Improved light-emitting device comprising a luminescent light concentrator

Abstract

The invention relates to a light-emitting device (1) comprising: a laser source (SL) which is suitable for generating an incident beam (FI); an optical pumping cavity (CP) which defines a closed volume that does not comprise the laser source (SL) and comprises at least one opening (O1, O2) such that the incident beam can pass through the at least one opening and be reflected a plurality of times on a surface (SR) of the optical pumping cavity, thereby creating what is referred to as pump radiation (RP); and a luminescent light concentrator (CL) which is arranged in the closed volume and is suitable for absorbing the pump radiation (RP) and then emitting luminescent radiation (LL) within the concentrator, the luminescent radiation having a lifetime TL, what is referred to as an outgoing light emission portion (Ls) of the luminescent radiation (LL) passing through what is referred to as an emitting face (FE) of the light concentrator and then passing through the at least one opening.

Description

DESCRIPTION Title of the invention: Improved light emitting device comprising a luminescent light concentrator Technical field:

[0001] The present invention relates to the field of laser source-pumped luminescent concentrators. Previous technique:

[0002] Light sources can be classified into two categories: so-called "classical" sources of large geometric extent (product of the emitting surface by the solid angle of emission) such as halogen lamps, flash lamps or LEDs and sources of small geometric extent such as lasers, laser diodes or fiber laser sources.

[0003] Figure 1 is a schematic representation of the power of some sources as a function of their geometric extent. For imaging or metrology, laser sources can pose difficulties of use because of their small geometric extent (related to spatial coherence) and their spectrum (related to temporal coherence). Random interference effects (speckle) can make the illumination inhomogeneous and reduce the quality of the images. Conversely, incoherent sources do not produce speckle but can be difficult to use to illuminate an area of ​​interest: their large extent makes the collection of their light rays difficult through an optical system.

[0004] It is possible to reduce the transverse dimension of a beam to reduce the extent but at the cost of a reduction in power. Indeed, the conservation of luminance (L) indicates that the power (P) and the extent (e) are proportional: P = L x e. Thus, by reducing the transverse dimension of the beam or by varying the emitted power, a predetermined source will make it possible to address the power-extent pairs which are below the line P=L e (in black in figure 1 for flash lamps for example).

[0005] Figure 1 shows that all incoherent sources are below a limiting line that passes through the brightest and most powerful sources: flashlamps and luminescent concentrators.

[0006] Luminescent concentrators are light sources that combine both high power and high luminance. They are optically pumped materials typically shaped into a parallelepiped. They contain phosphors distributed throughout the volume of the concentrator. They are polished on all sides to allow total internal reflections that guide the light emitted by the phosphors to the output face.

[0007] 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) region 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.

[0008] Figure 2A schematically represents a perspective view of an emission module ME0. The emission module ME0 comprises a set of LEDs intended to emit in a first spectral band and a light concentrator CL. The concentrator CL is a fluorescent parallelepiped crystal, having at least one illumination face SI1, SI2, of dimensions L xw, illuminated by the electroluminescent radiation L d emitted by the LEDs. The illumination faces SI1, SI2 are also called “large faces”. The thickness of the concentrator is noted e.

[0009] The concentrator crystal is configured to absorb said electroluminescent radiation L dThe luminous flux emitted by the LEDs is directed towards the illumination face. It is absorbed by the Lum phosphors 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: - the trapped rays noted L p : 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 eventually exit the crystal. They are divided into two subcategories: guided rays denoted L gwhose characteristic is to be guided by RTI and to exit on one of the faces of the concentrator and the unguided rays noted L out which come out directly from the concentrator without being reflected on the faces.

[0010] Figure 2B is a representation of the angular diagram of the rays emitted and trapped in the concentrator. 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 concentrator crystal CL 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.

