Multispectral partial interferogram sensor for multispectral partial interferogram imager

Abstract

The invention relates to a multispectral partial interferogram sensor (100) for an imager (1) comprising: an optical component (200), a spectral filtering device (710, 720), and a matrix detector (500). The optical component is a single-piece unit and is in the form of a body comprising: an input face A, reflective faces B, C, F, G, a semi-reflective surface D, and an output face H, arranged opposite the matrix detector. The input face A, the reflective faces B, C, and the semi-reflective surface D are arranged relative to each other to form a static two-way interferometer (300). The reflective faces E, F, G, and the output face H are arranged relative to each other to form an Öffner-type output optic (400). The interferometer is located upstream of the output optic. Figure for the abridged version: Fig. 1

Description

Title of the invention: Multispectral partial interferogram sensor for multispectral partial interferogram imager Technical field of the invention

[0001] The present invention falls within the field of observation instruments intended to be carried on satellites or aircraft.

[0002] The present invention relates to a multispectral partial interferogram imager, and more particularly to a multispectral partial interferogram sensor for a multispectral partial interferogram imager. Prior art

[0003] Observation instruments, such as for example multispectral partial interferogram imagers, allow the acquisition of images in order in particular to obtain a map of the gases contained in the atmosphere or to obtain a three-dimensional representation of information from the atmosphere, for example a gas concentration, a temperature profile or water vapor.

[0004] An example of a multispectral partial interferogram imager is described in French patent FR 1 701 392. Such an imager comprises: - a first Ll optic, - a static interferometer, of the Michelson type, with path difference steps, IM1, IM2, - a second L2 optical system, - a D photosensitive matrix sensor, and - means of recording and transforming images of multispectral partial interferograms into images of the atmosphere.

[0005] The static interferometer operates in the image plane, that is to say that the fringes Interference created by the static interferometer is generated in an image plane. This image plane of the static interferometer is conjugated to the plane of the photosensitive matrix sensor via the second optics.

[0006] The general principle of said imager consists of generating, by means of the first optics, an image of the observed scene in intermediate image planes of the static interferometer and then transporting, by means of the second optics, the images obtained in the intermediate image planes into the plane of the photosensitive matrix sensor. An optical filter located, for example, in front of the photosensitive matrix sensor selects a specific spectral band associated with each step difference created by the static interferometer.

[0007] Instead of acquiring the entire interferogram of the light spectrum of the observed scene, the imager, thanks to the static step-difference interferometer, will acquire only several portions of this interferogram in several specific spectral bands. More precisely, for each step-difference created by the static interferometer, the imager will acquire a portion of the interferogram in a specific spectral band. Each step-difference of the static interferometer has been defined for a specific spectral band. These acquired portions of the interferogram are called partial interferograms. Hence its name: multispectral partial interferogram imager.

[0008] The assembly formed by the static interferometer, the second optics, and the multispectral filter represents a significant portion of the imager's overall size. In particular, the second optics, which is constructed from several optical components with elementary mirrors arranged relative to each other in free space to ensure consistent imager performance, is particularly bulky. Presentation of the invention

[0009] The present invention aims to remedy the drawbacks of the solutions proposed by the prior art, in particular those set out above.

[0010] To this end, the present invention proposes a multispectral partial interferogram sensor, referred to as the sensor, for a multispectral partial interferogram imager comprising: - a housing, - an entry window, called a porthole, fixed to said housing, - an optical component, positioned in said housing, - a spectral filtering device, - a matrix detector, positioned in said housing,

[0011] the optical component being in the form of a body comprising: - an entrance face A, arranged opposite the porthole, - a reflective face B, - a reflective face C, - a semi-reflective surface D, - a reflective face E, - a reflective face F, - a reflective face G, - an output face H, arranged opposite the matrix detector.

[0012] The entrance face A, the reflective faces B and C and the semi-reflective surface D are arranged relative to each other so as to form an interferometer static two-way, called interferometer, the reflective face B being disposed on a first channel of the interferometer and the reflective face C being disposed on a second channel of the interferometer, the reflective face B having N reflective facets, the reflective face C having N reflective facets, N greater than or equal to 1.

[0013] The reflective faces E, F and G and the exit face H are arranged relative to each other so as to form an Offner type exit optic.

[0014] The interferometer is arranged upstream of the output optics. The interferometer includes: - on the first track, from a plane PI, N stacks of at least two plates, including a first plate and a second plate, of material having different refractive indices, each stack ending with a reflective facet, - on the second track, from a plane P2, N stacks of at least two plates, including a first plate and a second plate, of material having different refractive indices, said at least two plates each being beveled, each stack ending with a reflective facet,

[0015] the PI plane, on the first channel, and the P2 plane, on the second channel, corresponding to the same path of a light beam incident in the interferometer.

[0016] The spectral filtering device comprises: - a multi-spectral band filter arranged on one face of the window or on the input face A of the optical component, - N spectral band selection filters arranged on the first channel of the interferometer, each spectral band selection filter forming a reflective facet of the reflective face B, - N spectral band selection filters arranged on the second channel of the interferometer, each spectral band selection filter forming a reflective facet of the reflective face C.

[0017] The optical component is thus advantageously configured to receive an image of an observed scene in the form of an incident light beam and to transmit this incident light beam, via the interferometer and then the output optics, to the matrix detector.

[0018] The interferometer, with two stacked plates on each channel, allows advantageously fulfills both a function of creating step difference(s) and a function of opening compensation. These two blades of each stack and for each track are not air gaps. A step difference jump is created by the difference in thickness between the thickness of the first blade of a stack located on the first track and the thickness of the first blade of the stack located on the second track and arranged at the level of an area corresponding optically to said stack located on the first track. A step difference is advantageously chosen based on a specific spectral band that it is desired to analyze. The opening compensation is obtained, for each step difference jump, thanks to the difference in thickness between the thickness of the second blade of a stack located on the first track and the thickness of the second blade of the stack located on the second track and arranged at the level of an area corresponding optically to said stack located on the first track.

