Simulation technique enabling calibration-free frequency-modulation spectroscopy measurements of gas conditions and lineshapes with modulation frequencies spanning kHz to GHz.

Abstract

A simulation technique enabling calibration-free measurements of gas properties (e.g., temperature, mole fraction) and lineshapes via wavelength- or frequency-modulation spectroscopy (WMS or FMS) is presented. Unlike previously developed models, this simulation technique accurately accounts for (1) absorption and dispersion physics and (2) variations in the WMS/FMS harmonic signals, which can result from intensity tuning induced by scanning the laser's carrier frequency [e.g., via injection-current tuning of tunable diode lasers (TDLs)]. As a result, this approach is applicable to both WMS and FMS experiments employing a wide variety of light sources and any modulation frequency [typically kilohertz (kHz) to gigahertz (GHz)]. The accuracy of the simulation technique is validated via comparison with (1) simulated signals produced by established WMS and FMS models under conditions where they are accurate and (2) experimental data acquired under conditions where existing models are inaccurate. Under conditions where existing WMS and FMS models are accurate, this simulation technique yields nearly identical (within 0.1%) results. For experimental validation, the wavelength of a TDL emitting near 1392 nm was scanned across a single absorption line of H2O with a half-width at half-maximum of 350 MHz while frequency modulation was performed at 100 MHz. The best-fit first-harmonic (1f) signal produced by this simulation technique agrees within 1.6% of the measured 1f signal, and the H2O mole fraction and transition collisional width corresponding to the best-fit 1f spectrum agree within 1% of expected values.