SUBJECT/CODE:EC1402 : OPTICAL COMMUNICATION BRANCH:VII-SEM-ECE
DURATION : 3hrs. MAX.MARKS:100
MODEL EXAMINATION-III OCT.2010
ANSWER ALL QUESTIONS
PART-A 10X2 = 20Marks
1. Give the refractive index expression of a graded index fiber.
n ( r ) = n1[ 1-2Δ(r/a)α]1/2 for 0≤ r ≤a
= n1[ 1-2Δ] 1/2 = n1[ 1 - Δ] = n2 for r ≥ a
2. A point source of light if 12 cm below the surface of a large body of water (n=1.33). What is the radius of the largest circle on the water surface through which the light can emerge?
Find θc from Snell's law
n1*sinθ1=n2*sinθ2=1
n2=1.33
then θc=48.75 degree
Find r from tan θc= r / 12cm
r =13.7 cm.
n1*sinθ1=n2*sinθ2=1
n2=1.33
then θc=48.75 degree
Find r from tan θc= r / 12cm
r =13.7 cm.
3. Give the expression of the effective number of modes that are guided by a curved multimode fiber of radius ‘a’.
Neff = N∞{ 1- (α+2)/(2αΔ)[(2a/R)+(3/2ncladkR)2/3]}
Where α defines the graded index profile
Δ is the core-cladding index difference, k is the propagation constant
R is the radius of the curvature of the bent fiber.
a is the fiber core radius
N∞ is the total number modes in straight fiber.
4. Define normalized propagation constant.
Normalized propagation constant is defined by
b = {β2/k2-n2clad}/ {n2core - n2clad }
Where k is propagation constant
5. An LED has radiative and non radiative recombination times of 30 and 100ns respectively. Determine the internal quantum efficiency.
The internal quantum efficiency in the active region is the fraction of electron hole pairs that recombine radiatively. It’s given by
ηi=Rr / (Rr + Rnr)
ηi is the internal quantum efficiency
Rr is the radiative recombination per unit volume.
Rnr is the non radiative recombination rate
6. Define internal quantum efficiency of a LED.
The internal quantum efficiency in the active region is the fraction of electron hole pairs that recombine radiatively. It’s given by
ηi=Rr / (Rr + Rnr)
ηi is the internal quantum efficiency
Rr is the radiative recombination per unit volume.
Rnr is the non radiative recombination rate
7. An APD generates a current of 100nA when the incident power is 5nW. The operating wavelength is 1.5 µm. Find its responsivity. If the quantum efficiency is 0.7, find the multiplication factor.
Ip = RPo = (ηq/hv) Po
= (ηqλ/hc) Po
= {(0.7)(1.6 x 10-19)(1.5 x 10-6)/(6.625 x 10-34 x 3 x 108)}5 x 10-9
= 4.263nA
The multiplication factor(M) = Im / Ip = 100nA/4.263nA = 23.46
8. Define Responsivity.
R = Ip/P0 = ηq/hν where Ip-average photocurrent generated
P0- incident optical power level
η -quantum efficiency
9. What are solitons?
The term “Soliton” refers to special kinds of waves that can propagate undistorted over long distances and remain after collisions with each other. John Scott Russell made the first recorded observation of a soliton in 1838,when he saw a peculiar type of wave generated by boats in narrow Scottish canals.
In optical communication systems, solitons are very narrow, high-intensity optical pulses that retain their shape through the interaction of balancing pulse dispersion with the nonlinear properties of an optical fiber.
The set of pulses that do not change in shape are called fundamental solitons, and those that undergo periodic shape changes are called higher-order solitons.
10. What is the significance of rise time budget?
A rise time budget analysis is a convenient method for determining the dispersion limitation of an optical fiber link
tsys = ( ttx2 + tmod 2+ tmat 2+ trx2 ) 1/2
PART-B(16x5 = 80)
11. a. (i) Derive an expression for numerical aperture of a step index fiber(10)
(ii) The relative refractive index difference between the core axis and the cladding of a guided index fiber is 0.7% when the refractive index at the core axis 1.45. Estimate values for the numerical aperture of the fiber along the axis when the index profile is assumed to be triangular.(6)
a. i. NA = nsinθ0,max = sqrt(n12 – n22 ) = n1 sqrt(2Δ)
OR
b. Discuss the theory of graded index fibers. Derive the expression for numerical
aperture of graded index fiber.(16)
Refer Page No. from 65 to 66 (in Keiser)
n ( r ) = n1[ 1-2Δ(r/a)α]1/2 for 0≤ r ≤a
= n1[ 1-2Δ] 1/2 = n1[ 1 - Δ] = n2 for r ≥ a
NA( r ) = NA(0) [1-(r/a)α]1/2 for r ≤a
= 0 for r > a
12.a. Explain with suitable diagrams the different mechanism that contribute to
attenuation in optical fiber. (16)
Absorption (4)
Scattering (4)
Bending (4)
Core and cladding (4)
Refer Page No. from 94 to 103 (in Keiser)
OR
b. i) A continuous 40-km-long optical fiber link has a loss of 0.4 dB/Km
(a) What is the minimum optical power level that must be launched into the
fiber to maintain an optical power level of 2 mw at the receiving end?
