Systems and methods for addressing pumping of thermal interface materials in high-power laser systems

Abstract

In various embodiments, laser devices feature means, such as fasteners, for attaching a laser package to a cooling plate, which allow motion of the laser package in response to thermal cycles resulting from operation of a beam emitter therewithin. Embodiments of the invention additionally or instead include laser devices featuring segmented barrier layers for electrically isolating the laser package from the cooling plate.

Claims

1. A laser device comprising: a beam emitter; a laser package comprising (i) a thermally conductive top laser cooler disposed above and in thermal contact with the beam emitter and (ii) a thermally conductive bottom laser cooler disposed below and in thermal contact with the beam emitter; disposed below the laser package, a thermally conductive cooling plate for conducting heat away from the laser package; disposed between the laser package and the cooling plate, an electrically isolating barrier layer for preventing electrical conduction between the laser package and the cooling plate; a first thermal-interface material disposed between the laser package and the electrically isolating barrier layer; a second thermal-interface material disposed between the electrically isolating barrier layer and the cooling plate; and means for attaching the laser package to the cooling plate, with the electrically isolating barrier layer and first and second thermal-interface materials therebetween, and allowing motion of the laser package in response to thermal cycles resulting from operation of the beam emitter, whereby motion of the first and second thermal-interface materials away from the interface between the laser package and the cooling plate is reduced, wherein the electrically isolating barrier layer comprises a plurality of discrete and spaced-apart areal sections with gaps therebetween, each gap extending from the first thermal-interface material disposed above the gap to the second thermal-interface material disposed below the gap.

2. The device of claim 1, wherein the attachment means comprises an elastic member and a fastener, the fastener attaching the laser package to the cooling plate and compressing the elastic member, wherein the elastic member is configured to be additionally compressed in response to thermally induced expansion of the laser package.

3. The device of claim 2, wherein the fastener comprises a screw.

4. The device of claim 2, wherein the elastic member comprises at least one spring.

5. The device of claim 1, wherein the beam emitter comprises a diode bar configured to emit multiple discrete beams.

6. The device of claim 1, wherein at least one of the top laser cooler or the bottom laser cooler comprises copper.

7. The device of claim 1, wherein the cooling plate comprises aluminum.

8. The device of claim 1, wherein the electrically isolating barrier layer comprises aluminum nitride.

9. The device of claim 1, wherein at least one of the first thermal-interface material or the second thermal-interface material comprises a gel, a solder, a paste, or a liquid.

10. The device of claim 1, wherein at least one of the first thermal-interface material or the second thermal-interface material comprises a phase-change material.

11. The device of claim 1, further comprising: a second laser package disposed over the cooling plate; and a bus bar electrically connecting the laser package to the second laser package.

12. The device of claim 1, wherein the first thermal-interface material and the second thermal-interface material comprise the same material.

13. The device of claim 1, wherein the first thermal-interface material and the second thermal-interface material comprise different materials.

14. The device of claim 1, wherein at least one of the first thermal-interface material or the second thermal-interface material is electrically insulating.

15. The device of claim 1, wherein at least one of the gaps is devoid of material therewithin.

16. A laser device comprising: a beam emitter; a laser package comprising (i) a thermally conductive top laser cooler disposed above and in thermal contact with the beam emitter and (ii) a thermally conductive bottom laser cooler disposed below and in thermal contact with the beam emitter; disposed below the laser package, a thermally conductive cooling plate for conducting heat away from the laser package; disposed between the laser package and the cooling plate, an electrically isolating barrier layer for preventing electrical conduction between the laser package and the cooling plate; a first thermal-interface material disposed between the laser package and the electrically isolating barrier layer; a second thermal-interface material disposed between the electrically isolating barrier layer and the cooling plate; and a fastener for attaching the laser package to the cooling plate, with the electrically isolating barrier layer and first and second thermal-interface materials therebetween, and allowing motion of the laser package in response to thermal cycles resulting from operation of the beam emitter, whereby motion of the first and second thermal-interface materials away from the interface between the laser package and the cooling plate is reduced, wherein (i) the fastener incorporates or mechanically engages with an elastic member, (ii) the fastener is configured to compress the elastic member at room temperature when the beam emitter is not operating, thereby applying a force to the laser package that is equal to a nominal force, and (iii) the elastic member is configured to be additionally compressed in response to thermally induced expansion of the laser package during operation of the beam emitter, thereby increasing the force applied to the laser package by no more than 100% of the nominal force.

