Optical Fiber

Physics: Optical fiber guides light via total internal reflection: TIR occurs when θ > θ_c = arcsin(n₂/n₁). The numerical aperture NA = √(n₁²−n₂²) ≈ 0.12 for SMF-28. The V-number V = (2πa/λ)·NA determines mode count; single mode requires V < 2.405. SMF core diameter ≈ 8.2 μm; multimode fibers support hundreds of modes. Attenuation minimum: 0.2 dB/km at 1550 nm (Rayleigh scatter + OH absorption). EDFA amplifiers use Er³⁺-doped fiber to overcome attenuation in long-haul links.