Design of multipass cell with dense spot patterns and its performance in a LITES-based methane sensor

Gas sensing technology is the key of the development of artificial olfactory system and has broad application prospects. Laser absorption spectroscopy is an analytical method that uses the "fingerprint" absorption characteristics of gas molecules. The composition and concentration of gas are determined by measuring the attenuation of the intensity of a specific wavelength laser after passing through the gas. This technology has the advantages of high sensitivity, high selectivity and non-destructive detection, and is playing an increasingly important role in the field of gas sensing. Improving the effective absorption path length is an important means to enhance the detection sensitivity of laser absorption spectroscopy gas sensors.

 

In a new paper published in Light: Advanced Manufacturing, a team of scientists, led by Professor Yufei Ma from National Key Laboratory of Laser Spatial Information, Department of Aerospace, Harbin Institute of Technology, China, and co-workers have made new progress in the field of gas sensing based on light-induced thermoelastic spectroscopy (LITES). A ray tracing model based on the law of reflection in vector form was established to calculate the transmission trajectory of laser beams in multipass cell (MPC) and types of MPCs with dense spot patterns were developed. An MPC with a four-concentric-circle pattern exhibited the longest optical path length of approximately 38 m and an optimal ratio of optical path length to volume of 13.8 cm^-2. It was used to build a methane sensor that achieves high sensitivity. With an integration time of 100 s, the minimum detection limit the sensor can reach 59.5 ppb.

 

The MPC with dense spot patterns developed in the paper adopt a coaxial double spherical mirrors structure, which has the advantages of low cost and stable structure. It achieves multiple reflections of the laser beam in a confined space through a unique optical design, significantly increasing the effective interaction length between the laser and gas. The scientists have achieved a perfect integration of their designed MPC with LITES-based gas sensor. LITES is a rapidly developing novel trace gas detection technology in recent years. This technology is based on the light-induced thermoelastic effect of quartz and uses a small, low-cost, and wavelength selective quartz tuning fork as the photoelectric transducer. In addition, a Raman fiber amplifier has also been introduced into LITES system, which can achieve power amplification of near-infrared laser around 1.65 μm, further improving system performance.

 

This study achieved methane detection performance using near-infrared laser sources comparable to systems using mid-infrared laser sources. It is believed that methane sensors based on LITES will play a greater role in environmental monitoring and fire warning in the future.

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