Laser amplifier module for solid laser system and manufacturing method of them

Abstract

To provide a module that amplifies a laser beam capable of effectively cooling a heat sink connected to a lid state disc.SOLUTION: A monolithic laser amplifier module 1 contains: a monolithic composite 6 containing a solid state disc 2, a heat sink 3, and a reflection film 4 that is constructed so as to at least partially reflect an injection beam 5 which is transmitted in the solid state disc 2 within a wavelength range λ of 200nm to 10 μm, that is the monolithic composite 6 in which the reflection film 4 is accumulated onto a front surface of the heat sink by an accumulation method, and the heat sink 3 contains a specific heat thermal conductivity, a Young rate, and a thickness of the heat sink. Each of the solid state disc and the monolithic composite, includes front surfaces 61 and 21 each having a PV flatness level of <210nm, includes a surface roughness RMS of <2nm. The solid state disc 2 and the front surfaces 21 and 61 of the monolithic composite 6 are directly permanently connected.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a laser amplification module for solid-state lasers. More specifically, the present invention relates to a module for amplifying a laser beam that can effectively cool a heat sink coupled to a solid-state disk.

[0002] In a further embodiment, the present invention relates to a method for manufacturing the same. [Background technology]

[0003] High-average-power laser systems are particularly plagued by the heat generated in the laser components, especially during beam amplification. Typically, solid laser disks are bonded or soldered to water-cooled heat sinks, and heat is transferred from the solid laser disks to the heat sink. This heat dissipation technique is not very efficient, and beam pointing becomes inadequate in most high-average-power laser systems, especially as the average power approaches the kW class.

[0004] EP 2 996 211 A1 discloses a solid-state laser active medium, also known as a solid-state disk directly bonded to a transparent heatsink. The heatsink has a thermal conductivity of 149 W / m*K or greater, where the solid-state laser active medium and heatsink exhibit a root mean square (RMS) surface roughness of less than 1 nm, and the optical gain material is attached to the transparent heatsink by direct bonding. The document also discloses a monolithic semiconductor or amorphous structure including a solid-state laser disk and a reflective layer on the disk. The highly reflective layer can be attached to the solid-state laser disk by direct deposition methods such as sputtering, evaporation, or chemical vapor deposition, or it can be directly bonded to the solid disk. Figure 1 schematically shows two embodiments disclosed in the prior art described above. Figure 1A, the first embodiment, discloses a module for a solid-state laser beam comprising a solid-state disk 2 directly bonded to a reflective layer 4 (indicated by a dashed line) and directly bonded to a transparent heatsink 3. The optical beam 5 is incident on the solid-state disk 2 and propagates through the reflective layer 4 and the transparent heat sink 3. The amplified beam 51 is emitted from the transparent heat sink 3. Figure 1B, a second embodiment, schematically shows a solid-state laser beam module including a solid-state disk 2 and a reflective layer 4 (shown as a full line) deposited on the solid-state disk 2, which is directly coupled (shown as a dashed line) to the transparent heat sink 3. The optical beam 5 is incident on the solid-state disk 2 and propagates through the reflective layer 4 and the transparent heat sink 3. The amplified beam 51 is emitted from the transparent heat sink 3. However, both embodiments are disadvantageous in the case of heat removal by a water jet coming from the bottom of the heat sink 2.

[0005] Therefore, an object of the present invention is to provide a laser amplification module for a solid-state laser system that is easier to manufacture than embodiments of the prior art, while efficiently removing heat. More effective heat removal inevitably leads to higher and more stable laser performance. Surface activation allows for at least 0.5 J / m³ at temperatures above 500°C. 2The adhesive strength is guaranteed. Next, both surfaces are aligned with each other and bonded under atmospheric pressure or vacuum to facilitate manufacturing. [Overview of the project] [Means for solving the problem]

[0006] A monolithic laser amplification module for a solid-state laser system, according to a first aspect of the present invention, provides a solution to the above-mentioned objectives and is defined by claim 1.

[0007] The laser amplification module according to the present invention can be implemented in a solid-state laser system for emitting laser beams, particularly high-power laser beams for industrial or scientific applications such as laser drilling, laser shock peening, and particle acceleration.