[0011] The power emitted by a luminescent concentrator depends on the number of LEDs that can be placed near its large faces. For example, with a rectangular concentrator measuring 200 mm x 14 mm, it is possible to place more than 2000 LEDs to simultaneously pump the concentrator. In this case, it is possible to obtain output powers reaching kW with a range remaining limited to a value less than 1 cm 2 . sr~ 1 (see P.Pichon, A.Barbet, JP Blanchot, F.Druon, F.Balembois, P.Georges, Optica, Vol 5, N°10, pp 1236-1239 (September 2018)).

[0012] To increase the peak power of a concentrator's emission, a first option is to send current peaks into the LEDs. The shorter the current peaks, the more it is possible to increase the peak power. This method is effectively used to have an emission of the order of 4W per LED with 15 ps pumping pulses (see P.Pichon, A.Barbet, JP Blanchot, F.Druon, F.Balembois, P.Georges, Optica, Vol 5, N°10, pp 1236- 1239 (September 2018)). However, it is limited by the Auger effect in semiconductor layers of LEDs. For example, the potential for increasing peak power is only a factor of two by multiplying the current by 10 and reducing the pump duration from 15 ps to 20 ns, this to avoid thermal effects. This increase therefore comes at the cost of a significant drop in efficiency. In addition, driving thousands of LEDs at a current reaching hundreds of amperes would make the system very complex and very inefficient.

[0013] To increase the peak emission power of a concentrator, the second option is to increase the dimensions of the concentrator in order to enlarge the surface exposed to the LEDs and therefore increase the number of LEDs. Since the maximum size of the materials is given by their processing conditions (e.g. 100mm*50mm on Ce:YAG crystals), the surface increase can be achieved by optically bonding several concentrators together on the lateral faces. A longer concentrator implies an increase in the propagation distance of the rays in the structure and therefore a decrease in efficiency, due to losses in the material and the glue interface between the two structures. A wider concentrator does not change the propagation distance. However, in this case, the output face becomes increasingly flattened, which can make it difficult to use in a rotationally symmetric optical system.

[0014] Finally, a last option to increase the peak power of the emission is to increase the extraction efficiency of the concentrator. Figure 3A shows that it is possible to extract more rays than those belonging to the output cone (related to total internal reflection) by using mirrors. In a parallelepiped structure, a mirror on the face opposite the output face allows a gain of up to a factor of two. By adding an additional face and mirrors on all the lateral faces, it is possible to gain up to a factor of 4 (see for example Pierre Pichon, Lisa Lopez, Maxime Nourry-Martin, Stéphane Darbon, Frederic Druon, Patrick Georges and François Balembois, "Light extraction and brightness enhancement of luminescent rectangular slabs" Adv. Photonics Res. 2100356 (2022)).Figure 3B shows that it is possible to increase the exit cone by gluing a non-imaging optic to the exit face and selectively straightening the most inclined rays so that they can pass through the last diopter (as for example in S.Roelandt, Y. Meuret, D. K. G. de Boer, D. Bruis, P. Van De Voorde, and H. Thienpont, "Incoupling and outcoupling of light from a luminescent rod using a compound parabolic concentrator", Optical Engineering 54(5), 055101 (May 2015). These improvements in extraction allow a gain of up to a factor of 4 on the emitted power compared to a conventional concentrator.

[0015] The invention aims to provide a high-power, low-range source using luminescent concentrators. For this purpose, the invention relates to a light-emitting device comprising a luminescent light concentrator pumped by a laser source. The concentration properties of the laser radiation make it possible to have very high pump powers, which results in the emission of a high output power by the concentrator. In the configuration of the invention, the coupling between the pump beam and the concentrator is carried out with an optical pump cavity defining a closed volume in order to increase the volume density of pump power while avoiding laser optical damage and photobleaching effects on the fluorescent material. Summary of the invention:

[0016] For this purpose, an object of the invention is a light emission device comprising: - a laser source suitable for generating an incident beam - an optical pumping cavity defining a closed volume not including the laser source and comprising at least one opening so that the incident beam can pass through said at least one opening and be reflected a plurality of times on a surface of the optical pumping cavity, thus creating so-called pump radiation, - a luminescent light concentrator arranged in the closed volume and adapted to absorb said pump radiation and then emit luminescent radiation within said concentrator having a lifetime T L , a portion called outgoing light emission of said luminescent radiation passing through a face called emitting face of the light concentrator then passing through said at least one opening.