[0019] The interferometer with, on each channel, a third blade by stacking, allows to fulfill an additional function of insensitivity to the variation of temperature of the body forming the optical component. Insensitivity to temperature variation is achieved, for each step difference, thanks to the difference in thickness between the thickness of the third blade of a stack located on the first track and the thickness of the third blade of the stack located on the second track and arranged at a level optically corresponding to said stack located on the first track

[0020] The spectral filtering device (comprising the spectral multi-band filter and the spectral band selection filters forming the reflective face B and the reflective face C) allows a specific spectral band to be selected associated with each step difference jump created by the interferometer. The spectral multiband filter is configured to transmit N spectral bands. These N spectral bands advantageously correspond to the N spectral bands associated with each of the path difference step ranges of the interferometer. Each spectral band selection filter arranged on a stack, for each channel, is configured to, on the one hand, reflect the spectral band associated with the path difference step range created at that stack and, on the other hand, transmit the flux outside this spectral band.

[0021] The sensor according to the invention thus advantageously makes it possible to generate partial interferograms with homogeneous spectral response.

[0022] Thanks to an Ôffner type output optic, the sensor is also advantageously compact.

[0023] According to particular embodiments of the invention, the invention further meets the following characteristics, implemented separately or in each of their technically operative combinations.

[0024] In particular embodiments of the invention, on each channel, the interferometer comprises only the first blade and the second blade per stack, said first and second blades of each stack being successive, without space or air gap between them. The incident light beam passing through the optical component thus advantageously propagates only within the body forming the optical component. The number of interfaces encountered by the incident light beam in the sensor is limited. The only interfaces traversed by the incident light beam in the sensor are the input face A and the output face H of the body forming the optical component, the window, and the matrix detector.

[0025] In particular embodiments of the invention, the sensor comprises light traps arranged respectively opposite the reflective face B and the reflective face C. A first light trap is configured to absorb the flux not included in the desired spectral bands and transmitted by the N spectral band selection filters 720 of the reflective face B. A second light trap is configured to absorb the flux not included in the desired spectral bands and transmitted by the N spectral band selection filters of the reflective face C.

[0026] In particular embodiments of the invention, to minimize the variations in reflection along the two polarization axes, the interferometer has the general shape of a hexagonal prism.

[0027] In particular embodiments of the invention, the interferometer is formed of two joined elementary blocks, referred to as the first elementary block and the second elementary block, on which are assembled at least two plates forming the N stacks on the first channel and at least two plates forming the N stacks on the second channel, and in which the output optics are formed in one elementary block. Said elementary block is assembled by optical bonding or molecular adhesion to the second elementary block of the interferometer to stiffen the body forming the optical component.

[0028] In particular embodiments of the invention, the reflective faces E and G are made on a common surface of the body forming the optical component.

[0029] In particular embodiments of the invention, the reflective faces E, F and G have a spherical, aspherical or free shape.

[0030] In particular embodiments of the invention, to further reduce the size of the sensor, the output optics include a reflecting prism. The output optics thus comprise, in addition to the elementary block assembled by optical bonding or molecular adhesion to the second elementary block of the interferometer, another elementary block forming the reflecting prism. The reflecting prism has the output face H and is arranged opposite the matrix detector.

[0031] In particular embodiments of the invention, the filtering device comprises a rejection filter arranged on one face of the window, separate from the face of the window containing the spectral multiband filter. The rejection filter is associated with the spectral multiband filter and is configured to reject the flux not included in the N selected spectral bands.

[0032] The invention also relates to an imager comprising an input optic, a multispectral partial interferogram sensor meeting one or more of the characteristics described above and / or below, and an image processing module configured to analyze the images acquired by the matrix detector. Such an imager can be carried on a satellite or aircraft and advantageously allow the acquisition of images of an observed scene presenting a characteristic optical spectrum, such as for example the atmosphere. Brief description of the figures

[0033] The invention will be better understood upon reading the following description, given by way of non-limiting example, and made with reference to the following figures:

[0034] [Fig-1] schematically illustrates a cross-sectional view of an interferogram imager multispectral partials, the imager comprising a multispectral partial interferogram sensor according to an embodiment of the invention, and represents the ray path of a light beam forming the beam at the center of the field of view of an interferometer of the multispectral partial interferogram sensor,

[0035] [Fig.2] illustrates the imager of [Fig.1], in more detail and without the ray path of a light beam,

[0036] [Fig.3] schematically illustrates a cross-sectional view of a multispectral partial interferogram imager, the imager comprising a multispectral partial interferogram sensor according to an embodiment of the invention,

[0037] [Fig.4] schematically illustrates a cross-sectional view of a multispectral partial interferogram imager, the imager comprising a multispectral partial interferogram sensor according to an exemplary embodiment of the invention, and represents the ray path of two light beams forming the beams at the edge of the field of view of the interferometer of the multispectral partial interferogram sensor.

[0038] In these figures, identical numerical references from one figure to another designate identical or analogous elements. Furthermore, for reasons of clarity, the drawings are not to scale unless otherwise stated. Description of the implementation methods

[0039] Figure 1 schematically represents a cross-sectional view of a multispectral partial interferogram imager according to an embodiment of the invention. In the following description, the multispectral partial interferogram imager will simply be referred to as imager 1.

[0040] The imager 1 is intended, in general, to be carried on a satellite or an aircraft.

[0041] The imager 1 is intended to acquire images of an observed scene exhibiting a characteristic optical spectrum. This observed scene is preferably the atmosphere. By way of non-limitation, the imager 1 can, for example, provide a map of the concentrations of gases contained in the atmosphere or a three-dimensional representation of atmospheric information.

[0042] The imager 1 operates by moving the image in a direction called the scrolling direction, this scrolling allowing the reconstruction of partial interferogram images. The image scrolling can, for example, be achieved by moving the satellite or aircraft.