[0008] A solid-state laser is a type of laser that uses a solid gain medium. According to the present invention, the solid medium is a solid-state disk, also known in the prior art as a thin-disk laser, which is a type of diode-pumped solid-state laser characterized by a heat sink and laser output realized on the opposite side of a thin layer of the active gain medium. The shape of the laser disk is arbitrary, preferably circular or rectangular. The thickness of the solid-state disk is considerably smaller than the laser beam diameter. In the context of the present invention, the term gain medium should be understood as a material capable of generating optical gain, where optical gain is understood as describing the optical amplification process in the material. The gain medium can be both doped and undoped crystals, such as YAG, sapphire, silicate, garnet, vanadate, tungstate, or phosphate glass, preferably doped with laser-active ions, ceramic materials, or semiconductor materials such as GaAs, InGaAs, or GaN. The dopant content may vary depending on the specific application of the solid-state laser system, preferably 0.01 to 20 mol(atomic)%. In another embodiment, the solid-state disk may be derived from an amorphous gain material such as phosphate glass or amorphous silica. The solid-state disk may have any thickness, but advantageously, the module, resp. method according to the present invention is provided to be able to provide a thickness as low as 5 μm.

[0009] The monolithic composite is understood as a continuous solid-state element comprising a heat sink and a reflective film. The reflective film is deposited on the heat sink by prior art deposition methods such as physical vapor deposition, sputtering, evaporation, chemical vapor deposition, and crystal growth techniques such as molecular beam epitaxy. The reflective film is configured to reflect the incident beam, particularly the pump beam, at least partially. The reflective film reflects beams having wavelengths in the range of 200 nm to 10 μm. The reflective film can be selected from a group of materials such as zinc sulfide, titanium dioxide, tantalum pentoxide, silicon dioxide, hafnium oxide, gallium arsenide, and aluminum gallium arsenide.

[0010] The heat sink according to the present invention has a lateral thermal conductivity of at least 100 W / m*K, a Young's modulus of at least 100 GPa, and a heat sink thickness of at least 1 mm. The heat sink may be made from diamond, boron nitride, silicon, silicon carbide (SiC), ceramic, metal, metal-diamond composite, metal-boron nitride composite, or silicon-diamond composite. In a more preferred embodiment, the Young's modulus of the heat sink is at least 300 GPa, and / or its thickness is at least 2 mm. A heat sink thickness of at least 2 mm prevents bending during the coating process.

[0011] The surfaces of both the monolithic composite and the solid-state disk have a PV flatness of less than 210 nm and a roughness RMS of less than 2 nm. The above surfaces are directly and permanently bonded together. The direct bonding process should be understood as directly bonding the laser disk to a heat sink without any intermediate adhesive layers such as adhesives or metal solder. The heat sink can be understood as a heat spreader or final carrier substrate having a thermal conductivity of 100 W / m*K or higher at room temperature.

[0012] PV (peak-to-trough) flatness is defined as the absolute difference between the highest and lowest points of an optical surface relative to an ideal surface. RMS is a parameter well known in the prior art, generally defined as the root mean square of surface waviness on a nanometer scale. PV flatness and RMS can be measured using prior art methods such as laser interferometry or atomic force microscopy.

[0013] The monolithic laser amplification module for a solid-state laser system as defined in claim 1 provides a thermal conductivity of more than 0.1 W / (mK), preferably more than 1 W / (mK), for a solid-state disk temperature <500°C due to laser operation.

[0014] In a preferred embodiment, the monolithic composite includes at least two layers that form a reflective film. More preferably, the monolithic composite includes multiple layers. The specific number of layers depends on the industrial application. A person skilled in the art of a particular application of a solid-state laser will know how many layers are sufficient to meet the desired mirror specifications. For an SiO2 / Ta2O5 based film, calculated for 1030 nm at an incident angle of 0°, the reflectivity can vary from 60% for all four layers (2xSiO2+Ta2O5), over 90% for all eight layers, and 99.9% for 20 layers. The layers forming the reflective film alternate with respect to refractive index. The first and second layers have different refractive indices. In embodiments including multiple layers, the first and second layers have alternating refractive indices such that the first layer has n=1.46 (SiO2), the second layer has n=2.1 (Ta2O5), and the third layer again has n=1.46. The final upper layer is directly and permanently bonded to the solid-state disk. In a preferred embodiment, the lowest refractive index n low = 1.35 and highest refractive index n high A high-reflectivity film can be manufactured using a value of 5.6.

