GB2286041A - High resolution infared spectroscope - Google Patents

Abstract

An infrared spectroscopic gas analyser employs a high resolution filter 8 such as a guided mode resonance filter, diffraction grating, or Fabrey-Perot etalon to filter the output of a broadband infrared radiation source 6 and pass infrared radiation at a plurality of discreet frequencies through a gas cell 11, the discreet frequencies corresponding to absorption lines of the spectra of a gas of interest. A guided mode resonance filter may comprise a transparent substrate supporting a layer of higher refractive index incorporating a phase grating made by periodic variation of refractive index or period undulation of the surface. A plate 12 has a plurality of slots which pass light of only the discrete frequencies of interest to a detector 13. The plate is oscillated to produce a periodic output from the detector. <IMAGE>

Description

HIGH RESOLUTION INFRARED SPECTROSCOPE This invention relates to a high resolution infrared spectroscope and particularly to the use of a high resolution infrared spectroscope for gas analysis.

In spectroscopic gas analysis a beam of light having a known range of wavelengths, typically 50 nm or more in width, is produced and passed through a gas sample of interest, the quantity of a constituent gas in the sample having a known absorption characteristic at the wavelength range of the light beam can then be calculated by measuring the amount of light absorbed by the sample. This measurement of the amount of light absorbed is generally made by measuring the intensity of the light after passage through the gas cell or by directly measuring the amount of energy absorbed by the gas using photoacoustic techniques.

A difficulty with systems of this type is that the absorption bands of most gases are relatively broad in wavelength and in many cases overlap. As a result, it can be difficult to find a wavelength band at which a gas of interest can be sensed by this technique at which there is no absorption band of any other gas which might also be present. One example of this is the difficulty of measuring the relatively low concentrations of nitrogen oxides in car exhaust gases because the absorption bands of the nitrogen oxides overlap in wavelength with the absorption bands of water vapour which is present in car exhaust gases in large amounts and as a result complex and expensive water vapour removing equipment has to be used in spectroscopic sensors of this type.

One particular band of wavelengths of interest is the infrared region and particularly the infrared region having a wavelength of 3 to 5.5 pm, but it has not been possible to produce spectroscopic gas analysis equipment operating in this wavelength region commercially because of the lack of any suitable source to produce infrared radiation at a suitable range of frequencies in this region. Although tunable laser diodes operating in this frequency range are known, they require cryogenic cooling and are expensive, inconvenient to use and bulky as a result.

This invention was intended to provide a high resolution infrared spectroscope at least partially overcome these problems.

This invention provides an infrared spectroscopic gas analyser comprising a broad band infrared radiation source, a gas cell, an infrared sensor and a high resolution filter arranged such that the infrared radiation passing from the radiation source and through the gas cell to the infrared sensor is limited by the high resolution filter to a plurality of discrete frequencies corresponding in frequency to absorption lines of a gas to be sensed by the analyser.

This allows a simple incandescent infrared source to be filtered to provide an infrared beam having a spectrum of discrete line frequencies corresponding to the absorption lines of gases to be sensed and so enables more accurate infrared absorption gas sensors to be constructed than the prior art without excessive expense or complexity being necessary.

Systems employing the invention will now be described by way of example only with reference to the accompanying diagrammatic figures in which: Figure 1 shows an explanatory graph of absorptivity against frequency for a typical gas, Figure 2 shows a general view of a guided mode resonance filter, Figure 3 shows an infrared spectroscopic gas analyser for sensing a single gas, Figure 4 shows an explanatory graph showing the transmittivity of water vapour against frequency, Figure 5 is a further explanatory diagram showing the transmittivity of nitrous oxide against frequency, Figure 6a shows a second form of spectroscopic infrared gas analyser for sensing a single gas, Figure 6b shows another view of the gas analyser of Figure 6a, Figure 7 shows a spectroscopic gas analyser employing the invention and able to detect four gases simultaneously, Figure 7a shows a detail of the gas analyser of Figure 7, Figure 7b shows an altemative construction to that shown in Figure 7a, Figure 8 shows a further form of spectroscopic gas analyser according to the invention able to sense four different gases simultaneously, Figure 9 shows another form of spectroscopic infrared gas analyser according to the invention able to sense a single gas, Figure 1 Oa shows an explanatory graph of the absorptivity of carbon monoxide against frequency, Figure lOb shows an explanatory diagram of the filter spectrum of a Fabry Perot etalon, Figure 11 shows a further example of a spectroscopic infrared gas analyser according to the invention able to sense a single gas, and Figure 12 shows a further example of a spectroscopic infrared gas analyser according to the invention able to detect four gases simultaneously, similar parts have the same reference numerals throughout.

