Highly-folding pendular optical cavity

Abstract

An optical cavity includes: a first elliptical mirror, having a first focal axis A.sub.1, and designed to reflect a light beam emitted by a light source; a second elliptical mirror, having a second focal axis A.sub.2; a third elliptical mirror, having a third focal axis A.sub.3, the light beam exiting from the third elliptical mirror being designed to be received by a detector; a first reflector, arranged to reflect the light beam exiting from first elliptical mirror in the direction of the second elliptical mirror, and arranged to reflect the light beam exiting from second elliptical mirror in the direction of the third elliptical mirror; the first, second and third elliptical mirrors being arranged so that A.sub.1, A.sub.2 and A.sub.3 have a point of intersection F, corresponding to a focus common to the first, second and third elliptical mirrors.

Claims

1. Optical cavity comprising: a first elliptical mirror having a first focal axis noted A.sub.1, and designed to reflect a light beam emitted from a first point toward a first reflector; a second elliptical mirror having a second focal axis noted A.sub.2; and a third elliptical mirror having a third focal axis noted A.sub.3, a light beam exiting from the third elliptical mirror being designed to be received at a second point; wherein the first reflector is arranged to reflect a light beam coming from the first elliptical mirror toward the second elliptical mirror, and arranged to reflect a light beam coming from the second elliptical mirror toward the third elliptical mirror; and wherein the first, second and third elliptical mirrors being arranged so that A.sub.1, A.sub.2 and A.sub.3 have a point of intersection, noted F, corresponding to a focus common to the first, second and third elliptical mirrors.

2. Optical cavity according to claim 1, wherein A.sub.2 and A.sub.3 are identical.

3. Optical cavity according to claim 1, wherein a half-line FA.sub.3 designed to be directed towards the detector and a half-line FA.sub.1 designed to be directed towards the light source form an angle, noted a, comprised between 45 and 120.

4. Optical cavity according to claim 3, wherein the angle is comprised between 45 and 90.

5. Optical cavity according to claim 4, wherein the angle is equal to 90.

6. Optical cavity according to claim 1, further comprising a second reflector arranged to reflect the light beam coming from the second elliptical mirror toward the first reflector; the first reflector being arranged to reflect the light beam coming from the second reflector toward the third elliptical mirror.

7. Optical cavity according to claim 6, wherein the second reflector is a plane mirror.

8. Optical cavity according to claim 6, wherein the second reflector extends as a continuation of the first elliptical mirror, in a longitudinal direction parallel to A.sub.1, and presents a dimension, noted d.sub.2, in the longitudinal direction and starting from the second focal axis verifying: $d_2\frac{b^2}{2a}$ where: a is a semi-major axis of the second elliptical mirror, b is a semi-minor axis of the second elliptical mirror.

9. Optical cavity according to claim 1, wherein the first reflector is a plane mirror.

10. Optical cavity according to claim 1, wherein the first reflector extends along the second focal axis A.sub.2 and presents a dimension, noted d.sub.1, along A.sub.2 verifying:
d.sub.12c where c is a distance between the centre of the second elliptical mirror and the common focus F.

11. Optical cavity according to claim 1, wherein the second elliptical mirror has an ellipticity verifying 0<<0.25.

12. Optical cavity according to claim 1, further comprising two opposite reflecting ends designed to reflect the light beam and separated by the first, second and third elliptical mirrors and by the first reflector so as to form a waveguide.

13. Spectroscopic sensor comprising: an optical cavity according to claim 1, the first elliptical mirror presenting a focus, noted F.sub.1, different from the common focus F, the third elliptical mirror presenting a focus, noted F.sub.3, different from the common focus F; a light source designed to emit a light beam and arranged at focus F.sub.1; a detector arranged at focus F.sub.3 to receive the light beam exiting from the third elliptical mirror.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages will become apparent from the detailed description of the different embodiments of the invention, the description being accompanied by examples and referring to the appended drawings.

(2) FIG. 1 is a schematic view in transverse cross-section of a spectroscopic sensor comprising a optical cavity according to a first embodiment of the invention. The dotted lines extending the elliptical mirrors are construction lines illustrating a semi-ellipse. The dotted and dashed lines correspond to the focal axes of the elliptical mirrors.

(3) FIG. 2 is a schematic view in transverse cross-section of a sensor, comprising an optical cavity according to a second embodiment of the invention.

