We propose a new approach to range-resolved remote gas sensing in the atmosphere based on a combination of a DIAL and tunable-laser diode spectroscopy (TDLS) methods. To add range-resolving capabilities to a TDLS sensor we propose to arrange a group of retroreflectors (RRs) dividing an absorption path into adjacent measurement sections similar to those utilized by conventional DIAL systems. We implemented two techniques for the interrogation of the RRs: 1) scanning a beam of a continuous-wave laser over RRs sequentially; 2) using a time delay between returns from different RRs illuminated with a pulsed laser. We employed scanning technique with a vertical-cavity surface-emitting laser (VCSEL) operating near 1389 nm. A single-pulse interrogation method was demonstrated with a 10.9-&mgr;m quantum cascade laser (QCL) suitable for detection of ammonia, ethylene and water vapor in the atmosphere. Gas sensing and ranging was performed over distances varying from ~ 1 m up to ~ 1 km. Using VCSEL we attained a 0.5-s time resolution in gas concentration profiling with a 10-cm spatial resolution. Minimum interrogation time of a group of RRs was ~ 9 ms. A new generation of differential absorption LIDARs can be developed for range-resolved gas sensing in the atmosphere over distances up to ~ 1 km. The instruments can be used for a variety of applications ranging from fencing industrial areas to monitor fluxes of atmospheric pollutants to continuous air quality control in populated areas
We present a novel concept design of a differential absorption LIDAR for open path trace gas sensing in the atmosphere. To perform a range-resolved gas sensing we propose to arrange a set of retroreflectors in the laser beam path to measure a differential absorption in adjacent sections. In validation experiments we used a pulsed DFB quantum cascade laser fabricated by Alpes Lasers. The laser was excited with 200-ns current pulses with a repetition rate of 10 kHz. The frequency chirp rate was found to increase from 7.7 to 1.0 cm-1/μs as peak injection current was increased from 7.1, to 7.8 A. We utilized the frequency chirp at laser substrate temperature of 24.0 °C to scan the 967.0 - 968.5 cm-1 spectral interval containing the absorption lines of CO2 and NH3. We detected ~ 0.25 ppmv of NH3 in nitrogen at atmospheric pressure using a double-pass gas cell with an effective absorption path of 2.4 m. Digital filtering of the spectra was shown to be effective in eliminating a high-frequency noise. To demonstrate range-resolved capabilities of the sensor we used two retroreflectors inserted into the laser beam. A differential absorption of CO2 at 967.7 cm-1 was measured with the gas cell placed in one of the sections. Our experiments indicate that the frequency chirped LIDAR can be used for open path spectroscopy of NH3 over the ranges up to ~ 1 km with a spatial resolution of ~ 30 m and detection limit of ~ 20 ppbv per a 30-m section.
We have approximated with second-order polynomials the frequency tuning curves of long-wavelength single-mode VCSELs operating near 1654 and 1512 nm. Fitting coefficients were calculated using experimental data on injection currents and heat sink temperatures required to tune lasers to frequency markers generated by gas absorption lines. To measure temperature tuning rates, we tuned the lasers by temperature sequentially to pairs of absorption lines with
known frequency separations. To determine fitting coefficients associated with linear and non-linear frequency tuning, we varied the laser injection currents and temperatures simultaneously in such a way that made a laser emit exactly the same frequency. Linear and non-linear tuning coefficients were then calculated from the data on effects of relatively small and large variation of laser operation parameters on laser frequency. Lasers were calibrated by tuning them on narrow absorption lines with frequencies accurately known from previous studies. The simulated tuning curves were demonstrated to fit frequency markers generated over spectral intervals up to 40 cm-1 with an accuracy of ± 0.10 cm-1(1654-nm laser) and ± 0.15 cm-1 (1512-nm laser). A temperature dependence of injection current tuning rates of the 1654-nm laser was determined from the best fits of simulated tuning curves to a series of CO2 absorption lines in the whole operation temperature range of the laser (0 - 50 °C). A simple and accurate method developed to describe tuning properties of long-wavelength VCSELs can be applied to quantitatively characterize any narrow-linewidth tunable laser.
