Optical refrigerator

Abstract

An optical refrigerator (1), particularly for cooling an infrared sensor or detector, the refrigerator comprising a laser radiation source (2) and a cooling crystal (3) configured to receive the laser radiation from the source and cool itself by anti-Stokes fluorescence, characterized in that the laser radiation source and the crystal are configured to allow total internal reflection within the crystal of the laser radiation emitted by the source and received by the crystal. Figure to be published with the abbreviation: Fig. 3

Description

Title of the invention: Optical refrigerator FIELD OF INVENTION

[0001] The present invention relates to an optical refrigerator, particularly for cooling a component, for example, a photonic, electronic, or superconducting component, especially an infrared sensor or detector. The refrigerator comprises a laser radiation source and a cooling crystal configured to absorb the laser radiation from the source and cool itself by anti-Stokes fluorescence. The invention also relates to a detection assembly comprising a detector and a detector refrigerator. STATE OF THE ART

[0002] For Earth observation by satellite in the infrared range, it is necessary to cool infrared detectors or photodetectors to cryogenic temperatures, typically below 200 K. Current solutions are based either on the use of passive systems or on active systems based on mechanical actuators.

[0003] A known optical cooling method is based on anti-Stokes displacement or "anti-Stokes fluorescence." This type of cooling is described, for example, in US9362712, US60441610, or US8720219. This solution is based on the following physical principle: when the emission and absorption bands of a material (crystal) are superimposed, the excitation of the upper part of the absorption spectrum leads to the emission of photons with higher energy than the absorbed photons. This energy difference (emission / absorption) is due to the annihilation of phonons in the material's crystal lattice, the vibrational quanta for solids. In the solid state, thermal energy is mainly contained in the vibrational modes of the lattice. Indeed, thermal agitation causes the atoms to oscillate around their equilibrium position. Consequently, heat is extracted from the material by anti-Stokes emission, which induces a cooling effect. See [reference].See also the publication by M. Sheik-Bahe and RI Epstein, "Optical Refrigeration," Nat. Photonics 1, 12, 693 (2007). This refrigeration technology is potentially applicable to the cooling of sensors in the space sector.

[0004] Cooling an object by Anti-Stokes fluorescence has many advantages, for example it allows miniaturization, avoids vibrations, and has a low cost.

[0005] To date, the use of a fiber laser coupled to a multi-pass cavity (Herriot cell) represented the most efficient way of pumping the crystal.

[0006] Fig. 1 illustrates the structure and operating principle of an optical refrigerator.

[0007] Such an optical refrigerator 1 comprises a laser radiation source 2 configured to emit radiation towards a cooling crystal 3. The cooling crystal 3 is framed between at least two reflective mirrors 7 to concentrate the light into the crystal 3. The assembly, in particular the type of cooling crystal 3, is configured to generate cooling of the cooling crystal 3 by the "Anti-Stokes" effect.

[0008] The crystal 3 is put in heat exchange (contact) with an element 4 to be cooled via a thermal link 6.

[0009] The configuration of the two mirrors and the cooling crystal forms a non-resonant or multi-pass cavity. The use of the non-resonant cavity is dictated by the need to maximize laser absorption by the crystal in order to increase the overall efficiency of the refrigerator. Ideally, the entire laser beam is absorbed, and it is necessary to minimize parasitic losses in the cell. Therefore, the laser beam must be linearly polarized along the c-axis of the crystal to maximize absorption, and the laser beam must strike the crystal at Brewster incidence to minimize parasitic Fresnel reflections. This requires a high-power, polarized, transversely single-mode laser. To date, only fiber lasers meet these requirements.

[0010] Thus, the main obstacle to the use of optical refrigerators is energy efficiency or efficiency at the outlet. This efficiency is the combination of the efficiency of each of the optical refrigerator's components.

[0011] The electro-optical efficiency of fiber lasers is in the order of 20% to 30%, impacting the overall efficiency of the optical refrigerator.

