Abstract
The invention relates to a LASER amplification module for a solid-state laser system and method for manufacturing thereof. The present invention relates to a laser amplification module for a solid-state laser. More particularly, the present invention relates to the module amplifying laser beam capable to provide effective cooling of a heat sink bonded to a solid-state disk. The monolithic laser amplification module (1) comprises a solid-state disk (2); a monolithic composite (6) comprising a heat sink (3) and a reflecting coating (4) configured to at least partially reflect an incident beam (5) propagated in the solid-state disk (2) in a wavelength range λ from 200 nm-10 μm, wherein the reflecting coating (4) is deposited on surface of the heat sink by a deposition method, wherein the heat sink (3) has: transverse thermal conductivity at least 100 W/m*K, Young's modulus at least 100 GPa, preferably at least 300 GPa; and thickness of the heat sink at least 1 mm, preferably at least 2 mm; and wherein the solid-state disk and the monolithic composite have surfaces (61 and 21) having PV-flatness<210 nm and have a surface roughness RMS<2 nm; and wherein the surfaces (21 and 61) of the solid-state disk (2) and the monolithic composite (6) are directly and permanently bonded together.
Claims
1. A monolithic laser amplification module for a solid-state laser system comprising a solid-state disk; a monolithic composite comprising a heat sink and a reflecting coating configured to at least partially reflect an incident beam propagated in the solid-state disk in a wavelength range λ from 200 nm-10 μm, wherein the reflecting coating is deposited on surface of the heat sink by a deposition method, wherein the heat sink has: transverse thermal conductivity at least 100 W/m*K, Young's modulus at least 100 GPa, preferably at least 300 GPa; and thickness of the heat sink at least 1 mm, preferably at least 2 mm; and wherein the solid-state disk and the monolithic composite have surfaces having PV-flatness<210 nm and have a surface roughness RMS<2 nm; and wherein the surfaces of the solid-state disk and the monolithic composite are directly and permanently bonded together.
2. The module according to claim 1, wherein the directly bonded monolithic laser amplification module has a thermal conductivity of >0.1 W/(m K), preferably >1 W/(m K), for solid-state disk temperatures <500° C. caused by laser operation.
3. The module according to claim 1, wherein the monolithic composite comprising plurality of alternating layers forming the reflective coating, wherein layers are made of materials having alternating refractive indexes and wherein the top layer is bonded to the solid-state disk.
4. The module according to claim 1, wherein the monolithic composite comprises a sacrificial layer on top of the reflective coating, wherein the material and the thickness of the sacrificial layer decreases the reflectivity of the coating less than 10%.
5. The module according to claim 4, wherein the sacrificial layer of the monolithic composite comprises a plurality of layers forming a film stack, wherein at least one sacrificial layer is structured by micro or nano-pattern increasing mechanical and/or thermal properties of the solid-state disk.
6. The module according to claim 4, wherein the sacrificial layer of the monolithic composite comprises a plurality of layers forming a film stack, wherein this film stack is configured to increase the laser-induced damage threshold of the module.
7. The module according to claim 1, wherein the surface of the monolithic composite directly and permanently bonded to the solid-state disk is curved.
8. The module according to claim 1, wherein the surface of the solid-state disk and the surface of the monolithic composite have PV<70 nm and RMS<0.8 nm.
9. The module according to claim 1, wherein the surface of the module is curved and wherein the curved surface serves for direct and permanent bonding having PV<210 nm and RMS<2 nm; and wherein the curved surface is a part of the total surface of the monolithic composite.
10. The module according to claim 1, wherein the solid-state disk comprises a doped laser crystal or undoped laser crystal selected from the group consisting of: garnets, vanadates, tungstates, sapphire, chalcogenides or ceramic materials or semiconductor gain material.
11. The module according to claim 1, wherein the heat sink is transparent for wavelengths between 200 nm-10 μm with an attenuation coefficient <1 cm.sup.−1.
12. The module according to claim 1, wherein the heat sink is made of diamond, boron nitride, silicon, silicon carbide, ceramic, metal, metal-diamond composite, metal-boron nitride composite or silicon-diamond composite.
13. The module according to claim 1, wherein the solid-state disk comprises an anti-reflective coating on the surface opposite to the surface directly and permanently bonded to the monolithic composite.
14. The module according to claim 1, wherein an edge of the module is roughened and/or bevelled.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 represents an embodiment according to the state of the art.
[0041] FIG. 2 represents an embodiment according the state of the art.
