An optical time-domain reflectometer with low-power InGaAsP diode laser

An optical time-domain reflectometer (OTDR) suited for operation with low-power (of the order of 1-mW peak power) InGaAsP lasers is described. To launch maximum average power into the fiber, the laser is modulated by a pseudonoise binary sequence, and the back-scattered signal is averaged and then analyzed by a microcomputer. Initial tests of the reflectometer with a germanium avalanche photodiode (Ge APD) detector demonstrate backscattering signal levels at 1300 nm from 50-μm core diameter graded-index fibers of more than 15 dB above the system noise.