Ultrastable optical frequency reference at 1.064 /spl mu/m using a C/sub 2/HD molecular overtone transition

Saturated molecular overtone transitions in the visible and near infrared can be detected with record-high sensitivities using our cavity-enhanced frequency modulation technique. We have stabilized a Nd:YAG laser onto the P(5) line in (/spl nu//sub 2/+3/spl nu//sub 3/) overtone band of C/sub 2/HD at 1.064 /spl mu/m and established its absolute frequency. Excellent signal-to-noise ratio produced an Allan variance of 3.4/spl times/10/sup -13/ at 1 s averaging, improving to 1/spl times/10/sup -14/ after 800 s. Selection of slow molecules with low power and gas pressure gave a linewidth 13 times below the room temperature transit time limit.