Throughout this specification the term light" should be interpreted as including, among other things, infrared radiation.

A typical gas absorption spectrum is shown in detail in Figure 1, in this case the spectrum is for carbon monoxide at 1% concentration at atmospheric pressure. As can be seen, the absorption band is made up of a large number of very narrow absorption lines grouped together in a symmetrical arrangement. Generally, for simple gas molecules, each absorption band consists of a central wavelength corresponding to a transition between two vibrational states of the gas molecule with a spectrum of regularly spaced absorption lines on either side corresponding to transmissions between rotational states, the absorption band of CO shown is centred at 4.7 pm wavelength. As can be seen, the absorption at the absorption line frequencies is very high but the absorption between these lines is relatively low.In normal spectroscopy, which generally employs radiation having a bandwidth (spectral resolution) of 50 nm or more, this bandwidth will cover the whole of the absorption band. As a result such a conventional system will only sense the average absorption across the whole band, which is much lower than the absorption at the wavelength of each of the absorption lines. If radiation could be used at frequencies which could be defined narrowly enough to cover only the individual absorption lines in the absorption band, much greater levels of sensitivity could be achieved by spectral gas analysis techniques.

In a first form of the invention, a guided mode resonance filter is used to produce light at a plurality of discrete frequencies corresponding to absorption lines of the gas to be sensed.

The basic construction of a guided mode resonance filter is shown in Figure 2.

The guided mode resonance filter is formed by a thin transparent layer 1 of high refractive index material deposited on a transparent substrate 2 of lower dielectric constant. The transparent layer 1 of high refractive index material has a phase grating incorporated into it. In the example of Figure 2 this phase grating is formed by a periodic variation in the refractive index of the layer 1 shown as shaded regions 1A of higher refractive index than the intervening clear regions 1 B. Although this is shown for simplicity as a series of discrete regions, the index would in fact be varied sinusoidally between maximum and minimum values with a fixed periodicity. Instead of a varying refractive index the phase grating could instead be generated by a periodic surface undulation of the layer 1. When a collimated beam of light 3 is incident normal to the filter surface, the filter generates a polarised reflected beam 4 and a transmitted beam 5 due to internal resonance within the filter. At the resonance frequency of the filter all of the incident light is reflected and no light is transmitted while at all wavelengths on either side of resonance reflectivity decreases rapidly to near zero so the incident light is almost all transmitted. As a result, the guided mode resonance filter acts as a narrow bandwidth polarising reflective filter. It is possible to produce guided mode resonance filters with a reflected light line width of only one or two parts per million of the centre wavelength.If the incident light is not normal to the filter surface, the same effect occurs but the resonance wavelength will reduce as the angle of incidence (the angle between the incident light and the normal to the filter surface) increases.

If non-collimated light is used the guided mode resonance filter reflects a continuous spectrum of wavelengths, with a maximum wavelength being reflected normal to the filter and lower wavelengths being reflected at increasing angles of incidence.

The resonance wavelength of the filter is dependent on the periodicity of the periodic index modulation in the layer 1.

If desired, index modulations at several different periodic frequencies can be incorporated into a single filter in order to generate several different resonant wavelengths simultaneously, if such a structure is used with non-collimated light several continuous spectra will be generated, each having a different maximum wavelength and reducing at a different rate with increasing angles of incidence.

The above description assumes that only the first order diffracted wave is encountered, this condition can be guaranteed by ensuring that the spatial frequency of the periodic index modulation is high enough for all higher order diffracted waves to be cut off.

