Patent Application
20230238762
The present invention is in the field of high-power laser amplifiers. More specifically, it concerns solid-state laser amplifiers of the slab type.
Slab laser amplifiers constitute a class of solid-state laser amplifiers characterized by a gain medium having the form of a slab, with one dimension being substantially smaller than the other dimensions. In a slab amplifier, the laser signal which is amplified propagates inside the gain medium, substantially parallel to the slab plane, often performing a zig-zag path inside the slab and achieving significant amplification.
Slab laser amplifiers are particularly well suited for high-power applications, since the geometry of the gain medium, featuring a large surface relative to its volume, allows for an efficient heat extraction. This feature together with the compactness, mechanical robustness and high gain characteristics of this technology makes it particularly attractive for industrial applications.
Slab amplifiers can be classified according to the pumping geometry in two main categories: face-pumped, when the pump light is provided through the main (large) faces of the slab, and edge-pumped, when the pump light is provided through the narrow faces of the slab. Face pumping is particularly well adapted for lamp pumping systems, which used to be the standard technology a few decades ago. Typically, the slab would be cooled by a pump-transparent refrigerating liquid, directly in contact with the gain medium. More recently, with the emergence of high-power laser diodes, the edge-pumping scheme has become the gold standard. Indeed, the good spatial coherence of laser diodes allows an efficient injection of the pump radiation through the narrow edges of the slab. The large faces of the slab are thus available for heat extraction through a solid heat sink, typically a cooled copper mount. This has important advantages in terms of compactness and simplicity of the assembly, compared to direct liquid cooling of the gain medium.
Face-pumping of slab amplifiers based in diode laser pumping has been also proposed, for example in US2014211301A, the pump is provided through one face of the slab, whereas the opposite face is used for heat extraction through a heat sink. In this scheme, only one of the two large faces of the slab is available for heat extraction, whereas the other one is available for pump entry. This “single-side” limitation can be critical when dealing with high power amplification, above some hundreds of Watts of pump power.
Another reason why edge-pumping schemes are more often implemented than face-pumping concerns the optical path of the pump across the gain medium. In order to achieve high pump absorption, thus high gain, the path of the pump within the gain medium must be maximized. If the pump is simply launched perpendicular to the thin slab plane, only a small fraction of it may be absorbed and contribute to the amplification of the laser signal. In an edge-pumping configuration, the pump is launched along the plane of the slab and can be efficiently absorbed.
Despite these advantages, edge-pumping schemes also present drawbacks with respect to face-pumping. In edge-pumping schemes not only the seed, but also the pump beam must be carefully aligned across the slab geometry. Many prior art documents deal with schemes for simplifying the issue of pump alignment into slab laser amplifiers. For example, in U.S. Pat. No. 6,094,297A the pump beam is provided at the end of the slab, perpendicular to its plane like in a face pumping scheme, but it is internally reflected along the slab plane by end mirrors provided at an angle of 45°. Within the slab, the pump then propagates harnessed by total internal reflection, as it is often implemented in edge-pumping configurations.
Another disadvantage of the edge-pumping schemes is the un-even absorption of the pump across the gain medium. Close to the entry side, the pump intensity is more important than towards the center or opposite edge of the slab. This may lead to an inhomogeneous gain profile across the slab with possible thermal gradients and thermal lensing effects which may result in the distortion of the seed beam in the amplifier.
There is thus a need for a solution featuring the simplicity of assembly and alignment of a face-pumped slab amplifier, without running into the problems known from the prior art; namely, the limited available surface for heat extraction, and the loss of pump power due to a short optical path of the pump through the absorbing gain medium.
An aim of the present invention is to provide a face-pumped solid-state slab laser amplifier which overcomes the problems of the prior art. To this end, a laser amplifier device is disclosed here comprising an amplification element, which comprises a gain medium where a laser signal can be amplified. Said amplification element comprises a first and a second main faces, separated from each other by a distance which is smaller than their lateral dimensions.
In one aspect, the amplification element comprises a first and a second main faces, separated from each other by a distance which is at least 10 times smaller than their lateral dimensions. This thin-plate geometry of the amplification element allows for an efficient heat extraction.
