Abstract
A solid-state laser active medium comprising an optical gain material; a heat sink, wherein the heat sink is transparent, in particular over a wavelength range of 200 nm to 4000 nm, preferably with an absorption coefficient of <1 cm.sup.−1; the heat sink having a high thermal conductivity, in particular ≧149 W/(m*K); wherein the optical gain material and the heat sink exhibit a root-mean square, RMS, surface roughness of <1 nm; wherein the optical gain material is attached to the transparent heat sink by direct bonding.
Claims
1. A laser active medium for a solid-state laser system comprising: an optical gain material; a heat sink, wherein the heat sink is transparent within a threshold; the heat sink having a high thermal conductivity within a threshold; wherein the optical gain material and the heat sink exhibit a root-mean square surface roughness of <1 nm; and wherein the optical gain material is attached to the transparent heat sink by direct bonding.
2. The laser active medium according to claim 1, wherein the laser active medium further comprises a high reflectivity thin-film stack, wherein the optical gain material is combined with the high reflectivity thin-film stack.
3. The laser active medium according to claim 1, wherein the optical gain material comprises a semiconductor structure or a doped laser crystal.
4. The laser active medium according to claim 3, wherein the semiconductor structure is monolithic, combining a high reflectivity stack of alternating high and low index material and semiconductor-based optical gain material in one continuous structure.
5. The laser active medium according to claim 4, wherein the semiconductor-based optical gain material comprises a semiconductor quantum structure.
6. The laser active medium according to claim 3, wherein an active element comprises a composite structure including a doped laser crystal combined with a multilayer stack of alternating high and low index materials yielding a high reflectivity mirror.
7. The laser active medium according to claim 3, wherein the doped laser crystal comprises at least one of YAG doped with Nd, Yb, Er, Tm or combinations thereof; neodymium doped vanadates, neodymium doped tungstates, ytterbium doped tungstates, titanium doped sapphire, chromium doped Al2O3, chromium doped chalcogenides.
8. The laser active medium according to claim 1, wherein the direct bonding of the optical gain material to the transparent heat sink comprises van der Waals forces.
9. The laser active medium according to claim 1, wherein the heat sink comprises a transparent and high thermal conductivity material including one of diamond, SiC, and AlN for visible and IR operation in a range of wavelengths from 200 nm to 4000 nm, or single-crystal silicon for near and mid-IR applications in a range of wavelengths from 1200-4000 nm.
10. A method for manufacturing a laser active medium comprising a semiconductor-based optical gain material, a heat sink, the heat sink having a high thermal conductivity within a threshold, and being optically transparent within a threshold; the method comprising the steps of: providing the semiconductor-based optical gain material, the optical gain material having a root-mean square surface roughness of <1 nm, on a first substrate, wherein the first substrate comprises GaAs, InP, GaN, AlN, Si, or Ge; providing the heat sink, the heat sink having a polished surface with a root-mean square roughness below 1 nm; detaching the optical gain material from the first substrate; attaching the optical gain material to the heat sink by direct bonding.
11. A method for manufacturing a laser active medium based on a doped laser crystal, comprising the doped laser crystal as the optical gain material, a heat sink, wherein the heat sink exhibits high thermal conductivity within a threshold, and being optically transparent within a threshold; the method comprising the steps of: providing the doped laser crystal with a surface roughness below 1 nm root-mean square; providing the transparent heat sink with a surface roughness below 1 nm root-mean square; attaching the laser-crystal based optical gain material to the heat sink by direct bonding.
12. The method according to claim 10, wherein the laser active medium further comprises a high reflectivity thin-film stack, wherein the optical gain material is combined with the high reflectivity thin-film stack; wherein the optical gain material comprises a semiconductor structure or a doped laser crystal.
13. The method according to claim 10, wherein the optical gain material is directly attached to the heat sink by van der Waals forces and/or covalent bonding.
14. A solid-state laser system comprising: at least two mirrors for laser feedback; a laser active medium according to claim 1; and a pump light source providing a pump beam, the pump beam being incident on the laser active medium.
