Nondispersive infrared gas detection sensor

Abstract

A sensor including an optical cavity capable of receiving the gas, and defined by first and second opposite ends and a connecting portion connecting said ends; a light source arranged to emit infrared light in the optical cavity; at least one infrared detector arranged to detect the infrared light; at least one mirror arranged in the optical cavity to guide the infrared light towards said at least one infrared detector; the sensor being remarkable in that it includes first and second reflective elements respectively extending at the first and second ends of the optical cavity, and having an infrared light reflection coefficient greater than or equal to 75% for any angle of incidence.

Claims

1-25. (canceled)

26. A nondispersive infrared gas detection sensor, comprising: an optical cavity capable of receiving the gas, and defined by: first and second opposite longitudinal ends, and a connecting portion extending transversely between the first end and the second end to connect the first and second ends; a light source arranged to emit infrared light in the optical cavity; at least one infrared detector arranged to detect the infrared light; at least one mirror arranged in optical cavity to guide the infrared light towards at least one infrared detector; first and second reflective elements respectively extending at the first and second ends of the optical cavity, and having an infrared light reflection coefficient greater than or equal to 75% for any angle of incidence; the sensor being designed so as to arrange the light source between the first end and the second end, the light source being further directed towards the connecting portion.

27. The sensor of claim 26, wherein the first and second ends and the connecting portion form a cylinder defining the optical cavity; the first and second ends forming the bases of the cylinder.

28. The sensor of claim 26, wherein the connecting portion extends transversely between the first and second ends along a curvilinear direction.

29. The sensor of claim 26, wherein the connecting portion extends transversely between the first and second ends along a curvilinear direction and wherein the curvilinear direction forms an arc of a circle, and wherein the first and second ends and the connecting portion form a truncated torus defining the optical cavity; the torus being truncated by a plane perpendicular to the curvilinear direction.

30. The sensor of claim 26, wherein the first and second ends and the connecting portion form a frustum with parallel bases defining the optical cavity, the first and second ends forming said parallel bases.

31. The sensor of claim 26, wherein said reflection coefficient is greater than or equal to 80% for any angle of incidence.

32. The sensor of claim 26, wherein said reflection coefficient is greater than or equal to 95% for any angle of incidence smaller than 45°.

33. The sensor of claim 26, wherein the first and second reflective elements are planar mirrors.

34. The sensor of claim 26, wherein the first and second reflective elements each comprise a reflective surface having said reflection coefficient.

35. The sensor of claim 26, wherein the first and second reflective elements each comprise a reflective surface having said reflection coefficient, and wherein the reflective surface is made of a metallic material.

36. The sensor of claim 26, wherein the first and second reflective elements each comprise a reflective surface having said reflection coefficient, the reflective surface being made of a metallic material and wherein the reflective surface is coated with a layer of protection against a corrosion of the metallic material.

37. The sensor of claim 26, comprising a reference infrared detector, and said at least one mirror has a surface roughness greater than a threshold from which the infrared light diffused by said at least one mirror can be detected by the reference infrared detector.

38. The sensor of claim 26, wherein the reflective surface is planar.

39. The sensor of claim 26, wherein the reflective surface is planar and wherein the reflective surfaces of the first and second reflective elements are parallel.

40. The sensor of claim 39, wherein the reflective surface is planar and the reflective surfaces of the first and second reflective elements are parallel, and wherein the connecting portion extends transversely between the first and second ends along the normal to the reflective surfaces (50), to within ±3°.

41. The sensor of claim 26, wherein the optical cavity comprises no lenses.

42. The sensor of claim 26, wherein the gas is selected from the group comprising carbon monoxide, carbon dioxide, at least a hydrocarbon, a hydrochlorofluorocarbon, a chlorofluorocarbon, nitrogen monoxide, nitrogen dioxide, sulfur dioxide, ozone.

43. The sensor of claim 26, wherein said at least one mirror is arranged in the connecting portion.

44. A method of manufacturing a nondispersive infrared sensor to detect a gas, comprising the steps of: a) forming an optical cavity capable of receiving the gas, and defined by: first and second opposite ends, and a connecting portion connecting the first and second ends; b) arranging a light source to emit infrared light in the optical cavity; c) arranging at least one infrared detector to detect the infrared light; d) arranging at least one mirror in the optical cavity to guide the infrared light towards said at least one infrared detector; the method comprising a step of forming first and second reflective elements respectively extending at the first and second ends of the optical cavity, and having an infrared light reflection coefficient greater than or equal to 75% for any angle of incidence.

