along the
fiber, and is called as steady-state mode distribution.

20. What does micro-bending losses depend on?

a) Core material
b) Refractive index
c) Diameter
d) Mode and wavelength

Answer: d
Explanation: Micro-bending losses cause differential expansion or contraction. These losses
are mode dependent. The number of modes is a function inverse to the wavelength of the
transmitted light and thus micro-bending losses are wavelength dependent.
21. The fiber should be________________ to avoid deterioration of the optical transmission
characteristics resulting from mode-coupling-induced micro-bending.
a) Free from irregular external pressure
b) Coupled with plastic
c) Large in diameter
d) Smooth and in a steady state

Answer: a
Explanation: Micro-bending losses results from environmental effects such as temperature
variation. The irregular external pressure deteriorates the quality of transmission through the
fiber. Thus, controlled coating and cabling of the fiber is essential in order to reduce the
cabled fiber attenuation.

22. The diffusion of hydrogen into optical fiber affects the ______________
a) Transmission of optical light in the fiber
b) Spectral attenuation characteristics of the fiber
c) Core of the fiber
d) Cladding of the fiber

Answer: b
Explanation: The hydrogen absorption by an optical fiber increases optical fiber losses. It
forms absorption peaks where the hydrogen diffuses into interstitial spaces in the glass. At
high temperatures, these losses can increase and reduced if the hydrogen source is removed.
23 __________ can induce a considerable amount of attenuation in optical fibers.
a) Micro-bending
b) Dispersion
c) Diffusion of hydrogen
d) Radiation Exposure
Answer: d
Explanation: The optical transmission characteristics of the fiber cables can be degraded by
exposure to nuclear radiation. The nature of this attenuation depends upon fiber structures,
optical intensity, wavelength etc. The radiation-induced attenuation comprises both
 permanent and temporary components which makes the exposure irreversible and reversible
respectively.

23. A measure of amount of optical fiber emitted from source that can be coupled into a fiber
is termed as
a) Radiance
b) Angular power distribution
c) Coupling efficiency
d) Power-launching
Answer: c
Explanation: Coupling efficiency depends upon the type of fiber attached to the source
which should consider the parameters such as numerical aperture, core size, R.I. profile,
radiance, core-cladding index difference. All these parameters relate to the performance
of the fibers determined by power coupled into the fiber to power emitted by the source.
This is called coupling efficiency ηwhich is given by
η= PF/Ps
Where PF= power coupled into the fiber
Ps= power emitted by the source.
24. The ratio r = (n1 – n)/ (n1 – n) indicates
a) Fresnel reflection
b) Reflection coefficient
c) Refraction coefficient
d) Angular power distribution coefficient
Answer: b
Explanation: The ratio, r = (n1-n)/ (n1-n) is known as Reflection coefficient. It relates the
amplitude of the reflected ray to the amplitude of the incident wave.

25. A particular GaAs fiber has a Fresnel reflection magnitude of 17.6% i.e. 0.176. Find the
power loss between the source and the fiber?
a) 0.86 dB
b) 0.78 dB
c) 0.84 dB
d) 0.83 dB
Answer: c
Explanation: The optical losses in decibels at the joint is given by
Loss = -10log10(1-r)
Where L = loss due to Fresnel reflection
R = magnitude of Fresnel reflection.
26. Two joined step index fibers are perfectly aligned. What is the coupling loss of numerical
aperture are NAR= 0.26 for emitting fiber?
a) -0.828 dB
b) -0.010 dB
c) -0.32 dB
d) 0.32 dB
Answer: b
Explanation: Coupling loss for two joined step index fibers is given by
LF(NA)= -10 log (NAR/NAE) 2
Where LF= coupling loss
NAR= Numerical aperture of receiving fiber
NAE= Numerical aperture of emitting fiber.
