Multiple-Pass Enhanced Raman Spectroscopy For Fast Industrial Trace Gas Detection And Process Control

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 (3 sigma) of 12.0 (N-2), 13.3 (O-2), 12.2 (CO2), 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. Based on the experience gained in this prototype setup, various improvements are currently being investigated and implemented, aiming at higher sensitivity and stability. The results obtained with this newly designed Raman system are very promising, as sensitive subsecond monitoring and process control of low-pressure hydrogen isotopologues based on Raman spectroscopy seems to be within reach in a fusion reactor. Besides, the analysis and monitoring of other important gas species (e.g., environmental gas) can also be benefited from the current design.