Multiple‐pass‐enhanced multiple‐point gas Raman analyzer for industrial process control applications

Abstract

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.

A Versatile Multiple-Pass Raman System for Industrial Trace Gas Detection

“…Recently, Godot et al have introduced a commercial Raman analyzer for process control in a tritium facility, and the LOD was reported as 200 Pa for hydrogen isotopologues, with an acquisition time of 120 s [32]. Based on our previous investigations [29][30][31], the LOD of the current setup for hydrogen isotopologues can be safely estimated to be lower than that of H 2 O. Thus, the current system is preferred in low-pressure gas applications.…”

“…The laser head (Laser Quantum OPUS660) is stabilized by a water cooler, which maintains the base plate temperature at 24 degrees Celsius. The OPUS660, in fact, was first chosen for hydrogen isotopologues monitoring applications in our previous systems [29][30][31]. We use 660 nm instead of a shorter wavelength (e.g., 532 nm) because, in our previous design, the gas chamber was located between the cavity mirrors, and thus, fluorescence generated from optical windows reduced the signal-to-noise ratio.…”

“…In the past few years, various Raman systems have been designed and implemented, aiming at lowering limit of detection (LOD) of gas molecules. Examples of such systems are cavity-enhanced Raman spectroscopy (CERS) [11][12][13][14][15], fiber-enhanced Raman spectroscopy (FERS) [16][17][18][19][20][21], Purcell-enhanced Raman spectroscopy [22,23] and multiple-pass-enhanced Raman spectroscopy [24][25][26][27][28][29][30][31].…”

“…Recently, instead of using side detection geometry, Velez et al employed a collinear detection geometry for their near-concentric multiple-pass cavity, and 34 ppm was achieved for CO2 in 5 s [27]. We have recently introduced a variant of multiple-pass Raman spectroscopy with enhanced sensitivity and stability for industrial long-term monitoring applications [29][30][31]. We take advantage of the large collection area of fiber bundles, which relaxes the laser beam overlap requirements inside a multiple-pass cell.…”

Review on Hollow-Core Fiber Based Multi-Gas Sensing Using Raman Spectroscopy

“…In parallel, the Raman spectroscopy offers advantages in multi-gas detection. In recent years, the Raman spectroscopy has become a potent tool for qualitative and quantitative analysis of critical gases [16][17][18][19][20][21][22][23][24][25][26][27]. Researchers have explored ways like harnessing more potent lasers or intensifying the interaction between laser beams and gas molecules to enhance the Raman signal.…”