Assertion (A) : Attenuation and dispersion have negative effects on the propagation of signal in the optical fibres. Reason (R) : Optical signal degradation is caused due to structural imperfections of the fibre material. Select your answer using the codes given below.
Both (A) and (R) are true, but (R) is not the correct explanation of (A).
Examine the assertion — true. The two quantities are exactly the pair that limit an optical link:
| Mechanism | Effect | Limits |
|---|---|---|
| Attenuation | Optical power falls as \(P(z)=P_0e^{-\alpha z}\) | Repeater spacing |
| Dispersion | Pulses spread in time and overlap | Maximum bit rate |
Together they set the bandwidth-distance product of the fibre, so both are unambiguously harmful. The assertion is true.
Examine the reason — true, but partial. Structural imperfections certainly do degrade the signal. Density and composition fluctuations frozen into the glass cause Rayleigh scattering, which varies as
\(\alpha_R \propto \dfrac{1}{\lambda^{4}}\)
and dominates the loss of modern silica fibre at short wavelengths; bubbles, microcracks and core-diameter variations add further scattering, and microbends couple light out of guided modes. So the statement is a true description of one class of loss mechanism.
Does the reason explain the assertion? No — it accounts for only part of it. Structural imperfections explain some attenuation, but they do not explain the principal mechanisms at all:
Absorption is chemical, not structural — OH− ion impurities and transition-metal contaminants absorb at specific wavelengths, and infrared absorption comes from the intrinsic vibration of the Si–O bond.
Intermodal dispersion is purely geometric: different ray paths have different lengths, which happens in a structurally perfect fibre.
Material dispersion arises because the refractive index of pure silica varies with wavelength — again an intrinsic property, present in flawless glass.
Since dispersion in particular occurs in a perfect fibre, the reason cannot be the correct explanation of the assertion. Both are true, but the link fails: code 2.
The design consequence. The 1550 nm window is used because Rayleigh scattering has fallen as 1/λ4 while infrared absorption has not yet risen, giving a loss minimum near 0.2 dB/km; dispersion at that wavelength is then managed separately with dispersion-shifted fibre or compensating modules.
Hence, both (A) and (R) are true, but (R) is not the correct explanation of (A).
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