Two-tone optical heterodyne spectroscopy with diode lasers: theory of line shapes and experimental results

This paper presents a theoretical analysis of the line shapes and signal-to-noise ratios obtained in two-tone optical heterodyne spectroscopy with tunable lead-salt diode lasers. The theory is described in terms of the frequency-modulation (FM) index β, the amplitude-modulation (AM) index M, their relative phase shift ψ, and the ratio of modulation frequency to the absorption-line half-width ν¯m. Synthetic spectra are presented for both Gaussian and Lorentzian line shapes and show considerable structural variation with the theoretical parameters. Experimental two-tone optical heterodyne spectra were obtained by modulating a specially modified lead-salt diode laser in the radio-frequency region. The experimental spectra obtained from NH3 absorption lines confirm the theoretical results.

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