[0017] Preferably, the surface of the optical pumping cavity diffusely reflects a percentage greater than or equal to 90% of the incident radiation and has a reflection coefficient greater than 90%, preferably greater than 95% for the incident radiation.

[0018] According to one embodiment, the surface of the optical cavity is made of spectralon or polytetrafluoroethylene.

[0019] According to one embodiment, the laser source is a pulsed source delivering laser pulses of duration T P . Preferably, the lifetime T L of the luminescent radiation is less than or equal to the duration T P pulses. Preferably, the laser source and the optical pump cavity are adapted so that a fluence of the pump radiation within the light concentrator is less than 5 J / cm 2 , preferably less than or equal to 1 J / cm 2 .

[0020] According to one embodiment, the dimensions of the optical pumping cavity are adapted according to a lifetime T L luminescent radiation so that said outgoing light emission is a pulsed light emission of a predetermined duration, for example having a width at half height of between 50 and 250 ns.

[0021] According to one embodiment, the light emitting device comprises a non-imaging optical device attached to the emitting face in order to collect rays of said outgoing light emission by reducing a solid angle associated with the outgoing light emission so as to form an output beam. Preferably, the non-imaging optical device is adapted so that a solid angle of the output beam is less than or equal to sr.

[0022] According to one embodiment, said at least one opening has a diameter less than or equal to 10 mm.

[0023] According to one embodiment, the optical pumping cavity is entirely closed except for a single opening forming said at least one opening.

[0024] Alternatively, according to another embodiment, the optical pump cavity is entirely closed except for a first opening and a second opening forming said at least one opening, the incident beam passing through the first opening and the outgoing light emission passing through the second opening.

[0025] According to one embodiment, the concentrator is a parallelepiped having a length 10 times greater than its width and its thickness.

[0026] Another object of the invention is a method for generating an output beam with improved peak power with a luminescent light concentrator comprising the following steps: - generate a laser beam called an incident beam with a laser source - directing the incident beam through at least one opening formed in an optical pump cavity defining a closed volume not including the laser source, the incident beam reflecting a plurality of times on a surface of the optical pump cavity thus creating so-called pump radiation, - absorbing said pump radiation with the luminescent light concentrator arranged in the closed volume so as to emit luminescent radiation having a lifetime T Lwithin said concentrator, a portion called outgoing light emission of said luminescent radiation passing through a face called emitting face of the light concentrator then passing through said at least one opening. - collecting rays from said outgoing light emission by reducing a solid angle associated with the outgoing light emission so as to form an output beam. Brief description of the figures:

[0027] 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:

[0028] [Fig.1], a schematic representation of the power of certain sources as a function of their geometric extent,

[0029] [Fig.2A] a schematic view of an example of a light emission module known from the prior art,

[0030] [Fig.2B] a representation of the rays emitted in a luminescent concentrator, known from the prior art,

[0031] [Fig.3A], a schematic view of examples of extraction light emission modules improved by coupling several cones towards the output with mirrors, known from the prior art,

[0032] [Fig.3B], a schematic view of an example of an extraction light emission module improved by increasing the size of the output cone with a non-imaging interface optic, known from the prior art,

[0033] [Fig.4A], a schematic view of a light emitting device according to the invention,

[0034] [Fig.4B], a schematic perspective view of the concentrator only of the light emitting device according to the invention,

[0035] [Fig.4C], a schematic view of a light emission device according to the invention with an optical pumping cavity adapted to the geometry of the concentrator,

[0036] [Fig.4D], a schematic view of a light emitting device according to one embodiment of the invention, with an optical pumping cavity comprising a single opening

