Brillouin OTDR employing optical frequency shifter using side-band generation technique with high-speed LN phase-modulator

In order to measure spontaneous Brillouin backscattered lights, which are related to fiber-distributed tensile strain and temperature, by using a highly sensitive self-heterodyne coherent detection technique, a wideband optical frequency shifter is required to reduce the beat signal frequency between the Brillouin backscattered signal and the LO light to the IF bandwidth of the conventional coherent receiver. We constructed a highly stable optical frequency shifter with a 14 GHz bandwidth and 20 kHz frequency resolution which employs side-band light generated by a high-speed LN phase-modulator. The structure is simple and the insertion loss is low. We applied this shifter to B-OTDR and measured the distributed tensile strain in a 30-km dispersion-shifted optical fiber with a 100-m spatial resolution.