Abstract
We report the theoretical and experimental study of calibration-free heterodyne phase-sensitive dispersion spectroscopy (HPSDS) in the mid-infrared using a direct current modulated mid-infrared quantum cascade laser (QCL). The modulation of QCL current at several hundred MHz or higher generates the synchronous frequency and intensity modulation of the QCL emission. An analytical model of the phase of the beat note signal in HPSDS is derived by considering the absorption and dispersion processes and incorporating the QCL modulation parameters. In the experiment, a 4.5 μm QCL modulated at 350 MHz was used to measure N2O at 200 Torr in a 10 cm gas cell. The N2O concentrations inferred from the analytical model were compared with the nominal values to show good agreement over the concentration range of 189−805 ppm with a standard deviation <3%. When the QCL wavelength was locked at the line-center of the molecular transition, it was of interest to find that the theoretical model was simplified to that used for near-infrared HPSDS with an electro-optical modulator for laser modulation.
Highlights
Molecular dispersion spectroscopy has received more attention in the recent decade by measuring the refractive index in the vicinity of a molecular resonance [1,2,3,4]
Representative detection schemes adopted for laser dispersion spectroscopy include chirped laser dispersion spectroscopy (CLaDS) [1], heterodyne phase-sensitive dispersion spectroscopy (HPSDS) [2], and multi-heterodyne dispersion spectroscopy [12,13]
The slight asymmetry of the measured spectra was mainly due to the asymmetry of the sidebands generated by intensity modulation (IM) and frequency modulation (FM)
Summary
Molecular dispersion spectroscopy has received more attention in the recent decade by measuring the refractive index in the vicinity of a molecular resonance [1,2,3,4]. Dispersion spectroscopy enhanced by using a high-finesse optical cavity, known as noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy (NICE-OHMS), has achieved a sensitivity of 5 × 10−13 of integrated absorption [4] Among these detection schemes, HPSDS is an attractive technique that features a simpler. The model construction starts with the E-field emission reported for CLaDS [18,19] because both dispersion techniques use the high-frequency injection-current-modulated QCL. In QCL-based dispersion spectroscopy, a three-color laser beam is generated by directly modulating (angular frequency Ω) the injection current of the laser. The laser beam is transmitted through the gas sample where each wavelength component experiences the corresponding phase shift and attenuation because of the molecular dispersion and absorption. Thethe laser transmission through a gas sample due to the simultaneous absorption
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