A method to lower the detection limit for optical beat signals from a frequency-dithered stabilized laser

A narrow absorption feature in an atomic or molecular gas (such as iodine o r methane) is used as the frequency reference in many stabilized lasers. As part of the stabilization scheme an optical frequency dither is applied to the laser. In optical heterodyne experiments, this dither is transferred t o the RF beat signal, reducing the spectral power density and hence the sig nal to noise ratio over that in the absence of dither. We removed the dithe r by mixing the raw beat signal with a dithered local oscillator signal. Wh en the dither waveform is matched to that of the reference laser the output signal from the mixer is rendered dither free. Application of this method to a Winters iodine-stabilized helium-neon laser reduced the bandwidth of t he beat signal from 6 MHz to 390 kHz, thereby lowering the detection thresh old from 5 pW of laser power to 3 pW. In addition, a simple signal detectio n model is developed which predicts similar threshold reductions.