[0043] The imager 1 comprises an input optic 10, a multispectral partial interferogram sensor 100 and an image processing module (not shown in [Fig. 1]). In the remainder of the description, the multispectral partial interferogram sensor will simply be referred to as the sensor.

[0044] In a preferred embodiment, the input optics 10 is a refracting telescope (with lenses), a reflecting telescope (with mirrors) or a catadioptric telescope (with lenses and mirrors).

[0045] Sensor 100 preferably comprises: a 101 case, an entrance window, subsequently referred to as porthole 102, an optical component 200, a 500 matrix detector, a spectral filtering device.

[0046] The porthole 102, the optical component 200 and the matrix detector 500 are separate parts from each other.

[0047] The porthole 102 is fixed to the housing 101.

[0048] Preferably, the porthole 102 is arranged on a wall of the housing 101, at the level of a through orifice in said wall.

[0049] In one embodiment, the porthole 102 is assembled to the housing 101 by gluing or brazing. When the porthole 102 is assembled by gluing, the adhesive is, for example, a flexible optical adhesive, such as RTV 560 or RTV 566 silicone adhesive.

[0050] The optical component 200 and the matrix detector 500 are positioned in the housing 101.

[0051] The optical component 200 interfaces preferentially directly with the window 102 and the matrix detector 500. By directly, we mean that there is no other separate part interposed between the optical component 200 and on the one hand the window 102 and on the other hand the matrix detector.

[0052] The optical component 200 comprises: - a two-way static interferometer, - an image transport optic, called output optic 400.

[0053] In the remainder of the description, the two-way static interferometer will simply be referred to as the interferometer 300.

[0054] The optical component 200 according to the invention is a single-piece unit.

[0055] By monobloc part, it is understood that the optical component 200 is formed of a plurality of elements linked together so that the optical component 200 cannot be split without causing the deterioration of at least one of these elements.

[0056] The optical component 200 is in the form of a body, advantageously having a solid internal volume.

[0057] The interferometer 300 is disposed, in the body forming the optical component 200, in upstream of the 400 output optics.

[0058] The general principle of the imager 1 according to the invention consists, as illustrated in [Fig. 1], in generating, by means of the input optics 10, an image of the scene observed in intermediate image planes of the interferometer 300, and then in transporting, via the output optics 400, the images obtained in the intermediate image planes of the interferometer 300 into the plane of the matrix detector 500.

[0059] Preferably, the body forming the optical component 200 comprises, as illustrated in [Fig. 1]: - a face, called the entrance face A, a reflective face B, a reflective face C and a semi-reflective surface D arranged relative to each other to form the interferometer 300, - a reflective face E, a reflective face F, a reflective face G and a face, called the exit face H, arranged in relation to each other to form together the exit optic 400.

[0060] Thus, as illustrated in [Fig. 1], the window 102 is arranged opposite the input optics 10. The optical component 200 is arranged in the sensor 100 so that the input face A of the body is positioned opposite the window 101 and the output face H of the body is positioned opposite the matrix detector 500. The sensor 100 is arranged at the focus of the input optics 10. More precisely, the interferometer 300 is placed in a focal plane of the input optics 10. The optical component 200 is thus advantageously configured to receive an image of the observed scene in the form of an incident light beam and transmit this incident light beam, via the interferometer 300 and then the output optics 400, to the matrix detector 500, as illustrated in [Fig.1].

[0061] The input faces A and output faces H of the body forming the optical component 200 are preferably treated with an anti-reflective coating to eliminate parasitic reflections in the internal volume of said body.

[0062] The matrix detector 500 conventionally comprises a plurality of photoelements. The photoelements are preferably arranged in a matrix according to rows and columns. Each photoelement is intended to receive the incident light beams from the observed scene.

[0063] The image processing module is configured to transform the images acquired by the matrix detector into multispectral partial interferogram images and then into images of the atmosphere. Interferometer

[0064] The interferometer 300 is preferably a Michelson interferometer.

[0065] The reflective face B of the body forming the optical component 200 corresponds to the reflective face of the first channel, or reference channel, of the interferometer 300. The reflective face C of the body forming the optical component 200 corresponds to the reflective face of the second channel of the interferometer.

[0066] In one embodiment, the reflective face B, the reflective face C, and the entrance face A of the body are formed at the body surfaces at the interface with the external environment. The semi-reflective surface D is formed within the solid internal volume of the body.

[0067] The 300 interferometer is a step-difference-adjustable and aperture-compensated interferometer.

[0068] The interferometer 300 comprises: - on the first track, from a plane PI, N stacks of at least two blades 361, 362, - on the second track, from a plane P2, N stacks of at least two blades 351, 352,

[0069] N being an integer greater than or equal to 1.

[0070] Preferably, N is an integer greater than or equal to 2.

[0071] The PI plane, on the first channel, and the P2 plane, on the second channel, correspond to the same path of a light beam incident in the interferometer.

[0072] In other words, the PI plane and the P2 plane correspond to an equal thickness of material traversed in the body 200 forming the optical component, and are represented in dotted line on the [Fig.2].

[0073] Regardless of the interferometer channel, each stack preferably comprises the same number of plates.

[0074] Preferably, the interferometer comprises, as illustrated in [Fig.2]: - on the first track, from a plane PI, N stacks of two blades 361, 362, - on the second track, from a plane P2, N stacks of two blades 351, 352.

[0075] Such an interferometer 300, with only two blades per stack and on each channel, makes it possible to advantageously fulfill the functions of creating path difference jump(s) and aperture compensation. The two blades 351, 352, 361, 362 of each stack and on each track are not air gaps. Furthermore, on each track, and for each stack, these two blades are consecutive. By consecutive blades, we mean that the two blades of the same stack are placed side by side, without any space between them. In other words, there is no air gap between the different blades of each stack.

