Optical Communication
UNIT 1
INTRODUCTION TO OPTICAL FIBERS
1. What are the major components and addition components of optical fiber
communication system?
Major components-Transmitter ,receiver ,optical fiber link
Addition components- connector, optical splice, optical coupler or beam splitter, optical
amplifier, regenerator, repeater
2. What is the wavelength range of windows in spectrum of optical fiber?
First window range is 800 nm to 900 nm
Long wavelength regions range is 1100 nm to 1600 nm
The two windows defined inside the long wavelength region are second window centered
around 1310 nm &third window around 1550 nm.
3. Define wave front.
Wave front is defined as locus of all points in the wave train which have the same
phase. They are also called as phase front.
4. Define Geometrical optics.
Large scale optics effect such as reflection & refraction can be analysed by the
simple geometrical process of ray tracing .This view of optics is referred to as ray or
geometrical optics.
5. When did the light waves said to be transverse?
When the wave motion is perpendicular to the direction in which the wave
travels , the light waves said to be transverse.
6. State law of reflection
According to law of reflection ,the angle of which the incident ray makes with the
interface is equal to the angle which the reflected ray makes with the same interface.
7. Define critical angle.
Critical angle is defined as the angle of incidence for which the refracted ray just
traverses the interface between the 2 media .
8. What is intermodal dispersion?
The modes in a given optical pulse arrive at the fiber end at slightly different times
,thus causing the pulse to spread out in time as it travels along the fiber. This is intermodal
dispersion or intermodal distortion.
9. What is the index difference for the graded index fiber?
Index difference=n12-n22/2 n12=(n1- n2)/ n1
10. State any two function of cladding
It reduces scattering losses which occur as a result of discontinuities in the core
surface
It prevents core from absorbing surface contaminants which comes in contact with
core
11. What are the imperfections occur in the practical situation of the fiber?
1. non circular core
2. variations in the refractive index
3. asymmetrical lateral stresses
12. What are leaky modes?
Leaky modes are partially confined to the core region &attenuate continuously by
radiating their power out of the core as they propagate along the fiber. This power
radiation out of the wave guide is due to tunnel effect.
13. State the cutoff conditions of the fiber modes
A mode is referred to as being cutoff when it is no longer bound to the core of the
fiber ,so that its field no, longer decays on the outside of the core.
14. What is ray congruence?
The wave front is made up of large number of plane waves. The family of plane
waves is called as ray congruence
15. What is meridional rays?
It is the ray which is move in straight line or moving by reflection.
16. What is the typical value of index difference for step index fiber?
For single mode fiber-0.2% to 1%
For multimode fiber -1% to 3%
17. What is the basis of weakly guiding fiber approximation?
In a typical step index fiber the difference between the indices of refraction of the
core and cladding is very small .This is the basis of weakly guiding fiber
approximation .
18. What is the useful feature of the LP modes designation ?
The useful feature of the LP modes designation is the ability to readily visualize a
mode.
19. What is the mode field diameter?
The fundamental parameter of single mode fiber is the mode field diameter
(MFD).This parameter can be determined from the mode field distribution of the
fundamental LP01 modes.
20. Give the requirements in selecting material for optical fibers
It must be possible to made long, thin flexible fibers from the material
The material must be transparent at a particular wavelength in order for the
fiber to guide light efficiently
Physically compatible materials having slightly different refractive indices
for the core and cladding must be available.
21. What is WKB method?
WKB method is the most widely used analysis of modes in a graded index fiber
named after Wenzel, Kramers and Brillouin
22. What is fiber beat length?
If light is injected into the fiber so that both vertical and horizontal modes are
excited then one will be decayed in phase relative to the other as they propagate .When
this phase difference is an integral multiple of 2Π, the two modes will beat at this point
and the input polarization state will be reproduced. The length over which this beating
occurs is the fiber beat length.
