INFRARED RADIATION DETECTOR

Abstract

This infrared radiation detector comprises: – a cryostat equipped with a cold finger, suitable for heat exchange with a cold source, and a window transparent to the infrared radiation to be detected; – a cold plate, mechanically fixed and in heat exchange with the cold finger; – a detection block comprising at least one detection matrix sensitive to the range of infrared wavelengths to be detected, and in heat exchange directly or indirectly with the cold plate; – a cold screen mechanically fixed and in heat exchange with the cold plate and suitable for limiting parasitic radiation, the cold screen being equipped with at least one diaphragm or baffle (9), oriented radially with respect to the general orientation of the detector, and equipped with an opening suitable for allowing the transit of the optical path resulting from the incident radiation.The free edge (13) of said at least one diaphragm or baffle (9) is structured so as to define non-linear patterns (14). Figure for the abbreviation: Fig 6.

Description

Title of the invention: INFRARED RADIATION DETECTOR Scope of the invention

[0001] The invention relates to infrared detectors, and more particularly those operating at low temperature (quantum detectors), that is to say typically at temperatures between 50 and 200 Kelvin.

[0002] Such low-temperature infrared detectors operate according to quantum physics and are generally associated with a cryostatic chamber placed under vacuum, also called a cryostat. The detector consists of a plurality of individual or elementary detectors or sensors, called photosites, and typically a photodiode array, also called an FPA (Focal Plane Array), this detector being associated with a readout circuit. The assembly, commonly called the detection block, is mechanically fixed and in heat exchange with the cold finger of the cryostat, so as to bring the detector to the desired temperature.

[0003] The sensor itself, typically the FPA array, is therefore positioned within the vacuum chamber defined by the cryostat. The detector includes a window transparent to infrared radiation for the transmission of this radiation to the sensor, said window being traditionally mounted on an end wall of the cold finger in which the heat exchanger providing cryogenic cooling of the sensor is housed.

[0004] Such a detector is schematically represented in figures 1 and 2, respectively in exploded perspective and in section.

[0005] This detector (1) comprises a cold finger (2) connected to a cold source (in particular a Stirling or Joule-Thomson machine - not shown). An infrared sensor (3) is mounted on the end wall (4) of the cold finger, and thus in heat exchange with it. This sensor (3), square or rectangular in shape, is located directly above an optical window (5), transparent to the radiation of interest, and in this case infrared, and received at the upper end of a vacuum or Dewar flask (6).

[0006] The detector also includes a cold screen (7), provided with a through-hole (8), attached to the cold finger (2), and therefore in heat exchange with the latter, and surrounding the sensor (3). This cold screen (7) is intended to limit the flux of infrared radiation likely to reach said sensor (3), in addition to trapping incident rays from off-field sources.

[0007] In order to obtain the most accurate image possible of the radiative scene, typically consisting of a thermal source, it is necessary to limit the cone of perception of the sensor by excluding external sources of infrared radiation, which are described as off-field and constitute parasites.

[0008] For this purpose, the detector traditionally includes the cold screen described above, open at its top, said opening defining a field diaphragm.

[0009] The main objective of the invention is to eliminate noise from the parasitic off-field infrared flux, which is reflected on the internal elements of the cold screen. Prior state of the art

[0010] In order to overcome this difficulty, and therefore to minimize this parasitic flux, it has been proposed for example to cover certain internal components of the cold screen with a material that absorbs the wavelengths of the parasitic infrared radiation, and typically with a coating called NPC (nickel phosphorus converted).

[0011] It has also been proposed to attach, to the inner face of the cold screen, a number of diaphragms, also called baffles, directed radially, whose function is to stop, or reflect outwards, the incident rays in a direction other than that of the nominal field of view of the sensor or detector. Such a configuration is schematically represented in [Fig. 3], in which the cold screen (7) comprises two diaphragms or baffles (9).

