Optical sources and fibre losses

Optical sources

Light sources used in optical fibre communication:

1. Light emitting diode (LED): surface-emitting (Burrus) LED and edge-emitting LED; incoherent, broad spectrum, cheap.
2. Laser diode (LD): Fabry–Perot laser, distributed feedback (DFB) laser, distributed Bragg reflector (DBR) laser, and vertical-cavity surface-emitting laser (VCSEL); coherent, narrow spectrum, high power and speed.
3. Other lasers used in special systems: fibre lasers (erbium-doped) and Nd:YAG solid-state lasers.

Losses in optical fibre

Attenuation is the decrease of optical power along the fibre, measured in dB/km:

α (dB/km) = (10 / L) · log(Pin / Pout)

Modern silica fibre has about 0.35 dB/km at 1310 nm and 0.2 dB/km at 1550 nm.

1. Absorption losses

Light energy is absorbed by the glass and converted to heat.

• Intrinsic absorption: by the pure silica itself; electronic absorption in the ultraviolet and molecular vibration absorption in the infrared (beyond ~1.6 µm). It sets the long-wavelength limit.
• Extrinsic absorption: by impurities such as transition metal ions (Fe, Cu, Cr) and especially OH⁻ (water) ions, which give absorption peaks near 950, 1240 and 1380 nm.
• Atomic defect absorption: defects in the glass structure, e.g. caused by radiation.

2. Scattering losses

Light is scattered out of the guided path by small non-uniformities in the glass.

• Rayleigh scattering: by density and composition fluctuations much smaller than λ; loss ∝ 1/λ⁴. It is the dominant loss at short wavelengths and the main reason long links use 1310 and 1550 nm.
• Mie scattering: by larger imperfections (core–cladding irregularities, bubbles), reduced by careful manufacture.
• Non-linear scattering (stimulated Brillouin and Raman) appears only at high optical power.

3. Bending (radiation) losses

• Macrobending: bends with radius much larger than the fibre diameter (e.g. cable round a corner). On the outer side of a sharp bend the ray angle falls below the critical angle, so light leaks into the cladding. Loss rises steeply once the bend radius goes below a critical value.
• Microbending: small random bends of the fibre axis (µm scale) caused by uneven cabling pressure or temperature changes. They couple light from guided to radiating modes.

4. Coupling, splice and connector losses

Power is lost where light enters the fibre and where fibres are joined, because of lateral, angular and gap misalignment, core size or NA mismatch, and Fresnel reflection at end faces (about 0.05–0.5 dB per joint).