Consider the following statements : Losses in optical fibers are caused by 1. Impurities in the fibre material Of these statements :
2. Microbending
3. Splicing
4. Step index profile
1, 2 & 3 are correct.
The step-index profile causes dispersion, not loss — which is what separates statement 4 from the rest. The correct set is 1, 2 and 3, option 3.
| Cause | Mechanism | Loss? |
|---|---|---|
| 1. Impurities | Absorption by OH− ions and metals | ✓ |
| 2. Microbending | Guided rays exceed the critical angle and escape | ✓ |
| 3. Splicing | Misalignment and Fresnel reflection at the joint | ✓ |
| 4. Step index profile | Different path lengths for different modes | ✗ Dispersion |
Statement 4 — the important distinction. A step-index multimode fibre lets rays travel by many paths of different length, so a launched pulse arrives spread out in time. That broadening limits the bit rate, because adjacent pulses overlap; but the energy still arrives. Loss is measured in dB/km and describes power lost; dispersion is measured in ns/km and describes power delayed. Grading the index profile parabolically reduces dispersion — the rays taking the longer outer paths travel through a lower-index region and so go faster — but it does not reduce attenuation.
Statement 1 — the dominant intrinsic loss. The hydroxyl ion has vibrational absorption peaks near 1380 nm, and transition-metal impurities absorb across the spectrum. Reducing OH content is what opened the low-loss telecom windows, and modern fibre reaches about 0.2 dB/km at 1550 nm — close to the Rayleigh scattering floor set by frozen-in density fluctuations in the glass itself.
Statement 2 — microbending. Tiny random distortions of the axis, caused by pressure from the cable jacket or by uneven spooling, change the local angle of incidence. Rays that met the total-internal-reflection condition no longer do, and their energy passes into the cladding and is lost. Macrobending — a coil of too small a radius — does the same on a larger scale.
Statement 3 — splicing. Every joint costs power through lateral or angular misalignment, an end gap, or reflection at the glass-air interface. A fusion splice loses about 0.05 dB, a mechanical connector 0.3 to 0.5 dB — and in a link with many connectors this can exceed the loss of the fibre itself.
Hence, statements 1, 2 and 3 are correct.
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3. Scattering
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