[0012] There is therefore a need to increase the energy efficiency of optical refrigerators.

[0013] BRIEF DESCRIPTION OF THE INVENTION

[0014] The invention aims to meet this need and achieves this by means of an optical refrigerator, in particular for the cooling of an infrared sensor or detector, the refrigerator comprising a laser radiation source and a cooling crystal configured to receive the laser radiation from the source and to cool itself by Anti-Stokes fluorescence, in which the laser radiation source and the crystal are configured to allow total internal reflection in the crystal of the laser radiation emitted by the source and received by the crystal.

[0015] Advantageously, the configuration of the laser radiation source and the crystal allowing total internal reflection within the crystal reduces the configuration constraints of the optical refrigerator and makes the polarization constraints obsolete. and transverse single-mode for the laser radiation source. The optical refrigerator according to the invention therefore makes it possible to use diode lasers with an efficiency of around 50% as a laser radiation source.

[0016] According to one aspect of the invention, the cooling crystal included in the optical refrigerator is in a vacuum chamber, with a refractive index substantially equal to 1.

[0017] According to one aspect of the invention, the cooling crystal included in the optical refrigerator has the form of a straight cylinder, the laser and the crystal being configured so that the crystal receives the laser radiation in a direction substantially perpendicular to the axis of the cylinder and at an angle allowing total internal reflection in the crystal of the laser radiation received by the crystal, for example tangentially to the surface of the height of the cylinder.

[0018] According to one aspect of the invention, the cooling crystal included in the optical refrigerator has the shape of a cylinder having elliptical bases, for example circular.

[0019] According to one aspect of the invention, the cooling crystal included in the optical refrigerator comprises at least one notch on its height, said at least one notch being configured to receive the laser radiation from the laser radiation source.

[0020] According to one aspect of the invention, the optical refrigerator is configured so that the laser radiation is brought to said at least one notch through the free space or an optical fiber.

[0021] In particular, according to one aspect of the invention, the laser radiation is brought to the crystal by means of a lensed optical fiber either directly connected to said at least one notch or at a short distance from said at least one notch. According to one embodiment, said at least one notch may include a receiving means, for example a hole to accommodate the lens of the optical fiber.

[0022] According to one aspect of the invention, the cooling crystal included in the optical refrigerator has a substantially constant height greater than or equal to 0.5 mm, for example greater than or equal to 1 mm, and less than or equal to 4 mm, for example less than or equal to 3 mm.

[0023] According to one aspect of the invention, the cooling crystal included in the optical refrigerator has a diameter greater than or equal to 5 mm, for example greater than or equal to 7.5 mm and less than or equal to 20 mm, for example less than or equal to 15 mm.

[0024] According to one aspect of the invention, the laser is a diode laser.

[0025] According to one aspect of the invention, the laser is a fiber laser, namely a solid-state laser, in which the light and the booster pump are guided in optical fibers.

[0026] According to one aspect of the invention, at least one face of the crystal included in the optical refrigerator is covered with an optical treatment allowing at least partial, for example total, reflection of the light radiation generated by the crystal.

[0027] According to one aspect of the invention, the optical refrigerator further comprising at least one screen for absorbing the light radiation generated by the crystal, the at least one screen being arranged around the crystal.

[0028] According to one aspect of the invention, the optical refrigerator further comprising at least one thermal link in heat exchange on the one hand with the crystal and, on the other hand, an element to be cooled, in order to transfer cooling from the crystal to the element to be cooled, the element to be cooled being a cold finger or a focal plane intended to transfer cooling to a charge.

[0029] According to one aspect of the invention, the thermal link is transparent. In particular, according to one aspect of the invention, the optical refrigerator is devoid of a screen for absorbing the light radiation generated by the crystal.

[0030]

[0031] According to one aspect of the invention, the crystal is in direct thermal contact with an element to be cooled, for example glued to the element to be cooled, with a mirror on the glued face.

