SK288772B6 - Method of rapid and reliable detection of gases using differential absorption LIDAR - Google Patents

Abstract

Method for fast and reliable gas detection by means of differential absorption LIDAR uses two or more lasers, of which at least one is always tuned to a wavelength λON strongly absorbed by the detected gas and at least one is tuned to a wavelength λOFF weakly absorbed by the detected gas, wherein all lasers successively emit one laser pulse and the first concentration of the detected substance is determined from the return energies. Subsequently, all lasers are retuned from the wavelengths λON to the wavelengths λOFF and vice versa. A second series of pulses is then sent and the second concentration is similarly determined from the results, the average of the first and second concentrations being taken as the result. Subsequently, the concentrations of the detected substance for each laser separately are determined from the measured return energies, the detection being considered positive only if the concentrations determined from both series for each of the lasers individually are positive and exceed a specified minimum value.

Description

Technical field

The invention relates to a method for the rapid and reliable detection of gases by measuring the concentrations of gaseous substances in the atmosphere by means of a differential absorption LIDAR with two or more lasers. The invention belongs to the laser technique.

Prior art

DIAL systems using one tunable laser determine the concentration of the delegated substance in the measuring path by first tuning the laser to the wavelength λοΝ, which is strongly absorbed by the substance, and sends a laser pulse to the target through the examined atmosphere. The laser radiation reflected from the target is captured by the optical receiver of the system and the voltage signal Uon is converted into it. Subsequently, the laser is tuned to the wavelength Zoff, which is weakly absorbed by the substance, the procedure is repeated and a voltage signal Uoff is obtained. The concentration of the measured gaseous substance is then determined from the Uon / Uoff size ratio. The main disadvantage of the DIAL system working in this way is the fact that laser radiation at the Zoff wavelength passes through the atmosphere with a considerable delay against radiation at the wavelength λ on (the delay is given by the maximum laser repetition frequency, which is usually small, ie rarely above 10 pulses per second) ; however, the optical properties of a turbulent atmosphere change faster, which causes large measurement errors with such a DIAL system.

DIAL systems that use two or more lasers eliminate this problem by tuning at least one laser to the wavelength λ on and at least one to the wavelength Zoff. Then the lasers start with a very small delay, i.e. the delay of the pulse from the laser tuned to the Zoff wavelength versus the pulse from the laser tuned to the Zon wavelength is much shorter (usually by several orders of magnitude) and the effect of turbulence does not occur. Otherwise, the function of the system is similar.

The concentration of the measured gaseous substance in the route can again be determined from the attenuation ratio at the Zon wavelength and at the Zoff wavelength (see, e.g., Killinger, DK et al .: Optical and Laser Remote Sensing. SpringerVerlag, Berlin 1983, 383 p .; Measures, RM: Laser Remote Sensing. John Wiley and Sons, New York, 1983, 510 s .; CO2 Lasers and Applications. Proc. Of SPIE, Vol. 1042, Los Angeles 1989, 139 p.). One complete measurement usually consists of a large number of individual measurements. Individual measurement is obtained by sending one pulse from each laser.

To calculate the concentration of the delegated substance in the measuring path, the ratio is used:

P = Uon / Uoff

With such a DIAL system, measurement errors can occur due to technical imperfections of the differential absorption LIDAR and errors caused by inhomogeneity of the measurement material.

These errors are suppressed by the solution described in Slovak patent no. 282 472 entitled "Method for determining the concentration of gases with differential absorption LIDAR-suppressing measurement errors". The solution according to this patent suppresses the mentioned measurement errors, while the "erroneous" measurement can be detected by statistical processing of the results. By "erroneous" measurement is meant a measurement that has been affected by accidental disturbances within the DIAL system or in the atmosphere - along the measuring path, for example by accidental shielding of one laser pulse by vegetation. As a result, "erroneous" will occur, ie. false measurement: the substance is not delegated even though it is present on the measurement path or vice versa. If at least four laser pulses from each laser are used in the use of the said patent, the statistical processing of the results of the individual measurements clearly reveals the "erroneous" measurement - the statistically calculated measurement error is greater than the measured concentration. The use of fewer pulses does not detect the "erroneous" measurement in the manner described.