[0037] [Fig.5], the time profile of the outgoing light emission for two different diameters of a spherical optical pumping cavity,

[0038] [Fig.6], a schematic view of a light emission device according to an embodiment of the invention with a non-imaging extractor device at the output,

[0039] [Fig.7], a schematic view of a light emission device according to an embodiment of the invention with a conventional optic located at the output,

[0040] In the figures, unless otherwise indicated, the elements are not to scale. Detailed description:

[0041] Figure 4A shows a schematic view of a light emission device 1 according to the invention comprising a light concentrator CL luminescent optically pumped by an SL laser source via an optical pump cavity CP.

[0042] Figure 4B illustrates a schematic perspective view of the CL concentrator only, when it is optically pumped. As a non-limiting example, in the embodiment of Figures 4A and 4B, the CL concentrator is a rectangular parallelepiped of length L, width w and thickness e having two large faces FE1, FE2 and four lateral faces FE, FO, FL1, FL2.

[0043] As will be explained later, the light emission device 1 according to the invention makes it possible to obtain an outgoing light emission L spresenting a high peak power and a low spread, two characteristics that the light sources of the prior art did not allow to obtain simultaneously to date.

[0044] The laser source SL is adapted to generate an incident beam Fl. This source SL is not specific to the invention and can be any laser source known to those skilled in the art. Preferably, the source SL is a pulsed source delivering laser pulses of duration T P . A pulsed SL source makes it possible to obtain an outgoing light emission L s higher peak power than a continuous laser source, while reducing thermal problems within the concentrator.

[0045] According to one embodiment, the laser source SL is a Q-switched laser source. As a non-limiting example, the laser source SL is a frequency-tripled Q-switched Nd:YAG laser delivering pulses with an energy of 15 mJ at 355 nm and with a pulse duration T P = 5 ns and with a rate of 20 Hz.

[0046] The optical pump cavity CP has a surface SR adapted to reflect the incident beam Fl. The surface SR of the cavity CP defines a closed volume comprising the concentrator CL and in which the laser source SL is not included. The closed volume defined by the surface SR of the cavity CP comprises at least one opening 01 so that the incident beam can pass through the opening 01 and be reflected or diffused a plurality of times on the surface SR of the optical pump cavity, thus creating pump radiation RP.

[0047] By "closed volume" is meant here that the surface SR of the cavity CP is entirely closed except for one or more openings having a ST ST total surface ST such that — < 5%, preferably less than — < 1%. The use of a closed cavity makes it possible to avoid losses suffered by the incident beam Fl during its propagation in the cavity CP, which is essential for generating an outgoing light emission L s exhibiting high peak power.

[0048] The concentrator CL is arranged in the cavity CP and comprises phosphors Lum adapted to absorb the pump radiation RP so as to emit the luminescent radiation L L within the CL concentrator. We note T L the lifetime of the luminescent radiation L .

[0049] As mentioned above, in a manner known per se, a portion L s of luminescent radiation L L— called outgoing light emission - passes through a face called the emitter FE of the light concentrator CL, then passes through the opening 02. This portion Ls includes the guided portion L g reflected on the faces of the concentrator and the unguided portion L out associated with the emitting face FE passing directly through the latter, without being reflected on the faces of the concentrator.

[0050] The optical pump cavity CP allows the pump radiation RP to pass through the concentrator medium CL a plurality of times, which increases the absorption of the pump radiation RP by the concentrator. Thus, the peak power of the outgoing light emission L is increased. s . Knowing the absorption coefficient of the concentrator for the pump radiation (cc=1 cm -1for example), the absorption length can be defined as 1 / oc necessary to absorb 63% of the pump power. By using a cavity with a surface having a high reflection or scattering coefficient resulting in multiple reflections / scatterings on the surface of the cavity CP and therefore multiple passes in the concentrator, it is possible to use a concentrator CL in a material having an absorption length for the incident beam Fl much smaller than the thickness e of the concentrator. For example, it is possible to use a concentrator material with an absorption coefficient of the order of cm -1 or a fraction of cm -1 and a thickness e of the order of mm, implying an absorption length at least 10 times greater than the thickness.