[0015] The monolithic composite preferably includes a sacrificial layer. The functional roles of this sacrificial layer are (a) protection of the reflective film, (b) improvement of the laser-induced damage threshold, and (c) provision of a buffer layer that can be post-processed to achieve the desired surface specifications. The sacrificial layer is made of a material that reduces the reflectivity of the reflective film to less than 10%. Such a protective sacrificial layer may be SiO2, Si3N4, HfO2, an oxide of any metal, particularly Al2O3, GaAs, AlGaAs, NiP, or a photoresist material, which can be polished to achieve the desired surface shape, particularly a PV flatness of less than 210 nm and an RMS surface roughness RMS < 2 nm, preferably PV < 70 nm and RMS < 0.8 nm. Reflectivity from 90% to 99.9% is particularly advantageous. At least one sacrificial layer having a thickness of 407 nm, for example made from SiO2, can increase the LIDT of the underlying multilayer reflective film by about 7 times.

[0016] More preferably, the monolithic composite includes multiple sacrificial layers forming a film stack. The multiple sacrificial layers may be configured to increase the laser-induced damage threshold of the monolithic laser amplification module. Film stacks of sacrificial layers are particularly advantageous in combination with films containing AlGaAs or GaAs. These materials exhibit a low damage threshold for pulsed lasers with pulse lengths of less than 10 ns. More preferably, in the film stack, at least one layer is micro- or nano-structured by a pattern. This pattern improves the mechanical and / or thermal properties of the solid-state disk. The pattern may be any kind of pattern that conforms together with the pattern on the opposite surface, which may be the surface of a reflective film or further sacrificial layers or sacrificial layers and a heat sink and / or any combination thereof. The pattern may be groove-like.

[0017] In another embodiment, the LIDT of a multilayer reflective film can be increased by fabricating nanolaminates that co-evaporate at the interface.

[0018] The monolithic composite is preferably polished. The polished surfaces of the monolithic composite and the solid-state disk, which are directly and permanently bonded to each other, have a flatness PV < 210 nm. The polishing method can be selected according to prior art, such as mechanical, magnetoviscous, chemical, or ion-assisted, provided that the flatness PV and RMS of the above parameters are achieved.

[0019] The surface of the monolithic composite is preferably curved. More preferably, the surface of the monolithic composite is curved, and surfaces having a PV < 210 nm value and RMS < 2 nm cover an area greater than 80% of the total surface.

[0020] The surfaces of solid-state disks and monolithic composites have a PV < 70 nm and RMS < 0.8 nm. These values ​​provide higher bonding strength.

[0021] The heat sink is preferably transparent to wavelengths between 200 nm and 10 μm and has an attenuation coefficient < 1 cm -1 -1. A transparent heat sink can provide an embodiment in which the output coupling of the laser beam is provided through the transparent heat sink. In another embodiment, the heat sink can be opaque to the laser wavelength(s) such that a reflective output coupling of the laser beam can be provided.

[0022] The heat sink preferably comprises a material having a thermal conductivity of at least 100 W / (m·K), preferably diamond, boron nitride, silicon, silicon carbide, ceramic, metal, metal-diamond composite, metal-boron nitride composite, or silicon-diamond composite.

[0023] The solid state disk preferably includes an anti-reflection film on a surface opposite to the surface and is directly and permanently bonded to the monolithic composite.

[0024] The side edges of the monolithic laser amplification module are preferably roughened and / or chamfered. By roughening and / or chamfering the edges, the amplified spontaneous emission (ASE) from the laser active medium is minimized.

[0025] A second aspect of the present invention is a method of manufacturing a laser amplification module as defined in claim 15.

[0026] The method includes the step of providing a solid state disk, at least one of whose surfaces has a surface flatness with a root mean square (RMS) and peak-to-valley (PV) of less than 2 nm and less than 210 nm, respectively, and a thickness of at least 5 μm.

[0027] The next step is to provide a heat sink, at least one of whose surfaces has a surface flatness with an RMS and PV of less than 2 nm and less than 210 nm, respectively, and a thickness of at least 1 mm.

[0028] Subsequently, the heat sink is coated with a reflective film that at least partially reflects the incident beam with a wavelength range λ of 200 nm to 10 μm, thereby forming a monolithic composite.

[0029] The final step involves aligning and directly bonding the surfaces of both the monolithic composite and the heatsink, which have surface flatness of less than 2 nm RMS and PV < 210 nm.

[0030] The step of coating the heat sink with a reflective film preferably includes multiple coating steps to create a film stack of alternating materials having alternating refractive indices in order to form an interference film with a reflectivity of more than 90%. The multiple coating steps can be provided by conventional means such as evaporation, ion assistance, ion beam sputtering, magnetron sputtering, or molecular beam epitaxy.

