Actively cooled end-pumped solid-state laser gain medium.

Abstract

The actively cooled, end-pumped solid-state laser gain device includes a bulk solid-state gain medium. An input end of the gain medium receives a pump laser beam incident thereon and propagating in a direction toward an opposing output end. A metal foil is disposed across a face of the gain medium extending between the input and output ends. A housing cooperates with the metal foil to form a coolant channel on the face of the gain medium. The coolant channel has an inlet and an outlet configured to conduct a flow of coolant along the metal foil from the input end toward the output end. The metal foil is affixed between the gain medium and a portion of the housing extending adjacent the coolant channel. The metal foil provides a secure thermal contact and imparts little or no stress on the bulk gain medium.

Description

[Technical field]

[0001] (Priority) This application claims priority to U.S. Provisional Application No. 63 / 203,438, filed July 22, 2021, the disclosure of which is incorporated herein by reference in its entirety.

[0002] (Technical field of the invention) The present invention relates generally to active liquid cooling of solid-state laser gain media in lasers and laser amplifiers, and more particularly to active liquid cooling of bulk solid-state laser gain media that experience significant and non-uniform heat loads from a pump laser beam. [Background technology]

[0003] (Discussion of Background Art) The gain medium of a solid-state laser or laser amplifier is a solid host material doped with optically active ions that can generate or amplify laser radiation when stimulated. The host material is generally glass or crystalline, and the optically active ions are typically rare earth or transition metal ions such as neodymium, erbium, ytterbium, or titanium. The gain medium may be in the form of an optical fiber or a bulk crystal / glass. Most bulk gain media are shaped as rods or slabs.

[0004] Typically, solid-state laser gain media are optically pumped, i.e., optically active ions are optically excited to provide the required population inversion for lasing action. Traditionally, the optical pump source was a flash lamp. However, currently, many solid-state laser gain media are pumped by laser radiation, as laser pumping tends to be more efficient than lamp pumping. Diode lasers are an increasingly popular choice for pump laser sources, especially due to their many advantages, such as efficiency, compactness, long life, and low cost. Diode lasers can provide pump powers as high as hundreds of watts or even kilowatts. Some systems utilize arrays of laser diodes to provide the required pump power.

[0005] In the case of diode-laser-pumped bulk gain media, several different pumping geometries are possible: in end-pumping, the pump laser radiation is co-propagating (or, less commonly, counter-propagating) with the output laser radiation; in side-pumping, the pump laser radiation is directed into the gain medium, e.g., a slab or rod, through a face that is parallel to the propagation direction of the output laser beam, such that the propagation direction of the pump laser radiation is approximately perpendicular to that of the output laser radiation.

[0006] When the pump laser power is high, cooling of the bulk gain medium is necessary to limit the detrimental thermal effects resulting from the absorption of the pump laser radiation. Without cooling, the temperature of the bulk gain medium will rise in a significant and spatially non-uniform manner. This temperature rise and non-uniform temperature distribution are associated with undesirable effects that may hinder the performance of the system. Some of these undesirable effects are related to thermal lensing. Thermal lensing is mainly due to the thermo-optic effect, which is the temperature dependence of the refractive index of the gain medium as well as the thermal expansion of the gain medium. Thermal lensing can be accommodated in the optical design of the laser. However, the temperature dependence of the thermo-optic constants and the thermal conductivity causes aberrations of the thermal lens, which will ultimately limit the output power and degrade the beam quality of the laser. These aberrations are mitigated by minimizing the maximum temperature inside the gain medium. In addition, the non-uniform temperature distribution causes non-uniform thermal expansion, which, when combined with the external mechanical pressure on the bulk gain medium, leads to mechanical stresses in the gain medium. In the worst case, the bulk gain medium may crack.

[0007] End pumping is an advantageous geometry from the standpoint of cooling, since the side surfaces of the bulk gain medium can be in contact with the cooling elements without interfering with the propagation paths of either the pump laser radiation or the output laser radiation. However, at high pump powers, end pumping generates a thermal lens in the path of the laser radiation. This thermal lens tends to become increasingly aberrated with increasing temperature. Although it is possible to operate a laser or laser amplifier with some thermal lensing effect in the gain medium, it is preferable to keep the thermal lens relatively weak, and in particular to prevent any significant aberrations of the thermal lens.