27. Two joined graded index fibers that are perfectly aligned have refractive indices αR=1.93for
receiving fiber αE= 2.15 for emitting fiber. Calculate the coupling loss
a) 0.23 dB
b) 0.16 dB
c) 0.82 dB
d) 0.76 dB
Answer: a
Explanation: Coupling loss for two joined and perfectly aligned graded index fiber is given by
LF(α)= -10log10αR(αE+2) / αE(αR+2)
Where LF(α)= Coupling loss
αR= refractive index of receiving fiber
αE= refractive index of emitting fiber.
28. How many types of misalignments occur when joining compatible fiber?
a) One
b) Two
c) Five
d) Three
Answer: d
Explanation: There are three layers of fiber misalignments and they are: Longitudinal, lateral and
angular misalignments.
29. Losses caused by factors such as core-cladding diameter, numerical aperture, relative
refractive index differences, different refractive index profiles, fiber faults are known as
a) Intrinsic joint losses
b) Extrinsic losses
c) Insertion losses
d) Coupling losses
Answer: a
Explanation: There are inherent connection problems while joining fibers. These connection
problem cause different losses in the fibers and are called as Intrinsic joint losses.
30. A permanent joint formed between two different optical fibers in the field is known as a
a) Fiber splice
b) Fiber connector
c) Fiber attenuator
d) Fiber dispersion
Answer: a
Explanation: The jointing of two individual fibers is called as fiber splicing. It is used to
establish long-haul optical fiber links by joining two small length fibers.
31. How many types of fiber splices are available?
a) One
b) Two
c) Three
d) Four
Answer: b
Explanation: Splices are divided into two types depending upon the splicing technique used.
These are fusion splicing (welding) and mechanical splicing.
32. The insertion losses of the fiber splices are much less than the Fresnel reflection loss at a
butted fiber joint. State whether the given statement is true or false.
a) True
b) False
Answer: a
Explanation: The Fresnel reflection loss is usually more because there is no large step change in
refractive index with the fusion splice as it forms a continuous fiber connection. Also, some
method of index matching tends to be utilized with mechanical splices.
.33. What is the main requirement with the fibers that are intended for splicing?
a) Smooth and oval end faces
b) Smooth and square end faces
c) Rough edge faces
d) Large core diameter
Answer: b
Explanation: A curved mandrel is used which cleaves the fiber to achieve end preparation. The
edges must be smooth and have square face at the end for splicing purpose.
34. The heating of the two prepared fiber ends to their fusing point with the application of
required axial pressure between the two optical fibers is called as
a) Mechanical splicing
b) Fusion splicing
c) Melting
d) Diffusion
Answer: b
Explanation: Fusion splicing is also called as welding. It refers to the welding of two fiber ends.
It is essential for fusion splicing that the fiber ends are adequately positioned and aligned in order
to achieve good continuity of the transmission medium at the junction point.
35. Demountable fiber connectors are more difficult to achieve than optical fiber splices. State
whether the given statement is true or false.
a) True
b) False
Answer: a
Explanation: Fiber connectors must maintain tolerance requirements similar to splices in order to
couple light efficiently between the fibers. Also, fiber connectors must accomplish this in a
removable fashion. The connector design must allow repeated connection and disconnection
without any problems of fiber alignment.
36. How many categories of fiber connectors exist?
a) One
b) Three
c) Two
d) Four
Answer: c
Explanation: Fiber connectors are separated into two broad categories. They are butt-jointed
connectors and expanded beam connectors. Butt-jointed connectors rely upon alignment of the
two fiber ends butted to each other whereas expanded beam connectors uses interposed optics at
the joint.
37. 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.
38. 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.
39. 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.
40. 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.
41. 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.
42. ________ 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.
43. The most common method for manufacturing couplers is ____________
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.
44. Couplers insertion loss is same as that of excess loss. State whether the given statement is
true or false.
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.
45.A four-port multimode fiber FBT coupler has 50 μWoptical 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.
46. 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.
47. 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).
48. 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.