[0012] The drawback of these baffles or diaphragms (9) lies in the fact that they are traditionally made from a metallic sheet, typically nickel (10), approximately 50 micrometers thick, which is cut by laser or chemically to define an aperture. This sheet is then coated with an NPC layer (11) typically 15 micrometers thick. This results in a total thickness of approximately 80 micrometers for the baffle or diaphragm, which in turn defines a lateral edge (12) of roughly the same length, capable of promoting specular reflections that can be directed towards the FPA matrix, as illustrated in [Fig. 4]. These specular reflections generate a secondary image that is an unwanted image on the FPA matrix.Furthermore, part of the incident radiation is also reflected specularly on the areas formed by the fillets of the NPC coating, i.e. the rounded edge with a radius of curvature corresponding to the thickness of the NPC coating, typically a radius of 15 micrometers, acting as a convex cylindrical mirror and creating a substantially linear drag on the FPA matrix by uniaxial divergence.

[0013] Figure 5 illustrates the optical scheme of secondary image formation resulting from parasitic reflections on the edge or lateral edge of such a diaphragm.

[0014] Thus, if we consider an off-screen light source (15) that is a point source at infinity, the principal image is projected onto the image plane at a radial position r, greater than the lateral dimensions of the FPA matrix (3). This light source is seen as an image cone that is partially reflected onto the edge (12) of the diaphragm (9). Part of the image cone is reflected specularly (16) onto a substantially flat facet and focuses onto the FPA matrix (3) to form a secondary parasitic image (17), and another part of the image cone is reflected specularly (18) onto the areas formed by the fillets (rounded edges) of the NPC coating (11), acting as a convex cylindrical mirror, creating a linear trail (19) on the FPA matrix by uniaxial divergence.

[0015] To overcome this drawback, it has been proposed to mask the radiation from the off-screen light source upstream of the entrance pupil of the lens on which the detector is equipped, using a sunshade. However, this solution is not viable due to the increased size it creates.

[0016] Another solution has also been proposed, consisting of making the free edge of the diaphragm or baffle as thin as possible in order to achieve the greatest possible reduction of the lateral edge (12) on which the radiation is likely to be reflected, and thus generate the drawbacks described above. However, it turns out that this solution is complex to implement industrially, and that in any case, a residual right side remains on which at least part of the radiation is likely to be reflected. Description of the invention

[0017] The invention therefore relates to an infrared radiation detector which comprises: - a cryostat equipped with a cold finger, suitable for heat exchange with a cold source, and a window transparent to the infrared radiation to be detected; - a cold plate, mechanically fixed and in heat exchange with the cold finger; - a detection block comprising at least one detection matrix sensitive to the range of infrared wavelengths to be detected, and in heat exchange directly or indirectly with the cold plate; - a cold screen mechanically fixed and in heat exchange with the cold plane and capable of limiting parasitic radiation, the cold screen being equipped with at least one diaphragm or baffle, oriented radially with respect to the general orientation of the detector, and equipped with an opening capable of allowing the transit of the optical path resulting from the incident radiation.

[0018] According to the invention, the free edge of said diaphragm or of said baffles is structured in such a way as to define non-linear patterns.

[0019] In other words, the invention consists of structuring the free edge of the diaphragm(s), thereby avoiding the presence of faces or facets opposite the FPA matrix, so as to diverge the energy of the image cone—resulting from an intense, off-screen point source at infinity—rather than focusing it onto the FPA matrix. As a result, the image cone from an off-screen source strikes the edge or slice of at least one diaphragm, not along a single facet opposite the FPA matrix, but along a plurality of such surfaces, thus comprising a plurality of diverging facets, so that the secondary stray image is dispersed. While this secondary stray image is not eliminated, the stray flux is diffused more evenly into a more uniform background, preventing significant impact on the scene image of the incident flux that one wishes to capture.

[0020] According to the invention, the patterns resulting from the structuring of the edge of said at least one diaphragm are produced in a periodic or quasi-periodic manner, this periodicity being able to vary from a few tens to a few hundred micrometers.

[0021] By quasi-periodic, we mean that the repetition step (or equivalently the size of the patterns) is not necessarily constant, but can vary, including randomly, allowing a reasonable variation from the average value of said step, and typically between 50 and 200% of said average value.

[0022] According to another advantageous feature of the invention, each of the motifs consists of two circular arcs of radius of curvature converging in the direction of the opening, so as to form convex motifs, particularly lace-like patterns. Brief description of the figures

[0023] The manner in which the invention can be implemented and the resulting advantages will be more apparent from the following example of implementation, given by way of illustration and not limitation, in support of the attached figures.