[0051] In addition, the CP cavity allows the pumping to be "distributed" on all the faces of the concentrator simultaneously, without resorting to complex optical systems to distribute the light on the different lateral faces. Thus, the power density (or fluence in the pulsed case) of the pump radiation RP on the faces of the concentrator (and within the concentrator) is limited and the problems of thermal effect, optical damage threshold and photo-bleaching of the phosphors Lum of the concentrator CL are avoided. Conversely, pumping the concentrator CL directly with the incident beam Fl would result in a much higher power density in the pumped area which could cause optical damage effects and photo-bleaching problems. For example, a fluorescent medium using organic molecules pumped by laser is limited in luminance and power because of the aforementioned effects.

[0052] The distribution of the pumping on all the side faces also makes it possible to limit the surface area of ​​the large pumping faces FE1 FE2 and therefore to limit the width w of the concentrator in order to have the most symmetrical outlet face possible (ideally a square).

[0053] Thus, the light emission device 1 of the invention makes it possible to generate an outgoing light emission L s having high power and low range. For this, it is essential that the laser source SL is located outside the cavity CP so that it does not obstruct the multiple reflections of the incident beam Fl on the surface SR of the cavity CP, which would reduce the effective pump radiation power RP pumping the concentrator CL.

[0054] In a first approximation, the device 1 according to the invention works for any shape of concentrator CL. However, the shape of the concentrator directly influences the outgoing light emission L s , in particular its luminance and geometric extent. It is therefore preferable to choose a concentrator shape suitable for: - produce a significant concentrating effect of the luminescent radiation by total internal reflections in the structure. Thus, the luminance of the outgoing light emission L s This parameter is controlled by the ratio L / e between the length and thickness of the concentrator in the example of Figure 4B. - produce an outgoing light emission L s with a geometric extent adapted to the chosen application. This parameter is controlled by the shape and dimension of the emitting face FE.

[0055] Also, preferably, the concentrator is in the form of a "rod", that is to say a parallelepiped having a length at least 5 times, preferably 10 times, even more preferably 20 times, greater than its width and its thickness in order to achieve a significant "concentrating" effect by total internal reflections in the structure. The length of the concentrator is however limited by the propagation losses intrinsic to the concentrator. It is therefore preferable to find an optimal compromise between maximum concentrating effect and high propagation losses.

[0056] Even more preferably, the emitting face has a square or close to a square shape with an area adapted to the imaging system which will follow the source (for example a few mm 2 ).

[0057] In a preferred embodiment, the surface of the optical pump cavity CP diffusely reflects a percentage greater than or equal to 90% of the incident radiation and has a reflection coefficient greater than 90%, preferably greater than 95% for the incident radiation. Thus, the most homogeneous pump radiation possible is obtained and efficiently pumps a larger volume of the concentrator CL. This characteristic is critical to guarantee a high peak power of the outgoing light emission L. swhile limiting the pump power density volume within the concentrator. Conversely, a surface of the optical pump cavity CP specularly reflecting a majority of the incident beam would generate less homogeneous pump radiation that could lead to hot spots in the concentrator where the power density volume could exceed the threshold for optical damage or photobleaching.

[0058] For example, the surface of the optical cavity is made of Spectralon or polytetrafluoroethylene. Spectralon has a reflection coefficient of 99% for a spectral range from 400 nm to 1500 nm. This implies that the incident beam Fl can be reflected by scattering 100 times before losing 63% of its power.

[0059] As a non-limiting example, in the example of Figure 4A, the pumping cavity CP is a sphere of diameter D comprising two openings O1, O2.

