Sub-Doppler optical frequency reference at 1.064 mu m by means of ultrasensitive cavity-enhanced frequency modulation spectroscopy of a C2HD overtone transition

Using our cavity-enhanced frequency modulation technique, we obtained the saturated signal of the weakly absorbing gas C2HD for the P(5) line of the (v(2) + 3v(3)) overtone band at 1.064 mu m. 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 X 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). gave a linewidth four times narrower than the room-temperature transit-time limit. (C) 1996 Optical Society of America