[0076] The addition of additional blades in each stack, and on each channel, allows the interferometer to perform additional functions, in addition to the functions of creating path difference jump(s) and aperture compensation. Thus, for example, the interferometer may include on each channel, and for each stack, an additional plate, called a third plate (not shown in the figures). Such an interferometer allows, in addition to advantageously fulfilling the functions of creating path difference jumps and aperture compensation, a function of insensitivity to temperature variations of the body 200.

[0077] The N stacks on the first track are preferably arranged in an adjoining manner. Similarly, the N stacks on the second track are preferably arranged in an adjoining manner.

[0078] Each stack on the first channel comprises at least one first blade 361 and one second blade 362. The first blade 361 and the second blade 362 are not air blades. Each stack may comprise at least one third blade. The first blade 361 of each stack on the first channel will always be the blade from which the stack begins from the plane PL. Each stack on the first channel terminates in a reflective facet 380. Each reflective facet 380 is preferably planar. The N reflective facets 380 form the reflective face B (i.e., the reflective face of the first channel of the interferometer 300). Each stack on the second channel comprises at least one first blade 351 and one second blade 352. The first blade 351 and the second blade 352 are not air gaps. Each stack may include at least one third blade. The first blade 351 of each stack on the second channel will always be the blade from which the stack begins, starting from plane P2. Each stack on the second channel terminates in a reflecting facet 370. Each reflecting facet 370 is preferably planar. The N reflecting facets 370 form the reflecting face C (i.e., the reflecting face of the second channel of the interferometer 300).

[0079] The two blades 351, 352 of each stack on the second channel are made of materials having different refractive indices, but with similar coefficients of thermal expansion. The same is true for the two blades 361, 362 of each stack on the first channel.

[0080] The first plates 351, 361 of each stack on each channel are made of the same material having a refractive index ni. The second plates 352, 362 of each stack on each channel are made of the same material having a refractive index n2. The third plate of each stack on each channel is made of the same material having a refractive index n3. The third plate of each stack on each channel can be inserted between the first plate and the second plate or is placed on the second plate.

[0081] In one embodiment, the third blade can be an air gap. An air gap is created using shims, also called "shimplants," not shown in the figures. The shims are preferably made of a material having the same refractive index as the first blade 351, 361 of each stack on each channel.

[0082] The interferometer 300 has, on each channel, the same number of stacks. Each stack on the second channel of the interferometer 300 is located in an area that optically corresponds to a stack on the first channel of the interferometer 300.

[0083] In the non-limiting example of [Fig.2], the interferometer 300 comprises, on each channel, three stacks.

[0084] The reflective face on the first channel of the interferometer 300 (i.e. the reflective face B of the body formed by the N facets 380) thus has as many reflective facets as the reflective face on the second channel of the interferometer 300 (i.e. the reflective face C of the body formed by the N facets 370).

[0085] Each reflective facet 380 of the reflective face B of the body (i.e., the reflective face on the first channel of the interferometer 300) is thus arranged respectively at the level of each zone optically corresponding to a reflective facet 370 of the reflective face C (i.e., the reflective face on the second channel of the interferometer 300). In other words, to the image, by the semi-reflective surface 340, of each zone of the reflective face C having a reflective facet 370 corresponds a zone of the reflective face B having a reflective facet 380.

[0086] In the non-limiting example of [Fig. 1], the reflective face B and the reflective face C each have three reflective facets.

[0087] As previously written, the 300 interferometer is preferably a step-difference-compensated interferometer.

[0088] According to the invention, the interferometer 300 is configured to create N distinct path difference jumps between its two channels. A path difference jump is created by the difference in thickness between the thickness of the first blade 361 of a stack located on the first channel and the thickness of the first blade 351 of the stack located on the second channel and arranged at a level of an area optically corresponding to said stack located on the first channel.

[0089] The thickness of each first blade 351, and therefore the step difference jump, is advantageously chosen according to a specific spectral band that it is desired to analyze.

[0090] In the non-limiting example of [Fig. 2], the interferometer 300 comprises three stacks on each of its channels with different first-plate thicknesses 351, 361. The interferometer 300 is thus configured to create three path difference jumps from the three stacks located on the first channel and the respective three stacks located on the second channel.

[0091] The 300 interferometer is aperture-compensated. By "aperture compensation," it is meant that the interferometer is configured to compensate for variations in optical path differences within the interferometer as a function of the angle of incidence of the rays forming the incident light beam. In other words, the interferometer thus advantageously makes the path difference independent of the angle of incidence of the rays forming the incident light beam. According to the invention, the interferometer 300 is configured to compensate for path difference variations for each path difference jump created between the two paths of the interferometer 300. Compensating for these path difference variations advantageously ensures correct focusing of the rays forming the incident light beam on each facet 370, 380 of the faces reflective of each channel of the interferometer 300. The contrast of the interference fringes obtained on the matrix detector 500 is maximum and independent of the angle of incidence of the rays forming the incident light beams. The opening compensation is obtained, for each step difference jump, thanks to the difference in thickness between the thickness of the second blade 362 of a stack located on the first track and the thickness of the second blade 352 of the stack located on the second track and arranged at the level of an area corresponding optically to said stack located on the first track.

[0092] In one embodiment, the interferometer 300 can be configured to be insensitive to the temperature variation of the body 200, for each step difference jump created between the two channels of the interferometer 300. Insensitivity to temperature variation is obtained, for each step difference jump, thanks to the difference in thickness between the thickness of the third blade of a stack located on the first track and the thickness of the third blade of the stack located on the second track and arranged at the level of an area corresponding optically to said stack located on the first track.

