Pumping assembly and laser system comprising LED-pumped luminescent concentrators

Abstract

Pumping assembly (EP) intended to emit pump radiation (Rp) capable of pumping a laser medium (ML), said pumping assembly comprising a plurality of light-emitting modules (ME) stacked at least in one direction, each light-emitting module comprising: a plurality of light-emitting diodes (LEDs) configured to emit light-emitting radiation (Ld), a luminescent concentrator (CL) adapted to absorb said light-emitting radiation (Ld), the concentrator having an emitting face (SE) such that an outgoing portion (Ls) of said luminescent radiation can pass through the emitting face (SE), a non-imaging optical element (PC) adapted to collect the outgoing portion and then form an output beam (Fs) having an emission angle along a so-called horizontal plane () reduced relative to an emission angle of the outgoing portion but not reduced along a so-called vertical plane () perpendicular to direction,said optical element (PC) being adapted so that a coupling efficiency of said outgoing portion in the laser medium is maximum, said output beams (FS) of the light emission modules forming said pump radiation (RP). [Fig.3A],

Description

Title of the invention: Pumping assembly and laser system comprising luminescent concentrators pumped by LEDs Technical field

[0001] The present invention relates to the field of luminescent concentrators and more particularly to laser systems pumped by luminescent concentrators and more particularly to that of high energy laser systems. Prior art

[0002] High energy lasers (kJ-MJ) currently use glass plates doped with neodymium ions pumped by flash lamps. This type of pumping poses many problems: - Flash lamps emit a spectrum in the UV and visible range, only part of which corresponds to the absorption of neodymium. The efficiency of energy transfer between the pump source and the laser medium is therefore limited. In addition, the emission in the UV corresponds to absorption in the mass of the glass, without energy transfer to the neodymium ions. This induces significant and unnecessary heating of the laser plate. It severely limits the firing rate because it is necessary to wait for the laser plates to cool before launching the next shot. Typically, the firing rate is 1 shot per hour. - Flash lamps operate with very high voltages (around 25 kV) which require specific power supplies and restrictive usage conditions linked to high voltage. - The lifespan of flash lamps is limited in number of shots (typically between 20,000 and 30,000 for operation at full flash capacity). This increases replacement maintenance when seeking to increase the rate of fire.

[0003] High energy lasers are based on an "oscillator-amplifier" configuration, with many amplifiers in series, one behind the other. The cost of a high energy laser is mainly linked to its final (high energy) amplifiers which have sizes of several tens of cm (and up to 100 cm) in order to reach the required energies without going above the optical damage threshold of the materials crossed (a few 10 J / cm2). The beams to be amplified have square sections, optimized to limit the size of the amplifiers as much as possible. For example, the widest beams used to date are on very high energy lasers (the megajoule) in France with the LMJ and in the United States with the NIF, with a section of 40 cm x 40 cm.

[0004] The "final" amplifiers are in the form of plates in order to have homogeneous pumping at all points of the beam. [Fig.l] illustrates an example of such a "final" amplifier. Typically, the beam to be amplified Famp has a square section with a side of 40 cm. The ML laser plate of Nd: glass has for example a dimension of 73 cm x 40 cm. The LF flash lamps, in the form of tubes, are placed horizontally one above the other, opposite the ML laser plate. A diffuser or an MD mirror is placed on the rear face of the LF flashes to return part of the emitted light towards the ML laser plate.

[0005] In these systems, there is no coupling optics between the LF flashes and the ML laser plate. The coupling of the light in the Nd:glass plate is done by direct propagation for typically less than half of the emitted rays (rays denoted RI) and by multiple diffusions / reflections for a significant part of the rays (rays denoted R2) which largely re-cross the plasma of the tube in which the absorption losses are significant (of the order of cm-1). The collection of the R2 rays towards the ML laser plate is therefore partial. In addition, a part of the rays (rays denoted R3) does not reach the ML laser plate. Thus, it is estimated that less than 50% of the light emitted by the flashes reaches the Nd:glass plate.

[0006] Pumping by flash lamps is therefore far from being an optimum solution.

[0007] It is important to note that, in order to ensure sufficient gain in the amplifiers, the population inversion density must be sufficient in the Nd:glass. For example, to achieve a gain of 1.15 per amplifier (which is the reference value with a flash-pumped amplifier amplifying 40 cm x 40 cm beams), a pump density of the order of 500 W / cm2 would be required. This lower bound implies that it is impossible to use power LEDs that emit a maximum density of the order of 100 to 200 W / cm2.

[0008] Replacing flashes with laser diodes emitting around 800 nm (Nd absorption band) could be a solution. Indeed, the output power density of laser diodes assembled in panels can reach 1 to 10 kW / cm2, which is suitable for high-performance Nd:glass amplifiers. For example, the HAPLS L3 ELI system (see for example: https: / / www.eli-beams.eu / facility / lasers / laser-3-hapls-l-pw-30-j-10-hz / ) uses diode-pumped Nd:glass amplifiers. Their output power is 200 J at 1053 nm, which remains very far from the targeted energy (kJ-MJ). Currently, the energy increase using laser diodes is blocked due to the cost of laser diodes and for availability reasons. A rough calculation shows that the laser diode industry does not have the means to produce enough components to meet the challenge. Consider a laser emitting 1 MJ.Assuming a 10% laser efficiency (which would already be very good), a pump energy of 10 MJ is therefore required. The . pumping time being adapted to the lifetime of Nd:glass (300 ps), a peak pumping power of 33 GW is therefore required. Currently, the annual production capacity of one of the largest companies supplying laser diodes in the world reaches 140 MW. It is planned to increase to 400 MW within 3 years: there is still a factor of 80 to be gained.

