Pumping assembly and laser system including LED-powered luminescent concentrators

Abstract

Pumping assembly (PE) intended to emit pump radiation (Rp) suitable for 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 electroluminescent radiation (Ld); a luminescent concentrator (CL) adapted to absorb said electroluminescent 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 about a so-called horizontal plane reduced with respect to an emission angle of the outgoing portion but not reduced about a so-called vertical plane perpendicular to the direction.said optical element (PC) being adapted so that a coupling efficiency of said output portion in the laser medium is maximized, said output beams (FS) of the light-emitting modules forming said pump radiation (RP). [Fig.3A]

Description

Title of the invention: Pumping assembly and laser system comprising LED-pumped luminescent concentrators 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. Previous technique

[0002] High-energy lasers (kJ-MJ) currently use neodymium-doped glass plates pumped by flash lamps. This type of pumping poses numerous problems: - Flash lamps emit a spectrum in the UV and visible ranges, only a portion of which corresponds to neodymium absorption. The efficiency of energy transfer between the pump source and the laser medium is therefore limited. Furthermore, the UV emission corresponds to absorption within the glass itself, without any energy transfer to the neodymium ions. This induces significant and unnecessary heating of the laser plate. It severely limits the firing rate because the laser plates must be allowed to cool before the next firing. Typically, the firing rate is one firing per hour. - Flash lamps operate with very high voltages (around 25 kV) which require specific power supplies and restrictive usage conditions related to the high voltage. - The lifespan of flash lamps is limited in the number of shots they can fire (typically between 20,000 and 30,000 for operation at full flash capacity). This increases replacement maintenance when trying to increase the firing rate.

[0003] High-energy lasers are based on an oscillator-amplifier configuration, with numerous amplifiers connected in series. The cost of a high-energy laser is primarily due to its final (high-energy) amplifiers, which are several tens of centimeters in size (and up to 100 cm) in order to achieve the required energies without exceeding the optical damage threshold of the materials traversed (approximately 10 J / cm²). The beams to be amplified have square cross-sections, optimized to minimize the size of the amplifiers. For example, the largest beams used to date are found in very high-energy lasers (megajoule level) in France with the LMJ and in the United States with the NIF, with a cross-section of 40 cm x 40 cm.

[0004] The "final" amplifiers are in the form of plates to ensure homogeneous pumping at all points of the beam. Figure 1 illustrates an example of such a "final" amplifier. Typically, the beam to be amplified, Famp, has a square cross-section of 40 cm on each side. The Nd:glass laser plate ML, for example, has dimensions of 73 cm x 40 cm. The LF flash lamps, in the form of tubes, are placed horizontally, one above the other, in front of the laser plate ML. A diffuser or mirror MD is placed on the rear face of the LF flash lamps to reflect some of the emitted light back towards the laser plate ML.

[0005] In these systems, there is no coupling optics between the LF flashes and the ML laser plate. The coupling of light in the Nd:glass plate is achieved by direct propagation for typically less than half of the emitted rays (rays denoted RI) and by multiple scattering / reflections for a significant portion of the rays (rays denoted R2), many of which re-enter the tube plasma where absorption losses are substantial (on the order of cm⁻¹). The collection of R2 rays towards the ML laser plate is therefore partial. Furthermore, a portion of the rays (rays denoted R3) do 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 approximately 500 W / cm² would be required. This lower limit implies that it is impossible to use high-power LEDs that emit a maximum density of approximately 100 to 200 W / cm².

[0008] Replacing flashes with laser diodes emitting around 800 nm (the absorption band of Nd) 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 ELI HAPLS L3 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 is still far from the target energy (kJ-MJ). Currently, increasing the energy using laser diodes is limited due to the cost and availability of the laser diodes. A rough calculation reveals that the laser diode industry lacks the capacity to produce enough components to meet the demand. Consider, for example, 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. Since the pumping time is adapted to the lifetime of Nd:glass (300 ps), a peak pumping power of 33 GW is required. Currently, the annual production capacity of one of the world's largest laser diode suppliers is 140 MW. It is projected to increase to 400 MW within 3 years: a further 80-fold increase is still needed.

[0009] Another way to illustrate the problem is to estimate the annual global production of laser diodes in watts. The laser diode market is worth approximately €10 billion. Estimating an average cost of €10 per watt for a laser diode, we can therefore estimate that companies worldwide produce about 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). Therefore, a factor of 33 is missing to produce a single MJ laser.

