Wavelength-modulation dispersion spectroscopy of NO with heterodyne phase-sensitive detection.

Heterodyne phase-sensitive dispersion spectroscopy (HPSDS) provides an agile method for gas detection by measuring the phase of an amplitude modulation signal. However, previous HPSDS gas sensors have shown limited sensitivity. In this work, we report a new, to the best of our knowledge, dispersion spectroscopic technique, named wavelength-modulation heterodyne phase-sensitive dispersion spectroscopy (WM-HPSDS), to improve the detection sensitivity. As a proof-of-principle demonstration, a quantum cascade laser (QCL) at 5.26 µm is used to exploit the absorption line of nitric oxide (NO) in a 35-cm-long hollow-core fiber. In addition to modulating the injection current of the QCL at 1 GHz to generate the three-tone beam, a 10-kHz sinusoidal waveform is superimposed on the laser current to produce an additional wavelength modulation. We achieve a noise-equivalent concentration of 40 ppb NO using WM-HPSDS at an integration time of 90 s, corresponding to a noise-equivalent absorption (NEA) coefficient of 6.9 × 10 cm. Compared with the conventional HPSDS technique, the developed WM-HPSDS improves the sensitivity by a factor of 8.3.