The laser amplifier further comprises a solid-state heat spreader thermally connected to the first main face of the amplification element and substantially covering the surface of said first main face. Said heat spreader is optically transparent to a pump light being able to optically excite the gain medium of the amplification element. Said heat spreader features a good thermal conductivity and is also in thermal contact with a heat sink of the laser amplifier.
The laser amplifier further comprises a first reflector substantially covering and facing the first main face of the amplification element, and a second reflector substantially covering and facing the second main face of the amplification element; said reflectors being able to reflect the pump light for at least a range of incidence angles.
More particularly, the laser amplifier device is characterized by the fact that when the pump light is directed into the amplification element, it passes through the transparent heat spreader and through the first reflector. Furthermore, the first and second reflectors are configured to produce multiple reflections of said pump light across the amplification element, between the first and second reflectors. The first reflector is not in physical contact with the amplification element and the solid-state heat spreader.
According to an advantageous embodiment, the amplification element of the laser amplifier device may comprise a gain medium layer, sandwiched between two surrounding layers; the gain medium layer constituting the gain medium of the amplification element, and the surrounding layers being made from a transparent material approximately matching the index of refraction of the gain medium layer.
In such embodiment, the gain medium layer may advantageously feature a thickness between 100 μm and 3 mm; preferably, between 200 μm and 300 μm, whereas each of the two surrounding layers may present a thickness between 200 μm and 1 mm, preferably between 200 μm and 300 μm.
Also, in such advantageous embodiment, the gain medium layer, may comprise a doped ceramic or crystalline material, whereas the surrounding layers may comprise the same undoped ceramic or crystalline material.
In particular, said doped ceramic or crystalline material of the gain medium may comprise Yb-doped YAG, whereas the ceramic or crystalline material of the other two layers may comprise undoped YAG.
In one advantageous embodiment, the heat spreader of the laser amplifier device may be made of a material comprising diamond or sapphire.
The first reflector may be advantageously provided as an independent element, either a self-standing part of the system, or fixed on another part.
In another aspect related to a preferred embodiment, the first reflector of the laser amplifier device may comprise an array of small apertures allowing the passage of a corresponding array of locally spatially confined pump beams into the amplification element.
In that case, the first reflector may comprise a substrate comprising an array of tap-holes defining said array of small apertures and coated on at least one side with a reflective coating.
Alternatively, in such preferred embodiment, the first reflector may comprise a transparent substrate, comprising a patterned highly reflective coating defining said array of small apertures.
In one alternative embodiment, the pump light for the laser amplifier device may comprise a collimated beam, oriented relative to the laser amplifier device such as to provide a predetermined angle of entrance of the collimated beam into the amplification element. In this embodiment, the second reflector is configured making a non-zero reflector angle relative to the first reflector, such that the multiple reflections across the amplification element occur at other angles of incidence different from the predetermined angle of entrance. In this embodiment, the first reflector comprises a dielectric angle-dependent optical coating having transmitting properties for the collimated beam at the predetermined angle of entrance, and having reflective properties for the collimated beam at the different angles of incidence of the multiple reflections.
In another alternative configuration, the laser amplifier device may further comprise a second solid-state heat spreader thermally connected to the second main face of the amplification element and substantially covering the surface of said second main face. In such embodiment, the second heat spreader is also in contact with a heat sink and is optically transparent to the pump light, such that additional pump light can be further directed into the amplification element through the second heat spreader and through the second reflector of the laser amplifier device. This advantageously results in a symmetric device, where both main faces of the amplification element are used for optical pumping and heat extraction, simultaneously.
The present invention further relates to a system comprising a laser amplifier device as described in the previous paragraphs, and a light source configured to generate a pump light adapted to optically excite the gain medium of the amplification element of the laser amplifier device. In one aspect of such system, the pump light is provided to the laser amplifier device, substantially covering the surface of its first main face.
In one preferred embodiment of such system, the light source is configured to generate an array of locally spatially confined pump beams and the first reflector comprises an array of small apertures configured to allow the passage of said array of locally spatially confined pump beams into the amplification element.
