Suppression of Systematic Errors in Brillouin Optical Correlation Domain Analysis Based on Injection-Locking

Brillouin optical correlation domain analysis (BOCDA) is an established method for the measurement of Brillouin frequency distribution, where direct modulation of a distributed feedback laser diode (DFB-LD) is usually adopted to implement a frequency-modulated light source. Recently, we reported that a phase delay occurring between the amplitude and frequency variations, called AM-FM phase delay, in the output of the modulated DFB-LD can cause considerable errors in the Brillouin frequency measured by a BOCDA system. The error induced by the AM-FM phase delay is systematic since it cannot be suppressed by averaging process while constantly distorts the distribution map of Brillouin frequency. In this paper, we demonstrate the suppression of the adverse effects of the AM-FM phase delay using an injection-locked DFB-LD as a light source of BOCDA system. Theoretical explanation on the origin of the systematic error in the BOCDA measurement is provided by simulations. The investigation on the modulation characteristics of the master and slave LD's in the injection-locking is performed, the result of which shows that the injection locking not only reduces the depth of amplitude modulation, but also keeps the phase delay of near 180° in a wide range of modulation frequency. In test measurements, the systematic errors are reduced by more than 70% on average using the slave LD as a common light source of the BOCDA system.