[0093] The interferometer 300 also allows for chromatic aberration compensation. It is known that the refractive index of a material is a function of wavelength. The interferometer 300 according to the invention is thus configured to compensate for the variations in the refractive indices of the materials of the different plates 351, 352, 361, 362 as a function of the wavelengths of the rays forming the incident light beam. Chromatic aberration compensation advantageously ensures correct focusing of the rays forming the incident light beam on each facet 370, 380 of the reflective faces of each channel of the interferometer 300. The contrast of the interference fringes obtained on the matrix detector 500 is then maximum and independent of the wavelength of the rays forming the incident light beam.

[0094] In summary, the 300 interferometer is configured to perform at least the functions of generating N path difference jumps and aperture compensation. It can also be configured to perform, in addition, the function of insensitivity to temperature variation. These functions depend on the choice of the plate indices and the differences in plate thicknesses between the two channels of the interferometer.

[0095] These functions are further performed achromatically by creating a shift in the reflective facets 720 on the first channel. This shift is intended to compensate for the chromatic variations in the refractive indices of the plates. The shift in the reflective facets 720 on the first channel is visible in [Fig. 2], as the reflective facets 720 are not in the same plane.

[0096] A person skilled in the art is able to perform calculations of the thicknesses of the plates of each stack, according to the refractive indices of said plates, to obtain the desired path differences, the aperture compensation for each path difference, where appropriate the insensitivity to temperature variation, and the chromatic compensation.

[0097] According to the invention, each blade 351, 352 of each stack on the second channel, and therefore consequently the associated reflective facet 370 of the reflective face C of the body (i.e., the reflective face on the second channel of the interferometer 300), is beveled (not shown in the figures). By beveled blades, it is understood that the blade has beveled faces. The beveled faces of the blade are parallel to each other.

[0098] The at least two blades 351, 352 of a stack on the second track, and therefore the associated reflective facet 370, are beveled at the same angle.

[0099] The at least two blades 351, 352 of a stack on the second track can be beveled at a different angle than the at least two blades 351, 352 of another stack on the second track. Each reflective facet 370 of the reflective face C can thus be beveled at a different angle than the other reflective facets 370 of the reflective face C.

[0100] The bevel of the reflective facets 370 of the reflective face C of the body advantageously allows to create, for each step difference jump, a range (or variation) of step difference.

[0101] The bevel of at least two blades 351, 352 of each stack on the second track advantageously compensates for the opening both for the step difference jump and for the step difference variation.

[0102] In one embodiment, the interferometer 300 is made from two joined elementary blocks, called first elementary block 301 and second elementary block 302, on which are assembled the at least two blades 351, 352 forming the N stacks on the first channel and the at least two blades 361, 362 forming the N stacks on the second channel.

[0103] In one embodiment, as illustrated in [Fig. 1], the first elementary block 301 comprises, for example, the entrance face 310. The first elementary block 301 comprises a face on which are attached the first blades 361 of each stack, then the second blades 362 of each stack, optionally additional blades, and then the reflective facets 380 forming the reflective face B. The second elementary block 302 comprises a face on which are attached the first blades 351 of each stack, then the second blades 352 of each stack, optionally additional blades, and then the reflective facets 370 forming the reflective face C. The first block Elementary block 301 has a face intended to be joined to a face of the second elementary block 302, one of these two faces having a treatment such that once the two elementary blocks 301, 302 are joined by said two faces, the common face created forms the semi-reflective surface D of the interferometer 300. The first plates 351, respectively 361, are assembled by optical bonding or molecular adhesion onto the first elementary block 301, respectively the second elementary block 302. The second plates 352, respectively 362, are assembled by optical bonding or molecular adhesion onto the first plates 351, respectively 361.

[0104] Preferably, the two elementary blocks 301, 302 are made of the same material.

[0105] Preferably, the two elementary blocks 301, 302 are made of the same material as the first blades 351, 361 of each stack on each channel of the interferometer. In this configuration, the second elementary block 302 can be made directly to include the first blades 351 of each stack on the second channel. Similarly, the first elementary block 301 can be made directly to include the first blades 361 of each stack on the first channel.

[0106] In one embodiment, as illustrated in [Fig.3], the interferometer 300 has the general shape of a cube.

[0107] In a preferred embodiment, as illustrated in Figures 1 and 2, the interferometer 300 has the general shape of a hexagonal prism. Such a shape minimizes reflection variations along the two polarization axes. Spectral filtering device

[0108] As explained previously, sensor 100 includes a spectral filtering device.

[0109] The spectral filtering device includes firstly a spectral multi-band filter 710. By multi-band filter, we mean a filter whose bandwidth includes distinct bandwidths spaced apart from each other.

[0110] The spectral multiband filter 710 is configured to transmit N spectral bands. The N spectral bands advantageously correspond to the N spectral bands associated with each of the path difference jump ranges of the interferometer 300.

[0111] In other words, the spectral multiband filter 710 has a bandwidth including the spectral bands associated with each of the ranges of each step difference jump created in the interferometer 300.

[0112] The spectral multiband filter 710 is preferably arranged upstream of the interferometer 300.

[0113] In one embodiment, the spectral multiband filter 710 is arranged on one of the faces of the window 102, preferably on a face, called the external face 103, of the window located outside the sensor.

[0114] In another embodiment, the spectral multiband filter 710 is arranged on the input face 310 of the optical component 200.

[0115] The spectral filtering device further comprises N spectral band-selective filters 720 arranged on the second channel of the interferometer. More specifically, a spectral band-selective filter 720 is arranged on each stack of the reflecting face C (i.e., the reflecting face of the second channel of the interferometer 300) and forms a reflecting facet 370 of the reflecting face C. Each spectral band-selective filter 720 arranged on a stack is configured to, on the one hand, reflect the spectral band associated with the path difference jump range created at that stack and, on the other hand, transmit the flux not included in that spectral band.

[0116] In embodiment examples, the spectral band selection filters 720 are "notch" filters or dichroic filters.

[0117] A dichroic filter is used when the interferometer is a two-spectral-band interferometer. A notch filter is used when the interferometer is a more than two-spectral-band interferometer.