17. The device of claim 16, wherein the elastic member comprises at least one spring.

18. The device of claim 16, wherein the fastener comprises a screw.

19. The device of claim 16, wherein the first thermal-interface material and the second thermal-interface material comprise the same material.

20. The device of claim 16, wherein the first thermal-interface material and the second thermal-interface material comprise different materials.

21. The device of claim 16, wherein the elastic member is configured such that the additional compression of the elastic member during operation of the beam emitter increases the force applied to the laser package by no more than 50% of the nominal force.

22. The device of claim 16, wherein the elastic member is configured such that the additional compression of the elastic member during operation of the beam emitter increases the force applied to the laser package by no more than 10% of the nominal force.

23. The device of claim 16, further comprising an electrically insulating material disposed between the elastic member and the laser package to prevent electrical conduction between the elastic member and the laser package.

24. The device of claim 16, wherein the elastic member is electrically insulating.

25. The device of claim 1, wherein: a first region of the electrically insulating barrier material is disposed vertically beneath the beam emitter; and a second region of the electrically insulating barrier material is not disposed vertically beneath the beam emitter.

26. The device of claim 25, wherein a width of one or more areal sections of the electrically insulating barrier layer in the first region is smaller than a width of one or more areal sections of the electrically insulating barrier layer in the second region.

27. The device of claim 25, wherein a width of one or more gaps between areal sections of the electrically insulating barrier layer in the first region is different from a width of one or more gaps between areal sections of the electrically insulating barrier layer in the second region.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which:

(2) FIG. 1 is a perspective view of an exemplary laser package in accordance with various embodiments of the invention;

(3) FIG. 2 is an exploded view of the laser package of FIG. 1;

(4) FIG. 3 is a view of an exemplary laser package installed on a cooling plate in accordance with various embodiments of the invention;

(5) FIG. 4 is a schematic cross-sectional view of a an exemplary laser package installed on a cooling plate in accordance with various embodiments of the invention;

(6) FIG. 5 is a schematic plan view of a divided electrically insulating barrier in accordance with various embodiments of the invention; and

(7) FIG. 6 is a schematic view of a wavelength beam combining laser system incorporating a packaged laser in accordance with various embodiments of the invention.

DETAILED DESCRIPTION

(8) FIGS. 1 and 2 depict an exemplary laser package 100 utilized in accordance with embodiments of the present invention. As shown, in laser package 100 a beam emitter 105 is sandwiched between a bottom laser cooler 110 and a top laser cooler 115. The beam emitter 105 may include, consist essentially of, or consist of, e.g., a laser diode, a diode bar, an array of laser diodes, an array of diode bars, or one or more vertical cavity surface-emitting lasers (VCSELs). The laser coolers 110, 115 are thermally connected to the beam emitter 105 and each electrically connected to one of the electrodes (i.e., the anode and the cathode) of the beam emitter 105. For example, the bottom laser cooler 110 may be electrically connected to the anode of beam emitter 105 and the top laser cooler 115 may be electrically connected to the cathode of beam emitter 105, or vice versa. The laser coolers 110, 115 are typically highly thermally and electrically conductive; thus, in various embodiments, the laser coolers 110, 115 include, consist essentially of, or consist of one or more metals such as copper, silver, or gold. An electrically insulating layer and/or material may be disposed between the laser coolers 110, 115 (or portions thereof) such that the only electrical connection therebetween is through the beam emitter 105 itself.

(9) One or both of laser coolers 110, 115 may include features and/or materials as described in U.S. patent application Ser. No. 14/666,438, filed on Mar. 24, 2015, the entire disclosure of which is incorporated by reference herein. One or both of laser coolers 110, 115 and/or laser package 100 may be actively liquid-cooled, for example as described in U.S. patent application Ser. No. 15/627,917, filed on Jun. 20, 2017, the entire disclosure of which is incorporated by reference herein.

