Wavelength Division Multiplexing Questions and Answers

Q: The physical mechanism behind the operation of an Acousto-optic filters in WDM is the :

Acousto-optic filters work by the photoelastic effect.An acoustic wave launched into a crystal creates a periodic pattern of compression and rarefaction. Through the photoelastic effect, this strain changes the refractive index periodically, forming a moving diffraction grating. Only light of the wavelength that satisfies the Bragg condition is diffracted, and changing the acoustic frequency tunes the selected wavelength, which is how the filter picks out WDM channels.The photoelectric effect is the emission of electrons by light, the electro-optic effect is a change of refractive index with an electric field (used in modulators), and electromagnetic effects are not the mechanism here.

Q: Grating based demultiplexers use the phenomenon of .............. from an optical grating.

Grating demultiplexers use diffraction (Bragg).A diffraction grating sends each wavelength in a different direction, given by the grating equation, so the wavelengths combined in a WDM signal are spread out in angle and can be collected by separate output fibres. Fibre Bragg gratings use the same principle along the fibre, reflecting a narrow band around the Bragg wavelength.Scattering spreads light in random directions without separating wavelengths in a controlled way, and plain reflection or transmission does not separate wavelengths at all.

Q: The following can be used as WDM component in optical communication :

The key gives optical filters.Wavelength division multiplexing sends several channels on one fibre, each at its own wavelength. Separating or combining them needs wavelength-selective components, and optical filters, such as thin-film interference filters, fibre Bragg gratings and arrayed waveguide gratings, are the basic WDM components that pass one wavelength and reflect or block the rest.A general fibre coupler splits or combines power without selecting wavelength, a graded index lens is used to collimate and focus light between fibres, and an epoxy splice simply joins two fibres. None of them separates wavelengths on its own.

Direction for [ Question No: 1 To 1 ] :

Directions : The following items consist of two statements, one labelled the "Assertion (A)" and the other labelled the "Reason (R)". You are to examine these two statements carefully and decide if the Assertion (A) and the Reason (R) are individually true and if so, whether the Reason is a correct explanation of the Assertion. Select your answers to these items using the codes given below and mark your answer accordingly.

Assertion (A) : By using wavelength division, multiplexing the capacity of an optical fibre can be enhanced.

Reason (R) : The separation between two adjacent channels in WDM is of the order of 50 nm.

A.Both (A) and (R) are true and (R) is the correct explanation of (A).
B.Both (A) and (R) are true, but (R) is not correct explanation of (A).
C.(A) is true, but (R) is false.
D.(A) is false, but (R) is true.

Exam Year(s)

UGC NET - 2012,

Answer: C

The assertion is true and the reason is false (option C). Wavelength division multiplexing sends several wavelengths over one fibre, so it raises the capacity.

The channel spacing is not of the order of 50 nm. In dense WDM it is a fraction of a nanometre to about 1.6 nm, and in coarse WDM it is 20 nm, so 50 nm is too wide. Options A and B accept the reason, and D rejects the true assertion.

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The physical mechanism behind the operation of an Acousto-optic filters in WDM is the :

A.Photoelectric effect
B.Electro-optical effect
C.Electromagnet effect
D.Photoelastic effect

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: D

Acousto-optic filters work by the photoelastic effect.

An acoustic wave launched into a crystal creates a periodic pattern of compression and rarefaction. Through the photoelastic effect, this strain changes the refractive index periodically, forming a moving diffraction grating. Only light of the wavelength that satisfies the Bragg condition is diffracted, and changing the acoustic frequency tunes the selected wavelength, which is how the filter picks out WDM channels.

The photoelectric effect is the emission of electrons by light, the electro-optic effect is a change of refractive index with an electric field (used in modulators), and electromagnetic effects are not the mechanism here.

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Grating based demultiplexers use the phenomenon of .............. from an optical grating.

A.Diffraction (Bragg)
B.Scattering
C.Reflection
D.Transmission

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: A

Grating demultiplexers use diffraction (Bragg).

A diffraction grating sends each wavelength in a different direction, given by the grating equation, so the wavelengths combined in a WDM signal are spread out in angle and can be collected by separate output fibres. Fibre Bragg gratings use the same principle along the fibre, reflecting a narrow band around the Bragg wavelength.

Scattering spreads light in random directions without separating wavelengths in a controlled way, and plain reflection or transmission does not separate wavelengths at all.

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The following can be used as WDM component in optical communication :

A.Fibre coupler
B.Optical filters
C.Graded index lens
D.Epoxy splicer

Exam Year(s)

MH SET - 2023, SET Exam - 2023,

Answer: B

The key gives optical filters.

Wavelength division multiplexing sends several channels on one fibre, each at its own wavelength. Separating or combining them needs wavelength-selective components, and optical filters, such as thin-film interference filters, fibre Bragg gratings and arrayed waveguide gratings, are the basic WDM components that pass one wavelength and reflect or block the rest.

A general fibre coupler splits or combines power without selecting wavelength, a graded index lens is used to collimate and focus light between fibres, and an epoxy splice simply joins two fibres. None of them separates wavelengths on its own.

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Match the following lists :

List - I	List - II
(a) Optical fiber communication	(i) FDM
(b) Mobile communication	(ii) TDM
(c) Digital communication	(iii) CDMA
(d) Analog communication	(iv) Wavelength-Division Multiplexing (WDM)

Codes :
(a) (b) (c) (d)

A.(i) (iii) (ii) (iv)
B.(iv) (iii) (ii) (i)
C.(ii) (iv) (i) (iii)
D.(iii) (i) (ii) (iii)

Exam Year(s)

UGC NET - 2013,

Answer: B

The correct match is a-iv, b-iii, c-ii, d-i, the second option. Optical fibre communication uses wavelength division multiplexing, mobile communication uses CDMA, digital communication uses time division multiplexing, and analog communication uses frequency division multiplexing.

The other options pair FDM or TDM with optical or mobile systems, which is not the usual association, and the fourth option repeats (iii) twice.

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List - I	List - II
a. 1550 nm	i. 100 GHz
b. 0.8 nm	ii. Visible light
c. 400-700 nm	iii. Ultra violet
d. 200 nm	iv. 193 T Hz

A.a-iv, b-i, c-ii, d-iii
B.a-iv, b-ii, c-i, d-iii
C.a-i, b-ii, c-iii, d-iv
D.a-iv, b-iii, c-i, d-ii

Exam Year(s)

UGC NET - 2014,

Answer: A

The correct matching is a-iv, b-i, c-ii, d-iii. The frequency of 1550 nm light is \(c/\lambda = 3 \times 10^{8}/1550 \times 10^{-9} \approx 193\) THz (iv). A wavelength spacing of 0.8 nm near 1550 nm corresponds to \(\Delta f = c\Delta\lambda/\lambda^{2} \approx 100\) GHz (i), the standard channel spacing in dense WDM. 400 to 700 nm is visible light (ii), and 200 nm lies in the ultraviolet (iii).

The other options place visible light or ultraviolet against the wrong wavelengths.

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Wavelength Division Multiplexing Questions and Answers

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