Simple phase noise measurement scheme for cavity-stabilized laser systems

We describe a simple method for measuring the residual fast phase noise of a cavity-stabilized laser using the cavity as a reference. The method is based on generating a beat note between the laser output and the strongly filtered light transmitted through the cavity. The beat note can be directly analyzed without requiring further calibration of system parameters. We apply the method to measure the residual phase noise of an external-cavity diode laser (ECDL) locked to a reference cavity and compare the results with an analysis of the in-loop error signal of the feedback system.