Quartz-Enhanced Photothermal Spectroscopy-Based Methane Detection in an Anti-Resonant Hollow-Core Fiber

Abstract

In this paper, the combination of using an anti-resonant hollow-core fiber (ARHCF), working as a gas absorption cell, and an inexpensive, commercially available watch quartz tuning fork (QTF), acting as a detector in the quartz-enhanced photothermal spectroscopy (QEPTS) sensor configuration is demonstrated. The proof-of-concept experiment involved the detection of methane (CH4) at 1651 nm (6057 cm−1). The advantage of the high QTF Q-factor combined with a specially designed low-noise amplifier and additional wavelength modulation spectroscopy with the second harmonic (2f-WMS) method of signal analysis, resulted in achieving a normalized noise-equivalent absorption (NNEA) at the level of 1.34 × 10−10 and 2.04 × 10−11 W cm−1 Hz−1/2 for 1 and 100 s of integration time, respectively. Results obtained in that relatively non-complex sensor setup show great potential for further development of cost-optimized and miniaturized gas detectors, taking advantage of the combination of ARHCF-based absorption cells and QTF-aided spectroscopic signal retrieval methods.

Photothermal gas detection using a miniaturized fiber Fabry-Perot cavity

“…6b shows the full 2f spectra registered for four chosen values of the modulation depth with a 20 s long sweep of the pump center wavelength. The highest 2f PTS signal was observed for a pump wavelength modulation depth equal to 10.7 GHz, which is comparable to the values achieved in other sensors targeting this CH4 transition in the near-IR at ambient pressure values [37]. The full-width at half-maximum (FWHM) of the CH4 transition at 775 Torr is 5.1 GHz, therefore, the experimentally determined value of modulation depth vs. FWHM is close to the theoretical value of m = 2.2 [38].…”