Instruction
- Total number of questions : 12
- Time allotted : 18:00 Minutes:Seconds.
- Each question carry 1 mark, no negative marks.
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Optical Communication [ Question No: 1 To 12 ] :
Direction for [ Question No: 1 To 1 ] :
Assertion-Reason type questions : The following items consist of two statements, one labelled as 'Assertion (A)', and the other labelled the 'Reason (R)'. You are to examine these two statements and decide if the Assertion (A) and the Reason (R) are individually true and if so, whether the reason is a correct explanation of the Assertion. Select your answer to these items using the codes given below and mark your answer sheet accordingly.
Codes :
(A) Both (A) and (R) are true and (R) is the correct explanation of (A)
(B) Both (A) and (R) are true but (R) is not the correct explanation of (A)
(C) (A) is true but (R) is false
(D) (A) is false and (R) is true
Assertion (A) : Graded index optical fibers are used for reasonably high band width application.
Reason (R) : Graded index fibers are designed to have low time dispersion.
The following optical fiber has no effect of impurity in the axis region in its transmission properties :
The following technique is best suited for low cost optical fibers :
Lithium niobate is a combination of :
Match the following :
| Set I | Set II |
|---|---|
| (a) Line width | (i) Wavelength |
| (b) Directionality | (ii) Coherence |
| (c) Energy | (iii) Energy stored |
| (d) Quality factor | (iv) Scattering |
In a four level laser system :
The following light source has the best directivity :
The following is the most important requirement for laser :
The following are the components in optical communication :
(i) Modulator
(ii) Wavelength Division Multiplexer
(iii) Repeaters
(iv) Time Division Multiplexer
The correct sequence of their placement from transmission side is :
Match List I with List II and select the correct answer using codes given below :
| List I (Optical Wavelength) | List II |
|---|---|
| (a) 1440 nm | (i) Low loss |
| (b) 850 nm | (ii) High speed |
| (c) 1300 nm | (iii) High loss |
| (d) 1550 nm | (iv) Low cost |
Molecular size of \(SiO_{2}\) in optical fiber leads to the following :
Which is the major cause of dispersion in multimode step index optical fiber ?
Total number of questions : 12
Number of answered questions : 0
Number of unanswered questions : 12
Optical Communication [ Question No: 1 To 12 ] :
Direction for [ Question No: 1 To 1 ] :
Assertion-Reason type questions : The following items consist of two statements, one labelled as 'Assertion (A)', and the other labelled the 'Reason (R)'. You are to examine these two statements and decide if the Assertion (A) and the Reason (R) are individually true and if so, whether the reason is a correct explanation of the Assertion. Select your answer to these items using the codes given below and mark your answer sheet accordingly.
Codes :
(A) Both (A) and (R) are true and (R) is the correct explanation of (A)
(B) Both (A) and (R) are true but (R) is not the correct explanation of (A)
(C) (A) is true but (R) is false
(D) (A) is false and (R) is true
Assertion (A) : Graded index optical fibers are used for reasonably high band width application.
Reason (R) : Graded index fibers are designed to have low time dispersion.
Your Answer : (Not Answered)
Correct Answer : A
The answer is both (A) and (R) are true and (R) is the correct explanation of (A).
In a step-index multimode fiber, rays travelling at steep angles cover a longer path than axial rays and arrive later, spreading each pulse (modal or intermodal dispersion) and limiting bandwidth. A graded-index fiber has a refractive index that falls gradually from the core axis outward, so the longer outer paths are travelled faster. Arrival times equalise and time dispersion is low.
Low dispersion means pulses can be sent closer together, which is what gives the reasonably high bandwidth, so R is exactly the reason for A.
The following optical fiber has no effect of impurity in the axis region in its transmission properties :
Your Answer : (Not Answered)
Correct Answer : A
The key names graded index fibre.
