Double frequency modulation spectroscopy: high modulation frequency with low-bandwidth detectors

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Abstract
In this paper, we describe experimental and theoretical investigations of two variations of frequency modulation (FM) spectroscopy that use two electrooptic modulators. In the first variation, both modulators are frequency modulators (FM–FM), and, in the second, one is a frequency modulator and one is an amplitude modulator (FM–AM). The essential advantage of FM–FM and FM–AM spectroscopy is that sensitive lowbandwidth detectors, such as photomultiplier tubes, can be used to detect signals generated by the absorption of sidebands displaced from the carrier by frequencies far above the detector cutoff frequency. These two variations are complementary in the sense that, in situations where optical power is at a premium, the FM–FM scheme is most appropriate, and in situations where modulator drive power is at a premium, the FM–AM scheme is most appropriate. Using either of these variations, we have detected the absorption of 700-MHz sidebands with photomultiplier tubes whose cutoff frequencies lie below 100 MHz.