WO1994016311A1

WO1994016311A1 - Gas analyser

Info

Publication number: WO1994016311A1
Application number: PCT/GB1994/000040
Authority: WO
Inventors: Robert Clyde Newbury
Original assignee: Pollution Monitoring Systems Ltd.
Priority date: 1993-01-12
Filing date: 1994-01-10
Publication date: 1994-07-21
Also published as: GB9300483D0; GB2274163B; GB2274163A

Abstract

This invention provides a long open path optical gas analyser having broad band light emitting and detecting means. Light of selected wavelengths are produced from the light source by wavelength selecting means, passed through an open space sample, are reflected back through the sample area and are directed to detecting means where signal processing means are provided to indicate the presence of a particular gas or gases and the concentration thereof. In a preferred embodiment the broad band input light beam is directed into an acousto-optic cell to which acoustic signals are applied and which interact with the input beam to diffract a selected band width of light which is subsequently directed through said open path.

Description

GAS ANALYSER

This invention relates to optical gas analysers- More particularly, this invention relates to an optical gas analyser having a long open path and which is suitable for environmental gas monitoring.

There are several types of gas analysers currently available. These devices are produced for short path length situations. Such devices may cater for the real time analysis of industrial combustion products where the emitting and detecting apparatus are arranged across an exhaust flue. Other devices may cater for the remote sensing of gas where a sample cell of a gas mixture is interrogated at a location remote from the sampling area.

Optical gas analysers utilise the particular wavelength absorption spectrum characteristics of a gas. Every type of molecule has a characteristic absorption spectrum which is dissimilar to that of all other types of molecules. The spectra of mixtures of different types of molecules are cumulative and the absorption is proportional to the concentrations of the molecules. Optical absorption spectra can be obtained from any type of sample, be it solid, liquid or gas, so long as the sample is optically transmissive. Almost all gas molecules absorb infra-red radiation and each molecule has its absorption at specific characteristic wavelengths. By monitoring the magnitude of the absorption at specific wavelengths, the amount of a specific gas can be determined. In the case of an exhaust flue gas analyser infra-red radiation can be transmitted across the flue from an emitter to a detector. The wavelength of the transmitted radiation is preferably tuned to specific wavelengths, especially when the species of gas are known. The strength of the detected signal at a given wavelength can be correlated to the concentration of one or more absorbing gases having an absorption peak at that wavelength.

The selective tuning of the infra-red radiation can be accomplished either by the insertion of narrow band interference filters in the path, by inserting cells containing appropriate gas fills or by employing an acousto-optic tunable filter (AOTF) in the path of a broad band light source. The first two methods involve mechanical motion and the number of cells or filters that can be employed is limited:- reliability, together with accuracy, may suffer from the use of such methods. Acousto-optic materials can also be conveniently used as a filter in a spectrum analyser. An acoustic transducer is coupled to an acousto-optic crystal through which optical and infra-red radiation can be passed at a predetermined angle relative to the crystal optic axis. RF energy of variable frequency may be applied to the acousto-optic transducer material which is preferably a piezo-electro cell fixed to the acousto-optic cell, whereby acoustic waves are launched into the crystal. These acoustic waves interact with polarised infra-red radiation and the tuned narrow bandwidth radiation output from the cell is a function of the frequency of an RF energy source.

It is an object of the present invention to provide a long open path gas analyser, which utilises an acousto- optic tunable filter as a monochrometer. Unlike conventional spectrometers where there is a gas sample chamber or a short path length test area; the environment under test is an extensive open path. A retroreflector is placed at the far end of the open path which directs transmitted light back to the instrument.

In the field of environmental pollution monitoring there are many applications where open path measurements are far more suitable than remote testing of gas samples. When the source of the gas emission is not known, such as at a landfill site, petroleum plant, factory perimeter etc., the pollutants tend to move around the site in plumes being carried by the wind. With sampling systems these plumes are difficult to locate and may be blown away from the sampling instrument, in which cases, the sampling instrument is of little use. The large open path analyser, however, can detect these plumes as they cross the path of the light beam. Presently, infra-red open path analysers are either fixed wavelength devices allowing quantitative measurements of one preselected gas to be made or are fitted with monochrometers such as diffraction gratings, rotating filter wheels or moving mirrors for comparative measurements to be made. All of these techniques require mechanical movement of the monochrometer which are usually precision micro-movement devices and as such are susceptible to shock and vibration.