[0032] The invention also relates to a cooling crystal configured to receive laser radiation from a source and to cool itself by Anti-Stokes fluorescence, the crystal being configured to allow total internal reflection within the crystal of the laser radiation received by the crystal.

[0033] According to one aspect of the invention, the cooling crystal has the form of a straight cylinder, the laser and the crystal being configured so that the crystal receives the laser radiation in a direction substantially perpendicular to the axis of the cylinder and at an angle allowing total internal reflection in the crystal of the laser radiation received by the crystal, for example tangentially to the surface of the height of the cylinder.

[0034] According to one aspect of the invention, the cooling crystal has the shape of a cylinder having elliptical bases, for example circular.

[0035] According to one aspect of the invention, the cooling crystal comprises at least one notch on its height, said at least one notch being configured to receive the laser radiation from the laser radiation source.

[0036] According to one aspect of the invention, the cooling crystal has a substantially constant height greater than or equal to 0.5 mm, for example greater than or equal to 1 mm, and less than or equal to 4 mm, for example less than or equal to 3 mm.

[0037] According to one aspect of the invention, the cooling crystal has a diameter greater than or equal to 5 mm, for example greater than or equal to 7.5 mm and less than or equal to 20 mm, for example less than or equal to 15 mm.

[0038] The invention also relates to a detection assembly comprising a detector and detector refrigerator, wherein the refrigerator is in accordance with the invention.

[0039] In particular, according to one aspect of the invention, the optical refrigerator further comprising at least one thermal link in heat exchange on the one hand with the crystal and, on the other hand, an element to be cooled, in order to transfer cooling from the crystal to the element to be cooled, the element to be cooled being a cold finger or a focal plane intended to transfer cooling to a charge.

[0040] According to one aspect of the invention, the thermal bond is transparent. In particular, according to one aspect of the invention, the optical refrigerator is devoid of a screen for absorbing the light radiation generated by the crystal. BRIEF DESCRIPTION OF THE FIGURES

[0041] The invention will be better understood in the light of the following description, which is given by way of example only and is not intended to limit the said invention, together with the figures below:

[0042] [Fig.1] schematically represents an optical refrigerator according to the prior art,

[0043] [Fig.2] schematically represents an optical refrigerator according to the invention, and

[0044] [Fig.3] represents a crystal according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0045] Fig. 2 illustrates a structure and the operating principle of an optical refrigerator according to the invention.

[0046] Such an optical refrigerator includes a laser radiation source 2 configured to emit laser radiation towards a cooling crystal 3.

[0047] According to one aspect of the invention, the laser source is a multimode laser source, for example, the laser source is a laser diode. Advantageously, the use of a multimode laser source makes it possible to increase the electro-optical efficiency of the optical refrigerator.

[0048] Alternatively, the laser source can be a fiber laser, namely a solid-state laser, in which the light and booster pump are guided in optical fibers.

[0049] The cooling crystal 3 is configured to receive laser radiation from the source 2. Furthermore, the cooling crystal 3 is configured to cool itself by anti-Stokes fluorescence. For example, the cooling crystal 3 is a LiYF4 crystal doped with 7.5% Yb3+ or doped with 1% Ho3+.

[0050] According to an embodiment shown in [Fig.2], the crystal is placed in a vacuum chamber, having a refractive index substantially equal to 1. The optical refrigerator according to the invention is intended to cool a thermal load 4. This thermal load can also be disposed in the vacuum chamber.

[0051] The cooling crystal 3 and the laser radiation source 2 are configured to allow total internal reflection in the crystal of the laser radiation emitted by the source and received by the crystal.

[0052] Thus, the object of the invention is to radically change the optical pumping architecture. Rather than using a parallelepiped crystal cut at Brewster incidence, as shown in [Fig. 1], the idea of ​​the invention is to use a crystal disk as shown in [Fig. 3].

