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

Abstract

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.

Trace Analysis of C4F7N Insulating Gas Mixtures by Spontaneous Raman Spectroscopy and Gas Chromatography

“…However, numerous recent efforts into the development of enhancement techniques by way of optical capillaries, − fibers, − microcavities, and conventional resonator cavities − that increase the interaction strength and/or the SRS light collection efficiency have dramatically improved the prospects of SRS for trace gas sensing. Among these enhancement strategies, the utilization of a nonresonant multipass cavity offers particular robustness. − This technique has demonstrated trace sensitivity well into the parts-per-billion range, , while requiring minimal power consumption and no active optical resonance stabilization.…”

Multipoint Raman Spectrometer Based on a Time-Resolved CMOS SPAD Sensor

“…Typically, only a single point in a process is monitored by one spectrometer, but multipoint measurement setups have also been reported. Gas detection from two measurement points was shown in [7], and a waste water measurement from ten points was demonstrated in [8]. In these systems, the capability for measuring Raman spectra from multiple points is based on the two dimensions of a 2D pixel array of a CCD sensor.…”

Natural Gas Analysis Using Polarized Raman Spectroscopy

“…For a long time, the use of this method for gas analysis was limited by the low level of useful signals. However, small-sized high-power lasers and highly sensitive photodetectors have now become available, and various techniques are being developed to increase the intensity of Raman signals. − In this regard, the detection limit of modern Raman gas analyzers is close to 1 ppm. ,,,, This level of sensitivity makes it possible to measure the composition of the gas in accordance with the requirements specified in ISO 6974. However, the Raman spectrum of NG is quite complex, and the bands of various components strongly overlap each other .…”