Frequency offset of a stabilized laser due to modulation distortion

The response of a phase-sensitive detector is examined when its input signal is derived by modulation of a laser-saturated absorption line shape. Neglecting the laser background power curve, it is shown that second-harmonic distortion in the modulation gives rise to the largest laser frequency offset, 0.2% of such distortion yielding approximately 4.5 kHz frequency offset. The distortion effects are also examined when the background power curve is included.