SE450913B - GAS CORRELATED SUFFER - Google Patents

Description

450 2013 450 913 dispersive correlation spectroscopy, and gas correlation spectroscopy4v5. Simultaneous recording of several wavelengths can also be achieved by using multi-channel optical techniques or beam splitting systems. Gas correlation spectroscopy is a particularly simple and powerful technique, in which incoming light passes either directly to a detector or first passes through a cell containing an optically thick sample of the gas to be studied. In the event that the atmosphere is free of this gas, the light intensities in a selected wavelength range are balanced out by means of lock-in or electric bridge techniques. With the gas atmosphere, the light passing through the gas cell is still the same, while obtaining additional absorption in the direct beam which results in an imbalance in the electronics, which after calibration can be directly expressed in ppm-m of the gas.

Description of the present invention The gas correlation concept can be easily applied to the lidar technique and thereby leads to important system simplifications and improvements in the signal-to-noise ratio.

In particular, only a relatively broadband laser is needed and no laser tuning is required between the pulses. The on and off resonant wavelengths are emitted and detected simultaneously. To describe the gas correlation lidar technique, we choose a simple model example. We will consider the case of atmospheric (atomic) mercury registration, for which we have recently reported conventional dial measurements 10. The description will follow with reference to Fig. 1.

The laser is tuned to the Hg resonance line at 2357 Å. (A pulsed, frequency-doubled dye laser can be used.) The range of Hg absorption (taking into account isotope shear, hyperfine structure, Doppler and print width) is about 0.05 Å. that it is about three times this value. If a short pulse (få ns) is used, no pronounced mode structure will be obtained, and an even spectral distribution of the pulse is assumed for simplicity as indicated in the figure. The laser pulse is emitted into the atmosphere through an Hg cloud at some distance from the sufferer system and finally hits a topographic target or a retroreflector. Reflected light is received by an optical telescope and the overlap between the transmission and detection lobes is obtained at some distance from the system. For a homogeneous atmosphere, a 1 / R2 dependence of the registered intensity is obtained. With an interference filter, the spectral region of interest is isolated from the iso 913 region for attenuation of backlight. In contrast to the normal dial system, a beam splitter and two detectors were now used instead of one. One of the jets passes a gas filter correlation cell - in the selected example containing Hg with sufficient vapor pressure - to block out the central part of the resonant line. In dial language, the resonant signal is registered in this detection arm (in reality, two adjacent wavelengths are preferably used simultaneously).

In the second detection arm, the entire spectral distribution is measured, which, provided that no atmospheric mercury is present, is the same as the transmitted spectral distribution. In 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, while the signal in the gas cell arm is unaffected. The imbalance between the two arms indicates the presence of the external gas.

The figure shows the spectral and time dependence at different points in the system to illustrate the measurement process. In particular, the spectral distributions can be studied for final measurement cones. By dividing the signals as shown in the figure - a method which is also common in dial7 - a deviation from is obtained. 1 in the presence of external Hg. Note that the ratio Q (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 that are recorded distance resolved for all distances. For a fast moving platform, this is a great advantage. Note that the percentage deviation from 1 in the divided signal is the same as that which would have been obtained in a dial measurement where the laser would once be tuned to the absorption line, and a bunch is completely tuned by that line. Since a line width greater than the absorption line width is used, the relevant absorption cross sections are dependent on the actual laser line width, and an optical depth dependence (deviation from Beer-Lambert's law) is also present.

Thus, a gas correlation conductor system is best calibrated by inserting cells with known ppm ~ m numbers in the beam path between the telescope and the detector arrangement in direct connection with the current measurement.