[0118] The spectral filtering device further comprises N spectral band-selective filters 720 arranged on the first channel of the interferometer 300. More specifically, a spectral band-selective filter 720 forms a reflective facet 380 of the reflective face B (i.e., the reflective face of the first channel of the interferometer 300). The spectral band-selective filter 720 forming each reflective facet 380 of the reflective face B, arranged respectively at each zone optically corresponding to a facet 370 of the reflective face C, is identical to the spectral band-selective filter 720 forming the reflective facet 370 of said reflective face B.

[0119] In an improved embodiment, the spectral filtering device may include a rejection filter 730 upstream of the interferometer 300. This rejection filter 730 is advantageously associated with the spectral multiband filter 710 and is configured to reject the flux not included in the N selected spectral bands.

[0120] In a preferred embodiment, the rejection filter 730 is arranged on one face of the window 102. When the spectral multiband filter 710 is arranged on one face of the window 102, the rejection filter 730 is arranged on the other face of the window 102.

[0121] In one embodiment, the spectral multiband filter 710 is arranged on the external face 103 of the window 102 located outside the sensor 100 and the filter of rejection 730 is arranged on a face called internal face 104, located inside the sensor 100.

[0122] According to the invention, the arrangement of a spectral filtering device on the one hand on the window 102 and on the other hand at the level of the interferometer 300, and not at the level of the matrix detector 500, advantageously allows the use of a standard matrix detector, which makes it possible to significantly reduce the cost of the imager 1.

[0123] Preferably, as illustrated in Figures 1, 3 and 4, the sensor 100 comprises two light traps 600, 601. The two light traps 600, 601 are independent parts of the body forming the optical component 200. A first light trap 600 is arranged opposite the reflective face B (i.e. the reflective face of the first channel of the interferometer 300) and a second light trap 601 is arranged opposite the reflective face C (i.e. the reflective face of the second channel of the interferometer 300). The first light trap 600 (respectively the second light trap 601) is configured to absorb the flux not included in the desired spectral bands and transmitted by the N spectral band selection filters 720 of the reflective face B (respectively of the reflective face C). The two light traps 600, 601 are arranged on or in the housing 101. Output optics

[0124] As explained previously, the optical component 200 includes an output optic 400.

[0125] The output optics 400 are located downstream of the interferometer 300, in the direction of propagation of the incident light beam. In other words, the output optics 400 are located at the output of the interferometer 300. The output optics 400 receive the incident light beams from the interferometer 300 and direct them to the matrix detector 500, via the output face H.

[0126] The reflective face E, the reflective face F and the reflective face G are preferably mirrors.

[0127] In one embodiment, the reflective face E, reflective face F, reflective face G, and exit face H of the body forming the optical component 200 are formed on surfaces of the body at the interface with the external environment. A reflective coating is applied to these surfaces respectively to form the reflective faces E, F, and G. Similarly, the exit face H of the body forming the optical component 200 is formed on a surface of the body at the interface with the external environment. An anti-reflective coating is applied to this surface to eliminate unwanted reflections within the internal volume of the body.

[0128] In a preferred embodiment, the output optic 400 is of the Offner type. The reflective face F is placed between the reflective face E and the Reflective face G, in the direction of propagation of the light beams. Reflective face E and reflective face G are two concave mirrors. Reflective face F is a convex mirror.

[0129] The reflective face E, the reflective face F and the reflective face G are placed in a concentric geometry.

[0130] The reflective faces E, F and G can be configured so that the magnification of the Ôffner type output optics is equal to 1.

[0131] The reflective faces E and G can be made on a common surface of the body forming the optical component which has been locally treated to obtain each reflective face E and G, as illustrated in Figures 1 and 2. The reflective face F and the common surface treated locally to obtain the reflective faces E and G are spherical or aspherical.

[0132] Alternatively, as illustrated in [Fig.3], the reflective faces E and G can be made on two separate surfaces of the body forming the optical component 200. The reflective faces E and G are spherical, aspherical or freeform.

[0133] The Öffner-type output optics are also telecentric. Telecentric means that the optical beams are not inclined relative to the field of view of the interferometer 300 but have the same axis. Thus, it is possible to obtain a uniform spectral response by stepping the path difference. Therefore, it is possible to obtain a spectral response independent of the position within the interferometer's field of view. The reflective face F is positioned at the foci of the reflective faces E and G. The periphery of the reflective face F acts as a diaphragm. The position and absorbing periphery of the reflective face F advantageously create a telecentric exit optic.

[0134] In one embodiment, as illustrated in [Fig.2], the output optic 400 is made from an elementary block 401 on which a reflective treatment is applied to form each of the three reflective faces E, F, G.

[0135] Preferably, in order to avoid inducing spurious reflections, the elementary block 401 forming the output optic 400 is made of the same material as the two elementary blocks 301, 302 forming the interferometer 300. The elementary block 401 forming the output optic 400 is assembled by optical bonding or by molecular adhesion to the second elementary block 302 of the interferometer 300.

[0136] Optical bonding assembly ensures high operational stability between the interferometer 300 and the output optics 400.

[0137] The output optic 400 may include a reflecting prism, as illustrated in [Fig. 1]. The use of such a reflecting prism advantageously contributes to making the more compact 100 sensor. The output optics 400 thus comprise, in addition to the elementary block 401, another elementary block 402. This elementary block 402 forms the reflecting prism and includes the output face H. Preferably, in order to avoid inducing unwanted reflections, the elementary block 402 is made of the same material as the elementary block 401. The elementary block 402 is assembled by optical bonding or molecular adhesion to the elementary block 401 of the output optic 400. Example of material / dimensioning

[0138] The body forming the optical component 200 is made of materials having different refractive indices.

