Intensity-independent wavelength locking of diode lasers to a spectral slope of a fiber-optic sensor for ultrasonic detection.

We propose and demonstrate a simple and low-cost method to lock the wavelength of a diode laser to a point with a particular normalized slope on the spectrum of a fiber-optic sensor. The wavelength locking point is independent of the laser intensity variations. The locking involves simultaneously and independently modulating both the laser wavelength and the laser intensity. On the spectral slope, the wavelength modulation is also converted to intensity modulation that is superimposed to the direct intensity modulation. The error signal is the amplitude of the overall intensity modulation. We demonstrate a potential application of the locking method in a fiber-optic ultrasonic detection system using a distributed feedback diode laser and a phase-shifted fiber Bragg grating sensor.