1. The InGaAsP/InP double channel planar DFB-BH laser with a quarter wavelength shifted first order grating provides a single frequency operation and incorporates a phase shift of ______________
a) π/2 Radians
b) 2π Radians
c) π Radians
d) 3π/2 radians
Explanation: The performance of DFB laser is improved by modifying a grating, which in turn introduces an optical phase shift. The phase shift depends on the wavelength used. A quarter wavelength shifted first order grating incorporates the phase shift of π/2 in the corrugation at the center of laser cavity.
2. The narrow line-width obtained under the CW operation for quarter wavelength shifted DFB laser is ________________
a) 2 MHz
b) 10 MHz
c) 3 MHz
d) 1 MHz
Explanation: A quarter wavelength shifted DFB laser provides a large gain difference between the central mode and side modes. It provides improved dynamic single mode stability. Narrow line-width of around 3 MHz can be obtained under CW operation.
3. Line-width narrowing is achieved in DFB lasers by a strategy referred as _______________
a) Noise partition
b) Grating
c) Tuning
d) Bragg wavelength detuning
Explanation: Line-width narrowing is achieved in DFB lasers by detuning the lasing wavelength towards the shorter wavelength side of gain peak. It increases the differential gain between the central mode and nearest side mode. This strategy is called as Bragg wavelength detuning.
4. _________________ is a technique used to render the non-conducting material around the active cavity by producing permanent defects in the implanted area.
a) Dispersion
b) Ion de-plantation
c) Ion implantation
d) Attenuation
Explanation: Ion implantation approach concentrates the injection current in active region. Current confinement is realized by ion implantation. Ions are implanted into a selective area of a semiconducting material to make it non-conducting.
5. 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
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.
6. 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
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).
7. 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
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
8. High strain in strained MCQ structure should be incorporated.
a) True
b) False
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
9. The parameter that prevents carrier from recombination is __________
a) Auger recombination
b) Inter-valence band absorption
c) Carrier leakage
d) Low temperature sensitivity
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.
10. 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
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.