This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “Mid Infrared and Far Infrared Photodiodes”.
1. In the development of photodiodes for mid-infrared and far-infrared transmission systems, lattice matching has been a problem when operating at wavelengths ____________
a) 1 µm
b) Greater than 2 µm
c) 2 µm
d) 0.5 µm
View Answer
Explanation: Lattice matching for alloy materials is obtained at wavelengths above 2 µm. For example, a lattice-matched alloy material system (GaSb) was utilized in a p-i-n photodiode for high speed operation at wavelengths up to 2.3 µm.
2. What is generally used to accommodate a lattice mismatch?
a) Alloys
b) Attenuator
c) Graded buffer layer
d) APD array
View Answer
Explanation: The use of indium alloy cause inherent problems of dislocation-induced junction leakage and low quantum efficiency. To avoid these problems, a compositionally graded buffer layer is used to accommodate a lattice mismatch.
3. HgCdTe material system is utilized to fabricate long-wavelength photodiodes.
a) True
b) False
View Answer
Explanation: HgCdTe family alloys allow resonant characteristics via hole ionization. Its band gap energy variation enables optical detection to far-infrared. Thus, this material can be used for fabrication of long-wavelength photodiodes.
4. Avalanche photodiodes based on HgCdTe are used for ______________ in both the near and far infrared.
a) Dispersion
b) Dislocation
c) Ionization
d) Array applications
View Answer
Explanation: Avalanche photodiodes based on HgCdTe are used for array applications. The materials of APDs based on HgCdTe possess uniform avalanche gain across an array. This variation in gain is variation in gain is lower in HgCdTe as compared with silicon.
5. The detection mechanism in ____________ relies on photo excitation of electrons from confined states in conduction band quantum wells.
a) p-i-n detector
b) Quantum-dot photo detector
c) p-n photodiode
d) Avalanche photodiodes
View Answer
Explanation: Quantum-dot photo detector’s detection mechanism involves photo excitation of electrons. This process of photo excitation in photo detectors is similar to that in the Quantum-dot semiconductor optical amplifier. The dots-in-well in Quantum-dot detector is called as DWELL structure.
6. When determining performance of a photo detector ___________ is often used.
a) No. of incident photon
b) No. of electrons collected
c) Responsivity
d) Absorption coefficient
View Answer
Explanation: The expression for quantum efficiency does not include photon energy. Thus for characterizing performance of photo detector, Responsivity is used.
7. The important parameter for exciting an electron with energy required from valence band to conduction band is?
a) Wavelength
b) Absorption coefficient
c) Responsivity
d) Band gap energy
View Answer
Explanation: As wavelength of incident photon becomes longer, the photon energy is less than energy required to excite electron. Mostly parameters of photodiode are dependent on wavelength.
8. __________ is less than or unity for photo detectors.
a) Absorption coefficient
b) Band gap energy
c) Responsivity
d) Quantum efficiency
View Answer
Explanation: Quantum efficiency determines the absorption coefficient of semiconductor material of photo detector. It is not all incident photons are absorbed to create electron-hole pairs. Thus quantum efficiency must be less than unity.
9. There must be improvement in __________ of an optical fiber communication system.
a) Detector
b) Responsivity
c) Absorption Coefficient
d) Band gap energy
View Answer
Explanation: If proper and improved and highly efficient detector is utilized, it will then reduce the repeated stations. It will also lower down both capital investment and maintenance cost.
Sanfoundry Global Education & Learning Series – Optical Communications.
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