Frequency domain optical reflectometer using a GaAs optoelectronic mixer

A frequency domain optical reflectometer which uses a GaAs interdigital photodetector both for optical detection and for mixing is described. A simple receiver with a postdetection bandwidth of the order of 10 kHz suffices for distance resolution of 10 m with this technique, greatly simplifying the electronics by comparison with pulsed reflectometers or swept frequency reflectometers employing electronic mixing. With available lasers the instrument could detect discrete optical reflections of -57 dB at a distance of 1 km. It is intended for remote measurement of refractive index in fluids, using the Fresnel reflection from a square-cleaved fiber as the sensor.