[0008] Active water cooling is an effective method for cooling the sides of a bulk gain medium. In one approach, water is flowed along the side of the bulk gain medium that is in direct contact with it. In another approach, a copper block is placed in thermal contact with the side of the bulk gain medium, absorbing heat from it while the copper block is cooled by flowing water. Indium is sometimes interposed between the copper block and the bulk gain medium. Indium is a metal and therefore a thermal conductor, but is relatively soft. Compared to copper, this softness allows indium to better conform to the surface of the gain medium, which is generally not perfectly smooth. Indium's softness also provides conformality to better maintain thermal contact between the gain medium and the copper block in the presence of dissimilar thermal expansions. Summary of the Invention [Means for solving the problem]

[0009] SUMMARY OF THEINVENTION Disclosed herein is a solid-state laser gain device based on a solid-state bulk gain medium that is actively cooled and configured for end-pumping. The disclosed laser gain device is suitable for use in solid-state lasers as well as in solid-state laser amplifiers. At least one side surface of the bulk gain medium is in thermal contact with a metal foil that is actively cooled by a liquid coolant flow, such as a water flow. The metal foil may be a copper foil. Compared to a solid metal block, e.g., a copper block, the flexibility of the present metal foil allows the metal foil to conform to the bulk gain medium to achieve excellent thermal contact between the coolant and the bulk gain medium. In particular, the metal foil provides a more reliable thermal contact that is less subject to both (a) mechanical stresses due to non-uniform thermal expansion of the bulk gain medium, and (b) variations in the assembly process. Furthermore, compared to a solid metal block, the metal foil imparts less stress on the bulk gain medium.

[0010] In operation, the bulk gain medium is laser pumped in an end-pumped geometry, i.e., a pump beam is incident on the input end of the bulk gain medium and propagates in a direction towards the opposing output end of the bulk gain medium. The coolant flows over the metal foil in the same direction, i.e., from the input end towards the output end. This coordination of the coolant flow direction and the pump beam propagation direction promotes optimal cooling of the portion of the gain medium closest to the input end, and therefore experiences the greatest heat load from the pump beam.

[0011] In one aspect, an actively cooled, end-pumped solid-state laser gain device includes a solid-state gain medium, a metal foil, and a housing. The solid-state gain medium has opposing first and second ends and a first face extending between the first and second ends. The first end is configured to receive a pump laser beam incident thereon and propagating in a direction toward the second end. The metal foil is disposed across the first face of the gain medium. The housing cooperates with the metal foil to form a coolant channel from the first end of the gain medium toward the second end of the gain medium. The coolant channel has an inlet and an outlet configured to conduct a flow of coolant along the metal foil from the first end toward the second end. The metal foil is affixed between the gain medium and a portion of the housing extending adjacent to the coolant channel in a direction between the first and second ends. [Brief description of the drawings]

[0012] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate preferred embodiments of the invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

[0013] [Figure 1] FIG. 1 illustrates in cross-sectional side view an actively cooled end-pumped solid-state laser gain device with a slab-shaped bulk solid-state gain medium and two active cooling elements, each configured to actively cool the gain medium using a liquid-cooled metal foil, according to an embodiment.

[0014] [Diagram 2] 2A-C illustrate example spatial relationships between the gain medium of the device of FIG. 1 and either of its cooling elements.

[0015] [Diagram 3] FIG. 3 is a cross-sectional side view of a portion of the device of FIG. 1 showing how the cooling body channels of each cooling element extend beyond the ends of the gain medium.

[0016] [Figure 4] FIG. 4 is a cross-sectional side view of a portion of an alternative laser gain device with a truncated coolant channel according to an embodiment.

[0017] [Diagram 5] FIG. 5 is a cross-sectional end view of the laser gain device of FIGS.

[0018] [Figure 6] FIG. 6 is a cross-sectional end view of an actively cooled, end-pumped solid-state laser gain device based on a rod-shaped gain medium, according to an embodiment.

[0019] [Figure 7] FIG. 7 illustrates a cooling element in which a metal foil is pressed against the housing to enclose and seal the cooling body channels, according to an embodiment.

[0020] [Figure 8] FIG. 8 illustrates in cross-sectional side view a cooling element for cooling the gain medium of either of the laser gain devices of FIGS. 1 and 4 with enhanced cooling efficiency at the laser pumped end of the gain medium in accordance with an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Detailed Description of the Invention Referring now to the drawings, in which like components are designated by like numbers, Figure 1 is a cross-sectional side view of an actively cooled, end-pumped solid-state laser gain device 100. The device 100 includes a bulk solid-state gain medium 110 and two active cooling elements 120(1, 2) for cooling the gain medium 110. The device 100 is configured for end-pumping by a pump laser beam 162. In operation, the pump beam 162 is incident on an input end 114(1) of the gain medium 110 and propagates in a direction toward an opposing output end 114(2) of the gain medium 110.

