Accurate Measurement of Optical Resonator Finesse

We report on a novel and general measurement technique to characterize the finesse of optical resonators. The method retrieves the cavity transmission profile coefficient by using an amplitude modulation of the probe beam and a synchronous detection scheme. Both the real and imaginary parts of the transmission profile can be accurately measured. Validation of the method is performed using two different cavity configurations, a linear and a ring resonator, probed by both continuous-wave and mode-locked laser sources. A comparison with respect to conventional cavity-ring-down measurement scheme shows the accuracy of the proposed method.