The present invention relates in general to active liquid cooling of solid-state laser gain media in lasers and laser amplifiers. In particular, the present invention relates to active liquid cooling of a bulk solid-state laser gain medium that is subject to a significant, non-uniform heat load from a pump laser beam.
The gain medium of a solid-state laser or laser amplifier is a solid host-material doped with optically active ions capable of generating or amplifying laser radiation when excited. The host material is generally glass or crystalline, and the optically-active ions are typically rare earth or transition metal ions, such as neodynium, 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 a rod or a slab.
Commonly, solid-state laser gain media are optically pumped, that is, the optically active ions are optically excited to provide the needed population inversion for lasing action. Historically, the source of optical pumping was a flash lamp. At present, however, many solid-state laser gain media are pumped by laser radiation, since laser pumping tends to be more efficient than lamp pumping. Diode lasers are a particularly popular choice for the pump laser source due to their many advantages, e.g., efficiency, compactness, long lifetime, and low cost. Diode lasers may provide pump powers as high as hundreds of watts or even kilowatts. Some systems utilize arrays of laser diodes to provide the needed pump power.
In the case of diode-laser-pumped bulk gain media, several different pump geometries are possible. In end-pumping, the pump laser radiation is co-propagating (or, less commonly, counter-propagating) with the output laser radiation. Side-pumping entails directing the pump laser radiation into the gain medium, e.g., 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 generally perpendicular to that of the output laser radiation.
When the pump laser power is high, cooling of the bulk gain medium is necessary to limit adverse thermal effects resulting from absorption of the pump laser radiation. Without cooling, the temperature of the bulk gain medium will rise significantly and in a spatially non-uniform fashion. This temperature rise and non-uniform temperature distribution is associated with undesirable effects that may hamper the performance of the system. Some of these undesirable effects are related to thermal lensing. The thermal lens is primarily due to the thermo-optic effect, which is the temperature dependence of the refractive index of the gain medium, as well as thermal expansion of the gain medium. The thermal lens can be accommodated in the optical design of a laser. However, temperature dependences of the thermo-optic constant and the thermal conductivity cause aberrations in the thermal lens, which will ultimately limit the output power and degrade the beam quality of a laser. These aberrations are mitigated by minimizing the highest temperature inside the gain medium. In addition, the non-uniform temperature distribution causes non-uniform thermal expansion which, when combined with external mechanical pressure on the bulk gain medium, leads to mechanical stress in the gain medium. In worst case, the bulk gain medium may crack.
End-pumping is an advantageous geometry from a cooling perspective as the side surface(s) of the bulk gain medium may be in contact with cooling element(s) without interfering with the propagation paths of either one of the pump laser radiation and the output laser radiation. At high pump powers, however, end-pumping generates a thermal lens in the path of the laser radiation. This thermal lens tends to become increasingly aberrated with increasing temperature. While it is possible to operate a laser or laser amplifier with some degree of thermal lensing in the gain medium, it is preferable to keep the thermal lens relatively weak and, especially, prevent any significant aberration of the thermal lens.
Active water-cooling is an effective method for cooling the sides of a bulk gain medium. In one scheme, water is flowed along the side of the bulk gain medium in direct contact therewith. In another scheme, a copper block is placed in thermal contact with a side of the bulk gain medium to absorb heat therefrom while the copper block is cooled by flowing water. Indium is sometimes interposed between the copper block and the bulk gain medium. Indium, while being metallic and thus a thermal conductor, is relatively soft. As compared to copper, this softness allows indium to better conform to the surface of the gain medium, which is generally not perfectly smooth. The softness of indium also provides compliance to better maintain thermal contact between the gain medium and the copper block in the presence of dissimilar thermal expansion.
Disclosed herein are solid-state laser gain devices based on a solid-state bulk gain medium that is actively cooled and configured for end-pumping. The disclosed laser gain devices are 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. As 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 a superior thermal contact between the coolant and the bulk gain medium. Particularly, the metal foil provides a more reliable thermal contact that is less susceptible to both (a) mechanical stress due to non-uniform thermal expansion of bulk gain medium and (b) variation in the assembly process. Furthermore, as compared to a solid metal block, the metal foil imparts less stress on the bulk gain medium.