(10) FIG. 3 depicts an exemplary embodiment of the present invention in which the laser package 100 is installed on a cooling plate 300. As shown, an electrically insulating barrier 310 may be disposed between the laser package 100 and the cooling plate 300, and a thermal-interface material 320 may be disposed between the laser package 100 and the electrically insulating barrier 310. In various embodiments, an additional layer of thermal-interface material may be disposed between the electrically insulating barrier 310 and the cooling plate 300. In various embodiments, either or both of the thermal-interface materials may include, consist essentially of, or consist of, for example, a metallic liquid or paste (e.g., a liquid or paste including, consisting essentially of, or consisting of indium), and/or a thermally conductive, electrically insulating solid film or foil (e.g., a film or foil including, consisting essentially of, or consisting of silicone incorporating one or more filler materials such as boron nitride).

(11) The cooling plate 300 may include, consist essentially of, or consist of one or more thermally conductive materials such as copper, aluminum, and/or silver. In various embodiments, the cooling plate may incorporate or define therewithin recesses and/or channels (not shown in FIG. 3) for flow of a cooling fluid (e.g., water or a glycol) to provide active cooling to laser package 100. A coolant source and coolant sink may be connected to the cooling channels, and a coolant reservoir and, e.g., a heat exchanger, may be fluidly connected to the cooling channel and provide coolant thereto. Such cooling systems are conventional and may be utilized with embodiments of the present invention without undue experimentation. Cooling channels within the cooling plate 300 may be simple conduits and/or conduits or conduit networks that include complex features such as turns, branches, etc.

(12) In various embodiments, the electrically insulating barrier 310 may be omitted, and only a single layer of thermal-interface material 320 may be disposed between the laser package 100 and the cooling plate 300. For example, the cooling plate may be electrically resistive or have disposed thereon an electrically insulating layer, and/or the bottom laser cooler 110 may have an electrically insulating layer disposed at least on its bottom surface to prevent electrical conduction from the laser package 100 to the cooling plate 300.

(13) As shown in FIG. 3, the bottom laser cooler 110 and the top laser cooler 115 may be fastened together via one or more fasteners 330, e.g., screws. In various embodiments, the fasteners extend into holes defined in the cooling plate 300 and may be threaded to engage with complementary threads defined in the holes in the cooling plate 300. As also shown in FIG. 3, in embodiments in which multiple laser packages 100 are attached to a shared cooling plate 300, the laser packages 100 may be electrically connected to each other via one or more bus bars 340. For example, as shown, the laser coolers 110, 115 may incorporate or have attached thereto electrically conductive posts to which the bus bars 340 are connected. The bus bars 340 may include, consist essentially of, or consist of an electrically conductive material, e.g., a metal such as copper. In various embodiments, the bus bars 340 are configured (e.g., sized and shaped) to carry current from laser package 100 with minimal resistive loss. As shown in FIG. 3, adjoining laser packages 100 may be electrically connected in series, i.e., the top laser cooler from one (and thus one electrode of the corresponding beam emitter) may be connected to the bottom laser cooler from the next (and thus the opposite electrode of the next beam emitter). Bus bars 340 may also be utilized to connect laser packages 100 to other electronic devices and/or sources of electrical power (e.g., current sources).

(14) As shown in FIG. 3, in various embodiments of the present invention, the beam emitter 105 (disposed between the bottom laser cooler 110 and top laser cooler 115, and not clearly visible in FIG. 3) is associated with (e.g., attached or otherwise optically coupled to) a fast-axis collimator (FAC)/optical twister microlens assembly that collimates the fast axis of the emitted beams while rotating the fast and slow axes of the beams by 90°, such that the slow axis of each emitted beam is perpendicular to the WBC dimension downstream of the microlens assembly. As shown, the microlens assembly may include, consist essentially of, or consist of a FAC 350 and an optical twister (or optical rotator) 360, which may be mechanically supported by a holder 370. The microlens assembly may also, in WBC embodiments, converges the chief rays of the emitters from the beam emitter 105 (and/or other beam emitters in the laser device) toward a downstream dispersive element for combining into a single multi-wavelength beam. Suitable microlens assemblies are described in U.S. Pat. No. 8,553,327, filed on Mar. 7, 2011, and U.S. Pat. No. 9,746,679, filed on Jun. 8, 2015, the entire disclosure of each of which is hereby incorporated by reference herein.