15. The laser active medium according to claim 1, wherein the heat sink is transparent over a wavelength range of 200 nm to 4000 nm, and wherein the heat sink thermal conductivity is greater than or equal to 149 W/m*K.
16. The laser active medium according to claim 15, wherein the heat sink has an absorption coefficient of <1 cm-1.
17. The laser active medium according to claim 8, wherein the direct bonding of the optical gain material to the transparent heat sink comprises van der Waals forces and/or covalent bonding.
18. The method of manufacturing a laser active medium according to claim 10, wherein the heat sink is optically transparent over a wavelength range of 200 nm to 4000 nm, and wherein the heat sink thermal conductivity is greater than or equal to 149 W/m*K.
19. The method according to claim 18, wherein the heat sink has an absorption coefficient of <1 cm-1.
20. The method of manufacturing a laser active medium according to claim 11, wherein the heat sink is optically transparent over a wavelength range of 200 nm to 4000 nm, and wherein the heat sink thermal conductivity is greater than or equal to 149 W/m*K.
21. The method according to claim 20, wherein the heat sink has an absorption coefficient of <1 cm-1.
22. The method according to claim 13, wherein the direct bonding of the optical gain material to the transparent heat sink comprises van der Waals forces and/or covalent bonding.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 A first embodiment of a laser active medium for a solid-state laser system according to the invention.
[0040] FIG. 2: A further embodiment of a laser active medium for a solid-state laser system according to the invention.
[0041] FIG. 3: A modification of the laser active medium for a solid-state laser system of FIG. 2.
[0042] FIG. 4 A further embodiment of a laser active medium for a solid-state laser system according to the invention.
[0043] FIG. 5 A further embodiment of a laser active medium for a solid-state laser system according to the invention.
[0044] FIG. 6 A further embodiment of a laser active medium for a solid-state laser system according to the invention.
[0045] FIG. 7 A further embodiment of a laser active medium for a solid-state laser system according to the invention.
DETAILED DESCRIPTION
[0046] In the following it should be understood that the same reference numerals refer to the same elements.
[0047] FIG. 1 shows a first embodiment of a laser active medium 12″ for a solid-state laser system 100.
[0048] The laser active medium 12″ is shown as being part of the solid-state laser system 100. In FIG. 1 a pump beam 19U and 19L is shown, which may be produced by a pump light source. This pump beam 19U and 19L is incident substantially perpendicular, i.e. normal to the surface of the laser active medium 12″.
[0049] In FIG. 1, the laser active medium 12″ comprises the optical gain material 1″ which comprises a doped laser crystal 1A″. The doped laser crystal 1A″ may comprise at least one of YAG doped with Nd, Yb, Er, Tm or combinations thereof; neodymium doped vanadates, neodymium doped tungstates, ytterbium doped tungstates, titanium doped sapphire, chromium doped Al.sub.2O.sub.3, chromium doped chalcogenides. It should be understood, that while this embodiment explicitly shows an optical gain material 1″ comprising a doped laser crystal 1A″, the optical gain material 1″ may be substituted with optical gain material of FIG. 2-7. In other words, the optical gain material 1″ can be substituted by a semiconductor-based optical gain material 1 and 1′ as is discussed with regard to the other Figures.
[0050] The laser optical gain material 1″ of the laser active medium 12″ as shown in FIG. 1 is bonded onto a heat sink 15. This bond is a direct bond 3. A direct bonding process yielding a direct bond 3 should be understood as joining of the optical gain material 1″ directly to the heat sink 15 without any intermediate adhesive layers, such as glues or metallic solders.
