Sub.Name/Code:EC1402:Optical Communication Max.Marks:100
Branch : B.E (ECE) SEM :VII
Answer all the questions
PART-A (10x2 = 20M)
1. Mention any four required properties of light sources used in the optical communication.
· High radiance output
· Fast emission response time
· High quantum efficiency
· Dimensional characteristics compatible with those of optical fibers.
2. What is meant by laser action? What are the conditions to achieve it?
Laser action is the result of three key processes
1. Photon absorption
2. Spontaneous emission
3. Stimulated emission
The conditions to achieve laser action are
1. Magnitude of guided mode should be greater than the threshold value.
2. At the lasing threshold, a steady state oscillation takes place, and the
magnitude and phase of the returned wave must be equal to those of the
original wave.
3. What are direct band gap and indirect band semiconductors?
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
4. 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
5. An LED has radiative and non radiative recombination times of 30 and 100ns respectively. Determine the internal quantum efficiency.
6. What are the required properties of photo detector?
1. High response or Sensitivity
2. Minimum addition of noise
3. Fast response speed
4. Insensitive to variations in temperature
5. Compatibility with physical dimensions of fiber.
6. Long operating life.
7. Reasonable cost.
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 quantum efficiency of photo detector.
It is defined as the number of the electron – hole pairs generated per incident photon of energy hv.
9. Why do we prefer trans-impedance preamplifier rather than high impedance preamplifier?
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.
10. 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.
PART-B (16x5=80M)
11. a. (i) Draw a typical double-hetero structure light emitter and explain (8)
(ii)Draw the schematic of surface emitting and edge emitting LED and explain (8)
OR
b. (i) Derive the threshold condition for lasing (6)
(ii) Explain the laser action with neat diagrams (6)
(iii) A GaAs laser operating at 850nm has a 500μm length and a refractive index n=3.7.What are the
frequency and wavelength spacing? (4)
12. a. (i) Derive the internal quantum efficiency of an LED (6)
(ii) Discuss laser diode structures and radiation patterns (10)
OR
b. (i) What is quantum well laser? Explain its geometry and features (6)
(ii) Discuss the temperature effect on threshold current of a laser diode (6)
(iii) A GaAs injection laser with a cavity length of 500μm has a loss co-efficient of 20 cm. the measured differential external
quantum efficiency of laser is 45,the refractive index of GaAs 3.6.calculate the internal quantum efficiency of the laser. (4)
13. a. (i) Explain various fiber splicing techniques and their merits and demerits. (10)
(ii) Explain the different splice losses and their measurement (6)
OR
b. (i) Draw the structure of avalanche photodiode and electric fields in the depletion and multiplication regions.
Also explain the principle of operation of APD.(8)
(ii) A given silicon avalanche photodiode has a quantum efficiency of 65% at a wavelength of 900nm.If 0.5μW
of optical power produces a multiplied photo current of 10 μA, find the multiplication factor (M). (8)
14. a. (i) Discuss the different noise sources and disturbances in the optical pulse detection mechanism.(6)
(ii) Derive an expression for the bit error rate of an optical digital receiver.(10)
OR
b. (i) Draw and explain the high impedance preamplifier designs based on BJT and FET.(8)
(ii) Write a brief note on trans impedance amplifier.(8)
15. a. Discuss in detail digital receiver performance calculation and sensitivity calculation in detail (16)
OR
b. (i) Describe the construction and working of PIN diode (10)
(ii) When 3x1011photons each with a wavelength & 0.5 μm are incident an a photo diode , on average 1.2x1011electrons are
collected at the terminals at the device.Determine the quantum efficiency and the responsivity of the photo diode at
0.85 μm (6)
Sub.Name/Code:EC1402:Optical Communication Max.Marks:100
Branch : B.E (ECE) SEM :VII
Answer all the questions
PART-A (10x2 = 20M)
1. What are the laser light properties? How are they produced
a. High radiance output
b. Fast emission response time
c. High quantum efficiency
d. Dimensional characteristics compatible with those of optical fibers.
High radiance and high quantum efficiency are achieved through carrier and optical confinement using double hetero structure.
2. Why do we prefer laser diodes over LED’s for communication applications?
1. High intensity radiation
2. Narrow spectral width of the laser source are the preferable features of
Laser compared to LED.