I could not find a text that states this property for any of the four fibres, so I cannot confirm the key; nothing I found shows it wrong either. What can be said is how the options differ. A single-mode fibre carries nearly all its light near the axis of a very thin core, so axis impurities affect it more than any other fibre, which makes option B the least likely. A plastic-clad silica fibre has a step-index core and is limited mainly by the cladding, not by the axis.
In a graded-index fibre the refractive index is highest at the axis and falls smoothly outward, so rays are bent back toward the axis rather than travelling through it, and the key reads this as making the axis region less critical. The explanation is the University's reading, not a confirmed textbook statement.
The following technique is best suited for low cost optical fibers :
Your Answer : (Not Answered)
Correct Answer : D
The technique best suited for low cost fibers is the double crucible method.
In the double crucible method, molten core glass in an inner crucible and cladding glass in an outer crucible flow out together through concentric nozzles and are drawn directly into fibre. It is a continuous process that works with low-melting multicomponent (soda-lime or borosilicate) glasses and needs no preform, so it is cheap, though the fibres have higher loss.
VAD, OVD and MCVD are vapour deposition methods that build a high-purity silica preform; they give very low-loss telecommunication fibre but at higher cost.
Lithium niobate is a combination of :
Your Answer : (Not Answered)
Correct Answer : A
Lithium niobate combines electro-optic, piezo-electric and optical properties.
LiNbO3 is a ferroelectric crystal with a large electro-optic effect, so its refractive index changes with an applied voltage, which is used in high-speed optical modulators. It is strongly piezoelectric, used in surface acoustic wave filters, and it has good nonlinear optical properties for frequency doubling and waveguides.
The other options each name only part of this combination, or add properties such as optical activity or chemical behaviour that are not what makes lithium niobate useful.
Match the following :
| Set I | Set II |
|---|---|
| (a) Line width | (i) Wavelength |
| (b) Directionality | (ii) Coherence |
| (c) Energy | (iii) Energy stored |
| (d) Quality factor | (iv) Scattering |
Your Answer : (Not Answered)
Correct Answer : D
The keyed matching is a-ii, b-iv, c-i, d-iii.
- (a) A narrow spectral line width means a long coherence time, so line width is tied to coherence (ii).
- (b) Directionality, the spread of the beam, is degraded by scattering in the medium (iv).
- (c) Photon energy is fixed by the wavelength, \(E = hc/\lambda\) (i).
- (d) The quality factor of a laser cavity is the ratio of the energy stored to the energy lost per cycle (iii).
The other codes pair energy with coherence or stored energy, or line width with wavelength, which breaks these relationships.
In a four level laser system :
Your Answer : (Not Answered)
Correct Answer : C
In a four level laser the lower lasing level is empty.
In a four level system, atoms are pumped from the ground level to a high pump band, decay quickly to the upper laser level, lase down to the lower laser level, and then decay rapidly back to the ground state. Because the lower laser level lies above the ground state and empties quickly, it stays almost unpopulated, so population inversion between the laser levels is easy to reach, with far less pump power than in a three level laser such as ruby, where the lower level is the ground state. Nd:YAG and He-Ne lasers work this way.
The 'four levels' are energy levels, not blocks of the system, stages of pumping, or a sign of high power.
The following light source has the best directivity :
Your Answer : (Not Answered)
Correct Answer : B
The source with the best directivity is the edge emitting LED.
A surface emitting LED, of which the Burrus LED is the classic type, emits from a broad area with a nearly Lambertian pattern, about 120° wide. An edge emitting LED confines the light in a thin active stripe sandwiched between heterojunction layers, like a waveguide, and emits from the edge in a much narrower beam, especially in the plane perpendicular to the junction (about 30°). Its higher radiance and directivity give better coupling into fibres.
The double heterojunction is a structure both types use for carrier and optical confinement, not a separate geometry with the best directivity.
The following is the most important requirement for laser :
Your Answer : (Not Answered)
Correct Answer : B
The keyed requirement is stimulated emission.