45. The method of claim 44, wherein step a) comprises the steps of: a.sub.1) providing first and second substrates of a material; a.sub.2) hollowing each of the first and second substrates so as to form a recessed surface (50) and to keep a surface portion; a.sub.3) assembling the first and second substrates so that: the recessed surfaces form first and second opposite ends of optical cavity, the surface portions kept at step a.sub.2) form the connecting portion connecting the first and second ends.

46. The method of claim 45, wherein the first and second reflective elements are formed by deposition of a metallic material on the recessed surfaces of the first and second substrates.

47. The method of claim 45, wherein said at least one mirror arranged at step d) is formed by deposition of a metallic material on a lateral edge of a surface portion kept at step a.sub.2).

48. The method of claim 44, wherein step a) comprises the steps of: a.sub.1) providing first and second molds respectively comprising an impression of first and second parts each comprising a base topped with a surface portion; a.sub.2) injecting a plastic material into the first and second molds to obtain the first and second parts; a.sub.3) assembling the first and second parts so that: the bases form first and second opposite ends of the optical cavity, the surface portions form the connecting portion connecting the first and second ends.

49. The method of claim 48, wherein the first and second reflective elements are formed by deposition of a metallic material on the bases of the first and second parts.

50. The method of claim 48, wherein said at least one mirror arranged at step d) is formed by deposition of a metallic material on a lateral edge of a surface portion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0081] The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of different embodiments of the invention, in connection with the accompanying drawings, among which:

[0082] FIG. 1 is a schematic cross-section view of a NDIR sensor according to the invention;

[0083] FIG. 2 is a schematic exploded perspective view of a NDIR sensor according to the invention;

[0084] FIG. 3 is a schematic exploded perspective view of the NDIR sensor illustrated in FIG. 2 according to a different viewing angle;

[0085] FIG. 4 is a graph showing in abscissas the infrared light reflection coefficient (in %) of the first and second reflective elements, and in ordinates the light flow (in % of the flow emitted by the light source) received on the infrared detector(s);

[0086] FIG. 5 is a schematic top view illustrating the path of the light beams within the optical cavity for a specific arrangement of mirrors and of infrared detectors;

[0087] FIG. 6 is a schematic perspective view illustrating the path of the light beams within the optical cavity, for the same specific arrangement as FIG. 5,

[0088] FIGS. 7 and 8 are schematic side views, respectively, of the first and second reflective elements,

[0089] FIGS. 9 and 10 are schematic top views, respectively, of the first and second reflective elements,

[0090] FIGS. 11 to 13 are schematic top views illustrating the path of the light beams within the optical cavity for different arrangements of mirrors and of infrared detectors.

DETAILED DESCRIPTION

[0091] For the different embodiments, the technical characteristics described hereafter are to be considered alone or according to any technically possible combination; the same reference numerals are used for identical elements or elements carrying out the same function, to simplify the description.

[0092] FIGS. 1 to 13 illustrate a nondispersive infrared gas detection sensor (NDIR), comprising: [0093] an optical cavity 1 capable of receiving the gas, and defined by: [0094] first and second opposite longitudinal ends 100, 101, and [0095] a connecting portion 6a, 6b extending transversely between first end 100 and second end 101 to connect first and second ends 100, 101; [0096] a light source 2 arranged to emit infrared light in optical cavity 1; [0097] at least one infrared detector 3a, 3b arranged to detect the infrared light; [0098] at least one mirror 4a, 4b arranged in optical cavity 1 to guide the infrared light towards said at least one infrared detector 3a, 3b.

[0099] The NDIR sensor comprises first and second reflective elements 5a, 5b respectively extending at the first and second ends 100, 101 of optical cavity 1, and having an infrared light reflection coefficient greater than or equal to 75% for any angle of incidence.

[0100] Further, light source 2 is arranged between first end 100 and second end 101. Light source 2 is arranged to be directed towards connecting portion 6a, 6b.

[0101] First and Second Reflective Elements

[0102] First and second reflective elements 5a, 5b preferably have an infrared light reflection coefficient greater than or equal to 80%, preferably greater than or equal to 85%, more preferably greater than or equal to 90%, for any angle of incidence. Advantageously, the infrared light reflection coefficient is greater than or equal to 95%, preferably greater than or equal to 98%, for any angle of incidence smaller than 45°.