(15) FIG. 4 is a cross-sectional schematic diagram of a laser package 100 attached to cooling plate 300 in accordance with embodiments of the present invention. As shown, embodiments of the invention feature a first thermal-interface material 320-1 between the laser package 100 and the electrically insulating barrier 310, as well as a second thermal-interface material 320-2 between the electrically insulating barrier 310 and the cooling plate 300. In various embodiments, the thermal-interface materials 320-1, 320-2 include, consist essentially of, or consist of the same material, while in other embodiments, the thermal-interface materials 320-1, 320-2 include, consist essentially of, or consist of different materials. In various embodiments, the electrically insulating barrier 310 may be divided into discrete sections, as detailed below. Thus, in embodiments of the invention, the electrically insulating barrier 310 may not occupy the entire area of the interface between the laser package 100 and the cooling plate 300.

(16) As shown in FIG. 4, a spring 400 may be disposed between a top section (e.g., a head) of fastener 330 and the laser package 100, thereby enabling the laser package 100 to more freely move (e.g., expand and/or contract) during, for example, thermal cycles associated with operation of the beam emitter 105. For example, the laser package 100 may move, expand, and/or contract laterally or longitudinally (i.e., along the interface) and/or transversely (i.e., away from, e.g., perpendicular to, the interface). In various embodiments, the spring 400 may include, consist essentially of, or consist of a metal (e.g., stainless steel), a metal alloy, or a high-temperature-resistant polymer, fluoropolymer, elastomer, or plastic material. In various embodiments, the springs 400 may be electrically conductive or electrically insulating. In various embodiments, an electrically insulating barrier layer or barrier material may be disposed between the spring 400 and the laser package 100 to prevent electrical conduction therebetween. For example, a washer including, consisting essentially of, or consisting of an electrically insulating material (e.g., a thermoplastic or polymeric material such as polyetherimide) may be disposed on or around the fastener 330 and/or between the spring 400 and the laser package 100.

(17) As utilized herein, the term “spring” includes any elastic entity, member, or object that reversibly stores mechanical energy. Exemplary springs 400 include coil springs, wave springs, disc springs, leaf springs, Belleville springs (i.e., coned disc springs), and/or bellows. The spring 400 may operate in accordance with Hooke's law when compressed and/or stretched as contemplated herein, and may thus be characterized by a spring constant k. In various embodiments, the spring 400 is configured to therefore exert a nominal force F.sub.n=k×x.sub.n when compressed at a compression x.sub.n from the rest length (where x.sub.n is within the deformation range in which Hooke's law governs). The spring 400 thereby compresses the thermal-interface material 320 (e.g., thermal-interface material 320-1 and/or 320-2, if one or both are present) and maintains the thermal connection between the laser package 100 and the cooling plate 300.