50. It is a passive device which allows the flow of optical signal power in only one direction and
preventing reflections in the backward direction.
a) Fiber slice
b) Optical fiber connector
c) Optical isolator
d) Optical coupler
Answer: c
Explanation: Ideally, an optical isolator transmits the signal power in the desired forward
direction. Material imperfections in the isolator medium generate backward reflections. Optical
isolators can be implemented by using FBG.
51. Which feature of an optical isolator makes it attractive to use with optical amplifier?
a) Low loss
b) Wavelength blocking
c) Low refractive index
d) Attenuation
Answer: b
Explanation: Optical isolators are made using FBGs. Since FBGs are wavelength dependent, the
optical isolators can be designed to allow or block the optical signal at particular wavelength.
The wavelength blocking feature makes the optical isolator a very attractive device for use with
optical amplifier in order to protect them from backward reflections.
52. A device which is made of isolators and follows a closed loop path is called as a
____________
a) Circulator
b) Gyrator
c) Attenuator
d) Connector
Answer: a
Explanation: Isolator can be connected together to form multiport devices. A circulator is formed
from isolators connected together to form a closed circular path. In circulator, the signal
continues to travel in closed loop and does not get discarded unlike isolator.
53. 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
Answer: a
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.
54. How many types of sources of optical light are available?
a) One
b) Two
c) Three
d) Four
Answer: c
Explanation: Three main types of optical light sources are available. These are wideband
sources, monochromatic incoherent sources. Ideally the optical source should be linear.
55. Which process gives the laser its special properties as an optical source?
a) Dispersion
b) Stimulated absorption
c) Spontaneous emission
d) Stimulated emission
Answer: d
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.
56. 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
Answer: c
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.
57. __________________ 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
Answer: a
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.
58. Population inversion is obtained at a p-n junction by
a) Heavy doping of p-type material
b) Heavy doping of n-type material
c) Light doping of p-type material
d) Heavy doping of both p-type and n-type material
Answer: d
Explanation: Population inversion at p-n junction is obtained by heavy doping of both p-type and
n-type material. Heavy p-type doping with acceptor impurities causes a lowering of the Fermi-
level between the filled and empty states into the valence band. Similarly n-type doping causes
Fermi-level to enter the conduction band of the material.
59. The absence of _______________ in LEDs limits the internal quantum efficiency.
a) Proper semiconductor
b) Adequate power supply
c) Optical amplification through stimulated emission
d) Optical amplification through spontaneous emission
Answer: c
Explanation: The ratio of generated electrons to the electrons injected is quantum efficiency. It is
greatly affected if there is no optical amplification through stimulated emission. Spontaneous
emission allows ron-radiative recombination in the structure due to crystalline imperfections and
impurities.
60. Determine the internal quantum efficiency generated within a device when it has a radiative
recombination lifetime of 80 ns and total carrier recombination lifetime of 40 ns.
a) 20 %
b) 80 %
c) 30 %
d) 40 %
Answer: b
Explanation: The internal quantum efficiency of device is given by
ηint= τ/τr= 40/80 ×100 = 80%
Where
τ= total carrier recombination lifetime
τr= radiative recombination lifetime.
61. The Lambertian intensity distribution __________ the external power efficiency by some
percent.
a) Reduces
b) Does not affects
c) Increases
d) Have a negligible effect
Answer: a
Explanation: In Lambertian intensity distribution, the maximum intensity I 0 is perpendicular to
the planar surface but is reduced on the sides in proportion to the cosine of θ i.e. viewing angle as
apparent area varies with this angle. This reduces the external power efficiency. This is because
most of the light is tapped by total internal refraction when radiated at greater than the critical
angle for crystal air interface.
62. For a GaAs LED, the coupling efficiency is 0.05. Compute the optical loss in decibels.
a) 12.3 dB
b) 14 dB
c) 13.01 dB
d) 14.6 dB
Answer: c
Explanation: The optical loss in decibels is given by-
Loss= -10log10 ηc
Where,
ηc= coupling efficiency.
