GB2391310A

GB2391310A - Gas sensors

Info

Publication number: GB2391310A
Application number: GB0313471A
Authority: GB
Inventors: Stanley Desmond Smith; Alexander Vaas
Original assignee: EDINBURGH INSTR
Current assignee: EDINBURGH INSTR
Priority date: 2002-06-11
Filing date: 2003-06-11
Publication date: 2004-02-04
Also published as: GB0313471D0; GB0213326D0

Abstract

A gas sensor using the principle of infra-red (IR) absorption is disclosed in which the optical source 10. emits IR radiation into a gas cell 18. The IR radiation is reflected and focussed such that said IR radiation makes multiple passes within the gas cell before reaching the detector 12. Further aspects of the invention include multiple focussing reflectors, diffraction gratings and flame-proof housing for all or part of the sensor.

Description

1 "Gas Sensors" 3 The invention relates to gas sensors using the 4

principle of infra red (JR) absorption by which the 5 intensity of IR radiation transmitted through the 6 gas is reduced according to the concentration of the 7 gas being measured. The invention relates more 8 particularly to improvements in the arrangement of 9 reflector/focussing elements in such sensors.

10 Certain aspects of the invention are concerned ll particularly with the use of diffraction elements to 12 replace some or all of the conventional optical 13 reflectors employed in such sensors.

15 It is well known to measure the concentration of a 16 gas by measuring the absorption of IR radiation 17 passing through a volume of the gas between an IR 18 source and one or more detectors. It is also known 19 to use arrangements of reflectors to increase the 20 optical path length between the IR source and the 21 detector by multiple reflections of the IR radiation

1 through the gas volume, so as to reduce the size of 2 the sensor device.

4 The present invention provides gas sensors with 5 improved optical and source/detector arrangements.

7 In accordance with a first aspect of the invention, a gas sensor comprises a gas cell having a first end 9 and a second end, an optical source and at least one 10 detector sensitive to radiation from the source 11 disposed outside and adjacent to the first end of 12 the gas cell, a window at the first end of the gas 13 cell in front of the source and detector, the window 14 having a reflective surface adapted to reflect 15 radiation within the gas cell and having apertures 16 transparent to radiation of selected wavelengths 17 aligned with each of the source and detector, and at 18 least one focussing reflector at the second end of 19 the gas cell and defining at least one optical axis 20 extending from the first end to the second end of 21 the gas cell, the source and detector being offset 22 from the optical axis such that radiation from the I 23 source makes multiple passes between the window and 24 the reflector before reaching the detector.

26 In certain embodiments, the focussing reflector 27 comprises a curved (preferably spherical) mirror or 28 an equivalent diffractive optical component, and the 29 source and detector are located at or close to the 30 focal plane of the reflector and disposed 31 symmetrically on opposite sides of the optical axis.

32 Diffractive optical components for this purpose may

( 1 comprise a Fresnel mirror or lens or an angled or 2 curved diffraction grating.

4 In other embodiments, the focussing reflectors 5 comprise a plurality of curved mirrors or equivalent 6 diffractive optical components arranged in a row 7 across the width of the gas cell, and the source and 8 detector are located on a plane at or close to twice 9 the focal length of the reflectors, the source being 10 offset to the outside of the optical axis of the 11 reflector closest to one side of the gas cell and 12 the detector being offset to the outside of the 13 optical axis of the reflector closest to the 14 opposite side of the gas cell.

16 In still other embodiments, the focussing reflector 17 and the reflective surface of the window each 18 comprises a curved (preferably spherical) mirror or 19 equivalent diffractive optical component.

21 In each of the various embodiments, the window may 22 be formed from a material that is substantially I 23 transparent to wavelengths of interest, with a 24 reflecting coating on either its inner or outer 25 surface. A diffraction grating may also be formed 26 on either the inner or outer surface and the 27 diffraction grating may be set at a fixed or 28 adjustable angle.

30 In accordance with another aspect of the invention, 31 there is provided a gas sensor of the type 32 comprising a gas cell, an optical source, at least

1 one detector sensitive to radiation from the source, and an optical system for directing radiation from 3 the source through the gas cell to the detector, in 4 which at least one component of the optical system 5 comprises a diffractive optical component.

