Innovative Substrate-Integrated Hollow Waveguide Coupled Attenuated Total Reflection Sensors for Quantum Cascade Laser Based Infrared Spectroscopy in Harsh Environments

An innovative mid-infrared spectroscopic sensor system based on quantum cascade lasers has been developed. The system combines the versatility of substrate-integrated hollow waveguides (IHWGs) with the robustness of attenuated total reflection (ATR) crystals employed as internal reflection waveguides for evanescent field sensing. IHWGs are highly reflective metal structures that propagate infrared (IR) radiation and were used as light pipes for coupling radiation into the ATR waveguide. The combined IHWG-ATR device has been designed such that the utmost stability and robustness of the optical alignment were ensured. This novel assembly enables evanescent field absorption measurements at yet unprecedently harsh conditions, that is, high pressure and temperature. Combining these advantages, this innovative sensor assembly is perfectly suited for taking ATR spectroscopy into the field where the robustness of the assembly and optical alignment is essential.

In-Situ Evaluation of the Pitch of a Reflective-Type Scale Grating by Using a Mode-Locked Femtosecond Laser

Major modifications are made to the setup and signal processing of the method of in-situ measurement of the pitch of a diffraction grating based on the angles of diffraction of the diffracted optical frequency comb laser emanated from the grating. In the method, the improvement of the uncertainty of in-situ pitch measurement can be expected since every mode in the diffracted optical frequency comb laser can be utilized. Instead of employing a Fabry-Pérot etalon for the separation of the neighboring modes in the group of the diffracted laser beams, the weight-of-mass method is introduced in the method to detect the light wavelength in the Littrow configuration. An attempt is also made to reduce the influence of the non-uniform spectrum of the optical comb laser employed in the setup through normalization operation. In addition, an optical alignment technique with the employment of a retroreflector is introduced for the precise alignment of optical components in the setup. Furthermore, a mathematical model of the pitch measurement by the proposed method is established, and theoretical analysis on the uncertainty of pitch measurement is carried out based on the guide to the expression of uncertainty in measurement (GUM).

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)<sup> [1-4]</sup> 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<sup> [5]</sup>. 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 4×4×6 cm<sup>3</sup>, 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<sup>-2</sup>. 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 μm 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 “1331KSC”. 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>