# Optical Communication - Two mark Questions

Please find the two mark questions with answers below for optical communication subject related to electronics and communication (ECE) branch. Please forgive for the formulas. Please do cross refer it before using it. These is for the anna university syllabus - regulation 2008. Please feel free to share you feedback and comments so that i can improve this blog.

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1. Write the expression for the refractive index in graded index fibers.
n(r)= n1[1-2?(r/a)a]1/2 for 0<=r<=a
n1(1-2?)1/2 ~ n1(1-?) =n2 for r>=a
r radial distance from fiber axis
n1 refractive index at the core
n2 refractive index at the cladding
a shape of the index profile
? index difference

2. Define Numerical aperture of a step index fiber.
Numerical aperture (N.A) of the fiber is the light collecting efficiency
of the fiber and is the measure of the amount of light rays that can be accepted by the
fiber. It is equal to the sine of acceptance.
N.A=sin Fmax =(n12-n22)1/2
where n1 and n2 are the refractive indices of core and cladding respectively.

3. Define Mode-field diameter.
The fundamental parameter of a single mode fiber is the mode-field
diameter. This can be determined from the mode field distribution of the
fundamental LPo1 mode.

4. Give the expression for linearly polarized waves.
The electric or magnetic field of a train of plane polarized waves
travelling in a direction k can be represented in the general form
A(x,t) = eiAoexp[j(wt-k.x)]
with x=xex+yey+zez representing a general position vector and
k=kxex+kyey+kzez representing the wave propagation vector.

6.What is Snell’s law?
The relationship at the interface is known as Snell’s law and is given by
n1sinF1=n2 sinF2

7.What is the necessity of cladding for an optical fiber?
a) To provide proper light guidance inside the core
b) To avoid leakage of light from the fiber
c) To avoid mechanical strength for the fiber
d) To protect the core from scratches and other mechanical damages
8. What are the uses of optical fibers?
a) To transmit the information which are in the form of coded signals of the
telephone communication, computer data, etc.
b) To transmit the optical images (Example : Endoscopy)
c) To act as a light source at the inaccessible places.
d) To act as sensors to do mechanical, electrical and magnetic measurements.

9. What is the principle used in the working of fibers as light guides?
The phenomenon of total internal reflection is used to guide the light in the
optical fiber. To get total internal reflection, the ray should travel from denser to rarer
i.e. from core to clad region of the fiber and the angle of incidence in the denser
medium should be greater than the critical angle of that medium.

10. what are step index and graded index fibers?
In the case of graded index fiber, the refractive index of a core is a constant
and is larger than the refractive index of the cladding. The light propagation is mainly
by meridional rays. In the case of graded index fiber (GRIN fiber) the refractive index
of the core varies parabolically from the centre of the core having maximum refractive
index to the core-cladding interface having constant minimum refractive index. Here
the light propagation is by skew rays.

11. Define acceptance angle.
The maximum angle ‘Fmax’ with which a ray of light can enter through the
entrance end of the fiber and still be totally internally reflected is called acceptance
angle of the fiber.

12. Why do we prefer step index single mode fiber for long distance communication?
Step index single mode fiber has a)low attenuation due to smaller core
diameter b) higher bandwidth and c) very low dispersion.

13. Define relative refractive index difference.
? = n12-n22 = n1-n2
2n12 n1
Thus relative refractive index difference is the ratio between the refractive index
difference (of core and cladding) and refractive index of core.

14. What are meridional rays?
Meridional rays are the rays following Zig Zag path when they travel through
fiber and for every reflection it will cross the fiber axis.

15. What are skew rays?
Skew rays are the rays following the helical path around the fiber axis when
they travel through the fiber and they would not cross the fiber axis at any time.
16. What is V number of fiber or normalized frequency of fiber?
V number of fiber or normalized frequency of fiber is used to find the number
of propagating modes through the fiber.
V= 2?a (N.A)
?
In step index fiber number of modes propagating through the fiber=V2
Taking the two possible polarizations, total number of possible modes propagating
through the fiber = V2 * 2 = V2

17. What are the conditions fro total internal reflection?
a) Light should travel from denser medium to rarer medium.
b) The angle of incidence should be greater than the critical angle of the
denser medium.

