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Following the recent development of a ground-based prototype quantum cascade laser heterodyne radiometer operating in the midinfrared, atmospheric ozone profile retrievals from a solar occultation measurement campaign performed at the Rutherford Appleton Laboratory on 21 September 2006 are presented. Retrieval is based on the optimal estimation method. High resolution (0.0073 [cm.sup.-1]) atmospheric spectra recorded by the laser heterodyne radiometer and covering a microw-indow (1033.8-1034.5 [cm.sup.-1]) optimized for atmospheric ozone measurements were used as measurement vectors. As part of the evaluation of this novel instrument, a comprehensive analysis of the retrievals is presented, demonstrating the high potential of quantum cascade laser heterodyne radiometry for atmospheric sounding. Vertical resolutions of 2 km near the ground and about 3 km in the stratosphere were obtained. The information content of the retrieval was found to be up to 48 bits, which is much higher than any other passive ground-based instrument. Frequency mismatches of several absorption peaks between the forward model and experimental spectra have been observed and significantly contribute to the retrieval noise error in the upper-troposphere lower-stratosphere region. Retrieved ozone vertical profiles were compared to ozonesonde data recorded at similar latitudes. The agreement is generally excellent except for the 20 to 25 km peak in ozone concentration, where ozonesonde data were found to be 20% lower than the amount retrieved from the laser heterodyne radiometer spectra. Quantum cascade laser based heterodyne radiometry in the midinfrared has been demonstrated to provide high spectral resolution and unprecedented vertical resolution for a passive sounder in a highly compact and mechanically simple package. OCIS codes: 010.4950, 120.5630, 040.2840, 040.3060, 120.0280.

An experimental investigation of high-frequency noise, i.e., up to 3 GHz, exhibited by a 9.7 [micro]m quantum cascade laser, is described. Noise characteristics and measurements of a liquid-nitrogen-cooled continuouswave distributed-feedback laser are presented. Well defined sets of narrow and intense resonance peaks have been observed in the 10-300 MHz range. Measurements of relative intensity noise have been performed. It is also shown that quantum-cascade lasers are sensitive to optical feedback. The excess noise generated by the feedback has been investigated under well defined conditions. A description of the experimental phenomenon is presented along with methods of minimizing optical feedback. OCIS codes: 140.5960, 140.3600, 270.2500.

Simultaneous exhaled carbonyl sulfide (OCS) and carbon dioxide concentration measurements in human breath are demonstrated with a compact pulsed quantum-cascade laser-based gas sensor. We achieved a noise-equivalent sensitivity (1[sigma]) of 1.2 parts per billion by measuring a well-isolated OCS P(11) absorption line in the [v.sub.3] band at 2057.6 [cm.sup.-1] using an astigmatic Herriott cell of 36-m optical path length and 0.4-s acquisition time. OCIS codes: 300.1030, 280.3420.

We report on the development and performance of a gas sensor based on a quantum cascade laser operating at a wavelength of ~10 [micro]m to measure ethylene ([C.sub.2][H.sub.4]) concentrations by use of arotational component of the fundamental [v.sub.7] band. The laser is thermoelectrically cooled and operates in a pulsed mode. The influence of pulse-to-pulse fluctuations is minimized by use of a reference beam and a single detector with time discriminating electronics. Gas absorption is recorded in a 100-m optical path- length astigmatic Herriott cell. With a 10-kHz pulse repetition rate and an 80-s total acquisition time, a noise equivalent sensitivity of 30 parts per billion has been demonstrated. The sensor has been applied to monitor [C.sub.2][H.sub.4] in vehicle exhaust as well as in air collected in a high-traffic urban tunnel. OCIS codes: 280.3420, 280.1120, 140.5960, 300.6320.

We deal with the design of a diode laser heterodyne radiometer and its application in a combustion process. We present some experimental results obtained with a C[H.sub.4]-air premixed flat flame as the optical source. The goal is to prove that heterodyne detection techniques are relevant in remote detection and diagnostics of combustion and can have important applications in both civil and military fields. To the best of our knowledge, it is the first time that this demonstration is made. The radiometer, in spite of the low-power lead-salt diode laser used as a local oscillator, enables us to record high-temperature water-vapor emission spectra in the region of 1315 [cm.sup.-1]. OCIS codes: 300.2140, 300.6310, 300.6320, 300.6340, 280.2470, 280.3420.