[0051] The heat sink 15 may be understood as a heat spreader or a final carrier substrate. It may also be realized in combination with a submount 27 as is shown in FIG. 1. The heat sink 15 typically has a thermal conductivity greater than or equal to 149 W/m*K at room temperature. The heat sink 15 may be planar. It may also be possible to use a heat sink having a curved structure with a predefined radius of curvature (not shown). Since the optical gain material 1, i.e. the active mirror, is directly bonded to the heat sink 15, i.e. no intermediate layers are inserted and no materials such as adhesives, glues, or glass frits are present, such that the thermal performance of the system is significantly improved. Therefore, the thermal performance of the laser active medium 12″ is ultimately limited only by the thermal conductivities of the optical gain material 1″ and/or heat sink 15, whichever is lowest. Moreover, glue-based structures may melt, flow or plastically deform. This is avoided through the use of direct bond 3, such that the overall mechanical properties of the laser system 100 may be enhanced. In an optimized direct bonding process, interfacial bond strengths on the order of 1 J/m.sup.2 are possible, approaching the bulk bond strength of a typical solid.
[0052] In FIG. 1 the heat sink 15 may be transparent, in particular over a wavelength range of 200 nm to 4000 nm, preferably with an absorption coefficient of <1 cm.sup.−1. In FIG. 1, to enable direct bonding, the optical gain material and the heat sink exhibit a root-mean square, RMS, surface roughness of <1 nm.
[0053] FIG. 1 shows a submount 27. Said submount 27 may be transparent having similar properties as the heat sink 15. As shown in FIG. 1, submount 27 may additionally or alternatively comprise at least two parts 27L and 27R. The two parts 27L and 27R may comprise substantially the same material. The at least two parts 27L and 27R of the submount 27 however may be not directly adjacently provided such that laser output light may also be emitted through a gap 27 G between the two parts 27L and 27R.
[0054] In FIG. 1, laser output light 13U and 13L is emitted from the laser active medium 12″. Laser output light 13U is emitted into the direction from which pump beam 19U is incident, i.e. substantially normal to the surface of the laser active medium 12″. In addition, in FIG. 1, laser output light 13L may be emitted in a diametrically opposite direction than laser output light 13U. Here, the direct bond 3 of the optical gain material 1″, bonded to the transparent heat sink 15, may reduce the overall optical absorption of the laser active medium 12 and thus of the laser system 100, and in particular enables light transmission through the heat sink 15, as well as through the submount 27 or through the gap 27G in between the two parts of the submount 27L and 27R.
[0055] FIG. 1 also shows two external mirrors 11U and 11L for focusing the laser output light 13U and 13L, respectively and more importantly for providing optical feedback for the laser cavity. External mirrors 11U and 11L may be substantially concave mirrors, having their optical axes aligned with the center axis normal to the laser active medium 12″. That is, external mirror 11L focuses laser output light 13U which emerges through the heat sink 15 and the submount 27 or the gap 27G between submount parts 27L and 27R, respectively. External mirror 11L may be substantially the same as external mirror 11R. External mirrors 11L and 11R thus may be partially transparent only for substantially collinear laser light, reflecting back some portion of the laser output light onto the laser active medium 12″.
[0056] In FIG. 2, a further embodiment is shown according to the present invention. FIG. 2 shows a laser active medium 2 which is different from the laser active medium 12″ as shown in FIG. 1.
[0057] FIG. 2 shows a laser active medium 2 for a solid-state laser system 200. The laser active medium 2 is shown as being part of the solid-state laser system 200. In FIG. 2 an off-axis pump beam 9, which may be a pump light source is shown. This pump beam 9 is incident on the laser active medium 2 at an angle, whereas the angle of incidence is different from 90°, i.e. the pump beam is not perpendicular to the surface of the laser active medium 2.