[0103] First and second reflective elements 5a, 5b are preferably each formed from a plate. The plates are provided with transversal openings 502 shaped to receive light source 2 and infrared detector(s) 3a, 3b when the plates are placed into contact and fastened to each other. Each of first and second elements 5a, 5b may comprise a reflective surface 50 having the infrared light reflection coefficient (see above for the quantification). Reflective surface 50 may be curved or textured. Preferably, reflective surface 50 is planar and first and second elements 5a, 5b advantageously form planar mirrors.

[0104] Each plate comprises a surface portion 6a, 6b topping reflective surface 50. Reflective surface 50 and surface portion 6a, 6b of each plate define a half-cavity. When the plates are placed into contact and fastened to each other: [0105] reflective surfaces 50 form first and second opposite ends 100, 101 of optical cavity 1, [0106] surface portions 6a, 6b form the connecting portion connecting the first and second ends 100, 101.

[0107] In other words, reflective surfaces 50 form the first and second opposite longitudinal ends of optical cavity 1, and surface portions 6a, 6b form transversal walls of the optical cavity connecting first and second ends 100, 101.

[0108] The variation of the light flow received by an infrared detector 3a, 3b (in % of the flow emitted by light source 2) is illustrated in FIG. 4 for different values of reflection coefficients. It can be observed that the variation of the light flow is not linear according to the reflection coefficient, and that the received light flow increases all the faster as there is a strong reflection coefficient.

[0109] Reflective surface 50 of each of the first and second reflective elements 5a, 5b is preferably planar. Reflective surface 50 of each of the first and second reflective elements 5a, 5b is advantageously made of a metallic material, preferably selected from the group comprising gold, silver, and aluminum. Reflective surface 50 of each of first and second reflective elements 5a, 5b is advantageously coated with a layer of protection against a corrosion of the metallic material. The protection layer is advantageously made of a material selected from the group comprising SiO.sub.2, SiN, Si.sub.3N.sub.4, a DLC (Diamond-like carbon) amorphous carbon, polytetrafluoroethylene (PTFE), Pt, TiN. Planar reflective surfaces 50 of first and second reflective elements 5a, 5b are advantageously parallel.

[0110] Mirror Assembly

[0111] The NDIR sensor advantageously comprises a mirror assembly 4a, 4b arranged to detect the infrared light. Mirror assembly 4a, 4b advantageously has an infrared reflection coefficient greater than or equal to 75% for any angle of incidence. Said reflection coefficient is advantageously greater than or equal to 80%, preferably greater than or equal to 85%, more preferably greater than or equal to 90%, for any angle of incidence. Advantageously, the infrared light reflection coefficient is greater than or equal to 95%, preferably greater than or equal to 98%, for any angle of incidence smaller than 45°.

[0112] According to an advantageous characteristic, at least one mirror from mirror assembly 4a, 4b has a surface roughness greater than a threshold from which the infrared light diffused by the surface of the corresponding mirror 4a, 4b can be detected by a reference infrared detector 3b. The inventors have surprisingly observed that the diffusion of infrared light by the rough surface of the corresponding mirror 4a, 4b is not prejudicial to correctly image light source 2 on infrared detectors 3a, 3b, and obtain good performances of the NDIR sensor. It is thus possible to take advantage of a quality defect of the corresponding mirror 4a, 4b to obtain a flow for a reference path, the flow being formed by the infrared light diffused by the rough surface of the corresponding mirror 4a, 4b.

[0113] According to a variation, at least one mirror from among mirror assembly 4a, 4b has a texture or waviness adapted so that the infrared light diffused by the waviness can be detected by a reference infrared detector 3b.

[0114] For this purpose, the waviness has an averaged period (noted “a”) verifying the following relation:

[00001]$a=\lambda .Math..Math.\frac{L}{l}$

where: [0115] L is the characteristic dimension (e.g. the length) of optical cavity 1, [0116] λ is the wavelength of the infrared light, [0117] l is the distance between the corresponding mirror 4a, 4b and reference infrared detector 3b.

[0118] Average period “a” is not constant all over the wavy surface to avoid inducing diffraction orders.

[0119] The inventors have observed that the diffusion of infrared light by such a wavy surface enables to correctly image light source 2 on infrared detectors 3a, 3b and obtain good performances of the NDIR sensor. It is thus possible to obtain a flow for a reference path, the flow being formed by the infrared light diffused by the wavy surface.

