Abstract: The feedback in a laser system is measured in near real time by inducing a frequency shift and the feedback being based on the phase shift undergone by the feedback beam in an external cavity. The frequency shift is induced by phase modulating the feedback beam, for example by varying the position of an optical element placed in the feedback beam upon applying a ramp signal to the optical element. Frequency is detected by a Fabry-Perot interferometer which generates an interference fringe pattern, the intensity of the central fringe being detected to determine the induced frequency shift in real time, the amount of feedback being proportional to the time of flight multiplied by the frequency shift.