In operation, the bulk gain medium is laser pumped in the end-pumping geometry, that is, with the pump beam incident on an input-end of the bulk gain medium and propagating in the direction toward an opposite output-end of the bulk gain medium. The coolant flows on the metal foil in the same direction, that is, in the direction from the input-end toward the output-end. This coordination of coolant flow direction with the pump beam propagation direction facilitates optimal cooling of the portion of the gain medium nearest the input-end and therefore subject to the greatest heat load from the pump beam.
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 opposite 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 the direction toward the second end. The metal foil is disposed over 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 towards 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 towards the second end. The metal foil is secured between the gain medium and portions of the housing running adjacent to the coolant channel in a direction between the first and second ends.
The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate preferred embodiments of the present invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain principles of the present invention.
Referring now to the drawings, wherein like components are designated by like numerals,
In one use scenario, device 100 functions as a gain medium of a solid-state laser, in which case the population inversion in gain medium 110 generated by pump beam 162 leads to the generation of an 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 use scenario, device 100 functions as a gain medium of a solid-state laser amplifier, wherein the population inversion instead leads to amplification of a laser beam propagating through gain medium 110 collinearly with pump beam 162. In this scenario, output beam 164 is an amplified version of an input laser beam incident on one of ends 114.
Gain medium 110 is made of crystal, or glass, doped with optically active ions. Gain medium 110 is a slab with two opposite faces 112(1) and 112(2). Although not depicted in
Each cooling element 120 includes a metal foil 130 and a housing 122. Metal foil 130 is disposed over a respective face 112 of gain medium 110. A surface 126 of housing 122 is coupled to metal foil 130, such that housing 122 forms a coolant channel 140 on metal foil 130. Coolant channel 140 has an inlet 142 and an outlet 144, and accommodates a coolant flow 172 from inlet 142 to outlet 144. Coolant flow 172 runs along metal foil 130 from input-end 114(1) at least partway to output-end 114(2). This direction of coolant flow 172 is preferable due to the greater heat load from pump beam 162 near input-end 114(1), as compared to output-end 114(2). The coolant may be pure water, an aqueous mixture, an aqueous solution, or a non-aqueous liquid.
The thickness of metal foil 130 may be less than 200 micrometers (μm), for example in the range between 50 and 100 μm. In one embodiment, metal foil 130 is made of copper, or a copper alloy, to conduct heat from gain medium 110 to coolant flow 172 with high efficiency. The copper (or copper alloy) foil may be plated with nickel and/or gold. In another embodiment, metal foil 130 is made of another metal with high thermal conductivity. For example, metal foil 130 may be made of, or include, nickel, silver, molybdenum, tantalum, and/or tungsten. As compared to a solid metal block, metal foil 130 is flexible and therefore conforms better to the surface of gain medium 110. In addition, when gain medium 110 and metal foil 130 undergo dissimilar thermal expansion or when gain medium 110 expands non-uniformly, metal foil 130 imparts little, if any, mechanical stress on gain medium 110. In contrast, a solid metal block is likely to impart stress on gain medium 110 in such scenarios. Stress on gain medium 110 may lead to birefringence in gain medium 110 and, as a result, polarization rotation or depolarization of output beam 164. Polarization changes typically cause loss and are undesirable.
Housing 122 may be made of stainless steel or another material that is relatively inert to the coolant flowing through coolant channel 140, e.g., plastic. Alternatively, housing 122 may be coated with an inert material.
Two portions 226P(1) and 226P(2) of surface 126, indicated in
Referring now to
In the embodiments illustrated in
In alternative configurations, not depicted in
In each of devices 100 and 400, the dimensions of gain medium 110 may be tailored as needed (dimensions are indicated in
As indicated in
Each one of devices 100 and 400 may be implemented in a laser gain system that, in addition to device 100/400, includes a pump laser 160 and a coolant delivery system 170.
Devices 100 and 400 may be modified to include only one of cooling elements 120. In such embodiments, the omitted cooling element 120 may be replaced by a fixture, for example for supporting gain medium 110. Gain medium 110 may be clamped in place between this fixture and the remaining cooling element 120.
While gain medium 110 is in the form of a slab, devices 100 and 400 are readily modifiable to accommodate end-pumped gain media of other shapes, for example a rod-shaped gain medium.
