Modulation-free laser frequency stabilization to molecular absorption using single acoustooptic frequency shifter

A modulation-free scheme for the laser frequency stabilization to molecular transitions is demonstrated in this letter. In this scheme, two diffracted beams of opposite order from a single acoustooptic frequency shifter were used to simultaneously probe the Doppler-broadened absorption of acetylene. A dispersion-like (DL) signal was obtained from the difference of the two absorption signals. By carefully balancing the two absorption signals, the deviation of the center of the DL signal from the center of Doppler-limited absorption can be less than 2.4 MHz. Using the DL signal as the error signal for frequency stabilization, we could lock a 1.53-μm external-cavity diode laser with an estimated stability of 3.1×10/sup -10/.