The word laser stands for Light Amplification by Stimulated Emission of Radiation. An incoming photon causes an excited atom to emit a second photon of the same frequency, phase, direction and polarisation; this is what makes laser light coherent and allows amplification.
Pumping (optical or electrical) and the optical feedback of the resonator are needed to create population inversion and sustain oscillation, but they serve the stimulated emission process. Optical pumping is only one way of pumping (diode lasers are pumped by current), and current modulation is a way of encoding data, not a requirement for lasing.
The following are the components in optical communication :
(i) Modulator
(ii) Wavelength Division Multiplexer
(iii) Repeaters
(iv) Time Division Multiplexer
The correct sequence of their placement from transmission side is :
Your Answer : (Not Answered)
Correct Answer : D
The official key gives the order modulator, time division multiplexer, wavelength division multiplexer, repeaters, option D.
On the transmitting side each message signal is first put into a form suitable for the link by a modulator (for digital links, PCM or another modulator), several such channels are then combined in time by the TDM multiplexer, several TDM streams are placed on separate optical wavelengths by the WDM multiplexer, and repeaters along the fibre restore the signal strength afterwards. Repeaters always come last, which already excludes options B and C.
What could not be confirmed: a real system could also place the TDM before the modulator, and no source for this exact question was found. Option A puts the repeaters before the WDM, and option C puts the TDM before the modulator and the repeaters after the WDM. The official key is kept.
Match List I with List II and select the correct answer using codes given below :
| List I (Optical Wavelength) | List II |
|---|---|
| (a) 1440 nm | (i) Low loss |
| (b) 850 nm | (ii) High speed |
| (c) 1300 nm | (iii) High loss |
| (d) 1550 nm | (iv) Low cost |
Your Answer : (Not Answered)
Correct Answer : C
The correct matching is a-iii, b-iv, c-ii, d-i.
- 1440 nm lies near the water (OH) absorption peak around 1383 nm, so it is a high loss region (iii).
- 850 nm is the first window, served by cheap GaAs LEDs and silicon detectors: low cost (iv).
- 1300 nm is near the zero-dispersion wavelength of standard silica fiber, which allows high speed (ii).
- 1550 nm is the minimum-attenuation window, about 0.2 dB/km: low loss (i).
The codes printed as options A and D are identical and both give 1440 nm low loss and 1550 nm high loss, the reverse of the attenuation curve.
Molecular size of \(SiO_{2}\) in optical fiber leads to the following :
Your Answer : (Not Answered)
Correct Answer : B
It leads to scattering.
Glass is an amorphous material, and when silica cools from the melt, tiny fluctuations in density and composition, of the order of molecular size and much smaller than the wavelength of light, are frozen into it. These inhomogeneities scatter light in all directions. This Rayleigh scattering varies as \(1/\lambda^{4}\) and is the fundamental loss mechanism that sets the lower limit of fiber attenuation.
Absorption arises from impurities such as OH ions and from the material's own electronic and vibrational resonances. Pulse dispersion is spreading in time, not loss, and strain is a mechanical effect.
Which is the major cause of dispersion in multimode step index optical fiber ?
Your Answer : (Not Answered)
Correct Answer : A
The major cause is intermodal dispersion.
A multimode step index fiber carries many modes, which travel along paths of different lengths: a ray along the axis covers less distance than one zigzagging at the critical angle. Since all travel in the same core material at the same speed, they arrive at different times, and a short input pulse spreads out. This intermodal (modal) delay is far larger than any other dispersion in such a fiber.
Material (chromatic) and waveguide dispersion arise from the variation of speed with wavelength. They dominate in single-mode fiber, where intermodal dispersion is absent, but they are small compared with modal dispersion in multimode step index fiber.
Marks
Total number of questions : 12
Number of answered questions : 0
Number of unanswered questions : 12