LP=2Π /β
23. Define V number
V number of fiber indicates the number of possible propagation modes in the core.
Vnum=(2Π/λ)a*NA
a is the radius of core.
24. Define Birefringence
The modes propagate with different phase velocities and the difference between
their effective refractive indices is called as fiber birefringence.
25. Write Maxwell’s equation for a linear isotropic dielectric material
1. ▼xE= -ðB/ðt
2. ▼x H=ðD/ðt
3. ▼.D=0
4. ▼.B=0
UNIT -2
1. what are the other names for signal attenuation?
Fiber loss & signal loss are the other names for signal attenuation.
2. what is atomic defects?
Atomic defects are imperfections of the atomic structure of the fiber material such as
missing molecules and high density crystals.
3. what are the basic attenuation mechanisms?
(i) absorption
(ii) scattering
(iii)radiation losses
4 .define attenuation?
Signal attenuation is defined as the ratio of the optical output power from a fiber of
length L to the optical input power .This is the function of wavelength.
Attenuation is given by,
= 10/L log (Pin/Pout)
5. what are the classification of absorption?
1. atomic defects.
2. extrinsic absorption
3. intrinsic absorption
6. reasons for occurrence of scattering losses?
Scattering losses arise from microscopic variations in the material
density, from compositional fluctuations, and from structural inhomogenities
occurring during fiber manufacture.
7. State the two effects which gives rise to refractive index variations which
occur within the glass?
a)glass is composed of a randomly connected network of molecules
which contains regions in which the molecular density is either higher or lower
than the average density in the glass.
b)since glass is made up of several oxides such as SiO2,
GeO2, P2O5, compositional fluctuations can occur.
8 .give the reasons for scattering loss ?
1. microscopic variations in the material density.
2. structural inhomogenities.
3. defects due to manufacturing.
9. What is macroscopic bending loss?
The loss which occurs when a fiber cable turns a corner is known as
macroscopic bending loss.
10. What is microscopic bending loss?
The loss which occurs due to the microscopic ends of the fiber axis that can
arise when the fibers are incorporated into cables.
11 .define information capacity determination.
A measure of the information capacity of an optical waveguide is usually
specified by the bandwidth –distance product in MHz.km.
12. What is group delay?
When the light propagates along the fiber ,each spectral component can be
assumed to travel independently, and to undergo a time delay or group delay
per unit length.
13. How material dispersion occurs?
Material dispersion occurs because the index of refraction varies as a
function of the optical wavelength.
14. What is inter modal dispersion?
Intermodal dispersion is a signal degradation which is a result of different
values of the group delay for each individual mode at a single frequency.
15. Define effective cutoff wavelength.
The effective cutoff wavelength is defined as the largest wavelength at
which the order mode LP11 mode power relative to the fundamental LP01
mode power is reduced to 0.1 dB.
16. advantages of mode coupling.
Mode coupling tends to average propagation delay associated with the
modes thereby reduce intermadal dispersion.
17. What are the advantages of single mode fiber?
1. long installation lifetime
2. very low attenuation.
3. high quality signal transfer.
4. large bandwidth.
18. design optimization characters.
1. refractive index profile.
2. cutoff wavelength.
3. dispersion.
4. mode field diameter.
5. bending loss.
19. dispersion flattening fiber.
It is the alternate method to reduce fiber dispersion by spreading the
dispersion over a broader range ,this is known as dispersion flattening . It is
used for optical wavelength division multiplexing.
20. What are the 2 types of dispersion flattening fiber?
1)double clad (or) w profile
2)quadrupole-clad profile
21. What are the 2 types of distortion?
1. intermodal distortion
2. intramodal distortion
22 .What are the main causes of intramodal dispersion?
Material dispersion ,which arise from the variation of the refractive index
of the core material as a function of wavelength.
Waveguide dispersion which occurs because a single mode fiber only confines
about 80% of the optical power to the core.
23. define intermodal delay?
