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We demonstrate quantitative measurements of methane (CH 4 ) mole fractions in a low-pressure fuel-rich premixed dimethyl ether/oxygen/argon flat flame (Φ = 1.87, 37 mbar) using mid-infrared (IR) polarization spectroscopy (IRPS). Non-intrusive in situ detection of CH 4 , acetylene (C 2 H 2 ), and ethane (C 2 H 6 ) in the flame was realized by probing the fundamental asymmetric C–H stretching vibration bands in the respective molecules in the spectral range 2970–3340 cm −1 . The flame was stabilized on a McKenna-type porous plug burner hosted in a low-pressure chamber. The temperature at different heights above the burner (HAB) was measured from the line ratio of temperature-sensitive H 2 O spectral lines recorded using IRPS. Quantitative measurements of CH 4 mole fractions at different HAB in the flame were realized by a calibration measurement in a low-pressure gas flow of N 2 with a small admixture of known amount of CH 4 . A comprehensive study of the collision effects on the IRPS signal was performed in order to quantify the flame measurement. The concentration and temperature measurements were found to agree reasonably well with simulations using Chemkin. These measurements prove the potential of IRPS as a sensitive, non-intrusive, in situ technique in low pressure flames.

We present a scheme to estimate the noise equivalent power (NEP) of the frequency upconversion detectors (UCDs), detecting mid-infrared (MIR) light. The NEP of UCD is a combined contribution of NEPs from the upconversion process and from the photodetector, used for detecting the upconverted signal. The 2 − 5 µm MIR range is particularly investigated in this letter using a bulk periodically poled lithium niobate based CW-intracavity UCD. We measured the NEP of UCD as 20 fW/ √ Hz at MIR wavelength of 3.39 µm. We showed that the limiting factor here is not the noise from the upconversion process (estimated NEP is 2.3 fW/ √ Hz at 3.39 µm), but from the electrical noise in the photodetector itself. We also compared the performance of our UCD with previously published results and with market available direct MIR detectors. Additionally, we measured the optical noise of the UCD over its working spectral range (2.9 − 3.6 µm) and compared with numerical simulation.

This paper compares the signal-to-noise ratio obtained using an InSb photodiode for infrared (IR) polarization spectroscopy to that obtained using an upconversion detector, and shows a factor 64 improvement by the change. Upconversion detection is based on using sum frequency generation to move the IR optical signal to near-visible wavelengths to improve the sensitivity.

It has previously been demonstrated that the ratio of the degenerate four wave mixing signal from two hot water line groups near 3231 cm–1 can be used for seedless flame temperature measurements. This paper presents an investigation of the impact of saturation effects on the measured signal intensity from each line group, as well as an estimation of the accuracy of the method. The saturation effects observed here would result in a large systematic error if they are not taken into account when using the degenerate four-wave mixing intensity of these water line groups to calculate the flame temperature.