Optical Communications Multiple Choice Questions on “Mid Infrared and Far Infrared Lasers”.
1. The parameters having a major role in determining threshold current of efficiency of injection laser are ___________
a) Angle recombination and optical losses
b) Frequency chirping
c) Relaxation oscillation
d) Mode hopping
Answer: a
Explanation: Optical losses due to free carrier absorption are more because of their dependence on square of the wavelength. Also irradiative recombination through Auger recombination contributes to it. Both these effects cause more problems in md-infrared wavelengths and so are of much importance art high temperature due to high concentration of free carriers. They also limit maximum operating temperatures.
2. Auger current is mostly ___________________ for material with band gap providing longer wavelength emission.
a) Unaffected
b) Lesser
c) Larger
d) Vanishes
Answer: c
Explanation: The total current required for injection laser threshold is more than that provided to radioactive recombination as Auger current is added. This current depends on electronic band structure of material and often consists of different Auger transitions. So it is larger for materials with band gaps providing longer wavelength emission.
3. Injection lasers operating in smaller wavelengths are subjected to increased carrier losses.
a) True
b) False
Answer: b
Explanation: Injection lasers operating in longer wavelengths (mid and far infrared) are subjected to increased carrier losses as compared to devices operating up to 1.6μm. This is from nonradiative recombination through Auger interaction. This recombination energy is dissipated as thermal energy to other free carriers. If band gap of semiconductor is increased, occurrence of these events gets increased.
4. Devices based on quaternary PbSnSeTe and their ternary compounds, emit at wavelength?
a) Between 3-4 μm
b) Longer than 4 μm
c) Between 3.5 to 4.2 μm
d) Between 2 to 3 μm
Answer: b
Explanation: Quaternary devices emit at wavelength longer than 4μm. Auger effects are less in these alloys which provide lower current thresholds and higher maximum operating temperature.
5. Replacing Sn with Eu, Cd or Ge in some _________________ the band gap.
a) Remove the band gap
b) Does not affect
c) Decreases
d) Increases
Answer: d
Explanation: When in a particular alloy laser for example PbSnSeTe, if Sn is replaced with Eu, Cd or Ge, there is an increase in band gap. This increase in band gap provides the laser to operate in shorter wavelength.
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6. Lasing obtained in __________ when 191 mW of pump light at a wavelength of 0.477 μm is launched into laser.
a) Ternary PbSnSeTe alloy laser
b) Quaternary PbSnSeTe alloy laser
c) Doped Fluoro-zirconate fiber
d) Ternary PbEuTe alloy laser
Answer: c
Explanation: When Fluoro-zirconate fiber lasers are doped with Erbium helium or thulium, there are emission at 2-3 μm wavelength range. But lasing was obtained in this doped Fluorozirconate fiber at a wavelength of 0.477μm.
7. The thulium doped fiber laser when pumped with alexandrite laser output at 0.786 μm, the laser emits at ___________
a) 0.6 μm
b) 0.8 μm
c) 2.3 μm
d) 1.2μm
Answer: c
Explanation: The thulium system emits at 2.3 μmwhen subjected to alexandrite laser at 0.786 μm. this system is four levels in which the pump band is upper lasing level at 2.3μm.
8. The diode-cladding-pumped Erbium praseodymium-doped fluoride device operates at wavelength.
a) Around 3 μm
b) 4 μm
c) 2.6 μm
d) 1.04 μm
Answer: a
Explanation: The diode-cladding-pumped Erbium praseodymium-doped fluoride device operates at a wavelength of 3 μm. This laser is capable of producing a very high output power of about 1W or more. It consists of double clad fluoride fiber.
9. A technique based on inter-sub band transition is known as ___________
a) Auger recombination
b) Frequency chirping
c) Inter-valence band absorption
d) Quantum cascading
Answer: d
Explanation: The quantum cascaded laser is a layered semiconductor device having a series of coupled quantum wells grown on GaAs or Imp substrate. This principle of QC lasers provides emission of an optical signal around full wavelength range. Quantum mechanical band structure determines the emitted wavelength.
10. In a QC laser, a same electron can emit number of photons.
a) True
b) False
Answer: a
Explanation: The QC laser operates by pumping a energy level and then using the energy in a controlled manner. This gives some energy each time over several steps. And since a QC laser structure includes a series of energy levels the same electron emits a number of photons while cascading down through each energy level.
11. The phenomenon resulting in the electrons to jump from one state to another each time emitting of photon is known as ___________
a) Inter-valence band absorption
b) Mode hopping
c) Quantum cascading
d) Quantum confinement
Answer: d
Explanation: In Quantum confinement, charge carriers are trapped in a small area and this occurs in quantum wells at nanometer scale. When the quantum layer size raises to a size comparable to emission wavelength, the electron motion becomes perpendicular to plane of layer. Due to this, the electrons jump from one state to another each time from one state to another.
12. A QC laser is sometimes referred as ___________
a) Unipolar laser
b) Bipolar laser
c) Gain guided laser
d) Non semiconductor laser
Answer: a
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.
13. 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
Answer: d
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.
14. QC lasers ______________ the performance characteristics.
a) Have negligible effects
b) Does not affects
c) Improves
d) Degrades
Answer: c
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.
15. 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
Answer: c
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.
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