Acetylene sensing system based on wavelength modulation spectroscopy using a triple-row circular multi-pass cell.

A sensitive acetylene (CH) sensing system based on a novel triple-row circular multi-pass cell (CMPC) was demonstrated. This CMPC has an effective optical length of 21.9 m within an extremely small volume of 100.1 mL. We utilized wavelength modulation spectroscopy (WMS) for absorption spectroscopy detection of CH. The distance between the two minima of the second harmonic was used to normalize the maximum value of it, which makes the time to obtain stable output for continuous detection shorten dramatically. A fiber-coupled distributed feedback (DFB) diode laser emitting at 1.5316 µm was employed as a light source. An Allan deviation analysis yielded a detection sensitivity of 76.75 ppb with a normalized noise equivalent absorption coefficient of 8.8 × 10 cm Hz during an average time of 340 s. With a fast stable time, reduced size and high detection sensitivity, the proposed sensing system is suitable for trace gas sensing in a weight-limited unmanned aerial vehicle (UAV) and an exhalation diagnosis for smoking test.