Determination of radio-frequency phase in harmonic frequency modulation spectroscopy

We have devised two useful schemes to determine the rf phase of the modulation waveforms in harmonic FM spectroscopy, a novel technique that we reported recently for the removal of residual amplitude modulation.The first method is based on the study of the signal line shapes as a function of the relative rf phase of the twomodulating signals. By comparing the experimental results with the theoretical ones, the rf phase can beaccurately inferred. The second method requires an analysis of the laser FM sideband spectrum. Since thetwo harmonic rf waveforms are coherently generated and applied simultaneously to the modulator, thefrequency-modulated sideband spectrum should show an interference pattern that reveals the rf phaserelationship. With the rf phase information available, a suitable feedback control may then be easilyincorporated into the spectroscopic detection system.