63. The amount of radiance in planer type of LED structures is
a) Low
b) High
c) Zero
d) Negligible
Answer: a
Explanation: Planer LEDs are fabricated using liquid or vapor phase epitaxial processes. Here p-
type is diffused into n-type substrate which creates junction. Forward current flow through
junction provides Lambertian spontaneous emission. Thus, device emits light from all surfaces.
However a limited amount of light escapes the structure due to total internal reflection thus
providing low radiance.
64. In a multimode fiber, much of light coupled in the fiber from an LED is
a) Increased
b) Reduced
c) Lost
d) Unaffected
Answer: c
Explanation: Optical power from an incoherent source is initially coupled into large angle rays
falling within acceptance angle of fiber but have more energy than Meridional rays. Energy from
these rays goes into the cladding and thus may be lost.
65. The internal quantum efficiency of LEDs decreasing _______________ with
________________ temperature.
a) Exponentially, decreasing
b) Exponentially, increasing
c) Linearly, increasing
d) Linearly, decreasing
Answer: b
Explanation: The light emitted from LEDs decreases. This is due to increase in p-n junction
temperature. Thus, this results in exponentially decreasing internal quantum efficiency with
temperature increment.
66. To utilize _____________________ of SLDs at elevated temperatures, the use of
thermoelectric coolers is important.
a) Low-internal efficiency
b) High-internal efficiency
c) High-power potential
d) Low-power potential
Answer: c
Explanation: The output characteristics of SLDs are typically of nonlinear in nature. This is
observed with a knee becoming apparent at an operating temperature around 20 degree c. Thus,
to utilize high-power potential of these devices at elevated temperature, thermoelectric coolers
are necessarily used.
67. The optical bandwidth is _____________ the electrical bandwidth.
a) Smaller
b) Greater
c) Same as
d) Zero with respect to
Answer: b
Explanation: The difference between optical and electrical bandwidth In terms of frequency
depends on the shape of the frequency response of the system. If the system response is assumed
to be Gaussian, then optical bandwidth is a factor of √2 greater than electrical bandwidth.
68. ____________ converts the received optical signal into an electrical signal.
a) Detector
b) Attenuator
c) Laser
d) LED
Answer: a
Explanation: A detector is an essential component of an optical fiber communication system. It
dictates the overall system performance. Its function is to convert optical signal into an electrical
signal. This electrical signal is then amplified before further processing.
69. The first generation systems of optical fiber communication have wavelengths between
___________
a) 0.2 and 0.3 μm
b) 0.4 and 0.6 μm
c) 0.8 and 0.9 μm
d) 0.1 and 0.2 μm
Answer: c
Explanation: The first generation systems operated at a bit-rate of 45 Mbps with repeater spacing
of 10 km. It operates at wavelengths between 0.8 and 0.9μm. These wavelengths are compatible
with AlGaAs laser and LEDs.
70. Which of the following does not explain the requirements of an optical detector?
a) High quantum efficiency
b) Low bias voltages
c) Small size
d) Low fidelity
Answer: d
Explanation: The size of the detector must be small for efficient coupling to the fiber. Also,
ideally, the detector should not require excessive bias voltages and currents. The fidelity and
quantum efficiency should be high.
71. The ___________ process takes place in both extrinsic and intrinsic semiconductors.
a) Avalanche multiplication
b) External photoemission
c) Internal photoemission
d) Dispersion
Answer: c
Explanation: During intrinsic absorption, the received photons excite electrons from the valence
band and towards the conduction band in the semiconductor. Extrinsic absorption involves
impurity centers created with the material. Generally, intrinsic absorption is preferred for internal
photoemission.
72. P-n photodiode is forward biased. State whether the given statement is true or false.
a) True
b) False
Answer: b
Explanation: p-n photodiode includes p and n regions. The electric field developed across the p-n
junction sweeps holes and electrons to p and n regions respectively. P-n photodiode is thus
reverse biased due to reverse leakage current.
