A persistent problem in the application of frequency-modulation laser absorption spectroscopy (FMS) to high-sensitivity detection has been the appearance of a background signal attributed to residual amplitude modulation (RAM) in the electrooptic modulation process. The results of an approach to RAM elimination in which a special rf waveform is applied to the modulator are reported. The waveform consists of a wave at frequency omega and another wave at frequency 2 omega , derived from a common source. The omega wave is amplified and phase-shifted with respect to the 2 omega wave. When an optical wave is phase-modulated by the combined omega plus 2 omega waveform, it is possible to adjust the modulating waveform so that these two parts of the RAM are equal in amplitude and opposite in phase, and hence cancel in the photocurrent. Conventional frequency modulation spectroscopy performed using the new source validates the method.
|Original language||English (US)|
|Title of host publication||Unknown Host Publication Title|
|Publisher||Optical Soc of America|
|State||Published - Jan 1 1987|
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