18.Give the relation between numerical aperture of skew rays and meridional rays.
(N.A)skew = cos ?(N.A)meridional when the fiber is placed in air.
Here ? is the half of the angular change in every reflection.

19. State Goos-Haenchen effect.
Goos-Haenchen effect states that there is a lateral shift of the reflected ray
at the point of incidence at the core-cladding interface. This lateral shift is called the
Goos-Haenchen shift.

20. When do you have phase shift during total internal reflection of light.
When the light ray travels from denser medium to rarer medium, if the
angle of incidence is greater than the critical angle of core medium, there is a phase
shift for both TE and TM waves.

21. What are hybrid modes? Give two examples.
Hybrid modes are the mixture of TE and TM modes that can be traveled
through the optical fiber.
Examples:
1. HE1m modes in which |Ez|>|Hz|
2. EH1m modes in which |Hz|>|Ez|

22. Define cutoff wavelength of the fiber.
The cutoff wavelength is defined as the minimum value of wavelength that
can be transmitted through the fiber. The wavelengths greater than the cutoff
wavelength can be transmitted.
? cutoff = 2?a (N.A)
V

23. Mention the rule distinguishing ‘mode’ and ‘order’.
The rule states that the smaller the modes propagating angle, the lower the
order of the mode. Hus the mode traveling precisely along the fiber’s central axis is
zero mode.
24.What is fiber birefringence?
Imperfections in the fiber are common such as asymmetrical lateral stress,
non circular imperfect variations of refractive index profile. These imperfections
break the circular symmetry of ideal fiber and mode propagate with different phase
velocity and the difference between their refractive index is called fiber birefringence.
B=ko(ny-nx)

25. Give the expression for numerical aperture in graded index fibers.
N.A(r)=N.A.(0) (1-(r/a)a)1/2 for r<=a
where N.A(0) = axial numerical aperture = (n12-n22)1/2
a is core radius and a is the refractive index profile.

26.What is Intra Modal Dispersion?
Intra Modal dispersion is pulse spreading that occurs with in a single mode.
The spreading arises from finite spectral emission width of an optical source. This
phenomenon is also called as group velocity dispersion.

27.What are the causes of intra modal dispersion?
There is two main causes of intra modal dispersion. They are:
??Material dispersion
??Wave guide dispersion

28.What is material dispersion?
Material dispersion arises from the variation of the refractive index of the
core material as a function of wavelength. Material dispersion is also referred to as
chromatic dispersion. This causes a wavelength dependence of group velocity of
given mode. So it occurs because the index of refraction varies as a function of
optical wavelength. Material dispersion is an intra modal dispersion effect and is for
particular importance for single ode wave-guide.

29.What is waveguide dispersion?
Wave guide dispersion which occurs because of a single mode fiber confines
only about 80% of optical power to the core. Dispersion this arises since 20% of
light propagates in cladding travels faster than the light confined to the core.
Amount of wave-guide dispersion depends on fiber design. Other factor for pulse

30. What is group velocity?
If L is the distance traveled by the pulse, ? is the propagation constant along
axis then the group velocity in the velocity at which energy is a pulse travels along
the fiber.
Vg = C. (d? / dk)

31.What is group delay?
In an optical fiber there are various modes present. Then the optical input,
which is propagated along the fiber, will travel in various modes. Because of these
modes the velocity of the signal will vary also there may be a delay in the optical
signal of these various modes. This is called as the ‘ Group Delay’.

32.What is polarization?
It is a fundamental property of an optical signal .It refers to the electric field
orientation of a light signal which can vary significantly along the length of a fibre.

Dispertion induced signal distortion is that a light pulse will broaden as it
travels along the fibre.This pulse broadening causes a pulse to overlap with
neighbouring pulses.After a time ‘t’,the adjacent pulses can no longer be individually
distinguished at the receiver and error will occur.

34.What is polarization Mode Dispersion(PMD)?
The difference in propagation times between the two orthogonal polarization
modes will result in pulse spreading.This is called as polarization Mode Dispersion.

35.What is Mode Coupling?
It is another type of pulse distorsion which is common in potical links.The pulse
distortion will increase less rapidly after a certain initial length of fibre due to this mode
coupling and differential mode losses.In initial length coupling of energy from one
mode to another arises because of structural irregularities,fibre dia. etc.

