GAS CORRELATION LIDAR

Abstract: A method for controlling and sensing in order to monitor atmospheric gases using a lidar technique whereby a laser beam is emitted at a fixed wavelength, whereby a reflected laser pulse is optionally passed through an interference filter for isolating a desired spectral region, whereafter the laser beam pulse is passed through a beam splitter and two laser beam detectors, whereby one of the beams is passed through a gas correlation cell comprising the compound to be determined in order to block out the central part of a resonance line whereby the off resonance signal is determined and recorded, and whereby the second beam is used for measuring the whole spectral distribution, any inbalance in the two determinations made indicates the presence of an external gas to be detected.

Description: GAS CORRELATION LIDAR DESCRIPTION The ^differential absorption |idar (dial) technique* " ^ is a powerful and widely used remote-sensing method for monitoring of atmospheric gases, e.g. air pollutants. The object of the present invention is to obtain an improved and simplified lidar technique for remote control and sensing for monitoring of atmospheric gases using a combination of lidar and gas filter correlation techniques^. Basic operational considerations are given below and preliminary remote-sensing experiments on mer¬ cury are described. In normal dial experiments, pulsed laser radiation is transmitted into the atmosphere at two alternate wavelengths, one on an absorption line of the species of interest and one off the absorption line but still close in wavelength (reference wavelength). The range-dependent backscattering, which is mainly due to Mie scattering from particles, is recorded with an optical telescope equipped by a detector and time- resolving electronics. Atmospheric turbulence, which has a correlation time * of less than 10 ms will largely determine dial performance. By using a pulsed laser of a repetition rate of 10 Hz and switching between the two wavelengths between all pulses (See e.g. Ref. 7), large-scale inhomogeneities are cancelled out in normal ground-based applications but the signal-to-noise ratio is clearly impaired by turbu¬ lence, requiring some additional signal averaging for- such a relatively simple but still very useful system. By using two separate, individually tuned laser systems fired at an interval of some tens of microseconds, "frozen" atmospheric conditions are achieved and high-quality data are obtained for this rather complex system, which can still operate with a single detection system. (Both wavelengths pass the same narrow-band filter; the two lidar returns are captured on a single transient digitizer sweep. See e.g. Refs. 8,6.) For a monitoring system on a fastmoving platform a dual laser approach has been nedessary. In non-laser (passive) long-path optical absorption monitoring the effects of atmos¬ pheric turbulence can be eliminated by fast scanning such as in doas differential £ptical absorption spectroscopy^, dispersive correlation spectroscopy^ and gas filter correlation spectroscopy4>5. Simultaneous "on/off" monitoring can also be achieved using optical multichannel (array) techniques or systems with beam-splitters. Gas correlation spectroscopy is a particularly simple and powerful technique, where the incoming light is passed either directly to a detector or first passing through a cell containing an optically thick sample of the gas to be studied. For the case of an atmospheric path free from the gas, the light intensities in a selected wavelength region are balanced out using lock-in or electrical bridge techniques. With the gas present in the atmosphere the light passing through the gas cell is still the same, whereas the additional absorption in the direct beam results in an inbalance in the electronics, which after calibration can be directly expressed as a ppπrm atmospheric gas burden. Disclosure of the present invention The gas correlation concept can readily be applied to the lidar configuration leading to important system simplifications and improvements in signal-to-noise ratio. In particular, only one fairly broadband laser is needed and no laser tuning is necessary between pulses. On- and off resonance wavelengths are transmitted and detected simultaneously. In order to describe the gas correlation lidar technique we chose a simple model example. We will consider the case of atmospheric (atomic) mercury monitoring, for which we have recently reported ordinary dial measurements 0. The description will follow with reference to Fig. 1. The laser is tuned to the 2537 A Hg resonance line.