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 two modulating signals. By comparing the experimental results with the theoretical ones, the rf phase can be accurately inferred. The second method requires an analysis of the laser FM sideband spectrum. Since the two harmonic rf waveforms are coherently generated and applied simultaneously to the modulator, the frequency-modulated sideband spectrum should show an interference pattern that reveals the rf phase relationship. With the rf phase information available, a suitable feedback control may then be easily incorporated into the spectroscopic detection system.
Shum
C M
CM
Whittaker
E A
EA