According to one aspect of the invention there is provided a long open path optical gas analyser comprising: light emitting means, light detecting means, reflecting means and signal processing means; wherein the light emitting means comprises a broad band light source, filtering means and light focussing means and wherein the filter can select from the broad band beam a beam of a distinct wavelength or several wavelengths; wherein said beam of a distinct wavelength or several wavelengths is directed by the focussing means through the long open path towards the reflecting means from which it is reflected towards the detector, the detector being connected to the signal processing means which can process the detected optical signal and thereby indicate the presence of a particular gas.

Preferably, the wavelength of the beam is selected by directing the beam from the broad band source through an acousto-optic tunable filter to which radio frequency signals are applied such that a wavelength or several wavelengths are then produced and output from the device. The output can be modulated. Alternatively, the output can scan a selected wavelength spectrum. Preferably, the processing means comprises control means and display means for displaying the analysis details. The detecting device can be a cooled photovoltaic or photoconductive detector such as PbSe, PbS, Si, Ge, InSb, HgCdTe or, alternatively, is a charge coupled device (CCD).

According to a preferred aspect of the invention, the broad band optical source operates substantially in the infra-red. The broad band source may, alternatively, operate substantially in the visible spectrum or substantially in the ultra-violet spectrum.

According to another aspect of the invention, several acousto-optic tunable filters operate as narrow band filters, each acousto-optic tunable filter operating over one octave of light.

In another aspect of the invention, several sets of output and detecting means are used together with several sets of reflectors in order to comprehensively analyse the gas composition of an air space.

The invention also provides a method of analysing the composition of gases in a long open path which comprises the steps of: generating an output signal from a broad band optical source; passing the output of said broad band optical source through a filter to select a narrow bandwidth light beam; separating the zero order component from the light beam; passing narrow bandwidth light beam through a long open path sample area; reflecting said narrow bandwidth light beam along substantially the same path to detecting means; detecting and analysing the reflected signal by means of the detecting means together with analysing means; and representing the data on display means by means of a microprocessor.

Preferably, the filter is an acousto-optic tunable filter (AOTF), where the acousto-optic tunable filter is controlled by a radio frequency transducer; by varying the frequency of the radio frequency signal, the wavelength of the output light beam from the acousto-optic tunable filter may also be varied. The output from the acousto- optic tunable filter may be varied to scan a selected wavelength spectrum. By varying the amplitude of the radio frequency signal, the amplitude of the output light beam from the acousto-optic tunable filter may also be varied. The amplitude of the output light beam from the acousto-optic tunable filter may be varied to provide a means of amplitude modulating the light beam.

Preferably, the separated narrow bandwidth light beam is passed through a beam splitter in order that a portion of the beam can be used for comparative purposes. The angle of deflection, for comparative purposes, is preferably 90° to the axis of propagation of the undeflected beam. Preferably, the undeflected light beam is passed through a light collimating mechanism, such as a Newtonian, Cassegrain, Herschel or off-axis collimating mechanism. On the return of the reflected beam, this collimating mechanism acts as a telescope, gathering the reflected light beam. This beam is then focussed onto a beam splitter which directs the beam onto the detecting means.

A laser operating in the visible wavelength spectrum in conjunction with a sighting arrangement may be mounted such that a laser beam will follow a path coaxial to that of the infra-red analysis beam. The light from the laser will be directed towards the retroreflector and the return beam may be directed towards the detector by means of adjusting the retroreflector and/or the position of optical components within the analyser unit.

The analyser combines an infra-red acousto-optic tunable filter based optical system with an open path analysis arrangement. In adopting this approach the AOTF is used for filtering and determining the pulse form of the light for analysis and also provides the method for modulating the light for determining the distance between the output and detecting means and the retroreflector. A laser for calibration of the detector means can be provided which may be switched into the optical path automatically.

The system can provide greater accuracy over broad band short path analysers. By extending the length of the sample path, the accuracy of the instrument is significantly improved, enabling much lower gas concentrations to be detected than is possible with conventional analysers. This is a function of the Beer- Lambert gas absorption law which defines the relationship between the light absorption of the gas, the path length and the intensity of the incident light.

The Beer-Lambert gas absorption law may be expressed as T=10^"ec =I/Io and ecb=A=-log(T) where:- A=Absorbance, T=Transmittance, I=Intensity of incident light, Io=Intensity of transmitted light, e=Molar absorptivity of a given gas at a fixed wavelength of light and is a constant, c=Concentration (MOL l^-1), b=Path length (cm) .

