Absolute frequency stabilization of an extended-cavity diode laser by means of noise-immune cavity-enhanced optical heterodyne molecular spectroscopy

We implemented an optical frequency standard based on noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) at 1.39 mu m. The emission frequency of an extended-cavity diode laser was actively stabilized against the center of the 4(4,1) -> 4(4,0) transition of the H-2 O-18 v(1) + v(3) band, under optical saturation conditions. The nonlinear regime of laser-gas interaction was reached by using an optical cavity with a finesse of about 8700. By filling it with an O-18-enriched water sample at a pressure of a few Pa, the Lamb dip could be observed with a full width at half-maximum of about 2 MHz. Absolute frequency stabilization was obtained by locking the cavity resonance to the center of the sub-Doppler signal, which was provided by the NICE-OHMS technique under the dispersion regime of operation. An Allan deviation analysis demonstrated a relative frequency stability of similar to 5 x 10(-13) for an integration time of 1 s. For longer integration times, the flicker frequency noise floor set the stability at the level of 4 x 10(-14). (C) 2014 Optical Society of America