Ultrasensitive Detections of Weak Resonances ─ Application to Optical Frequency Standards

We present our latest results on the ultrasensitive molecular overtone spectroscopy using the cavity-enhanced frequency modulation (FM) technique. The principle of this technique makes use of a high-finesse external cavity to enhance the intrinsic resonance contrast, while an FM modulation approach provides shot-noise limited signal recovery. Working with a 1.064 m Nd:YAG laser, we have obtained subDoppler overtone resonances of C2HD, C2H2 and CO2 molecules. A detection sensitivity of 5 x10 of integrated absorption (1 x 10 / cm) over 1-s averaging time has been achieved. The resultant high signal-to-noise ratio of the weak resonance produces excellent laser frequency stabilization.