GB2153994A - Gas detection system - Google Patents

Abstract

A differential absorption gas detection system in which a single laser (1) is arranged to transmit bursts of a plurality of discrete wavelengths in a single beam. Part of the output is passed through an area (7) to be monitored and part is passed through a reference cell (4) containing a sample of a gas to be detected. The presence or absence of that gas in said area is determined by electronic comparison of the signals detected downstream of the area under test, at detector (D3), and of the reference cell at detector (D2). <IMAGE>

Description

SPECIFICATION Gas detection system This invention relates to the detection of gases by the use of differential absorption techniques.

The use of differential absorption lidar (DIAL) techniques for the remote detection of trace gases is well known. This technique depends upon the propagation of two collinear laser beams consisting of two wavelengths. One of these wavelengths is chosen to have a resonant absorption with the molecule it is desired to detect, while the other wavelength is off-resonance and not absorbed.

Consequently, if the beams pass through a volume of gas containing the specific absorber, the relative intensities of the two beams will be changed, so indicating the presence of the trace gas in the beam path.

Such systems may be single leg or two leg systems, employing either direct or heterodyne detection receivers.

One major limitation of such systems derives from the use of only two wavelengths.

This has two major disadvantages. Firstly, the structure of molecules which absorb radiation mainly in the near and mid infrared regions is complex, with the result that there are many molecules which exhibit some absorption at any given wavelength. Consequently, with only two wavelengths available to detect a gas using the differential absorption techniques, it is possible to produce ambiguous results. Secondly, a pair of laser wavelengths is required which is characteristic of each particular gas.

If it is desired to detect traces of other gases, the lasers must somehow be tuned to other specific lines to achieve this objective.

It is an object of the present invention to provide a system which removes the aforementioned problems of the known systems and which may allow the detection of a number of molecules, with minimal adjustment of the system.

In a differential absorption gas detection system according to the present invention, a single laser is arranged to transmit bursts of a plurality of discrete wavelengths in a single beam, part of the laser output being passed through an area to be monitored and part being passed through a reference cell containing a sample of a gas to be detected, the presence or absence of that gas in said area being determined by electronic comparison of the signals detected downstream of the area under test and of the reference cell.

The present invention also provides a method for detecting the presence of trace gases, comprising tuning a single laser so that it transmits bursts of a plurality of discrete wavelengths in a single beam, transmitting a first part of said beam to a first detector, transmitting a second part of said beam to a second detector by way of a reference cell containing a sample of a gas or gases to be detected, transmitting a third part of said beam to a third detector via a space where the presence of that gas or gases is sought, electronically comparing the first and second detector signals to establish an electronic signal representative of the particular gas or gases in said reference cell and electronically comparing the latter signal with the signal from the third detector to determine the presence or absence of such gas in said space.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of one embodiment of a remote sensing system for trace gases in accordance with the present invention; Figures 2 and 3 are schematic diagrams of two embodiments of rapidly tunable laser for use with the present system; and Figure 4 is a diagram showing a typical output from the laser arrangements of Figs. 2 and 3.

The system illustrated in Fig. 1 enables many (perhaps 100) discrete wavelengths to be transmitted in a rapid burst in a single beam. The relative absorption of a given molecule to each wavelength can be determined and stored as digital information. In this way a spectral "footprint" of a particular gas can be established. Then, the return signal can be examined to see which characteristic "footprint" is present, so enabling the particular molecule to be detected. The same approach can be used to detect the presence of any gas with significant spectral features in the spectral region covered by the burst of wavelengths produced by the laser. In addition, in principle it is possible to detect a number of gases simultaneously where a combined footprint can be stored for comparison.

The system includes a laser 1 which is designed to allow very rapid wavelength tuning across all the relevant spectral features in the laser emission spectrum of the lasing species. In one example of this, with carbon dioxide lasing mixtures, the output spectrum would consist of more than 50 (depending on detailed laser design) discrete wavelengths scanned in a suitable time frame, which could for example, be in the range 1001lsec-10 millisec. This burst of multi-wavelength radiation is split at beamsplitters 2 and 3. A detector D1 provides a measure of the intensity variation of the emission spectrum of the laser. Some of the radiation is directed via the beam splitter 3 through a sample cell 4 which contains a suitable buffered concentration of a gas which it is desired to detect with this system at some remote location.The modification to the basic emission spectrum of the laser caused by absorption in the gas can be determined by comparison of the signals at detectors D1 and D2 via A/D converters 5 and a digital computer 6.

The main part of the laser beam following the beamsplitter 2 is transmitted across a region 7 in which it is desired to search for trace concentration of the gas whose spectral footprint has been electronically stored. The detector D3 (which in principle could be part of either a direct or coherent receiver) is used to detect the signal after passage across the region 7. The output of detector D3 is processed via signal processing apparatus 8 and fed into the computer 6 where it is electronically compared to the trace gas absorption footprint already in store. A positive correlation will give rise to an output reflecting the presence of the particular absorber gas.