[0139] In a preferred, non-limiting embodiment, the elementary blocks 301, 302, 401 forming the interferometer 300 and the output optics are made of a first material having a refractive index n1. The first plates 351, 361 of the stacks on each of the interferometer channels are also made of this first material. The second plates 352, 362 of the stacks on each of the interferometer channels are made of a second material having a refractive index n2.

[0140] The choice of materials is advantageously made in the following manner: - the materials are transparent to the wavelengths of the incident light beam (light spectrum of the observed scene), more particularly to the wavelengths associated with the spectral bands linked to each of the ranges of each step of the path difference created in the interferometer 300, - the thermal expansion of the materials compensates each other, so as to make the component 200 insensitive to the external temperature, thus minimizing the variations of the path difference in the optical component due to temperature, - the refractive indices are close to minimize unwanted reflections, but the difference between the refractive indices must be sufficient to perform the functions of creating path difference jump(s) and aperture compensation in a small footprint.

[0141] It is clear that the choice of materials is within the reach of a person skilled in the art.

[0142] The refractive indices ni and n2 can be selected from the group of lenses including the following glass references: N-F2, N-SF5; N-SF14, N-LAK22; silica, Zerodur ®

[0143] The housing 101 of the sensor 100 is preferably intended on the one hand to protect the optical component 200 and the matrix detector 500, in particular from dust, and on the other hand to thermally insulate them from the outside.

[0144] When the optical component 200 is mainly made of glass referenced N-F2, N-SF5, N-SF14, N-LAK22, the housing 101 can for example be made of titanium.

[0145] When the optical component 200 is mainly made of silica or Zerodur®, the housing 101 can for example be made of Invar®.

[0146] The body forming the optical component 200 and preferably assembled by optical bonding with the housing 101.

[0147] When the materials for the elementary blocks forming the optical component 200 are made of glass, in particular the glasses referenced N-F2, N-SF5, N-SF14, N-LAK22, and the housing is made of titanium, the thermal expansion between the bonding points of the optical component 200 with the housing 101 remains on the order of 1 pm for a maximum non-operational temperature variation of -60°C.

[0148] The window 102 is preferably assembled to the housing 101 by bonding or brazing. Bonding the window advantageously seals the housing 101 against external contamination. The seal is preferably complete, so as to provide both particulate and molecular protection.

[0149] The window 102 is advantageously made of a material transparent to the wavelengths of the incident light beam, more particularly to the wavelengths associated with the spectral bands linked to each of the ranges of each step difference jump created in the interferometer 300.

[0150] The window 102 can be made of transparent glass in the spectral bands to be measured, for example of silica in the visible and near IR range, of germanium or of zinc selenide (ZnSe) for thermal infrared.

[0151] A design of athermal optical treatments in which the positions of spectral bands do not depend on temperature may require more specific window material choices, such as for example the F7, WMS-02.

[0152] A surface treatment is applied to one of the faces of the window, for example the external face 103, to form the spectral multi-band filter.

[0153] A surface treatment is applied to one of the faces of the window, preferably the inner face 104, to form the rejection filter. Example of dimensioning#:

[0154] The sensor 100 according to the invention advantageously has reduced dimensions. The optical component 200 and the matrix detector 500 can be contained without difficulty in a housing 101 having, for example, a cubic shape, on the order of 15cm on each side.

[0155] In one embodiment, the elementary blocks 301, 302 of the interferometer, the first blades 351, 361 of each stack and on each channel, and the elementary block 401 (and where applicable the elementary block 402) of the output optics are The first two blades (351, 352, 362) of each stack and on each track are made of N-SF57 (also called Flint). The width of each stack of the first and second blades (351, 352, 361, 362) on each track is between 7 and 9 millimeters. The thickness of the first and second blades (351, 352, 361, 362) of each stack and on each track is between 3 and 8 millimeters. The bevel of the first and second blades of each stack on the second track is between 10 and 20 milliradians. Functioning

[0156] The following description illustrates an example of propagation of light beams from the scene to be analyzed in sensor 100.

[0157] By way of illustration only, [Fig. 1] shows an example of the trajectory of a light beam incident in the sensor 100, and more specifically in the optical component 200. For clarity, only the two rays of the incident light beam forming the beam at the center of the field of view of the interferometer 300 are shown. Arrows on the rays indicate the direction of propagation of these rays. In [Fig. 4], only three rays from two incident light beams forming the beams at the edge of the field of view of interferometer 300 are shown for clarity. This [Fig. 4] highlights the telecentric characteristic of the Offner.

[0158] Returning to [Fig. 1], incident light beams from the scene to be analyzed are collected by the input optics 10, which transmits them towards the window 102. The window 102 only transmits incident light beams with wavelengths corresponding to the desired spectral bands. The incident light beams transmitted by the window 102 are then directed into the optical component 200, via the input face A.

[0159] The light beams are transmitted into the body forming the optical component 200, first passing through the interferometer 300 and then the output optics 400.

[0160] The light beams entering the body forming the optical component 200 are split by the semi-reflective surface 340. Part of the light beams is reflected by the semi-reflective surface D, then reflected by the reflective face B, and then transmitted by the reflective surface D to the output optic 400, more precisely to the reflective face E of the body. On the other hand, the other part of the light beams is transmitted by the semi-reflective surface D, then reflected by the reflective face C, and then reflected by the reflective surface D to the reflective face E of the body.

[0161] Each facet 370 of the reflective face B will reflect only light beams having wavelengths within the associated spectral band to the stack comprising said facet. The same applies to each facet 380 of the reflective face C.

[0162] The light beams reflected by the reflective face B and the light beams reflected by the reflective face C will form an interference pattern in the image plane of the matrix detector 500. The light beams reflected by the reflective face B and those reflected by the reflective face C are directed towards the reflective face E. The light beams are reflected by the reflective face E towards the reflective face F. The latter reflects the light beams towards the reflective face G, which reflects them towards the output face H of the body forming the optical component 200, which transmits them to the matrix detector 500. The output optics 400 thus focus the light beams into the plane of the matrix detector 500, forming distinct images on the matrix detector 500.

