Infrared radiation detector

Abstract

This infrared radiation detector comprises: - a cryostat provided with a cold finger, capable of ensuring heat exchange with a cold source, and a window transparent to the infrared radiation to be detected; - a cold plane, mechanically fixed to and in a heat exchange relationship with the cold finger; - a detection block comprising at least one detection matrix sensitive to the infrared wavelength range to be detected, and in a heat exchange relationship directly or indirectly with the cold plane; - a cold shield mechanically fixed to and in a heat exchange relationship with the cold plane and capable of limiting parasitic radiation, the cold shield being provided with at least one diaphragm or baffle (9), oriented radially with respect to the overall orientation of the detector, and provided with an opening capable of allowing the transit of the optical path resulting from the incident radiation. The free edge (13) of the at least one diaphragm or baffle (9) is structured such that non-linear patterns (14) are defined.

Description

[0001] INFRARED RADIATION DETECTOR

[0002] FIELD OF THE INVENTION

[0003] The invention relates to infrared detectors, and more particularly those operating at low temperature (quantum detectors), i.e. typically at temperatures between 50 and 200 Kelvin.

[0004] 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 unitary or elementary detectors or sensors, called photosites, and typically a matrix of photodiodes also called FPA for the English expression "Focal Plane Array", this detector being associated with a reading circuit. The assembly, commonly called a detection block, is mechanically fixed and in heat exchange with the cold finger of the cryostat, so as to bring said detector to the desired temperature.

[0005] The sensor itself, typically the FPA matrix, is therefore positioned within the vacuum enclosure 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 ensuring cryogenic cooling of the sensor is housed.

[0006] Such a detector is shown schematically in Figures 1 and 2, respectively in exploded perspective and in section.

[0007] This detector (1) comprises a cold finger (2) connected to a cold source (in particular a Stirling or Joule-Thomson engine - not shown). An infrared sensor (3) is mounted on the end wall (4) of the cold finger, and therefore in heat exchange with the latter. This sensor (3), of square or rectangular 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 chamber or Dewar (6). The detector also comprises a cold screen (7), provided with a through opening (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 flow of infrared radiation likely to reach said sensor (3), in addition to trapping the incident rays coming from the off-field sources.

[0008] 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 sensor's cone of perception by excluding external sources of infrared radiation, described as out-of-field and constituting parasites.

[0009] For this purpose, the detector traditionally comprises the cold screen previously described, open at its top, said opening defining a field diaphragm.

[0010] The main objective of the invention is to overcome the noise coming from the parasitic infrared flux outside the field, which is reflected on the internal elements of the cold screen.

[0011] PRIOR STATE OF THE ART

[0012] 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 absorbing the wavelengths of parasitic infrared radiation, and typically with a so-called NPC (nickel phosphorus converted) coating.

[0013] It has also been proposed to attach, at the level of the internal face of the cold screen, a certain number of diaphragms, also called baffles, directed radially, and whose function is to stop, or to reflect outwards, the incident rays in a direction other than that of the nominal field of vision of the sensor or detector. Such a configuration is schematically represented in Figure 3, in which the cold screen (7) comprises two diaphragms or baffles (9). The disadvantage of these baffles or diaphragms (9) lies in the fact that they are traditionally made from a metal sheet and typically nickel (10), with a thickness of around 50 micrometers, within which a laser or chemical cutting is carried out in order to define an opening. Then, this sheet is coated with an NPC layer (11) with a typical thickness of 15 micrometers.In doing so, this generates a total thickness of the baffle or diaphragm of around 80 micrometers, de facto defining a lateral edge (12), of substantially the same length, capable of promoting specular reflections likely to be directed towards the FPA matrix, as illustrated in Figure 4. These specular reflections generate a parasitic secondary image at the level of the FPA matrix. Furthermore, part of the incident radiation is also reflected specularly on the areas formed by the fillets of the NPC coating, that is to say 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.

[0014] Figure 5 illustrates the optical diagram of secondary image formation resulting from parasitic reflections on the edge or lateral edge of such a diaphragm.

[0015] Thus, if we consider a point light source (15) outside the field and at infinity, the main 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 according to an image cone which is partly reflected on the edge (12) of the diaphragm (9). A part of the image cone is specularly reflected (16) on a substantially flat facet and is focused on the FPA matrix (3) to form a parasitic secondary image (17), and another part of the image cone is specularly reflected (18) on 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.

[0016] In order to overcome this drawback, it has been proposed to mask the radiation from the off-field light source upstream of the entrance pupil of the objective with which the detector is equipped, using a sun visor. This solution is however not viable due to the bulk it generates. Another solution has also been proposed, consisting of refining as much as possible the free edge of the diaphragm or the baffle, in order to obtain the greatest possible reduction of the lateral edge (12) on which the radiation is likely to be reflected, and thus generate the drawbacks previously described. It turns out, however, that this solution is complex to achieve on an industrial level, and that in any case, there remains a residual right flank on which at least part of the radiation is likely to be reflected.