7 In accordance with still another aspect of the 8 invention, there is provided a gas sensor comprising 9 a gas cell having a first end and a second end, an 10 optical source and at least one detector sensitive 11 to radiation from the source disposed outside and 12 adjacent to the first end of the gas cell, a window 13 at the first end of the gas cell in front of the 14 source and detector, the window having a reflective 15 surface adapted to reflect radiation within the gas 16 cell and having apertures transparent to radiation 17 of selected wavelengths aligned with each of the 18 source and detector, said window forming part of a 19 flame-proof housing isolating the source and 20 detector from the gas cell.

22 Embodiments of the invention will now be described, I 23 with reference to the accompanying drawings in 24 which: 26 Fig. 1A is a schematic longitudinal cross section of 27 a first embodiment of a gas sensor in accordance 28 with the invention, Fig. 1B is an end view 29 illustrating the configuration of a mirror at one 30 end of a gas cell of the sensor and Fig. 1C is an 31 end view illustrating the configuration of a window 32 at the other end of the gas cell;

( 2 Fig. 2A is a schematic longitudinal cross section of 3 a second embodiment of a gas sensor in accordance 4 with the invention, Fig. 2B is an end view 5 illustrating the configuration of mirrors at one end 6 of a gas cell of the sensor and Fig. 2C is an end 7 view illustrating the configuration of a window at 8 the other end of the gas cell; 10 Fig. 3A is a schematic longitudinal cross section of 11 a third embodiment of a gas sensor in accordance 12 with the invention, Fig. 3B is an end view 13 illustrating the configuration of a mirror at one 14 end of a gas cell of the sensor and Fig. 3C is an IS end view illustrating the configuration of a mirror 16 at the other end of the gas cell; 18 Fig. 4 is a schematic longitudinal cross section of 19 a fourth embodiment of a gas sensor in accordance 20 with the invention, similar to Fig. 1 but in which 21 the window comprises a diffraction grating; 22 1 23 Fig. 5 is a schematic longitudinal cross section of 24 a fifth embodiment of a gas sensor in accordance 25 with the invention, similar to Fig. 1 but in which 26 the mirror is replaced by a Fresnel lens and the 27 window comprises a diffraction grating; 29 Fig. 6 is a schematic longitudinal cross section of 30 a sixth embodiment of a gas sensor in accordance 31 with the invention, similar to Fig. 4 but in which

( 1 the mirror is replaced by a curved diffraction 2 grating; 4 Fig. 7 is a schematic longitudinal cross section of a seventh embodiment of a gas sensor in accordance 6 with the invention, similar to Fig. 4 but with a 7 different type of diffraction grating; 9 Fig. 8 is a schematic longitudinal cross section of 10 an eighth embodiment of a gas sensor in accordance ll with the invention, similar to Fig. 5 but with a 12 tunable angled diffraction grating; and 14 Fig. 9 is a schematic longitudinal cross section of 15 a ninth embodiment of a gas sensor in accordance 16 with the invention, similar to Fig. 8 but in which 17 the Fresnel lens and tunable diffraction grating are 18 each replaced by an angled diffraction grating.

20 Referring now to the drawings, Fig. 1 shows a first 21 embodiment of a gas sensor in which an infra red 22 source 10 and detector 12 are located within a flame 23 proof housing 14 behind a generally planar window 16 24 of material transparent to appropriate wavelengths 25 of infra red radiation (which might be glass, 26 quartz, etc.). The use of a flame proof housing in 27 this and other embodiments is preferred but not 28 essential. However, the arrangement of the flame 29 proof housing in this and other embodiments 30 comprises one aspect of the invention.

31 Alternatively, in any of the embodiments described 32 herein a flameproof housing could enclose the

! 1 entire device, including the Optics/gas cell as well 2 as the source/detector etc. The front of the window 3 16 faces the interior of a gas cell 18 and either 4 the front or rear surface is coated with reflecting 5 diffractive, metallic or dielectric coatings such as 6 aluminium or gold. The coating has apertures 20, 22 7 aligned with the source 10 and detector 12 8 respectively to permit the passage of IR radiation 9 through the window 16.

