Abstract
Frequency modulation (FM) spectroscopy has been shown to be one of the fastest growing fields of highly sensitive spectroscopic techniques since it was first introduced by Bjorklund.1 Typically an external electro-optic phase modulator is used to modulate the laser output at radio frequencies where laser noise approaches the shot noise limit. This modulated beam is incident on the photodetector after passing through the absorbing medium. For pure FM, a beat- frequency absorption signal with zero background is obtained. Recently, Janik, Carlisle, and Gallagher2 have devised a new FM modulation/demodulation technique that retains the desirable high frequency modulation yet with low frequency detection. The technique involves simultaneous modulation of the laser beam with two different frequencies, thus the term two-tone modulation.
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