[0022] In one usage scenario, device 100 serves as the gain medium of a solid-state laser, where population inversion in gain medium 110, generated by pump beam 162, leads to the generation of output laser beam 164. Output beam 164 propagates collinearly with pump beam 162, either in the same direction as pump beam 162 or in the opposite direction. In another usage scenario, device 100 serves as the gain medium of a solid-state laser amplifier, where population inversion instead leads to amplification of a laser beam, which propagates through gain medium 110 collinearly with pump beam 162. In this scenario, output beam 164 is an amplified version of the input laser beam incident on one of ends 114.

[0023] The gain medium 110 is made of a crystal or glass doped with optically active ions. The gain medium 110 is a slab with two opposing faces 112(1) and 112(2). Although not depicted in FIG. 1, the gain medium 110 may include a coating on either one of the faces 112(1) and 112(2). The coating may be a metal coating, for example, including chromium, nickel, and / or gold. A cooling element 120(1) is disposed on the face 112(1), and a cooling element 120(2) is disposed on the face 112(2). Each cooling element 120 is thermally coupled to the gain medium 110 and serves to remove heat therefrom. In some cases, the absorption of the output beam 164 in the gain medium 110 generates a non-negligible heat load. However, the heat load typically results primarily from the absorption of the pump beam 162, specifically, the quantum defect in the oscillation of the optically active ions and any non-radiative losses thereof. As the pump beam 162 propagates from the input end 114(1) towards the output end 114(2) of the gain medium 110, optical absorption leads to a gradual attenuation of the pump beam 162. Thus, the heat load from the pump beam 162 is greatest near the input end 114(1). The resulting temperature distribution within the gain medium 110 is non-uniform not only in the dimension transverse to the propagation direction of the pump beam 162, but also in the dimension along the gain medium 110 from the input end 114(1) to the output end 114(2).

[0024] Each cooling element 120 includes a metal foil 130 and a housing 122. The metal foil 130 is disposed across a respective face 112 of the gain medium 110. A surface 126 of the housing 122 is bonded to the metal foil 130 such that the housing 122 forms a cooling body channel 140 on the metal foil 130. The cooling body channel 140 has an inlet 142 and an outlet 144 and accommodates a cooling body flow 172 from the inlet 142 to the outlet 144. The cooling body flow 172 extends at least partially along the metal foil 130 from the input end 114(1) to the output end 114(2). This direction of the cooling body flow 172 is preferred due to the greater heat load from the pump beam 162 proximate the input end 114(1) compared to the output end 114(2). The cooling body may be pure water, an aqueous mixture, an aqueous solution, or a non-aqueous liquid.

[0025] The thickness of the metal foil 130 may be less than 200 micrometers (μm), for example, in the range of 50-100 μm. In one embodiment, the metal foil 130 is made of copper or a copper alloy to conduct heat from the gain medium 110 to the coolant flow 172 with high efficiency. The copper (or copper alloy) foil may be plated with nickel and / or gold. In another embodiment, the metal foil 130 is made of another metal with high thermal conductivity. For example, the metal foil 130 may be made of or include nickel, silver, molybdenum, tantalum, and / or tungsten. Compared to a solid metal block, the metal foil 130 is flexible and therefore conforms better to the surface of the gain medium 110. Additionally, if the gain medium 110 and the metal foil 130 undergo dissimilar thermal expansion, or if the gain medium 110 expands non-uniformly, the metal foil 130 imparts little, if any, mechanical stress on the gain medium 110. In contrast, a solid metal block would likely impart stress on the gain medium 110 in such a scenario. Stress on the gain medium 110 can lead to birefringence in the gain medium 110 and, as a result, polarization rotation or depolarization of the output beam 164. Changes in polarization typically cause losses and are undesirable.

[0026] The housing 122 may be made from stainless steel or another material, such as plastic, that is relatively inert to the coolant flowing through the coolant channels 140. Alternatively, the housing 122 may be coated with an inert material.

