Effects of asymmetric frequency modulation in optical correlation-domain distributed Brillouin sensors

Direct current modulation of a laser diode (LD) is widely used in optical correlation-domain distributed Brillouin sensors such as Brillouin optical correlation domain analysis (BOCDA) and reflectometry (BOCDR) for the synthesis of optical coherence function. We report the direct modulation of a DFB-LD induces asymmetry in the frequency modulation that causes unique distortion in the Brillouin frequency map obtained by BOCDA and BOCDR systems. We present the simulation result on the effects of asymmetric frequency modulation in correlation-domain distributed sensors and the experimental result of measuring the asymmetry in the frequency modulation of the LD using an optical sampling method. Finally, the experimental result of distributed measurements by a BOCDA system with the asymmetric modulation is provided to compare with simulation data.