36.What is Profile Dispersion?
A fibre with a given index profile(alpha) will exhibit different pulse spreading
according to the sourse wavelength used.This is called as Profile Dispersion.

37.What is M-C fiber?
Fibers that have a uniform refractive index throughout the claddind is called as

38.What is D-C fiber?
In depressed cladding fiber the cladding portion next to the core has a lower
index than the outer cladding region.

39.Define depression shifted fiber
by creating a fiber with large negative waveguide dispersion & assuming the
same values for material dispersion as in a standard single mode fiber the addition of
waveguide & material dispersion can then shifted to zero dispersion point to long
wavelenth. The resulting optical fiber are known as dispersioin shifted fiber.

40.Define dispersion flattening?
The reduction of fiber dispersion by spreading the dispersion minimum out over
a widen range .this approach is known asd dispersion flattering.

41.What is effective cut-off wavelenth?
It is defined as the largest wavelenth at which the higher order LP11 mode power
relative to the fundamental LP01 mode power is redued to 0.1db.

42. Write a note on scattering losses.
Scattering losses in glass arise from microscopic variation in the material
density from compositional fluctuation and from structural in homogeneities or defects
occurring during fiber manufacture

43.What is Rayleigh scattering?
The index variation causes a Rayleigh type of scattering of light. Rayleigh
scattering in glass in the same phenomenon that scatters light from sun in the
atmosphere, giving rise to blue sky.
The expression for Rayleigh scattering loss is given by
?scat =(8?3/3?2)(n2-1)2kBTf?T ?
n = refractive index
kB = boltzman constant
?T= isothermal compressibility
Tf =fictive temperature
? =operative wavelength

44.What is intermodal dispersion?
Intermodal dispersion is a pulse spreading that occurs with in a single mode. the
spreading arises from finite spectral emission width of an optical source. it is called
group velocity dispersion or intermodal dispersion

45.what is intramodal delay?
The other factor giving rise to pulse spreading is intramodal delay which is a
result of each mode having a different value of Group velocity at a single frequency.

46.what is the measure of information capacity in optical wave guide?
It is usually specified by bandwidth distance product in MHz.For a step index fiber
the various distortion effects tend to limit the bandwidth distance product to 20MHz.

47.Mention the losses responsible for attenuation in optical fibers.
Absorption losses, Scattering losses and bending losses

48.What do you meant by Extrinsic absorption ?
Absorption phenomena due to impurity atoms present in the fiber.

49. Define microscopic bending?
Fiber losses occur due to small bending arise while the fiber is inserted into a
cable.

50. Define macroscopic bending?
If any bending present in the fiber while cabling , the optical power get radiated

51. What are the advantages of optical communication?
1. Low transmission loss.
2 Small size and weight.
3. No electromagnetic interference.
4. Electrical isolation.

52. Define direct band gap materials and indirect band gap materials.
In direct band gap materials direct transition is possible from valence band to
conduction band.e.g.GaAs,InP,InGaAs
In indirect band gap materials direct transition is not possible from valence band to
conduction.e.g.silicon,germanium.

53. What are the advantages of LED?
1. LEDs are less complex circuits than Laser diodes.
2. Fabrication is easier.
3. They have long life.

53. What are the two types of confinement used in LEDs?
1. optical confinement.
2. carrier confinement.

54. What are the two types of LED configurations?
1. homo junction
2. Single and double hetero junction.

55. What are the three requirements of Laser action?
1. Absorption
2. Spontaneous emission
3. stimulated emission.

56. What are the three types of Laser diode structures?
1. Gain indexed guide
2. Positive indexed guide
3. Negative indexed guide

57. What are the fundamental structures of Index guided lasers?
1. buried hetero structure.
2. Selectively diffused construction
3. Varying thickness structure
4. Bent layer configuration.

58.What are the three basic methods of current confinement?
1. Preferential dopant diffusion.
2. Proton implantation
3. Inner strip confinement
4. Re growth of back biased PN junction.

59. Define modulation.
The process of imposing information on a light stream is called modulation. This can
be achieved by varying the laser drive current.