(4) FIG. 3 is a similar view to FIG. 2, without plotting of the light beams, illustrating the geometric parameters of the optical cavity.

(5) What is meant by transverse is a direction perpendicular to the direction (noted Z) passing through the thickness of the optical cavity. The cutting plane is noted (X, Y) in the figures and corresponds to the plane of the optical cavity.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(6) Identical parts or parts performing the same function will bear the same reference numerals for the different embodiments, for the sake of simplification.

(7) One object of the invention is to provide an optical cavity 1 comprising: a first elliptical mirror M.sub.1, having a first focal axis noted A.sub.1, and designed to reflect a light beam 2 emitted by a light source S; a second elliptical mirror M.sub.2, having a second focal axis noted A.sub.2; a third elliptical mirror M.sub.3, having a third focal axis noted A.sub.3, the light beam 2 exiting from third elliptical mirror M.sub.3 being designed to be received by a detector D; a first reflector R.sub.1, arranged to reflect light beam 2 exiting from first elliptical mirror M.sub.1 in the direction of second elliptical mirror M.sub.2, and arranged to reflect light beam 2 exiting from second elliptical mirror M.sub.2 in the direction of third elliptical mirror M.sub.3;

(8) first, second and third elliptical mirrors M.sub.1, M.sub.2, M.sub.3 being arranged so that A.sub.1, A.sub.2 and A.sub.3 have a point of intersection, noted F, corresponding to a focus common to first, second and third elliptical mirrors M.sub.1, M.sub.2, M.sub.3.

(9) Elliptical Mirrors

(10) The focuses of first, second and third elliptical mirrors M.sub.1, M.sub.2, M.sub.3 extend in the direction of the thickness of optical cavity 1 (direction Z). Each elliptical mirror M.sub.1, M.sub.2, M.sub.3 has a focal axis A.sub.1, A.sub.2, A.sub.3. The two focuses of an elliptical mirror M.sub.1, M.sub.2, M.sub.3 extend in a plane defined by two directions: a first direction corresponding to the focal axis of the corresponding elliptical mirror and a second direction Z in the plane of the thickness of the corresponding elliptical mirror, or in the plane of the thickness of optical cavity 1.

(11) A.sub.2 and A.sub.3 are advantageously identical. In other words, the planes in which the focuses of second and third elliptical mirrors M.sub.2, M.sub.3 extend are advantageously identical. In other words, the focuses of second and third elliptical mirrors M.sub.2, M.sub.3 are advantageously coplanar. As illustrated in FIGS. 1 to 3, focuses F, F.sub.2 of second elliptical mirror M.sub.2 and focuses F, F.sub.3 of third elliptical mirror M.sub.3 advantageously extend in the plane (X, Z).

(12) Half-line FA.sub.3 designed to be directed towards detector D and half-line FA.sub.1 designed to be directed towards light source S advantageously form an angle, noted a, comprised between 45 and 120. In other words, the planes in which the focuses of third and first elliptical mirrors M.sub.3, M.sub.1 extend form a dihedral angle , comprised between 45 and 120. Angle is advantageously comprised between 45 and 90, and preferentially equal to 90. As illustrated in FIGS. 1 to 3, plane (X, Z) in which focuses F, F.sub.3 of third elliptical mirror M.sub.3 extend preferentially forms a dihedral angle equal to 90 with plane (Y, Z) in which focuses F, F.sub.1 of first elliptical mirror M.sub.1 are located.

(13) In the embodiment illustrated in FIG. 1, second elliptical mirror M.sub.2 comprises first and second parts arranged on each side of first elliptical mirror M.sub.1. In other words, first elliptical mirror M.sub.1 is inserted between the first and second parts of second elliptical mirror M.sub.2. As can be seen in FIG. 1, the ellipse defined by second elliptical mirror M.sub.2 presents an axis of symmetry extending in direction Y. The first and second parts of second elliptical mirror M.sub.2 extend in direction X on each side of axis of symmetry . The first part of second elliptical mirror M.sub.2 is preferentially in contact with third elliptical mirror M.sub.3. The second part of second elliptical mirror M.sub.2 is preferentially in contact with first elliptical mirror M.sub.1. First elliptical mirror M.sub.1 is arranged relatively to second elliptical mirror M.sub.2 so as not to reflect light beam 2 exiting from the first part of second elliptical mirror M.sub.2. To do this, as illustrated in FIG. 1, let us consider: I.sub.2 the end of the first part of second elliptical mirror M.sub.2 situated facing first elliptical mirror M.sub.1, located on first focal axis A.sub.1, the axis passing through I.sub.2 and F.sub.2, I.sub.1 the end of the first elliptical mirror M.sub.1 situated facing first reflector R.sub.1, d.sub.2 the distance between I.sub.1 and second focal axis A.sub.2.