A method of gas phase chemical analysis using direct absorption spectroscopy is implemented with long-wavelength vertical-cavity surface-emitting lasers (VCSELs) operating near 1577 nm. The method is based on a linear dependence of widths of collisionally broadened absorption lines on gas pressure. It is shown that the absolute gas concentrations in multicomponent gas mixtures can be extracted from the line widths of all compounds measured simultaneously. The concentrations of both absorbing and nonabsorbing compounds are extracted from the results of simultaneous measurements of peak absorption and line width of the absorbing compound. Long-wavelength VCSELs with a buried tunnel junction (Vertilas, Germany) are used, for the first time, for multispecies and trace gas detection. Continuous single-mode tuning of the VCSELs up to 30 cm–1 is achieved with temperature and injection current varied in the range 0 to 50 °C and 1.3 to 6.5 mA, respectively. A fractional absorption of ~10–4 (600 ppm of CO2 in air) is measured with a single-beam spectroscope. The method described can be used for laser chemical analysis of gas mixtures with relatively high concentrations of target compounds and for open-path trace gas sensing. Compact sensors based on long-wavelength VCSELs can be developed for environmental and industrial gas monitoring.
A pulsed distributed feedback quantum cascade laser operating near 970 cm-1 (10.3 μm) was coupled with the technique of cavity ring-down spectroscopy, as described here for the first time. The newly constructed set-up was tested by recording three relatively weak rotational lines of the 1000→0001 vibrational band of CO2 in the range from 966.75 cm-1 to 971.5 cm-1. The CO2 lines were recorded by measuring the decay time of a CO2 - N2 mixture flowing through an open sample tube placed between the cavity ring-down mirrors. The quantum cascade laser frequency was tuned at a rate of ~ 0.071 cm-1/K by changing the heat sink temperature in the range between -20 and 50 °C. The first results demonstrated the applicability and high sensitivity of the cavity ring-down spectroscopy - pulsed quantum cascade laser combination and encouraged us to extend our research to the study and detection of ammonia. We demonstrated that a detection limit of ammonia of ~ 25 ppbv can be attained with the current set-up. Basic instrument performance and optimization of the experimental parameters for sensitivity improvement are discussed.
We used direct absorption spectroscopy for characterization of long-wavelength VCSELs (VERTILAS, Germany). Gas mixture CO:CO2=1:1 (total pressure 0.1 - 0.5 bar) provided in the tuning ranges of lasers a multitude of absorption lines spaced by intervals from 1 to 60 GHz that allowed the absolute calibration of laser scans. We measured tuning rates of the VCSELs in 0-50 °C temperature interval in dependence on injection current and modulation frequency varied from 0 up to 6.5 mA and 500 kHz respectively. The continuous tuning range of the 1577.5-nm VCSEL with single-mode output of up to 1.5 mW was found to be of 9.61 nm (38.62 cm-1). The temperature tuning rates for all VCSELs under study were in the range between -0.4 and -0.5 cm-1/ °C. The current tuning rates, varied from laser to laser between -1.9 and -2.2 cm-1/mA at low injection currents and modulation frequencies, were found to be twice as big for all lasers at high injection currents and decreased by factor 2 and 5 at modulation frequencies of 300 and 500 kHz respectively. A phase shift between the amplitude and frequency modulations was measured by fine tuning VCSEL with a DC injection current to display the end of a laser scan in respect to a local maximum in laser power. The phase shift changed from 0 to -0.3 rad with modulation frequency raised from 500 Hz to 500 kHz. The results of phase shift measurements have been compared with those obtained recently for DFB lasers with different methods.
A long-wavelength VCSEL has been used for the first time for multi-species gas detection and for trace gas sensing. The VCSEL with a buried tunnel junction (VERTILAS, Germany) was capable of covering a spectral range of 7 nm or 28 cm-1 (central wavelength at 1576.3 nm) with the laser temperature and injection current varied between 0-50 °C and 0.5-5 mA respectively. The pressure of CO:CO2=3:2 gas mixtures buffered with N2 (N2 content 0 - 90 %) was varied from 1 mBar up to 1 Bar. To avoid a non-linear dynamic tuning, the combination of a direct injection current with a saw-tooth waveform was used to sweep the laser frequency across absorption lines. A LabVIEW-based computer code was developed for multi-species gas analysis in time domain. Absorption spectra were averaged over 102-103 laser scans. It has been shown that a cross interference from all collisional partners should be taken into account for accurate multi-component gas detection. A concentration of 600 ppm of CO2 in atmospheric air (fractional absorption ~ 10-4) was detected with laser output power of 120 uW. Long-wavelength VCSELs can be used both for multi-species gas detection in a wide range of pressures and for trace gas monitoring.
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