Frequency modulation spectroscopy with second harmonic detection

Attention is given to the use of second harmonic demodulation in FM spectroscopy to yield symmetric spectral features that may be useful in unscrambling complex spectra, and in allowing the modulation frequency to be effectively doubled (at the expense of modulator drive power). It is noted that at frequencies above 1 GHz, modulation frequency doubling becomes more difficult because of the phase-matching properties of electrooptic materials. Superior SNR with relatively broad linewidth lasers, allowing spectral scans across the entire dye curve, are achieved by the present method.