This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “LED Characteristics”.
1. Intrinsically _________________ are a very linear device.
a) Injection lasers
b) DH lasers
c) Gain-guided
d) LEDs
View Answer
Explanation: The ideal light output power against current characteristics for an LED linear. This tends to be more suitable for analog transmission where several constraints are put in linearity of optical source.
2. Linearizing circuit techniques are used for LEDs.
a) True
b) False
View Answer
Explanation: In practice, LEDs exhibit nonlinearities depending on configuration used. Thus, to allow its used in high quality analog transmission system and to ensure linear performance of device, linearizing circuit techniques is used.
3. The internal quantum efficiency of LEDs decreasing _______________ with ________________ temperature.
a) Exponentially, decreasing
b) Exponentially, increasing
c) Linearly, increasing
d) Linearly, decreasing
View Answer
Explanation: The light emitted from LEDs decreases. This is due to increase in p-n junction temperature. Thus, this results in exponentially decreasing internal quantum efficiency with temperature increment.
4. To utilize _____________________ of SLDs at elevated temperatures, the use of thermoelectric coolers is important.
a) Low-internal efficiency
b) High-internal efficiency
c) High-power potential
d) Low-power potential
View Answer
Explanation: The output characteristics of SLDs are typically of nonlinear in nature. This is observed with a knee becoming apparent at an operating temperature around 20 degree c. Thus, to utilize high-power potential of these devices at elevated temperature, thermoelectric coolers are necessarily used.
5. For particular materials with smaller bandgap energies operating in _____________ wavelength, the linewidth tends to ______________
a) 2.1 to 2.75 μm, increase
b) 1.1 to 1.7 μm, increase
c) 2.1 to 3.6 μm, decrease
d) 3.5 to 6 μm, decrease
View Answer
Explanation: For materials with smaller bandgap, linewidth increases to 50 to 160 nm. This increases in band gap is due to increased doping levels and formation of bandtail states.
6. The active layer composition must be adjusted if a particular center wavelength is desired.
a) True
b) False
View Answer
Explanation:There is a difference in output spectra between surface and edge emitting LEDs when devices have generally heavily doped and lightly doped active layers by reduction in doping.
7. In optical fiber communication, the electrical signal dropping to half its constant value due to modulated portion of optical signal corresponds to _______
a) 6 dB
b) 3 dB
c) 4 dB
d) 5 dB
View Answer
Explanation: Modulation bandwidth in optical communication is often defined in electrical/optical terms. So when considering electrical circuitry in optical fiber system, electrical 3 dB point or frequency at which output electrical power is reduced by 3 dB bandwidth with respect to input electrical power.
8. The optical 3 dB point occurs when currents ratio is equal to _____________
a) \(\frac{8}{3}\)
b) \(\frac{2}{2}\)
c) \(\frac{1}{2}\)
d) \(\frac{3}{4}\)
View Answer
Explanation: In optical regime, the bandwidth is defined by frequency at which output current has dropped to ½ output input current system.
9. The optical bandwidth is _____________ the electrical bandwidth.
a) Smaller
b) Greater
c) Same as
d) Zero with respect to
View Answer
Explanation: The difference between optical and electrical bandwidth In terms of frequency depends on the shape of the frequency response of the system. If the system response is assumed to be Gaussian, then optical bandwidth is a factor of √2 greater than electrical bandwidth.
10. When a constant d.c. drive current is applied to device, the optical o/p power is 320 μm. Determine optical o/p power when device is modulated at frequency 30 MHz with minority carrier recombination lifetime of LED i.e. 5ns.
a) 4.49*10-12
b) 6.84*10-9
c) 1.29*10-6
d) 2.29*10-4
View Answer
Explanation: The output o/p at 30 MHz is
Pc(30 MHz) = Pdc/(1+(wΓi)2)1/2
= 320*10-6/(1+(2π*30*10-6*5*10-9)2)1/2
= 2.29*10-4W.
11. The optical power at 20 MHz is 246.2 μW. Determine dc drive current applied to device with carrier recombination lifetime for LED of 6ns.
a) 3.48*10-4
b) 6.42*10-9
c) 1.48*10-3
d) 9.48*10-12
View Answer
Explanation: The optical output power at 20 MHz is
Pe(20 MHz) = Pdc/(1+(WTi)2)1/2
246.2*10-6 = Pdc/(1+(2π*20*10-6*5*10-9)2)1/2
Pdc = 3.48*10-4.
12. Determine the 3 dB electrical bandwidth at 3 dB optical bandwidth Bopt of 56.2 MHz.
a) 50.14
b) 28.1
c) 47.6
d) 61.96
View Answer
Explanation: The 3dB electrical bandwidth is given by
B = Bopt/ √2
= 56.2/2
= 28.1 MHz.
13. The 3 dB electrical bandwidth B is 42 MHz. Determine 3dB optical bandwidth Bopt.
a) 45.18
b) 59.39
c) 78.17
d) 94.14
View Answer
Explanation: The 3dB electrical bandwidth is
B = Bopt/√2
Bopt = B*√2
= 42*√2
= 59.39 MHz.
14. Determine degradation rate βrif constant junction temperature is 17 degree celsius.
a) 7.79*10-11
b) 7.91*10-11
c) 6.86*10-11
d) 5.86*10-11
View Answer
Explanation: The degradation rate βris determined by
βr = β0exp (-Ea/KT)
= 1.89*107exp (-1*1.602*10-19/1.38*10-23*290)
= 7.79*10-11 h-1.
15. Determine CW operating lifetime for LED with βrt = -0.58 and degradation rate βr = 7.86*10-11 h-1.
a) 32.12
b) 42
c) 22.72
d) 23.223
View Answer
Explanation: The CW operating lifetime is given by
t = Ln 0.58/7.86*10-11
= 22.72h-1.
Sanfoundry Global Education & Learning Series – Optical Communications.
To practice all areas of Optical Communications, here is complete set of 1000+ Multiple Choice Questions and Answers.
If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]