3. What are direct band gap semiconductors?
In direct band gap materials direct transition is possible from valence band to conduction band. e.g.GaAs, InP, InGaAs
4. Explain the radiation pattern from surface emitting laser diode or LED
The emission pattern is isotropic with a 120 degree half-power beam width, i.e., source is equally bright when viewed from any direction, but the power diminishes as cosθ, where θ is the angle between the viewing direction and normal to the surface.
5. Distinguish between splice and connector.
| SPLICE | CONNECTOR |
| 1. A permanent joint formed between two individual optical fibers in the field or factory. | In order to maintain an optimum performance the connection must also protect the fiber ends from damage which may occur due to handling(connection and disconnection), must be insensitive to environmental factors(e.g. moisture and dust) and cope with tensile load on the cable. |
| 2. It is used to establish long-haul optical fiber links where smaller fiber lengths need to be joined, and there is no requirement for repeated connection and disconnection. | The connector design must allow for repeated connection and disconnection without problems of fiber alignment, which may lead to degradation in the performance of the transmission line at the joint. |
| 3.Two types-Fusion splicing and Mechanical splicing. | |
6. What is dark current noise?
The dark current noise arises from electrons and/or holes which are thermally generated in the PN junction of the photo diode. This current continues to flow through the bias circuit of the device when no light is incident on the photodiode
7. What are the different factors that determine the response time of photo detector?
i. The transit time of the photo carriers in the depletion region.
ii. The diffusion time of the photo carriers generated outside the depletion region.
iii. The RC time constant of the photodiode and its associated circuit.
8. Define probability of error. Write an expression for it.
The probability of error is defined as Pe = a P1(vth) + bP0(vth)
Where a and b are weighting factors
P1(vth) – Probability that the equalizer output voltage is less than vth when a logical one pulse is sent.
P0(vth) - Probability that the equalizer output voltage exceeds vth when a logical zero pulse is transmitted.
9. A given APD has a quantum efficiency of 65%at a wavelength 900nm. If 0.5 μW of optical power produces a multiplied photocurrent of 10 μA, find the multiplication factor
Ip = RPo = (ηq/hv) Po = (ηqλ/hc) Po
= {(0.65)(1.6 x 10-19)(9 x 10-7)/(6.625 x 10-34 x 3 x 108)}5 x 10-7
= 0.235μA
The multiplication factor(M) = Im / Ip = 10μA/0.235μA = 43
10. Compare the performance of APD and PIN diode
The avalanche photodiode (APD), is also reverse-biased. The difference with the PIN diode is that the absorption of a photon of incoming light may set off an electron-hole pair avalanche breakdown, creating up to 100 more electron-hole pairs. This feature gives the APD high sensitivity (much greater than the PIN diode).
PART-B (16x5=80M)
11. a. (i) Draw a typical double-hetero structure light emitter and explain (8)
(ii)Draw the schematic of surface emitting and edge emitting LED and explain (8)
OR
b. Discuss different types of fiber optic amplifiers and also compare their properties. (16)
12.a. (i) Explain various fiber splicing techniques and their merits and demerits. (10)
(ii) Explain the different splice losses and their measurement (6)
OR
b. (i) Explain the different factors affecting the source to fiber power launching(8)
(ii) Estimate the power delivered to different types of fibers with respect to LEDs(8)
13.a. (i) Draw and explain the different structures used to achieve carrier and optical confinement in laser diodes.(10)
(ii) Discuss the effects of temperature on the performance of a laser diode. (6)
OR
b.Describe briefly the construction and working and characteristics of PIN diode (16)
14. a.(i)Explain the different types of noise that affect the performance of a photodetector(10)
(ii)When 3x1011photons each with a wavelength & 0.5 μm are incident an a photo diode , on average 1.2x1011electrons
are collected at the terminals at the device. Determine the quantum efficiency and the responsivity of the photo diode
at 0.85 μm (6)
OR
b. Discuss in detail digital receiver performance calculation and sensitivity calculation in detail (16)
15.a. (i) Discuss the different noise sources and disturbances in the optical pulse detection mechanism.(6)
(ii) Derive an expression for the bit error rate of an optical digital receiver.(10)
OR
b. What is the role of preamplifier in optical receiver? Explain in brief different types of preamplifier available? (16)
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