Intermodal delay is a result of each mode having a different value of group
velocity at a single frequency.
.
24. how structural inhomogenities are created & what are their forms?
Structural inhomogenities are created during fiber and can also cause
scattering of light out of the fiber. These defects may be in the form of trapped
gas bubbles, unreacted starting materials and crystallized regions in the glass.
25. why losses of multimode fibers are higher than there of single mode fibers?
The losses of multimode fibers are generally lesser than those of
single mode fibers. This is a result of higher dopant concentrations and the
accompanying larger scattering loss due to greater compositional fluctuation in
multimode fibers.
UNIT -3
FIBER OPTICAL SOURCES
1. What are the different classifications of semiconductors ?
1. Direct band gap materials
2. Indirect band gap materials
The recombination of an electron and hole is carried out by emission of
a photon .If the electrons and holes has the same momentum then it is called
as direct band gap materials.
For indirect band gap materials the conduction band minimum and valence
band minimum energy level occur at different values of momentum.
2. Define radiance or brightness
Radiance is a measure in watts of the optical power radiated in to a unit
solid angle per unit area of the emitting surface .High radiance are needed
to couple high optical power levels into a fiber .
3. What are the factors that an LED should have for useful fiber transmission
applications?
1. High radiance output
2. Fast emission response time
3. High quantum efficiency .
4. Define emission response time
Emission response time is define as the time delay between the application of
current pulse at the optical junction .This time delay is a factor limiting the band
width with which the source is modulated directly by varying the injected current .
5. Which are the two basic LED configurations used for fiber ?
1. Surface emitter
2. Edge emitter
For surface emitter the plane of active light emitting region is oriented
perpendicular to the axis of the fiber .
Edge emitter consist of an active junction and two guiding layers .the guiding
layers have refractive index lower than the active region .
6. Define population inversion
Stimulated emission of electrons occur only if the population of the excited
states is greater than the ground state .This condition is called population
inversion .Lasers use this technique for light emission .
7. What is internal quantum efficiency ?
Internal quantum efficiency is the fraction of electron hole pairs that recombine
radiatively .
Ŋ0 =Rr/(Rr+Rnr)
Rr – radiative recombination rate per unit volume
Rnr - non radiative recombination rate per unit volume
Ŋ0 = τ /τr
τ- Bulk recombination life time& τr-radiative recombination lifetime
8. What is diffusion length and how it is related to bulk recombination life time?
In a semiconductor light source, as the charge carriers diffuse through the
materials, some will disappear by recombination. They move a distance Le and Lh
For electrons and holes respectively .This distance is know as diffusion length .
L=(Dτ)1/2
Where
D - Diffusion co-efficient
τ - Bulk recombination life time
L – Diffusion length
9. Define modes of a cavity
The optical radiation within the cavity of a laser diode sets up a pattern of
electric and magnetic field lines called modes of the cavity .They are
conventionally separated in to two independent sets of TE and TM modes ,Each
set of modes can be described in terms of longitudinal ,lateral and transverse half
sinusoidal electro magneticfields along the major axis of the cavity.
10. Which are the compounds used as direct bandgap materials ?
Direct gap materials are made from compounds of group 3 elements (such as Al,
Ga or In) and group 5 elements (such as P, As or Sb ) .For operation in the 800 -
900 nm spectrum, the material used is GaAlAs . For longer wavelengths we use
InGaAsP .
11. Why GaAlAs and InGaAsP are chosen to make semiconductors light sources ?
This is because it is possible to match the lattice parameters of the hetero
structure interfaces by using proper combination of binary ,tertiary and quaternary
materials.
12. Define external differential quantum efficiency ?
The external quantum efficiency is defined as the number of photons emitted
per radiative electron hole pair recombination above the threshold.