60. Define external quantum efficiency.
The external quantum efficiency is defined as the number of photons emitted per
radiative electron-hole pair recombination above threshold.

61. Define threshold current.
The threshold current is conventionally defined by extrapolation of the lasing region of
the power-versus-current curve. At high power outputs, the slope of the curve decreases
because of junction heating.

62. Define longitudinal modes.
Longitudinal modes are associated with the length of the cavity and determine the
typical spectrum of the emitted radiation.

63. Define lateral modes.
These modes lie in the plane of the pn junction. They depend on the sidewall preparation and the width of the cavity. It determines the shape of the lateral profile of the laser beam.

64. Define transverse modes.
Transverse modes are associated with the electromagnetic field and beam profile in the
direction perpendicular to the plane of the pn junction. They determine the laser
characteristics as the radiation pattern and the threshold current density.

65. Define population inversion.
Stimulated emission will exceed absorption only if the population of the excited states
is greater than that of the ground state. This condition is called as population inversion

66. Define internal quantum efficiency.
The internal quantum efficiency is the fraction of the electron-hole pairs that recombine
rateis Rnr, then the internal quantum efficiency is the ratio of the ratio of the radiative
recombination rate to the total recombination rate.

67. Differentiate LEDs and Laserdiodes.

68. What is mass action law?
Pn=ni*ni
Where
p- concentration of holes.
n - Concentration of electrons.
Ni- intrinsic concentration.

69. What is an intrinsic and extrinsic semiconductor material?
Intrinsic semiconductors have no impurities.
Extrinsic semiconductors contain impurities like boron and phosphorus.

70.Define responsivity
The performance of an avalanche photodiode is characterized by its
responsivity
RAPD= ?qM = Ro M
hv
where Ro is the unity gain responsivity.

71.Define long wavelength cut off related to photodiode.
The upper wavelength cutoff (?c) is determined by the band gap energy
Eg of the material. If Eg is expressed in units of electron volts(eV),then ?c is given in
units of micrometers (µm) by
?c(µm) = hc = 1.24
Eg Eg(eV)

72.A given APD has a quantum efficiency of 65% at a wavelength of 900 nm. If 0.5
µW of optical power produces a multiplied photocurrent of 10µA, find the
multiplication M.
R= ?e?
hc
Ip=PoR
M = I
Ip

73. Give some types of photodetectors.
1.      Photomultipliers
2.      Pyroelectric detectors
3.      Semiconductor- based detectors
4.      Phototransistors
5.      Photodiodes

74. What are the advantages of photodiodes?
a. Small size
b. Suitable material
c. High sensitivity
d. Fast response time

75. What are the types of photodiodes?
??PIN photodetector
??Avalanche photodiode(APD)

76.Define photocurrent.
The high electric field present in the depletion region causes the carriers
to separate and be collected across the reverse-biased junction. This gives to a
current flow in the external circuit, with one electron flowing for every carrier
pair generated. This current flow is known as photocurrent.

77. Define quantum efficiency.
It is defined as the number of the electron – hole pairs generated per
incident photon of energy hv and is given by
n=No.of electron-hole pairs generated
No. of incident photons

78. Define impact ionization.
In order for carrier multiplication to take place, the photo generated
carriers must traverse a region where a very high electric field is present. In this high
field region, a photo generated electron or hole can gain energy so that it ionizes bound
electrons in the valence band upon colliding with them. This carrier multiplication
mechanism is known as impact ionization.

79. Define avalanche effect.
The newly created carriers are accelerated by the high electric field, thus
gaining enough energy to cause further impact ionization. This phenomenon is called
avalanche effect.

80. What is p+ ? p n+ reach- through structure?
The reach –through avalanche photodiode (RAPD) is composed of a
high resistivity p-type material deposited as an epitaxial layer on a p+ substrate. A p-
type diffusion is then made in the high resistivity material, followed by the construction
of an n+ layer. The configuration is called p+ ? p n+ reach- through structure.

81. Define ionization rate.
The avg. no. of electron hole pairs created by a carrier per unit distance
traveled is called ionization rate.
82. What are the conditions to be met for a high signal- to- noise ratio in a
photodetector?
??The photodetector must have a high quantum efficiency to
generate a large sign al power
??The p and amplifier noises should be kept as low as possible.