In Figure 3 a spectroscopic gas analyser sensing a single gas and employing a guided mode resonance filter is shown. A thermal radiation source 6 emits broad band infrared radiation, the source 6 is an electrically heated ceramic coated wire at a temperature of around 1500K. The broad band infrared radiation emitted by the source 6 is incident on a curved focusing mirror 7 which roughly focuses the radiation onto a guided mode resonance filter 8. The uncollimated infrared radiation incident on the guided mode resonance filter 8 from the focusing mirror 7 has significant angular divergence and as a result the resonance filter 8 generates a divergent beam of reflected infrared radiation having a continuous spread of frequencies varying with angular position. This is because, as explained above, the resonant frequency of the filter varies with angular position.

The reflected light from the filter 8 thus consists of a continuous spectrum of wavelengths, each wavelength constituting a plane wave travelling at a specific angle to the filter surface, this reflected spectrum is incident on an accurate aspheric reflector mirror 9 which brings all of the reflected radiation to a focus in a focal plane 10. In the focal plane 10 a continuous spectrum of the radiation emitted by the source 6 is generated by the aspheric mirror 9 with each position in the focal plane being the focal point of a specific wavelength.

Before reaching the focal plane 10 the radiation reflected from the aspheric mirror 9 passes through a gas cell 11. A gas mixture to be analyzed enters the gas cell 11 through an inlet pipe 11 a and leaves it through an outlet pipe 11 b. A code plate 12 (shown in more detail in Figure 3a) is situated in the focal plane 10 and is substantially radiation blocking but has a plurality of narrow transmissive slots 12a.

The position of each of the transmissive slots 12a corresponds to the point in the focal plane at which infrared radiation at a wavelength corresponding to one of the absorption lines of the gas to be sensed by the system is focused and as a result only infrared radiation at these discrete frequencies of interest passes through the code plate 12 and is incident on a pyroelectric detector 13.

Pyroelectric detectors are sensitive only to changes in temperature so a piezoelectric actuator 14 is attached to the code plate 12 to periodically move it a short distance back and forth along the focal plane 10 at a constant frequency. This movement varies the intensity of the infrared radiation reaching the pyroelectric detector 13, because the transmissive slots in the code plate are moved to positions in the focal plane corresponding to frequencies which do not correspond to absorption lines of the gas of interest. The frequencies transmitted by the code plate and the degree of movement of the actuator 14 must be selected so that this periodic movement does not allow radiation at frequencies corresponding to absorption lines of other gases to reach the pyroelectric detector.The periodic interruption of the radiation at the absorption line frequencies will cause the radiation incident on the pyroelectric detector 13 to vary periodically and as a result the electrical signal generated by the pyroelectric detector 13 will be a periodic waveform. The concentration of the gas of interest within the gas cell 11 can be calculated from the amplitude of this periodic waveform using well established spectroscopic techniques.

It would be possible to use an infrared detector of some other type, but the use of a pyroelectric detector is preferred because of its cheapness. Similarly a transmissive optical system employing lenses could be used instead of focusing mirrors but the great cost of infrared transmissive lenses would generally make the use of mirrors more desirable.

The use of an AC signal generating infrared sensor is preferred because the AC signal produced is directly proportional to the optical absorption in the gas cell so that very low values of absorption can be measured and the DC signal effects of leakage currents, temperature changes or stray light (for example) have no effect on the final readings.

Because a system of this type employs a plurality of discrete frequencies corresponding to absorption lines in the absorption spectrum of the gas to be sensed, the apparent absorptivity of the gas is much higher than in a broad band spectral system so greater sensitivity can be achieved. There is a further advantage of such a system and this is illustrated with reference to Figures 4 and 5.

Figure 4 shows the absorption spectrum of water vapour while Figure 5 shows the absorption spectrum of nitrous oxide, both in the wavelength region 5.1 to 5.5 pm.

