# Optical Communications Questions and Answers – Absorption

This set of Optical Communications Multiple Choice Questions & Answers (MCQs) focuses on “Absorption”.

1. The absorption of photons in a photodiode is dependent on __________
a) Absorption Coefficient α0
b) Properties of material
c) Charge carrier at junction
d) Amount of light

Explanation: Absorption in a photodiode is for producing carrier pans. Thus, photocurrent is dependent on absorption coefficient α 0of the light in semiconductor used to fabricate device.

2. The photocurrent in a photodiode is directly proportional to absorption coefficient.
a) True
b) False

Explanation: The absorption of photons produces carrier pairs. Thus, photocurrent is dependent on absorption coefficient and is given by
I = Po e(1-h)/hf(1-exp (-α rd))
Where r = Fresnel coefficient
D = width of absorption region.

3. The absorption coefficient of semiconductor materials is strongly dependent on __________
a) Properties of material
b) Wavelength
c) Amount of light
d) Amplitude

Explanation: In some common semiconductors, there is a variation in absorption curves for materials. It is found that they are each suitable for different wavelength and related applications. This is due to difference in band gap energies. Thus absorption coefficient depends on wavelength.

4. Direct absorption requires assistance of photon.
a) True
b) False

Explanation: Indirect absorption requires photon assistance resulting in conversation of energy and momentum. This makes transition probability less likely for indirect absorption than direct absorption where no photon is included.

5. In optical fiber communication, the only weakly absorbing material over wavelength band required is?
a) GaAs
b) Silicon
c) GaSb
d) Germanium

Explanation: The transition over wavelength band in silicon is due to indirect absorption mechanism. This makes silicon weakly absorbent over particular wavelength band.

6. The threshold for indirect absorption occurs at wavelength __________
a) 3.01 μm
b) 2.09 μm
c) 0.92 μm
d) 1.09 μm

Explanation: The band gap for silicon is 4.10 eV corresponding to threshold of 0.30 μm in ultraviolet. Thus it’s outside wavelength range is the one which is required.

7. The semiconductor material for which the lowest energy absorption takes place is?
a) GaAs
b) Silicon
c) GaSb
d) Germanium

Explanation: Germanium absorption is by indirect optical transition. The threshold for direct absorption is at 1.53μm. Below this, germanium becomes strongly absorbing to corresponding link.

8. The wavelength range of interest for Germanium is __________
a) 0.8 to 1.6 μm
b) 0.3 to 0.9 μm
c) 0.4 to 0.8 μm
d) 0.9 to 1.8 μm

Explanation: Germanium is used in fabrication of detectors over the whole wavelength range i.e. first and second generation 0.8 to 1.6 μm while specially taking into consideration that indirect absorption will occur up to a threshold of 1.85 μm.

9. A photodiode should be chosen with a ________________ less than photon energy.
a) Direct absorption
b) Band gap energy
c) Wavelength range
d) Absorption coefficient

Explanation: A photodiode selection must be made by choosing that diode having band gap energy less than photon energy corresponding to longest operating wavelength. This provides high absorption coefficient which ensures a good response and limits the thermally generated carriers to obtain low dark current with no incident light.

10. ________________ photodiodes have large dark currents.
a) GaAs
b) Silicon
c) GaSb
d) Germanium

Explanation: Germanium photodiodes provide narrow band gaps as compared to other semiconductor materials. This is main disadvantage with use of germanium photodiodes at shorter wavelength and thus they have large dark current.

11. For fabrication of semiconductor photodiodes, there is a drawback while considering _________________
a) GaAs
b) Silicon
c) GaSb
d) Germanium

Explanation: Due to drawback with germanium to be used as fabricating material, there
is an increased investigation of direct band gap III and V alloys for longer wavelength region.

12. _________________ materials are potentially superior to germanium.
a) GaAs
b) Silicon
c) GaSb
d) III – V alloys

Explanation: The band gap energies for III – V alloys materials can be tailored to required wavelength. This can be achieved by changing relative concentration of their constituents which results in low dark currents. Thus, III – V alloys are superior potentially to germanium.

13. ____________ alloys such as InGaAsP and GaAsSb deposited on InP and GaSb substrate.
a) Ternary
b) Quaternary
c) Gain-guided
d) III – V alloys

Explanation: Ternary alloys are used to fabricate photodiodes for longer wavelength band. Thus, these alloys such as InGaAsP and GaAsSb are deposited on InP and GaSb substrates.

14. _________________ alloys can be fabricated in hetero-junction structures.
a) InGaSb
b) III – V alloys
c) InGaAsP
d) GaAsSb

Explanation: III – V alloys enhances the high speed operations of hetero-junction structures. Thus these structures can be fabricated with III-V alloys.

15. The alloys lattice matched to InP responds to wavelengths up to 1.7μm is?
a) InAsSb
b) III – V alloys
c) InGaSb
d) InGaAs

Explanation: Although there were difficulties in growth of IOnGaAs alloys, the problems are now reduced. These alloys lattice matched to InP responding to wavelength around 1.7 μmare widely utilized for fabrication of photodiodes operating around 1.7μm.

Sanfoundry Global Education & Learning Series – Optical Communications.

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