In order that the invention may more readily be understood, reference will now be made to the accompanying Figures, wherein:-

Figure 1 shows a block diagram of a long open path gas analyser in accordance with one embodiment of the invention.

Figure 2 shows an AOTF arrangement allowing separation of the light beam with the use of polarisers; and

Figure 3 shows an AOTF arrangement allowing separation of the light beam without the use of polarisers.

Referring to Figure 1, a broad band light source (1) is focused onto an acousto-optic tunable filter (6) by means of a focussing arrangement (2) . The randomly polarised light from the broad band light source is polarised by a polariser (5) prior to passing through the AOTF (6). The filter (6) is tuned by selecting a suitable acoustic frequency. This is achieved by means of a programmable radio frequency RF synthesiser (26) which is controlled by a microprocessor (27). The RF signal is amplified and then converted to an acoustic wave by means of a piezo electric transducer which is bonded to the optically birefringent material of the AOTF (6) . Modulation is applied to the light beam by amplitude modulation of the radio frequency signal. The source of amplitude modulation is an oscillator (24) which is used to modulate the RF signal from the radio frequency synthesiser (26) . A specific wavelength of light is filtered and modulated by the AOTF (6). Note that the incident orthogonal polarised light is split into ordinary birefringent rays (o-rays) containing the zero order light and extraordinary birefringent rays (e-rays) containing a narrow band of filtered light which comprises first order diffracted light.

In an alternative embodiment of the invention there are no polarisers, the transmitted zero order light from the AOTF being spatially separated from the filtered first order light. Referring to Figure 2 which illustrates this arrangement, broad band randomly polarised light enters a non-colinear AOTF (6). On leaving the AOTF, four rays of light emerge, these being the first order vertically polarised e-ray, the first order horizontally polarised o- ray, the zero order e-ray polarised vertically and the zero order o-ray polarised horizontally. The geometry of the AOTF (6) is arranged such that the angular separation between the first order e-ray and the first order o-ray is large (of the order of 32 degrees). The zero order o-ray and e-ray have a small separation (of the order of 2 degrees) and are spaced between the two first order rays (in the region of 16 degrees between the two zero order rays and the first order rays). A beam dump (40) is fixed some distance away from the exit aperture of the AOTF (6) and is aligned such that both zero order rays and the first order ray are completely absorbed, such that they do not interfere with any measurements. The filtered and modulated beam (8) is directed through a beam splitter (9) to collimating and gathering optics (15). A focussing lens (10) focuses the filtered and modulated beam (8) onto a secondary mirror (11) which reflects the light beam onto a spherical mirror (12) which expands and colli ates the light beam. The light beam is directed through an open path (15). At the far end of the open path (15) is placed a retroreflector (16) which returns the light along the same path to the collimating and gathering optics (13). The returned light beam (62) is deflected by the beam splitter (9) at 90° onto a detector (18) by focussing lenses 17.

Gases in the path of the light beam (8) are impinged by the filtered light. If a gas (14) is present which absorbs light at the wavelength of the filtered beam (8), there will be a reduction in the amplitude of the light reaching the detector (18). This reduction is related to the concentration of the gas (14) in the light path by the Beer-Lambert gas absorption law. The electrical signals sensed at the detector are amplified by a pre-amplifier (19) and a phase sensitive lock-in amplifier (20). A reference signal is provided for the phase sensitive detector of the lock-in amplifier (20) from the oscillator (24) which has the same frequency as the amplitude modulation of the filtered light beam (8) and consequently the same frequency as the measured electrical signal at the detector (18). Only signals modulated at the frequency of the oscillator (24) will be amplified and other background signals and noise will be suppressed.

The output voltage from the lock-in amplifier will be related to the amplitude of the filtered and modulated light (8) reaching the detector and, consequently, is related to the concentration of detected gas (14) in the light path. An analog digital converter (22) is used to obtain a digital representation of this voltage. The digital signal is sent to a microprocessor (27) which processes the data, calculates and, if required, averages the gas concentrations. Digital signal processing is applied to the data in order to identify and quantify individual gases in gas mixtures and also to annul the effects of atmospheric gases and water vapour.

