Dual-sideband heterodyne of dispersion spectroscopy based on phase-sensitive detection

Abstract

A methane sensor based on dispersion spectroscopy is presented in this paper. A standard Mach-Zehnder modulator working in carrier suppression mode is adopted to generate a spectrum of a carrier and two sidebands. We aim at detecting the phase shift of the beatnote generated by the two sidebands in a methane concentration evaluation process. We put forward an analytical model to describe the dual-sideband heterodyne scheme and carry out experiments to demonstrate the model. Long-term tests show that the sensor has a minimum detection limit of 0.4 ppm·mHz^-0.5 at an average time of 1 s. And in the condition of 1 atm and room temperature, a linear measurement range from 0.4 to 44955 ppm·m is achieved.

Heterodyne Phase-Sensitive Dispersion Spectroscopy in the Mid-Infrared with a Quantum Cascade Laser

“…It is worth noting that, for optimum performance (maximization of the HPSDS signal), the modulation frequency has to be 0.58 times the full width at half-maximum (fwhm) of the target spectral absorption profile, given the profile is a Lorentzian . Lower modulation frequencies will reduce the HPSDS signal. , …”

“…In the past few years, several demonstrations have proved the many advantages of new molecular dispersion spectroscopic methods over traditional absorption measurement techniques for gas detection and analysis based on tunable lasers. , Detection approaches based on molecular dispersion spectroscopy not only overcome baseline and normalization problems , but are capable of providing an output linearly dependent on gas concentration . A direct consequence of this is an enhanced dynamic range .…”

Theoretical and Experimental Study of Heterodyne Phase-Sensitive Dispersion Spectroscopy with an Injection-Current-Modulated Quantum Cascade Laser

“…because ω ≫ Ω [2,5]. The carrier has no extra phase shift induced by the target molecule; thus, Ψ0 = 0.…”

“…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]. Compared with absorption spectroscopy, it provides the advantages of inherent immunity to laser power fluctuations and large dynamic range [5]. Dispersion spectroscopy has been used in many applications such as remote open-path gas sensing [6][7][8], isotope analysis [9], and combustion diagnostics [10,11].…”

“…The refractive index of gas medium is then measured by phase-sensitive detection using a lock-in amplifier. The spectroscopic principle, theoretical model, and experimental setup for near-infrared HPSDS were well documented elsewhere [2,5]. Besides that, the MEMS-based vertical-cavity surface-emitting laser (VCSEL) was also applied in HPSDS for multispecies detection due to the mode-hop-free broadband wavelength tunability of the MEMS-VCSEL [3].…”