[0024] Fig. 1 is a schematic exploded perspective representation of a prior art quantum infrared detector.

[0025] The [Fig.2] is a schematic sagittal section representation of the anterior art detector of the [Fig.1].

[0026] The [Fig.3] is a schematic perspective representation of a prior art cold screen.

[0027] The [Fig.4] is a schematic representation in partial cross-section of a diaphragm or baffle conforming to the prior art.

[0028] Fig. 5 is a schematic illustration showing the optical path and the formation of the secondary image by implementing a diaphragm conforming to figures 3 and 4.

[0029] Fig. 6 is a schematic representation of a part of the diaphragm or baffle according to the invention.

[0030] Fig. 7 is a schematic perspective representation of a part of the diaphragm or baffle according to the invention. Detailed description of the invention

[0031] Figures 6 and 7 show the free edge (13) of a diaphragm or baffle (9), attached to the inner face of the cold screen (7) according to the invention. As can be seen in these two figures, this edge (13) is not linear, but has a transverse structure and, in this case, typically in the form of a succession of arcs of circles converging in the direction of the optical axis of the cold screen (7) within which these diaphragms or baffles are attached, i.e., convex in shape.

[0032] In the example described, these structures or patterns (14) are of a periodic nature, and typically according to a step of 0.2 millimeter.

[0033] However, this periodicity is not imperative, provided that there is an irregular structure at the edge (13) of the diaphragm or baffle.

[0034] In parallel, this particular pattern does not have any facets in the direction of the FPA matrix, but on the other hand, has curved facets in such a way as to obtain convex lace, so that the edge, i.e. the edge (13) acts like a convex mirror, capable of diverging the energy from the image cone rather than focusing said cone on the FPA matrix.

[0035] Due to this structuring of the edge of the baffle or diaphragm, the image cone attacks said edge (13) not according to a facet, but according to a plurality of patterns defining a plurality of diverging facets, resulting in a bursting of the secondary parasitic image, constituting a process similar to diffusion, and therefore of a more uniform nature, whereas according to the prior art, this parasitic image cone is concentrated into a secondary parasitic image and a linear trail, affecting the quality of the image obtained.

[0036] In other words, the implementation of the structured edge of the diaphragm or baffle generates a destructuring of the secondary parasitic image, allowing less pollution of the scene image that one wishes to detect.

[0037] We can therefore understand the full advantage of the present invention which, by means of the structuring of the inner edge of the baffle and the diaphragm, typically obtained by means of a femtosecond laser, makes it possible to improve the quality of the main image to be detected.

[0038] Moreover, in terms of industrialization, the process of making such baffles or diaphragms is not impacted, except by the cutting path of the nickel sheet from which these elements are made.

Claims

Demands

1. An infrared radiation detector comprising: - a cryostat equipped with a cold finger (2), suitable for heat exchange with a cold source, and a window (5) transparent to the infrared radiation to be detected; - a cold plate, mechanically fixed and in heat exchange with the cold finger (2); - a detection block comprising at least one detection matrix (3) sensitive to the range of infrared wavelengths to be detected, and in heat exchange directly or indirectly with the cold plate; - a cold screen (7) mechanically fixed and in heat exchange with the cold plate and suitable for limiting stray radiation, the cold screen being equipped with at least one diaphragm or baffle (9), oriented radially with respect to the general orientation of the detector, and equipped with an opening (8) suitable for allowing the transit of the optical path resulting from the incident radiation;characterized in that the free edge (13) of said at least one diaphragm or baffle (9) is structured in such a way as to define non-linear patterns (14).

2. Infrared radiation detector according to claim 1, characterized in that the patterns (14) resulting from the structuring of the free edge (13) of said at least one diaphragm or baffle (9), are made in a periodic or quasi-periodic manner.

3. Infrared radiation detector according to claim 2, characterized in that the periodicity is between a few tens and a few hundred micrometers.

4. Infrared radiation detector according to any one of claims 1 to 3, characterized in that each of the patterns consists of two circular arcs of radius of curvature converging so as to form convex patterns

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