[0031] The coating step may introduce undesirable curvature into the heat sink and result in a high surface roughness of RMS > 1 nm. To compensate for the introduced curvature, those skilled in the art may further provide: a) a heat sink having a Young's modulus of at least 300 GPa and a thickness of at least 2 mm, such that curvature caused by the coating is very unlikely; or b) a high reflectivity film may further comprise a sacrificial layer on top of the reflective film. Chemical mechanical polishing or ion-assisted polishing processes may help reduce void density while simultaneously improving surface quality (PV flatness, RMS surface roughness). Generally, voids are understood to be empty spaces (holes) resulting, for example, from the release of gases from the material. Defects are considered errors in the crystal structure.

[0032] The manufacturing method preferably includes the step of providing a sacrificial layer on top of the reflective film and polishing it to PV < 210 nm and RMS < 1 nm. Polishing of the protective sacrificial layer can be performed by ion beam assisted polishing technique. In this method, argon ions are typically generated in an ion gun and accelerated toward the protective sacrificial layer. The impact on the protective sacrificial layer leads to sputtering of surface atoms. A reduction in surface roughness can be observed. The specific substrate material, incident angle, and ion beam voltage have a significant impact on the final quality of the surface roughness. This processing method can also be used to precisely modify the surface shape, in particular to etch the substrate to a very high degree of flatness (PV < 10 nm for fused silica substrates). To avoid surface voids during processing, the surface is kept very clean, and soft beam parameters (low ion energy) are selected accordingly.

[0033] Another embodiment, which involves providing a sacrificial layer on top of the reflective film and polishing it, is chemical mechanical polishing. Chemical mechanical polishing processes are common procedures in the semiconductor industry, applied to various wafer materials. This is a combination of chemical etching for atomic-scale material removal and mechanical abrasion for achieving overall flatness. As a result, the surface can achieve flatness of PV < 70 nm and surface roughness RMS < 0.5 nm, depending on the material.

[0034] The direct bonding step involves providing a monolithic composite while simultaneously over-cleaning and surface-activating the solid-state disks. Surface activation can be chemical, plasma, or ion beam activation. The surfaces are then pressed together. The pressure applied may vary depending on the thickness of the substrate, but is typically in the range of 2 kPa to 10 MPa. For curved substrates, high bonding strength can be achieved by aligning with an accuracy of less than 5 μm.

[0035] The method according to the present invention is 0.1 J / m 2 , advantageously 0.5 J / m 2This can provide a bonding strength. This allows for the mechanical polishing and / or thinning of the second surface of the solid-state disk to a desired thickness (typically greater than 5 μm, ideally greater than 100 μm). After thinning and / or polishing, the second surface of the solid-state disk exhibits high flatness (ideally <70 nm) and low RMS surface roughness (ideally RMS <0.8 nm).

[0036] The method may preferably include the step of coating a surface configured to receive a light beam from a pump source. This surface may then be coated with an anti-reflective coating for efficient laser operation. The coating may be carried out by conventional means such as evaporation, ion-assisted, ion beam sputtering, or magnetron sputtering. Alternatively, one may also use a semiconductor-based anti-reflective coating (AlGaAs / GaAs).

[0037] In a further preferred embodiment, the method further includes side roughening and / or chamfering of the bonded disks. Side roughening and / or chamfering minimizes amplified spontaneous emission (ASE) from the laser gain medium when the edges are roughened. Higher surface roughness creates small reflection sites, resulting in significant diffraction loss of reflected light. Another way to suppress ASE is to bevel the edges of the thin-film disks. [Brief explanation of the drawing]

[0038] [Figure 1] A and B represent two prior art embodiments. [Figure 2] This represents a first embodiment of the present invention. [Figure 3] This represents a preferred embodiment including two layers that form a reflective film. [Figure 4] This represents a preferred embodiment that includes a plurality of alternating layers forming a reflective film. [Figure 5] This represents a preferred embodiment that includes a sacrificial layer on top of the film. [Figure 6]This represents a preferred embodiment including a nanostructured sacrificial layer. [Figure 7] This represents a preferred embodiment including an anti-reflective coating. [Figure 8] Figures 8A and 8B show preferred embodiments illustrating a module according to the present invention. [Figure 9] This shows an embodiment of the laser amplification module according to the present invention in a solid-state laser system. [Figure 10] Figures 10A and 10B show experimental examples according to the present invention in which the thickness of the sacrificial layer on the reflective film is varied. [Figure 11] Figures 11A and 11B show comparative examples of the present invention. [Figure 12] Comparative study - Examples using prior art are shown. [Modes for carrying out the invention]