(14) First elliptical mirror M.sub.1 is advantageously arranged relatively to second elliptical mirror M.sub.2 so that I.sub.1 belongs to , with d.sub.2 verifying the relation

(15) $d_2=\frac{b^2}{2a},$
where a is the semi-major axis of second elliptical mirror M.sub.2 and b is the semi-minor axis of second elliptical mirror M.sub.2. First elliptical mirror M.sub.1 can thus collect a maximum amount of light rays originating from light source S (when light source S is not directive), without reflecting light beam 2 exiting from the first part of second elliptical mirror M.sub.2. First elliptical mirror M.sub.1 can be arranged relatively to second elliptical mirror M.sub.2 so that

(16) $d_2>\frac{b^2}{2a},$
with the drawback of a loss of the number of light beams received by first elliptical mirror M.sub.1 when light source S is not directive (e.g. isotropic) such as a heat source. This drawback can be overcome by using a directive light source S such as a quantum cascade laser.

(17) In the embodiment illustrated in FIG. 2, the second part of second elliptical mirror M.sub.2 is eliminated.

(18) For non-restrictive example purposes, the dimensions (a, b) of the ellipses of first, second and third elliptical mirrors M.sub.1, M.sub.2, M.sub.3 are set out in the table below:

(19) TABLE-US-00001 Dimension Mirror a (m) b (m) First elliptical mirror M.sub.1 79.6 48 Second elliptical mirror M.sub.2 100 90 Third elliptical mirror M.sub.3 45 15

(20) where a is the semi-major axis, and b is the semi-minor axis. Dimension c of the ellipses, which is the distance between the centre and a focus of the corresponding ellipse, can be calculated by means of the following formula: c={square root over (a.sup.2b.sup.2)}.

(21) The dimensions (a, b, c) of the ellipses of first, second and third elliptical mirrors M.sub.1, M.sub.2, M.sub.3 are chosen in particular according to the length of interaction I required between the fluid and light beam 2 when the envisaged application is a fluid sensor.

(22) Each elliptical mirror M.sub.1, M.sub.2, M.sub.3 advantageously has a reflection coefficient of light beam 2 greater than or equal to 75% for any angle of incidence. Said reflection coefficient is advantageously greater than or equal to 80%, preferentially greater than or equal to 85%, more preferentially greater than or equal to 90%, for any angle of incidence. Advantageously, the reflection coefficient of light beam 2 is greater than or equal to 95%, preferably greater than or equal to 98% for any angle of incidence smaller than 45. What is meant by reflection coefficient is the reflection coefficient in intensity, for an angle of incidence comprised between 0 (normal incidence) and 90 (grazing incidence), for a given wavelength and taking account of the arithmetically averaged polarisations s and p when light source S is a heat source. The angle of incidence is the angle between the direction of propagation of light beam 2 and the normal to the reflecting surface of the corresponding elliptical mirror.

(23) Second elliptical mirror M.sub.2 advantageously has an ellipticity verifying 0<<0.25.

(24) Optical cavity 1 is advantageously devoid of lenses.

(25) First Reflector

(26) First reflector R.sub.1 is advantageously a plane mirror. First reflector R.sub.1 preferentially extends in direction A.sub.2, and presents a dimension noted d.sub.1, along A.sub.2, verifying:
d.sub.12c

(27) where c is the distance between the centre of the second elliptical mirror and common focus F.

(28) In the embodiment illustrated in FIG. 1, first reflector R.sub.1 is arranged to reflect light beam 2 exiting directly from second elliptical mirror M.sub.2 to third elliptical mirror M.sub.3.

(29) More precisely, first reflector R.sub.1 is arranged to send light beam 2 exiting directly from second elliptical mirror M.sub.2 back to second elliptical mirror M.sub.2 until light beam 2 reaches third elliptical mirror M.sub.3. The successive reflections of light beam 2 inside second elliptical mirror M.sub.2 via first reflector R.sub.1 can be qualified as pendular reflections. In other words, the light rays successively reflected by second elliptical mirror M.sub.2 and by first reflector R.sub.1 move between one end and the opposite end of second elliptical mirror M.sub.2, while being flattened.