As can be seen the two spectra overlap and any attempt to use broad band spectroscopy to sense nitrous oxide will have its accuracy severely impaired if any water vapour is present because the system will not be able to distinguish between absorption by these two different gases, as a result expensive water vapour removal equipment is required in nitrous oxide sensing spectrometers. However the absorption lines of the two spectra do not coincide, there are many nitrous oxide absorption lines which do not correspond to and are not immediately adjacent to water absorption lines and a narrow band IR spectrometer as described above can be set to sense only radiation corresponding to these nitrous oxide absorption lines and thus interference by any water vapour present can be avoided. As a result, water vapour absorption equipment is not necessary.

Note that in Figures 4 and 5 transmissivity rather than absorption is plotted against wavelength.

Referring to Figures 6a and 6b another form of gas analyser employing guided mode resonance filters is shown. The sensor is shown from above in Figure 6a and from the side in Figure 6b. Broad band infrared radiation is generated by a long, narrow electrically heated ceramic source 20 and the infrared radiation from the source 20 is roughly collimated and directed onto a guided mode resonance filter 21 by a low quality spherical mirror 22.

In this case the guided mode resonance filter 22 instead of having a phase grating of fixed period on its surface has a phase grating which varies in periodicity along its length to give the phase grating a chirped periodicity. The effect of this chirped periodicity is that when the guided mode filter is illuminated with light at a single wavelength it reflects this light in the form of a converging cylindrical wave which is focused to a diffraction limited line at the centre of curvature of the cylindrical waveform. This phenomenon is due to a combination of the variation in resonant frequency with incident angle effect described above for a non-chirped guided resonance mode filter and the change in reflected resonant frequency with changing periodicity.When the chirped guided mode resonance filter is illuminated with a range of wavelengths it focuses each wavelength as a diffraction limited line at a different point in a focal plane because the centre of curvature for each of the wavelengths will be different.

As a result, a chirped guided mode resonance filter can act as both a wavelength filter and a focusing lens simultaneously. This allows the focusing mirror 9 shown in the arrangement of Figure 3 to be dispensed with.

The infrared radiation reflected from the filter 21 passes through the gas cell 11 and is brought into focus as a continuous spectrum of lines arranged side by side, each line having a specific frequency and being at a specific point in the focal plane 23 of the filter 21. As before, a code plate 12 bearing a plurality of transmissive slots to select the specific wavelengths of interest is placed in the focal plane 23 and is oscillated periodically by a piezoelectric actuator 14 to generate a periodic output from a pyroelectric detector 13. The radiation passing through the code plate 12 is focused onto the pyroelectric detector 13 by a focusing mirror 24.

Strictly speaking, the wavefront curvature of the cylindrical waves generated by the chirped guided resonance mode filter will only be perfectly corrected for one specific wavelength but over narrow wavelength ranges the aberrations will be small enough not to affect the sensitivity of the sensor and systems of this type can operate quite satisfactorily using small ranges of wavelengths.

The system shown in Figures 6a and 6b can easily be adapted to simultaneously measure the concentrations of more than one gas in a mixture of gases. The code plate can obviously have transmissive slots arranged to transmit wavelengths corresponding to absorption lines of two or more different gases but it is necessary to also include some means by which the changes in absorption produced by one of the gases can be distinguished from changes in absorption produced by another gas.

A first solution to this problem is shown in Figure 7.

Referring to Figure 7 a guided resonance mode filter gas sensor able to sense four different gases simultaneously is shown. This sensor is based on the chirped filter single gas sensor shown in Figures 6a and 6b and comprises a source 20, chirped guided mode resonance filter 21, spherical mirror 22, gas cell 11, focusing mirror 24 and pyroelectric detector 13 as before. The code plate 12 of Figures 6a and 6b has been replaced by four separate code plates 25a to 25d spaced apart in the focal plane parallel to the line foci produced by the filter 21, the code plates 25a to 25d are shown in more detail in Figure 7a. Each of the four code plates 25a to 25d bears a plurality of transmissive slots at positions in the focal plane 23 corresponding to absorption lines of a different gas.In order to allow the signals generated in the pyroelectric detector 13 by these four different gases to be distinguished, each of the code plates 25a to 25d is moved periodically in the focal plane 23 at a different frequency by a respective piezoelectric actuator 26a to 26d. The DC electrical signals generated in the output of the pyroelectric detector 13 by each of the code plates 25a to 25d can easily be separated using conventional signal processing techniques such as employing a plurality of electrical filters each of which is locked to the frequency of one of the code plates 25.