- (A pulsed frequency-doubled dye laser could be used.) The region of Hg absorption (considering isotope shifts, hyperfine structure, Doppler and pressure-broadening) is about 0.05 A. The laser band-width is chosen to be about three times this value. If a short pulse (few ns) is used, no pronounced mode structure will be obtained and a smooth spectral distribu¬ tion for the pulse is assumed for simplicity as indicated in the figure. The laser pulse is transmitted into the atmosphere through a Hg cloud at some distance from the lidar system and is finally hitting a topographic target or a retroreflector. Back- scattered light is received by an optical telescope and overlap between the trans¬ mission and detection lobes is obtained after some distance from the system. For a homogeneous atmosphere, a 1/R^ fall-off of the recorded intensity is then received. With an interference filter the interesting spectral region is isolated for background light suppression. In contrast to the normal dial system, a beam splitter and two detectors are now used instead of one. One of the beams passes a gas filter correlation cell - in the chosen example containing Hg of sufficient vapor pressure - to block out the central part of the resonance line. In dial language, in this detection arm the off-resonance signal is recorded (actually, preferably two close-lying reference wavelengths are used simultaneously). In the other detection arm the whole spectral distribution is measured, which for the case of no atmospheric mercury is the same as the transmitted spectral distribution. For this case the detected signals in the two arms can be made equal (balanced out as in passive gas correlation) by beam attenuation or gain adjustments. If external Hg is present less signal is detected in this arm whereas the signal in the gas cell arm is unaffected. The inbalance between the two arms indicates the presence of the external gas. In the figure spectral and temporal curves at different points in the system are shown illustrating the measurement process. In particular, spectral distributions could be considered for the final target echoes. By dividing the signals as illustrated in the figure - a procedure which is also common in dial? - a deviation from 1 is obtained in the presence of external Hg. Note that the ratio (R) is independent of the laser pulse energy, turbulence effects etc, since the measurements are performed simultaneously on the same pulse, this is true for the signals recorded range resolved at any one delay. For the fast moving platforms this is a great advantage. Note, that the percentage deviation from 1 in the divided signal is the same as the one that would have been obtained in a dial measurement where the laser would be used once tuned on the absorption line and once tuned completely off the line. Since a linewidth larger than the absorption linewidth is used the relevant absorption- cross-sections are dependent on the actual laser linewidth, and an optical depth dependence (deviation from the Beer-Lambert law) also persists. Thus, a gas correlation lidar system is best calibrated by inserting cells with known ppm-m numbers in the light path between the telescope and the detector arrangement in direct connection with the actual measurements. For practical laser the spectral distribution within the laser bandwidth will vary from pulse to pulse and this fact will result in a strongly increased noise level, since the two detection arms can no longer be balanced out. However, it is possible to monitor the relevant spectral fluctuations of the laser by detecting the ratio Q 0 of the intensity of the laser beam for a direct path to a detector and when passing an identical gas correlation cell. No special arrangement is needed for this. The prompt signals due to light scattering in the telescope can be adjusted to a proper level and can be isolated from an atmospheric backscattering background by an initial separation of the transmitted laser beam from the telescope optical axis. The signals are recorded together with the atmospheric returns as indicated in Fig. 1. If a low external gas concentration can be assumed close to the telescope and a laser power yielding a sufficient atmospheric backscatter as in the figure is used, the 0 value can also be obtained from the close-range backscattering. It can easily be shown that exp(-2roύ n(r)dr) « 1 ^ 1 <!> • where <=£ is the effective absorption coefficient in the used bandwidth of the studied species of concentration n(r). k is the ratio of the signals for wavelengths not absorbed by the gas correlation cell at the gas cell detector and direct pass detectors, respec¬ tively. If q(R) andQ 0 are recorded for every pulse the integrated concentration value is not affected by turbulence, laser spectral fluctuations etc, and a very noise-free measurement situation has been achieved. Practically, and from the point of view of the approximate nature of Equation (1), the system is most conveniently calibrated by inserting cells of known absorption in front of the beamsplitter. Problems with possible Fabry-Perot fringes are then also largely eliminated. In order to demonstrate the gas correlation lidar concept some preliminary experi¬ ments on mercury with an experimental set-up similar to the one in Fig. 1 were performed. An excimer-pumped dye laser, frequency-doubled to the 254 nm region, was used. The lidar set-up was similar to the one described in our previous paper on Hg l l The laser beam was directed through a remote 2 m long open-ended chamber (70 m distance), where a Hg-containing atmosphere could be obtained by introducing Hg droplets. After passing the chamber the beam was retro-reflected back to the lidar telescope, which had a diameter of 25 cm. The signals from the two detectors were recorded