73. The depletion region must be ____________ to allow a large fraction of the incident light to
be absorbed in the device(photodiode).
a) Thick
b) Thin
c) Long
d) Inactive
Answer: a
Explanation: In p-n photodiode, intrinsic conditions are created in the depletion region. The
depletion region must be thick in order to achieve maximum carrier pair generation. Also, its
width must be limited to enhance the speed of operation of the p-n photodiode.
74. The process of excitation of an electron from valence band to conduction band leaves an
empty hole in the valence band and is called as
a) Detection
b) Absorption
c) Degeneration of an electron-hole pair
d) Regeneration of an electron-hole pair
Answer: d
Explanation: A photon is incident in the depletion region of a device has an energy greater than
or equal to the band gap energy of the fabricating material. This will cause excitation of an
electron from valence to the conduction band. This creates an empty hole in valence band which
is referred to as photo-generation of an electron-hole pair.
75. The electron hole pairs generated in a photodiode are separated by the ____________
a) Magnetic field
b) Electric field
c) Static field
d) Depletion region
Answer: b
Explanation: Electric field separates the electron-hole pairs in a photodiode. The electric field
distribution is determined by an internal and an external field component. A reverse bias voltage
is usually applied to the p-n photodiode.
76. The photocurrent of an optical detector should be __________
a) Less
b) More
c) Linear
d) Non-linear
Answer: c
Explanation: A linear relationship must exist between the intensity of the incident light and the
photocurrent. This makes the photodiode free of noise. It increases the system performance.
77. 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.
78. 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.
79. 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.
80. ________________ 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.
81. The fraction of incident photons generated by photodiode of electrons generated collected at
detector is known as
a) Quantum efficiency
b) Absorption coefficient
c) Responsivity
d) Anger recombination
Answer: a
Explanation: Efficiency of a particular device is obtained by ratio of input given to that of output
obtained. Thus, similarly, in photodiode, input i.e. incident photon and output generated
electrons and their ratio is quantum efficiency.
82. In photo detectors, energy of incident photons must be ________________ band gap energy
a) Lesser than
b) Greater than
c) Same as
d) Negligible
Answer: b
Explanation: While considering intrinsic absorption process, the energy of incident photon must
be greater than band gap energy of material fabricating photo detector.
83. Quantum efficiency is a function of photon wavelength. Determine the given statement is
true or false.
a) True
b) False
Answer: a
Explanation: Quantum efficiency is less than unity as all of incident photons are not absorbed to
create electrons holes pairs. For example quantum efficiency of 60% is equivalent to 60% of
electrons collected per 100 photons. Thus efficiency is a function of photon wavelength and must
be determined at a particular wavelength.
84. The quantum efficiency of photodiode is 40% with wavelength of 0.90*10-6. Determine the
responsivity of photodiodes
a) 0.20
b) 0.52
c) 0.29
d) 0.55
Answer: c
Explanation: Responsivity of photodiodes is given by
R = ηe λ/hc
= 0.4*1.602*10-19* 0.90*10-6/ 6.626*10-34* 3*108
= 0.29 AW-1.
85. The phenomenon leading to avalanche breakdown in reverse-biased diodes is known as
_______
a) Auger recombination
b) Mode hopping
c) Impact ionization
d) Extract ionization
Answer: c
Explanation: In depletion region, almost all photons are absorbed and carrier pairs are generated.
So there comes a high field region where carriers acquire energy to excite new carrier pairs. This
is impact ionization.
86. _______ is fully depleted by employing electric fields.
a) Avalanche photodiode
b) P-I-N diode
c) Varactor diode
d) P-n diode
Answer: a
Explanation: APD is fully depleted by electric fields more than 10 4V/m. This causes all the
drifting of carriers at saturated limited velocities.
87. When determining performance of a photo detector ___________ is often used.
a) No. of incident photon
b) No. of electrons collected
c) Responsivity
d) Absorption coefficient
Answer: c
Explanation: The expression for quantum efficiency does not include photon energy. Thus for
characterizing performance of photo detector, Responsivity is used.