[0042] FIG. 3 represents a first embodiment according to the present invention.
[0043] FIG. 4 represents a preferred embodiment comprising two layers forming the reflecting coating.
[0044] FIG. 5 represents a preferred embodiment comprising a plurality of alternating layers forming the reflecting coating.
[0045] FIG. 6 represents a preferred embodiment comprising a sacrificial layer on top of the coating.
[0046] FIG. 7 represents a preferred embodiment comprising a nanostructured sacrificial layer.
[0047] FIG. 8 represents a preferred embodiment comprising an anti-reflective coating.
[0048] FIG. 9 represents preferred embodiments depicting the module according to the present invention.
[0049] FIG. 10 represents a preferred embodiment depicting the module according to the present invention.
[0050] FIG. 11 represents an implementation of the laser amplification module according to the present invention in a solid-state laser system.
[0051] FIG. 12 shows experimental examples according to the present invention with varying thicknesses of the sacrificial layer on top of the reflective coating.
[0052] FIG. 13 shows experimental examples according to the present invention with varying thicknesses of the sacrificial layer on top of the reflective coating.
[0053] FIG. 14 shows a comparative study—the example according to the present invention.
[0054] FIG. 15 shows a comparative study—another example according to the present invention.
[0055] FIGS. 16 and 17 show a comparative studies according to the state of the art.
DETAILED DESCRIPTION
[0056] Compared to the state-of-the art, the reflective coating forming a highperformance reflector (>98%) are deposited on the solid-state disk by a deposition method, in particular sputtering. This introduces an intrinsic film stress inside the coating leading to an excessive solid-state disk curvature. The thinner and elastic the solid-state disk is, the more it is bent. However, in accordance with the present invention, a heat sink having Young's modulus at least 100 GPa and a thickness of >1 mm ensures that the heat sink is mechanically rigid so that the bending is not that profound and the PV flatness can still be used or corrected for the direct bonding with the solid-state disk.
[0057] FIG. 3 schematically shows a first embodiment according to the present invention. A monolithic laser amplification module 1 for a solid-state laser system is shown. The module 1 comprises a thin solid-state disk 2 made from optical gain medium, such as doped or undoped crystal for example, YAG, sapphire, silicate, garnets, vanadates, tungstates, or phosphate glasses, preferably doped with laseractive ions, ceramic materials or semiconductor material such as GaAs, InGaAs or GaN. In general, a skilled person in the art of laser designing knows which species of the gain medium is suitable for 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 pumping source of the solid-state disk laser. The pump beam can be another laser beam. The opposite surface 21 is treated so that, the PV flatness<210 and RMS<2 nm is achieved. The treatment resulting with these parameters are, for example annealing, polishing, or any of the mechanical- and/or chemical treatment. The surface 21 is configured with the above-mentioned parameters to provide a direct and permanent bond with a monolithic composite 6. The monolithic composite 6 comprises a heat sink 3 and a reflective coating 4. The monolithic composite 6 has the surface 61 being treated so that the PV-flatness<210 nm and RMS<2 nm. The direct and permanent bonding provides joining of the solid-state disk 2 directly to the monolithic composite 6 without any intermediate adhesive layers, such as glues or metallic solders. This bonding provides strength from 0.1 J/m.sup.2, preferably >0.5 J/m.sup.2. The heat sink 3 has transverse thermal conductivity at least 100 W/m*K, Young's modulus at least 100 GPa, preferably at least 300 GPa, and thickness of the heat sink 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. The thickness of the heat sink at least 2 mm prevent bending during the coating process, which can be proved by the experiment described in accordance with FIGS. 14, 15, and 16. The monolithic composite 6 further comprises a reflecting coating 4 deposited on top of the heat sink 3. The reflective coating 4 can be deposited on the heat sink 3 by state-of the art deposition method such as chemical vapor deposition, sputtering, evaporation, chemical vapor deposition, or even crystal growth techniques such as molecular beam epitaxy. The reflective coating is configured to at least partially reflect an incident beam 5 propagated through the solid-state disk 2, particularly a pump beam. The reflective coating 4 reflects the beam having the wavelength from the range from 200 nm to 10 μm. The reflective coating can be selected from the group of: zinc sulfide, titanium dioxide, tantalum pentoxide, silicon dioxide, hafnium oxide, gallium arsenide, aluminum gallium arsenide etc. The reflective coating 4 is thus sandwiched between the heat sink 3 and the solid-state disk 2.