(30) Second Reflector

(31) In the embodiment illustrated in FIG. 2, optical cavity 1 advantageously comprises a second reflector R.sub.2 arranged to reflect light beam 2 exiting from second elliptical mirror M.sub.2 to first reflector R.sub.1. First reflector R.sub.1 is then arranged to reflect light beam 2 exiting from second reflector R.sub.2 to third elliptical mirror M.sub.3. First reflector R.sub.1 is arranged to reflect light beam 2 exiting indirectly (i.e. after reflection with second reflector R.sub.2) from second elliptical mirror M.sub.2 to third elliptical mirror M.sub.3. More precisely, first reflector R.sub.1 is arranged to send light beam 2 exiting indirectly from second elliptical mirror M.sub.2 back to second elliptical mirror M.sub.2 until light beam 2 reaches third elliptical mirror M.sub.3. The successive reflections of light beam 2 inside second elliptical mirror M.sub.2 via first reflector R.sub.1 and second reflector R.sub.2 can be qualified as pendular reflections. In other words, the light rays successively reflected by second elliptical mirror M.sub.2 and by first and second reflectors R.sub.1, R.sub.2 move between one end and the opposite end of second elliptical mirror M.sub.2, while being flattened.

(32) As stated above, in the embodiment illustrated in FIG. 2, the second part of second elliptical mirror M.sub.2 is eliminated and is replaced by second reflector R.sub.2.

(33) Second reflector R.sub.2 is advantageously a plane mirror. Second reflector R.sub.2 preferentially extends in the continuation of first elliptical mirror M.sub.1 in the longitudinal direction parallel to A.sub.1, and presents a dimension, noted d.sub.2, in the longitudinal direction and starting from second focal axis A.sub.2 verifying:

(34) $d_2\frac{b^2}{2a}$

(35) where: a is the semi-major axis of second elliptical mirror M.sub.2, b is the semi-minor axis of second elliptical mirror M.sub.2.

(36) Dimension d.sub.2 corresponds to the distance d.sub.2 between I.sub.1 and second focal axis A.sub.2 in the first embodiment illustrated in FIG. 1.

(37) Second reflector R.sub.2 is advantageously arranged so that d.sub.2 verifies the relation

(38) $d_2=\frac{b^2}{2a},$
where a is the semi-major axis of second elliptical mirror M.sub.2 and b is the semi-minor axis of second elliptical mirror M.sub.2. In this way, first elliptical mirror M.sub.1 can collect a maximum amount of light rays originating from light source S (when the light source is not directive), without reflecting light beam 2 exiting from second elliptical mirror M.sub.2.

(39) Second reflector R.sub.2 can be arranged so that

(40) $d_2>\frac{b^2}{2a},$
with the drawback of a loss of the number of light rays received by first elliptical mirror M.sub.1 when light source S is not directive (e.g. isotropic) such as a heat source. This drawback can be overcome by using a directive light source S such as a quantum cascade laser.
Waveguide

(41) Optical cavity 1 advantageously comprises two opposite reflecting ends (not visible in the figures) designed to reflect light beam 2 and arranged on each side of first, second and third elliptical mirrors M.sub.1, M.sub.2, M.sub.3 and of first reflector R.sub.1 so as to form a waveguide. When optical cavity 1 comprises a second reflector R.sub.2, the two reflecting ends are arranged on each side of first, second and third elliptical mirrors M.sub.1, M.sub.2, M.sub.3 of first reflector R.sub.1 and of second reflector R.sub.2 so as to form a waveguide. The two reflecting ends guide light beam 2 in direction Z of the thickness of optical cavity 1. First, second and third elliptical mirrors M.sub.1, M.sub.2, M.sub.3, first reflector R.sub.1, and second reflector R.sub.2 if applicable, join the two reflecting ends to one another.

(42) Each reflecting end comprises a reflecting surface having a reflection coefficient of light beam 2 greater than or equal to 80%, preferably greater than or equal to 85%, more preferentially greater than or equal to 90%, for any angle of incidence. Advantageously, the reflection coefficient of light beam 2 is greater than or equal to 95%, preferably greater than or equal to 98% for any angle of incidence smaller than 45. What is meant by reflection coefficient is the reflection coefficient in intensity, for an angle of incidence comprised between 0 (normal incidence) and 90 (grazing incidence), for a given wavelength and taking account of the arithmetically averaged polarisations s and p when light source S is a heat source. The angle of incidence is the angle between the direction of propagation of light beam 2 and the normal to the reflecting surface of the corresponding reflecting end. The reflecting surface of each reflecting end is preferentially flat. The flat reflecting surfaces of the reflecting ends are advantageously parallel.