11 A curved (preferably spherical) mirror 24 is 12 disposed opposite the front surface of the window 16 13 at the other end of the gas cell 18. The optical 14 axis 26 of the mirror 24 is substantially 15 perpendicular to the plane of the window 16 and the 16 mirror 24 is spaced from the source 10 and detector 17 12 by a distance substantially equal to the focal 18 length of the mirror (i e. the source 10 and 19 detector 12 lie substantially on the focal plane of 20 the mirror). The source 10 and detector 12 are 21- located close to the focal point of the mirror 24, 22 offset from the optical axis by substantially equal 23 distances on opposite sides thereof.

25 Because the source 10, is close to the focal point 26 of the mirror 24, radiation from the source 27 propagates through the first aperture 20 to fill the 28 mirror 24, thus providing a return beam which is 29 substantially collimated or parallel but at a slight 30 angle offset from the optical axis 26 as determined 31 by the offset of the source 10. The offset parallel 32 return beam falls upon the entirety of the window

1 16, returns again to the mirror 24 and is then -

2 substantially focussed through the second aperture 3 22 in the window 16 onto the detector 12 (or a 4 multiplicity of detectors) which may, for example, 5 be a pyro-electric detector or a thermopile or 6 semiconductor detector. Radiation from the source 7 1O thus travels from the source to the mirror 24, 8 back to the window 16, again to the mirror 24 then 9 back to the detector 12 making four passes through 10 the gas cell 18. The sensor thus contains an 11 absorption path four times the length between the 12 window 16 and mirror 24, thus providing a gas sensor 13 that is sensitive while having a small volume to 14 path-length ratio.

16 In the preferred embodiments, and in accordance with 17 one aspect of the invention, the window 16 itself is 13 constructed in a robust manner so that it forms one 19 end of the flame-proof enclosure 14 which includes -

20 the source 10 and detector(s) 12. Some electronic 21 components may also be contained within this 22 enclosure, provided with a suitable set of pin out 23 plugs. -

25 In this and other embodiments a second detector (not 26 shown) with a spectral coverage not significantly 27 including the gas absorption band can be included to 28 give a double beam ratio measuring system. One 29 embodiment of this can be a similar infra red 30 detector with an interference filter designed for 31 this purpose. The second embodiment can use a

( 1 semiconductor detector again operating at different -

2 wavelengths from the gas absorption.

4 Fig. 2 shows a second embodiment of a gas sensor 5 using similar absorption measuring principles to the 6 first embodiment. This includes a source 210, 7 detector(s) 212, housing 214, window 216, gas cell 8 218 and apertures 220 and 222 as before. In this -

9 case, the optical system is modified to include two 10 or more curved (preferably spherical) mirrors 224 11 arranged in a row across the width of the sensor, 12 and the source 210, detector 212 and corresponding 13 apertures 220, 222 are located close to the 14 periphery of the window on opposite sides of the 15 central axis of the sensor. The mirrors 224, -

16 source 210 and detector are so aligned that 17 radiation from the source makes multiple passes (in 18 excess of the four passes of the first embodiment) 19 through the gas cell 218, thus lengthening the 20 absorption path.

22 The source 210 and detector 212 are positioned at a 23 distance from the mirrors 224 substantially equal to 24 twice the focal length of the mirrors 224 and offset 25 to the outside of the optical axes of the outermost 26 mirrors 224 at either side of the sensor so that the 27 radiation from the source 210 traverses the width of 28 the sensor from the source 210 to the detector 212 29 as it is reflected back and forth between the 30 mirrors 224 and the window 216.

( 1 Fig. 3 shows a third embodiment using the same -

2 principles as the first, including the source 310, 3 detector 312, etc., but uses a window 316 in the 4 form of a curved (preferably spherical) mirror 5 instead of a planar mirror, with the source 310 and 6 detector 312 located close to the periphery of the 7 window 316 and with a lens 328 in front of and a reflector 329 behind the source 310 to provide a 9 collimated source output beam. The optical system 10 in this case operates on the principle of a Herriot 11 cell or White cell in which multiple passes are 12 focussed at the mirrors and proceed stepwise around 13 the circumference (as indicated by the shaded 14 circles 330 and 332 in Figs. 3B and 3C) or across 15 the surface giving multiple passes in a small 16 volume.

18 Figs. 4 to 9 illustrate further embodiments that 19 operate on similar principles to the first 20 embodiment, but in which one or both of the mirror 21 24 and window 16 is replaced by/comprises 22 diffractive optical components working on principles 23 of optical diffraction and the Fresnel lens / -

24 mirror.