The microprocessor (27) controls the operation of the RF synthesiser (26) which in turns selects an optical wavelength of light for transmission through the AOTF (6) and into the gas path (14). The microprocessor (27) may be programmed to either sweep through a range of wavelengths thereby providing a spectra of gas absorption lines or alternatively may be programmed to select fixed wavelengths of light which correspond to absorption lines of specific gases. The distance of the open path (14) is determined by measuring the phase shift of the amplitude modulation of the returned light in the detector in relation to the phase of the modulating signal at the AOTF (6). The resultant phase shift gives rise to a DC voltage for ranging data at the output of the phase shift detector (33), of the lock-in amplifier (20), which is related to the distance between the AOTF (6) and the retroreflector (16). The voltage at the output of the phase detector (33), the ranging data signal (31) is applied to the second channel of a multiplexed analogue to digital converter (22). The ranging data signal (31) is applied to the digital to analogue converter (22) under control of the microprocessor (27) which selects either the measurement data (21) or the ranging data (31) to be digitised. The digital output is applied to the microprocessor (27) where the ranging data is used in an algorithm to calculate gas concentrations using the Beer- Lambert gas absorption law.

Information detailing the ranging, gas presence etc., may be shown either by means of a display (29) of operator selected gases, showing gas concentrations or a spectrograph of light wavelength versus light absorption. Limits may be set for maximum or minimum concentrations of particular gases and an alarm or controlling signal to other equipment being generated if the limits are out of bounds.

The light produced by the broad band light source (1) may alternatively be wavelength modulated by frequency modulation of the RF carrier signal which is applied to the AOTF (6). This is achieved by the microprocessor (27) continually re-tuning the RF synthesiser (25) in small increments about a narrow band of frequencies. This will enable the instrument to employ second derivative spectroscopy techniques, whereby the wavelength modulated light is repeatedly scanned about a fixed centre wavelength at the modulation frequency. If a gas (14) exhibits infra-red absorption at the wavelength about which the modulation is centred, the detector (18) will output a signal at twice the frequency of the modulating signal. Second derivative spectroscopy provides a method of improving spectral detail which may be lost when conventional spectroscopy is employed.

The maximum length of the long open path is restricted by the level of light that can be transmitted into the path and by the effects of absorption by water vapour etc. on the light path. As the path length or the water vapour content in the path are increased the amount of light received at the detector decreases and degrades the signal to noise ratio. The effect of water vapour may be compensated for by periodically tuning the AOTF (6) to a reference wavelength where there is absorption by water vapour but not by measurand gases. The water vapour content of a spectra can then be nulled by digitally processing a measured spectra to null the effects of water absorption measured at the reference wavelength.

Acousto-optic tunable filters vary in their geometry, composition and optical arrangement and may be described as being either colinear or non-colinear. Figure 2 shows a colinear arrangement: the acoustic wave and the optical ray transverse the crystal in the same plane. This arrangement necessitates the use of an input polariser (7a) such that all light entering the crystal is polarised in the same plane. Also, an output polariser (7b) is required, referred to as an analyser which separates the diffracted polarised output ray from the undiffracted polarised output ray. Non-colinear acousto-optic filters have an alternative arrangement, as shown in Figure 3, whereby the acoustic wave is perpendicular to the input light beam axis. This arrangement enables the acousto- optic interaction to occur over an extended optical aperture thereby facilitating a greater acceptance angle to the input light beam. This, in turn, allows a greater throughput of light. Although the diffracted output ray may be separated from the zero order light by the use of polarisers at the incident face and output face of the filter, it is possible for the non-colinear acousto-optic tunable filter to dispense with the need for polarisers. This is provided that the crystal geometry has been altered to enable the diffracted output light beam to be spatially separated from the zero order light beam.

In the arrangement of Figure 3, the diffracted ray and the zero order rays are spatially separated in an arrangement using a non-colinear acousto-optic tunable filter.

Ordinarily, variations in acoustic frequency give rise to an angular change in the output beam with optical wavelength. The exit face of the non-colinear acousto- optic tunable filter may be cut in such a way as to reduce variations in the angle of the diffracted output beam with wavelength change. It is recognised that in an open path analyser, significant angular variations in the exit light beam from the acousto-optic tunable filter could give rise to a loss of alignment of the light beam in the open path between the collimating device and retroreflector or between the acousto-optic filter and the collimating device. The crystal of the acousto-optic filter can be cut in such a way as to reduce angular variations in the diffracted output light beam over the tunable waveband.

It is also recognised that the use of a phase sensitive lock in amplifier connected to the detector will improve signal to noise ratio by means of reducing the bandwidth of the amplification such that only signals at the modulation frequency are amplified thereby reducing both system noise and noise from background radiation.