[0009] Another way to illustrate the problem is to estimate the annual global production of laser diodes in W. The laser diode market is around €10 billion. By estimating an average cost per watt at €10 for a laser diode, we can therefore estimate that companies worldwide produce around 1 GW per year in the form of laser diodes at all wavelengths (and not just at 800 nm which is the wavelength of choice for pumping Nd:glass). This therefore leaves a factor of 33 to make a single MJ laser.

[0010] To explain this relatively "low" production of laser diodes per year, it is necessary to take into account the complexity of the manufacturing process of a laser diode. Indeed, it is a component which emits from the edge and which requires many operations during its production, starting from the semiconductor layers (cleaving, passivation of the facets, installation of the gold wire contacts, mounting on a base, cooling by micro-channels, micro-optics to collimate the output radiation, etc.).

[0011] The invention aims to overcome some of the aforementioned problems of the prior art.

[0012] For this purpose, the invention proposes a pumping assembly (and a corresponding laser system) based on stacked light-emitting modules. In the invention, each emission module comprises a luminescent concentrator pumped with light-emitting diodes and comprises a non-imaging optical element for reducing an emission angle associated with the light emission of the concentrator. Given the structure of the non-imaging optical element, this reduction of the emission angle is carried out along a single plane. This arrangement allows for optimal coupling efficiency of the light emission of the concentrators while allowing pump radiation to be obtained having an optimal cross-section for pumping the laser medium.

[0013] Furthermore, compared to pumping with flash lamps or laser diodes, the solution of the invention is more efficient and less expensive. The invention makes it possible to produce high-energy laser systems with reduced consumption, thus improving their throughput and accessibility. Summary of the invention

[0014] To this end, an object of the invention relates to a pumping assembly intended to emit pump radiation capable of pumping a laser medium, said pumping assembly pumping comprising a plurality of light emitting modules stacked at least in one direction x, each light emitting module comprising: - a plurality of light-emitting diodes configured to emit light-emitting radiation - a luminescent concentrator adapted to absorb said electroluminescent radiation so as to emit luminescent radiation, the concentrator having an emitting face such that an outgoing portion of said luminescent radiation can pass through the emitting face - a non-imaging optical element adapted to collect the outgoing portion and then form an output beam having an emission angle along a so-called horizontal plane reduced relative to an emission angle of the outgoing portion but not reduced along a so-called vertical plane perpendicular to the x direction, said output beams of the light emission modules forming said pump radiation.

[0015] According to one embodiment, each optical element is adapted so that a coupling efficiency of the outgoing portion collected by said optical element in the laser medium is greater than 90%.

[0016] According to one embodiment, each optical element has a parabolic section along the horizontal plane which is substantially constant along a direction y perpendicular to the horizontal plane, said optical element not having a parabolic section which is substantially constant along the vertical plane.

[0017] According to one embodiment, each optical element has an input face and an output face crossed by a respective output beam, and in which a dimension of the output face in the x direction relative to a dimension of the input face in the x direction is adapted to maximize a coupling efficiency of said respective output beam.

[0018] According to one embodiment, for each transmission module, the associated concentrator has a body in the general shape of a plate extending along the vertical plane and has a first large face and a second large face opposite the first large face.

[0019] Preferably, in the previous embodiment: - the light-emitting diodes of each transmission module are arranged so as to illuminate the first and second large faces of the concentrator of said transmission module, - the emission modules are stacked so that, for emission modules, the light-emitting diodes illuminating the first large face of the concentrator of a first light-emitting module, and the light-emitting diodes illuminating the second large face of the concentrator of a second light emission module share the same electrical connection plate arranged between the concentrator of the first light emission module and the concentrator of the second light emission module, the second light emission module being adjacent to the first light emission module in the stack, - the emission modules are stacked so that, for emission modules, the light-emitting diodes illuminating the first large face of the concentrator of a first light emission module, and the light-emitting diodes illuminating the second large face of the concentrator of a second light emission module share the same cooling plate arranged between the concentrator of the first light emission module and the concentrator of the second light emission module, the second light emission module being adjacent to the first light emission module in the stack.

[0020] According to one embodiment, the transmission modules are arranged in the form of a matrix of columns and rows, each row being in the x direction, and each column being in a y direction perpendicular to the horizontal plane.

[0021] Preferably, in the preceding embodiment, the optical element of the emission modules of each row are formed by a respective region of the same monolithic optical component. Even more preferably, said monolithic optical component is obtained by an additive printing process.

[0022] According to one embodiment, each concentrator has a face opposite the emitting face which is functionalized so as to increase the extraction efficiency of the outgoing portion.

[0023] According to one embodiment, a structure of the optical elements is adapted so that a filling rate of the pump radiation in a so-called transverse plane perpendicular to the horizontal plane and to the vertical plane is greater than 90%, preferably greater than 95%.

[0024] Another object of the invention is an amplifier laser system comprising a laser medium intended to amplify an incident laser beam and a pump assembly according to the invention.