According to one example of such preferred embodiment, the light source may comprise an array of optical fibers, each optical fiber comprising an output face emitting one of the locally spatially confined pump beams, and each output face being aligned in front of each of said small apertures.
According to another example of such preferred embodiment, the light source may comprise an array of micro-lenses generating the array of locally spatially confined pump beams, the array of micro-lenses being arranged to focus said locally spatially confined pump beams into said small apertures.
In an alternative embodiment of a system according to the invention, the light source may be configured to generate a collimated beam of pump light.
In another alternative embodiment of the laser amplifier device according to the invention, the first reflector may comprise a dielectric angle-dependent optical coating featuring reflecting or transmitting properties for the pump light according to the angle of incidence.
Further details of the invention will appear more clearly upon reading the description below, in connection with the following figures which illustrate:
In one aspect, the amplification element comprises a first main face 5 and a second main face 6, separated from each other by a distance which is at least 10 times smaller than their lateral dimensions. This thin-plate geometry of the amplification element allows for an efficient heat extraction.
In one aspect, the thickness, i.e., the distance between the first and second main faces 5, 6, of the amplification element can be smaller than 3 mm, preferably smaller than 1 mm. The lateral dimensions of the amplification element can be between 1 cm and 20 cm.
As known from the state of the art, the laser signal 3 may propagate within the amplification element 1 following a zig-zag path to maximize the interaction length with the gain medium, therefore maximizing the process of optical amplification. The zig-zag path may be implemented by multiple reflections of the laser signal 3 on external mirrors (not shown in
The amplification element 1 is thermally connected to a heat spreader 8, enabling heat transfer from the amplification element 1 to the heat spreader 8. The thermal contact is provided through the first main face 5 of the amplification element 1. The contact surface between the amplification element 1 and the heat spreader 8, substantially covers the whole area of said first main face 5, at least over the region where the amplification element 1 is optically pumped.
The thermal contact between the amplification element 1 and the heat spreader 8 is achieved by intimate mechanical contact over the surfaces. The surfaces can be either directly contacted or bonded. Either the amplification element 1 or the heat spreader 8, or both parts may be provided with a coating having specific optical or mechanical or chemical properties extending along the surface of contact 5. Also, a thin layer of adhesive may be applied between the two parts to provide the mechanical bond and the heat transfer function.
According to one aspect of the invention, the heat spreader 8 is made of an optically transparent material for the wavelength of a pump light 12 used to excite the gain medium of the amplification element 1. A face-pumping scheme of the slab is implemented by transmitting the pump light 12 through the heat spreader 8 onto the main face 5 of the amplification element 1.
Advantageously, according to one aspect of the present invention, the pump light 12 is distributed along substantially the whole surface of the main face 5 of the amplification element 1. In terms of optical pumping, this enables the delivery of very high optical powers into the amplifier without implementing an extreme focusing of the pump light into the entry faces of the slab, as happens in edge-pumping schemes. At the same time, the distribution of the incident pump light 12 across the large surface 5 of the amplification element 1, results in a higher homogeneity of the available pump energy across the whole volume of the gain medium, than can be obtained in other configurations.
Examples of suitable materials for the heat spreader 8 can be: diamond or sapphire.
As shown in
Further heat extraction from the system may be provided by an additional heat sink 11, thermally connected to the amplification element 1 through the second main face 6 of the amplification element 1.
Given the thin geometry of the amplification element 1, the pump light 12 traveling in a direction perpendicular to the main faces 5, 6 undergoes a rather short interaction with the gain medium in a single passage through the amplification element 1. This can result in limited absorption of the pump 12 by the gain medium and hence to low efficiency. This problem is solved by the provision of two reflectors 14, 16, disposed on both sides of the amplification element 1, substantially covering the main faces 5, 6 of the amplification element 1. As will be explained in greater details in the following paragraphs, the two reflectors 14, 16 produce multiple reflections of the pump light 12 and therefore, multiple passages of the pump light 12 through the amplification element 1, ensuring high pump absorption. The reflectors 14, 16 act thus as an optical trap, enabling an efficient absorption of the pump energy by the gain medium.