[0027] 2A-C are a series of perspective views illustrating one example spatial relationship between the gain medium 110 and either one of the cooling elements 120. FIG. 2A shows the housing 122 with an upwardly facing surface 126. The surface 126 surrounds the recessed surface 124 and the channels that form the inlet 142 and the outlet 144. The surface 124 is also shown in FIG. 1 and is on the opposite side of the cooling body channel 140 from the face 112 of the gain medium 110. As shown in FIG. 2B, the metal foil 130 is located on the surface 126 with a contact interface between the metal foil 130 and the surface 126 that surrounds the recessed surface 124, the inlet 142, and the outlet 144. The metal foil 130 is sealed to the surface 126 such that the cooling body channel 140 is enclosed away from the openings provided by the inlet 142 and the outlet 144. Surface 124 and metal foil 130 may be considered as the floor and ceiling (or vice versa), respectively, of cooling body channel 140. Metal foil 130 may be crimped and / or screwed into housing 122 to completely seal against surface 126, optionally with a compliant seal therebetween. Alternatively, metal foil 130 may be soldered or brazed to surface 126.

[0028] Two portions 226P(1) and 226P(2) of the surface 126, shown in FIG. 2A, extend adjacent to the coolant channel 140 from the inlet 142 to the outlet 144. The associated portions of the housing 122 form two separate walls on either side of the coolant channel 140. The width 210W of the gain medium 110 exceeds the width 240W of the coolant channel 140. The gain medium 110 is contacted by the metal foil 130, as shown in FIG. 2C, with a contact interface between the gain medium 110 and the metal foil 130 extending over each of the surface portions 226P(1) and 226P(2). The metal foil 130 is thereby secured between (a) the corresponding face 112 of the gain medium 110 and (b) the surface portions 226P(1) and 226P(2). The contact between the gain medium 110 and the metal foil 130 may be direct or indirect with one or more intervening layers disposed therebetween. The bond between the gain medium 110 and the surface 126 locks the position of the otherwise floating gain medium 110 within the device 100.

[0029] 1 and 2A-C in combination, gain medium 110 is positioned between cooling elements 120(1) and 120(2) in device 100. In certain embodiments, gain medium 110 is pressed into position between cooling elements 120(1) and 120(2). In such embodiments, surface 126 of each cooling element 120 applies pressure onto a portion of gain medium 110 within the footprint of gain medium 110 on surface 126.

[0030] 1 and 2A-C, the length 210L of the gain medium 110 is less than the length 240L of the coolant channel 140 along the metal foil 130 and the surface 126. The associated footprint 232 of the gain medium 110 on the surface 126 and the metal foil 130 is shown in FIG. 2B. This relationship between the lengths of the gain medium 110 and the coolant channel 140 is illustrated in more detail in FIG.

[0031] FIG. 3 is a partial cross-sectional side view of a portion of device 100 showing only cooling element 120(1) and not cooling element 120(2). Cooling element 120(2) has similar properties to cooling element 120(1) in relation to gain medium 110, but is omitted from FIG. 3 for clarity of illustration. Length 240L of the section of cooling body channel 140 extending along metal foil 130 and surface 126 exceeds length 210L of gain medium 110. Cooling body channel 140 extends beyond input end 114(1) by a distance 360(1) and beyond output end 114(2) by a distance 360(2). Each of distances 360 may be in the range of 1 to 5 millimeters (mm). This configuration ensures active liquid cooling of the entire length of gain medium 110 between ends 114(1) and 114(2). Additionally, in embodiments in which gain medium 110 is compressed between cooling elements 120(1) and 120(2), the present configuration confines the compression pressure on gain medium 110 to its extreme width portions. Specifically, cooling elements 120(1) and 120(2) apply pressure only on the extreme width portions of gain medium 110 that overlap surface portions 226P(1) and 226P(2). As long as pump beam 162 is restricted to portions of gain medium 110 that do not overlap surface portions 226P(1) and 226P(2), cooling element 120 is prevented from applying external stress directly on regions of gain medium 110 that transmit pump beam 162 and / or output beam 164. Such a scenario would be construed as a lateral 1 / e 2 The width 262W of the intensity profile is depicted in FIG. 2C, which is within the width 240W of the cooling body channel 140.

[0032] In an alternative configuration, not depicted in FIGS. 1, 2A-C, and 3, the length 240L of the coolant channel 140 may match the length 210L of the gain medium 110, or even be inside one or both of the ends 114. It may be unnecessary to cool all the way to the output end 114(2), especially when there is strong attenuation of the pump beam 162 in the gain medium. In addition, although cooling is generally most needed in the area closest to the input end 114(1), practical considerations may favor a section of the coolant channel 140 extending along the metal foil 130 beginning slightly inside the input end 114(1) with little or no loss of cooling efficiency. However, this configuration may result in undesirable crimping pressure on the active region of the gain medium 110.

