Interferometric frequency stabilization for tunable lasers with very high accuracy verified by means of a frequency comb

The application of tunable single-frequency lasers often requires the measurement and stabilization of the frequency with high accuracy, e.g. for spectroscopy, distance measurement and so on. In an earlier DGaO conference we presented an interferometric method for laser locking, which uses the phase difference between a quadrature interferometer signal and some arbitrary computer-generated quadrature signal as an error signal. The latter is fed back to the laser in control loop, in order to keep the optical frequency stable. As the method can stabilize to an arbitrary target frequency in a wide range of several hundred nanometers, it was a challenge to specify the absolute uncertainty of the actual laser frequency for the whole range. This was now done using an optical frequency comb generated by a fiber laser. As a result, the deviation of the observed beat frequency between the locked laser and the comb to the postulated value was less than 1.6MHz RMS (maximum 6MHz) in a range of 750 to 795 nm. We also mention the calibration procedure which led to this high accuracy.