The disadvantage of this arrangement is that, since the guided mode filter 21 generates a continuous range of wavelengths in the focal plane 23, the separate code plates 25a to 25d must all operate in different ranges of wavelengths and so the sensing of several different gases employing interleaved absorption lines cannot be carried out.

There are two ways of overcoming this problem, one is to arrange the code plates as shown in Figure 7b where four code plates 27a to 27d each driven by a dedicated piezoelectric driver 28a to 28d are shown, the code plates being spaced apart in the focal plane parallel to the line foci generated by the filter 21, rather than being spaced apart perpendicularly to the line foci as shown in Figure 7a. As a result it is possible for the code plates to have transmissive slots arranged to transmit interleaved line frequencies.

A better method of overcoming this problem is to employ a guided mode resonance filter having a plurality of phase gratings of different periodicities embedded in it simultaneously so that multiple complete spectra are focused into different regions of the focal plane separated by a few millimetres, one spectrum being focused by each of the phase gratings. In this case an arrangement as shown in Figures 7 and 7a can be used with a complete frequency spectrum being focused into the region of each of the code plates 25a to 25d, thus allowing each code plate 25a to 25d to carry transmissive slots selecting any required wavelength, interleaving of the wavelengths transmitted by the different code plates being possible.

In any of the multiple gas sensors described above, the use of a single pyroelectric detector and electrical filter to separate out the signals corresponding to different gases could be replaced by employing four separate dedicated pyroelectric detectors each receiving only the radiation transmitted by one of the code plates 25a to 25d, there would then be no requirement for the code plates 25a to 25d to oscillate at different periodicities and they could all be driven by a single piezoelectric actuator.

In sensors employing multiple code plates some structure for blocking radiation passing between the code plates will be required, this is easily arranged and has been omitted in the figures for clarity.

An alternative approach to multiple gas sensing is shown in Figure 8 where another system able to sense four different gases simultaneously is shown.

Referring to Figure 8, a system based on that shown in Figure 6 is shown comprising a source 20, spherical mirror 22, a gas cell 11 and pyroelectric detector 15 as before. In this four gas sensor the infrared radiation from the source 20 is directed by the mirror 22 onto a guided mode resonance filter array 30. They array 30 comprises four separate guided mode resonance filters 30a, 30b, 30c and 30d arranged in parallel as shown in more detail in Figure 8a. The four guided mode resonance filters 30a to 30d have different filter periodicities so they generate spectra of infrared radiation in different regions of a common focal plane 23 separated by a few millimetres.A fixed code plate 31, shown in more detail in Figure 8b, is placed in the focal plane 23 and bears four separate groups of transmissive slots 31a to 31d, each corresponding to the region where the spectra of a respective one of the guided mode resonance filters 30a to 30d is focused. The infrared radiation transmitted by the four groups of transmissive slots 31a to 31d is incident on a condensing mirror 32 which directs it onto a pyroelectric detector 15.

In order to allow the signals produced by each of the different guided mode filters and corresponding to different gases to be distinguished, each of the guided mode resonance filters 30a to 30d in the array 30 moves periodically at a low frequency by oscillation of a semi-rigid cantilever 33a to 33d respectively linking it to a rigid support 34. This oscillation of each of the guided mode filters 30a to 30d is driven by a respective piezoelectric actuator 35a to 35d and the piezoelectric actuators 35a to 35d are each driven at different frequencies by different AC electrical signals so that each of the guided mode resonance filters 30a to 30d oscillates at a different frequency.

The oscillation of the guided mode resonance filters causes their focus spectra to move at a corresponding frequency by a small amount, effectively sweeping the spectra past the slots and generating an oscillating transmissive intensity as the frequency transmitted by each of the slots in the code plates changes. As a result the electrical signal generated by the pyroelectric detector will contain four different signals at four different frequencies, each corresponding to the concentration of a different gas in the gas cell, these four electrical signals can easily be separated, by spectra analysis by a filter bank or fast Fourier transform analyser for example, to provide separate signals at each of the different modulation frequencies. The amplitude of the separated signals can then be used to calculate the concentrations of the gases of interest in the gas cell.

