Wavelength Locking of a Diode Laser to the Maximal Slope of a $\pi$ -Phase-Shifted Fiber Bragg Grating for Acoustic Emission Detection

We theoretically and experimentally investigate a locking method for a fiber-optic acoustic emission (AE) sensor system based on a π-phase-shifted fiber Bragg grating (π-FBG) and a distributed-feedback diode laser. The π-FBG features a narrow notch in the middle of the reflection spectrum for high-sensitivity AE detection. The investigated locking method generates a signal that is proportional to the second derivative of this spectral notch, which is used as the error signal. The laser wavelength is automatically locked to one of the zero-crossing points of the error signal, which corresponds to the maximum slope of the selected side of the spectral notch. With the laser locked to the maximum slope, the optimal sensitivity for AE detection is obtained. We also show that the locking point is immune to low-frequency variations of the output power of the laser.