[0025] According to particular embodiments of the laser system according to the invention: - a number of emission modules per column is adapted so that a dimension of each column in the direction y is substantially equal to a dimension of the laser medium in the direction y, - a number of emission modules is adapted so that an illumination on the laser medium is greater than 1.5 kW / cm2, - the light emission modules are arranged at a distance from the laser medium so as not to intercept said incident laser beam. Brief description of the drawings

[0026] Other characteristics, details and advantages of the invention will emerge from reading the description given with reference to the appended drawings given by way of example and which represent, respectively:

[0027] [Fig. 1], an example of a “final” amplifier known from the prior art for high energy lasers,

[0028] [Fig.2], a schematic view of a laser amplifier system according to the invention.

[0029] [Fig.3A], [Fig.3B], a schematic view of a laser system according to a mode of preferred embodiment of the invention according to a top view (along a horizontal plane xz) and according to a side view (along a vertical plane respectively,

[0030] [Fig.4A], a schematic perspective view of a hub and LEDs of a transmission module according to a non-limiting example of the invention,

[0031] [Fig.4B], an angular diagram of the distribution of trapped rays in the concentrator of the example of [Fig.4A],

[0032] [Fig.5], a schematic side view of a laser system according to one embodiment of the invention,

[0033] [Fig.6], a perspective view obtained by ray tracing software of an optical element included in an emission module according to an embodiment of the invention,

[0034] [Fig.7A], [Fig.7B], two examples of parabolic concentric concentrators known from the prior art and not included in the invention,

[0035] [Fig.8], a schematic view of an emission module of an amplifier laser system according to an embodiment of the invention

[0036] [Fig.9], a front view of the pumping assembly according to one embodiment of the invention,

[0037] [Fig. 10], a front view of the outlet faces of nine parabolic concentric concentrators known from the prior art and not included in the invention

[0038] [Fig. 11], a schematic side view of a laser system according to one embodiment of the invention,

[0039] [Fig.l2A], [Fig.l2B], a schematic side view of a laser system according to two embodiments of the invention in which the face opposite the output face is functionalized to increase the extraction efficiency,

[0040] [Fig.l3A], [Fig.l3B], a schematic side view of a laser system according to two embodiments of the invention, for two different dimensions of the output face,

[0041] In the figures, unless otherwise indicated, the elements are not to scale and identical references designate identical elements.

[0042] Furthermore, unless otherwise indicated, the optional features described in the description and the figures may be combined with each other. Description of the embodiments

[0043] The invention relates to a pumping assembly EP - and an associated amplifier laser system 1 - based on emission modules ME each comprising a luminescent concentrator CL pumped with light-emitting diodes LED and a non-imaging optical element PC. This optical element PC has a structure adapted to reduce an emission angle associated with the light emission Ls of the concentrator CL according to one plane only.

[0044] Before describing the invention in detail, a brief reminder of the properties of the luminescent concentrators CL used in the emission modules ME of the invention is given below.

[0045] Luminescent concentrators are optically pumped materials typically cut into a parallelepiped shape to form light sources combining both high power and high luminance. They contain phosphors distributed throughout the volume of the concentrator and are polished on all faces to allow total internal reflections which will guide the light emitted by the phosphors to the output face.

[0046] It is known to pump concentrators with LEDs (see for example Barbet, Adrien, et al. "Light-emitting diode pumped luminescent concentrators: a new opportunity for low-cost solid-state lasers." Optica 3.5 (2016): 465-468.). An example of a concentrator is a fluorescent crystal in the visible (red-orange) range such as Ce:YAG which absorbs in the blue (around 450 nm), at a wavelength where LEDs are very efficient. The crystal is cut in the form of a plane, lined with hundreds (or even thousands) of LEDs on the two large surfaces and with emission from the edges. These concentrators make it possible to obtain luminance values ​​10 to 20 times higher than that of an LED.

[0047] With pumping by blue LEDs on the largest faces, a power density exceeding 10 kW / cm2 was obtained with a Ce:YAG concentrator (P. Pichon, A. Barbet, JP. Blanchot, F. Druon, F. Balembois and P. Georges, “Light emitting diodes: a new paradigm for Ti:sapphire pumping”, Optica, vol. 5, no. 10, pp 1236-1239 (2018).). This performance made it possible to pump laser media and in particular Nd:YAG, whose absorption spectrum is quite close to Nd:glass (P. PICHON, et al. "High-radiance light sources with LED-pumped luminescent concentrators applied to pump Nd:YAG passively Q-switched laser" Optics and Laser Technology, Vol 96 pp 7-12 (2017)).

[0048] As will be more clearly explained hereinafter, the invention allows a judicious implementation of the pumping of the laser medium by concentrators which is particularly suitable for producing pump radiation having a large cross-section. In the remainder of the description, we will detail different embodiments making it possible to obtain this result. It is understood that the different structures detailed in the present description and the figures are given by way of example and are intended to illustrate the invention. They should in no case be considered as limiting the scope of the invention. Various modifications and variations of the structures described will appear to those skilled in the art without departing from the scope and spirit of the invention. In particular, all equivalent means known to those skilled in the art for producing the non-imaging optical element are covered by the scope of the invention.

[0049] Figure 2 schematically and non-detailedly describes an amplifier laser system 1 according to the invention. Figures 3A and 3B illustrate in more detail the various constituent elements of the invention. More specifically, Figures 3A and 3B schematically illustrate a laser system 1 according to a preferred embodiment of the invention in a top view (along a horizontal plane) and in a side view (along a vertical plane J^) respectively.

