3D-Printed Miniature Fiber-Coupled Multi-pass Cell with Dense Spot Pattern for ppb-level Methane Detection Using a Near-IR Diode Laser

<p>Tunable diode laser absorption spectroscopy (TDLAS) based on multi-pass cell (MPC)^[1-4] is a powerful analytical tool for field applications in air quality monitoring, industrial process control and medical diagnostics. However, the conventional MPC as a core component in TDLAS devices has a large size, low utilization efficiency of the mirror surfaces and tight optical alignment tolerances^[5]. Design of miniaturized long-path MPC for the development of handheld portable high sensitivity sensing devices is one of the mainstream trends nowadays. In this work, we designed and fabricated a mini-MPC with an effective optical absorption path length of 4.2 m and dimensions of 4&#215;4&#215;6 cm³, which to our best knowledge is the current smallest MPC in terms of the same optical path length. The mini-MPC generates a seven-nonintersecting-circle dense spot pattern on two 25.4 mm spherical mirror surfaces providing a high fill factor of 21 cm^-2. A fiber-coupled collimator and an InGaAs photodetector are integrated into the mini-MPC via a high-resolution 3D-printed frame, hence removing the requirement of active optical alignment. Using a 1.65 &#956;m distributed-feedback laser, the performance of this mini-MPC for methane detection was evaluated in terms of linearity, flow response time, stability, minimum detectable limit and measurement precision. Continuous measurements of methane near a sewer and in the atmosphere were performed to demonstrate the stability and robustness of the highly integrated mini-MPC based gas sensor. This work paves the way towards a sensitive, low-cost, miniature trace gas sensor inherently suitable for large-scale deployment of distributed sensor networks and for handheld mobile devices.</p><p><strong>Acknowledgments</strong></p><p>The project is sponsored by National Key R&D Program of China (2017YFA0304203), National Natural Science Foundation of China (NSFC) (61622503, 61575113, 61805132, 11434007), Outstanding Innovative Teams of Higher Learning Institutions of Shanxi, Foundation for Selected Young Scientists Studying Abroad, Sanjin Scholar (2017QNSJXZ-04) and Shanxi &#8220;1331KSC&#8221;. Frank K. Tittel acknowledges support by the Robert Welch Foundation (Grant #C0586)<strong>.</strong></p><p><strong>References</strong></p><p>[1] L. Dong; F. K. Tittel; C. Li; N. P. Sanchez; H. Wu; C. Zheng, Y. Yu, A. Sampaolo, R. J. Griffin, Opt. Express <strong>24</strong> (2016) A528.</p><p>[2] K. Liu, L. Wang, T. Tan, G. S. Wang, W. J. Zhang, W. D. Chen, X. M. Gao, Sensor. Actuat. B-Chem. <strong>220</strong> (2015) 1000.</p><p>[3] R. Cui, L. Dong, H. Wu, S. Li, L. Zhang, W. Ma, W. Yin, L. Xiao, S. Jia, F. K. Tittel, Opt. Express <strong>26</strong> (2018) 24318.</p><p>[4] C. T. Zheng, W. L. Ye, J. Q. Huang, T. S. Cao, M. Lv, J. M. Dang, Y. D Wang, Sensor. Actuat. B-Chem. <strong>190</strong> (2014) 249.</p><p>[5] P. Weibring, D. Richter, A. Fried, J. G. Walega, C. Dyroff, Appl. Phys. B <strong>85</strong> (2006) 207.</p>