It will be obvious that the technique shown in Figure 8 of moving the guided mode resonance filter and holding the code plate stationary could be employed in any of the preceding examples instead of holding the filter steady and moving the code plate to modulate the output signal or vice-versa. Which of these techniques is used is a matter of design choice in each case. Similarly in the example of Figure 8 the four guided mode resonance filters used are all chirped guided mode resonance filters in order to eliminate the need for separate focusing optics, however if it was felt desirable fixed periodicity guided mode resonance filters could be used together with appropriate focusing optics.The use of a single pyroelectric detector 15 and condensing mirror 32 in the system of Figure 8 could of course be replaced by the use of four separate pyroelectric detectors each generating a signal corresponding to concentration of a different gas in the gas cell.

It will be clear to the reader that various features used in different ones of the examples described could be combined in different combinations to produce a wide range of gas analysers.

Instead of guided mode resonance filters similar systems can be produced employing infrared grating spectrometers. These operate in a similar manner to guided mode resonance filters but require the use of collimated light from a source, as a result the guided mode resonance filter will generally be preferred because of the much higher optical throughput which can be achieved.

Referring to Figure 9 a spectrographic gas analyser employing a grating spectrometer is shown. An incandescent infrared source 40 formed by an electrically heated ceramic element emits infrared radiation which is focused by a focusing mirror 41 onto a slit 42 the infrared radiation transmitted through the slit 42 is incident on a reflective region 43a of a Littrow mounted grating 43. The reflected radiation passes into a gas cell 44 through an infrared transmissive window 44a, travels the length of the gas cell 44 and is incident on a parabolic mirror 45 which focuses the infrared radiation into a collimated beam which passes for a second time along the length of the gas cell 44 and illuminates the grating 43.The grating 43 is inclined at an angle of 45 to this collimated beam and as a result the principal diffracted beam is returned parallel to the incident beam from the mirror 45, this diffracted beam passes back along the length of the gas cell 44 for a third time and is reflected from the parabolic mirror 45, passes along the gas cell 44 for a fourth time, passes through a small reflective region 43b on the grating 43 and is brought to a focus in a focal plane 46.

Because of the presence of the Littrow mounted grating 43 in the optical system the infrared radiation focused into the focal plane 46 is refocused into a continuous line spectrum in the plane of the input slit 42. A code plate 47 is placed in the focal plane 46 bearing transmissive slots at positions where light at wavelengths corresponding to absorption lines of the gas it is desired to sense are focused so that only infrared radiation at these frequencies is transmitted through the code plate 47 to a pyroelectric detector 48.

In order to provide an AC output signal from the pyroelectric detector 48 the code plate 47 is oscillated periodically in the focal plane by a piezoelectric actuator 49 driven by a low frequency signal source 50. An electrical filter arrangement 51 is also supplied with the signal from the low frequency source 50 allowing it to use synchronous detection techniques to extract the AC signal at the same frequency from the output of the pyroelectric detector 48 and supply it as an output along lead 52 having an amplitude proportional to the concentration of the gas being sensed within the gas cell 44.

The Littrow mounted prism 43 is shown mounted at an angle of 450 to the axis of the gas cell 44 and parabolic mirror 45 but other large angles could also be used.

The folded optical path shown in Figure 9 shows the infrared radiation to pass four times along the length of the gas cell 44 and so increases the sensitivity of the system, but many other optical arrangements could clearly be used to produce the same line output spectrum with the infrared radiation passing a different number of times through the gas cell 44.

Although the use of a Littrow mounted grating is useful grating spectrometers mounted in other ways could be used instead.

In order to increase the optical throughput of a grating spectrometer system of this type multiple input slits could be used but the analysis of the output spectra of such systems can be difficult because the output is the cross-correlation of the input and output slit patterns, so unless input and output slit patterns having slits spaced by the same regular amounts are used this will generally not be a preferred technique.

