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 P(12) absorption line of the H(79)Br 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 approximately 112 kHz is obtained over an observation time of 10(3) s.