[0050] The laser system 1 comprises a laser medium ML intended to amplify an incident laser beam FL. As a non-limiting example, in the illustration of [Fig.2], the laser medium ML has a body in the general shape of a plate, but it is understood that the invention applies to any form of laser medium ML.

[0051] To pump the laser medium ML and thus enable amplification of the incident laser beam FI, the laser system 1 comprises a pump assembly EP intended to emit pump radiation Rp and itself comprising a plurality of light emission modules ME stacked in a direction x. Each emission module is adapted to emit a respective output beam Fs so that all of the beams Fs form the pump radiation Rp illuminating the laser medium ML.

[0052] According to the invention and as visible in FIGS. 3A and 3B, each light-emitting module ME comprises a plurality of light-emitting diodes LED configured to emit electroluminescent radiation Ld.

[0053] Each emission module ME comprises a luminescent concentrator CL adapted to absorb the electroluminescent radiation Ld so as to emit luminescent radiation (not shown) within the concentrator. In order to be able to optically pump the laser medium ML, the luminescent radiation has wavelengths in a spectral range overlapping with the absorption spectral band of the ML laser medium.

[0054] Figure 4A represents a schematic perspective view of a concentrator CL and the LEDs of an emission module ME according to a non-limiting example of the invention. The concentrator CL is a luminescent parallelepiped crystal, for example fluorescent, having at least one illumination face FEi, FE2, of dimensions LX w, illuminated by the electroluminescent radiation Ld emitted by the LEDs. The illumination faces FEi, FE2 are also called “large faces”. The thickness of the concentrator is noted e.

[0055] The concentrator crystal is configured to absorb the electroluminescent radiation Ld emitted by the LEDs illuminating the illumination faces FEi and FE2. This radiation is absorbed by the phosphors Lum of the fluorescent crystal which are distributed throughout the volume of the crystal and which then emit fluorescence radiation inside the crystal. The emitted rays can be classified into two main categories: - trapped rays denoted Lp: these rays are trapped in the crystal due to total internal reflection (TIR) ​​on the different faces of the crystal. These rays exist if the crystal is a parallelepiped with 6 faces parallel in pairs and perpendicular to each other. The trapped rays never leave the crystal, apart from imperfections in the latter. - untrapped rays are the rays that end up exiting the crystal. They are divided into two subcategories: guided rays denoted Lg whose characteristic is to be guided by RTI and to exit on one of the faces of the concentrator and unguided rays denoted Lout which exit directly from the concentrator without being reflected on the faces.

[0056] [Fig.4B] is a representation of the angular diagram of the rays emitted and trapped in the CL concentrator according to the example of [Fig.4A]. The dark caps represent the angles corresponding to the untrapped rays (guided and unguided) and the light areas represent the angles corresponding to the trapped rays. In this representation given as an example, the medium chosen as the CL concentrator crystal is a Ce:YAG crystal (index n2=1.82) having a critical angle of 33° when the ambient medium is air. The percentage of radiation trapped by RTI compared to the untrapped radiation is fixed by the index of the crystal and that of the ambient medium by the Snell-Descartes law.

[0057] Thus, as mentioned above, a portion Ls of the luminescent radiation - called the outgoing portion - passes through a face called the emitter SE of the light concentrator CL. This portion Ls comprises the guided portion Lg reflected on the faces of the concentrator and the unguided portion Lmü associated with the emitting face SE passing directly through the latter, without being reflected on the faces of the concentrator.

[0058] Furthermore, each emission module ME comprises a non-imaging optical element PC associated with a respective concentrator for collecting the outgoing portion Ls and then forming an output beam Fs. By its structure, the optical element PC is adapted so that the output beam Fs has an emission angle along the horizontal plane xz which is less than the emission angle of the outgoing portion Ls.

[0059] As a reminder, the emission by the luminophores Lum within the concentrator being Lambertian, the total half-angle of emission of the outgoing portion Ls at mid-height in emitted intensity is worth 3^ = 60 °.

[0060] In order to ensure optimal coupling efficiency between each output beam Fs and the laser medium ML, the optical element PC has a structure adapted to reduce the emission angle by a factor Qhs greater than 1.2, preferably between 1.33 and 4. It is understood that the optimal value of this reduction factor 3LJ 6^ is dependent on the geometry of the laser medium ML and the arrangement of the emission modules relative to the laser medium. However, by simulations and calculations, the inventors have determined that a reduction factor greater than 1.2 and preferably between 1.33 and 4 was optimal for an amplification laser system 1 having typical dimensions of ML laser medium and FI incident laser beam for high energy laser systems.

[0061] Notably, and unlike certain non-imaging optical components known to those skilled in the art allowing a reduction in the emission angle, the optical element PC of the invention only allows a reduction in the emission angle along the horizontal plane xz. In other words, along the vertical plane yz, the emission angle of the output beam is identical to that of the outgoing portion θ^. The various structures allowing this reduction in the emission angle to be obtained along a single plane will be described later.

[0062] The coupling efficiency of the pumping light is defined as the ratio between the light received by the laser medium and the light emitted by the pump source. According to the invention, the structure of each optical element PC and the arrangement of the emission modules ME are such that a coupling efficiency of each outgoing portion in the laser medium is greater than 90%. Indeed, the control of the emission angle along the horizontal plane by the optical elements PC makes it possible to ensure optimal coupling between the radiation emitted by the concentrators CL and the laser medium ML.