ŋext = ŋi(gth-ά)/gth
13. Define gain-guided lasers?
The injection of electrons and holes into a device alters the refractive index of
the active layer directly below the stripe. The profile of these injected carriers
creates a weak , complex waveguide that confines the light laterally. This device is
commonly referred to as gain-guided lasers.
14. Define index guided lasers?
The dielectric waveguide structures are fabricated in lateral directions. The
variation of the real refractive index of various materials in these structures control
the lateral structures in the laser. These devices are called as index guided lasers.
15. Which are the two types of index guided lasers?
1. Positive index
2. Negative index
In positive index waveguide, the central region has more refractive index than the
Outer regions. In negative index waveguides, the active layer has lower refractive
index than outer regions.
16 .Define single mode lasers?
If a particular index guided laser supports only the fundamental transverse mode
and the fundamental longitudinal mode, then that lasers are called as single mode
lasers.
17. Which are the three types of laser configurations in single mode fibers?
1. Distributed feedback lasers
2. Distributed Bragg reflector lasers
3. Distributed reflector lasers.
18. What are the modulation methods used to vary the optical output from laser
diodes?
1. Pulse modulation in digital systems
2. Amplitude modulation in analog data transmissions.
19. What are the four basic current configuration methods?
1. Preferential dopant diffusion
2. Proton implantation
3. Inner stripe confinement
4. Regrowth of back biased PN junction .
20. List the four methods for the construction of lasers?
1. Burried hetero structure
2. Selectively diffused construction
3. Varying thickness structure
4. Bent layer configurations
21 What is the special feature of the vertical cavity surface emitting lasers?
The special feature is that light emission is perpendicular to a semi conductor surface.
This feature facilitates the integration of multiple lasers on to a single chip in one or two
dimensional arrays which make them attractive for wavelength division multiplexing
applications.
22 .Define photon lifetime
Photon life time is an average lifetime that the photon resides in the lasing cavities before
being lost either by absorption or by emission through the facets. It is denoted by τph
23. What are the advantages of the laser diodes over LEDs?
1. faster response time, so that much greater modulation rates are possible with the laser
diodes.
2. A narrower spectral width of the output which implies less dispersion induced signal
distortion .
3. A much higher optical power level that can be coupled into the fiber with the laser
diode, thus allowing greater transmission distances.
24. What are the draw backs of laser diodes?
1. Their construction is more complicated .
2. The optical output power level is strongly dependent on temperature
3. They are susceptible to catastrophic face at degradation which readily reduces the
device life time
25. Brief on temperature effects laser diodes.
The threshold current of laser diodes are temperature dependent. This parameter
increases with temperature in all types semiconductor lasers because of various complex
temperature dependent factors. Ith(T) = Ize(T/To)
To-relative temperature insensitivity and Iz is a constant
UNIT-4
1.To achieve high signal to noise ratio, what are the conditions should be satisfied?
The photodetector must have a high quantum efficiency to generate a large signal
power.
The photodetector and amplifier noises should be kept as low as possible.
2.What is minimum detectable optical power?
The sensitivity of a photodetector in an optical fiber communication system is
desirable in terms of the minimum detectable optical power. This is the optical power
necessary to produce a photocurrent of the same magnitude as the root mean square of
the total noise current .
3.What is quantum efficiency?
The quantum efficiency ή is the number of electron- hole carrier pairs generated
per incident photon of energy hν and is given by
number of electron –hole pairs generated
ή =
number of incident photons
4.Define ionization rate.
The average number of electron-hole pairs created by a carrier per unit distance
travelled is called ionization rate.
5.What is impact ionization ?
In high field region,in avalanche photodiodes electron or hole can gain enough
energy so that it ionizes bound electrons in the valence band upon colliding with
them.This carrier multiplication mechanism is known as impact ionization.
6.What is meant by photo carriers?Why is it called so?
When an incident photon has an energy greater than or equal to the bandgap
energy of the semiconductor material, the photon can give up its energy and excite an
electron from the valence band to the conduction band.This process generate free
electron hole pairs ,which are known as photocarriers since they are photon generated
charge carriers.