In practice, however, it happens that even sending four pulses from each laser takes too long; either the user requires a faster response of the DIAL system to the situation in the atmosphere, or the time instability of the concentration of the delegated substance in the atmosphere cancels the measurement.

The essence of the invention

The above-mentioned drawbacks are largely eliminated by the method for fast and reliable detection of gases by means of differential absorption LIDAR-a according to the present invention, the essence of which consists in that the detection takes place in such a way that LIDAR-a contains at least two or more tunable lasers at least one is tuned to the wavelength Zon strongly absorbed by the detected gas and at least one is tuned to the wavelength Zoff weakly absorbed by the delegated gas All lasers successively emit one laser pulse through the test atmosphere with minimal possible delay and the first concentration of delegated substance is determined from the return energies.

S K 288772 B6

Subsequently, all lasei are retuned from Zon wavelengths to Loff wavelengths and vice versa. A second series of pulses is then sent and the results are similarly determined from the second concentration, the average of the first and second concentrations being taken as the result.

The measurement results are evaluated according to the method described in the invention SK 282 472, i.e. as the average of two individual measurements, each of which is obtained using all the lasers of the system. In addition, however, the concentrations of the delegated substance for each laser separately are determined from the measured return energies, and detection is considered positive only if the concentrations determined from both series for each of the lasers individually are positive and exceed a specified minimum.

The advantages of the method for the rapid and reliable detection of gases by means of the differential absorption LIDAR according to the invention are evident from its external effects. The effects are that, although the data measured by each laser individually are significantly less accurate than the data obtained according to patent SK 282 472, they make it possible to detect "erroneous" measurements, ie. measurement degraded by a fault inside or outside the DIAL system. The use of the present solution according to the invention makes it possible to reduce the detection time by at least half, while maintaining a high probability of detecting an "erroneous" measurement.

Examples of embodiments of the invention

In the DIAL system with two lasers, the laser "A" is tuned to the wavelength Lon (strongly absorbed by the delegated substance) and the laser ..B is tuned to the wavelength Loff (slightly absorbed by the delegated substance). Both lasers emit one laser pulse (a second laser with a small delay) and the return energies for both wavelengths are measured. Subsequently, the laser originally tuned to the wavelength Lon is retuned to the wavelength Loff and the laser originally tuned to the wavelength Loff ^is retuned to the wavelength Lon. The laser pulses are emitted again and the return energies are measured again. Then 3 concentrations are determined:

- The concentration determined as half of the sum of the concentration determined from the first pair of pulses and the concentration determined from the second pair of pulses;

- Concentration obtained from data for laser "A", ie. for a laser in the first pair of pulses tuned to the wavelength LoNav to an expensive pair of pulses tuned to the wavelength Loff;

- Concentration obtained from data for laser "B", ie. for a laser in the first pair of pulses tuned to the wavelength LoFF and for the second pair of pulses tuned to the wavelength Lon.

The first concentration (from data from both lasers) is considered to be the result of the measurement, but detection is considered positive only if the concentrations determined from both series for each of the lasers individually are positive and exceed a specified minimum value.

Claims (1)

PATENT CLAIMS Method for rapid and reliable detection of gases by means of differential absorption LIDAR with two or more lasers, of which at least one is always tuned to the wavelength λ on strongly absorbed by the delegated gas and at least one is tuned to the wavelength λορρ weakly absorbed by the delegated gas, all the lasers emit successively one laser pulse and the first concentration of the delegated substance is determined from the return energies; subsequently, all lasers are retuned from the wavelengths λ on to the wavelengths λορρ and vice versa, then a second series of pulses is sent and the results are similarly determined with a second concentration, the average of the first and second concentrations being taken as the result, The concentrations of the delegated substance for each laser separately shall also be determined, with detection being considered positive only if the concentrations determined from both series for each of the lasers individually are positive and exceed a specified minimum value.