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

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

[0012] To this end, 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 includes a non-imaging optical element to reduce an emission angle associated with the light emission from the concentrator. Given the structure of the non-imaging optical element, this reduction of the emission angle is achieved along a single plane. This arrangement allows for optimal coupling efficiency of the light emission from the concentrators while enabling the production of pump radiation with 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 create high-energy laser systems with reduced power consumption, thus improving their speed 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 assembly of pumping comprising a plurality of light-emitting modules stacked at least along one x-direction, each light-emitting module comprising: - a plurality of light-emitting diodes configured to emit electroluminescent 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 with respect to an emission angle of the outgoing portion but not reduced along a so-called vertical plane perpendicular to direction x, 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 about the horizontal plane substantially constant along a direction y perpendicular to the horizontal plane, said optical element not having a parabolic section substantially constant about the vertical plane.

[0017] According to one embodiment, each optical element has an input face and an output face through which a respective output beam is traversed, and in which 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.

[0018] According to one embodiment, 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 to the first large face.

[0019] Preferably, in the preceding 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 The concentrators of a second light-emitting module share the same electrical connection plate 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, - 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 cooling plate 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.

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

[0021] Preferably, in the preceding embodiment, the optical element of the emission modules in each row is 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 an 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 and vertical planes is greater than 90%, preferably greater than 95%.

[0024] Another object of the invention is a laser amplifier system comprising a laser medium for amplifying 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: - the number of emission modules per column is adapted so that one dimension of each column along the y direction is substantially equal to one dimension of the laser medium along the y direction, - the number of emission modules is adapted so that the 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 features, details and advantages of the invention will become apparent from the description given with reference to the accompanying drawings provided by way of example, which represent, respectively:

[0027] [Fig. 1], an example of a known prior art "final" amplifier 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 concentrator and LEDs of a emission module according to a non-limiting example of the invention,

[0031] [Fig.4B], an angular diagram of the distribution of rays trapped 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 concentrators known in the prior art and not included in the invention,

[0035] [Fig.8], a schematic view of an emission module of a laser amplifier 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 in 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.12A], [Fig.12B], 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 extraction efficiency,

[0040] [Fig.13A], [Fig.13B], 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] In addition, unless otherwise indicated, the optional features described in the description and figures can be combined with each other. Description of the implementation methods

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

[0044] Before describing the invention in detail, we proceed below to a quick reminder of the properties of the luminescent CL concentrators implemented in the ME emission modules of the invention.

[0045] Luminescent concentrators are optically pumped materials typically shaped like parallelepipeds, enabling the formation of light sources that combine both high power and high luminance. They contain phosphors distributed throughout the volume of the concentrator and are polished on all faces to allow for total internal reflections that guide the light emitted by the phosphors to the output face.

[0046] It is known to pump LED concentrators (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) such as Ce:YAG, which absorbs in the blue (around 450 nm), at a wavelength where LEDs perform very well. The crystal is cut into a flat shape, covered with hundreds (or even thousands) of LEDs on its two large surfaces, and emitted edge-on. 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 that of 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 explained more clearly later, the invention allows for a judicious implementation of pumping the laser medium by concentrators, which is particularly suited to producing pump radiation with a large cross-section. In the following description, we will detail various embodiments for achieving this result. It is understood that the various structures detailed in this description and the figures are given by way of example and are intended to illustrate the invention. They should in no way be considered as limiting the scope of the invention. Various modifications and variations of the described structures will be apparent 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 implementing the non-imaging optical element are covered by the scope of the invention.

[0049] Figure 2 schematically and without detail describes a laser amplifier 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) respectively.

[0050] The laser system 1 comprises a laser medium ML intended to amplify an incident laser beam FL. By way of 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 shape 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 along a direction x. Each emission module is adapted to emit a respective output beam Fs so that the set of beams Fs forms the pump radiation Rp illuminating the laser medium ML.

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

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

[0054] Figure 4A shows a schematic perspective view of a CL concentrator and the LEDs of an ME emission module according to a non-limiting embodiment of the invention. The CL concentrator is a luminescent, for example fluorescent, parallelepiped-shaped crystal, having at least one illumination face FEi, FE2, of dimensions LXw, 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 denoted e.