[0053] As shown in [Fig.3], the crystal can have the shape of a straight cylinder.

[0054] A "cylinder" is defined as a solid bounded by a cylindrical surface and by two parallel planes. A "cylinder" is said to be right when these planes are substantially perpendicular to the generatrices, for example at an angle between 80° and 90°.

[0055] According to one embodiment of the invention, the laser and the crystal are configured so that the crystal receives the laser radiation in a direction substantially perpendicular to the axis of the cylinder and at an angle allowing total internal reflection in the crystal of the laser radiation received by the crystal, for example tangentially to the surface of the height of the cylinder.

[0056] Typically, the crystal has the shape of a cylinder having elliptical bases, for example circular.

[0057] As shown in [Fig. 3], the crystal may include at least one notch along its height, said at least one notch 8 being configured to receive the laser radiation from the laser radiation source. Advantageously, the notch 8 allows the laser radiation to enter the crystal at a nearly tangential angle of incidence. The propagation of light within the crystal then occurs by total internal reflection.

[0058] According to different embodiments of the invention, the optical refrigerator can be configured so that the laser radiation is brought to said at least one notch through the free space or an optical fiber.

[0059] According to an aspect of the invention not shown, the laser radiation is brought to the crystal by means of a lensed optical fiber either directly connected to said at least one notch or at a short distance from said at least one notch. In one embodiment, said at least one notch may include a receiving means, for example a hole to accommodate the lens of the optical fiber.

[0060] Preferably, the cooling crystal has a substantially constant height greater than or equal to 0.5 mm, for example greater than or equal to 1 mm, and less than or equal to 4 mm, for example less than or equal to 3 mm.

[0061] According to one embodiment of the invention, the diameter of the cooling crystal is greater than or equal to 5 mm, for example greater than or equal to 7.5 mm and less than or equal to 20 mm, for example less than or equal to 15 mm.

[0062] The invention also relates to the cooling crystal itself having a particular shape enabling it to receive laser radiation with an almost tangential angle of incidence, the propagation of light in the crystal then taking place by total reflection on the internal surfaces of the crystal.

[0063] As shown in [Fig.2], the optical refrigerator according to the invention may further comprise at least one thermal link 6 in heat exchange on the one hand with the cooling crystal 3 and, on the other hand, an element to be cooled, for example a thermal load 4, in order to transfer cooling from the cooling crystal 3 to the element to be cooled 4. The element to be cooled 4 is typically a cold finger or a focal plane intended to transfer cooling to a load.

[0064] As illustrated in patent application FR3064828, the thermal link 6 may comprise two blades having first ends respectively in heat exchange, for example direct contact, respectively with two distinct surfaces of the crystal 3, for example the two parallel planes of the cylinder forming the crystal, the two blades 6 having second ends in heat exchange, for example by direct contact, with the element 4 to be cooled.

[0065] Preferably the second ends of the two blades 6 are in heat exchange respectively with two distinct surfaces or sides of the element 4 to be cooled.

[0066] The blades 6 have for example a thickness greater than or equal to 0.5 mm and less than or equal to 5 mm, a width greater than or equal to 1 mm and less than or equal to 10 mm and a length greater than or equal to 2 mm and less than or equal to 40 mm.

[0067] At each end the blades 6 are in contact with the crystal 3 and the element to be cooled over a surface for example comprising 1 mm2 and 75 mm2 and 1 mm2 and 100 mm2 respectively.

[0068] The two plates 6 can be made, for example, of MgF2 or YLF or sapphire or any other suitable transparent material: glass... By transparent we mean in particular: transparent at appropriate wavelengths, for example to allow the radiation emitted by the crystal to pass through, which is symbolized by lines or arrows in Figures 1 and 2.

[0069] In order to limit the leakage of anti-stokes radiation emitted by the cooling crystal, at least one face of the crystal, for example at least the two parallel faces, can be covered with an optical treatment allowing at least partial, for example total, reflection of the light radiation generated by the crystal.

