D. Courtois

Design and characteristics of a differential Helmholtz resonant photoacoustic cell for infrared gas detection

The Helmholtz resonant photoacoustic (PA) cell is a very convenient design of PA system for air pollution monitoring based on infrared molecular absorption. A simple differential Helmholtz resonator designed for flow measurements is presented in this work. The investigation of the PA systems characteristics based on this design includes experimental study of the responsitivity both of the separate photoacoustic cell and the whole photoacoustic system applied to trace gases detection. The experimental observations are compared to the theoretical predictions. A simple arrangement to enhance the photoacoustic signal of the whole system by a factor of 2 is presented.

Pressure broadening and shift coefficients of H2O due to perturbation by N2, O2, H2 and He in the 1.39 μm region: experiment and calculations

Near-infrared diode laser spectrometry was used to determine the pressure broadening and shift effect on H2O due to N2, O2, H2 and He in the 1.39 µm region. Halfwidths and shifts of water vapour were measured for six transitions. These lines are from the ν 1+ν 3 and 2ν 1 bands. A complete set of H2O transitions with various J values was investigated, including the lines selected to monitor in situ stratospheric H2O with the SDLA, a balloon-borne diode laser spectrometer. Experimental results are compared with theoretical calculations based on a semi-empiric technique that incorporates various corrections to the Anderson theory. This approach is performed in the framework of the impact approximation, which makes interpretation of the collision process simpler and allows reliable results to be obtained.

Free-running 9.1-mu m distributed-feedback quantum cascade laser linewidth measurement by heterodyning with a (CO2)-O-18 laser

We report spectral linewidth measurements of a 9.1‐µm distributed-feedback quantum cascade laser (QCL). The free-running QCL beam was mixed with a waveguide isotopic C18O2 laser onto a high-speed HgCdTe photomixer, and beat notes were recorded from a radio-frequency spectral analyzer. Beating was performed at two operating conditions, first near the QCL laser threshold (beating with the C18O2R10 line) and then at a high injection current (beating with the C18O2R8 line). Overall, beat note widths of 1.3–6.5 MHz were observed, which proves that a free-running QCL can have a short-term spectral width near 1 MHz.

Optimisation of photoacoustic resonant cells with commercial microphones for diode laser gas detection.

The theoretical and experimental study of the differential Helmholtz resonant (DHR) cell sensitivity under variation of the total gas pressure is made for various commercial microphones. Near-infrared lasers (room-temperature diode lasers) were used to measure the response of DHR cell versus pressure of the absorbing gas and frequency of the laser radiation modulation. Several molecular absorbers like H2O, CH4, mixed with molecular buffer gases were used to investigate the behavior of the photoacoustic (PA) signal characteristics with a DHR cell. The experimental data are compared with the results of computer simulation. The minimal detectable concentrations of gases were determined for the DHR cell for each commercial microphone.

Atmospheric laser heterodyne detection

Abstract We describe in this paper what has been done for ten years at Reims in the area of atmospheric heterodyne laser spectrometry. The recent technical progress allows now to use this technique in two different ways. The first is devoted to the radiometry with low spectral resolution [0.1 cm −1 ] of “cold” gases (300 K) in the troposphere. The second is devoted to the transmission spectrometry of the atmosphere (high resolution 0.002 cm −1 ) with the sun as a blackbody source. In the particular case of ozone, the gas absorption lines are in the neighborhood of the CO 2 laser lines. The earlier ozone concentration measurements were precision limited because of the atmospheric CO 2 absorption and the total integration time (>30 min) of the recorded spectrum. An alternative improved method is presented where the atmospheric CO 2 absorption is eliminated with the use of a 18 O 12 O 18 O laser. An acousto-optic spectrometer (AOS) gives the multiplex advantage and reduces the total duration of a spectrum to only one minute.

Water-vapor isotope ratio measurements in air with a quantum-cascade laser spectrometer

A spectrometer was used in the laboratory to study water-vapor isotope ratio measurements in air: H2 18O/H2 16O and HDO/H2 16O near 6.7 microm. The spectral region ranging from 1483 to 1487 cm(-1), which is suitable for the in situ laser sensing of major water-vapor isotopologues in the middle atmosphere from airborne or balloonborne platforms, was investigated by use of a continuous-wave distributed feedback quantum-cascade laser. The concentrations obtained were compared with the concentrations obtained with a hygrometer. The sigma(18O) values were found to be in excellent agreement with the standard value for two individual lines. The sigma(D) value was slightly higher than the standard value.

Precise and absolute intensity measurement of the ν3 (10, 5, 6)-(9, 5, 5) Ozone line with high resolution spectrometers

Abstract A new Tunable Diode Laser Spectrometer (T.D.L.S.) has been developed at the Reims G.S.M.A. Laboratory (France). In order to check the preliminary results obtained by this apparatus, a comparison between spectra obtained with T.D.L.S. and a Laser Heterodyne Spectrometer (L.H.S.) has been made. The study of a spectral ozone line by these two high resolution techniques is presented and the results are compared and discussed.

Heterodyne spectrometer for the 10 micron region

We first consider the interest of infrared heterodyne spectroscopy to study the vertical distribution of atmospheric constituents such O3. Then an heterodyne spectrometer is described. It operates in the 10 micron region with a CO2 laser and gives a resolution of 5 MHz. A laboratory spectrum of C2H4 is presented as a preliminary result.

High quality infrared (8 μm) diode laser source design for high resolution spectroscopy with precise temperature and current control

Abstract The performance of a tunable diode laser (DL) spectrometer is directly determined by the DL qualities and its tunability control. Emission characteristics are very sensitive to current and temperature drifts. In order to obtain a high quality source for spectroscopy, we designed our own liquid nitrogen cryostat containing the DL. Moreover we compared current and temperature control and we found that temperature tuning can be more efficient. We check and demonstrate it by recording SO 2 spectra in the 1168–1169 cm –1 spectral region.

Dual-beam laser heterodyne spectrometer: Ethylene absorption spectrum in the 10 μm range

A laser heterodyne spectrometer for the 10 μm region has been built. A 5 MHz apparatus function is obtained. The improvement of this spectrometer using a dual-beam technique is described. The folding effects in heterodyne spectroscopy are discussed. The recorded lineshapes are explained. An ethylene spectrum is shown as an example.