This paper details infra-red pump and probe studies on nitric oxide conducted with two continuous wave quantum cascade lasers both operating around 5 μm. The pump laser prepares a velocity selected population in a chosen rotational quantum state of the v = 1 level which is subsequently probed using a second laser tuned to a rotational transition within the v = 2 ← v = 1 hot band. The rapid frequency scan of the probe (with respect to the molecular collision rate) in combination with the velocity selective pumping allows observation of marked rapid passage signatures in the transient absorption profiles from the polarized vibrationally excited sample. These coherent transient signals are influenced by the underlying hyperfine structure of the pump and probe transitions, the sample pressure, and the coherent properties of the lasers. Pulsed pump and probe studies show that the transient absorption signals decay within 1 μs at 50 mTorr total pressure, reflecting both the polarization and population dephasing times of the vibrationally excited sample. The experimental observations are supported by simulation based upon solving the optical Bloch equations for a two level system. [ABSTRACT FROM AUTHOR]

The Total Carbon Column Observing Network (TCCON) has been the baseline network of instruments that record solar absorption spectra from which accurate and precise column-averaged dry air mole fractions of CO2 (XCO2), CH4 (XCH4), CO (XCO) and other gases are retrieved. The TCCON data have been widely used for carbon cycle science and validation of satellites measuring greenhouse gas concentrations globally. The number of stations in the network (currently about 25) is limited and the stations are distributed mostly in Northern America, Europe, Japan and Oceania leaving gaps in the global coverage. A denser distribution of ground-based solar absorption measurements is needed to cover various atmospheric conditions (humid, dry, polluted, presence of aerosol), various surface conditions (high and low albedo) and a larger latitudinal distribution. More stations in the southern hemisphere are also needed but a further expansion of the network is limited by its costs and logistical requirements. For this reason several groups are investigating supplemental portable low-cost instruments. The European Space Agency (ESA) funded campaign Fiducial Reference Measurements for Ground-Based Infrared Greenhouse Gas Observations (FRM4GHG) at the Sodankylä TCCON site in northern Finland aims at characterising the assessment of several low-cost portable instruments for precise solar absorption measurements of XCO2, XCH4 and XCO. The test instruments under investigation are three Fourier transform spectrometers (FTS): a Bruker EM27/SUN, a Bruker IRcube and a Bruker Vertex70; as well as a Laser Heterodyne spectro-Radiometer (LHR) developed by the UK Rutherford Appleton Laboratory. All four remote sensing instruments performed measurements simultaneously next to the reference TCCON instrument, a Bruker IFS 125HR, for a full year in 2017. The TCCON FTS was operated in its normal high-resolution mode (TCCON data set) and in a special low-resolution mode (HR125LR data set), similar to the portable spectrometers. The remote sensing measurements have been complemented by regular AirCore launches performed from the same site. They provide in-situ vertical profiles of the target gas concentrations as auxiliary reference data for the column retrievals which is traceable to the WMO SI standards. The timeseries, the bias relative to the reference instrument and its scatter and the seasonal and the day-to-day variations of the target gases are shown and discussed. The comparisons with the HR125LR data set gave useful analysis of the resolution dependent effects on the target gas retrieval. The solar zenith angle dependence of the retrievals is shown and discussed. The reference measurements performed with the Bruker IFS 125HR (TCCON and HR125LR data sets) were found to be affected by non-linearity. A non-linearity correction of the TCCON data was performed and compared with the test instruments and AirCore. The non-linearity corrected TCCON data show a better match with the test instruments and AirCore data as compared to the reference TCCON data. The intercomparison results show that the LHR data have a large scatter and biases with a strong diurnal variation relative to the TCCON and other FTS instruments. The LHR is a new instrument under development and these biases are being currently investigated and addressed. The campaign helped to characterise and identify the instrumental biases and possibly retrieval biases which are currently under investigation. Further improvements of the instrument are ongoing. The EM27/SUN, the IRcube, the modified Vertex70 and the HR125LR provided stable and precise measurements of the target gases during the campaign with quantified small biases. The bias dependence on the humidity along the measurement line-of-sight has been investigated and no dependence was found. These three portable low-resolution FTS instruments are suitable to be used for campaign deployment or long-term measurements from any site and offer the ability to complement the TCCON and expand the global coverage of ground-based reference measurements of the target gases. [ABSTRACT FROM AUTHOR]