[0039] Compared to conventional technologies, reflective films that form high-performance reflectors (>98%) are deposited on solid-state disks by deposition methods, particularly sputtering. This generates inherent film stress within the coating, causing the solid-state disk to bend excessively. The thinner and more elastic the solid-state disk, the more it bends. However, according to the present invention, a heatsink having a Young's modulus of at least 100 GPa and a thickness of more than 1 mm ensures the mechanical rigidity of the heatsink so that the bending is not too deep, and it is still possible to use or compensate for PV flatness for direct bonding with the solid-state disk.

[0040] Figure 2 schematically illustrates a first embodiment according to the present invention. A monolithic laser amplification module 1 for a solid-state laser system is shown. Module 1 includes a thin solid-state disk 2 made from, for example, doped or undoped crystals such as an optical gain medium, e.g., YAG, sapphire, silicate, garnet, vanadate, tungstate, or phosphate glass, preferably doped with laser-active ions, ceramic material, or semiconductor material such as GaAs, InGaAs, or GaN. Generally, those skilled in the art of laser design know which type of gain medium is suitable for a particular industrial application. The solid-state disk 2 has two surfaces. The first surface serves as a plane for an incident beam 5, such as a pump beam from a pump source of a solid-state disk laser. The pump beam may be another laser beam. The opposite surface 21 is treated so that a PV flatness < 210 and RMS < 2 nm are achieved. Treatments that achieve these parameters may be, for example, annealing, polishing, or mechanical and / or chemical treatments. Surface 21 is configured with the parameters described above to provide a direct and permanent bond with the monolithic composite 6. The monolithic composite 6 includes a heat sink 3 and a reflective film 4. The monolithic composite 6 has a surface 61 that has been treated to have a PV flatness < 210 nm and an RMS < 2 nm. The direct and permanent bond provides direct bonding of the solid-state disk 2 to the monolithic composite 6 without any intermediate adhesive layers such as adhesives or metal solder. This bond has a conductivity of 0.1 J / m 2 Therefore, preferably >0.5 J / m 2The heat sink 3 provides strength. The heat sink 3 has a lateral thermal conductivity of at least 100 W / m*K, a Young's modulus of at least 100 GPa, preferably at least 300 GPa, and a heat sink thickness of at least 1 mm, preferably at least 2 mm. The heat sink can be made from diamond, boron nitride, silicon, silicon carbide (SiC), ceramic, metal, metal-diamond composite, metal-boron nitride composite, or silicon-diamond composite. A heat sink thickness of at least 2 mm prevents bending during the coating process, which can be demonstrated by experiments described according to Figures 11A, 11B, and 12. The monolithic composite 6 further includes a reflective film 4 deposited on the heat sink 3. The reflective film 4 can be deposited on the heat sink 3 by prior art deposition methods such as chemical vapor deposition, sputtering, evaporation, and even crystal growth techniques such as molecular beam epitaxy. The reflective film is configured to at least partially reflect an incident beam 5 propagating through the solid-state disk 2, particularly a pump beam. The reflective film 4 reflects beams having wavelengths in the range of 200 nm to 10 μm. The reflective film can be selected from the group including zinc sulfide, titanium dioxide, tantalum pentoxide, silicon dioxide, hafnium oxide, gallium arsenide, and aluminum gallium arsenide. In this way, the reflective film 4 is sandwiched between the heat sink 3 and the solid state disk 2.

[0041] Figure 3 shows a more preferred embodiment, in which the reflective film 4 includes at least two layers 41 and 42. The first layer 41 may be made of Si having a refractive index of about 3.97, and the second layer 42 may be made of SiO2 having a refractive index of about 1.46.

[0042] Figure 4 shows yet another preferred embodiment, in which the reflective film 4 comprises multiple layers of materials having alternating refractive indices. In the embodiment shown in Figure 4, four reflective layers are selected. The first layer 41 may have a refractive index n=2.1, the second layer may have a refractive index n=1.5, the third layer may have a refractive index n=2.1, and the top layer 42 may have a refractive index n=1.5. In another preferred embodiment, the set of alternating layers may be taken from EP3076208.