(43) The reflecting surface of each reflecting end is preferentially made from a metallic material. The metallic material is preferably selected from the group comprising gold, silver, aluminium and copper. The reflecting surface of each reflecting end is advantageously coated with a protective layer to prevent corrosion of the metallic material. The protective layer is advantageously made from a material selected from the group comprising SiO.sub.2, SiN, Si.sub.3N.sub.4, a diamond-like carbon (DLC), polytetrafluoroethylene (PTFE), Pt, and TiN.

(44) The two reflecting ends are preferentially both manufactured in the form of a plate. The plates are advantageously provided with openings shaped to receive light source S and detector D when the plates are placed in contact and fixed to one another. The two reflecting ends advantageously form plane mirrors.

(45) Fabrication of the Optical Cavity

(46) A first fabrication method of an optical cavity 1 according to the invention comprises the following steps:

(47) a) providing first and second substrates made from a material, the material preferably being semi-conducting, more preferentially silicon;

(48) b) hollowing out both the first and second substrates so as to form a bottom and to keep a superficial part;

(49) c) assembling the first and second substrates so that: the superficial parts kept in step b) form first, second and third elliptical mirrors M.sub.1, M.sub.2, M.sub.3, first reflector R.sub.1, and second reflector R.sub.2 if applicable, the bottoms form the opposite reflecting ends of optical cavity 1.

(50) Step b) is advantageously performed by deep ion etching. Step b) is preferentially comprises a prior step consisting in depositing a photoresist on the surface of the first and second substrates. The recesses can then be obtained by photolithography and etching steps. Step b) is advantageously performed so that the recesses obtained enable flat bottoms to be formed. Hollowing out two substrates instead of one, in step b), enables the thickness of optical cavity 1 formed in step c) to be increased in order to reduce optical losses by reflection.

(51) The reflecting surface of the reflecting ends is advantageously formed by deposition of a metallic material on the bottoms of the first and second substrates, the deposition preferably being performed by cathode sputtering. Deposition of the metallic material can also be performed by vacuum evaporation or by electrolysis. Deposition of the metallic material is performed before step c).

(52) The reflecting surfaces of elliptical mirrors M.sub.1, M.sub.2, M.sub.3 and of reflectors R.sub.1, R.sub.2 are advantageously formed by deposition of a metallic material on a lateral edge of a superficial part, the deposition preferably being performed by cathode sputtering. Deposition of the metallic material can for example be performed by vacuum evaporation or by electrolysis. Deposition of the metallic material is performed before step c).

(53) The recesses of the first and second substrates obtained in step b) thereby enable two half-cavities to be obtained. Optical cavity 1 is formed in step c) by assembling the first and second substrates in order to join the two half-cavities. Forming a recess, for example by reactive ion etching, in a substrate made from semiconductor material is an inexpensive and viable solution as it enables an incline of the superficial parts relatively to the normal to the first and second substrates to be obtained that is typically about 1 to 2.

(54) A second method for fabricating an optical cavity 1 according to the invention comprises the steps:

(55) a) providing first and second moulds respectively comprising an imprint of first and second parts each comprising a base capped by a superficial part;

(56) b) injecting a plastic material in the first and second moulds so as to obtain the first and second parts;

(57) c) assembling the first and second parts so that: the superficial parts form first, second and third elliptical mirrors M.sub.1, M.sub.2, M.sub.3, first reflector R.sub.1, and second reflector R.sub.2 if applicable, the bases form the opposite reflecting ends of the optical cavity 1.

(58) Step a) is preferably executed so that the first and second moulds each comprise a fixed part and a movable part. Step b) is preferably performed using an injection press.

(59) The reflecting surface of the reflecting ends is advantageously formed by deposition of the metallic material on the bases of the first and second substrates, the deposition preferably being performed by cathode sputtering. Deposition of the metallic material can also be performed by vacuum evaporation or by electrolysis. Deposition of the metallic material is performed before step c).

(60) The reflecting surfaces of elliptical mirrors M.sub.1, M.sub.2, M.sub.3 and of reflectors R.sub.1, R.sub.2 are advantageously formed by deposition of the metallic material on a lateral edge of a superficial part, the deposition preferably being performed by cathode sputtering. Deposition of the metallic material can also be performed by vacuum evaporation or by electrolysis. Deposition of the metallic material is performed before step c).