[0063] Thus, the laser system 1 according to the invention allows more efficient, simpler and less expensive pumping of the laser medium than the prior art using flash lamps or laser diodes. Indeed, compared to laser diodes, LEDs are surface components that require far fewer operations for their production. Furthermore, as will be explained later, the solution of the invention is particularly suited to the geometry of high-energy laser systems and in particular to the pumping of large ML laser plates (typically 40 cm x 40 cm). The invention thus makes it possible to produce high-energy laser systems with reduced consumption, thus improving their throughput and accessibility.

[0064] The optical element PC only reduces the emission angle along the horizontal plane. Also, it is understood that the emission angle of the output beam Fs along the vertical plane remains equal to that obtained at the concentrator output (i.e. that of a Lambertian source). In order to redirect the rays diverging within the laser medium ML, the laser system 1 comprises a pair of mirrors Ml arranged substantially perpendicular to the direction y as illustrated in [Fig.3B].

[0065] According to one embodiment, as illustrated in FIG. 3B, each mirror M1 is placed on either side of the pumping assembly EP perpendicular to the direction y, so as to cover only the optical elements PC and the laser medium ML. Thus, the mirrors M1 reflect only the output beams Fs along the xz plane to redirect them into the medium ML. This embodiment makes it possible to increase the coupling efficiency of the pumping assembly EP.

[0066] Alternatively, according to another embodiment illustrated in Figure 5, the mirrors M1 are adapted to cover the concentrators CL, the optical elements PC and the laser medium ML. As in the embodiment of Figure 3B, the mirrors M1 reflect the output beams Fs along the xz plane to redirect them into the medium ML. In addition, for each concentrator, they reflect the outgoing portion of the luminescent radiation associated with the large lateral faces (faces of dimensions Exe in the embodiment of [Fig.4A]) within this concentrator. Without the presence of these mirrors, this outgoing portion would be “lost” and would not participate in the pumping of the laser medium. The embodiment of [Fig.5] makes it possible to further increase the coupling efficiency of the pumping assembly EP.

[0067] It is noted that the use of such mirrors instead of the optical element PC to reduce the emission angle along the horizontal plane is not possible given the large cross-section of the laser beams to be amplified in high-energy laser systems. Indeed, [Fig.3A] shows that the pump beams must propagate throughout the area where the laser beam passes before reaching the ML medium.

[0068] Preferably, as illustrated in [Fig.3A], the electrical connection plate PCB of the LEDs is shared between two adjacent light emission modules in the stack of emission modules. In other words, the emission modules ME are stacked so that the light-emitting diodes illuminating the first large face FEi of a concentrator of a first module emission, and the light-emitting diodes illuminating the second large face FE2 of the concentrator of a second light-emitting module share the same electrical connection plate PCB. This electrical connection plate is arranged in an interposed space El between the concentrator of the first light-emitting module and the concentrator of the second light-emitting module. This arrangement allows for better compactness of the EP pumping assembly.

[0069] It should be noted that the dimension along the stacking direction x of this intercalary space El is directly dependent on the dimension of the output face FO of the optical element PC. In other words, by adapting the dimension of the face FO of the optical element PC, the electrical connection plates PCB can be inserted without changing the pump power coupled into the laser medium ML. This is a consequence of the conservation of the geometric extent and the luminance in the optical systems. This modularity by adapting the dimension of the output face will be illustrated in particular in Figures 13A and 13B.

[0070] Preferably, the emission modules ME of the EP pumping assembly according to the invention are arranged in the form of a matrix of columns and rows, each row being in the direction x, and each column being in the direction perpendicular to the horizontal plane xz.

[0071] For example, as illustrated in Figures 3A and 3B, the EP pumping assembly comprises a matrix of nine ME emission modules distributed in three rows and three columns.

[0072] It is understood that a different number of emission modules ME is possible in the invention. Indeed, as those skilled in the art will understand, the arrangement of the pumping system is extremely modular and makes it possible to easily adapt the EP pumping assembly and the EP pump radiation that it produces to the dimensions of the laser medium. For example, in the EP pumping assembly, the number of emission modules ME per column is adapted so that a dimension of each column along the direction is substantially equal to a dimension of the laser medium along the y direction.

[0073] Furthermore, the arrangement of the pumping system is compact given the possibility of bringing the stacked concentrators closer together and of pooling certain elements of the pumping assembly.

[0074] Preferably, the laser medium ML is arranged so that the incident beam FI is incident on the laser medium according to the Brewster angle to limit losses at the interfaces and reduce the power density on the surface of the laser medium ML.

[0075] [Fig.6] illustrates a perspective view obtained by ray tracing software of an optical element PC included in an emission module ME according to an embodiment of the invention.

[0076] As mentioned above, each optical element PC allows a reduction of the emission angle of the outgoing portion Ls collected along the horizontal plane only. For this, preferably, each optical element PC according to the invention is a “one-dimensional parabolic concentrator” as illustrated in Figures 3A, 3B and 6. By “one-dimensional parabolic concentrator” is meant here that each optical element PC has a parabolic section along the horizontal plane xz that is substantially constant along the direction J' perpendicular to the horizontal plane (see Figures 3A and 6) but does not have a parabolic section that is substantially constant along the vertical plane xz (see [Fig. 3B] and 6). The optical element PC therefore does not have spherical or cylindrical symmetry of revolution.Thus, along the vertical plane the optical element PC has a typically rectangular section while in the horizontal plane the optical element PC has a parabolic section in order to reduce the emission angle of the outgoing portion Ls collected along this plane.

