Photon Tube - An Optical-Fiber Time-Domain Reflectometer

Time-domain reflectometers for locating breaks in optical fibers have been implemented by a number of researchers 1,2 . Typically, these instruments have incorporated GaAs injection lasers to produce pulses of light having a high peak power and a narrow width and have incorporated avalanche photodiode (APD) receivers. The present instrument incorporates a gated photomultiplier receiver, specifically designed for this application. The photomultiplier allows increased sensitivity compared to the APD receivers and its gating action allows the user to "look behind" large reflections that otherwise would cause undesirable saturation of the receiver. The instrument is capable of detecting echos from breaks or imperfections that are 65 dB below the 4-percent echo from a perfect break. The distance resolution is 0.5 m.* By using a transmitted pulse that is wider or narrower (the present pulse is 5 ns in duration), a trade-off between sensitivity and resolution can be made. In addition, the backward Rayleigh scattering, which occurs roughly uniformly along the fiber, can be used to estimate the loss as a function * The delay per unit length of light in glass is 5 ns/m. The 5-ns full-width transmitted pulse gives a delay resolution of 0.5 m without special signal processing. 355