This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “Some Injection Laser Structures”.
1. In multimode injection lasers, the construction of current flow to the strip is obtained in structure by __________
a) Covering the strip with ceramic
b) Intrinsic doping
c) Implantation outside strip region with protons
d)Implantation outside strip region with electrons
Explanation: The current flow is realized by implanting the region outside strip with protons. This implantation makes the laser highly resistive and gives superior thermal properties due to absence of silicon dioxide layer.
2. What is the strip width of injection laser?
a) 12 μm
b) 11.5 μm
c) Less than 10 μm
d) 15 μm
Explanation: A strip width less than or equal to 10 μm is usually preferred in injection lasers. This width range provides the lasers highly efficient coupling into multimode fibers as comapred to single mode fibers.
3. Some refractive index variation is introduced into lateral structure of laser.
Explanation: Gain guided lasers possess several undesirable characteristics, nonlinearities in light output versus current characteristics, high threshold current, low differential quantum efficiency, movement of optical a;ong junction plane. This problems can be reduced by introducing refractive index variations into lateral structure of lasers so that optical mode is determined along the junction plane.
4. Buried hetero-junction (BH) device is a type of _____________ laser where the active volume is buried in a material of wider band-gap and lower refractive index.
a) Gas lasers.
b) Gain guided lasers.
c) Weak index guiding lasers.
d) Strong index guiding lasers.
Explanation: In strong index guiding lasers, a uniformly thick, planar active waveguide is achieved by lateral variations in confinement layer thickness or refractive index. In Buried hetero-junction (BH) devices, strong index guiding along junction plane introduces transverse mode control in injection lasers.
5. In Buried hetero-junction (BH) lasers, the optical field is confined within __________
a) Transverse direction
b) Lateral direction
c) Outside the strip
d) Both transverse and lateral direction
Explanation: Optical field is strongly confined in both transverse and lateral direction. This provides strong index guiding of optical mode along with good carrier confinement.
6. A double-channel planar buried hetero-structure (DCP BH) has a planar active region, the confinement material is?
a) Alga AS
Explanation: The planar active region made up of InGaAsP can be seen in double-channel planar buried hetero-structure (DCP BH). This material confinement provides a very high power operation with CW output power up to 40 mW in longer wavelength region.
7. Problems resulting from parasitic capacitances can be overcome __________
a) Through regrowth of semi-insulating material
b) By using oxide material
c) By using a planar InGaAsP active region
d) By using a AlGaAs active region
Explanation: The use of reverse-biased current confinement layers introduces parasitic capacitances which reduces high speed modulation of BH lasers. This problem can be reduced by regrowth of semi-insulating material or deposition of dielectric material. This causes increase in modulation speeds of 20 GHz.
8. Quantum well lasers are also known as __________
a) BH lasers
b) DH lasers
c) Chemical lasers
d) Gain-guided lasers
Explanation: DH lasers are known as Quantum well lasers. The carrier motion normal to active layer is restricted in these devices. This results in quantization of kinetic energy into discrete energy levels for carriers moving in that direction. This phenomenon is similar to quantum mechanical problem of one dimensional potential well which is seen in DH lasers.
9. Quantum well lasers are providing high inherent advantage over __________
a) Chemical lasers
b) Gas lasers
c) Conventional DH devices
d) BH device
Explanation: Quantum well lasers exhibit high incoherent advantage over conventional DH lasers. In Quantum well laser structures, the thin active layer results in drastic changes in electronic and optical properties over conventional DH laser structures. This changes are due to quantized nature of discrete energy levels with step-like density and also allow high gain and low carrier density.
10. Strip geometry of a device or laser is important.
Explanation: Near fluid intensity distribution corresponding to single optical output power level in plane of junction can be seen in GaAs or AlGaAs lasers. This distribution is in lateral direction and is determined by the nature of lateral waveguide. The single intensity maximum shows the fundamental lateral mode is dominant.
11. Better confinement of optical mode is obtained in __________
a) Multi Quantum well lasers
b) Single Quantum well lasers
c) Gain guided lasers
d) BH lasers
Explanation: As compared to all lasers including single quantum well lasers, multi-Quantum well lasers are having better confinement of optical mode. This results in a lower threshold current density for these devices.
12. Multi-quantum devices have superior characteristics over __________
a) BH lasers
b) DH lasers
c) Gain guided lasers
d) Single-quantum-well devices
Explanation: Lower threshold currents, narrower bandwidths, high modulation speeds, lower frequency chirps and less temperature dependence are parameters determining characteristics of a particular laser. All the above parameters make multi-quantum devices superior over DH lasers.
13. Dot-in-well device is also known as __________
a) DH lasers
b) BH lasers
c) QD lasers
d) Gain guided lasers
Explanation: Quantum well lasers are devices in which device contains a single discrete atomic structure or Quantum-dot. These are elements that contain electron tiny droplets which forms a quantum well structure.
14. A BH can have anything from a single electron to several electrons.
Explanation: Quantum-dot lasers are fabricated using semiconductor crystalline materials. They have a particular dimension ranging from nm to few microns. The size, shape of these structures and number of electrons they contain are precisely controlled.
15. QD lasers have a very low threshold current densities of range __________
a) 0.5 to 5 A cm-2
b) 2 to 10 A cm-2
c) 10 to 30 A cm-2
d) 6 to 20 A cm-2
Explanation: Low-threshold current density between 6 to 20 A cm-2 is obtained with InAs/InGaAs QD lasers which emit at a wavelength of 1.3 μm and 1.5 μm Such low values of threshold current densities make these lasers possible to create stacked or cascaded QD structures. These structures provide high optical gain for short-cavity transmitters and vertical cavity surface-emitting lasers.
Sanfoundry Global Education & Learning Series – Optical Communications.
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