[0077] This characteristic implies that the structure of the optical element PC is different from the concentric parabolic concentrators (CPC in English) of the prior art. As an example, [Fig. 7A] illustrates a concentric parabolic concentrator CPC1 known from the prior art and not included in the invention which is spherically symmetrical. Similarly, [Fig. 7B] illustrates a concentric parabolic concentrator CPC2 known from the prior art and not included in the invention which is cylindrically symmetrical with parabolas of different dimensions in the two output directions. In both cases and unlike the invention, the concentric parabolic concentrators CPC1 and CPC2 have a parabolic section along the horizontal plane and along the vertical plane.

[0078] Compared to the CPC concentrators of the prior art, the fact that the PC optical element according to the invention allows a reduction of the emission angle along a single plane has several advantages. This firstly allows an effective reduction of the emission angle along a dimension of interest while being less bulky than the CPC concentrators of the prior art.

[0079] Indeed, the inventors used ray tracing software to validate the efficiency of emission angle reduction by a one-dimensional parabolic concentrator PC according to the invention. For this, the inventors compared the emission of a luminescent concentrator in Ce:YAG (index n=1.83) assumed to be perfect, without losses, glued to a two-dimensional concentrator according to the example of [Fig.7A] (not included in the invention) and glued to a one-dimensional concentrator according to the embodiment of the invention illustrated in [Fig.8]. The indices of the glue CI and the concentrators are equal and are worth n=1.5, a value which is easily accessible for non-imaging optics made of glass or plastic. To optimize the extraction, a mirror M2 is added in both cases on the face opposite the emitting face SE.

[0080] Ray tracing simulations show that the two-dimensional parabolic concentrator extracts 39.5% of the rays emitted in the luminescent concentrator CL. With the one-dimensional concentrator PC of the invention, the extraction reaches 35%. This result is counterintuitive. Indeed, since the one-dimensional concentrator PC only acts in the horizontal direction, one would have expected that the vertical dimension would undergo a conventional extraction following the passage of two plane and parallel interfaces without straightening of the rays (i.e. approximately 14.5%). By averaging the two outputs for the horizontal and vertical rays, one would have found an extraction of 27%. In the one-dimensional concentrator PC of the invention, the extraction efficiency is better because the parabolic concentrator PC also acts on rays outside the horizontal plane.

[0081] In addition to allowing an effective reduction of the emission angle along the horizontal plane, the optical element PC according to the invention makes it possible to facilitate the design of the pumping assembly without compromising the quality of the pump radiation RP (and in particular its filling rate) as explained below.

[0082] Figure 9 illustrates a front view (along a transverse plane TZ) of the pumping assembly EP according to an embodiment of the invention. This embodiment is a variant of the embodiment illustrated in Figures 3A and 3B in which the optical element PC of the emission modules of each row are formed by a respective region RS of the same monolithic optical component CM. In [Fig.9], the concentrators CL are visible by transparency through the monolithic optical components CM.

[0083] By pooling the use of the monolithic optical component CM, it is possible to simplify the design of the pumping assembly.

[0084] Preferably, the monolithic optical component CM is obtained by an additive printing process. Thus, the monolithic optical component CM is easily designed and inexpensive.

[0085] Furthermore, Figure 9 illustrates that the one-dimensional parabolic concentrators PC make it possible to optimize the filling factor of the pump radiation RP. The filling factor of the pump radiation RP is defined by the total surface area occupied by the output faces FO of the optical elements PC in the transverse plane over the total surface area ST of the pumping assembly in the transverse plane.

[0086] In the embodiment of [Fig.9], it is noted that the filling factor is very close to 1 (greater than 90%). This implies that the RP pump radiation is capable of pumping a large surface area homogeneously in the laser medium.

[0087] In the invention, obtaining a fill factor close to 1 is made easy by the possibility of adhering the optical elements PC of the same emission module column as illustrated in [Fig.9]. This is possible even when the optical elements PC are “individual” and are not formed by a respective region RS of the same monolithic optical component CM for each row.

[0088] Furthermore, it is possible to place the PC optical elements of two different columns side by side. In other words, it is also possible to place the PC optical elements of the same row side by side as illustrated in [Fig.3A]. Thus, it is possible to obtain a filling factor equal to 1.

[0089] To illustrate the comparative advantage of the one-dimensional parabolic concentrators of the invention on the shape of the pump radiation, [Fig. 10] illustrates the FO output face of nine parabolic concentric concentrators CPC1 known from the prior art and not included in the invention. As an example, each parabolic concentric concentrator CPC1 is according to the example of [Fig.7A].

[0090] The parabolic concentric concentrators CPC1 are optimally arranged in the most compact arrangement possible. However, in the example of [Fig. 10], it is noted that the filling factor of the pump radiation produced by the set of parabolic concentric concentrators CPC1 is significantly lower than that produced by the embodiment of [Fig. 9] (approximately 0.77). Indeed, given the spherical symmetry of the parabolic concentric concentrators CPC1 of [Fig. 7A], there necessarily exist spaces between the concentrators CPC1 in which no pump radiation is emitted. Conversely, in the invention, it is possible to optimize the arrangement of the pumping assembly EP so that such spaces do not exist.

