Match List I with List II and select the correct answer using codes given below :List I (Optical Wavelength)List II(a) 1440 nm(i) Low loss(b) 850 nm(ii) High speed(c) 1300 nm(iii) High loss(d) 1550 nm(iv) Low cost

The correct matching is a-iii, b-iv, c-ii, d-i.1440 nm lies near the water (OH) absorption peak around 1383 nm, so it is a high loss region (iii).850 nm is the first window, served by cheap GaAs LEDs and silicon detectors: low cost (iv).1300 nm is near the zero-dispersion wavelength of standard silica fiber, which allows high speed (ii).1550 nm is the minimum-attenuation window, about 0.2 dB/km: low loss (i).The codes printed as options A and D are identical and both give 1440 nm low loss and 1550 nm high loss, the reverse of the attenuation curve.

Attenuation in Optical Fibres Questions and Answers

Q: Importance of 1.3 µm range in optical communication arises to the fact that the :

The importance of the 1.3 µm range is that the bandwidth is highest.The passage says that at 1.3 µm material dispersion becomes extremely small, so the information-carrying capacity is limited only by intermodal dispersion, and single mode fibre removes even that. Less pulse spreading means a higher bit rate for a given length, that is, a higher bandwidth.Attenuation (A) is smaller at 1.3 µm than at 0.8 µm, as the passage says, but it is not the lowest: the loss spectrum printed with the passage reaches its minimum near 1.55 µm. Component availability (C) is not the physical reason. Impurities (D) do affect transmission; the water-related loss peak near 1.4 µm in the same figure is one.

Q: Molecular size of \(SiO_{2}\) in optical fiber leads to the following :

It leads to scattering.Glass is an amorphous material, and when silica cools from the melt, tiny fluctuations in density and composition, of the order of molecular size and much smaller than the wavelength of light, are frozen into it. These inhomogeneities scatter light in all directions. This Rayleigh scattering varies as \(1/\lambda^{4}\) and is the fundamental loss mechanism that sets the lower limit of fiber attenuation.Absorption arises from impurities such as OH ions and from the material's own electronic and vibrational resonances. Pulse dispersion is spreading in time, not loss, and strain is a mechanical effect.

Q: Rayleigh scattering is a fundamental loss mechanism arising from :

Rayleigh scattering arises from local microscopic fluctuations.Glass freezes from a melt with tiny random variations in density and composition, on a scale much smaller than the wavelength of light. These local fluctuations in refractive index scatter light in all directions, with a loss proportional to \(1/\lambda^{4}\), and they set the fundamental lower limit on fibre attenuation at short wavelengths.The loss is a property of the glass structure itself, not of electrical, electronic or mechanical fluctuations; mechanical effects such as bends cause separate bending losses.

Q: Silica fibres can change their optical properties permanently when they are exposed to :

Silica fibres change permanently when exposed to intense radiation from a laser in the blue/ultraviolet spectrum.This is photosensitivity: germanium-doped silica absorbs ultraviolet light, around 240 nm, and its refractive index changes permanently. The effect is used to write fibre Bragg gratings by exposing the core to an ultraviolet interference pattern.Red and green light are not absorbed strongly enough to cause the change, and electrical or electrostatic shocks do not alter the optical properties of glass in this way.

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Loss in dB/km (log scale) against wavelength in um for a typical optical fibre: about 5 at 0.8 um, about 1 at 1.3 um, a water peak above 10 near 1.4 um, a minimum of about 0.2 near 1.55 um, then the infrared tail rising; a dotted Rayleigh scattering line falls from 1.5 to 0.1

The shifting of the operating wavelength to the 1.3 µm range has another important advantage. The corresponding loss is much smaller. Fig. above shows the loss spectrum for a typical optical fibre and as can be seen the losses at λ₀ ~ 1.3 µm are ~ 1 dB/km which is smaller by a factor of about 3 when compared with losses at λ₀ ~ 0.8 µm. Optical communication systems around 1981 used graded index multimode fibres with 1.3 µm sources; typical sources used were InGaAsP/InP LEDs (Δλ₀ ≈ 25 nm) and laser diodes (Δλ₀ ≈ 2 nm) with Ge APDs as detectors. The losses in the fibres were ~ 1 dB/km. These systems are usually said to belong to second generation optical communication systems. In a typical operating system the repeater spacing was 30 km and the bit rate was ~ 45 Mbit/s. Thus, in comparison to first generation systems, the repeater spacing was increased by a factor of about 2.5 which was primarily due to the decrease in pulse dispersion as well as in the losses in optical fibres.

