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

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.

Modulation Characteristics of Period-One Oscillations in Quantum Cascade Lasers

“…However, the modulation characteristics of P1 oscillations described in this work deviate significantly from both required features; hence, it is difficult to apply P1 oscillations in WMS. On the other hand, the modulation frequency of HPSDS is in the range of MHz to GHz [25,32], and that of FMS is of 100 MHz to several GHz [26][27][28][29]32], respectively. Both modulation frequencies can be easily reached by the P1 oscillations in QCLs.…”

“…HPSDS mainly requires AM, while FMS requires FM, but the co-existence of AM and FM is allowed in both practical applications [27,50]. HPSDS requires only a small modulation depth, which satisfies the modulation characteristics of P1 oscillations [27,28]. In contrast, FMS requires a modulation depth spreading from several percent up to tens of percent for achieving optimal performance [50,53].…”

“…QCLs have been employed for gas sensing applications using a large variety of spectroscopy techniques [24]. Among these techniques, the modulation spectroscopy requires a sinusoidal modulation of the laser sources, including wavelength modulation spectroscopy (WMS) [25], frequency modulation spectroscopy (FMS) [26], and heterodyne phase-sensitive dispersion spectroscopy (HPSDS) [27,28]. The QCLs are usually modulated by direct current modulation or by external modulation using modulators [27,[29][30][31] with modulation frequencies ranging from kHz up to several GHz [25][26][27][28][29]32].…”

“…Among these techniques, the modulation spectroscopy requires a sinusoidal modulation of the laser sources, including wavelength modulation spectroscopy (WMS) [25], frequency modulation spectroscopy (FMS) [26], and heterodyne phase-sensitive dispersion spectroscopy (HPSDS) [27,28]. The QCLs are usually modulated by direct current modulation or by external modulation using modulators [27,[29][30][31] with modulation frequencies ranging from kHz up to several GHz [25][26][27][28][29]32]. Since the P1 oscillations also modulate the light in a sinusoidal waveform, this modulation might be helpful for developing modulation spectroscopy systems.…”

A Laser-Based Multipass Absorption Sensor for Sub-ppm Detection of Methane, Acetylene and Ammonia

“…Over the past decades, various techniques have been developed for quantitative measurements of gas concentrations, including electrochemical method [14], Fourier transform spectroscopy [15], non-dispersive infrared method [16], laser-based absorption and dispersion spectroscopy [17][18][19][20], photoacoustic and photothermal spectroscopy [21,22]. Many of these techniques have been transferred to commercially-available sensors and have been continuously improved to satisfy the increasing demand for higher detectivity, sensitivity and robustness.…”