[0058] FIG. 4 further shows a preferred embodiment, wherein the reflective coating 4 comprises at least two layers 41 and 42. The first layer 41 can be made from Si having a refractive index about 3,97 and the second layer 42 can be made from SiO.sub.2 having the refractive index about 1.46.
[0059] FIG. 5 further shows another preferred embodiment, wherein the reflective coating 4 comprises a plurality of layers with materials having alternating refractive index. In an embodiment shown in FIG. 5, four reflective layer are chosen. The first layer 41 can have refractive index n=2.1, the second layer can have refractive index n=1.5, the third layer can have refractive index n=2.1 and the top layer 42 can have a refractive index n=1.5. In another preferred embodiment, the set of alternating layers can be adopted from EP3076208.
[0060] FIG. 6 further shows another preferred embodiment, wherein the monolithic composite 6 further comprises a sacrificial layer 7 provided on top of the reflective coating and having the surface with the PV-flatness<210 nm and RMS<2 nm so that, the direct and permanent bonding with the solid-state disk 2 can be set. The sacrificial layer 7 serves for protection of the reflective coating 4. The sacrificial layer 7 can be preferably made of material decreasing the reflectivity of the reflective coating less than 10%. The sacrificial layer 7 can be made of SiO.sub.2, Si.sub.3N.sub.4, HfO.sub.2, any oxides of any metals, particularly Al.sub.2O.sub.3, GaAs, AlGaAs, NiP or photoresist material, which can be polished to reach the required surface topography, in particular PV flatness less than <210 nm and RMS surface roughness RMS<2 nm, preferably PV<70 nm and RMS<0.8 nm. The reflectivity from 90% up to 99.9% is particularly advantageous. At least one sacrificial layer, e.g. made from SiO.sub.2, having thickness 407 nm can increase the LIDT of the underlying multilayered reflecting coating by a factor of about 7 as shown in experimental result according to Schlitz et al., Applied Optics 56 (4), C136-C139.
[0061] FIG. 7 comprises a plurality of sacrificial layers 71, 72 for protection of the reflective coating 4 forming a film stack. The plurality of sacrificial layers 71, 72 can be configured to increase the laser-induced damage threshold of the monolithic laser amplification module. More preferably, in a film stack, at least one layer is micro- or nano-structured by a pattern. The pattern increases mechanical and/or thermal properties of the solid-state disk. The pattern can be a grove fitting to each other in order to increase the surface. The increased surface area can provide higher thermal transfer, which provides better laser stability. The particular pattern in terms of shape, size, periodicity can be achieved by the state of the art method such as lithography. The nesting groves may have a rectangular shape, for example. The nano or micro pattern can be provided either on respective surfaces of sacrificial layers, a surface of the reflecting coating which does not serve for direct bonding and the sacrificial layer or on the interface between the coating and the heat sink.
[0062] In an embodiment, not depicted in the drawings, the surface 61 of the monolithic composite directly and permanently bonded to the solid-state disk 2 has flatness of PV<70 nm. Such a flatness of the PV can be achieved by polishing by the state of the art method, such as mechanical, chemical or ion-assisted polishing.
[0063] 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 can be >0.05 m, more preferably >0.5 m.
[0064] In yet another embodiment, the surface 21 of the solid-state disk 2 and the surface 61 of the monolithic composite 6 have PV<70 nm and RMS<0.8 nm.
[0065] In yet another embodiment, the surface 61 of the monolithic composite 6 is curved, the flatness PV<210 nm and RMS<2 nm are provided on the curved surfaces of the monolithic composite 6 and the solid disk 2, wherein the curved surfaces are a part of the total surface of the monolithic composite, such as more than 80% of the total surface the monolithic composite. Therefore, it is not needed to provide the direct boding overall interface.
[0066] In yet another embodiment, the heat sink (3) is transparent for wavelengths between 200 nm-10 μm with an attenuation coefficient>1 cm.sup.−1 and/or opaque to the laser wavelength(s).
[0067] FIG. 8 shows an embodiment, wherein an antireflective coating 22 is deposited on the surface serving for increasing the transmission and decreasing the amount of unwanted reflected light.
[0068] In another embodiment, not shown in the drawings, an edge of the module 1 is roughened and/or bevelled. This embodiment provides minimization of minimize amplified spontaneous emission (ASE) from the laser gain media.
[0069] FIGS. 9 and 10 show two exemplary embodiments of combination of features according to the present invention.
