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Gas correlation lidar

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Publication number
WO1986001295A1
WO1986001295A1 PCT/SE1985/000305 SE8500305W WO8601295A1 WO 1986001295 A1 WO1986001295 A1 WO 1986001295A1 SE 8500305 W SE8500305 W SE 8500305W WO 8601295 A1 WO8601295 A1 WO 8601295A1
Authority
WO
Grant status
Application
Patent type
Prior art keywords
laser
gas
beam
correlation
lidar
Prior art date
Application number
PCT/SE1985/000305
Other languages
French (fr)
Inventor
Hans Georg Edner
Sune Roland Svanberg
Leif Peter UNÉUS
Erik Wilhelm Wendt
Original Assignee
Boliden Aktiebolag
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date

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Classifications

    • GPHYSICS
    • G01MEASURING; TESTING
    • G01SRADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S17/00Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
    • G01S17/88Lidar systems specially adapted for specific applications
    • G01S17/95Lidar systems specially adapted for specific applications for meteorological use
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using infra-red, visible or ultra-violet light
    • G01N21/17Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infra-red light
    • G01N21/3504Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infra-red light for analysing gases, e.g. multi-gas analysis
    • G01N21/3518Devices using gas filter correlation techniques; Devices using gas pressure modulation techniques
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using infra-red, visible or ultra-violet light
    • G01N21/17Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/39Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
    • Y02A90/19

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 Q0 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) andQ0 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 Hgl 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*!. χne 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 C02 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

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.
PCT/SE1985/000305 1984-08-10 1985-08-08 Gas correlation lidar WO1986001295A1 (en)

Priority Applications (2)

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SE8404064 1984-08-10
SE8404064-1 1984-08-10

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Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2264169A (en) * 1992-02-07 1993-08-18 Alan John Hayes Collinear-beam drift-compensated methane detector
DE4300853A1 (en) * 1993-01-15 1994-07-21 Daimler Benz Ag Spectroscopic determination of nitrogen oxide content in measurement chamber
DE4324154A1 (en) * 1993-07-19 1995-02-02 Kayser Threde Gmbh Device and method for analysis, with high spatial resolution, of at least one gas component in a gas mixture
EP1793220A1 (en) * 2005-12-01 2007-06-06 Pergam-Suisse Ag Mobile remote detection of fluids by a laser
US7411196B2 (en) * 2005-08-18 2008-08-12 Itt Manufacturing Enterprises, Inc. Multi-sensors and differential absorption LIDAR data fusion
FR2916849A1 (en) * 2007-05-29 2008-12-05 Univ Claude Bernard Lyon I Eta Method of remote sensing optical compounds in a medium
US7884937B2 (en) * 2007-04-19 2011-02-08 Science & Engineering Services, Inc. Airborne tunable mid-IR laser gas-correlation sensor
CN102353650A (en) * 2011-07-06 2012-02-15 南京信息工程大学 Method and system for detecting liquid explosive based on laser radar technology
CN103293116A (en) * 2013-05-03 2013-09-11 中国科学院合肥物质科学研究院 Automatic continuous detection device of micro-pulse differential absorption lidar water vapor spatial and temporal distribution
CN103575675A (en) * 2013-10-30 2014-02-12 中国科学院安徽光学精密机械研究所 Onboard multi-angle region pollution distribution scanning detection device

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1205985B1 (en) 2000-02-09 2014-08-13 NGK Insulators, Ltd. Lithium secondary cell

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4126396A (en) * 1975-05-16 1978-11-21 Erwin Sick Gesellschaft Mit Beschrankter Haftung, Optik-Elektronic Device for the non-dispersive optical determination of the concentration of gas and smoke components
DE3007236A1 (en) * 1980-02-27 1981-09-10 Messerschmitt Boelkow Blohm Atmospheric emission supervision - by laser beam with intermediate reflectors preceding terminal reflector
EP0102282A2 (en) * 1982-08-03 1984-03-07 Office National d'Etudes et de Recherches Aérospatiales (O.N.E.R.A.) Method and device for measuring small amounts of gaseous components
DE3334264A1 (en) * 1982-09-25 1984-04-05 Showa Denko Kk Method and meter for measuring the concentration of methane in a gas mixture

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4126396A (en) * 1975-05-16 1978-11-21 Erwin Sick Gesellschaft Mit Beschrankter Haftung, Optik-Elektronic Device for the non-dispersive optical determination of the concentration of gas and smoke components
DE3007236A1 (en) * 1980-02-27 1981-09-10 Messerschmitt Boelkow Blohm Atmospheric emission supervision - by laser beam with intermediate reflectors preceding terminal reflector
EP0102282A2 (en) * 1982-08-03 1984-03-07 Office National d'Etudes et de Recherches Aérospatiales (O.N.E.R.A.) Method and device for measuring small amounts of gaseous components
DE3334264A1 (en) * 1982-09-25 1984-04-05 Showa Denko Kk Method and meter for measuring the concentration of methane in a gas mixture

Cited By (17)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2264169A (en) * 1992-02-07 1993-08-18 Alan John Hayes Collinear-beam drift-compensated methane detector
GB2264169B (en) * 1992-02-07 1995-08-02 Alan John Hayes Fluid monitoring
DE4300853A1 (en) * 1993-01-15 1994-07-21 Daimler Benz Ag Spectroscopic determination of nitrogen oxide content in measurement chamber
DE4300853C2 (en) * 1993-01-15 2003-09-04 Daimler Chrysler Ag A method for the spectroscopic determination of the nitrogen oxide content
DE4324154A1 (en) * 1993-07-19 1995-02-02 Kayser Threde Gmbh Device and method for analysis, with high spatial resolution, of at least one gas component in a gas mixture
US7411196B2 (en) * 2005-08-18 2008-08-12 Itt Manufacturing Enterprises, Inc. Multi-sensors and differential absorption LIDAR data fusion
WO2007062810A1 (en) * 2005-12-01 2007-06-07 Pergam-Suisse Ag Mobile remote detection of fluids by a laser
EP1793220A1 (en) * 2005-12-01 2007-06-06 Pergam-Suisse Ag Mobile remote detection of fluids by a laser
US7884937B2 (en) * 2007-04-19 2011-02-08 Science & Engineering Services, Inc. Airborne tunable mid-IR laser gas-correlation sensor
US7965391B2 (en) * 2007-04-19 2011-06-21 Science & Engineering Services, Inc. Airborne tunable mid-IR laser gas-correlation sensor
FR2916849A1 (en) * 2007-05-29 2008-12-05 Univ Claude Bernard Lyon I Eta Method of remote sensing optical compounds in a medium
WO2008152286A3 (en) * 2007-05-29 2009-02-19 Centre Nat Rech Scient Method of optical teledetection of compounds in a medium
WO2008152286A2 (en) * 2007-05-29 2008-12-18 Universite Claude Bernard Lyon I Method of optical teledetection of compounds in a medium
US8514378B2 (en) 2007-05-29 2013-08-20 Universite Claude Bernard Lyon I Method of optical teledetection of compounds in a medium
CN102353650A (en) * 2011-07-06 2012-02-15 南京信息工程大学 Method and system for detecting liquid explosive based on laser radar technology
CN103293116A (en) * 2013-05-03 2013-09-11 中国科学院合肥物质科学研究院 Automatic continuous detection device of micro-pulse differential absorption lidar water vapor spatial and temporal distribution
CN103575675A (en) * 2013-10-30 2014-02-12 中国科学院安徽光学精密机械研究所 Onboard multi-angle region pollution distribution scanning detection device

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