An external cavity diode laser sensor

A simple compact optical sensor system consisting of a near-field external cavity coupled to a semiconductor laser is reported. In this device, a change of phase in the light fed back into the laser cavity by the perturbed external mirror modulates the output of the laser by effectively changing the laser facet reflectivity. Phase shifts of
9\cdot10^{-8}
rad and 10
^-6
rad were measured at 10 kHz and 100 Hz, respectively, using a 1-Hz bandwidth and a 98-percent reflector placed less than 10 μm from the laser facet. The sensitivity of these devices is shown to be limited by the intrinsic amplitude noise of the laser. The implementation of this sensor configuration is reported as an acoustic sensor, hydrophone, magnetic field, and current sensor as well as an accelerometer.