[0070] FIG. 11 shows an exemplary implementation of the module according to the present invention as a part of the disk laser. A pump beam 5 is directed to the module 1 according to the present invention through a lens 81. A part of the pump beam 5 is absorbed and amplified in the module 1 and part if the beam is reflected to a mirror 82 reflecting the beam back to the module 1. An amplified laser beam is emitted from the module 1 to a partially reflecting element 83 capable to at least partially reflect the laser beam back to the module 1 and partially transmitting the laser beam 51.
[0071] FIG. 12 represents a simulation results carried with the module 1. The simulation shows an example reflectivity of the reflective coating 4 with a sacrificial layer 7 on top of it. Here the reflective coating consists of alternating layers of SiO.sub.2 and Ta.sub.2O.sub.5 with the corresponding refractive indices of 1.46 and 2.1. Here the reflective coating is designed for a central wavelength of 1000 nm leading to a SiO.sub.2 layer thickness of 171.2 nm and a Ta.sub.2O.sub.5 layer thickness of 119 nm. A high reflectivity of 99.9% is achieved through 20 alternating layers. On top of the reflective coating a SiO.sub.2 sacrificial layer is simulated. FIG. 10A shows the different reflectivity behaviours of the coating at different thicknesses of the sacrificial layer.
[0072] FIG. 13 represents the dependence of the reflectivity of the coating to the thickness of the sacrificial layer thickness. The coating is configured in the same way as for FIG. 12. It is shown that the thickness of the sacrificial layer only has a marginal effect on the final reflectivity performance of the coating. Based on this fact, the sacrificial layer can be used for improving the surface quality, establishing a PV<210 nm and RMS surface roughness of <2 nm.
[0073] FIG. 14 shows an experimental proof of a coated heatsink. In this example the heat sink has a Young's modulus of >300 GPa and a thickness of 2 mm with 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. The flatness measurement is carried out with a laser interferometer operating at 632.8 nm. The surface roughness is measured with an atomic force microscopy in the non-contact mode. A high reflectivity coating of a total thickness of approximately of 4 μm consisting of SiO.sub.2/Ta.sub.2O.sub.5 with a 340 nm SiO.sub.2 sacrificial layer on top is applied onto the heat sink providing a reflectivity of >99.9% for 1030 nm. The coating introduces a spherical curvature. The measured PV flatness is about 130 nm and the surface roughness is measured to be around RMS=0.5 nm. Comparing the coating-induced bending with state-of the art technology as depicted in FIG. 16, the invention described here provides an almost a factor of 100 less bending. In case the PV flatness of 130 nm is still too high, a sacrificial layer can be co-deposited and subsequently polished to the required surface quality.
[0074] FIG. 15 shows an experimental proof of a coated heatsink with a polished sacrificial layer. In this example the heat sink has a Young's modulus of >300 GPa and a thickness of 2 mm with 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. The flatness measurement is carried out with a laser interferometer operating at 632.8 nm. The surface roughness is measured with an atomic force microscopy in the non-contact mode. A high reflectivity coating of a total thickness of approximately of 4 μm consisting of SiO.sub.2/Ta.sub.2O.sub.5 with a 340 nm SiO.sub.2 sacrificial layer on top is applied onto the heat sink providing a reflectivity of >99.9% for 1030 nm. The top sacrificial layer is then polished via an ion-beam assisted polishing method. With this method, a surface with a PV flatness of <40 nm and a RMS surface roughness of <0.4 nm can be achieved. Comparing the ion assisted post-polishing method with the sate-of-the art technology as depicted in FIG. 16, the invention described here provides more than a factor of 300 better surface flatness.
[0075] On the other hand, FIGS. 16 and 17 shows the experimental result according to the state of the art, in particular approach EP 2 996 211 in which a solid-state disk of a thickness of 200 μm is coated. To minimize the curvature, an anti-stress coating was applied here. Nevertheless, one can see that solid-state disk is excessively curved up to PV=10 μm. This high bending will decrease the bonding strength due to counteracting molecular forces. The topography is measured across the whole diameter of the solid-state disk (blue line).
REFERENCE SIGNS
[0076]
TABLE-US-00001 1 monolithic laser amplification module 2 solid-state disk 21 surface of the solid-state disk 22 antireflective coating 3 heat sink 4 reflecting coating 41 first layer 42 second layer 5 incident beam 6 monolithic composite 61 surface of the monolithic composite 7 sacrificial layer 71 first sacrificial sub-layer 711 nano or micro pattern on the sacrificial layer 72 second sacrificial sub-layer 81 lens 82 mirror 83 partially reflecting element 9 cooling
