Multi-mode absorption spectroscopy using a quantum cascade laser for simultaneous detection of NO and H2O

Detection of multiple transitions in NO and H2O using multi-mode absorption spectroscopy, MUMAS, with a quantum cascade laser, QCL, operating at 5.3 μm at scan rates up to 10 kHz is reported. The linewidth of longitudinal modes of the QCL is derived from pressure-dependent fits to experimental MUMAS data. Variations in the spectral structure of the broadband, multi-mode, output of the commercially available QCL employed are analysed to provide accurate fits of modelled MUMAS signatures to the experimental data.

Multi-species sensing using multi-mode absorption spectroscopy with mid-infrared interband cascade lasers

The application of an interband cascade laser, ICL, to multi-mode absorption spectroscopy, MUMAS, in the mid-infrared region is reported. Measurements of individual mode linewidths of the ICL, derived from the pressure dependence of lineshapes in MUMAS signatures of single, isolated, lines in the spectrum of HCl, were found to be in the range 10-80 MHz. Multi-line spectra of methane were recorded using spectrally limited bandwidths, of approximate width 27 cm^-1, defined by an interference filter, and consist of approximately 80 modes at spectral locations spanning the 100 cm^-1 bandwidth of the ICL output. Calibration of the methane pressures derived from MUMAS data using a capacitance manometer provided measurements with an uncertainty of 1.1 %. Multi-species sensing is demonstrated by the simultaneous detection of methane, acetylene and formaldehyde in a gas mixture. Individual partial pressures of the three gases are derived from best fits of model MUMAS signatures to the data with an experimental error of 10 %. Using an ICL, with an inter-mode interval of ~10 GHz, MUMAS spectra were recorded at pressures in the range 1-10 mbar, and, based on the data, a potential minimum detection limit of the order of 100 ppmv is estimated for MUMAS at atmospheric pressure using an inter-mode interval of 80 GHz.

Mid-infrared multi-mode absorption spectroscopy using interband cascade lasers for multi-species sensing

An interband cascade laser (ICL) operating at 3.7 μm has been used to perform multimode absorption spectroscopy, MUMAS, at scan rates up to 10 kHz. Line widths of individual modes in the range 10-80 MHz were derived from isolated lines in the MUMAS signatures of HCl. MUMAS data for methane covering a spectral range of 30 nm yielded a detection level of 30 μbar·m for 1 s measurement time at 100 Hz. Simultaneous detection of methane, acetylene, and formaldehyde in a gas mixture containing all three species is reported.

Thermal grating velocimetry

A method is reported for time- and space-resolved nonintrusive velocimetry of high-speed gas flows by measurement of the Doppler shift of light scattered from a laser-induced thermal grating. The principle is demonstrated by use of a pulsed frequency-doubled Nd:YAG laser to induce a thermal grating in NO(2) seeded into an argon flow. Signals are generated by Bragg scattering of probe beams at the fundamental frequency of the same Nd:YAG laser. Flow velocities in the range 30-180 ms(-1) are measured, in agreement with values obtained with a Pitot tube. The measurement uncertainties obtained indicate that a precision of 1% is feasible for flows at Mach 1.

Imaging of trace species distributions by degenerate four-wave mixing: diffraction effects, spatial resolution, and image referencing

We describe the theory of imaging by degenerate four-wave mixing (DFWM) using a standard diffraction theory of imaging by coherent light. We demonstrate that, even with the phase-conjugating geometry, no aberration correction can be achieved by DFWM imaging. We demonstrate the coherent nature of DFWM image formation using spatially modulated signals generated in flame OH in the phase-conjugating geometry. The intensity distribution in the Fourier plane of a telecentric lens system is shown to be the spatial Fourier transform of the object distribution characteristic of coherent imaging. The brightness of the DFWM signals exceeds that of similar laser-induced fluorescence signals that can be discriminated by restricting the aperture of the imaging system while still allowing a spatial resolution of approximately 70 ?m. DFWM imaging with the forward-folded boxcars geometry is demonstrated and used in a simple referencing scheme to compensate for structure on the images imposed by nonuniformity of the laser beams employed. Images formed in NO are used to illustrate that structure on a scale of less than 100 ?m arising from beam inhomogeneity can be removed by this referencing technique.

Multiplex H2 coherent anti-Stokes Raman scattering thermometry with a modeless laser

The use of a modeless laser as the Stokes source for multiplex coherent anti-Stokes Raman scattering in molecular hydrogen is reported. The elimination of noise associated with mode competition in conventional standing wave lasers is shown to result in reliable and accurate single-shot thermometry of H(2) in a microwave-assisted diamond chemical vapor deposition plasma reactor. Single-shot temperatures are recorded with a precision of 7.3%. Possible improvements to this precision are discussed and applications of the technique for on-line process monitoring are briefly presented.

Velocimetry of gas flows using degenerate four-wave mixing

A method of velocimetry of gaseous flows based on resonant degenerate four-wave mixing (DFWM) is described. Spatially resolved velocity measurements are derived from Doppler shifts of DFWM signals generated in NOseeded into flows of air or N(2). Velocities in the range of 60 to 200 ms(-1) have been measured with an uncertainty of +/-30 ms(-1).

Two-dimensional mapping of temperature in a lame by enerate four-wave mixing in OH

Phase conjugate images are produced by resonant degenerate four-wave mixing from rotationally excited OH molecules in a premixed, laminar methane/air flame. By comparing signal intensities in the images produced using transitions from different rotational levels, the temperature map of a planar section of the flame is derived.

Improved precision of single-shot temperature measurements by broadband CARS by use of a modeless laser

The use of a novel modeless laser as the broadband source in multiplex CARS thermometry, by reducing the laser noise on the CARS spectra, is shown to give a precision of approximately 1 % in single-shot temperature measurements in nitrogen at 1200 K.

Multiplex degenerate four-wave mixing in a flame

The principle of multiplex degenerate four-wave mixing (DFWM) is demonstrated by recording, with a single laser shot, DFWM signals from both D lines of atomic sodium in a seeded flame. Applications of the technique for singleshot temperature measurements for combustion diagnostics are discussed.

Two-dimensional phase-conjugate imaging of atomic distributions in flames by degenerate four-wave mixing

We report the demonstration of a technique using resonant degenerate four-wave mixing to produce high-resolution, background-free images of atomic distributions in flames. Comparison with laser-induced-fluorescence methods and applications of the method to species of interest in combustion are briefly discussed.