83. Define minimum detectable optical power.
It is defined as the optical power necessare to produce a photocurrent of
the same magnitude as the root mean square of the total current.

84. Define quantum noise.
It is not possible to predict exactly how many electron-hole pairs are
generated by a known optical power incident on the detector is the origin of the type of
short noise called quantum noise.

85. What is meant by error rate?
An approach is to divide the number Ne of errors occurring over a
certain time interval t by the number Nt of pulses transmitted during this interval. This
is called either the error rate or the bit error rate.
Bit error rate BER = Ne =Ne
Nt Bt
Where B= 1
Tb

86. Define quantum limit
It is possible to find the minimum received optical
power required for a specific bit error rate performance in a digital system. This
minimum received power level is known as quantum limit.

87. Give the classifications of preamplifiers.
??Low impedence(LZ) preamplifier
??High impedence(HZ) preamplifier
??Transimpedence preamplifier

88. What is meant by excess noise factor?
The ratio of the actual noise generated in an avalanche photodiode to the
noise that would exist if all carrier pairs were multiplied by exactly m is called the
excess noise factor (F).
F = (m2) = (m2)
(m)2 m2

89. What is meant by inter symbol interference(ISI) ?
ISI results from pulse spreading in the optical fibre. The presence of this
energy in adjacent time slots results in an interfering signal. Hence it is called ISI.

90.Give the advantages of Pin photodiodes.
??Very low reverse bias is necessary
??High quantum efficiency
??Large bandwidth
??Low noise level

91.What do you mean by thermal noise?
Thermal noise is due to the random motion of electrons in a conductor.
Thermal noise arising from the detector load resistor and from the amplifier electronics
tend to dominate in applications with low signal to noise ratio.

92.Give the equation for mean square shot noise.
The mean square shot noise is given by
n
2>=2qIB
I - average output current
B B bandwidth of the amplifier
93. Define multiplication M.
The multiplication M for all carriers generated in the photo-diode is defined by
M = IM
IP
IM average value of the total multiplied output current
IP primary un-multiplied photo current

94. What is current mode of operation of photo diode?
In photo conducting mode, the photo current is slightly dependent on the
reverse bias. For a constant reverse bias, the current is linear. This is called current
mode of operation of the photo diode.

95. What are the system requirements?
The following are the key system requirements.
??The desired or possible transmission distance
??The data rate or channel bandwidth
??Bit error rate(BER)
96. What are splices? What are the requirements of splices?
The splices are generally permanent fiber joints, whereas connectors are temporary
fiber joints. Splicing is a sort of soldering. The requirements of splices are:
??Should cause low attenuation
??Should be strong & light in weight
??Should have minimum power loss
??Should be easy to install

97. What are the methods of fiber splicing?
There are 3 methods of fiber splicing. They are:
??Electric arc fusion splicing or fusion splicing
??Mechanical splicing
??V-groove splicing or loose tube splicing

98. What are connectors? What are the types of connectors?
The connectors are used to join the optical sources as well as detectors to the optical
fiber temporarily. They are also used to join two optical fibers. The 2 major types of
connectors are:
??Lensed type expanded beam connector
??Ferrule type connector

99. What are the requirements of a good connector?
The requirements of a good connector are as follows:
??Low loss
??Repeatability
??Predictability
??Ease of assembly and use
??Low cost & reliability
??compatibility

100. Give the 2 analysis that are used to ensure system performance?
The 2 analysis that are used to ensure system performance are:
??rise time budget analysis

In the optical power loss model for a pt-to-pt link, the optical power rxed at the photo
detector depends on the amount of light coupled into the fiber & losses occurring in the
fiber at the connectors & splices. The link loss budget is derived from the sequential loss
contribution of each element in the link.
Loss=10 log (Pout)
(Pin)
The total optical power loss is, PT = PS - PR

102. Give the range of system margin in link power budget?
The system margin is usually (6-8) dB. A positive system margin ensures proper
operation of the circuit. A negative value indicates that insufficient power will reach the
detector to achieve the required bit error rate, BER.

