We report a variant of multiple‐pass Raman spectroscopy with enhanced sensitivity for engineering applications. A four‐mirror multiple pass cell is introduced and compared with the conventional two (concave)‐mirror configuration. With this system, gas samples in static cells are measured and monitored. The results show that the noise equivalent detection limit (3σn) close to 100 ppm for hydrogen isotopologues can be achieved within two seconds with a 1.5‐W red laser. The relative precision is characterized by long‐term measurement, and the results indicate high system stability. The signal‐to‐noise ratio (SNR) in different configurations are investigated, and results show that high (greater than nine) SNR gain is achieved in the configuration of 12 passes. This newly designed Raman system is highly stable and suitable for fast and precise in‐line monitoring of hydrogen isotopologue mixtures, as well as other (hazardous) gas samples.

We report an advanced multiple-pass Raman spectroscopy setup with enhanced sensitivity for industrial in situ monitoring and process control applications. A multiple-pass cavity with 20 total passes is constructed using one concave mirror and two high-reflection mirrors. The multiple-pass cavity is fully compatible with gas chamber with dead volume as small as 2.5 ml. The setup is simple, reliable, and robust, which is important for practical industrial applications. With this system, gas samples in static cells are tested to show the analytical potential of this multiple-pass setup. The results show that the noise equivalent detection limit (3σ) of 12.0 (N 2 ), 13.3 (O 2 ), 12.2 (CO 2 ), and 5.8 Pa (hydrogen isotopologues), which corresponds to relative abundance by volume at 1 bar total pressure of 120, 133, 122, and 58 ppm can be achieved in 1 s with a 1.5-W red laser. We further demonstrate a modified version of current multiple-pass Raman spectroscopy, which can afford an even lower detection limit for nonhazardous gas samples. The sensitivity of the current setup can be further increased, and different approaches are discussed in detail.

We report an advanced multiple-pass multiple-point Raman spectroscopy setup with enhanced sensitivity for industrial in situ monitoring and process control applications. The design of this multiple sampling setup enables high laser effective energy utilization ratio, and full laser energy is available for each sampling point. As a result, high sensitivity is obtained in every sampling point. The setup is simple, reliable, and robust, which is important for practical industrial applications. With eight passes configuration, gas samples in static cells are tested to show the analytical potential of this multiple sampling setup. The back-to-back experimental results show that the noise equivalent detection limit (3σ) of 40.6 (N 2), 46.0 (O 2), 22.9 (H 2 O), and 19.1 Pa (hydrogen isotopologues), which corresponds to relative abundance by volume at 1 bar total pressure of 406, 459, 229, and 191 ppm, respectively, can be achieved in 1 s with a 1.5-W red laser. The analysis indicates that similar or even better sensitivity can be achieved for every sampling point in a practical 3-point system. The system precision is characterized by long-term measurement of static samples, and the results indicate high system stability. Although a 3-point system is demonstrated in this investigation, this setup can be easily upgraded to incorporate more sampling positions by increasing the number of lenses inside the multiple-pass cavity, which is important for practical applications. Important aspects regarding instrumentation engineering are also briefly discussed. The results obtained with this multiple-pass-enhanced multiple-point Raman system are very promising. The system can be applied to hydrogen isotopologues monitoring and process control in international thermonuclear experimental reactor (ITER), as well as China fusion engineering test reactor (CFETR). Other industrial applications like automotive engine diagnosis and logging gas detection can also be benefited from current design.

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The fast and in-line multigas detection is critical for a variety of industrial applications. In the present work, we demonstrate the utility of multiple-pass-enhanced Raman spectroscopy as a unique tool for sensitive industrial multigas detection. Instead of using spherical mirrors, D-shaped mirrors are chosen as cavity mirrors in our design, and 26 total passes are achieved in a simple and compact multiple-pass optical system. Due to the large number of passes achieved inside the multiple-pass cavity, experiments with ambient air show that the noise equivalent detection limit (3σ) of 7.6 Pa (N2), 8.4 Pa (O2) and 2.8 Pa (H2O), which correspond to relative abundance by volume at 1 bar total pressure of 76 ppm, 84 ppm and 28 ppm, can be achieved in one second with a 1.5 W red laser. Moreover, this multiple-pass Raman system can be easily upgraded to a multiple-channel detection system, and a two-channel detection system is demonstrated and characterized. High utilization ratio of laser energy (defined as the ratio of laser energy at sampling point to the laser output energy) is realized in this design, and high sensitivity is achieved in every sampling position. Compared with single-point sampling system, the back-to-back experiments show that LODs of 8.0 Pa, 8.9 Pa and 3.0 Pa can be achieved for N2, O2 and H2O in one second. Methods to further improve the system performance are also briefly discussed, and the analysis shows that similar or even better sensitivity can be achieved in both sampling positions for practical industrial applications.