[0091] Thus, by an optimal arrangement, it is possible to achieve a total illumination greater than or equal to 1.44 kW / cm2 on the FO output faces of the pumping assembly of the invention. As illustrated by the numerical example below, this makes it possible to ensure a gain of 1.14 sufficient to achieve amplification in the laser medium.

[0092] Indeed, LEDs are typically arranged on CL concentrators with a filling rate of 40% (see for example P. Pichon et al. “Light emitting diodes: a new paradigm for Ti:sapphire pumping”, Optica, vol. 5, no. 10, pp 1236-1239 (2018).). As an example, suppose that each CL concentrator is a 10 x 100mml plate. There can therefore be 800 LEDs (400 per large face) pumping each concentrator.

[0093] Each LED typically emits a power of 4.5 W. Estimating a luminescent concentrator efficiency of 20% (which takes into account losses, wavelength conversion efficiency and the amount of light coupled to the output), the output power is approximately 720 W. According to the embodiment of Figures 3A and 3B, the pumping assembly comprises nine luminescent concentrators. The total power is 720 x 9 W = 6480 W

[0094] As an example, we assume a filling factor equal to 1 and therefore an output surface ST equal to the sum of the surface of the nine output faces FO of the optical elements PC, i.e. 15 x 30 mm z . This gives an illumination of the pump radiation Rp at the level of the output faces FO equal to 1.44 kW / cm2.

[0095] By optimizing the emission angles and with the mirrors Ml, it is calculated that the illumination received by the laser medium ML is 1.2 kW / cm2. By arranging two pumping assemblies EP, EP' on each side of the laser medium ML as illustrated in [Fig. 11], this received illumination value makes it possible to ensure a gain of 1.14 of the same order of magnitude as that obtained with the flash lamps.

[0096] One of the key points for the illumination at the FO output faces is the optimization of the extraction efficiency outside the luminescent concentrator CL. The extraction efficiency of 20% proposed in the previous dimensioning corresponds to a "classic" value that can be found on the luminescent concentrators in Ce:YAG of the prior art.

[0097] In order to improve this extraction efficiency, according to an embodiment illustrated in FIG. 12A, each concentrator CL has a face opposite the emitting face SE cut in the shape of a roof and a set of mirrors M2 covering this opposite face and a pair of mirrors M1 covering the lateral faces which are along the horizontal plane xz. Indeed, as demonstrated in application WO2017157742A1, the content of which is included by reference in the present application, the mirrors M2 placed on the roof-shaped face and the lateral mirrors M1 force the trapped rays to turn in the structure until they have the angle suitable for exiting through the emitting face SE.

[0098] Alternatively, according to another embodiment illustrated in [Fig.l2B], a diffusing medium MD is arranged on the face opposite the emitting face. This diffusing medium makes it possible to redirect the rays trapped within the concentrator in order to give them the right angles to allow an exit through the emitting face SE. This technique makes it possible to increase the extraction efficiency. The embodiment of [Fig.l2B] comprises a mirror M2 placed behind the diffusing face, in the case where the diffusion would not be sufficient to return the rays into the concentrator, in the direction of the emitting face. It is understood that this diffusing medium MD can be arranged on any of the lateral faces of the concentrators CL in order to contribute to improving the extraction efficiency.

[0099] The functionalization of the face opposite the outlet face as illustrated in figures 12A and 12B makes it possible to gain a factor of approximately two on the extraction efficiency of the concentrator.

[0100] As mentioned above, the invention allows for great structural and design modularity given the intrinsic modularity of the PC optical elements. Indeed, as illustrated in FIG. 3A, the dimension of the interposed space El along the x direction is limited by the dimension of the output faces FO along the x direction when the parabolic concentrators are placed side by side. This can pose a problem if the mechanical supports and the PCB connection plates of the LEDs are thicker than this dimension.

[0101] The invention makes it possible to parameterize the dimension of the output face FO along the x direction so as to insert various elements into the intercalary space EL. This adaptation is permitted without changing the pump power coupled into the laser medium ML.

[0102] Figures 13A and 13B illustrate the advantage of this modularity. More specifically, Figures 13A and 13B schematically illustrate a top view of an EP pumping assembly, according to an embodiment of the invention similar to that of Figure 3A, for two different dimensions of the outlet face FO. As a non-limiting example, the dimension of the outlet face FO in the x direction is 5 mm for [Fig. 13A] and 10 mm for [Fig. 13B].

[0103] Given the conservation of the extent within an optical component, the emission angle at the output of the optical element PC is lower in the embodiment of figure 13B (6^ = 20 °) than for figure 13A = 45 0)

[0104] This increase in the dimension of the output face FO in the x direction implies an increase in the intercalary space EL. Thus, in the embodiment of [Fig.l3B] a water circulation system CR is inserted in the intercalary space El. This water circulation system CR is for example a block of copper or aluminum cooled by water arranged so as to be in contact with the LEDs to dissipate the heat emitted by the latter. It should be noted that the arrangement of the LEDs in superimposed plates makes it possible to share this cooling block for two PCB electrical connection plates of the LEDs.

[0105] Compared to the embodiment of Figure 13A, the power density on the light wall is typically halved in the embodiment of Figure 13B. However, as mentioned above, it is possible to modify the arrangement in the system of the invention. Indeed, thanks to a smaller emission angle, it is possible to move the light emission modules ME away from the laser medium to ensure that the value of the coupling between the pump radiation EP and the ML laser medium remains optimal. Thus, the number of ME emission modules illuminating the ML laser medium remains identical in the embodiment of Figures 13A and 13B and the only difference between these two embodiments is the dimension of the pumping assembly in the x direction, which is doubled.

