Cavity-enhanced Raman spectroscopy with optical feedback cw diode lasers for gas phase analysis and spectroscopy

Abstract

A variant of cavity-enhanced Raman spectroscopy (CERS) is introduced, in which diode laser radiation at 635 nm is coupled into an external linear optical cavity composed of two highly reflective mirrors. Using optical feedback stabilisation, build-up of circulating laser power by 3 orders of magnitude occurs. Strong Raman signals are collected in forward scattering geometry. Gas phase CERS spectra of H(2), air, CH(4) and benzene are recorded to demonstrate the potential for analytical applications and fundamental molecular studies. Noise equivalent limits of detection in the ppm by volume range (1 bar sample) can be achieved with excellent linearity with a 10 mW excitation laser, with sensitivity increasing with laser power and integration time. The apparatus can be operated with battery powered components and can thus be very compact and portable. Possible applications include safety monitoring of hydrogen gas levels, isotope tracer studies (e.g., (14)N/(15)N ratios), observing isotopomers of hydrogen (e.g., radioactive tritium), and simultaneous multi-component gas analysis. CERS has the potential to become a standard method for sensitive gas phase Raman spectroscopy.

Intracavity Phase-matched Coherent Anti-stokes Raman Spectroscopy for Trace Gas Detection

“…A previous study reported a detection limit of 480 ppm for hydrogen using CERS based on spontaneous Raman scattering with an incoherent Raman process. 11 Therefore, we expect that our approach will achieve a detection limit on the ppb level because it is based on a CARS process with an intrinsic coherent Raman process. This approach has the potential to lead to a high-sensitivity, highly selective, and real-time monitoring tool for the detection of trace isotopes, especially diatomic molecules, including radioactive hydrogen isotopes, which are infrared inactive.…”

“…11 In fact, coherent anti-Stokes Raman scattering (CARS) microscopy has excellent sensitivity, comparable to that of fluorescent microscopy, but without the need for staining. 13 Since the coherent Raman process generally generates signals 10 5 -10 7 times higher than the incoherent spontaneous Raman process, CERS incorporating a coherent Raman process has great potential as a detection tool for gas-phase chemical species that are otherwise difficult to detect using conventional absorption-based spectroscopic methods and will become an attractive alternative to CEAS and CRDS.…”

“…10,11 CERS also benefits from the cavity enhancement effect, and is an attractive tool, for example, for the detection of homonuclear molecules that have no absorption band in the infrared region. However, CERS often suffers from its relatively low sensitivity for detection of molecules because of the lower Raman cross-sections (~10 -30 cm 2 ) compared with the absorption cross-sections (~10 -16 cm 2 ) of the molecules.…”

Recent Advances in the Characterization of Gaseous and Liquid Fuels by Vibrational Spectroscopy

“…Consequently, a laser with relatively low power could be employed (0.1 W, argon ion). High sensitivity with a low-power laser was also achieved by Hippler et al [54]. They built a cavity, in which the radiation of a diode laser (10 mW, 635 nm) was enhanced by three orders of magnitude to measure hydrogen, methane and benzene vapor.…”

Technical Readiness and Gaps Analysis of Commercial Optical Materials and Measurement Systems for Advanced Small Modular Reactors

“…Sample gas is pumped into the optical cavity to interact with the intense laser light. A recent study demonstrated low ppm or even ppb level detection limit using cavity-enhanced Raman spectroscopy to detect H 2 , N 2 , and other gases (Salter et al 2012). In this experiment, a 10-mW single-mode diode laser at 635 nm was used.…”