Multiple‐pass enhanced Raman spectroscopy for fast industrial trace gas detection and process control

Abstract

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.

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

“…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.…”

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

“…A major obstacle that limits the applicability of Raman spectroscopy compared with other techniques is its weak signal strength due to small cross section of gas molecules and low molecular density in the gas phase. In the past few years, different techniques are developed by different groups to raise the sensitivity of Raman spectroscopy for trace gas detection, among them are cavity-enhanced Raman spectroscopy (CERS), [3][4][5][6] fiberenhanced Raman spectroscopy (FERS), [7][8][9][10][11][12] multiplepass-enhanced Raman spectroscopy, [13][14][15][16] and nonlinear Raman spectroscopy. [17] Most Raman systems empowered by above techniques only allow for single point measurement.…”

“…We have recently demonstrated two multiple-pass Raman spectroscopy setups with enhanced sensitivity for industrial applications. [15,16] These systems are highly stable and suitable for fast and precise in-line monitoring of (hazardous) gas samples. In this article, we report a multiple-pass multiple-point Raman spectroscopy setup with high utilization ratio of laser energy.…”