7.What is the mechanism used in APD?
APD’s internally multiply the primary signal photocurrent before it enters the
circuitry of the following amplifier.Inorder for carrier multiplication to take place,the
photo generated carriers must traverse the region where a very high electric field is
present.In this high field region a photogenerated electron or hole can gain enough energy
so that it ionizes bound electron in the valence band upon colliding with them.This carrier
multiplication mechanism is known as impact ionization.
8.What are the three factors determining the response time of a photodiode together
with its output circuit?
1.The transit time of the photocarriers in the depletion region .
2.The diffusion time of the photo carriers generated outside the depletion region
3.The RC time constant of the photodiodes and its associated circuit.
9.How do you measure the photodetector performance by using ionization rate?
The ratio of hole ionization rate to the electron ionization rate is the measure of
photodetector performance
β
K=
Α
10.What is meant by photodetector?
The first element of optical receiver photodetector,senses the luminescent
power falling upon it and converts thenvariation of this power into correspondingl varying
elelctric current.
Eg:Pin photodiode,Avalanche photodiode
11.What is non zero extinction ratio ε?
Non zero extinction ratio is defined as the ratio of the optical power in a
zero pulse to the power in a one pulse.
Ε=boff /bon
12.What is quantum limit?
The minimum received optical power required for a specific bit error rate
performancein a digital system is called as the quantum limit.
13.What makes the evaluation of the shot noise more difficult than that of the thermal
noise?
The non Gaussian nature of the photo detection and the avalanche
multiplication noise makes the evaluation of the shot noise more difficult than that of the
thermal noise.
14.Define pulse penalty?
The additional or excess power required for pulse shape other than the
impulses is normally defined as the power penalty. It is measured in decibels.
bon nonimpulse
Δp=10 log
bon impulse
15.Define bit error rate(BER)
The ratio of number Ne of errors occurring over a certain limit interval t,by
the number Nt of pulses transmitted during this interval.
BER=Ne /Nt
16.Give an application of analog amplifier
Extension of microwave and satellite links
CATV
Transmission system
17.Mention the variations of on the SAM (separate absorption and multiplication)structure
due to the addition of other layers to the device
-Using the grading layer between the absorption and multiplication regions to
increase the response time and bandwith of the device
-Adding a charge layer that provides better control of the electric field
profile
-Incorporating the resonant cavity that decouples the optical and electrical path
lengths to achieve high quantum efficiency and bandwidths
18.Explain the effect of temperature an avalanche gain
If the operating temperature decreases and the applied bias voltage is kept
constant the ionization rate for electrons and holes will increase and so will the avalanche
gain.
19. What are the wavelengths required for short and long distance?
The wavelength provided for the devices to operate for a short distance is 850nm and that
for long link is 1300nm &1550nm
20.Define signal to noise ratio at the output of an optical receiver
S/N = signal power from photocurrent /(photo detector noise
power+amplifier noise power)
21.Define excess noise factor F
The ratio of actual noise generated in an avalanche photodiodes ot the noise that wopuld
exist if all carrier pairs were multiplied by exactly m is called excess noise factor.
F=<m2> /<m>2
=<m2> /M2
22.Define photo diode dark current
The photo dioe dark current is the current that continues to flow through the
bias circuit of the device when no light is incident on the photodiodes.
23.What is meant by surface dark current?
Surface dark current is also referred to as surface leakage current or simply
leakage current.It depends on surface defects ,cleanliness,bias voltage and surface areas
24.Why semiconductor based photodetector is used almost for fiber system?
The semiconductor based photodetector have the following properties
1.small size
2.suitable material
3.high sensitivity
4.fast response time
25.Define responsivity
R=Ip /Po
=nq /nv
where Ip=average photocurrent generated by a steady average
optical power Po incident on a photodetector.
.