[0017] STATEMENT OF THE INVENTION

[0018] The invention therefore relates to an infrared radiation detector which comprises:

[0019] • a cryostat equipped with a cold finger, capable of ensuring heat exchange with a cold source, and a window transparent to the infrared radiation to be detected;

[0020] • a cold plane, mechanically fixed and in thermal exchange with the cold finger;

[0021] • a detection block comprising at least one detection matrix sensitive to the range of infrared wavelengths to be detected, and in direct or indirect heat exchange with the cold plane;

[0022] • a cold screen fixed mechanically and in thermal exchange with the cold plane and capable of limiting parasitic radiation, the cold screen being provided with at least one diaphragm or baffle, oriented radially relative to the general orientation of the detector, and provided with an opening capable of allowing the transit of the optical path resulting from the incident radiation.

[0023] According to the invention, the free edge of said diaphragm or said baffles is structured in such a way as to define non-linear patterns.

[0024] In other words, the invention consists in structuring the free edge of the diaphragm(s), thus avoiding the presence of faces or facets facing the FPA matrix, so as to diverge the energy of the image cone - the result of an intense and out-of-field point source at infinity, rather than focusing it on the FPA matrix. This results in the image cone resulting from an out-of-field source attacking the edge or the edge of said at least one diaphragm, not along a single facet facing the FPA matrix, but along a plurality of such surfaces thus comprising a plurality of diverging facets, so that the secondary parasitic image is split. Admittedly, this secondary parasitic image is not eliminated, but the parasitic flux is more diffused into a more uniform background, making it possible not to significantly impact the scene image of the incident flux that it is thus desired to capture.

[0025] 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 hundreds of micrometers.

[0026] By quasi-periodic, we mean the fact that the repetition step (or equivalently the size of the patterns) is not necessarily constant, but can vary, including randomly, allowing a reasonable variation compared to the average value of said step, and typically between 50 and 200% of said average value.

[0027] According to another advantageous characteristic of the invention, each of the patterns consists of two arcs of a circle with a radius of curvature converging in the direction of the opening, so as to form patterns, in particular in the form of convex lace.

[0028] BRIEF DESCRIPTION OF THE FIGURES

[0029] The manner in which the invention can be implemented and the advantages which result therefrom will emerge more clearly from the following example of implementation, given for informational and non-limiting purposes, with the support of the appended figures.

[0030] Figure 1 is an exploded perspective schematic representation of a prior art quantum infrared detector.

[0031] Figure 2 is a schematic representation in sagittal section of the prior art detector of Figure 1.

[0032] Figure 3 is a schematic perspective representation of a prior art cold shield.

[0033] Figure 4 is a schematic representation in partial cross-section of a diaphragm or baffle according to the prior art.

[0034] Figure 5 is a schematic illustration illustrating the optical path and the formation of the secondary image by the implementation of a diaphragm according to Figures 3 and 4. Figure 6 is a schematic representation of a part of the diaphragm or baffle according to the invention.

[0035] Figure 7 is a schematic perspective representation of a part of the diaphragm or baffle according to the invention.

[0036] DETAILED DESCRIPTION OF THE INVENTION

[0037] In relation to Figures 6 and 7, the free edge (13) of a diaphragm or baffle (9), attached to the internal face of the cold screen (7), according to the invention, has been shown. 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, that is to say of convex shape.

[0038] In the example described, these structures or patterns (14) are of a periodic nature, and typically in a step of 0.2 millimeters.

[0039] However, this periodicity is not imperative, as long as there is an irregular structure at the edge (13) of the diaphragm or baffle.

[0040] At the same time, 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 lacework, so that the edge, i.e. the edge (13) acts like a convex mirror, capable of diverging the energy coming from the image cone rather than focusing said cone on the FPA matrix.

[0041] Due to this structuring of the edge of the baffle or diaphragm, the image cone attacks said edge (13) not according to one facet, but according to a plurality of patterns defining a plurality of divergent facets, resulting in a splitting of the secondary parasitic image, constituting a process similar to diffusion, and therefore of a more uniform nature, even though according to the prior art, this parasitic image cone is concentrated into a parasitic secondary image and a linear trail, affecting the quality of the image obtained. In other words, the implementation of the thus structured edge of the diaphragm or baffle causes a destructuring of the parasitic secondary image, making it possible to less pollute the scene image that one wishes to detect.We can therefore understand the full interest of the present invention which allows, through the structuring of the internal edge of the baffle and the diaphragm, typically obtained by means of a femtosecond laser, to improve the quality of the main image to be detected.

[0042] Furthermore, in terms of industrialization, the process of producing such baffles or diaphragms is not impacted, except by the cutting path of the nickel sheet from which these elements are made.

Claims

CLAIMS 1. Infrared radiation detector which includes: • a cryostat equipped with a cold finger (2), capable of ensuring heat exchange with a cold source, and a window (5) transparent to the infrared radiation to be detected; • a cold plane, mechanically fixed and in thermal 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 direct or indirect heat exchange with the cold plane; • a cold screen (7) mechanically fixed and in thermal exchange with the cold plane and capable of limiting parasitic radiation, the cold screen being provided with at least one diaphragm or baffle (9), oriented radially relative to the general orientation of the detector, and provided with an opening (8) capable of 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 produced 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 hundreds of micrometers.

4. Infrared radiation detector according to claim 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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