[0055] The concentrator crystal is configured to absorb the electroluminescent radiation Ld emitted by the LEDs illuminating the illumination faces FE1 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 within the crystal. The emitted rays can be classified into two main categories: - Trapped rays, denoted Lp: these rays are trapped within 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 six faces that are parallel in pairs and perpendicular to each other. The trapped rays never escape the crystal, except for imperfections in the crystal itself. - Untrapped rays are the rays that eventually exit 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] Figure 4B is a representation of the angular diagram of the emitted and trapped rays in the concentrator CL, as shown in Figure 4A. The dark areas 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 by way of example, the medium chosen as the concentrator crystal CL is a Ce:YAG crystal (index n2 = 1.82) with a critical angle of 33° when the surrounding medium is air. The percentage of radiation trapped by RTI relative to the untrapped radiation is determined by the index of the crystal, and that of the surrounding medium by Snell's law.

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

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

[0059] As a reminder, the emission by the phosphors Lum within the concentrator being Lambertian, the half-angle of total emission of the outgoing portion Ls at half-height in intensity emitted is 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^ depends on the geometry of the laser medium ML and the arrangement of the emission modules relative to the laser medium. However, through 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 a laser amplification system 1 exhibiting typical laser medium dimensions ML and incident laser beam dimensions FI for high energy laser systems.

[0061] Notably, and unlike certain non-imaging optical components known to those skilled in the art that allow for a reduction of the emission angle, the optical element PC of the invention only allows for a reduction of 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 9^. The various structures for achieving this reduction of the emission angle 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 PC optical element and the arrangement of the ME emission modules are such that the coupling efficiency of each outgoing portion in the laser medium is greater than 90%. Indeed, controlling the emission angle along the horizontal plane by the PC optical elements ensures optimal coupling between the radiation emitted by the CL concentrators and the ML laser medium.

[0063] Thus, the laser system 1 according to the invention allows for 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-mounted components that require significantly fewer operations for their production. Furthermore, as will be explained later, the solution of the invention is particularly well-suited to the geometry of high-energy laser systems, and especially to the pumping of large ML laser wafers (typically 40 cm x 40 cm). The invention thus enables the creation of high-energy laser systems with reduced power consumption, thereby improving their speed and accessibility.

[0064] The optical element PC only reduces the emission angle along the horizontal plane. Therefore, 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 diverging rays within the laser medium ML, the laser system 1 comprises a pair of mirrors Ml arranged substantially perpendicular to the y direction as illustrated in [Fig. 3B].

[0065] According to one embodiment, as illustrated in Figure 3B, each mirror M1 is placed on either side of the pumping assembly EP perpendicular to the y-direction, 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 increases 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]) back into that concentrator. Without these mirrors, this outgoing portion would be "lost" and would not participate in pumping the laser medium. The embodiment of [Fig. 5] further increases the coupling efficiency of the pumping assembly EP.

[0067] It should be noted that the use of such mirrors instead of the PC optical element 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 entire area through which the laser beam passes before reaching the ML medium.

[0068] Preferably, as illustrated in [Fig. 3A], the electrical connection board PCB of the LEDs is shared between two adjacent light-emitting modules in the stack of emission modules. In other words, the emission modules ME are stacked such that the light-emitting diodes illuminating the first large face FEi of a concentrator of a first module The emission and LED components illuminating the second large face FE2 of the concentrator of a second light emission module share the same electrical connection board (PCB). This electrical connection board is located in an intercalated space El between the concentrator of the first light emission module and the concentrator of the second light emission module. This arrangement allows for a more compact overall EP pumping system.

[0069] It should be noted that the dimension along the stacking direction x of this interlayer 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 FO face of the optical element PC, the electrical connection boards PCB can be inserted without changing the power of the coupled pump in the laser medium ML. This is a consequence of the conservation of the extent geometry and luminance in optical systems. This modularity through adaptation of the output face dimension will be illustrated in particular in Figures 13A and 13B.

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

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

[0072] It is understood that a different number of ME emission modules 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 easy to adapt the EP pumping assembly and the EP pump radiation it produces to the dimensions of the laser medium. For example, in the EP pumping assembly, the number of ME emission modules per column is adapted so that one dimension of each column along the y-direction is substantially equal to one dimension of the laser medium along the y-direction.

[0073] In addition, the arrangement of the pumping system is compact given the possibility of bringing the stacked concentrators closer together and of sharing 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 at the Brewster angle to limit losses at interfaces and reduce the power density on the surface of the laser medium ML.