Each cooling element 620 includes metal foil 130 and a housing 622. Metal foil 130 is wrapped around a portion of gain medium 610. Housing 622 and metal foil 130 cooperatively form coolant channel 140 around the circumference of gain medium 610. Metal foil 130 is secured between gain medium 610 and surface portions 626P(1) and 626P(2) located adjacent coolant channel 140. Whereas coolant channel 140 of device 100/400 spans a linear width 240W, coolant channel 140 of device 600 has an angular span 640A. In the embodiment depicted in
In embodiments of device 600 that include both of 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 indium layer 150 between gain medium 610 and metal foil 130 of each cooling element 620, in a manner similar to that discussed above for device 100.
The remainder of this disclosure will be based on a slab-shaped gain medium. However, in a manner similar to the adaptation of the
Indium layer 150 may be integrated in cooling element 720. In one such implementation, indium layer 150 is clamped between metal foil 130 and bracket 770.
Many different options exist for affixing bracket 770 to housing 122. In one embodiment, bracket 770 is screwed or otherwise clamped onto surface 126. In another embodiment, bracket 770 extends beyond surface 126, and at least a portion of bracket 770 is affixed to other surfaces of housing 122, such as an end surface 722S. For example, bracket 770 may be screwed to portions of surface 126 running along the lengthwise dimension of coolant channel 140 parallel to length 240L, and wrap down along end surface 722S (and a similar opposite end surface of housing 122) to be affixed thereto. This example is advantageous for minimizing the bulk of bracket 770 at ends 114 of gain medium 110 where laser beams enter and exit gain medium 110. Housing 122 may have additional features, not shown in
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 surrounds recessed surface 124, inlet 142, and outlet 144, and may help ensure a tight seal between metal foil 130 and surface 126. Although not shown in
Coolant channel 140 of cooling element 820 has a non-uniform height 840H to impose a non-uniform coolant flow speed along the lengthwise dimension of gain medium 110. Specifically, the height of coolant channel 140 near input-end 114(1) is less than the height of coolant channel 140 near output-end 114(2), such that the speed of coolant flow 172 (see
In an alternative embodiment, a relatively shallow height 840H(1), needed to achieve sufficient cooling near input-end 114(1), is maintained along the entire length of coolant channel 140. In this embodiment, the pressure drop along coolant channel may be too great to maintain the desired coolant flow speed near input-end 114(1). This potential issue is prevented in cooling element 820 by increasing the height of coolant channel 140 after the initial shallow segment near input-end 114(1).
In one example, height 840H(1) is less than 1 mm, for example in the range between 0.1 and 1 mm. Height 840H(2) may be in the range between 1 and 5 mm. In one implementation, the height of coolant channel 140 along length 840(2) is inversely proportional to the local heat load in gain medium 110.
Due to height 840H(1) being relatively shallow, coolant flow 172 through this first segment of coolant channel 140, characterized by having height 840H(1), may be laminar. The cooling efficiency through this first segment of coolant channel 140 may be improved by incorporating protruding and/or recessed features 848 to introduce turbulence. In one implementation, protruding features 848 are implemented in the surface of housing 122 facing gain medium 110, as shown in
The performance of cooling element 820, with indium layer 150, was evaluated experimentally and compared to the performance 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 respective conventional water-cooled solid copper blocks. With an optical pump power of approximately 220 watts, the conventional solid copper blocks maintained a gain-medium temperature of approximately 100° C. When the same gain medium was implemented in device 100 and cooled by two cooling elements 820, it was possible to pump the gain medium with a higher pump power, approximately 250 watts, and yet maintain a lower gain-medium temperature of approximately 70° C.
Without departing from the scope hereof, any one of the laser gain devices disclosed above may be operated with a coolant flow propagating in the direction opposite to the propagation direction of pump beam 162, that is, with the coolant entering coolant channel 140 via outlet 144 and exiting via inlet 142. At least when the 1/e absorption length of pump beam 162 in gain medium 110 is less than length 210L of gain medium 110, the cooling performance of this counter-propagating coolant flow is likely inferior to that of the co-propagating coolant flow discussed above. However, even with a counter-propagating coolant flow, the laser gain devices still benefit from other advantages, such as excellent and reliable thermal contact between gain medium 110 and the coolant, as well as minimal mechanical stress on gain medium 110.
The present invention is described above in terms of a preferred embodiment and other embodiments. The invention is not limited, however, to the embodiments described and depicted herein. Rather, the invention is limited only by the claims appended hereto.
This application claims priority to U.S. Provisional Application Ser. No. 63/203,438, filed Jul. 22, 2021, the disclosure of which is incorporated herein by reference in their entirety.
Number | Date | Country | |
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63203438 | Jul 2021 | US |