Lilian Joly

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.

Laser diode absorption spectroscopy for accurate CO 2 line parameters at 2 μm: consequences for space-based DIAL measurements and potential biases

Space-based active sensing of CO(2) concentration is a very promising technique for the derivation of CO(2) surface fluxes. There is a need for accurate spectroscopic parameters to enable accurate space-based measurements to address global climatic issues. New spectroscopic measurements using laser diode absorption spectroscopy are presented for the preselected R30 CO(2) absorption line ((20(0)1)(III)<--(000) band) and four others. The line strength, air-broadening halfwidth, and its temperature dependence have been investigated. The results exhibit significant improvement for the R30 CO(2) absorption line: 0.4% on the line strength, 0.15% on the air-broadening coefficient, and 0.45% on its temperature dependence. Analysis of potential biases of space-based DIAL CO(2) mixing ratio measurements associated to spectroscopic parameter uncertainties are presented.

Photoacoustic spectroscopy for trace gas detection with cryogenic and room-temperature continuous-wave quantum cascade lasers

The main characteristics that a sensor must possess for trace gas detection and pollution monitoring are high sensitivity, high selectivity and the capability to perform in situ measurements. The photacoustic Helmholtz sensor developed in Reims, used in conjunction with powerful Quantum Cascade Lasers (QCLs), fulfils all these requirements. The best cell response is # 1200 V W−1 cm and the corresponding ultimate sensitivity is j 3.3 × 10−10 W cm−11 Hz−11/2. This efficient sensor is used with mid-infrared QCLs from Alpes Lasers to reach the strong fundamental absorption bands of some atmospheric gases. A first cryogenic QCL emitting at 7.9 μm demonstrates the detection of methane in air with a detection limit of 3 ppb. A detection limit of 20 ppb of NO in air is demonstrated using another cryogenic QCL emitting in the 5.4 μm region. Real in-situ measurements can be achieved only with room-temperature QCLs. A room-temperature QCL emitting in the 7.9 μm region demonstrates the simultaneous detection of methane and nitrous oxide in air (17 and 7 ppb detection limit, respectively). All these reliable measurements allow the estimated detection limit for various atmospheric gases using quantum cascade lasers to be obtained. Each gas absorbing in the infrared may be detected at a detection limit in the ppb or low-ppb range.

Diode laser spectroscopy of two acetylene isotopologues (12C2H2, 13C12CH2) in the 1.533 μm region for the PHOBOS-Grunt space mission

Several line intensities of the nu(1) + nu(3)(Sigma(u)(+)) - 0(Sigma(g)(+)) bands of (12)C(2)H(2) and (13)C(12)CH(2) at 1.533 microm have been revised at room temperature. These molecular transitions were selected to measure acetylene within the framework of the Martian space mission PHOBOS-Grunt. In the spectral region ranging from 6518 to 6530 cm(-1), 10 lines of both isotopologues have been analyzed using a high resolution tunable diode laser spectrometer. These transitions are well appropriate to the monitoring of C(2)H(2) by laser absorption spectroscopy with standard telecommunication laser diodes. Both the Voigt and the Rautian models are used to fit the molecular line shape and to provide accurate line strengths. Our data are thoroughly compared to existing database (including HITRAN08) and former experimental measurements.

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.

Characterization of the stray light in a space borne atmospheric AOTF spectrometer

Acousto-optic tunable filter (AOTF) spectrometers are being criticized for spectral leakage, distant side lobes of their spectral response function (SRF), or the stray light. SPICAM-IR is the AOTF spectrometer in the range of 1000-1700 nm with a resolving power of 1800-2200 operating on the Mars Express interplanetary probe. It is primarily dedicated to measurements of water vapor in the Martian atmosphere. SPICAM H(2)O retrievals are generally lower than simultaneous measurements with other instruments, the stray light suggested as a likely explanation. We report the results of laboratory measurements of water vapor in quantity characteristic for the Mars atmosphere (2-15 precipitable microns) with the Flight Spare model of SPICAM-IR. We simulated the measured spectra with HITRAN-based synthetic model, varying the water abundance, and the level of the stray light, and compared the results to the known amount of water in the cell. The retrieved level of the stray light, assumed uniformly spread over the spectral range, is below 1-1.3·10(-4). The stray may be responsible for the underestimation of water abundance of up to 8%, or 0.6 pr. µm. The account for the stray light removes the bias completely; the overall accuracy to measure water vapor is ~0.2 pr. µm. We demonstrate that the AOTF spectrometer dependably measures the water abundance and can be employed as an atmospheric spectrometer.