[0043] Figure 5 shows yet another preferred embodiment, in which the monolithic composite 6 further includes a sacrificial layer 7 having a surface with PV flatness < 210 nm and RMS < 2 nm, provided on the reflective film to establish a direct and permanent bond with the solid-state disk 2. The sacrificial layer 7 serves to protect the reflective film 4. The sacrificial layer 7 may preferably be made of a material that reduces the reflectivity of the reflective film to less than 10%. The sacrificial layer 7 may be made of SiO2, Si3N4, HfO2, oxides of any metal, particularly Al2O3, GaAs, AlGaAs, NiP, or photoresist materials, which may be polished to achieve the desired surface shape, particularly a PV flatness of less than 210 nm and an RMS surface roughness of RMS < 2 nm, preferably PV < 70 nm and RMS < 0.8 nm. Reflectivity from 90% to 99.9% is particularly advantageous. As shown in the experimental results by Schlitz et al., Applied Optics 56 (4), C136-C139, at least one sacrificial layer having a thickness of 407 nm, made of, for example, SiO2, can increase the LIDT of the underlying multilayer reflective film by approximately seven times.

[0044] Figure 6 includes a plurality of sacrificial layers 71, 72 for protecting the reflective film 4 forming the film stack. The plurality of sacrificial layers 71, 72 may be configured to increase the laser-induced damage threshold of the monolithic laser amplification module. More preferably, in the film stack, at least one layer is micro- or nano-structured by a pattern. This pattern improves the mechanical and / or thermal properties of the solid-state disk. The pattern may be interlocking grooves to increase the surface area. The increased surface area can provide higher heat transfer, which provides laser stability. Specific patterns in terms of shape, size, and periodicity can be achieved by prior art methods such as lithography. Nesting grooves may have, for example, a rectangular shape. Nano or micro-patterns may be provided on either the surface of each sacrificial layer, the surface of the reflective film and sacrificial layer that do not serve to directly bond, or the interface between the film and the heat sink.

[0045] In embodiments not shown, the surface 61 of the monolithic composite directly and permanently bonded to the solid-state disk 2 has a flatness of PV < 70 nm. Such flatness of PV can be achieved by polishing by prior art methods such as mechanical, chemical, or ion-assisted polishing.

[0046] In yet another embodiment, the surface 61 of the monolithic composite 6 directly and permanently bonded to the solid-state disk 2 is curved. The radius of curvature may be >0.05m, more preferably >0.5m.

[0047] In yet another embodiment, the surface 21 of the solid-state disk 2 and the surface 61 of the monolithic composite 6 have a PV < 70 nm and an RMS < 0.8 nm.

[0048] In yet another embodiment, the surface 61 of the monolithic composite 6 is curved, and flatness PV < 210 nm and RMS < 2 nm are provided on the curved surfaces of the monolithic composite 6 and the solid disk 2, and the curved surface is a part of the entire surface of the monolithic composite, such as more than 80% of the entire surface of the monolithic composite. Therefore, it is not necessary to provide an interface for the entire direct bond.

[0049] In yet another embodiment, the heat sink (3) is transparent to wavelengths between 200 nm and 10 μm with an attenuation coefficient > 1 cm, and / or opaque to the laser wavelength(s). -1

[0050] FIG. 7 shows an embodiment in which an antireflection film 22 is deposited on the surface to increase the transmittance and reduce the amount of unwanted reflected light.

[0051] In another embodiment not shown, the edges of the module 1 are roughened and / or chamfered. This embodiment provides minimization that minimizes amplified spontaneous emission (ASE) from the laser gain medium.

[0052] FIGS. 8A and 8B show two exemplary embodiments of combinations of features according to the present invention.

[0053] FIG. 9 shows an exemplary embodiment of a module according to the present invention as part of a disk laser. The pump beam 5 is directed towards the module 1 according to the present invention through the lens 81. When the beam is reflected by the mirror 82 so as to return to the module 1, a part of the pump beam 5 is absorbed and amplified within the module 1. The amplified laser beam is emitted from the module 1 to the partial reflection element 83 that can at least partially reflect the laser beam back to the module 1 and partially transmit the laser beam 51.

[0054] Figure 10A shows the simulation results performed in Module 1. The simulation shows the exemplary reflectivity of a reflective film 4 having a sacrificial layer 7 on top of the reflective film 4. Here, the reflective film consists of alternating layers of SiO2 and Ta2O5 with corresponding refractive indices of 1.46 and 2.1. Here, the reflective film is designed for a central wavelength of 1000 nm, with a thickness of 171.2 nm for the SiO2 layer and a thickness of 119 nm for the Ta2O5 layer. A high reflectivity of 99.9% is achieved by 20 alternating layers. On top of the reflective film, an SiO2 sacrificial layer is simulated. Figure 10A shows the different reflectivity behavior of the film at different thicknesses of the sacrificial layer.