26 The fourth embodiment shown in Fig. 4 is similar to 27 the first, but the plane infra red window 16 of Fig. 28 1 is replaced by a window 416 comprising a 29 diffraction grating 428 having reflective coating(s) 30 and IR-transparent apertures 420, 422. In this case 31 the diffraction grating 428 comprises a surface 32 diffraction pattern formed on the rear surface of

( l the window 316 (protecting it from contamination 2 from within the gas cell 418). However, as shown in 3 Fig. 7 (seventh embodiment, and also in the 4 embodiments of Figs. 5, 6 and 8), the diffraction 5 grating 728 could be formed on the front surface of 6 the window 716.

8 The two apertures 420 and 422 for the source 410 and 9 detector 412 are made in the diffraction pattern and 10 the beam again propagates to a curved (preferably I 11 spherical) mirror 424 and is reflected in a pseudo 12 collimated manner as in Fig. 1, but benefits from 13 the wavelength selection that can be designed into 14 the diffraction grating 428. Devices can thus be 15 made gas-specific without the need for optical 16 filters in front of the detectors.

18 In the fifth embodiment of Fig. 5, a Fresnel-like I l9 mirror or lens 524 is used instead of the curved 20 mirror of Fig. 1. This is a reflective/difEractive 21 optical device which, by having reflecting surfaces 22 with plane, inclined or curved steps, causes 23 interference between the reflected beams such that 24 the planar device operates as a spherical concave 25 mirror. The same geometry as in Fig. 1 above 26 applies with an offset source 510 and detector 512 27 and apertures 520 and 522 in a transparent material 28 window 516 which again forms the end of a flameproof 29 housing 514 for source and detector. The i 30 diffractive mirror 524 may have up to 16 'steps' 31 giving efficiencies as high as 90 or more and may T...1._

( 1 be replicated, cheaply, from a 'master' using I 2 techniques established in VLSI etc. 4 In this embodiment the Fresnel mirror may be made 5 out of plastic material coated with aluminium or 6 gold, etc. which may be reproduced by replica from a 7 master solid material pattern.

9 Fig. 6 shows a sixth embodiment similar to Figs. 1 10 and 7 except that the curved mirrors are replaced by I 11 a curved (preferably spherical) diffraction grating 12 624.

14 Fig. 8 shows an eighth embodiment, similar to the 15 fifth, except that the window,316 is adjustable 16 (tunable) to vary its angle to the optical axis 826 17 of the Fresnel lens 824. Varying the angle of the 18 diffraction grating 828 allows the sensor to be I 19 tuned to particular wavelengths.

21 Fig. 9 shows a ninth embodiment, similar to Fig. 8, 22 in which the Fresnel lens and tunable diffraction 23 grating are each replaced by parallel angled 24 diffraction gratings 924 and 928. In this case the 25 angles are fixed, being selected to suit particular 26 wavelengths. Using opposed diffraction gratings 27 with different periods also allows different free 28 spectral ranges to be selected.

29 i 30 The various embodiments provide multiple reflections 31 within the gas cell. The number of reflections can 32 be from 2 - n by design of the optics. This then

l gives an e-fold improvement in dispersion so that in 2 effect the miniature gas sensor behaves in a manner 3 analogous to a grating spectrometer which spreads 4 out the wavelengths. A consequence of the use of 5 diffractive optics for gas detection is that a 6 series of holes in the grating / window can be 7 arranged to put different wavelengths on to a series 8 of different detectors. The sensor can thus 9 simultaneously detect a number of gases. ' 10 1 11 Diffractive optical components can be used to 12 replace a variety of conventional curved reflective 13 or refractive components including spherical and 14 aspheric (cylinder, conic, toroid, tubular, 15 diffusive) mirrors and lenses. The special 16 attributes of diffractive optical components enable 17 them to be used to obtain multiple (including dual) 18 images from a single source, single images from I l9 multiple sources, or multiple images from multiple 20 sources, to provide wavelength selection and/or 21 polarization selection, and to use amplitude and/or 22 wavefront division. The optics may be made gas 23 selective by changing one or more optical parameters 24 including refractive index, polarization, 25 reflectivity, spectral reflectivity, transmission 26 and spectral transmission.

28 Diffractive optical components can be used in place 29 of any or all of the mirrors in the embodiments of i 30 Figs. 1 to 3 and may also be employed advantageously 31 in the optical systems of gas sensors other than 32 those particularly described herein.