[0106] In summary, the invention makes it possible to produce a laser system 1 comprising a pumping assembly EP in which luminescent concentrators CL are arranged so as to increase the pumping light collection surface, to manage mechanical constraints more easily and to increase thermal exchanges.

[0107] In addition, the emission angle at the output of the luminescent concentrators is reduced along the horizontal plane only by non-imaging optical elements PC in order to adapt the pumping assembly to the remote pumping of a laser medium ML which is a large laser plate. Thus, the EP pumping assembly is particularly suitable for high-energy laser systems.

[0108] Finally, the PC optical elements are modular and allow adaptation of the pumping assembly by modifying their FO output face.

Claims

Claims

1. Pumping assembly (EP) intended to emit pump radiation (Rp) capable of pumping a laser medium (ML), said pumping assembly comprising a plurality of light-emitting modules (ME) stacked at least in one direction x', each light-emitting module comprising: - a plurality of light-emitting diodes (LED) configured to emit light-emitting radiation (Ld) - a luminescent concentrator (CL) adapted to absorb said light-emitting radiation (Ld) so as to emit luminescent radiation,the concentrator having an emitting face (SE) such that an outgoing portion (Ls) of said luminescent radiation can pass through the emitting face (SE) - a non-imaging optical element (PC) adapted to collect the outgoing portion and then form an output beam (Fs) having an emission angle along a so-called horizontal plane (xz) reduced compared to an emission angle of the outgoing portion but not reduced along a so-called vertical plane (^) perpendicular to direction x, said output beams (FS) of the light emission modules forming said pump radiation (RP).,

2. Pumping assembly according to the preceding claim, in which each optical element (PC) is adapted so that a coupling efficiency of the outgoing portion collected by said optical element in the laser medium is greater than 90%.

3. Pumping assembly according to any one of the preceding claims, in which each optical element has a parabolic section along the horizontal plane (xz) which is substantially constant along a direction perpendicular to the horizontal plane, said optical element not having a parabolic section which is substantially constant along the vertical plane (xz)-

4. Pumping assembly according to any one of the preceding claims, in which each optical element (PC) has an input face (FE) and an output face (FO) crossed by a beam respective output beam, and wherein a dimension of the output face along the x direction relative to a dimension of the input face along the x direction is adapted to maximize a coupling efficiency of said respective output beam.

5. Pumping assembly according to any one of the preceding claims, in which, for each emission module, the associated concentrator has a body in the general shape of a plate extending along the vertical plane and has a first large face and a second large face opposite the first large face.

6. Pumping assembly according to the preceding claim, in which the light-emitting diodes of each emission module are arranged so as to illuminate the first and second large faces of the concentrator of said emission module.

7. Pumping assembly according to the preceding claim, in which the emission modules are stacked so that, for emission modules, the light-emitting diodes illuminating the first large face of the concentrator of a first light-emitting module, and the light-emitting diodes illuminating the second large face of the concentrator of a second light-emitting module share the same electrical connection plate (PCB) arranged between the concentrator of the first light-emitting module and the concentrator of the second light-emitting module, the second light-emitting module being adjacent to the first light-emitting module in the stack.

8. A pumping assembly according to claim 6 or 7, wherein the emission modules are stacked such that, for emission modules, the light-emitting diodes illuminating the first large face of the concentrator of a first light-emitting module, and the light-emitting diodes illuminating the second large face of the concentrator of a second light-emitting module share a common cooling plate (CR) disposed between the concentrator of the first light-emitting module and the concentrator of the second light-emitting module, the second light-emitting module being adjacent to the first light-emitting module in the stack.

9. A pumping assembly according to any preceding claim, wherein the emission modules are arranged under the form of a matrix of columns and rows, each row being in the x direction, and each column being in a direction perpendicular to the horizontal plane (x^).

10. Pumping assembly according to the preceding claim, in which the optical element of the emission modules of each row are formed by a respective region of the same monolithic optical component.

11. Pumping assembly according to the preceding claim, wherein said monolithic optical component is obtained by an additive printing process.

12. A pumping assembly according to any preceding claim, wherein each concentrator has a face opposite the emitting face which is functionalized so as to increase an extraction efficiency of the outgoing portion.

13. Pumping assembly according to any one of the preceding claims, in which a structure of the optical elements is adapted so that a filling rate of the pump radiation in a so-called transverse plane perpendicular to the horizontal plane and to the vertical plane is greater than 90%, preferably greater than 95%.

14. A laser amplifier system (1) comprising a laser medium (ML) for amplifying an incident laser beam (FI) and a pump assembly (EP) according to any one of the preceding claims.

15. Laser system according to the preceding claim in its dependency with one of claims 9 to 11, in which a number of emission modules per column is adapted so that a dimension of each column in the direction is substantially equal to a dimension of the laser medium in the direction y.

16. A laser system according to claim 14 or 15, wherein a number of emission modules is adapted so that an illumination on the laser medium is greater than 1.5 kW / cm2.

17. A laser system according to any one of claims 14 to 16, wherein the light emitting modules are arranged at a distance from the laser medium so as not to intercept said incident laser beam.

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