[0120] The mirror assembly preferably comprises a first elliptical mirror 4a, called measurement mirror, and a second elliptical mirror 4b, called reference mirror. Elliptical measurement mirror 4a is mounted on the plate of first reflective element 5a. More specifically, elliptical measurement mirror 4a is arranged on the reflective surface 50 of first reflective element 5a. Elliptical measurement mirror 4a preferably extends on a lateral edge of surface portion 6a of the plate of first reflective element 5a. Elliptical reference mirror 4b is mounted on the plate of second reflective element 5b. More specifically, elliptical measurement mirror 4b is arranged on the reflective surface 50 of second reflective element 5b. Elliptical reference mirror 4b preferably extends on a lateral edge of surface portion 6b of the plate of second reflective element 5b.

[0121] As a non-limiting example, dimensions (a, b, c) of the ellipses of first and second elliptical mirrors 4a, 4b are gathered in the following table:

TABLE-US-00001 Dimension a (mm) b (mm) c (mm) Measurement mirror 4a 15 14.58 3.5 Reference mirror 4b 18.28 17 6

[0122] where “a” is the semi major axis, “b” is the semi minor axis, and “c” is the distance between the center and the focus of the corresponding ellipse.

[0123] As illustrated in FIGS. 5 and 6, the two focuses of each of the first and second elliptical mirrors 4a, 4b are in planes respectively called measurement focal plane MFP and reference focal plane RFP, extending across the thickness of optical cavity 1 (that is, along Z or Z′). As a non-limiting example, the MFP and RFP planes form together a 45° dihedron angle.

[0124] The mirror assembly may have different configurations according to the interaction length 1 desired between the gas and the infrared light and the number of infrared detectors 3a, 3b used.

[0125] According to a first variation, it is possible to suppress an elliptical mirror 4a, 4b if a single infrared detector 3a, 3b is used, and this, to maximize the received flow for an application requiring a high sensitivity.

[0126] According to a second variation, it is possible to partition an elliptical mirror into a plurality of portions of elliptical mirrors having a common focus if a plurality of infrared detectors 3a, 3b is used.

[0127] According to a third variation illustrated in FIG. 11, it is possible to symmetrically arrange an assembly of elliptical mirrors 4a, 4b, 4c, 4d in plane (X′, Y′) of optical cavity 1 on either side of an axis of symmetry Δ, and to arrange a light source 2 along axis of symmetry Δ to optimize the electric power consumption of light source 2, light source 2 emitting in all directions.

[0128] According to a fourth variation illustrated in FIGS. 12 and 13, it is possible to replace elliptical mirrors 4a, 4b with off-axis parabolic mirrors M.sub.2 (that is, off the optical axis) and planar mirrors M.sub.1. A parabolic mirror M.sub.2 is arranged to collimate light source 2; the collimated beam is reflected by planar mirrors M.sub.1 and is then focused on detector 3a (FIG. 12) or detectors 3a, 3b (FIG. 13) by at least one additional parabolic mirror M.sub.2.

[0129] Optical cavity Preferably, elliptical measurement mirror 4a, elliptical reference mirror 4b, and reflective surfaces 50 of the first and second reflective elements 5a, 5b form a cylinder 10 defining optical cavity 1.

[0130] The directrix curve of cylinder 10, corresponding to a cross-section of cylinder 10 in a plane (X, Y)—or (X′, Y′)—, is an open curve comprising first and second elliptical portions (see FIG. 1). The first and second elliptical portions correspond to first and second elliptical mirrors 4a, 4b. First and second ends 100, 101 of cylinder 10 (that is, the bases of cylinder 10) are preferably closed by reflective surfaces 50 of first and second reflective elements 5a, 5b. The thickness of optical cavity 1 corresponds to height H of cylinder 10, along direction Z (or Z′). As an example, H=600 μm.

[0131] Other shapes may be envisaged for optical cavity 1. Reflective surfaces 50 form first and second ends 100, 101 of optical cavity 1. Reflective surfaces 50 and connecting portion 6a, 6b (possibly mirror(s) 4a, 4b extending from connecting portion 6a, 6b) define optical cavity 1.

