optical time-domain reflectometry
In optical fiber communications, the losses in optical fibers can be measured with optical time domain reflectometers. The measuring method works similar to radar but works with very low light levels and helps to localize poor fiber splices or faulty optical components. For measurement, a fast rise time pulse is injected into the cable system at one end. If the traveling impulses arrive at a location where the impedance differs from the characteristic impedance of the cable (e.g. at a faulty link), some of these signals will be reflected back to the source and can be detected with photon-counting detectors, photomultipliers, or avalanche photodiodes. With help of a fast ADC, the time parameters of the reflection are measured in order to determine the location of the impedance discontinuity.