[0060] It is important to note that the shape of the cavity can be adapted to the geometry of the concentrator. For example, Figure 4C shows a cylindrical diffusing pump cavity CP with a length adapted to the length L of the luminescent concentrator. In this compact structure, direct illumination of the concentrator by the laser source SL is avoided by diffusion of the incident beam Fl upon entering the pump cavity thanks to a diffusing medium MD placed in the cavity CP. In order to complete the confinement of the light inside the cavity, it is covered with a reflective layer SR. This cavity can also be adapted to allow air to circulate to cool the concentrator heating up due to optical pumping.

[0061] In the preferred embodiment where the laser source SL operates in pulsed mode, it is very advantageous that the lifetime T Lof the phosphors is less than or equal to the duration T P pulses. Thus, the emission from the concentrator is not "slowed down" by the lifetime of the excited level of the phosphor. This makes it possible to maximize the peak power of outgoing light remission L s .

[0062] Similarly, in the embodiment where the SL laser source operates in pulsed mode, it is preferable that the SL laser source and the optical pumping cavity (via its incident beam reflection characteristics) are adapted so that a fluence (or energy density) of the pump radiation within the light concentrator is less than 5 J / cm 2 , preferably less than or equal to 1 J / cm 2 . Indeed, the laser optical damage threshold is typically between 5 J / cm 2 and 10 J / cm 2 for a pulsed laser delivering 10 ns pulses, depending on the experimental conditions.

[0063] In the embodiment of Figure 4A, the optical pump cavity CP is fully closed except for a first and a second opening 01, 02, the incident beam passing through the first opening 01 and the outgoing light emission L out passing through the second opening 02. This embodiment has the advantage of being easy to implement.

[0064] Alternatively, according to another embodiment illustrated in Figure 4D, the optical pumping cavity CP is entirely closed except for a single opening 01 crossed by the incident beam Fl and by the outgoing light emission L s This embodiment is for example implemented by using a dichroic mirror MR arranged so as to reflect the incident beam Fl by directing it through the single opening, the dichroic mirror further transmitting the outgoing light emission L s. This embodiment has the advantage of minimizing the power losses of the pump radiation RP caused by the presence of two openings (as in the embodiment of Figure 4A for example). Even more preferably, as illustrated in Figure 4D, the mirror MD is arranged so that the incident beam Fl does not pass through the concentrator before a first reflection on the surface of the cavity CP in order to limit the power density of the laser beam during its first passage through the concentrator. This avoids the effects of photo-bleaching and laser optical damage.

[0065] Preferably, in all embodiments of the invention, the dimension of the opening of the optical cavity crossed by the incident beam Fl (e.g.: the opening 01 in the embodiment of FIG. 4A) is adapted according to the waist w0 of the incident beam Fl. More precisely, the opening is arranged so that the incident beam Fl crosses the opening at its smallest transverse dimension, that is to say when the incident beam has a transverse radius of dimension w0, the opening having a radius greater than or equal to w0 but less than 3w0. This makes it possible to limit the losses associated with the cavity CP by preventing the incident beam from being diaphragmed by the opening it crosses. More generally, the opening of the optical cavity crossed by the incident beam Fl has a diameter less than or equal to 10 mm in order to limit the losses associated with the cavity CP.

[0066] Similarly, preferably, the dimension of the opening of the optical cavity crossed by the outgoing light emission L s (e.g., aperture 02 in the embodiment of Figure 4A) is adapted taking into account the Lambertian remission of the concentrator, so that none of the rays of the outgoing light emission L s are not blocked by the opening. For example, the emitting face FE is attached to the opening of the optical cavity crossed by the outgoing light emission L s , the latter having the same dimensions as the emitting face FE (for example wxe in the embodiment of figure 4B).

