Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO 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
  • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
  • G01S17/88—Lidar systems specially adapted for specific applications
  • G01S17/95—Lidar systems specially adapted for specific applications for meteorological use
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
  • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
  • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
  • G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
  • G01N21/3518—Devices using gas filter correlation techniques; Devices using gas pressure modulation techniques
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
  • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
  • G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
  • Y02A90/00—Technologies having an indirect contribution to adaptation to climate change
  • Y02A90/10—Information and communication technologies [ICT] supporting adaptation to climate change, e.g. for weather forecasting or climate simulation

Definitions

• 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.
• 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.
• 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.
• 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 pp ⁇ rm atmospheric gas burden.
• the gas correlation concept can readily be applied to the lidar configuration leading to important system simplifications and improvements in signal-to-noise ratio.
• 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.
• 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.
• the whole spectral distribution is measured, which for the case of no atmospheric mercury is the same as the transmitted spectral distribution.
• 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.
• 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.
• a deviation from 1 is obtained in the presence of external Hg.
• 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.
• 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.
• 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.
• 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.
• 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
• a laser simultaneously emitting two close-lying wavelengths could be used together with gas filter techniques as described above.
• 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.

Landscapes

• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Health & Medical Sciences (AREA)
• Analytical Chemistry (AREA)
• Remote Sensing (AREA)
• Electromagnetism (AREA)
• Computer Networks & Wireless Communication (AREA)
• Life Sciences & Earth Sciences (AREA)
• Chemical & Material Sciences (AREA)
• Radar, Positioning & Navigation (AREA)
• Biochemistry (AREA)
• General Health & Medical Sciences (AREA)
• Immunology (AREA)
• Pathology (AREA)
• Investigating Or Analysing Materials By Optical Means (AREA)
• Spectrometry And Color Measurement (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=20356697

Family Applications (1)

Country Status (4)

Cited By (3)

Families Citing this family (15)

Family Cites Families (5)

• 1984
  • 1984-08-10 SE SE8404064A patent/SE450913B/sv unknown
• 1985
  • 1985-08-08 EP EP85904024A patent/EP0190280A1/en not_active Withdrawn
  • 1985-08-08 WO PCT/SE1985/000305 patent/WO1986001295A1/en unknown
  • 1985-08-08 AU AU47229/85A patent/AU4722985A/en not_active Abandoned

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events