88. The important parameter for exciting an electron with energy required from valence band to
conduction band is
a) Wavelength
b) Absorption coefficient
c) Responsivity
d) Band gap energy
Answer: a
Explanation: As wavelength of incident photon becomes longer, the photon energy is less than
energy required to excite electron. Mostly parameters of photodiode are dependent on
wavelength.
89.. A ________ is created by hetero-junction at collector-base junction.
a) Potential barrier
b) Depletion region
c) Parasitic capacitance
d) Inductance
Answer: a
Explanation: Potential barrier is created at emitter-base junction by hetero-junction. This
eliminates hole junction from base. This is achieved when junction is forward-biased and
provides good emitter-base efficiency.
90. Which are the two main sources of noise in photodiodes without internal gain?
a) Gaussian noise and dark current noise
b) Internal noise and external noise
c) Dark current noise & Quantum noise
d) Gaussian noise and Quantum noise
Answer: c
Explanation: The two main sources of noise in photodiodes without internal gain are dark current
noise and quantum noise. They are regarded as shot noise on the photocurrent. These noise are
together called as analog quantum noise.
91. For linear as well as in nonlinear mode _______________ are most important network
elements.
a) Optical amplifier
b) Optical detector
c) A/D converter
d) D/A converters
Answer: a
Explanation: In single-mode fiber system, signal dispersion is very small, hence there is
attenuation. These systems don’t require signal regeneration as optical amplification is sufficient
so optical amplifier are most important.
92. The more advantages optical amplifier is
a) Fiber amplifier
b) Semiconductor amplifier
c) Repeaters
d) Mode hooping amplifier
93. ________________ cannot be used for wideband amplification
a) Semiconductor optical amplifier
b) Erbium-doped fiber amplifier
c) Raman fiber amplifier
d) Brillouin fiber amplifier
Answer: d
Explanation: Brillouin fiber amplifiers provide a very narrow spectral bandwidth. These
bandwidth can be around 50 MHz, hence cannot be employed for wideband amplification.
94. ____________ is used preferably for channel selection in a WDM system.
a) Semiconductor optical amplifier
b) Erbium-doped fiber amplifier
c) Raman fiber amplifier
d) Brillouin fiber amplifier
Answer: d
Explanation: Brillouin fiber provides amplification of a particular channel. This amplification
can be done without boosting other channels besides that particular channel.
95. _____________ must be operated in stimulated emission region.
a) Injection laser
b) LED’s
c) Detector
d) Receiver
Answer: a
Explanation: Injection laser is a threshold device. In stimulated emission region, continuous
optical output power levels are in the range of 1 to 10mW.
96. Coherent radiation is relatively __________
a) Parabolic
b) Elliptic
c) Directional
d) Rectangular
Answer: c
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.
97. _____________ are capable of launching powers between 0.5 and several mW.
a) LED’s
b) Injection laser
c) Attenuator
d) Reflector
Answer: b
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.
98. Optical output power from an LED is directly proportional to the device junction
temperature. State whether the given statement is true or false.
a) False
b) True
Answer: b
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.
99. _____________ 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
Answer: a
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.
100. The _________ of the LED is twice that of the effective minority carrier lifetime.
a) Dwell time
b) Reflection scatters
c) Sensitivity
d) Rise time
Answer: d
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.
01. The finite spectral width of the optical source causes ___________
a) Depletion
b) Frequency burst
c) Pulse broadening
d) Efficient reflection
Answer: c
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.
02. ____________ limits receiver sensitivity.
a) Noise
b) Depletion layer
c) Avalanche
d) Current
Answer: a
Explanation: Receiver noise affects receiver sensitivity. It can dictate the overall system design.
The noise can be temperature, environmental factor or due to components.
03. A ____________ performs the linear conversion of the received optical signal into an electric
current.
a) Receiver
b) Converter
c) Detector
d) Reflector
Answer: c
Explanation: An optical signal is always fed to a detector. A detector is an opto – electronic
converter which linearly converts the received optical signal into an electric current.