[0067] According to an embodiment in which the source SL is pulsed, the cavity CP has dimensions adapted according to the lifetime T L of the luminescent radiation so that the outgoing light emission L sor a pulsed light emission of a predetermined duration. Indeed, it is possible to adapt the dimensions of the cavity in order to adapt the duration of pulsed light remission, the lower limit of the duration of the pulsed light emission being fixed by the lifetime T L of the luminescent radiation of the fluorophores. The upper bound is related to the average propagation time of a pump photon inside the pump cavity. This characteristic gives a modularity to the device according to the invention which is not usually available in light sources of the prior art. This result is illustrated in Figure 5 which represents the time profile of outgoing light remission L sfor two different diameters of a spherical CP cavity. As a non-limiting example, the curves in Figure 5 are obtained for a frequency-tripled SL Nd:YAG laser source delivering pulses with an energy of 15 mJ at 355 nm and with a pulse duration T P = 5 ns and with a rate of 20 Hz. In addition, the CL concentrator is a rectangular volume made of PMMA with the following dimensions L = 40 mm; w = 2 mm; e = 2 mm and featuring fluorophores with a lifetime T L of about 10 ns.

[0068] The time profile C1 is obtained for a spherical cavity with a diameter D = 50 mm while the time profile C2 is obtained for a spherical cavity with a diameter D = 100 mm. It can be seen that the time profile C1 has a width at half-maximum of approximately 27 ns. With the time profile C2 obtained for a sphere of diameter D = 100 mm, the duration of the outgoing light emission L sis longer (full width at half maximum of about 35 ns) because of the larger propagation distances in the sphere. The temporal broadening recorded with the larger diameter spherical cavity is due to the longer travel time of the pump photons between each scattering at the surface of the sphere.

[0069] In the example of figure 5, for profile C1 we measure a maximum peak power of 6 kW for the outgoing light emission L s , for a pump energy of 15 mJ. This corresponds to a luminance of 20 kW / cm 2 / sr, 5 times higher than the luminance obtained in the document M.Nourry-Martin, P.Pichon, F. Druon, S.Darbon, F.Balembois, and P. Georges "Light recycling in LED-pumped Ce:YAG luminescent concentrators" Optics Express Vol.29, No.16 25302-25313 (2021 ), with a Ce:YAG concentrator pumped by LED with 15 ps pulses. However, the inventors considered it preferable to limit the pump energy to 10 mJ in order to avoid the appearance of photo-bleaching phenomena of the dyes of the concentrators.

[0070] As is known, the luminescent emission in the concentrator is Lambertian. Also, the solid angle of emission of the outgoing light emission L sis worth Ti sr which makes it difficult to use this radiation for practical purposes, for example when it is desired to couple it to a conventional optical system. To overcome this problem, in the embodiment illustrated in Figure 6, the light emitting device 1 comprises a non-imaging optical device ONI attached to the emitting face FE in order to collect outgoing light remission rays by reducing a solid angle associated with the outgoing light emission so as to form an output beam FS. This non-imaging optical device ONI is preferably bonded to the emitting face FE with a high index glue to maximize the extraction efficiency of the outgoing light emission L sAnother advantage of using the non-imaging optical device ONI attached to the emitting face FE is that it allows the mechanical maintenance of the concentrator CL within the cavity CP without using an additional mechanical element which would reduce the effective pump power.

[0071] According to one embodiment, the non-imaging optical device ONI is a parabolic concentrator (concentrator parabolic compound in English) attached to the emitting face FE using a photo-curable UV glue in order to extract a maximum of light trapped in the concentrator and to straighten the rays in the output beam FS. As a non-limiting example, the parabolic concentrator ONI has an input diameter of 2.5 mm attached to the face FE and an output diameter of 5 mm. The solid angle at the output of the parabolic concentrator ONI is estimated at sr and the geometric extent of the output beam is therefore of the order of 0.1 cm2 sr.

[0072] More generally, it is preferable that the non-imaging optical device ONI has a structure adapted so that a solid angle of the output beam is less than or equal to sr in order to facilitate the use of the output radiation of the device 1. Thus, the output beam FS shaped by the non-imaging optical device ONI can be coupled into a conventional optical device SO as illustrated in FIG. 7 in order to provide a beam FE usable by an optical system of low numerical aperture for example.