[0055] Figure 10B shows the dependence of the film's reflectivity on the sacrificial layer thickness. The film is constructed similarly to Figure 10A. It is shown that the sacrificial layer thickness has only a slight effect on the film's final reflectivity performance. Based on this fact, a sacrificial layer can be used to improve surface quality and establish PV < 210 nm and RMS surface roughness < 2 nm.

[0056] Figure 11A shows the experimental results of a coated heat sink. In this embodiment, the heat sink has a Young's modulus greater than 300 GPa, a thickness of 2 mm, and a diameter of 12 mm. The heat sink is first polished to a flatness of PV < 70 nm and a surface roughness of RMS < 0.4 nm. Flatness is measured using a laser interferometer operating at 632.8 nm. Surface roughness is measured using an atomic force microscope in non-contact mode. A high-reflectivity film made of SiO2 / Ta2O5 with a total thickness of approximately 4 μm and a 340 nm SiO2 sacrificial layer on top is applied to the heat sink, providing a reflectivity of over 99.9% at 1030 nm. The coating introduces spherical curvature. The measured PV flatness was approximately 130 nm, and the surface roughness was measured to be approximately RMS = 0.5 nm. As shown in Figure 12, when comparing the bending induced by the coating with the prior art, the invention described herein reduces the bending to approximately 1 / 100th. If the flatness of the 130nm PV is still too high, a sacrificial layer can be co-deposited and then polished to the desired surface quality.

[0057] Figure 11B shows experimental proof of a coated heat sink with a polished sacrificial layer. In this embodiment, the heat sink has a Young's modulus greater than 300 GPa, a thickness of 2 mm, and a diameter of 12 mm. The heat sink is first polished to a flatness of PV < 70 nm and a surface roughness of RMS < 0.4 nm. Flatness was measured using a laser interferometer operating at 632.8 nm. Surface roughness was measured using an atomic force microscope in non-contact mode. A high-reflectivity film made of SiO2 / Ta2O5 with a total thickness of approximately 4 μm and a 340 nm SiO2 sacrificial layer on top is applied to the heat sink, providing a reflectivity of more than 99.9% at 1030 nm. Subsequently, the upper sacrificial layer is polished by ion beam-assisted polishing. This method can achieve a surface with a PV flatness < 40 nm and an RMS surface roughness < 0.4 nm. As shown in Figure 12, when comparing the ion-assisted polishing method with the prior art, the invention described herein provides a surface flatness more than 300 times greater.

[0058] On the other hand, Figure 12 shows experimental results from prior art, particularly approach EP 2 996 211, where a 200 μm thick solid-state disk is coated. An anti-stress coating was applied to minimize curvature. Nevertheless, it can be seen that the solid-state disk is excessively curved up to PV = 10 μm. This high curvature leads to a decrease in bond strength due to the cancellation of molecular forces. Topography is measured across the entire diameter of the solid-state disk (blue line). Reference Symbol 1. Monolithic laser amplification module 2 Solid State Disks 21 Surface of a solid-state disk 22 Anti-reflection coating 3 Heatsink 4 Reflective film 41. The first layer 42. Second Layer 5. Incident beam 6. Monolithic complex 61 Surface of a monolithic composite 7 layers of victims 71 First Sacrifice Sublayer 711 Nano or micro patterns on the sacrificial layer 72 Second Sacrificial Sublayer 81 Lens 82 Mirror 83 Partially reflective element 9 Cooling

Claims

1. A monolithic laser amplifier module (1) for a solid-state laser system, comprising: a solid state disk (2); A monolithic composite (6) comprising a heat sink (3) and a reflective film (4) configured to at least partially reflect an incident beam (5) propagating in said solid-state disk (2) in a wavelength range λ of 200 nm to 10 μm, said reflective film (4) being deposited on a surface of said heat sink by a deposition method, said heat sink (3) comprising: a lateral thermal conductivity of at least 100 W / m*K; a Young's modulus of at least 100 GPa, preferably at least 300 GPa, and a monolithic composite (6) comprising a thickness of said heat sink of at least 1 mm, preferably at least 2 mm; the solid state disk and the monolithic composite have surfaces (61 and 21) with a PV flatness < 210 nm and a surface roughness RMS < 2 nm; A monolithic laser amplifier module (1) for a solid-state laser system, wherein the surfaces (21 and 61) of the solid-state disk (2) and the monolithic composite (6) are directly and permanently bonded.