[0033] FIG. 4 is a partial cross-sectional side view of a portion of one laser gain device 400 having a truncated coolant channel 140. FIG. 4 utilizes the same view as FIG. 3. Device 400 is similar to device 100, except that coolant channel 140 is shortened at each of input end 114(1) and output end 114(2). The length 240L of the section of coolant channel 140 that runs along metal foil 130 is less than the length 210L of gain medium 110. The section of coolant channel 140 that runs along metal foil 130 begins a distance 460(1) inside input end 114(1) of gain medium 110 and ends a non-zero distance 460(2) before output end 114(2) of gain medium 110. Distance 460(1) may be in the range of zero to 2 mm. Distance 460(2) may be in the range of 1 mm to 25% of length 210L of gain medium 110.

[0034] In each of the devices 100 and 400, the dimensions of the gain medium 110 may be adjusted as necessary (dimensions are shown in FIG. 2C). Typically, the length 210L exceeds the height 210H of the gain medium 110. In one embodiment, the width 210W also exceeds the height 210H by as much as a factor of up to 5, 10, or more. Such an embodiment is compatible with a highly elongated pump beam 162, such as that generated by a laser diode bar, as illustrated in FIG. 2C. Such an embodiment may also be operated with a pump beam 162 characterized by a width 262W, which is less than the width 210W, as shown in FIG. 2C, to contain the pump beam 162 and the output beam 164 within a region of the gain medium 110 that is not subject to pressure from the housing 122. In one embodiment, the height 210H is in the range of 0.5 to 5 mm, the width 210W is in the range of 2 to 20 mm, and the length 210L is in the range of 5 to 20 mm.

[0035] As shown in FIGS. 1, 3, and 4, certain embodiments of each of the devices 100 and 400 further include an indium layer 150 between the metal foil 130 of each cooling element 120 and the corresponding surface 112 of the gain medium 110. The indium layer 150 serves to improve thermal contact between the metal foil 130 and the corresponding surface 112 of the gain medium 110. The indium layer 150 may be soldered in place between the metal foil 130 and the gain medium 110 to ensure good contact between the gain medium 110 and the metal foil 130 via the indium layer 150. Soldering of the indium layer 150 may be accomplished by heating the device 100 to a temperature that exceeds the melting temperature of indium, which is 157° C. Alternatively, the indium layer 150 may be held in place between the metal foil 130 and the surface 126 of the housing 122. In this case, the pressure of the coolant flow 172 may aid in thermal contact between the gain medium 110 and the surface 126 via the indium layer 150. The indium layer 150 may be incorporated into the device 100 / 400 in sheet or foil form. In one embodiment, the thickness of the indium layer 150 is in the range of 50-500 μm.

[0036] Each of the devices 100 and 400 may be implemented in a laser gain system that includes, in addition to the device 100 / 400, a pump laser 160 and a cooling body delivery system 170. FIG. 1 diagrammatically illustrates such a laser gain system 102 based on the device 100. The pump laser 160 generates a pump beam 162. The pump laser 160 may be based on various laser technologies. In one embodiment, the pump laser 160 uses one or more laser diodes to generate the pump beam 162. Due to their efficiency, affordability, reliability, and ease of use, laser diodes are often the preferred pump laser source. The cooling body delivery system 170 may include one or more fluid pumps and is coupled to the housing 122 of each cooling element 120 to generate the cooling body flow 172 through the cooling body channel 140. The system 102 may further include a controller 180 that governs the operation of the pump laser 160 and / or the cooling body delivery system 170.

[0037] Devices 100 and 400 may be modified to include only one of the cooling elements 120. In such an embodiment, the omitted cooling element 120 may be replaced by a fixture, for example, to support the gain medium 110. The gain medium 110 may be crimped into place between this fixture and the remaining cooling element 120.

[0038] 5 is a cross-sectional end view of device 100 / 400 with a cross section intersecting the slab-shaped gain medium 110 and the cooling body channel 140 of each cooling element 120. This cross-sectional end view is orthogonal to the cross-sectional side views of device 100 of FIGS. 1 and 3 and device 400 of FIG. 4. Within each cooling element 120, metal foil 130 is sealed to housing 122 to close the cooling body channel 140, and the footprint of gain medium 110 overlaps surface portions 226P(1) and 226P(2) of each cooling element 120. The cooling body channel 140 of each cooling element 120 spans width 240W across a portion of gain medium 110.

