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Sub-Doppler optical frequency reference at 1.064 microm by means of ultrasensitive cavity-enhanced frequency modulation spectroscopy of a C(2)HD overtone transition.

Using our cavity-enhanced frequency modulation technique, we obtained the saturated signal of the weakly absorbing gas C(2)HD for the P(5) line of the (nu(2) + 3nu(3)) overtone band at 1.064 microm. An absorption sensitivity of 1.2 x 10(-10) (1.8 x 10(-12)/cm) has been obtained. The absolute frequency of the line center is established (within +/-2.6 kHz) with respect to a doubled Nd:YAG laser/I(2) frequency reference. The short-term stability, limited by the available signal-to-noise ratio, currently is 1 + 10(-11) at 1 s, improving to 5 x 10(-13) at 1000 s. The measured pressure-broadening rate is 34.7(0.8) kHz/mTorr (260 +/- 6.1 kHz/Pa). Selection of slow molecules gave a linewidth four times narrower than the room-temperature transit-time limit.