Abstract
Laser dispersion spectroscopy provides an attractive way of gas sensing by measuring refractive index rather than absorption. The previous dispersion gas sensors were mostly developed with free-space optics. In this work, we demonstrated heterodyne phase-sensitive dispersion spectroscopy (HPSDS) for sensitive nitric oxide (NO) detection in a hollow-core fiber to take advantage of the enhanced light-gas interaction in a compact setup. A quantum cascade laser (QCL) at 5.26 µm was coupled into a 35 cm tellurite hollow-core antiresonant fiber to exploit the strong absorption line of NO. The injection current of the QCL was modulated at 1 GHz to generate the three-tone beam for dispersion measurements. We achieved a noise equivalent concentration (NEC) of 0.8 ppm at the measurement time of 80 s. A comparative study of HPSDS with wavelength modulation spectroscopy (WMS) was also conducted to evaluate these two methods in terms of sensitivity and dynamic range.
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