(b) What is the required input power if the fiber has a loss of o.6 db/km
ii) What is meant by waveguide dispersion? Derive the expression (8)
Refer Page No. from 109 to 111 (in Keiser)
13.a. Explain briefly the three key processes involved in the laser action. Describe for a fabry
perot resonator laser diode, modes and threshold conditions. Obtain its rate equations for
steady state output. (16)
Refer Page No. from 163 to 171 (in Keiser)
OR
b. (i) Discuss the different misalignment losses and their magnitudes.(8)
Refer Page No. from 218 to 223 (in Keiser)
(ii)A step-index fiber has a core refractive index 1.5 and a core diameter of 50μm
The fiber is joined with a lateral misalignment between the core axes of 5μm.
Estimate the lateral efficiency and the insertion loss at the joint due to lateral
misalignment.(8)
14.a. With neat diagram explain operation and characteristics of PIN photodiode.
(16)
Refer Page No. from 244 to 248 (in Keiser)
OR
b. (i) A silicon PIN photodiode incorporated into an optical receiver has a quantum efficiency of 60% at a wavelength of 0.9μm. The dark current is 3nA and load resistance is 4kΩ. The incident optical power is 200nW and the receiver bandwidth is 5MHz. Determine
1. Mean square quantum noise current.
2. Mean square dark current
3. Mean square thermal noise current at a temperature of 20ºC.(8)
(ii) The quantum efficiency of particular silicon RAPD is 80% for the detection
of radiation at a wavelength of 0.9μm, when the incident optical power is
0.5μW. The output current from the device (after avalanche gain) is 11μA.
Determine the multiplication factor of the photodiode under these conditions.
15.a Explain briefly link power budget and rise time budget analysis in optical
communication system. (16)
Refer Page No. from 325 to 330 (in Keiser)
OR
b. Explain the salient features of solitons using relevant expressions and diagrams.(16)
Refer Page No. from 506 to 510 (in Keiser)
SUBJECT/CODE:EC1402 : OPTICAL COMMUNICATION BRANCH:VII-SEM-ECE
DURATION : 3hrs. MAX.MARKS:100
ANSWER ALL QUESTIONS
PART-A 10X2 = 20Marks
1. Define acceptance angle and critical angle of the fiber.
If the angle of incidence Φ1 is increased a point will eventually be reached where the light ray in air is parallel to the glass surface. This point is known as critical angle of incidence Φc.
2. Calculate the number of modes in the graded index fiber at an operating wavelength of 850nm.
3. What is the need for mode coupling in optical fiber?
Coupling energy from one mode to another is called mode coupling. It arises because of structural imperfections, fiber diameter and refractive index variations and cabling induced in micro bends.
4. Define Group delay.
It is defined as the ratio of distance travelled by pulse to the group velocity,
τg = L/Vg
5. What is meant by indirect band gap?
In indirect band gap materials direct transition is not possible from valence band to conduction. e.g.silicon, germanium.
6. Define quantum efficiency of an LED.
The internal quantum efficiency in the active region is the fraction of electron hole pairs that recombine radiatively. It’s given by
ηi=Rr / (Rr + Rnr)
ηi is the internal quantum efficiency
Rr is the radiative recombination per unit volume.
Rnr is the non radiative recombination rate
7. 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.
8. What are the benefits of transimpedance amplifier?
Since the high impedance produces large input RC time constant, the front end bandwidth is less than the signal bandwidth. This drawback is overcome in the trans impedance preamplifier
9. What is the purpose of rise time budget analysis?
A rise time budget analysis is a convenient method for determining the dispersion limitation of an optical fiber link
tsys = ( ttx2 + tmod 2+ tmat 2+ trx2 ) 1/2
10. What is meant by Solitons?
The term “Soliton” refers to special kinds of waves that can propagate undistorted over long distances and remain after collisions with each other. John Scott Russell made the first recorded observation of a soliton in 1838,when he saw a peculiar type of wave generated by boats in narrow Scottish canals.
In optical communication systems, solitons are very narrow, high-intensity optical pulses that retain their shape through the interaction of balancing pulse dispersion with the nonlinear properties of an optical fiber.
The set of pulses that do not change in shape are called fundamental solitons, and those that undergo periodic shape changes are called higher-order solitons.