2. 2. The module (1) of claim 1, wherein the directly coupled monolithic laser amplifier module (1) has a thermal conductivity of more than 0.1 W / (mK), preferably more than 1 W / (mK), for solid-state disk temperatures of less than 500°C due to laser operation.

3. 2. The module (1) of claim 1, wherein the monolithic composite (6) comprises a plurality of alternating layers forming the reflective film (4), the layers (41, 42) being made of materials having alternating refractive indices, and an upper layer being bonded to the solid-state disk (2).

4. 2. The module (1) of claim 1, wherein the monolithic composite (6) includes a sacrificial layer (7) on top of the reflective film (4), the material and thickness of the sacrificial layer (7) reducing the reflectivity of the film (4) by less than 10%.

5. 5. The module (1) of claim 4, wherein the sacrificial layer (7) of the monolithic composite (6) comprises a plurality of layers (71, 72) forming a film stack, at least one sacrificial layer being structured with a micro- or nano-pattern (711) to improve the mechanical and / or thermal properties of the solid-state disk (2).

6. 5. The module (1) of claim 4, wherein the sacrificial layer (7) of the monolithic composite (6) comprises a plurality of layers (71, 72) forming a film stack, the film stack being configured to increase the laser-induced damage threshold of the module (1).

7. 2. The module (1) according to claim 1, wherein the surface (61) of the monolithic composite (6) directly and permanently bonded to the solid-state disk (2) is curved.

8. 2. The module (1) of claim 1, wherein the surface (21) of the solid-state disk (2) and the surface (61) of the monolithic composite have a PV<70 nm and an RMS<0.8 nm.

9. 2. The module (1) of claim 1, wherein the surface (61) of the module (1) is curved, the curved surface acts as a direct and permanent bond with PV<210 nm and RMS<2 nm, and the curved surface is part of the total surface of the monolithic composite (6).

10. 2. The module (1) of claim 1, wherein the solid-state disk (2) comprises a doped or undoped laser crystal selected from the group consisting of garnet, vanadate, tungstate, sapphire, chalcogenide, or ceramic materials, or semiconductor gain materials.

11. The heat sink (3) has a damping coefficient <1 cm －１ 2. The module (1) according to claim 1, wherein the wavelength is transparent to wavelengths between 200 nm and 10 μm.

12. The module (1) according to claim 1, wherein the heat sink (3) is made from diamond, boron nitride, silicon, silicon carbide, ceramic, metal, metal-diamond composite, metal-boron nitride composite, or silicon-diamond composite.

13. 2. The module (1) of claim 1, wherein the solid-state disk (2) includes an anti-reflective coating (22) on a surface opposite the surface (21) and is directly and permanently bonded to the monolithic composite (6).

14. The module (1) according to claim 1, wherein the edges of the module (1) are roughened and / or chamfered.

15. 2. A method for manufacturing a laser amplifier module (1) according to claim 1, said method comprising the steps of: providing a solid-state disk (2), at least one surface (21) of which has a surface roughness (Root Mean Square) RMS<2 nm and a surface flatness (PV)<210 nm; providing a heat sink (3), at least one surface (31) of which has a root mean square (RMS) of <2 nm and a surface flatness (PV) of <210 nm, and a thickness of at least 1 mm; coating said heat sink (3) with a reflective film (4) that at least partially reflects an incident beam (5) in the wavelength range λ of 200 nm to 10 μm, thereby forming a monolithic composite (6); cleaning the surfaces (21 and 41) of said solid state disk (2) and said monolithic composite (6); activating said surfaces (21 and 41); and pressing the surfaces (21 and 41) of both the monolithic composite and the heat sink, having a surface flatness of RMS<2 nm and PV<210 nm, together to form a direct and permanent bond.

16. providing a sacrificial layer (7); further coating the reflective film (4) with the sacrificial layer (7), thereby forming a monolithic composite (6); 16. The method of claim 15, further comprising the step of treating the sacrificial layer (7) of the monolithic composite (6) with chemical mechanical polishing or ion beam assisted polishing to provide a surface roughness of RMS<2 nm and a surface flatness of PV<210 nm and to reduce the number and size of defects or voids.

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