(18) In addition, in various embodiments, the spring 400 is configured (e.g., the spring constant k and/or nominal compression x.sub.n are selected) such that, during a typical thermal cycle (i.e. temperature change) ΔT of laser package 100 (e.g., during operation), the force exerted by the spring 400 is no more than approximately 100% more than (i.e., twice) the force F.sub.n. That is, in various embodiments the thermal cycle does not result in additional compression of the spring 400 sufficient to increase the force exerted by the spring 400 (i.e., when not heated, e.g., when at room temperature) by more than approximately 100%. In some embodiments, the force exerted by the spring 400 is no more than approximately 50% more, or no more than approximately 10% more than F.sub.n. The compression of the spring 400 resulting from the thermal expansion of the laser package 100 may be Δx=CTE×ΔT×x.sub.n, where CTE represents the CTE of the laser package 100. For example, in an illustrative example, the nominal compression x.sub.n of a spring 400 corresponding to a plurality of Belleville springs is approximately 200 μm, and the spring constant k is approximately 3.28×10.sup.6 N/m. For a beam emitter having a height h of 10 mm, and a laser package composed of copper (having a CTE of approximately 16 ppm), and for a ΔT of 100° C., the additional thermal compression corresponds to Δx=CTE×ΔT×h=approximately 16 μm. Thus, the additional force exerted by the spring 400 during the thermal cycle is ΔF=k×Δx=approximately 52.5 N. Based on the nominal compression of the spring 400, the nominal force F.sub.n is equal to k×Δx.sub.n approximately 656 N. Therefore, the force exerted by the spring 400 during the thermal cycle, in this illustrative example, is ΔF/F.sub.n, or approximately 8%.

(19) In various embodiments of the invention, therefore, the use of the springs 400 in conjunction with fasteners 330 may reduce or substantially eliminate the thermal pumping and concomitant loss of thermal-interface material 320 from the interface between the laser package 100 and the cooling plate 300. That is, the motion or loss of the thermal-interface material(s) 320 from the interface is reduced (i.e., compared to cases in which fasteners without springs 400 are utilized and/or thermal-induced motion of the laser package 100 is constrained or prevented) or substantially eliminated. In various embodiments, the spring 400 is separate from and disposed around the fastener 330, while in other embodiments the spring 400 is attached to, or even part of, the fastener 330. All or part of the portion of the fastener 330 extending into the cooling plate 300 may be threaded, and the hole defined in the cooling plate 300 may be complementarily threaded for anchoring of the fastener 330. The fasteners 330 may include, consist essentially of, or consist of, e.g., a metal such as stainless steel. An electrically insulating coating or layer may be disposed on the outside surface of the fastener 330 in order to prevent electrical connection or shorting between various components of laser system through the fastener 330.

(20) In addition to or instead of the use of springs 400 in conjunction with fasteners 300, laser systems in accordance with embodiments of the invention may incorporate an electrically insulating barrier 310 that is divided into multiple discrete area-wise (or “areal”) regions for reduction of thermal pumping effects (due to, e.g., CTE mismatch between the electrically insulating barrier and the beam emitter and/or laser package). FIG. 5 is a schematic plan view of a divided electrically insulating barrier 500 that may correspond to electrically insulating barrier 310 in FIG. 4. As shown, the electrically insulating barrier 500 may be divided into two or more discrete sections 510 separated by gaps 520 therebetween. In various embodiments, during thermal cycling of the laser system, each of the sections 510 may expand, e.g., in response to CTE mismatch-induced pumping, independent of the others. Thus, the total expansion or movement of the electrically insulating barrier 500 may be decreased by a factor corresponding to the number of sections 510 per unit length (e.g., dL/L in the exemplary embodiment illustrated in FIG. 5). The thermally induced pumping of the thermal-interface material 320-1 and/or thermal-interface material 320-2 may therefore decrease by approximately the same factor. In various embodiments, the sections 510 may not be equally spaced apart, i.e., the gaps 520 may vary in width. Moreover, the sizes of the sections 510 may vary across the area of the electrically insulating barrier 500. For example, areas of the electrically insulating barrier 500 disposed beneath regions of laser package 100 that experience higher maximum temperatures and/or larger thermal cycles may be divided into smaller sections 510, compared to other portions of electrically insulating barrier 500. In various embodiments, the gaps 520 between sections may be different (e.g., smaller or larger) beneath regions of laser package 100 that experience higher maximum temperatures and/or larger thermal cycles (e.g., directly below the beam emitter) than in other portions of the electrically insulating barrier.