For practically used lasers, the spectral distribution within the laser bandwidth will vary from pulse to pulse and this fact will result in a greatly increased noise level, since the two detection arms can no longer be balanced out. However, it is possible to record the relevant spectral variations of the laser by detecting the ratio Qo for the intensity of a laser beam going directly to the detector and for one passing an identical gas correlation cell. No special arrangement is needed for this. The immediate signals from light scattering in the telescope can be adjusted to a 'suitable level' and can be isolated from an atmospheric scattering by initial separation of the emitted laser beam from the optical axis of the telescope. The signals are registered together with the atmospheric reflex as stated in Fig. About a little. external gas concentration can be expected near the telescope and a laser effect which gives a sufficient atmospheric scattering as used in the figure, the Qof value can also be obtained from the nearby scattering. It can be easily shown that I 't exp (- 2Û / R0 (n (r) dr) k: (1) where the effective absorption coefficient of the bandwidth used of the studied substance at the concentration n (r). K is the ratio between If the Q (R) and Qo are detected for each pulse, the integrated concentration value is not affected by turbulence, the spectral variations of the laser, etc., and a very noise-free measuring situation has been achieved. use, and taking into account the approximate nature of Equation (1), the system is most appropriately calibrated by inserting cells of known concentration in front of the beam splitter. some preliminary experiments on mercury with an experimental setup similar to that in Fig. 1. An excimer-pumped dye laser, frequency doubled to the 254 nm region was used, the lidar arrangement was similar to that described in our previous work on Hgw. The laser beam was directed through a remotely arranged 2 m long open chamber (at a distance of 7.0 m), where a lig-containing atmosphere could be obtained by introducing lig-drops. After passing through the chamber, the beam was reflected back to the lidar telescope, which had a diameter of 25 cm. The signals from the two detectors were recorded on a two-channel boxcar integrator which was set to the reflector echo and connected to a mini-w * 1o 15 20 25 30 35 450 913 in a computer. illustrated. No attempt was made to calibrate the system in this demonstration, nor were the spectral variations (fluctuations) compensated.

Although the system concept description and the experiments have been Hg, it is obvious that the same principle is applicable to any other gas, where adjacent wavelength regions with strong differential absorption are present. NO with a sharp band? -head "at about 226 nm is one such case, where normal dial measurements have been performed.

The laser is precisely tuned to the tape head, thereby generating both the on and off resonant wavelength components. Also in the nearby Ill region, which is available for example by Raman shifting of fairly broadband dye lasers (cf. for example Ref.12), or by differential frequency generation, measurements on CH4, CO, HCl etc. can be possible. To achieve an even more dial-like measuring situation and for better laser control, a laser can be used which simultaneously emits two adjacent wavelengths (cf. for example Ref. 13 and the references therein) together with the gas filter technique described above.

The sharp absorption properties of gases allow a stable separation of signals at adjacent wavelengths without the use of sharp interference optics.

It has recently been proposed to take advantage of these properties in lidar systems that detect Mie and Rayliegh scattering separately to obtain atmospheric temperature ”. In the present work, the spectral correlation between an atmospheric gas and gas contained in a cell was used instead of air pollution determination. It should be noted that although perfect spectral matching is achieved, temporarily coinciding with an interfering molecular absorption line could cause an error due to the narrow spectral region containing essentially only one line. Using a broadband laser (tens of Å) such as those used in broadband coherent anti-Stokes Raman spectroscopy (cf. e.g. Ref. 15), true gas correlation4 with automatic suppression of interfering substances should be achieved, e.g. in a NO2 lidar system. Furthermore, the same idea should apply to appropriately selected wavelength regions for Herline HF / DF and C02 TEA lasers. 10. 11. 12. 13.. 14. is. 450 913 REFERENCES R M Scotland, in Proceedings of the Third Symposium on Remote Sensing I of the Environment (University of Michigan, Ann Arbor, 1964). and K W Rothe, g, 181 (1975). 3, 116 0974); U Brinkmann and H Walther, Appl. Phys.

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Claims (4)

10 15 20 25 450 913 'PATENT REQUIREMENTS A method of control and sensing for the purpose of detecting atmospheric gases using lidar technology, wherein a laser beam is emitted at a fixed wavelength, characterized in that a reflected laser pulse 'may pass through an interference filter to isolate a desired speck. - tral region, after which the light passes through a beam splitter and is detected by two detectors; wherein one of the beams passes through a gas correlation cell containing the compound to be determined in order to block out the central part of a resonant line, the off-resonance signal being determined and recorded, and the other beam being used to measure the whole spectral distribution, each imbalance between the two detecting signals _ indicate the presence of an external gas which is intended to be detected. A method according to claim 1, characterized in that the undivided, reflected laser beam passes through a calibration cell before passing the beam splitter. A method according to claim 1, characterized in that a broadband laser is used. A method according to claim 1, characterized by. that two laser beams with adjacent wavelengths are emitted and detected after retransmission.