Distributed Brillouin sensor system based on offset locking of two distributed feedback lasers

An offset locking technique, which uses an external optical delay line to tune the distributed feedback (DFB) laser frequency and a proportional-integral-derivative (PID) controller to lock the tuned frequency, is proposed for the first time, to the best of our knowledge, in the distributed Brillouin sensor system. This method provides large tuning range (greater than 1 GHz), high tuning speed (less than 100 mus per frequency step), and frequency tuning is independent of the laser frequency and power. The two DFB lasers are phase locked at the Brillouin frequency using a hardware PID controller. Using this offset locking with optical delay line, we demonstrated a high signal-to-noise ratio of 32 dB, which allows 1 m spatial resolution and better than 0.6 MHz frequency measurement accuracy (equivalent to 0.5 degrees C temperature resolution or 8 microepsilon strain resolution) over kilometers sensing length. The bias of the electro-optic modulator is controlled by a lock-in amplifier to provide high temperature or strain measurement accuracy.