by a dual-channel boxcar integrator, that was gated to the reflector echo and interfaced to a minicomputer. The ratio Q was plotted, illustrating the remote detection of the introduction and removal of the Hg droplets. No attempt to calibrate the system was made in this demonstration, nor were the spectral fluctua¬ tions compensated for. Although the system concept description and the experiments were related to Hg it is evident that the same principle works for any gas, where close-lying wavelength regions with strong differential absorption exists. NO with a sharp band-head at about 226 nm is such a case, where normal dial measurements previously have been performed*!. χ ne laser is tuned right to the band-head generating on- and off- resonance wavelength components simultaneously. Also in the near IR region, which is accessible, e.g. by rather broadband Raman shifted dye lasers (see e.g. Ref. 12), or through optical difference frequency generation, measurements on CH4, CO, HC1 etc. should be feasible. For achieving an even more dial-like measurement situa¬ tion and for better laser control, a laser simultaneously emitting two close-lying wavelengths (see e.g. Ref. 13 and refs. therein) could be used together with gas filter techniques as described above. The sharp absorption features of gases allow a stable separation of signals at close- lying wavelengths without using sharp interference optics. It has recently been suggested to take advantage of these features in lidar systems detecting Mie and Rayliegh scattering separately for assessing atmospheric temperature!^, in the present paper the spectral correlation between an atmospheric gas constituent and the gas contained in a cell is instead used for pollution monitoring. It should be noted that whereas a perfect spectral match is achieved, an accidental coincidence by an interfering molecular absorption line would cause an error because of the narrow spectral region containing essentially only one line. Using a broadband laser (tens of A), such as the ones used in broad-band coherent anti-Stokes Raman spectro- scopy (See e.g. Ref. 15), true gas correlation^ with automatic rejection of interfering species should be achievable, e.g. in a NO2 lidar system. Further, the same concept should apply for properly selected wave-length regions of multi-line HF/DF and C0 2 TEA lasers. References 1. R.M. Schotland, in Proceedings of the Third Synmposium on Remote Sensing of the Environment (University of Michigan, Ann Arbor, 1964). 2. K.W. Rothe, U. Brinkmann and H. walther, Appl. Phys. _3, 116 (1974); 4, 181 (1975). 3. W.B. Grant, R.D. Hake, Jr., E.M. Liston, R.C. Robbins and E.K. Proctor, Jr., Appl. Phys. Lett. 24, 550 (1974). 4. T.V. Ward and H.H. Zwick, Appl. Opt. U, 2896 (1975). 5. J.H. Davies, A.R. Barringer and R. Dick, in D.A. Killinger and A. Mooradian (Eds). "Optical and laser remote sensing", Springer Series in Optical Sciences Vol. ^"9 (Springer- Verlag, Heidelberg 1983). 6. N. Menyuk and D.K. Killinger, Opt. Lett. , 301 (1981). 7. K. Fredriksson, B. Galle, K. Nystrδm and S. Svanberg, Appl. Opt. 2O, 4181 (1981). 8. E.V. Browell, in the book quoted in Ref. 5. 9. U. Platt and D. Perner, in the book quoted in Ref. 5. * 10. M. Alden, H. Edner and S. Svanberg, Opt. Lett. 7, 221 (1982). 11. M. Alden, H. Edner and S. Svanberg, Opt. Lett. 7, 543 (1982). 12. H. Edner, K. Fredriksson, A. Sunesson and S. Svanberg, Lund Reports on Atomic Physics LRAP-27 (1983). 13. M. Alden, K. Fredriksson and S. Wallin, Appl. Opt., to appear. 14. H. Shimizu, S.A. Lee and C.Y. She, Appl. Opt. £2, 1373 (1983). 5. M. Alden, H. Edner and S. Svanberg, Phys. Scr. 27, 29 (1983). Figure Caption Figure 1. Conceptual diagram of gas correlation lidar.

Claims: CLAIMS 1. A method for controlling and sensing in order to monitor atmospheric gases using a lidar technique whereby a laser beam is emitted at a fixed wavelength, characterized in that a reflected laser pulse is optionally passed through an interference filter for isolating a desired spectral region, whereafter the laser beam pulse is passed through a beam splitter and two laser beam detectors, whereby one of the beams is passed through a gas corrrelation cell comprising the compound to be determined in order to block out the central part of a resonance line whereby the off resonance signal is determined and recorded, and whereby the second beam is used for measuring the whole spectral distribution, any inbalance in the two determinations made indi¬ cates the presence of an external gas to be detected. 2. A method according to claim 1, characterized in that the unsplitted reflected laser beam is passed through a calibration cell prior to passing the beam splitter. 3. A method according to claim 1, characterized in that a broadband laser is used. 4. A method according to claim 1, characterized in that two laser beams having close-lying wavelengths are emitted and determined when reflected.

Inventors: EDNER HANS GEORG, SVANBERG SUNE ROLAND, UNEUS LEIF PETER, WENDT ERIK WILHELM, EDNER, HANS, GEORG, SVANBERG, SUNE, ROLAND, UNEUS, LEIF, PETER, WENDT, ERIK, WILHELM
Applicants: BOLIDEN AB, BOLIDEN AKTIEBOLAG
Application year: 1985
Main classification: G01N21
IPC codes: G01N21, G01S17