[0039] Although the gain medium 110 is in the form of a slab, the devices 100 and 400 are easily modifiable to accommodate other shaped end-pumped gain media, for example, rod-shaped gain media. Figure 6 illustrates one exemplary modification of the devices 100 / 400 from implementing a slab-shaped gain medium to implementing a rod-shaped gain medium.

[0040] FIG. 6 is a cross-sectional end view of an actively cooled, end-pumped solid-state laser gain device 600 based on a rod-shaped gain medium 610. The device 600 is a modification of either one of the devices 100 and 400 adapted to accommodate the rod-shaped gain medium 610. Here, the rod has a circular cross-sectional shape. However, the rod may have a square or other polygonal shape. The device 600 includes one or two cooling elements 620. When including two cooling elements 620(1) and 620(2), these cooling elements may be disposed on opposite faces of the gain medium 610, as shown in FIG. 6. (When the gain medium 610 is a rod with a circular cross-sectional shape, these two faces are on either side of the cylindrical outer surface of the gain medium 610). Each cooling element 620 is an adaptation of the cooling element 120 that fits into the curved face of the gain medium 610.

[0041] Each cooling element 620 includes a metal foil 130 and a housing 622. The metal foil 130 is wrapped around a portion of the gain medium 610. The housing 622 and the metal foil 130 cooperatively form a cooling body channel 140 around the circumference of the gain medium 610. The metal foil 130 is secured between the gain medium 610 and surface portions 626P(1) and 626P(2) adjacent the cooling body channel 140. While the cooling body channel 140 of the device 100 / 400 spans a linear width 240W, the cooling body channel 140 of the device 600 has an angled span 640A. In the embodiment depicted in FIG. 6, the angled span 640A is less than 180 degrees, for example, in the range of 90 to 170 degrees. This angled span 640A allows for compression of the gain medium 610 between two cooling elements 620(1) and 620(2) or between one cooling element 620 and a fixture replacing the other cooling element 620.

[0042] In embodiments of device 600 that include both cooling elements 620(1) and 620(2), cooling elements 620(1) and 620(2) may utilize a common metal foil 130 rather than two separate metal foils 130. Device 600 may include an indium layer 150 between the gain medium 610 and the metal foil 130 of each cooling element 620 in a manner similar to that discussed above with respect to device 100.

[0043] The remainder of this disclosure will be based on slab-shaped gain media, however, in a manner similar to the adaptation of the configuration of Figure 5 to arrive at the configuration of Figure 6, the embodiments disclosed below are easily extended to other gain medium shapes, such as rod-shaped gain media.

[0044] FIG. 7 is an exploded view of one cooling element 720 in which the metal foil 130 is pressed against the housing 122 to enclose and seal the cooling body channel 140 (away from the inlet 142 and the outlet 144). The cooling element 720 is an embodiment of the cooling element 120 and may be implemented in either of the devices 100 and 400. FIG. 7 shows the components in a perspective view similar to that used in FIGS. 2A-C. The cooling element 720 includes the housing 122, the metal foil 130, the bracket 770, and optionally the indium layer 150. The thick dashed arrow in FIG. 7 shows how the components of the cooling element 720 spatially integrate when assembled. The bracket 770 is pressed against the surface 126 of the housing 122 with the metal foil 130 disposed between the bracket 770 and the surface 126. The bracket 770 forms an opening 772 that is sized to contain the footprint 232 of the gain medium 110. Once the cooling element 720 is assembled, the gain medium 110 may be placed on the cooling element 720 inside the opening 772 .

[0045] The indium layer 150 may be integrated into the cooling element 720. In one such implementation, the indium layer 150 is pressed between the metal foil 130 and the bracket 770.

[0046] There are many different options for attaching the bracket 770 to the housing 122. In one embodiment, the bracket 770 is threaded or otherwise crimped onto the surface 126. In another embodiment, the bracket 770 extends beyond the surface 126 and at least a portion of the bracket 770 is affixed to another surface of the housing 122, such as the end surface 722S. For example, the bracket 770 is threaded onto a portion of the surface 126 that extends along the longitudinal dimension of the cooling body channel 140 parallel to the length 240L and is enclosed along the end surface 722S (and a similar opposing end surface of the housing 122) to which it is attached. This embodiment is advantageous for minimizing the bulk of the bracket 770 at the end 114 of the gain medium 110 when the laser beam enters and exits the gain medium 110. Housing 122 may have additional features, not shown in FIG. 7, to facilitate mounting bracket 770 to other portions of housing 122 than surface 126 .