[0132] As non-limiting examples, connecting portions 6a, 6b may extend transversely between reflective surfaces 50 along a direction which is curvilinear (and thus non-rectilinear, conversely to cylinder 10). The curvilinear direction may form an arc of a circle, in which case reflective surfaces 50 and connecting portion 6a, 6b may form a torus truncated by a plane perpendicular to the curvilinear direction. The truncated torus may comprise openings capable of receiving light source 2 and infrared detector(s) 3a, 3b. “Torus” designates the surface generated by the revolution of a planar curve (open or curved) around an axis located in the plane of said planar curve.

[0133] Reflective surfaces 50 and connecting portion 6a, 6b may form a frustum with parallel bases, reflective surfaces 50 forming the parallel bases. The frustum may comprise openings capable of receiving light source 2 and infrared detector(s) 3a, 3b.

[0134] Optical cavity 1 advantageously comprises no lenses.

[0135] Light Source

[0136] Light source 2 preferably comprises an infrared radiation emission element. Light source facing connecting portion 6a, 6b means that said emitter element is arranged opposite connecting portion 6a, 6b. Preferably, the optical axis of light source 2 is directed towards connecting portion 6a, 6b.

[0137] Light source 2 advantageously comprises an emitter element, for example, of filament type, which is made to conduct an electric current so that the element heats up and emits an infrared radiation. The element has a dimension h along direction Z (or Z′) and preferably verifies:

100 μm≦h H≦1.5 mm, preferably 250 μm≦h≦H≦1200 μm.

[0138] The element preferably has the shape of a disk with a circular surface, having a 250-μm diameter (corresponding to dimension h).

[0139] The axis of light source 2, or the optical axis of light source 2, is defined as the normal to the circular surface. As a non-limiting example, the disk may have a 400-nm thickness along the axis of light source 2. The directrix curve of cylinder 10 images light source 2 in a single direction, that is, the plane of optical cavity 1. The image of light source 2 (that is, the disk-shaped element) is a rectangle having a 250-μm width and a 600-μm height (corresponding to the height of optical cavity 1). Preferably, the normal to said rectangle (image of light source 2) is directed towards connecting portion 6a, 6b. More preferably, the normal is interposed between first and second ends 100, 101, and it is substantially parallel to said ends 100, 101.

[0140] Infrared Detectors

[0141] The NDIR sensor preferably comprises two infrared detectors 3a, 3b: a measurement detector 3a, and a reference detector 3b. Measurement detector 3a and reference detector 3b are respectively in the MFP plane and in the RFP plane. Each infrared detector 3a, 3b has a surface preferably sensitive to infrared, arranged between first end 100 and second end 101, and arranged opposite connecting portion 6a, 6b.

[0142] As a non-limiting example, the sensitive surface may have the shape of a square having a 600-μm side length. The axis of an infrared detector 3a, 3b is defined as the normal to the sensitive surface. The axis of light source 2 and axis X (or X′) preferably form a 25° angle to maximize the light flow received by infrared detectors 3a, 3b. In other words, the axis of light source 2 preferably forms a 25° angle with the MFP. The axis of measurement detector 3a and axis X (or X′) preferably form a 25° angle to optimize the received light flow. In other words, the axis of measurement detector 3a preferably forms a 25° angle with the MFP. The axis of reference detector 3b is preferably perpendicular to axis Y (or Y′) to optimize the received light flow. The axis of reference detector 3b preferably forms a 45° angle with the RFP.

[0143] Measurement detector 3a is preferably equipped with an optical filter of bandpass type, centered on the spectral absorption band of the gas to be detected. As non-limiting examples, the gas is selected from the group comprising carbon monoxide, carbon dioxide, at least a hydrocarbon, a hydrochlorofluorocarbon, a chlorofluorocarbon, nitrogen monoxide, nitrogen dioxide, sulfur dioxide, ozone.

[0144] Reference detector 3b is preferably equipped with an optical filter centered on a spectral band which is absorbed by no gas to be detected.

[0145] As non-limiting examples, the gas may also be selected from the following gases, which absorb in a spectral absorption band in the range from 0.78 μm to 12 μm: [0146] 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, [0147] 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, [0148] AsH.sub.3, a polycyclic aromatic hydrocarbon, benzene, toluene, the three isomers of xylene, C.sub.2H.sub.4O, BCl.sub.3.

[0149] The spectral transmission band of conventional filters generally depends on the angle of incidence. In other words, for angles greater than a given threshold (for example, 45° or also 30°), the filter transmits wavelengths which are no longer in the absorption band of the gas to be detected.

