In one line
Attenuation in fibres is caused mainly by absorption, scattering and bending losses, plus losses at joints and connectors.
Explained simply
Light in a fibre is like water in a long pipe: some is soaked up by the pipe material (absorption), some splashes out at rough spots (scattering), some spills at sharp bends (bending loss), and some leaks at badly joined sections (splice and connector loss).
How it works
- Absorption:
- Intrinsic: by the silica itself, ultraviolet absorption (electronic) at short wavelengths and infrared absorption (molecular vibrations) at long wavelengths.
- Extrinsic: by impurities such as transition-metal ions (Fe, Cu, Cr) and especially OH− ions from water, which cause absorption peaks (notably near 1380 nm).
- Rayleigh scattering: tiny density and composition fluctuations frozen in the glass scatter light in all directions. It varies as \(1/\lambda^4\), so it is much stronger at short wavelengths. This is why longer wavelengths (1310 nm, 1550 nm) are used.
- Bending losses:
- Macrobending: a large visible bend makes rays meet the boundary below the critical angle, so they escape.
- Microbending: tiny kinks from uneven pressure or poor cabling scatter light out.
- Splice, connector and coupling losses: misalignment, gaps, end-face roughness or core-size mismatch at joints, and loss in coupling light from the source into the fibre.
Key formula
Rayleigh scattering loss \(\propto \dfrac{1}{\lambda^4}\), where \(\lambda\) is the wavelength.
Quick example
Comparing 850 nm and 1550 nm: \((1550/850)^4 \approx 11\), so Rayleigh scattering at 850 nm is about 11 times stronger. That is a major reason long-haul links use 1550 nm.
Common mistakes
- Saying cladding is the main absorber; most absorption is in the core material and impurities.
- Writing Rayleigh scattering \(\propto \lambda^4\); it is inversely proportional.
- Forgetting bending losses, which are often the easiest marks.
How MU asks it
Asked as "Explain the factors contributing to attenuation in optical fibre" (5 marks). Use the three headings absorption, scattering and bending (plus joint losses), with one or two lines and a sub-type for each. A sketch of a bent fibre with a ray escaping earns extra credit.