[0075] Figure 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 for a reduction in the emission angle of the outgoing portion Ls collected along the horizontal plane only. Preferably, for this purpose, 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," it is understood here that each optical element PC has a parabolic cross-section about 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 cross-section about the vertical plane xz (see [Fig. 3B] and 6). The optical element PC therefore does not exhibit spherical or cylindrical rotational symmetry.Thus, in the vertical plane the optical element PC has a typically rectangular cross-section, while in the horizontal plane the optical element PC has a parabolic cross-section in order to reduce the emission angle of the outgoing portion Ls collected in this plane.

[0077] This feature implies that the structure of the optical element PC differs from that of prior art concentric parabolic concentrators (CPCs). By way of example, [Fig. 7A] illustrates a prior art concentric parabolic concentrator CPC1, not included in the invention, which has spherical symmetry. Similarly, [Fig. 7B] illustrates a prior art concentric parabolic concentrator CPC2, not included in the invention, which has cylindrical symmetry with parabolas of different dimensions in the two output directions. In both cases, and unlike the invention, the CPC1 and CPC2 concentric parabolic concentrators have a parabolic cross-section along the horizontal and vertical planes, respectively.

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

[0079] Indeed, the inventors used ray tracing software to validate the efficiency of reducing the emission angle by a one-dimensional parabolic PC concentrator according to the invention. To this end, the inventors compared the emission of a Ce:YAG luminescent concentrator (index n=1.83), assumed to be perfect and lossless, bonded to a two-dimensional concentrator as shown in [Fig. 7A] (not included in the invention) and bonded to a one-dimensional concentrator according to The embodiment of the invention is illustrated in [Fig. 8]. The refractive indices of the adhesive CI and the concentrators are equal and are n = 1.5, a value readily achievable for non-imaging optics made of glass or plastic. To optimize 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 might have expected the vertical dimension to undergo conventional extraction following the passage of two parallel, flat interfaces without ray straightening (i.e., approximately 14.5%). Averaging the two outputs for the horizontal and vertical rays would have yielded 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 enabling an effective reduction of the emission angle along the horizontal plane, the PC optical element according to the invention makes it easier to design 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 EP pumping assembly 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 through the monolithic optical components CM.

[0083] By sharing 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 one-dimensional parabolic concentrators PC optimize the pump radiation fill factor RP. The pump radiation fill factor RP is defined as the total surface area occupied by the output faces FO of the PC optical elements in the transverse plane divided by 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 homogeneously pumping a large surface area in the laser medium.

[0087] In the invention, obtaining a fill factor close to 1 is made easy by the possibility of placing the optical elements PC of the same column of emission module side by side 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 from two different columns in pairs. In other words, the PC optical elements of the same row can also be placed in pairs as illustrated in [Fig. 3A]. Thus, it is possible to obtain a fill factor equal to 1.

[0089] To illustrate the comparative advantage of the one-dimensional parabolic concentrators of the invention with respect to the shape of the pump radiation, [Fig. 10] shows the FO outlet face of nine prior art parabolic concentrators CPC1 known in the prior art and not included in the invention. By way of example, each parabolic concentrator CPC1 is as shown in [Fig. 7A].

[0090] The parabolic concentric concentrators CPC1 are optimally arranged in the most compact configuration possible. However, in the example of [Fig. 10], it is observed that the pump radiation filling factor produced by the entire 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 in [Fig. 7A], there are necessarily gaps between the CPC1 concentrators in which no pump radiation is emitted. Conversely, in the invention, it is possible to optimize the arrangement of the EP pumping assembly so that such gaps 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 40% fill rate (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 100 mm plate. There can therefore be 800 LEDs (400 per large face) pumping each concentrator.

[0093] Each LED typically emits 4.5 W of power. Assuming a 20% efficiency for the luminescent concentrator (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 shown in Figures 3A and 3B, the pumping assembly comprises nine luminescent concentrators. The total power is 720 x 9 W = 6480 W

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

[0095] By optimizing the emission angles and with the mirrors M1, it is calculated that the illumination received by the laser medium ML is 1.2 kW / cm2. By arranging two pumping sets EP, EP' on each side of the laser medium ML as illustrated in [Fig. 11], this value of received illumination makes it possible to ensure a gain of 1.14 of the same order of magnitude as that obtained with flash lamps.