[0047] In one embodiment, cooling element 720 includes a compliant seal 780, such as a rubber gasket (e.g., an O-ring), between metal foil 130 and surface 126. Compliant seal 780 may surround recessed surface 124, inlet 142, and outlet 144 and help ensure a seal between metal foil 130 and surface 126. Although not shown in FIG. 7, compliant seal 780 may be seated in a groove in surface 126.

[0048] 8 illustrates one cooling element 820 for cooling the gain medium 110 with improved cooling efficiency at the input end 114(1) compared to the output end 114(2). The cooling element 820 is an embodiment of the cooling element 120 and may be implemented in either one of the devices 100 and 400, and the length 240L of the cooling body channel 140 may be longer, shorter, or the same as the length 210L of the gain medium 210.

[0049] The coolant channels 140 of the cooling element 820 have a non-uniform height 840H that imposes a non-uniform coolant flow velocity along the longitudinal dimension of the gain medium 110. Specifically, the height of the coolant channels 140 near the input end 114(1) is less than the height of the coolant channels 140 near the output end 114(2) such that the velocity of the coolant flow 172 (see FIG. 1 ) is higher at the input end 114(1) than at the output end 114(2). This height variation of the coolant channels 140 serves to reduce the coolant flow impedance in downstream portions of the coolant channels 140 adjacent to a portion of the gain medium 110 that experiences less heat load from the pump beam 162 while maximizing cooling efficiency in the region of the gain medium 110 closest to the input end 114(1), which experiences the greatest heat load from the pump beam 162. The cooling element 820 thereby reduces the coolant pressure drop between the inlet 142 and the outlet 144 while providing efficient cooling where it is needed most. The pressure drop determines the size, power, and cost of the fluid pump used to achieve a particular flow rate, and therefore it is advantageous to avoid very large pressure drops. In the embodiment depicted in FIG. 8, the coolant channel 140 has a relatively shallow height 840H(1) from the input end 114(1) through a first section of the length 840(1), and then has an increasing height in subsequent sections of the length 840(2) until it reaches height 840H(2) at the outlet 144. The increase in height through this subsequent section of the coolant channel 140 may be gradual as shown in FIG. 8, or may be stepwise. The length 840(1) may be equal to or exceed the 1 / e extinction length of the pump beam 162 in the gain medium 110.

[0050] In an alternative embodiment, the relatively shallow height 840H(1) required to achieve sufficient cooling near the input end 114(1) is maintained along the entire length of the coolant channel 140. In this embodiment, the pressure drop along the coolant channel may be too large to maintain the desired coolant flow rate near the input end 114(1). This potential problem is prevented by increasing the height of the coolant channel 140 after the initial shallow section near the input end 114(1) within the cooling element 820.

[0051] In one embodiment, height 840H(1) is less than 1 mm, for example, in the range of 0.1 to 1 mm. Height 840H(2) may be in the range of 1 to 5 mm. In one implementation, the height of coolant channel 140 along length 840(2) is inversely proportional to the local heat load within gain medium 110.

[0052] Due to the relatively shallow height 840H(1), the coolant flow 172 through this first section of the coolant channel 140, characterized by having a height 840H(1), may be laminar. Cooling efficiency through this first section of the coolant channel 140 may be improved by incorporating protruding and / or recessed features 848 to provide turbulence. In one implementation, the protruding features 848 are implemented in a surface of the housing 122 facing the gain medium 110, as shown in FIG. 8. Locating the features 848 on the housing 122 is typically preferred over locating the features 848 on the metal foil 130, at least because (a) it is more practical to fabricate such features in the material of the housing 122, rather than in the metal foil 130, and (b) the uniform thickness of the metal foil 130 is more likely to ensure a more consistent thermal conductivity between the gain medium 110 and the coolant flow 172.

[0053] The performance of the cooling element 820 with the indium layer 150 was experimentally evaluated and compared to that of a conventional solid copper block also implementing an indium layer. An end-pumped slab-shaped gain medium was cooled from two sides by two separate conventional water-cooled solid copper blocks. With an optical pump power of about 220 watts, the conventional solid copper block maintained a gain medium temperature of about 100° C. When the same gain medium was implemented in the device 100 and cooled by two cooling elements 820, it was possible to pump the gain medium with a higher pump power, i.e., about 250 watts, and still maintain a lower gain medium temperature of about 70° C.