[0058] The laser active medium 2 comprises an optical gain material 1. The optical gain material 1 comprises a semiconductor structure 1A and a high reflectivity thin-film stack 1S. It is also understood that the optical gain material may comprise a doped laser crystal combined with a high reflectivity stack. In all embodiments shown in the FIGS. 1-7 either structure, i.e. semiconductor-based optical gain material or doped crystal based optical gain material is possible. The semiconductor structure 1A is combined with the high reflectivity thin-film stack 1S, thereby forming an active mirror. The term “combine” refers to realization of direct, i.e. intimate contact of the high reflectivity thin-film stack 1S which may be a high-reflectivity multilayer with the semiconductor structure 1A of optical gain material 1. This may be achieved either by direct deposition methods, with select examples being sputtering, evaporation, chemical vapor deposition, or even crystal growth techniques such as molecular beam epitaxy, as well as direct bonding techniques in which a separately deposited multilayer is transferred onto the gain material via a direct bonding process. The semiconductor structure 1A may be monolithic, combining the high reflectivity stack 1S, which may be a high-reflectivity stack of alternating high and low index material and semiconductor-based optical gain structure 1A in one continuous structure representing the laser optical gain material 1.
[0059] In FIG. 2, one side of the laser optical gain material 1 corresponds to the semiconductor structure 1A, the other side corresponds to the high reflectivity stack 1S.
[0060] The laser optical gain material 1 of the laser active medium 2 as shown in FIG. 2 is bonded onto a heat sink 5. This bond is a direct bond 3.
[0061] A direct bonding process should be understood as joining of the optical gain material 1 directly to the heat sink 5 without any intermediate adhesive layers, such as glues or metallic solders. As in FIG. 1, the heat sink 5 may comprise the same material as the heat sink 15 discussed with regard to FIG. 1. The heat sink 5 may be understood as a heat spreader or a final carrier substrate. It may also be realized in combination with a submount 7 as shown in FIG. 2. The heat sink 5 typically has a thermal conductivity greater than or equal to 149 W/m*K at room temperature. The heat sink 5 may be planar. It may also be possible to use a heat sink having a curved structure with a predefined radius of curvature, though that is not shown here. Since the optical gain material 1, i.e. the active mirror, is directly bonded to the heat sink 5, i.e. no intermediate layers are inserted and no materials such as adhesives, glues or glass frits are present, such that the thermal performance of the system is significantly improved. Therefore, the thermal performance of the laser active medium 2 is ultimately limited only by the thermal conductivities of the optical gain material 1 or heat sink 5, whichever is lowest. Moreover, glue-based structures may melt, flow or plastically deform. This is avoided through the use of direct bond 3, such that the overall mechanical properties of the laser system 200 may be enhanced. In an optimized direct bonding process, interfacial bond strengths on the order of 1 J/m.sup.2 are possible, approaching the bulk bond strength of a typical solid.
[0062] In FIG. 2 the heat sink 5 may be transparent, in particular over a wavelength range of 200 nm to 4000 nm, preferably with an absorption coefficient of <1 cm.sup.−1. In FIG. 2, wherein the optical gain material and the heat sink exhibit a root-mean square, RMS, surface roughness of <1 nm.
[0063] FIG. 2 shows that the side of the laser optical gain material 1 that corresponds to the high reflectivity stack 1S, is the side with which the laser optical gain material is directly bonded to the heat sink 5 by direct bond 3. The heat sink 5 in FIG. 2 is further supported by a submount 7. The material of the submount 7 may be the same as that of submount 27, discussed with respect to FIG. 1. In particular, submount 7 may exhibit the same optical properties and parameters as the heat sink 5. It should be understood, however, that submount 7 may not be essential for the laser system 200 as shown in FIG. 2. The heat sink 5 may be attached to the submount 7 by conventional means.
[0064] FIG. 2 shows laser output 13 emerging from the laser optical gain material 1. The laser output 13 emerges substantially to the top side, i.e. the surface of the optical gain material 1, i.e. the active mirror. This is the side of the optical gain material 1, on which the semiconductor structure or doped laser crystal 1A is located. FIG. 2 shows an additional external mirror 11 for partly reflecting laser output light 13. The external mirror 11 may be substantially the same as external mirrors 11U and 11L discussed with respect to FIG. 1. External mirror 11 will be partially transparent only for substantially collinear laser light, reflecting back laser output light onto the laser active medium 2. The center axis of the collinear laser output light 13 is typically aligned with the center axis of the laser optical gain material 1 and the heat sink 5, and typically also the center axis or the center of the submount 7.