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

[0097] To improve this extraction efficiency, according to an embodiment illustrated in Figure 12A, each concentrator CL has a roof-shaped face opposite the emitting face SE and a set of mirrors M2 covering this opposite face and a pair of mirrors M1 covering the lateral faces that lie along the horizontal plane xz. Indeed, as demonstrated in application WO2017157742A1, the contents of which are incorporated 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 rotate within the structure until they reach the angle suitable for exiting through the emitting face SE.

[0098] Alternatively, according to another embodiment illustrated in [Fig. 12B], a diffusing medium MD is disposed on the face opposite the emitting face. This diffusing medium redirects the rays trapped within the concentrator to the correct angles for exiting through the emitting face SE. This technique increases extraction efficiency. The embodiment of [Fig. 12B] includes a mirror M2 placed behind the diffusing face, in case diffusion is insufficient to reflect the rays back into the concentrator towards the emitting face. It is understood that this diffusing medium MD can be disposed on any of the lateral faces of the concentrators CL to contribute to improving extraction efficiency.

[0099] Functionalizing the face opposite the outlet face as illustrated in Figures 12A and 12B allows us to gain approximately two times the extraction efficiency of the concentrator.

[0100] As mentioned previously, the invention allows for a high degree of structural and design modularity due to the inherent modularity of the PC optical elements. Indeed, as illustrated in Figure 3A, the dimension of the gap 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 boards of the LEDs are thicker than this dimension.

[0101] The invention allows the dimension of the output face FO to be parameterized along the x direction so as to insert various elements into the intercalated space EL. This adaptation is permitted without changing the power of the coupled pump in 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. By way of non-limiting example, the dimension of the outlet face FO along the x direction is 5 mm for [Fig. 13A] and is 10 mm for [Fig. 13B].

[0103] Given the conservation of 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 along the x-direction implies an increase in the gap space EL. Thus, in the embodiment of [Fig. 13B], a water circulation system CR is inserted into the gap space EL. This water circulation system CR is, for example, a water-cooled copper or aluminum block arranged to be in contact with the LEDs to dissipate the heat emitted by them. It should be noted that the arrangement of the LEDs in overlapping plates allows this cooling block to be shared by 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 lower emission angle, it is possible to move the ME light emission modules further away from the laser medium to ensure that the coupling value between the pump radiation EP and the laser medium ML remains optimal. Thus, the number of ME emission modules illuminating the laser medium ML 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 along the x direction, which is doubled.

[0106] In summary, the invention makes it possible to realize 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 stresses more easily and to increase heat exchanges.

[0107] Furthermore, the emission angle at the outlet of the luminescent concentrators is reduced along the horizontal plane solely by non-imaging PC optical elements in order to adapt the pumping assembly for remote pumping of a laser medium ML, which is a large laser plate. Thus, the EP pumping assembly is particularly well-suited to 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

Demands

1. Pumping assembly (PE) for emitting 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 (LEDs) configured to emit electroluminescent radiation (Ld) - a luminescent concentrator (CL) adapted to absorb said electroluminescent 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 with respect 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-emitting modules forming said pump radiation (RP).

2. Pumping assembly according to the preceding claim, wherein 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, wherein each optical element has a parabolic cross-section about the horizontal plane (xz) that is substantially constant in a direction perpendicular to the horizontal plane, said optical element not having a parabolic cross-section that is substantially constant about the vertical plane (xz)-

4. A pumping assembly according to any one of the preceding claims, wherein each optical element (PC) has an inlet face (FE) and an outlet face (FO) through which a beam passes of respective output, and in which a dimension of the output face along the x direction with respect 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, wherein, 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, wherein 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, 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 the same electrical connection board (PCB) 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.

8. 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 the same 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 one of the preceding claims, wherein the emission modules are arranged under the form of a matrix of columns and rows, each row being along the x direction, and each column being along a direction perpendicular to the horizontal plane (x^).

10. Pumping assembly according to the preceding claim, wherein the optical element of the emission modules of each row is 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. Pumping assembly according to any one of the preceding claims, 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, wherein 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 and vertical planes is greater than 90%, preferably greater than 95%.

14. 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 dependence on any one of claims 9 to 11, wherein a number of emission modules per column is adapted so that one dimension of each column along the direction is substantially equal to one dimension of the laser medium along the y direction.

16. 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. 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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