After every fibre optic cable installation or repair, you need to test for continuity and end-to-end loss. You may also need to troubleshoot installed fibre that’s not performing up to expectations.
To check fibre, you need to test it with a light source and a power meter and compare your results with an estimate of what a reasonable loss for that cable or link is. This estimate is called a loss budget and is calculated using typical losses expected for each part of the cable: the fibre itself, connectors, and splices, if any. If the measured loss exceeds the loss budget by a significant amount, there is a problem with the cable, most often at the connectors or a splice rather than the cable itself.
Two basic varieties of Fiber Light Source
A fibre light source is used to inject light into a fibre optic cable for the purpose of testing it. They come in two basic varieties: light emitting diodes (LEDs) and laser diodes. They’re further differentiated by the wavelength they produce and the type of cable they test.
LEDs are low cost, slower speed, easy to use, multimode-only, and have a wide output pattern. Because LEDs produce a less concentrated light than lasers and have a much lower power output than lasers, they’re difficult to couple into fibres, limiting them to multimode fibres. LEDs have less bandwidth than lasers and can achieve a maximum throughput of 1 Gbps.
Laser diodes are higher cost and faster speed, allow single-mode or multimode, and have a narrow output pattern. Lasers can achieve throughput up to and beyond 10 Gbps.