[0073] As mentioned previously, preferably, the concentrator CL is a parallelepiped with a square emitting face FE because this facilitates the coupling of the outgoing light emission L s with the non-imaging optical device ONI and possibly the conventional optical device SO.

Claims

Claims 1. Light emission device (1) comprising: - a laser source (SL) adapted to generate an incident beam (Fl) - a luminescent light concentrator (CL) adapted to absorb pump radiation (RP) then emit luminescent radiation (L L ) within said concentrator, a portion (L s ) called outgoing light emission of said luminescent radiation (L L ) passing through a so-called emitting face (FE) of the light concentrator, - an optical cavity defining a closed volume not comprising the laser source (SL) and comprising the concentrator and at least one opening (01, 02), the incident beam passes through said at least one opening and is reflected a plurality of times on a surface (SR) of the optical cavity, thus creating diffuse radiation in the optical cavity, said radiation being absorbed by the concentrator, and said light emission passing through said at least one opening.

2. Device according to claim 1, in which said surface of the optical cavity (CP) diffusely reflects a percentage greater than or equal to 90% of the incident radiation and has a reflection coefficient greater than 90%, preferably greater than 95% for the incident radiation.

3. Device according to claim 1 or 2, wherein said surface of the optical cavity is made of polytetrafluoroethylene.

4. Device according to any one of the preceding claims, in which the laser source (SL) is a pulsed source delivering laser pulses of duration T P .

5. Device according to claim 4, in which the luminescent radiation within the concentrator has a lifetime T L and said lifetime of the luminescent radiation is less than or equal to the duration T P impulses.

6. Device according to claim 4 or 5, in which the laser source (SL) and the optical pumping cavity are adapted so that a fluence of the pump radiation within the light concentrator is less than 5 J / cm 2 , preferably less than or equal to 1 J / cm 2 .

7. Device according to any one of claims 4 to 6, in which dimensions of the optical cavity (CP) are adapted according to a lifetime T Lluminescent radiation so that said outgoing light emission is a pulsed light emission of a predetermined duration, for example having a width at half height of between 50 and 250 ns.

8. Device according to any one of the preceding claims, comprising a non-imaging optical device attached to the emitting face in order to collect rays of said outgoing light emission by reducing a solid angle associated with the outgoing light emission so as to form an output beam (FS), preferably the non-imaging optical device is adapted so that a solid angle of the output beam is less than or equal to sr.

9. Device according to any one of the preceding claims, in which said at least one opening has a diameter less than or equal to 10 mm.

10. Device according to any one of the preceding claims, in which the optical cavity is entirely closed except for a single opening forming said at least one opening or a first and a second opening forming said at least one opening, the incident beam passing through the first opening and the outgoing light emission passing through the second opening.

11. Device according to any one of the preceding claims, in which the concentrator is a parallelepiped having a length 10 times greater than its width and its thickness.

12. Device according to any one of the preceding claims, in which the concentrator is made of polymethyl methacrylate (PMMA).

13. Device according to any one of the preceding claims, in which the optical cavity is spherical or cylindrical in shape.

14. A method for generating an output beam with enhanced peak power with a luminescent light concentrator (CL) comprising the following steps: - generate a laser beam called an incident beam (Fl) with a laser source (SL) - directing the incident beam through at least one opening (01, 02) formed in an optical cavity (CP) defining a closed volume not including the laser source (SL), the incident beam being reflected a plurality of times on a surface of the optical pumping cavity thus creating diffuse radiation in the optical cavity, - absorbing said diffuse radiation with the luminescent light concentrator (CL) arranged in the closed volume so as to emit luminescent radiation (L), a portion (Ls) called outgoing light emission of said luminescent radiation (L L) passing through a so-called emitting face (SE) of the light concentrator then passing through said at least one opening. - collecting rays from said outgoing light emission by reducing a solid angle associated with the outgoing light emission so as to form an output beam (FS).

Images