[0070] Alternatively or in addition to the optical treatment of at least one face of the crystal, the optical refrigerator according to the invention may include at least one screen for absorbing the light radiation generated by the crystal, the at least one screen being arranged around the crystal. According to one embodiment, the absorption screen may be configured so as to allow the conversion of the light radiation generated by The crystal is converted into electrical energy. This electrical energy can be stored in a battery.

[0071] The optical screen (sometimes called "optical baffles") is intended to isolate the crystal and in particular to absorb radiation around the crystal 3. This screen can be composed in particular of metallic material, copper, aluminium.

[0072] Alternatively, according to an embodiment not shown in the figures, the crystal is in direct thermal contact with an element to be cooled, for example glued to the element to be cooled, with a mirror on the glued face.

[0073] The optical refrigerator according to the invention makes it possible to increase the efficiency of the device, in particular by using a laser diode, and to simplify the device, in particular by eliminating the mirrors required in devices according to the prior art.

[0074] The invention also relates to a detection assembly comprising a detector and detector refrigerator, in which the refrigerator conforms to the invention.

[0075] The invention has been described above with the aid of embodiments shown in the figure, without limitation of the general inventive concept.

[0076] Many other modifications and variations are suggested of themselves to a person skilled in the art, after reflection on the different modes of embodiment illustrated in this application.

[0077] These embodiments are given by way of example and are not intended to limit the scope of the invention, which is determined exclusively by the claims below.

[0078] The mere fact that different features are listed in mutually dependent claims does not indicate that a combination of these features cannot be advantageously used. Finally, any reference used in the claims should not be interpreted as a limitation of the scope of the invention.

Claims

Demands

1. Optical refrigerator (1), particularly for cooling an infrared sensor or detector, the refrigerator comprising a laser radiation source (2) and a cooling crystal (3) configured to receive laser radiation from the source and cool itself by Anti-Stokes fluorescence, characterized in that the laser radiation source and the crystal are configured to permit total internal reflection in the crystal of the laser radiation emitted by the source and received by the crystal.

2. Optical refrigerator according to claim 1, wherein the crystal has the form of a straight cylinder, the laser and the crystal being configured so that the crystal receives the laser radiation in a direction substantially perpendicular to the axis of the cylinder and at an angle permitting total internal reflection in the crystal of the laser radiation received by the crystal, for example tangentially to the surface of the height of the cylinder.

3. Optical refrigerator according to claim 1 or 2, wherein the crystal has the shape of a cylinder having elliptical bases, for example circular.

4. Optical refrigerator according to any one of claims 2 or 3, wherein the crystal comprises at least one notch (8) on its height, said at least one notch being configured to receive laser radiation from the laser radiation source.

5. Refrigerator according to any one of the preceding claims, wherein the crystal has a substantially constant height greater than or equal to 0.5 mm, for example greater than or equal to 1 mm, and less than or equal to 4 mm, for example less than or equal to 3 mm.

6. Refrigerator according to any one of the preceding claims, wherein the crystal has a diameter greater than or equal to 5 mm, for example greater than or equal to 7.5 mm and less than or equal to 20 mm, for example less than or equal to 15 mm.

7. Refrigerator according to any one of the preceding claims, wherein the laser is a diode laser.

8. Refrigerator according to any one of the preceding claims, wherein at least one face of the crystal is covered with an optical treatment allowing at least partial, for example total, reflection of the light radiation generated by the crystal.

9. Refrigerator according to any one of the preceding claims, further comprising at least one thermal link (6) in heat exchange on the one hand with the crystal and, on the other hand, an element to be cooled (4), for transferring frigories from the crystal to the element to be cooled, the element to be cooled being a cold finger or a focal plane intended to transfer frigories to a load.

10. Detection assembly comprising a detector and detector refrigerator, wherein the refrigerator conforms to any one of the preceding claims.

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