04. __________ are provided to reduce distortion and to provide a suitable signal shape for the
filter.
a) Detector
b) Equalizer
c) Filters
d) Amplifier
Answer: b
Explanation: Optical detectors are linear devices. They do not introduce distortion themselves
but other components may exhibit nonlinear behaviour. To compensate for distortion, an
equalizer is provided in the receiver circuit.
05. Intermodal dispersion is nonexistent in ________ fibers.
a) Multimode
b) Single mode
c) Step index- multimode
d) Al-GU
Answer: b
Explanation: Intra-modal as the name suggests need multimode fibers to propagate. In single
mode fibers, only one mode is there to propagate. Hence, Intermodal dispersion is nonexistent in
single mode fibers.
06. In the single mode fibers, the dominant dispersion mechanism is ____________
a) Intermodal dispersion
b) Frequency distribution
c) Material dispersion
d) Intra-modal dispersion
Answer: d
Explanation: In single mode case, the dominant dispersion mechanism is chromatic. Chromatic
dispersion is called as intra-modal dispersion.
07. Intra-modal dispersion tends to be dominant in multimode fibers. State whether the given
statement is true or false.
a) True
b) False
Answer: b
Explanation: Intra-modal dispersion is dominant in case of single mode fibers. In case of
multimode fibers, intermodal dispersion comes handy and is dominant.
08. ______________ of the fiber is strongly dependent on the refractive index profile.
a) Amplitude
b) Tuning frequency
c) Diameter
d) Information carrying capacity
Answer: d
Explanation: Information carrying capacity is strongly dependent on the refractive index profile
same as that of the impulse response of the fiber. These characteristics predict the dispersion in
the fiber.
.09. ______________ is required in case of graded index fibers.
a) High amplitude
b) High frequency
c) High impulse response
d) Optimum profile
Answer: d
Explanation: In case of graded index fibers, it is essential that the fiber manufacturer is able to
produce particular profiles with higher accuracy. This profile is known as optimum profile as it
minimizes the dispersion rate.
10. A multimode fiber has many cutoff wavelengths. State whether the given statement is true or
false.
a) False
b) True
Answer: b
Explanation: A multimode fiber has many cutoff wavelengths. It is because the number of bound
propagating modes is usually large.
11.The _________ wavelength is defined as the wavelength greater than which the ratio of the
total power and the fundamental mode power has to be decreased to less than 0.1dB.
a) Magnetic
b) Quasi
c) Cut-off
d) EIA
Answer: c
Explanation: The cut off wavelength is usually measured by increasing the signal wavelength in
a fixed length of fiber until the mode is undetectable. It is usually called as effective cut-off
wavelength.
12. The ____________ affects the light gathering capacity and the normalized frequency of the
fiber.
a) Numerical aperture
b) Amplitude modulation
c) Responsivity
d) Quantum efficiency
Answer: a
Explanation: Numerical aperture is an important optical parameter as it dictates the important
characteristics of the optical fiber. This in turn dictates the number of propagating modes within
the fiber.
13. The numerical aperture for a step index fiber is sine angle of the ____________
a) Efficient angle
b) Aperture
c) Acceptance angle
d) Attenuation
Answer: c
Explanation: The numerical aperture of a step index fiber is given by –
NA = sinθa, where θa is the acceptance angle and NA is the numerical aperture.
14. The calculations of the numerical aperture from a refractive index data are less accurate for
the graded index fibers than for step index fibers. State whether the given statement is true or
false.
a) False
b) True
Answer: b
Explanation: The refractive indices of the core and cladding are fluctuating, thus causing the data
to be less efficient. For graded index fibers, it is usually less accurate than for the step index
fibers.
15. ____________ affects both the fiber attenuation and dispersion.
a) Refractive index
b) Micro-bending
c) Connectors
d) Splices
Answer: b
Explanation: Effects such as micro-bending with a resultant mode coupling affect both the fiber
attenuation and dispersion. It does not provide the overall characteristics of the transmission link.