The optical components of the analyser are preferably mounted some distance from the retroreflector (typically 100m) . The alignment of the optical head assembly with the retroreflector can be adjusted by means of a visible laser in conjunction with a sighting arrangement. The laser is mounted such that a laser beam will follow a path coaxial to that of the infra-red analysis beam. The light from the laser will be directed towards the retroreflector, the return beam being visible in the sighting arrangement. The optical head can then be adjusted for best visible alignment with the retroreflector.

To ensure optical alignment, an automatic sighting system can also be employed which uses the infra-red analysis beam. This can be achieved under the control of the microprocessor in an alignment mode. In this alignment mode, the acousto-optic tunable filter is set to select a wavelength of light at which there is substantially little absorption by atmospheric gases, thereby ensuring a peak signal detector. The light is modulated by the acousto-optic tunable filter and is transmitted to the retroreflector where it is reflected back along substantially the same path to the optical head. The returned light will fall on the detector where the amplitude of the modulating beam is converted to an electrical signal which is amplified, demodulated and converted to a digital representation. The microprocessor will display the amplitude of the returned signal in a convenient manner such as a bar graph or percentage of maximum expected value. The optical head can then be manipulated either automatically or manually with reference to the microprocessor display until a peak value is obtained, at which point the instrument and the retroreflector will be optimally aligned.

Claims

CLAIMS : -

1. A long open path optical gas analyser comprising: light emitting means, light detecting means, reflecting means and signal processing means; wherein the light emitting means comprises a broad band light source, filtering means and light focussing means and wherein the filter can select from the broad band beam a beam of a distinct wavelength or several wavelengths; wherein said beam of a distinct wavelength or several wavelengths is directed by the focussing means through the long open path towards the reflecting means from which it is reflected towards the detector, the detector being connected to the signal processing means which can process the detected optical signal and thereby indicate the presence of a particular gas.

2. A long open path optical gas analyser according to claim 1 wherein the filtering means comprises an acousto-optic cell having an acoustic transducer attached thereto wherein, in use, a broad band light input beam directed into the acousto-optic cell to which acoustic signals are applied such that said acoustic signals interact with a selected bandwidth of the input beam whereby said selected bandwidth is preferentially diffracted and is directed through said open path.

3. A long open path optical gas analyser according to claim 2 wherein the acoustic transducer is a piezo¬ electric cell to which radio frequency signals are applied which cause the piezo-electric cell to vibrate at acoustic frequencies.

4. A long open path optical gas analyser according to claim 2 wherein the light emitting means further comprises means to absorb any radiation not directed along said open path.

5. A long open path optical gas analyser according to any one of claims 1 to 4 wherein the signal processing means comprises: modulating means to modulate the output beam, amplifying means to amplify the signals received from the detector which is connected to demodulating and analog signal processing means, said demodulating and analogue signal processing means outputting signals to a phase shift detector and to an analogue to digital converter; microprocessing means which is connected to i) the modulating means, ii) analysing and ranging means and iii) the analog to digital converter which is connected in turn to analysing and ranging means; control means which is connected to the microprocessing means; and display means to display the analysis information.

6. A long open path optical gas analyser according to any one of claims 1 to 5 wherein the light emitting means and the light detecting means are combined in one unit.

7. A long open path optical gas analyser according to claim 6 wherein the analysis beam is reflected back along the same path by the reflector and wherein the light emitting means further includes a beam splitter in the path such that light reflected from the reflector is directed onto the detector.

8. A long open path optical gas analyser according to any one of the preceding claims further comprising a laser for calibration of the detector.

9. A long open path optical gas analyser according to any one of the preceding claims wherein the output beam from the emitting means is amplitude modulated by modulating means whereby the distance of the open path can be determined by measurement of the phase shift of the amplitude modulation of the returned light, detected by the detector in relation to the phase of the modulating signal of the modulating means.

10. A long open path gas analyser according to claim 9 wherein the modulation is effected by an acousto-optic tunable filter.

11. A long open path gas analyser according to claim 9 wherein the acousto-optic tunable filter can modulate the wavelength of the output beam by the application of a frequency modulated radio frequency signal to the acousto- optic transducer to allow the output to be scanned about a fixed sensor wavelength at the modulation frequency and thereby enable second derivative spectroscopy techniques to be employed.

12. An analyser according to any one of the preceding claims which includes a visible laser for adjustment and alignment of the reflector in conjunction with a sighting arrangement.

13. A long open path optical gas analyser substantially as described herein and with reference to any one of the Figures.