Optical Fibre Fundamentals and Principles of Light Transmission
Optical fibre fundamentals and principles of light transmission are tested through questions on total internal reflection, numerical aperture, modes, attenuation windows and dispersion.
Core rules
- A fibre guides light by total internal reflection when core refractive index n1 exceeds cladding n2 and the ray angle at the core–cladding boundary exceeds the critical angle.
- NA = √(n1² − n2²); it fixes the acceptance cone in air, θa = sin⁻¹(NA). Typical silica SMF: core 8–10 µm, cladding 125 µm, NA ≈ 0.12–0.14.
- Single-mode fibre supports one mode when V = (2πa/λ)√(n1²−n2²) < 2.405. Multimode 50/125 or 62.5/125 µm has many modes and modal dispersion.
- Attenuation minima: ~0.35 dB/km at 1310 nm and ~0.20 dB/km at 1550 nm for G.652D SMF. 1310 nm is the zero-chromatic-dispersion region; 1550 nm gives lowest loss but needs dispersion management.
- Bend loss is macrobend (tight coil) or microbend (microscopic deformation); PMD and chromatic dispersion limit high-bit-rate reach.
Worked example
A step-index fibre has n1 = 1.48 and n2 = 1.46. NA = √(1.48² − 1.46²) = √(2.1904 − 2.1316) = √0.0588 = 0.2425. Acceptance angle in air = sin⁻¹(0.2425) ≈ 14.0°. A ray arriving at 20° to the axis is outside the cone and will not be guided in the core mode.
Traps
- Using n1 + n2 instead of the squares; forgetting the square root.
- Mixing dB and dBm: attenuation is dB, absolute power is dBm.
- Assuming 1550 nm is always better: short-reach low-cost systems often use 1310 nm because of zero dispersion and cheaper optics.
- Confusing NA with core diameter; a larger NA accepts more light but increases multimode dispersion.