As shown above by operating at 1.3 µm wavelength, material dispersion has become extremely small and therefore the information-carrying capacity is limited by intermodal dispersion. This intermodal dispersion can be totally eliminated by using single mode fibres where only one guided mode is possible.

This will be discussed in the following section.

Importance of 1.3 µm range in optical communication arises to the fact that the :

A.attenuation is lowest
B.bandwidth is highest
C.components are easily available
D.impurities have no effect in transmission

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: B

The importance of the 1.3 µm range is that the bandwidth is highest.

The passage says that at 1.3 µm material dispersion becomes extremely small, so the information-carrying capacity is limited only by intermodal dispersion, and single mode fibre removes even that. Less pulse spreading means a higher bit rate for a given length, that is, a higher bandwidth.

Attenuation (A) is smaller at 1.3 µm than at 0.8 µm, as the passage says, but it is not the lowest: the loss spectrum printed with the passage reaches its minimum near 1.55 µm. Component availability (C) is not the physical reason. Impurities (D) do affect transmission; the water-related loss peak near 1.4 µm in the same figure is one.

Discuss in Forum

Molecular size of \(SiO_{2}\) in optical fiber leads to the following :

A.Absorption
B.Scattering
C.Pulse dispersion
D.Strain

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: B

It leads to scattering.

Glass is an amorphous material, and when silica cools from the melt, tiny fluctuations in density and composition, of the order of molecular size and much smaller than the wavelength of light, are frozen into it. These inhomogeneities scatter light in all directions. This Rayleigh scattering varies as \(1/\lambda^{4}\) and is the fundamental loss mechanism that sets the lower limit of fiber attenuation.

Absorption arises from impurities such as OH ions and from the material's own electronic and vibrational resonances. Pulse dispersion is spreading in time, not loss, and strain is a mechanical effect.

Discuss in Forum

Match List I with List II and select the correct answer using codes given below :

List I (Optical Wavelength)	List II
(a) 1440 nm	(i) Low loss
(b) 850 nm	(ii) High speed
(c) 1300 nm	(iii) High loss
(d) 1550 nm	(iv) Low cost

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

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: C

The correct matching is a-iii, b-iv, c-ii, d-i.

1440 nm lies near the water (OH) absorption peak around 1383 nm, so it is a high loss region (iii).
850 nm is the first window, served by cheap GaAs LEDs and silicon detectors: low cost (iv).
1300 nm is near the zero-dispersion wavelength of standard silica fiber, which allows high speed (ii).
1550 nm is the minimum-attenuation window, about 0.2 dB/km: low loss (i).

The codes printed as options A and D are identical and both give 1440 nm low loss and 1550 nm high loss, the reverse of the attenuation curve.

Discuss in Forum

Rayleigh scattering is a fundamental loss mechanism arising from :

A.Electrical fluctuations
B.Mechanical fluctuations
C.Electronic fluctuations
D.Local microscopic fluctuations

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: D

Rayleigh scattering arises from local microscopic fluctuations.

Glass freezes from a melt with tiny random variations in density and composition, on a scale much smaller than the wavelength of light. These local fluctuations in refractive index scatter light in all directions, with a loss proportional to \(1/\lambda^{4}\), and they set the fundamental lower limit on fibre attenuation at short wavelengths.

The loss is a property of the glass structure itself, not of electrical, electronic or mechanical fluctuations; mechanical effects such as bends cause separate bending losses.

Discuss in Forum

Silica fibres can change their optical properties permanently when they are exposed to :

A.Electrical shocks
B.High power electrostatic shock
C.Intense radiation from a laser operating in blue/ultraviolet spectrum
D.Intense radiation from a laser operating in a red and green spectrum

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: C

Silica fibres change permanently when exposed to intense radiation from a laser in the blue/ultraviolet spectrum.

This is photosensitivity: germanium-doped silica absorbs ultraviolet light, around 240 nm, and its refractive index changes permanently. The effect is used to write fibre Bragg gratings by exposing the core to an ultraviolet interference pattern.

Red and green light are not absorbed strongly enough to cause the change, and electrical or electrostatic shocks do not alter the optical properties of glass in this way.

Discuss in Forum

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Attenuation in Optical Fibres Questions and Answers

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