( 2 Improvements and modifications may be included 3 without departing from the scope of the invention.

Claims

( 1 Claims

3 1. A gas sensor comprising a gas cell having a first 4 end and a second end, an optical source and at 5 least one detector sensitive to radiation from 6 the source disposed outside and adjacent to the 7 first end of the gas cell, a window at the first 8 end of the gas cell in front of the source and 9 detector, the window having a reflective surface 10 adapted to reflect radiation within the gas cell 11 and having apertures transparent to radiation of 12 selected wavelengths aligned with each of the 13 source and detector, and at least one focussing 14 reflector at the second end of the gas cell and 15 defining at least one optical axis extending from 16 the first end to the second end of the gas cell,

17 the source and detector being offset from the 18 optical axis such that radiation from the source 19 makes multiple passes between the window and the 20 reflector before reaching the detector.

22
2. A gas sensor as claimed in claim 1, wherein the 23 at least one focussing reflector comprises a 24 curved mirror or an equivalent optical component.

26
3. A gas sensor as claimed in claim 2, wherein the 27 equivalent optical component is a Fresnel mirror 28 or lens or an angled or curved diffraction 29 grating. 31
4. A gas sensor as claimed in claim 2, wherein the 32 curved mirror is a spherical curved mirror.

(
5. A gas sensor as claimed in any preceding claim, 3 wherein the source and detector are located at or 4 close to the focal plane of the focussing 5 reflector. 7
6. A gas sensor as claimed in claim 5, wherein the 8 source and detector are located on opposite sides 9 of the optical axis.

11
7. A gas sensor as claimed in claim 1, wherein the 12 focussing reflectors comprise a plurality of 13 curved mirrors or equivalent optical components 14 arranged in a row across the width of the gas 15 cell. 17
8. A gas sensor as claimed in claim 7, wherein the 18 source and detector are located on a plane at or 19 close to twice the focal length of the focussing 20 reflectors. 22
9. A gas sensor as claimed in claim 8, wherein the 23 source is offset to the outside of the optical 24 axis of the reflector closest to one side of the 25 gas cell and the detector is offset to the 26 outside of the optical axis of the reflector 27 closest to the opposite side of the gas cell.

29
lO.A gas sensor as claimed in claim 1, wherein the 30 focussing reflector and the reflective surface of 31 the window each comprises a curved mirror or 32 equivalent optical component.

2
11. A yes sensor as claimed in claim 9, wherein the 3 curved mirror is a spherical curved mirror.

5
12. A gas sensor as claimed in any preceding claim, 6 wherein the window is formed from a material that 7 is substantially transparent to wavelengths of 8 interest. 10
13. A gas sensor as claimed in claim 12, wherein the 11 material has a reflective coating on its inner 12 and/or outer surface.

14
14. A gas sensor as claimed in claim 12, wherein the 15 material has a diffraction grating formed on its 16 inner or outer surface.

18
15. A gas sensor as claimed in claim 14, wherein the 19 diffraction grating is set at a fixed angle to 20 said optical axis.

22
16. A gas sensor as claimed in claim 13, wherein the 23 diffraction grating is set at an adjustable angle 24 to said optical axis.

26
17. A gas sensor as claimed in any preceding claim, 27 wherein the optical source and the at least one 28 detector are located within a housing which is 29 substantially flame proof.

31
18. A gas sensor as claimed in claim 17, wherein the 32 housing also incorporates the gas cell.

2
19. A gas sensor comprising a gas cell, an optical 3 source, at least one detector sensitive to 4 radiation from the source, and an optical system 5 for directing radiation from the source through 6 the gas cell to the detector, in which at least 7 one component of the optical system comprises a 8 diffractive optical component.

10
20. A gas sensor comprising a gas cell having a 11 first end and a second end, an optical source and 12 at least one detector sensitive to radiation from 13 the source disposed outside and adjacent to the 14 first end of the gas cell, a window at the first 15 end of the gas cell in front of the source and 16 detector, the window having a reflective surface 17 adapted to reflect radiation within the gas cell 18 and having apertures transparent to radiation of l9 selected wavelengths aligned with each of the 20 source and detector, said window forming part of 21 a flame-proof housing isolating the source and 22 detector from the gas cell.