Alternative method for gas detection using pulsed quantum-cascade-laser spectrometers

Pulsed quantum-cascade-laser (QCL) spectrometers are usually used to detect atmospheric gases with either the interpulse technique (short pulses, typically 5-20 ns) or the intrapulse technique (long pulses, typically 500-800 ns). Each of these techniques has many drawbacks, which we present. Particularly the gas absorption spectra are generally distorted. We demonstrate the possibility to use intermediate pulses (typically 50-100 ns) for gas detection using pulsed QCL spectrometers. IR spectra of ammonia recorded in the 10 microm region are presented in various conditions of pulse emission. These experiences demonstrate the large influence of the pulse shape on the recorded spectrum and the importance to use our alternative method for gas detection with pulsed QCL spectrometers.

Continuous-wave quantum cascade lasers absorption spectrometers for trace gas detection in the atmosphere

Mid infra-red absorption spectrometry based on continuous-wave distributed feedback (DFB) quantum cascade laser (QCL) is more and more widely used for trace gas detection and pollution monitoring. The main advantages of this technique are high sensitivity, high selectivity and a potential for extreme compactness. Various examples of trace gas detection for atmospheric detection will be presented in this paper. Commercial QCLs available on the shelves were first implemented. A cryogenic QCL emitting at 6.7 μm was used to demonstrate the detection of water vapor and its isotopes. A room-temperature QCL was then used to simultaneously detect methane and nitrous oxide at 7.9 μm. Recently, we have developed a room-temperature top grating DFB QCL designed around 4.5 μm for the demonstration of N2O detection in the ppb range. Atmospheric applications of these spectrometers will be presented. The improvements of QCL performances make it now possible to develop instruments that are more and more compact and therefore compatible with in situ applications.

Inter-comparison of 2 μm Heterodyne Differential Absorption Lidar, Laser Diode Spectrometer, LICOR NDIR analyzer and flasks measurements of near-ground atmospheric CO2 mixing ratio

Remote sensing and in situ instruments are presented and compared in the same location for accurate CO(2) mixing ratio measurements in the atmosphere: (1) a 2.064 microm Heterodyne DIfferential Absorption Lidar (HDIAL), (2) a field deployable infrared Laser Diode Spectrometer (LDS) using new commercial diode laser technology at 2.68 microm, (3) LICOR NDIR analyzer and (4) flasks. LDS, LICOR and flasks measurements were made in the same location, LICOR and flasks being taken as reference. Horizontal HDIAL measurements of CO(2) absorption using aerosol backscatter signal are reported. Using new spectroscopic data in the 2 microm band and meteorological sensor measurements, a mean CO(2) mixing ratio is inferred by the HDIAL in a 1 km long path above the 15m height location of the CO(2) in situ sensors. We compare HDIAL and LDS measurements with the LICOR data for 30 min of time averaging. The mean standard deviation of the HDIAL and the LDS CO(2) mixing ratio results are 3.3 ppm and 0.89 ppm, respectively. The bias of the HDIAL and the LDS measurements are -0.54 ppm and -0.99 ppm, respectively.

Self-induced pressure shift and temperature dependence measurements of CO2 at 2.05μm with a tunable diode laser spectrometer.

By using a high resolution tunable diode laser absorption spectrometer combined with a cryogenically cooled optical multi-pass cell, we have measured the self-induced pressure shift coefficients for 8 transitions in the R branch of the (20(0)1)(III)←(00(0)0)(I) band of carbon dioxide around 2.05μm. This spectral region is of particular interest for the monitoring of atmospheric CO(2) with Differential Absorption Lidars (DiAL). The measurement of these shift coefficients was realized at five different temperatures ranging from 218 to 292K in order to determine their temperature dependence. The results are thoroughly compared to previous values reported in the literature for the (20(0)1)(III)←(00(0)0)(I) band of CO(2). The temperature dependence of the self-induced pressure shifts are reported experimentally for the first time for this specific CO(2) band.