PART-B (5x16=80)Marks
11.(a) Derive an expression to determine the modes propagation in Step Index fiber.(16)
[OR]
(b)(i) Explain with neat block diagram the fundamentals of optical fiber
Communication (8)
(ii) Discuss the propagation of meridional rays along with the acceptance angle and
numerical aperture of step index fiber. (8)
12.(a) Explain with suitable diagrams the different mechanism that contribute to
attenuation in optical fiber. (16)
[OR]
(b)Write a brief note on design optimization with relevant expression and diagram(16)
13.(a) Explain the basic LED configuration used as optical source. Derive the expression
for quantum efficiency and optical power generated in LEDs. (16)
[OR]
(b)(i) Explain the steps involved in splicing the fiber. Discuss the various splicing
techniques employed between 2 fibers. (8)
(ii) Explain the lensing scheme used to improve optical source to fiber coupling
efficiency. (8)
14.(a) With neat diagram explain operation and characteristics of PIN photodiode. (16)
[OR]
(b)(i) Draw and explain high impedance preamplifier design based on BJT and FET.(8)
(ii) Write a brief note on transimpedance amplifier (8)
15.(a) Explain briefly link power budget and rise time budget analysis in optical
communication system. (16)
[OR]
(b)(i)Explain the principle of WDM? (8)
(ii)Explain the basic concept of SONET Network. (8)
| | | |
SUBJECT/CODE:EC1402 : OPTICAL COMMUNICATION BRANCH:VII-SEM-ECE
DURATION : 3hrs. MAX.MARKS:100
MODEL EXAMINATION-III OCT.2010
ANSWER ALL QUESTIONS
PART-A 10X2 = 20Marks
1. What are the linearly polarized modes?
These modes are having same electric field patterm and the propagation constant and the modes are said to be degenerate. The degenerate modes are called linearly polarized modes.
2. Calculate the numerical aperture of a step index fiber having n1 = 1.48 and n2 = 1.46.
NA=√ n12 – n22
3. What is meant by Polarization Mode Dispersion?
The difference in propagation times between two orthogonal polarization modes will results in pulse spreading. This is PMD.
4. What is intermodal dispersion? What does it cause?
Intermodal dispersion is pulse spreading that occurs within a single mode. It arises due to group velocity being a function of wavelength. The increasing spectral width of the optical source will increase the intramodal dispersion.
5. What is population Inversion?
Stimulated emission will exceed absorption only if the population of the excited status is greater than that of the ground state. This condition is known as population inversion.
6. Compare LED and LASER Diode.
LED: 1. Optical output is incoherent.2. Output radiation has a broad spectral width.
LASER: 1.Optical output is coherent 2. very directional.
7. Define Quantum efficiency and responsivity of photo detector.
Quantum efficiency is defined as the number of electron hole pair generated per incident photon of energy and responsivity is defined as the ratio of output photocurrent to the incident optical power.
8. Compare the performance of APD and PIN Diode.
PIN:1. Sensivity: less sensitive 2. Gain : no internal gain
APD: 1. more sensitive 2. internal gain
9. Define Modal Noise.
Modal noise arises when the light from a coherent lase is coupled into a multimode fiber.
10. What are the advantage of WDM?
1. capacity upgrade
2. transparency
PART-B (5x16=80)Marks
11.(a) (i)Derive the wave equation in Step Index fiber. (12)
(ii) Mention the advantage of optical fiber communication system (4)
[OR]
(b)(i) Discuss briefly about Linearly polarized mode. (8)
(ii)Draw the structure of single and multimode step index fiber and graded index
fiber with their typical dimension . (8)
12.(a) Discuss material and waveguide dispersion mechanisms with necessary mathematical
expressions. (16)
[OR]
(b)(i)Explain the scattering and bending losses that occur in an optical fiber with
relevant diagrams and expressions. (8)
(ii)Discuss polarization mode dispersion and its limitations. (8)
13.(a)(i) Draw and explain the different structure used to achieve carrier and optical
confinement in laser diodes. (10)
(ii) Explain the modulation process involved in LED and discuss its frequency
response. (6)
[OR]
(b)(i) Derive an expression for the power coupled from a surface emitting LED into step
index and graded index fibers. (10)
(ii) Explain mechanical misalignment that can occur between 2 joined fibers with
necessary diagrams. (6)
14.(a)With neat diagram explain operation and characteristics of avalanche photodiode. (16)
[OR]
(b)(i) Discuss with necessary expressions the different types of noise that affect the
performance of a photodetector. (10)
(ii) Write a brief note on transimpedance amplifier (6)
15.(a) (i)What are the system consideration in point to point links? Explain in detail. (10)
(ii) Explain the link power budget in optical communication system. (6)
[OR]
(b)Explain the salient features of solitons using relevant expression and diagrams. (16)
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