Among the benefits of this system, two are particularly important. First of all, the existence of many wavelengths allows the identification of a gas with considerably reduced ambiguity because of the increased number of pairs of wavelengths (any two from more than 50, as compared to only two with the DIAL system) which can be used to establish and correlate with the absorber footprint. Secondly, the large increase in spectral coverage allows many other gases to be sought with essentially the same system. To seek a different gas, only the sample gas cell constituents, and in some cases the computer software need to be changed.

A further embodiment utilises any other laser having a suitable range of possible output wavelengths, which can be sequentially selected by control of a suitable tuning element, to provide both detecting and reference signals.

Figs. 2 and 3 illustrate one example of a rapidly tunable laser for use as the laser 1 of the present system. This utilises an electrically controlled angle scanning device which is arranged in the resonator of the laser between the laser mirror and a dispersive element, such as a diffraction grating or prism, such that as the scanning angle changes, the resonator is in alignment sequentially for vibrations characteristic of each of a plurality of rotational vibrational transitions.

By this means, the laser output will consist of a sequence of radiation bursts, each burst consisting of a series of lines produced sequentially as the laser output sweeps across the available gain spectrum.

With reference to Fig. 2, the laser has a resonator designed to incorporate an electronically controlled angle scanning device 10 such as an electro-mechanical scanner, either of the linear or the resonant variety, or an acoustoptic deflector. Such devices are capable of scan rates of at least tens of kilohertz. This scanning device is disposed between the laser mirror 1 2 and a dispersive element 14, such as a diffraction grating or prism. The laser gain medium is indicated by the reference numeral 1 6.

As the scanner angle changes, the resonator will be in alignment sequentially for the wavelengths characteristic of each of the rotational vibrational transitions. Consequently, the output of the laser will consist of a sequence of pulses as illustrated in Fig. 3. Then, as the scanner reverses direction, the output will be a mirror image of the first scan. In this way, bursts of radiation, each burst consisting of a series of lines produced sequentially at the laser output, sweep across the available gain spectrum.

In the embodiment of Fig. 3, an acousto optic device 1 8 is used as the rear reflector and the grating, although these functions can be separated. A partially transmitting laser mirror 20 is disposed between the laser gain medium 16' and the modulator device 18.

The grating equation is: 2d sin 0 = nA where d = grating spacing o = incident angle A = wavelength By varying d and keeping 0 constant, the output wavelength can be adjusted. The grating spacing can be adjusted by simply altering the input frequency to the acousto optic device 1 8. It will be noted that the relatively low reflection efficiencies achievable by using an acousto optic modulator 1 8 as a grating are counteracted by employing a three-mirror cavity.

These laser devices have many possible applications in remote surveillance such as trace gas or pollution detection or any system where a rapidly tuned frequency-agile laser is required.

Claims (8)

1. A method of detecting the presence of trace gases, comprising tuning a single laser so that it transmits bursts of a plurality of discrete wavelengths in a single beam, transmitting a first part of said beam to a first detector, transmitting a second part of said beam to a second detector by way of a reference cell containing a sample of a gas or gases to be detected, transmitting a third part of said beam to a third detector via a space where the presence of that gas or gases is sought, electronically comparing the first and second detector signals to establish an electronic signal representative of the particular gas or gases in said reference cell and electronically comparing the latter signal with the signal from the third detector to determine the presence or absence of such gas in said space.

2. A differential absorption gas detection system in which a single laser is arranged to transmit bursts of a plurality of discrete wavelengths in a single beam, part of the laser output is passed through an area to be moni tored and part is passed through a reference cell containing a sample of a gas to be detected, the presence or absence of that gas in said area being determined by electronic comparison of the signals detected downstream of the area under test and of the reference cell.

3. A system as claimed in claim 2 in which the output spectrum from the laser comprises more than fifty discrete wavelengths scanned in a suitable time frame.

4. A system as claimed in claim 3 in which the time frame is in the range 1 00use to 10 millisec.

5. A system as claimed in any of claims 2, 3 or 4 in which the single beam is divided into parts by one or more beam splitters.

6. A system as claimed in any of claims 2, 3, 4 and 5 in which the plurality of discrete wavelengths transmitted by the laser are generated by arranging an electrically controlled angle scanning device in the resonator of the laser between the laser mirror and a dispersive element such that as the scanning angle changes the resonator is in alignment sequentially for vibrations characteristic of each of a plurality of rotational vibrational transitions.

7. A differential absorption gas detection system constructed and arranged substantially as hereinbefore described with reference to and as illustrated in Fig. 1 or Figs. 1 and 2 or Figs. 1 and 3.

8. A method of detecting the presence of trace gas substantially as hereinbefore described.