[0150] Thus, such a NDIR sensor according to the invention enables to increase, at the same time, the optical efficiency and the compactness of the optical cavity with respect to the state of the art, while optimizing the angular distribution of the infrared radiation collected by the detector(s). The ingenious arrangement of the different sensor elements, particularly, the light source, the infrared detector(s), the mirror(s), and the reflective elements extending at the ends of the optical cavity, enables to conjugate the light source with the infrared detector(s) in the plane of the optical cavity, to guide the infrared light across the thickness and according to an optimal angular distribution (angles of incidence smaller than said threshold for a majority of the incident radiations) to use at best the angular emission properties of the light source.

[0151] Manufacturing Method

[0152] A method of manufacturing a nondispersive infrared sensor to detect a gas, comprises the steps of:

a) forming an optical cavity 1 capable of receiving the gas, and defined by: [0153] first and second opposite ends 100, 101, and [0154] a connecting portion 6a, 6b connecting the first and second ends 100, 101;
b) arranging a light source 2 to emit infrared light in optical cavity 1;
c) arranging at least one infrared detector 3a, 3b to detect the infrared light;
d) arranging at least one mirror 4a, 4b in optical cavity 1 to guide the infrared light towards said at least one infrared detector 3a, 3b.

[0155] The method comprises a step of forming first and second reflective elements 5a, 5b respectively extending at the first and second ends 100, 101 of optical cavity 1, and having an infrared light reflection coefficient greater than or equal to 75% for any angle of incidence.

[0156] According to a first embodiment illustrated in FIGS. 7 and 8, step a) comprises the steps of:

a.sub.1) providing first and second substrates 500 of a material, the material being preferably semiconductor, more preferably silicon;
a.sub.2) hollowing each of the first and second substrates 500 so as to form a recessed surface 50 and to keep a surface portion 6a, 6b,
a.sub.3) assembling the first and second substrates 500 so that: [0157] recessed surfaces 50 form first and second opposite ends 100, 101 of optical cavity 1, [0158] surface portions 6a, 6b kept at step a.sub.2) form the connecting portion connecting the first and second ends 100, 101.

[0159] Step a.sub.2) is advantageously executed by deep reactive ion etching. Step a.sub.2) preferably comprises a previous step comprising depositing a resist at the surface of first and second substrates 500. Then, recesses 501 may be obtained by photolithography and etch steps. Step a.sub.2) is advantageously executed so that the obtained recesses 501 enable to form planar recessed surfaces 50.

[0160] First and second reflective elements 5a, 5b are advantageously formed by deposition of a metallic material on recessed surfaces 50 of first and second substrates 500, the deposition being preferably executed by cathode sputtering. The deposition of the metallic material may also be executed by vacuum evaporation or by electrolysis. The deposition of the metallic material is executed before step a.sub.3).

[0161] Mirror(s) 4a, 4b are advantageously formed by deposition of a metallic material on a lateral edge of a surface portion 6a, 6b, the deposition being preferably executed by cathode sputtering. The deposition of the metallic material may also be executed by vacuum evaporation or by electrolysis. The deposition of the metallic material is executed before step a.sub.3).

[0162] According to a second embodiment, step a) comprises the steps of:

a.sub.1) providing first and second molds respectively comprising an impression of first and second parts, each comprising a base 500 topped with a surface portion 6a, 6b;
a.sub.2) injecting a plastic material into the first and second molds to obtain the first and second parts,
a.sub.3) assembling the first and second parts so that: [0163] bases 500 form first and second opposite ends 100, 101 of optical cavity 1, [0164] surface portions 6a, 6b form the connecting portion connecting first and second ends 100, 101.

[0165] Step a.sub.1) is preferably executed so that the first and second molds each comprise a fixed portion and a mobile portion. Step a.sub.2) is preferably executed with an injection press.

[0166] First and second reflective elements 5a, 5b are advantageously formed by deposition of a metallic material on bases 500 of the first and second parts, the deposition being preferably executed by cathode sputtering. The deposition of the metallic material may also be executed by vacuum evaporation or by electrolysis. The deposition of the metallic material is executed before step a.sub.3).

[0167] Mirror(s) 4a, 4b are advantageously formed by deposition of a metallic material on a lateral edge of a surface portion 6a, 6b, the deposition being preferably executed by cathode sputtering. The deposition of the metallic material may also be executed by vacuum evaporation or by electrolysis. The deposition of the metallic material is executed before step a.sub.3).