[0065] FIG. 3 shows a further embodiment with a laser active medium 12 for a solid-state laser system 200. In FIG. 3, as in FIG. 2, an off-axis pump beam 9 is incident onto the laser active medium 12. The laser active medium 12 is similar to the laser active medium 2 as shown in FIG. 2. The laser active medium 12 is attached to an optically transparent heat sink 15. The heat sink 15 is optically transparent, in particular over a wavelength range of 200 nm to 4000 nm, preferably with an absorption coefficient of <1 cm.sup.−1. In FIG. 3, wherein the optical gain material 1 and the heat sink 15 exhibit a root-mean square, RMS, surface roughness of <1 nm. The optical gain material 1 is attached to the heat sink 15 by direct bond 3, which is the same as shown in FIG. 2. In FIG. 3, a submount 17 is shown which is optically transparent for laser light emitted from the laser active medium 12. The submount 17 as in FIG. 3 may comprise at least two parts 17L and 17R. The two parts 17L and 17R are similar to the submount 27L and 27R as shown in FIG. 1. The at least two parts 17L and 17R of the submount 17 are not directly adjacently provided such that laser output light may also be emitted through a gap 17 G between the two components 17L and 17R. The laser output light 13U is emitted similar as the laser output light 13 in FIG. 2. In addition, in FIG. 3, laser output light 13L may be emitted in a diametrically opposite direction than laser output light 13U. Thus, the direct bond 3 of the optical gain material 1, bonded to the transparent heat sink 15, may reduce the overall optical absorption of the laser active medium 12 and thus of the laser system 200, and in particular enables light transmission through the heat sink 15, as well as through the submount 17.
[0066] FIG. 4 shows the laser active medium 12 as shown in FIG. 3 for modified solid-state laser system 300. The setup as shown in FIG. 4 is similar to the setup shown in FIG. 3. In the solid-state laser system 300, however, a collinear pump beam 19U and 19L is shown instead of an off-axis beam 9. Thus the pump beam 19U and 19L is similar to the pump beam 19U and 19L as in FIG. 1. The collinear pump beam 19U and 19L is directly incident onto the laser active medium 12, as well as through a submount 27, which may be substantially similar to the submount 27 as shown in FIG. 1. The laser output light 13U and 13L thus is emitted collinearly to the incident pump beam 19U and 19L.
[0067] FIG. 5 shows the laser active medium 12 as shown in FIG. 3 and FIG. 4 for a further modified solid-state laser system 500.
[0068] FIG. 5 shows a similar set of externals mirrors 11U and 11L as in FIG. 1. As already discussed for FIG. 1, the optical gain material 1″ as shown in FIG. 1 is replace by an optical gain material 12 as shown in FIGS. 2 and 3.
[0069] FIG. 6 shows the laser active medium 12 as shown in FIG. 3 and FIG. 4 and FIG. 5 for further modified solid-state laser system 600.
[0070] FIG. 7 shows a further embodiment of a laser active medium 12′ for a solid-state laser system 700.
[0071] In FIGS. 6 and 7, the same setup of a solid-state laser system is shown as in FIG. 5. However, collinear pump beam 19U and 19L as shown in FIG. 5 are only incident from one side of the structure. That is, in FIG. 6, the collinear pump beam 19U is incident directly onto the laser gain medium 12. No collinear pump beam is passing the transparent heat sink 15 and or the submount 27. On the other hand, in FIG. 7, the opposite scenario is shown. Pump beam 19L is incident onto the laser gain medium 12 by passing through a gap 27G of the submount 27. and then, typically immediately afterwards, passing through heat sink 15. Furthermore, in FIG. 7, a laser active medium 12′ is shown in which the sequence of an optical gain material 1′ and a high reflectivity thin-film stack 1S′ is reversed as compared to the other embodiments. In these structures the thin film stack 1S and 1S′ are substantially highly reflective for the incident pump light.