[0054] Without departing from the scope of the present specification, any of the laser gain devices disclosed above may be operated with a coolant flow propagating in a direction opposite to the propagation direction of the pump beam 162, i.e., with the coolant entering the coolant channel 140 via the outlet 144 and exiting it via the inlet 142. When at least the 1 / e absorption length of the pump beam 162 in the gain medium 110 is less than the length 210L of the gain medium 110, the cooling performance of this counter-propagating coolant flow is likely to be inferior to that of the co-propagating coolant flow discussed above. However, even with a counter-propagating coolant flow, the laser gain device still benefits from other advantages such as excellent and reliable thermal contact between the gain medium 110 and the coolant, as well as minimal mechanical stress on the gain medium 110.

[0055] The present invention is described above in terms of preferred and alternative embodiments. However, the present invention is not limited to the embodiments described and depicted herein. Rather, the present invention is limited only by the claims appended hereto.

Claims

1. An active cooling type end-face pumped solid laser gain device, a solid gain medium having opposing first and second ends and a first surface extending between the first end and the second end, wherein the first end is configured to receive an excitation laser beam that is incident thereon and propagates in a direction toward the second end, the solid gain medium; a flexible metal foil disposed across the first surface of the gain medium; a housing that cooperates with the metal foil to form a coolant channel extending from the first end of the gain medium toward the second end of the gain medium, the coolant channel having an inlet and an outlet configured to conduct the flow of a coolant along the metal foil from the first end to the second end, the housing; comprising; the metal foil is fixed between the gain medium and a part of the housing that extends adjacent to the coolant channel in a direction between the first end and the second end, the device.

2. the metal foil is crimped onto the housing to form a cooling element therewith, the laser further comprises a fixture disposed on a second surface of the gain medium facing the first surface, and the gain medium is crimped between the cooling element and the fixture, the device according to claim 1.

3. the fixture is a second example of the cooling element in which the metal foil is disposed across the second surface of the gain medium to provide cooling of the gain medium through the second surface, the device according to claim 2.

4. the device according to claim 1, further comprising an indium layer between the metal foil and the first surface of the gain medium.

5. the device according to claim 4, wherein the indium layer is soldered between the metal foil and the first surface of the gain medium.

6. the device according to claim 4, wherein the thickness of the indium layer is in the range of 50 to 500 micrometers.

7. the device according to claim 1, wherein the metal foil is fixed between the first surface of the gain medium and two walls of the housing, each of the two walls extending between the first end and the second end of the gain medium on an individual side of the coolant channel.

8. the device according to claim 1, wherein the metal foil is coupled to a part of the housing via a flexible seal.

9. The device according to claim 1, wherein the metal foil is soldered or brazed to the housing.

10. The device according to claim 1, wherein the metal foil contains copper.

11. The device according to claim 1, wherein the thickness of the metal foil is 50 to 200 micrometers.

12. The height of the coolant channel above the metal foil is such that the flow rate of the coolant at the first end is higher than that at a location closer to the second end and lower than that at a location closer to the second end at the first end, in the device according to claim 1.

13. The device according to claim 12, wherein the height of the coolant channel is less than 1 millimeter through a first section of the coolant channel closest to the first end.

14. The device according to claim 13, wherein the first section spans from the first end to a location spaced from the first end by at least 1 / e of the absorption length of the excitation laser beam in the gain medium.

15. The device according to claim 13, wherein the height of the coolant channel in a second section extending at least to some extent from the first section to the second end increases as a function of the distance from the first end.

16. The device according to claim 13, wherein the surface of the housing facing the metal foil and forming the ceiling of the first section of the coolant channel has recessed or protruding features for inducing turbulence in the flow of the coolant.

17. The device according to claim 1, wherein the coolant channel extends at least the length of the gain medium from the first end to the second end.

18. The device according to claim 1, wherein the metal foil and the coolant channel extend beyond the first and second ends in a dimension parallel to the first surface of the gain medium.

19. The gain medium has a length L from the first end to the second end, and the portion of the metal foil pinched between the gain medium and the coolant channel extends from a location within 1 millimeter of the first end to a location within 0.25L of the second end, in the device according to claim 1.

20. A laser gain system, comprising: the device according to claim 1; and an excitation laser for generating the excitation laser beam. A coolant delivery system for pumping the coolant into the coolant channels through the inlet A laser gain system comprising the same.

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