Doppler broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry: optimum modulation and demodulation conditions, cavity length, and modulation order

摘要

Doppler broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry (Db-NICE-OHMS) has been scrutinized with respect to modulation and demodulation conditions (encompassing the modulation frequency, νm, the modulation index, β, and the detection phase, θ), the cavity length, L, and the modulation order, k (defined as νm∕νFSR, where νFSR is the free-spectral range of the cavity), primarily in the Doppler limit but also for two specific situations in the Voigt regime (for equal Doppler and homogeneous width and for purely Lorentzian broadened transitions), both in the absence and presence of optical saturation (the latter for the case in which the homogeneous broadening is smaller than the modulation frequency). It is found that, for a system with a given cavity length, the optimum conditions (i.e., those that produce the largest NICE-OHMS signal) for an unsaturated transition in the Doppler limit comprise ν_m∕Γ_D = 1.6 (where ΓD is the half-width at half-maximum of the Doppler width of the transition), β = 1.3, and θ = 0.78π. It is also found that the maximum is rather broad; the signal takes 95% of its maximum value for modulation frequencies in the entire 0.4 ? ν_m∕Γ_D ? 2.4 range. When optical saturation sets in, θ shifts toward the dispersion phase. The optimum conditions encompass k > 1 whenever L > 0.35L_D and 2.6L_D for the dispersion and absorption modes of detection, respectively [where L_D is a characteristic length given by c∕(2Γ_D)]. Similar conditions are found under pressure broadened conditions.