Optical Time Domain Reflectometer (OTDR) 60 comprises pulsed optical source 70 and beam splitter 80 directing a pulse to optical fibre 90. A backscattered signal is directed by beamsplitter 80 towards Variable Transmission Element (VTE) 100 and first pre-amplifier 106 in receiver 104. VTE 100 transmission is regulated by function generator 110 which produces control signal 120. VTE 100 comprises an electro-optic, acousto-optic or a magneto-optic modulator, Micro-Engineered Mechanical (MEMs) attenuator or optical amplifier. VTE 100 regulates the measured signal's dynamic range by varying its gain in inverse proportion to the backscattered signal. In alternate embodiments the bias of a photodetector (160, fig. 4), which has an internal gain, or a local oscillator signal derived from a laser (204, fig. 5) pulse and mixed with the backscatter signal can be varied in order to reduce the optical signal's dynamic range.
Optical time domain refectometry