Optical Fibres and their Applications

An optical fibre is a very thin, flexible strand of high-quality glass or plastic, engineered so that light entering one end undergoes total internal reflection over and over again along its length and emerges, with very little loss, from the other end, however much the fibre itself is bent or coiled along the way. The fibre is built from two coaxial layers: an inner core, made of a material of relatively high refractive index $n_1$, surrounded by an outer cladding of a slightly lower refractive index $n_2<n_1$. This deliberate step in refractive index at the core-cladding boundary is the whole working principle -- light launched into the core at one end strikes the core-cladding boundary at an angle greater than the critical angle for that boundary, so it undergoes TIR back into the core, strikes the opposite boundary again, undergoes TIR again, and so on continuously down the length of the fibre, however many times the fibre bends, as long as the bend is not sharp enough to violate the critical-angle condition. A protective outer plastic jacket is added purely for mechanical strength and is not part of the optical path. The two properties that follow directly from this all-internal-reflection guiding mechanism -- very low signal loss over long distances, and immunity to electrical/electromagnetic interference (since the signal is carried as light, not as an electrical current) -- are exactly what make optical fibres the backbone of modern long-distance telecommunications and internet data transmission, where a single hair-thin fibre can carry an enormous number of simultaneous digital signals as rapid pulses of ligh …