Fibre Optics
Principle
Light is guided by total internal reflection in a core of higher refractive index surrounded by cladding. Single-mode fibre has a core of about 9 micrometres and carries one path, allowing long distances at 1310 or 1550 nm with a laser source. Multimode fibre (50 or 62.5 micrometres) is simpler and cheaper, uses LED or VCSEL sources at 850 or 1310 nm, and is limited by modal dispersion; graded-index multimode reduces that dispersion. Detectors are PIN photodiodes or avalanche photodiodes (more sensitive, but needing temperature compensation).
Loss budget
Attenuation is quoted in dB/km. Connectors and splices each cost a fraction of a dB, while bending, dirt and misalignment cost much more. Losses in dB simply add.
Traps
Never look into a fibre or an unterminated connector: laser light is invisible and can damage the retina. Keep connectors capped and clean, because a fingerprint on an end face can lose more than kilometres of fibre. Fibre is immune to EMI and creates no sparks, but it is damaged by tight bend radius and crush loads.
Rule of thumb
3 dB halves the power; 10 dB reduces it tenfold. Compare the sum of all losses with source power minus receiver sensitivity. Test with a power meter and light source, and locate faults with an OTDR.
Worked example
A 10 km link at 1 dB/km with two connectors at 0.5 dB and two splices at 0.2 dB has a total loss of 10 + 1.0 + 0.4 = 11.4 dB. If the budget is 15 dB, the link passes.