SK32013A3 - Method of rapid and reliable detection of gases using differential absorption LIDAR - Google Patents

Abstract

The detection is carried out in such a way that at least one laser is tuned to a wavelength strongly absorbed by the detected gas - and at least one laser is tuned to a wavelength weakly absorbed by the detected gas -? OFF. Subsequently, all lasers of the system sequentially send one laser pulse and the concentration no. 1 of the detected substance. Then the lasers tuned to? ON are switched to OFF and the lasers tuned to? OFF are tuned to? ON, a second series of pulses is transmitted and the concentration no. 2. The resulting concentration is the average of concentrations no. 1 and no. 2, however, detection is considered positive only if both the positive and the concentration determined from both series of pulses for each of the lasers individually and exceed a certain minimum value.

Description

The invention relates to the detection and measurement of concentrations of gaseous substances in the atmosphere by means of a laser system operating with the DIAL (DIfferential Absorption LIDAR) technique with two or more lasers.

BACKGROUND OF THE INVENTION

DIAL systems using one tunable laser determine the concentration of the detected substance on the measurement path by first tuning the laser to the Xon wavelength, which is absorbed strongly by the substance, and sending a laser pulse through the atmosphere under investigation. The laser beam reflected from the target is captured by the optical receiver of the system and transformed into a Uon voltage signal. The laser then tunes to a wavelength Xoff, which is poorly absorbed by the substance, repeats the procedure to obtain a voltage signal Uoff. The concentration of the gaseous substance to be measured is then determined. The main disadvantage of such a DIAL system is that Xoff laser radiation passes through an atmosphere with a considerable delay to Xon radiation (the delay is given by the maximum repetition rate of the laser, which tends to be small, ie rarely above 10 pulses per second); however, the optical properties of the turbulent atmosphere change more rapidly, which in such a DIAL system causes major measurement errors.

DIAL systems that use two or more lasers eliminate this problem by having at least one laser tuned to Xon and at least one to Xoff · Then the lasers start with a very small delay, ie the pulse delay from the laser (s) tuned ( tuned) to Äoff against a pulse from a laser tuned to Xon is much shorter (usually several orders of magnitude) and the effect of turbulence does not occur. Otherwise, the function of the system is similar.

Again, the concentration of the measured gaseous material along the route can be determined from the attenuation ratio to Xon and Xoff (see, e.g., Killinger, DK et al .: Optical and Laser Remote Sensing. Springer-Verlag, Berlin 1983, 383 s .; Measures, RM: Laser Remote Sensing, John Wiley & Sons, New York, 1983, 510 p.; CO2 Lasers and Applications. Proc. Of SPIE, Vol. 1042, Los Angeles 1989, 139 p.).

One complete measurement typically consists of a plurality of individual measurements. Individual measurement is obtained by sending one pulse from each laser.

The ratio of:

P ^_ Uon / UqfF

Such a DIAL system may give rise to measurement errors resulting from the technical imperfections of the differential absorption LIDAR (= DIAL) and errors due to the inhomogeneity of the target used in the measurement.

These errors are suppressed by the patent method "Determination of gas concentration with differential absorption LIDAR suppressing measurement errors", no. No. 282,472, which came into effect on February 5, 2002. This patent suppresses the aforementioned measurement errors, while "erroneous" measurements can be detected by statistical processing of the results. A "false" measurement is a measurement that has been affected by accidental disturbances within the DIAL system or in the atmosphere - along the measurement path, for example by accidentally blocking one laser pulse through vegetation. As a result, "wrong", ie. false measurement: the substance is not detected, even if it is present on the measurement path or vice versa. If at least 4 laser pulses per laser are used with the patent, the statistical processing of the individual measurement results will clearly reveal a "false" measurement - the statistically calculated measurement error is greater than the measured concentration. The use of fewer pulses does not allow the detection of "erroneous" measurements as described.

In practice, however, it also takes too long to send 4 pulses from each laser; either the user requires a faster response from the DIAL system to the atmosphere situation, or the temporal instability of the concentration of the detected substance in the atmosphere cancels the measurement.

SUMMARY OF THE INVENTION

These drawbacks are largely overcome by the method of rapidly and reliably detecting gases using the differential absorption LIDAR of the present invention.

The detection is done by sending only two laser pulses from each LIDAR laser with at least 2 tunable lasers from one laser: one on X <, the other on Xoff: First, one laser is sent from each laser, wherein at least one laser is tuned to Xon and at least one to offoff, the pulses are transmitted with the minimum possible delay. Then all lasers are tuned, ie. from Xon to Xqff or vice versa, and a second series of pulses is sent.

The measurement results are evaluated as described in the invention "SK 282 472", i. as an average of two individual measurements, each of which is obtained using all the lasers of the system. In addition, however, the concentrations of the detected substance are determined for each laser separately, i. as in a single laser system. While such data are substantially less accurate than those obtained under the 'SK 282 472' patent, they make it possible to detect 'erroneous' measurements; Measurement impaired by failure inside or outside the DIAL system.

The use of the present invention makes it possible to reduce the detection time by at least half, while maintaining a high probability of detecting a "false" measurement.

DETAILED DESCRIPTION OF THE INVENTION

In the dual laser system DIAL, the laser "A" tunes to the wavelength λο _Ν (strongly absorbed by the detected substance) and the laser "B" to Xoff (poorly absorbed by the detected substance). Both lasers emit one laser pulse (second laser with low delay) and the return energies are measured for both wavelengths; subsequently, the laser initially tuned to Xon is tuned to Aoff and the laser tuned initially to Xoff is tuned to Xon, the laser pulses are again sent and the return energies are measured again. 3 concentrations are then determined:

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

- The concentration obtained from the data for the laser "A", ie. for a laser in the first pair of pulses tuned to Xon and in the second pair of pulses tuned to Xoff;

- Concentration obtained from data for laser "B", ie. for a laser in the first pair of pulses tuned to Xoff and in the second pair of pulses tuned to Xon ·

The first concentration (from both laser data) is considered as the result of the measurement, but detection is considered positive only if the concentrations for the individual lasers are positive and exceed the minimum detectable concentration of the DIAL system.

Patent claims

Claims (2)

A method of quickly and reliably detecting gases by means of differential absorption LIDAR with two or more lasers, at least one of which is tuned to a wavelength strongly absorbed by the detected gas - Äon and at least one tuned to a wavelength weakly absorbed by the detected gas - Äoff, By concentrating all the lasers one after the other in a single laser pulse, the measured return energies determine the concentration no. 1 of the detected substance, then all lasers are tuned from Xon to off and vice versa, then a second series of pulses is sent, from which the concentration no. 2, the average concentration no. 1 and no. 2, but detection is considered positive only if the concentrations determined from both series of laser pulses for each laser individually are