Another approach to high resolution infrared spectroscopy is to employ a Fabry Perot etalon.

A Fabry-Perot etalon consists of a pair of plane parallel semi-transparent mirrors separated by a dielectric medium and, as is well known, the etalon has regularly spaced transmission maxima at wavelengths for which the mirror spacing is equal to a half integral multiple of the wavelength in the dielectric medium.

If infrared radiation is filtered by a Fabry-Perot etalon the resulting transmitted frequency spectrum will comprise a plurality of discrete line frequencies separated periodically in wavelength by a fixed amount as shown in Figure 1 Ob. As described earlier the absorption bands of gases tend to be formed by a plurality of equally spaced absorption lines, a carbon monoxide absorption band is shown in Figure 1 0a as an example. By arranging for the periodic separation and wavelength of the transmission maxima of a Fabry-Perot etalon to coincide with the periodically spaced absorption lines of a gas of interest a highly sensitive gas sensor can be produced.

Referring to Figure 11 an incandescent infrared source 60 formed by an electrically heated ceramic element emits infrared radiation and this is collimated by a parabolic mirror 61. A conventional narrow band filter 62 placed in front of the incandescent source 60 limits the emitted radiation to a frequency band corresponding to the absorption band of the gas of interest. The collimated radiation is then incident normally on a Fabry-Perot etalon 63. The Fabry-Perot etalon 63 is formed by a plane parallel plate of infrared transparent material such as silicon with reflective multi-layer dielectric coatings on each of its plane faces. The radiation transmitted through the etalon 63 comprises a plurality of discrete frequencies with a fixed spacing between them in the transmission band of the conventional filter 62.The dielectric constant and thickness of the Fabry-Perot etalon 63 is chosen such that these discrete frequencies correspond to the evenly spaced absorption maxima of the gas to be sensed. This transmitted radiation passes through a gas cell 64 and is then focused by a focusing mirror 65 onto a pyroelectric detector 66.

In order to provide an AC modulated radiation intensity so that it can be detected by the pyroelectric detector 66 the Fabry-Perot etalon 63 is periodically oscillated through a small angle by an oscillating drive 67 oscillating at a frequency controlled by a low frequency source 68.

When a Fabry-Perot etalon is tilted away from the normal, relative to the collimated incident radiation, the wavelengths of the discrete spectral lines in the transmitted radiation change because the apparent thickness of the etalon (that is, the thickness of etalon material which the radiation passes through) varies. By arranging the angle through which the etalon is tilted to be such that the discrete frequency lines of the transmitted radiation are moved from absorption lines in the absorption band of the gas of interest to frequencies between these absorption lines forming absorption minima the radiation intensity incident on the pyroelectric detector 66 can be caused to oscillate.

It has been calculated that where a silicon etalon is used to generate a filter spectrum with transmission lines corresponding to the 10 nm separation between the principal absorption lines in the spectrum of nitrous oxide the required angular tilt would be some 8.5 .

The low frequency signal from the low frequency source 68 driving the oscillating drive 67 is supplied to an electrical filter circuit 69 which filters the output of the pyroelectric detector 66 to extract the AC signal at the frequency of the low frequency source 68 whose amplitude will correspond to the concentration of the gas of interest in the gas cell 64 and provides this as an output along line 70.

The narrow band filter used to limit the range of frequency lines received by the pyroelectric detector could of course be placed at any other convenient position along the transmitted radiation path from the source to the pyroelectric detector.

