Absolute Frequency Stabilization of a Tunable Tm:Ho:YAG Laser to the HBr P(12) Line at 2097 nm

We demonstrate absolute frequency stabilization of a widely tunable Tm:Ho:YAG laser by locking the oscillator to the P12 absorption line of the H79Br molecule at 2097.222 nm, using the fringe side-locking technique. We perform time and spectral analysis of open- and closed-loop error signals to evaluate the frequency noise. In closed-loop operation a long-term frequency stability of better than 2 MHz with rms frequency fluctuations of ∼112 kHz is obtained over an observation time of 103 s.