This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “Injection Laser Characteristics”.
1. The threshold temperature coefficient for InGaAsP devices is in the range of __________
a) 10-40 K
b) 40-75 K
c) 120-190 K
d) 150-190 K
View Answer
Explanation: The threshold temperature coefficient for InGaAsP devices is in between 40 and 75 K. This range shows higher temperature sensitivity due to intrinsic physical properties of InGaAsP material system, Auger recombination, inter-valence band absorption, carrier leakage effects over hetero-junctions.
2. The process where the energy released during the recombination of an electron-hole event getting transferred to another carrier is known as __________
a) Inter-valence bond absorption
b) Auger recombination
c) Carrier leakage effects
d) Exothermic actions
View Answer
Explanation: Auger recombination is a process where energy is released. This energy is released during the recombination of electron-hole and this released energy is transferred to another electron-hole event. During this process, when a carrier is excited to a higher energy level, it loses its excessive amount of energy by emitting a phonon in order to maintain thermal equilibrium. It consists of number of different processes each process involving three particles (2 electrons and 1 or 2 holes and 1 electron).
3. Auger recombination can be reduced by using __________
a) Strained MQW structure
b) Strained SQW structure
c) Gain-guided strained structure
d) Strained Quantum dots lasers
View Answer
Explanation: Auger recombination is a process where energy is released during recombination of electron-hole event is transferred to another event. This loss mechanism can be reduced by using strained by using MCQ laser structure. Strain can be either compressive or tensile, modifying the valence band energy levels of material and therefore can be used to increase energy.
4. High strain in strained MCQ structure should be incorporated.
a) True
b) False
View Answer
Explanation: Strain is introduced in thin layers of quantum wells by making small differences in lattice constants. High strain should be avoided because it causes damage in these thin-quantum layers. Also carrier leakage adds at high temperatures since it represents processes that prevent carrier from recombination thus reducing device efficiency.
5. The parameter that prevents carrier from recombination is __________
a) Auger recombination
b) Inter-valence band absorption
c) Carrier leakage
d) Low temperature sensitivity
View Answer
Explanation: Carrier leakage is the parameter that prevents carriers (electrons, holes) from recombination. At high temperatures, carrier leakage represents all those processes preventing carriers from recombination. It therefore increases the lasing threshold and thus reduces device efficiency.
6. Determine the threshold current density for an AlGaAs injection laser with T0=180k at 30°C.
a) 6.24
b) 9.06
c) 3.08
d) 5.09
View Answer
Explanation: The threshold current density for a laser is given by-
Ith = exp(T/T0)
For AlGaAs device,
Ith(30)=exp(T/T0) = exp(293/180) = 5.09.
7. The phenomenon occurring when the electron and photon population within the structure comes into equilibrium is known as __________
a) Auger recombination
b) Inter-valence band absorption
c) Carrier leakage
d) Relaxation oscillations
View Answer
Explanation: Phenomenon occurring when the electron and photon population within the structure comes into equilibrium is known as Relaxation oscillations. The application of a current state to device resulting in a switch delay which is followed by high frequency damped oscillations.
8. When a current pulse reaches a laser having parasitic capacitance after the initial delay time, that pulse will __________
a) Have no effect
b) Will get vanished
c) Becomes narrower
d) Gets broader
View Answer
Explanation: The pulse will be broadened when it will reach a laser with parasitic capacitance after initial time delay. This is because when a current pulse reaches the laser, the parasitic capacitance of laser provides a source of current over the period when there is high photon density. As electron density is repetitively built up and reduced quickly, there will be several pulses at laser output as photon density will be high resulting in relaxation oscillations.
9. Reducing delay time and ____________ are of high importance for lasers.
a) Auger recombination
b) Inter-valence band absorption
c) Carrier leakage effects
d) Relaxation oscillations
View Answer
Explanation: For lasers generally a switch-on delay time may last for 0.5ns and relaxation oscillations behind twice that period. This behavior can produce serious deterioration in shape of laser pulse at a data rate of 100Mbits. So time delay and Relaxation oscillations are highly desirable for lasers.
10. Dynamic line-width broadening under the direct modulation of injection current is known as __________
a) Auger recombination
b) Inter-valence band absorption
c) Carrier leakage effects
d) Frequency Chirping
View Answer
Explanation: Frequency Chirping is a phenomenon which is due to Dynamic line-width broadening under direct modulation of a single longitudinal mode. Semiconductor laser cause a dynamic shifting of peak wavelength emitted from device. Strong coupling between the free carrier density and refractive index of device present in semiconductor structure results in gain-induced variations which also causes Frequency Chirping.
11. A particular characteristic or parameter that occurs during analog transmission of injection lasers is?
a) Noise
b) Mode hopping
c) Carrier leakage effects
d) Frequency Chirping
View Answer
Explanation: During analog transmission, noise behavior of device is main thing that affects the operation of injection laser. This noise may be due to instabilities in kinks in light output versus current characteristics, reflection of light back to device and mode partition noise.
12. Intensity of output from semiconductor injection lasers leading to optical intensity noise is due to __________
a) Fluctuations in amplitude
b) Mode hopping
c) Carrier leakage effects
d) Frequency Chirping
View Answer
Explanation: Fluctuations in the laser output or intensity of laser output leads to optical intensity noise. These fluctuations are generally caused by temperature variations and spontaneous emission in the laser output. This randomness in fluctuations creates a noise source known as relative intensity noise (RIN).
13. In multimode lasers the optical feedback from unnecessary external reflections affecting stability of frequency and intensity is?
a) Remains unaffected
b) Increased gradually
c) Reduced
d) Gets totally vanished
View Answer
Explanation: The effect due to unwanted external reflections in multimode laser is reduced. This is because the reflections are spread along any fiber modes so they are weakly coupled back into laser mode.
14. Reduction in the number of modes in multimode fiber increases the mode partition noise.
a) False
b) True
View Answer
Explanation: Mode partition noise is a result of laser spectral fluctuations and so a reduce in number of modes results in low pulse-width spreading thus providing low values of intermodal dispersion in the fiber. And so, the mode partition noise is decreased in multimode fiber due to reducing the number of modes.
15. The behavior of laser occurring when current is increased above threshold particularly is?
a) Mode hopping
b) Auger recombination
c) Frequency chirping
d) Noise
View Answer
Explanation: Mode hopping results in the hopping of modes to a higher wavelength. This mode hopping occurs in all injection lasers and is due to increase in temperature. Mode hopping is not a continuous function of drive current but occurs above 1 to 2 mA. Mode hopping alters characteristics of laser and results in kinks in characteristics of single mode device.
Sanfoundry Global Education & Learning Series – Optical Communications.
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