Quantitative gas measurements using a versatile OPO-based cavity ringdown spectrometer and the comparison with spectroscopic databases

Abstract

A versatile OPO-based cavity ringdown spectrometer is reported for quantitative and sensitive gas measurement down to nmol/mol levels. The system is based on cavity ringdown spectroscopy (CRDS) in combination with a continuous wave optical parametric oscillator tunable from 2693 to 3505 nm. Using a single set of CRDS mirrors, spectra were recorded of methane, ethane, benzene, propane, water, acetone and formaldehyde. Gas mixtures were gravimetrically prepared in cylinders or via dynamic generation using diffusion tubes (formaldehyde). Results were compared with data from the Hitran, PNNL and NIST databases. Good agreement was found with PNNL and NIST data for most molecules while agreement with Hitran was less for ethane and formaldehyde.

Off-axis re-entrant cavity ring-down spectroscopy with a mid-infrared continuous-wave optical parametric oscillator

“…With the current setup, there is no significant difference between the detection limit of the on-axis and the off-axis technique. The detection limit is also of the same order of magnitude as to what has been previously reported for an on-axis CRDS setup based on a mid-infrared cw-OPO [33] or other rather simple CEAS methods, such as optical-feedback cavity-enhanced absorption spectroscopy based on a mid-infrared quantum cascade laser at 4.43 μm [34]. Advantage of a cw-OPO is that it can …”

Mid-infrared optical parametric oscillators and frequency combs for molecular spectroscopy

“…The effective absorption path length can be increased to several kilometres by coupling the mid-infrared beam into an optical cavity formed by highly reflective low-loss mirrors. Highquality dielectric mirrors with reflectivity of better than 0.9998 in the wavelength region 3-5 µm are commercially available [167]. A 1-m long cavity formed by two such mirrors leads to an optical path length of about 5 km -in other words, a photon coupled into the cavity bounces back and forth between the mirrors for 5 km on average, before escaping the cavity.…”

“…Typical ring-down times with highly reflective mirrors in the 3-5 µm band are in the order of 10 µs [167]. In practice, the NNEA of a CW mid-infrared CRDspectrometer is 10 -8 -10 -10 cm -1 Hz -1/2 , making this method very useful for the detection and analysis of small concentrations of trace gases [60,167]. As an example, von Basum et al have demonstrated ppt-level detection limit for ethane in air using a CW-OPO at 3.35 µm (NNEA = 1.6×10 -10 cm -1 Hz -1/2 ) [176].…”

“…Several aspects need to be taken into account for accurate quantification of trace gas concentrations, as is discussed, for example, in Refs. [167,191]. In practice, a laser spectrometer can be calibrated using a reference gas standard, but in many cases the absorption line strengths need to be known in order to calculate the trace gas amount fractions.…”

Multi-resonant optical parametric oscillator based on 2D-PPLT nonlinear photonic crystal

“…Their high efficiency and wide tuning range, which is only limited by the material transparency range, allowed the OPOs to offer highly attractive all-solid laser sources for various applications, especially in the range of wavelengths not covered by usual laser sources. They are very promising for technological applications such as highresolution spectroscopy [1], detection and diagnosis of gas flows [2][3][4], medicine [5,6], and communications systems [7]. They are particularly useful in the IR region (2-20 μm) in which different molecules have their absorption bands and where there is a lack of tunable laser sources.…”