103. The specifications of the light sources are converted to equivalent rise time in rise
time budget. Why?
A rise time budget is a convenient method to determine the dispersion limitation of an
optical link. This is particularly useful for digital systems. For this purpose, the
specifications of the light sources (both the fiber & the photo detector) are converted to
equivalent rise time. The overall system rise time is given in terms of the light source rise
time, fiber dispersion time & the photo detector rise time.

104. What are the system components of system rise time?
The 4 basic system components that contribute to the system rise time are:
??transmitter (source) rise time
??material dispersion time of the fiber
??modal dispersion time of the fiber link
All these 4 basic elements may significantly limit system speed.

105. Why the attenuation limit curve slopes downwards to the right?
As the minimum optical power required at the rxer for a given BER becomes higher
for increasing data rates, the attenuation limit curve slopes downward to the right.

106. What are the noise effects on system performance?
The main penalties are modal noise, wavelength chirp, spectral broadening,
mode- partition noise.

107. Define modal noise?
It arises when the light from a coherent laser is coupled in to a multimode fiber
operating at 400Mbps and higher. It mainly occurs due to mechanical vibrations and
fluctuations in the frequency of the optical source.

108. What are the measures to avoid modal noise?
The measures are
???use LEDs
???use LASER having more longitudinal modes
???use a fiber with large numerical aperture
???use a single mode fiber

109. Define mode partition noise?
The mode partition noise is associated with intensity fluctuations in the
longitudinal modes of a laser diode. It becomes more pronounced for the higher bit
rates.

110. What is meant by chirping?
It means that the dynamic line broadening (line broadening is a frequency chirp)
in the laser which oscillates in the single longitudinal mode under CW operation when
the injection current is intensity modulated.

111. What is the best way to minimize the chirping?
It is to choose the laser emission wavelength close to the zero-dispersion of the
wavelength of the fiber.

112. What is reflection noise?
It is the optical power that gets reflected at the refractive index discontinuities
such as in splices, couplers and filters, or connectors. The reflected signals can degrade
both the transmitter and receiver performance.

113. What are the effects of reflection noise in high speed systems?
They cause optical feedback which leads to optical instabilities that may lead to
inter symbol interference and intensity noise.

114. What are the techniques to reduce optical feedback?
Fiber end faces with a curved surface to the laser emitting facet.
Index matching oil or gel at air glass interfaces.
PC connectors
Optical isolators within the transmitter module.

115. What are the basic performances of the WDM?
??Insertion loss
??Channel width
??Cross talk

116. What is WDM? Define.
WDM is wavelength division multiplexing. The optical beam consists of
different wavelengths and several channel information is transmitted over a single
channel.

117. What is meant as bidirectional WDM?
A single WDM which operates as both multiplexing and de-multiplexing
Devices is said as the bidirectional WDM.

Radiance (or brightness) is a measure, in Watts, of the optical power radiated into a
unit solid angle per unit area of the emitting surface.

119. What is meant by ‘population inversion’?
In thermal equilibrium, the density of excited electrons is very small. Most
photons incident on the system will therefore be absorbed, so that stimulated emission
is essentially negligible. Stimulated emission will exceed absorption only if the
population of the excited states is greater than that of the ground state. This condition is
known as population inversion.

120. What are the factors to be considered in link power budget?
The factors to be considered in link power budget are:
??transmission speed
??optical sources & detectors
??optical fiber

121. What are the causes of absorption?
Normally, the system is in the ground state. When a photon of energy h?12
impinges on the system, an electron in state E1 can absorb the photon energy & be
excited to state E2.

122. What is meant by hetero junction?
In hetero junction, two different alloy layers are on each side of the active region.
Because of the sandwich structure of differently composed alloy layers, both the
carriers & optical field are confined in the central active layer.

123. What is meant by indirect band gap semiconductor material?
For indirect band gap materials, the conduction-band minimum & the valence-
band maximum energy levels occur at different values of momentum. Here, band-to-
band recombination must involve a third particle to conserve momentum, since the
photon momentum is very small. Phonon serve this purpose.

124. What is meant by ‘modal noise’?
It arises when light from a laser is coupled into the multi-mode fiber.

125.What is the necessity of cladding for an optical fiber?
a) To provide proper light guidance inside the core
b) To avoid leakage of light from the fiber
c) To avoid mechanical strength for the fiber
d) To protect the core from scratches and other mechanical damages