1. A QC laser is sometimes referred as ___________
a) Unipolar laser
b) Bipolar laser
c) Gain guided laser
d) Non semiconductor laser
Explanation: A QC laser utilizes only n-type of charge carriers. Their operation is entirely based on electrons and holes play no part in this, so they are known as unipolar lasers.
2. In QC lasers, it is possible to obtain different output signal wavelengths. This can be achieved by ___________
a) Inter-valence band absorption
b) Mode hopping
c) Quantum cascading
d) Selecting layers of different thickness
Explanation: In QC laser, electrons emit energy. This energy emitted at this stage determines wavelength of radiation and it depends only on thickness of the layer. Thus output signal wavelength is dependent on thickness of lasers.
3. QC lasers ______________ the performance characteristics.
a) Have negligible effects
b) Does not affects
c) Improves
d) Degrades
Explanation: QC lasers are based on inter sub band transition techniques. They have ability of carrying large amount of currents. A single electron is enough to generate number of photons. Thus, provides an increase in output signal power which is greater than thousands at same wavelength due to large number of cascaded stages.
4. An MQW cascaded laser is more advantageous because of ___________
a) Mode hopping
b) Auger recombination
c) Control over layers of material
d) Properties of material
Explanation: In MQW cascaded layers, cascading creates number of injector/collector and active region in single stage. Each region contains a single quantum wells. Such structures permit maximum injection/collection of current and thereby produce a large number of photons. This formation of any injector/collector and active regions is achieved through precise control of several hundreds of layers of the material, where each layer should only be few nanometers thick.
5. The amount of radiance in planer type of LED structures is ____________
a) Low
b) High
c) Zero
d) Negligible
Explanation: Planer LEDs are fabricated using liquid or vapor phase epitaxial processes. Here p-type is diffused into n-type substrate which creates junction. Forward current flow through junction provides Lambertian spontaneous emission. Thus, device emits light from all surfaces. However a limited amount of light escapes the structure due to total internal reflection thus providing low radiance.
6. In optical fiber communication _____________ major types of LED structures are used.
a) 2
b) 4
c) 6
d) 3
Explanation: Optical fiber communication involves the use of 6 different major LED structure. These are the surface emitter, edge emitter, the super luminescent, the resonant cavity LED, planar LEDs and Dome LEDs.
7. As compared to planar LED structure, Dome LEDs have ______________ External power efficiency ___________ effective emission area and _____________ radiance.
a) Greater, lesser, reduced
b) Higher, greater, reduced
c) Higher, lesser, increased
d) Greater, greater, increased
Explanation: In Dome LEDs, the diameter of dome is selected so as to maximum the internal emission reaching surface within critical angle of GaAs. Thus, dome LEDs have high external power efficiency. The geometry of Dome LEDs is such that dome is much larger than active recombination area, so it has greater emission era and reduced of radiance.
8. The techniques by Burros and Dawson in reference to homo structure device is to use an etched well in GaAs structure.
a) True
b) False
Explanation:Burros and Dawson provided a technique to restrict emission to small active region within device thus providing high radiance. Etched well in a GaAs substrate is used to prevent heavy absorption of emitted region and physically accommodating the fiber. These structures provide low thermal impedance allowing high current densities of high radiance.
9. In surface emitter LEDs, more advantage can be obtained by using ____________
a) BH structures
b) QC structures
c) DH structures
d) Gain-guided structure
Explanation: DH structures provide high efficiency from electrical and optical confinement. Along with efficiency, they provide less absorption of emitted radiation.
10. Internal absorption in DH surface emitter Burros type LEDs is ____________
a) Cannot be determined
b) Negligible
c) High
d) Very low
Explanation: The larger band gap confining layers and the reflection coefficient at the back crystal space is high in DH surface emitter Burros type LEDs. This provides good forward radiance. Thus these structure LEDs have very less internal absorption