(21) In various embodiments, the electrically insulating barrier 500 may be applied between the laser package 100 and the cooling plate 300 (e.g., with one or both of thermal-interface materials 320-1, 320-2 present) as a plurality of discrete sections 510. In other embodiments, the electrically insulating barrier 500 is applied as a single uniform layer at the interface, and portions of the material are removed to form the gaps 520 between the discrete sections 510. For example, one or more portions of the material may be removed via etching (e.g., after masking remaining regions). In various embodiments, one or more of the sections 510 may be regularly shaped (e.g., squares, other polygons, or circles), or one or more of the sections 510 may be irregularly shaped. Similarly, the gaps 520 may not be uniform in width across their entire lengths and may vary in width in a regular or irregular pattern. As shown in FIG. 5, when viewed in plan view, one or more, or even all, of the sections 510 may be regular polygons (or circles) in which all of the sides of the section 510 have approximately the same length. (That is, in various embodiments, the sections 510 have shapes different from irregular shapes that would result from the pressing of a liquid or gel material or the introduction of such a material within a small space.) In various embodiments, when viewed from the side or in cross-section, each section 510 may have substantially the same thickness across its lateral dimension (and two or more, or even all, of the sections 510 may have substantially the same thickness as each other), and/or the sidewalls of the section 510 may be substantially perpendicular to the top and bottom surfaces of the section 510, which may be parallel to each other. In various embodiments, one or more (or even all) of the sections 510 may extend across an entire lateral dimension (e.g., length or width) of cooler 110 (e.g., in the manner of “strips” or “stripes”), while one or more other sections may be smaller. In various embodiments, portions of one or more of the sections 510 may extend out from beneath the cooler 110, for example as shown in FIGS. 3 and 4.

(22) In various embodiments, the divided electrically insulating barrier layer 500 is utilized with thermal interface material 320-1 and/or thermal interface material 320-2 that is also electrically insulating, in order to prevent inadvertent electrical conduction between the laser package 100 and the cooling plate 300 within one or more of the gaps 520. In various embodiments, a compliant or flexible material (e.g., a compliant epoxy) may be disposed within one or more of the gaps 520 to prevent inadvertent electrical conduction between the laser package 100 and the cooling plate 300.

(23) In various embodiments, the thermal-interface material 320 (e.g., thermal-interface material 320-1 and/or thermal interface material 320-2) and/or portions of components in contact therewith may be sealed with a sealing material to further minimize or prevent creep or movement of the thermal-interface material, for example as described in U.S. patent application Ser. No. 15/006,733, filed Jan. 26, 2016, the entire disclosure of which is incorporated by reference herein.

(24) Packaged lasers in accordance with embodiments of the present invention may be utilized in WBC laser systems. FIG. 6 depicts an exemplary WBC laser system 600 that utilizes a packaged laser 605. The packaged laser 605 may include, consist essentially of, or consist of, for example, laser package 100 as detailed herein, which may be disposed on a cooling plate 300 as detailed herein. In the example of FIG. 6, laser 605 features a diode bar having four beam emitters emitting beams 610 (see magnified input view 615), but embodiments of the invention may utilize diode bars emitting any number of individual beams or two-dimensional arrays or stacks of diodes or diode bars. In view 615, each beam 610 is indicated by a line, where the length or longer dimension of the line represents the slow diverging dimension of the beam, and the height or shorter dimension represents the fast diverging dimension. A collimation optic 620 may be used to collimate each beam 610 along the fast dimension. Transform optic(s) 625, which may include, consist essentially of, or consist of one or more cylindrical or spherical lenses and/or mirrors, are used to combine each beam 610 along a WBC direction 630. The transform optics 625 then overlap the combined beam onto a dispersive element 635 (which may include, consist essentially of, or consist of, e.g., a diffraction grating such as a reflective or transmissive diffraction grating), and the combined beam is then transmitted as single output profile onto an output coupler 640. The output coupler 640 then transmits the combined beams 645 as shown on the output front view 650. The output coupler 640 is typically partially reflective and acts as a common front facet for all the laser elements in this external cavity system 600. An external cavity is a lasing system where the secondary mirror is displaced at a distance away from the emission aperture or facet of each laser emitter. In some embodiments, additional optics are placed between the emission aperture or facet and the output coupler or partially reflective surface.

(25) The terms and expressions employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.