(61) Optical cavity 1 is thus formed in step c) by assembling the first and second parts so as to join two half-cavities each of which is demarcated by the base and the corresponding superficial part. Plastic injection is an inexpensive and viable solution in so far as shape defects are not detrimental to imaging light source S correctly on detector D.

(62) Sensor

(63) One object of the invention is to provide a spectroscopic sensor 10 comprising: an optical cavity 1 in accordance with the invention, first elliptical mirror M.sub.1 presenting a focus, noted F.sub.1, different from the common focus F, third elliptical mirror M.sub.3 presenting a focus, noted F.sub.3, different from the common focus F; a light source S, designed to emit a light beam 2 and arranged at focus F.sub.1; a detector D, arranged at focus F.sub.3 to receive light beam 2 exiting from third elliptical mirror M.sub.3.

(64) The spectroscopic sensor 10 can be an infrared (e.g. non-dispersive) sensor to detect a fluid such as a gas. For non-restrictive example purposes, the gas can be selected from the group comprising carbon monoxide, carbon dioxide, at least one hydrocarbon, a hydrochlorofluorocarbon, a chlorofluorocarbon, nitrogen monoxide, nitrogen dioxide, sulphur dioxide, and ozone. The gas can also be selected from the following gases absorbing in a spectral absorption band comprised between 0.78 m and 12 m: HF, HCl, SO.sub.3, HBr, H.sub.2S, COS, C.sub.2H.sub.6, C.sub.3H.sub.8, C.sub.4H.sub.10, COCl.sub.2, BF.sub.3, CH.sub.4, HNO.sub.3, a volatile organic compound (e.g. C.sub.6H.sub.6, CH.sub.3COCH.sub.3), B.sub.2H.sub.6, CO, CS.sub.2, HCN, WF.sub.6, N.sub.2O, NH.sub.3, AsH.sub.3, a polycyclic aromatic hydrocarbon, benzene, toluene, the three xylene isomers, C.sub.2H.sub.4O, BCl.sub.3.

(65) The spectroscopic sensor 10 can also be a particle sensor or a biological sensor.

(66) In the case of a particle sensor, light source S preferentially emits in the visible range. Light source S can be of LED type. The operating principle is as follows. The particles generate waves diffused in optical cavity 1 at random angles, which will not be refocused onto the detector due to successive absorptions in particular by elliptical mirrors M.sub.1, M.sub.2, M.sub.3. This transmission loss measured by detector D is indicative of the type (index, size) and concentration of the particles in so far as the particles are generally not intrinsically absorbent.

(67) Light Source

(68) For non-restrictive example purposes, light source S can be a heat source or a quantum cascade laser. Light source S can be an infrared source. Light source S advantageously comprises an element, for example of filament type, in which an electric current is made to flow so that the element heats and emits an infrared radiation. The element presents a dimension, noted e, along the thickness of optical cavity 1 preferentially verifying:

(69) 100 meE1.5 mm, more preferentially 250 meE 1200 m

(70) where E is the thickness of optical cavity 1.

(71) The element preferentially presents the shape of a disk presenting a circular surface with a diameter of 250 m (corresponding to dimension e). The axis of light source S is defined as being the normal to the circular surface. For non-restrictive example purposes, the disk can present a thickness of 400 nm along the axis of light source S. The image of light source S (i.e. the disk-shaped element) is a rectangle presenting a width of 250 m and a height of 600 m (the height corresponding to the direction Z along the thickness of optical cavity 1).

(72) Detector

(73) Detector D can be an infrared detector. The infrared detector can for example be a bolometer or a pyrometer. The infrared detector can present a surface sensitive to infrared rays. For non-restrictive example purposes, the sensitive surface can have the shape of a square with sides measuring 600 m. Detector D is preferentially equipped with an optical bandpass filter centred on the spectral absorption band of the gas to be detected if this is the case, when light source S is a heat source. Detector D advantageously extends in direction Z, over the whole thickness of optical cavity 1, in order to receive a maximum amount of light rays, conjugation of the focuses of elliptical mirrors M.sub.1, M.sub.2, M.sub.3 being imperfect along Z.

(74) The invention is not limited to the embodiments set out herein. The person skilled in the art will be able to consider their technically operative combinations and to substitute equivalents for the latter.