Referring to Figure 12 a detector able to sense four gases simultaneously using the oscillating Fabry-Perot etalon technique is shown. This system is substantially similar to the system shown in Figure 11 for sensing a single gas and comprises an incandescent source 60, parabolic mirror 61 and gas cell 64 as shown in the system of Figure 11. In order to allow four gases to be sensed simultaneously a stack 71 of four Fabry-Perot etalons 71a to 71d is used, the entire stack 71 being oscillated together by an oscillating drive 67 controlled by low frequency source 68. Each of the Fabry-Perot etalons 71 a to 71d generates its own transmission spectrum of periodically spaced discrete line frequencies, the spacing and value of these line frequencies corresponding to the absorption lines of a different gas for each of the four etalons 71a to 71 d.Because the radiation incident on the etalon stack 71 has been collimated by the parabolic mirror 61 the four transmitted beams generated by the four etalons 71 a to 71d travel in parallel through the gas cell 64 to a filter stack 72 comprising four separate conventional narrow band filters 72a to 72d each having one of the transmitted beams from the corresponding one of the etalons 71 a to 71d incident on it. Each narrow band filter 72a to 72d limits the radiation transmitted through it to a frequency band corresponding to an absorption band of the gas it is intended to detect.

The radiation transmitted through the filter stack 72 is then incident on a stack of focusing mirrors 73 comprising four facets 73a, 73b, 73c and 73d each arranged to receive the radiation transmitted by a respective one of the narrow band filters 71 a to 71 d and focus it onto a respective one of four pyroelectric detectors 74a to 74d.

Thus, each of the pyroelectric detectors 74a to 74d produces an electrical output including an AC signal at the frequency of the low frequency source 68 and having an amplitude corresponding to the concentration of a different gas in the gas cell 64. The output of each of the pyroelectric detectors 74a to 74d is supplied to a corresponding locking amplifier 75a to 75d. All of the locking amplifiers 75a to 75d are supplied with the signal from the low frequency source 68 and they use this to separate the AC signal corresponding to gas concentration from their respective pyroelectric detector 74a to 74d and supply this as an output along a line 76a to 76d respectively.

A plurality of parallel narrow band filters, mirrors and pyroelectric detectors are used because all four Fabry-Perot etalons are oscillated at the same frequency. It would of course be possible to oscillate the Fabry-Perot etalons at different frequencies and use a single detector and a filter array to separate the signals at different frequencies corresponding to different gas concentrations in the gas cell but generally the additional mechanical complexity of this arrangement will outweigh the benefits particularly since it will normally be necessary to employ a plurality of separate narrow band filters in parallel in order to limit the range of discrete frequencies transmitted by each Fabry-Perot interferometer so that they do not include frequencies corresponding to absorption lines of gases other than those they are intended to detect.

The disadvantage of the Fabry-Perot etalon based systems compared to the grating and filter systems is that the etalon produces a spectrum of evenly spaced line frequencies, which may not be convenient, whereas the grating and filter systems can select line frequencies having any desired separations.

Claims (5)

1. An infrared spectroscopic gas analyser comprising a broad band infrared radiation source, a gas cell, an infrared sensor and a high resolution filter arranged such that the infrared radiation passing from the radiation source and through the gas cell to the infrared sensor is limited by the high resolution filter to a plurality of discrete frequencies corresponding in frequency to absorption lines of a gas to be sensed by the analyser.

2. A gas analyser as claimed in claim 1 in which the high resolution filter is a Fabry-Perot etalon.

3. A gas analyser as claimed in claim 1 in which the high resolution filter is a diffraction grating.

4. A gas analyser as claimed in claim 1 in which the high resolution filter is a guided mode resonance filter.

5. An infrared spectroscopic gas analyser substantially as shown in or as described with reference to the accompanying Figures.

5. An infrared spectroscopic gas analyser substantially as shown in or as described with reference to the accompanying Figures.

Amendments to the claims have been filed as follows 1. An infrared spectroscopic gas analyser comprising a broad band infrared radiation source, a gas cell, an infrared sensor and a high resolution filter arranged such that the infrared radiation passing from the radiation source and through the gas cell to the infrared sensor is limited by the high resolution filter to a plurality of discrete frequencies corresponding in frequency to absorption lines of a gas to be sensed by the analyser.

2. A gas analyser as claimed in claim 1 in which the high resolution filter is a Fabry-Perot etalon.

3. A gas analyser as claimed in claim 1 in which the Fabry-Perot etalon is tilted relative to the infrared radiation to alter the discrete frequencies of the infrared radiation passing from the radiation source to the infrared sensor.

4. A gas analyser as claimed in claim 1 in which the high resolution filter is a guided mode resonance filter.