[0163] When the interferometer 300 comprises, on each channel, for each stack, only the first blade (351; 361) and the second blade (352; 362), it is clear that, between the entrance face A and the exit face H of the body forming optical component 200, the path taken by the light beams is made only in the different materials constituting the body forming the optical component, without passing through air.

[0164] In other words, the imager 1 will generate, by means of the input optics 10, an image of the observed scene in intermediate image planes of the interferometer 300 (at the level of the reflecting face B and the reflecting face C) and then transport, by means of the output optics 400, the images obtained in the intermediate image planes into the plane of the matrix detector 500. The spectral filtering device (comprising the spectral multiband filter 710 and the spectral band selection filters 720 forming the reflecting face B and the reflecting face C) makes it possible to select a specific spectral band associated with each step difference jump created by the interferometer 300.

[0165] At the matrix detector 500, the images obtained correspond to the superposition of the image from the first channel of the interferometer (corresponding to a clear image of the observed scene) and the image from the second channel of the interferometer (corresponding to a slightly modified image of the observed scene, taking into account the various market difference jumps created). This superposition of the two images creates an image modulated by a sinusoid.

[0166] The imager 1 according to the invention will thus allow the acquisition of N partial interferograms in N specific spectral bands.

[0167] Synchronizing the acquisitions of the matrix detector 500 with the scrolling of the image of the scene observed in the plane of said matrix detector will allow to obtain the different path difference samples, associated with a one-pixel interferogram of the image.

[0168] The foregoing description clearly illustrates that, through its various features and their advantages, the present invention achieves its intended objectives. In particular, it presents a sensor for a multispectral partial interferogram imager that helps generate partial interferograms with a homogeneous spectral response and little stray light. The sensor is relatively insensitive to the thermal environment. The sensor is compact. By using a standard matrix detector, the development time and costs of the sensor are reduced.

Claims

Demands

1. Multispectral partial interferogram sensor (100) for multispectral partial interferogram imager (1) comprising: - a case (101), - an entry window, called a porthole (102), fixed to said housing, - an optical component (200), positioned in said housing, - a spectral filtering device (710, 720), - a matrix detector (500), positioned in said housing, the optical component (200) being in the form of a body comprising: - an entrance face A, arranged opposite the porthole (102), - a reflective face B, - a reflective face C, - a semi-reflective surface D, - a reflective face E, - a reflective face F, - a reflective face G, - an output face H, arranged opposite the matrix detector (500), the entrance face A, the reflective faces B and C and the semi-reflective surface D being arranged relative to each other to form a static two-way interferometer, called interferometer (300), the reflective face B being disposed on a first channel of the interferometer and the reflective face C being disposed on a second channel of the interferometer, the reflective face B having N reflective facets, the reflective face C having N reflective facets, N greater than or equal to 1, the reflective faces E, F and G and the output face H being arranged relative to each other to form an output optic (400) of the Ôffner type, the interferometer (300) being positioned upstream of the output optics (400), the interferometer (300) comprising: - on the first channel, from a plane PI, N stacks of at least two plates, including a first plate (361) and a second plate (362), of material having different refractive indices (ni, n2), each stack ending in a reflective facet (380), - on the second channel, from a plane P2, N stacks of at least two plates, including a first plate (351) and a second plate (352), of material having different refractive indices (ni, n2), said at least two plates (351, 352) each being beveled, each stack ending in a reflective facet (370), the plane PI, on the first channel, and the plane P2, on the second channel, corresponding to the same path of a light beam incident in the interferometer (300), the spectral filtering device comprising: - a spectral multiband filter (710) arranged on one face of the window (102) or on the entrance face A of the optical component (200),- N spectral band selection filters (720) arranged on the first channel of the interferometer (300), each spectral band selection filter forming a reflective facet (380) of the reflective face B, - N spectral band selection filters (720) arranged on the second channel of the interferometer (300), each spectral band selection filter forming a reflective facet (370) of the reflective face C.

2. Multispectral partial interferogram sensor (100) according to claim 1 wherein, on each channel, the interferometer (300) comprises only the first blade (351; 361) and the second blade (352; 362) per stack, said first and second blades of each stack being successive, without space or air gap between them.

3. Multispectral partial interferogram sensor (100) according to any one of the preceding claims comprising light traps (600, 601) arranged respectively opposite the reflective face B and the reflective face C.

4. Multispectral partial interferogram sensor (100) according to any one of the preceding claims wherein the interferometer (300) has the general shape of a hexagonal prism.

5. A multispectral partial interferogram sensor (100) according to any one of the preceding claims, wherein the interferometer (300) is formed of two joined elementary blocks, referred to as the first elementary block (301) and the second elementary block (302), on which are assembled the at least two blades (351, 352) forming the N stacks on the first channel and the at least two blades (361, 362) forming the N stacks on the second channel, and wherein the output optics (400) is formed in an elementary block (401), said elementary block (401) being assembled by optical bonding or molecular adhesion to the second elementary block (302) of the interferometer (300).

6. Sensor (100) with multispectral partial interferograms according to any one of the preceding claims wherein the reflective faces E and G are made on a common surface of the body forming the optical component (200).

7. Sensor (100) with multispectral partial interferograms according to the preceding claim in which the reflective faces E, F and G have a spherical, aspherical or free shape.

8. Multispectral partial interferogram sensor (100) according to any one of the preceding claims wherein the output optics (400) comprises a reflecting prism.

9. Multispectral partial interferogram sensor (100) according to any one of the preceding claims wherein the filtering device comprises a rejection filter (730) arranged on one face of the window (102), distinct from the face of the window comprising the spectral multiband filter (710).

10. Imager (1) comprising an input optic (10), a multispectral partial interferogram sensor (100) according to any one of the preceding claims, and an image processing module configured to analyze the images acquired by the matrix detector (500).

Images