Measurement error induced by the power-frequency delay of the light source in optical correlation-domain distributed Brillouin sensors.

Direct current modulation of a semiconductor laser is a key method adopted in most optical correlation-domain distributed Brillouin sensors such as Brillouin optical correlation-domain analysis (BOCDA) and reflectometry (BOCDR) for localizing the sensing position by synthesis of an optical coherence function. We report that distributed measurement of Brillouin frequency by the BOCDA or BOCDR system can suffer significant distortion caused by phase delay between output power and frequency variations of the modulated light source. We calculate characteristics of the distortion by numerical simulations, and compare the results with experimental data obtained by a BOCDA system for confirmation. Our results show that a maximum error of more than 30 MHz can occur in Brillouin frequency measured under ordinary operating conditions with MHz-order direct current modulation.