Obviously, the configuration of the reflectors 14 and 16 requires that the pump light 12 enters the space between the two reflectors before undergoing multiple reflections. Several strategies may be implemented to permit this entrance of the pump light 12 through the first reflector 14, before getting trapped in multiple reflections. One of these strategies is depicted in
As the locally confined pump beams 21 cross the reflector 14 through the corresponding apertures 20, they quickly diverge within the space between the two reflectors 14, 16. In the example of
It is understood that each reflection of the pump on the first reflector 14, results in optical losses through the array of entry apertures 20. If the pump beams 12 diverge sufficiently, the fraction of optical power lost at each reflection on the first reflector 14 is roughly proportional to the ratio of total aperture surface to total reflector surface. In a possible embodiment, the apertures may be circular, with 200 μm diameters, and be distributed in a square array separated by 2 mm (center to center) from the adjacent apertures—In this case, the ratio of total aperture surface to total reflector surface results in approximately 1% of optical losses per reflection on the first reflector 14. In contrast to these small losses, in each double passage through the amplification element 1, the pump may lose ca. 10% of its power through absorption. In this manner, after several roundtrips between the reflectors 14, 16, most of the pump power will end-up transferred to the gain medium, with only a minor fraction being lost through the entry apertures 20.
Possible secondary reflections of the pump light at the interface of the heat spreader 8 and the amplification element 1 (not represented in
Several solutions can be conceived for providing an array of locally confined pump beams 21, micro-metrically positioned to correspond with the array of apertures 20 presented in
The example of solution depicted in
The optical fibers 30, may be mounted in standard ferrules or be fixed by any other known means to the fixation mount 32. At the output face 37 of the fibers 30, the pump beams 21 are spatially confined with a waist diameter roughly corresponding to the diameter of the optical core of the fibers 30. Conveniently, high-power laser diodes are commonly coupled to highly multimode fibers featuring core diameters between 100 μm and 400 μm. The output beams 21 often present a top-hat transversal beam profile and diverge rapidly with typical numerical apertures above 0.15, all of which contribute to a rapid and homogeneous distribution of the pump power after crossing the reflector 14 through the apertures 20.
If no focusing means are provided, the first reflector 14 with the apertures can be conveniently arranged as close as possible to the output faces 37. In one advantageous embodiment of this solution, as illustrated in
It can be worth pointing out that the reflectors 14 and 16 need not necessarily be in direct contact with the amplification element 1 or the heat spreader 8, as long as the multiple reflections of the pump 12 between the two reflectors 14, 16, undergo multiple passages through the amplification element 1, according to the invention.
To reduce reflection losses at the heat spreader/air interface in the multiple passages that the pump beams 12 undergo between the reflectors 14, 16, the heat spreader 8 can be advantageously treated with an anti-reflection coating 34 at this interface.
In one advantageous embodiment of the example of
In this example, the first reflector 14 will be preferably arranged at the position of the focal plane of the focused pump beams 21. The distance between said focal plane and the lenses 36 can range from few tens of microns to several millimeters or even few centimeters. The first reflector 14 could be directly constructed on, or bond to an optical element of the laser amplifier device 100; for example, to the heat spreader 8, as in the examples of
The implementation example of the reflector 14 in
The substrate 38 in this example of
In some configurations of the laser amplifier device 100, for example in the embodiment of
It is understood that the techniques described above, allowing the fabrication of a reflective coating 41 comprising an array of apertures 20 on a transparent substrate 39, can be equally applied to fabricate such coating 41 on any specific transparent element of the laser amplifier device 100. For example, with reference to the embodiment depicted in
In the example of
In this example, the pump focussing element 50 has been configured as a transparent substrate of relevant thickness, where the split pump beams 21 propagate, converging towards their corresponding apertures 20. It is understood that alternative embodiments can be contemplated, wherein the pump focussing element 50 comprises a thin substrate, or a support with tap holes where an array of small lenses can be mounted, having no physical contact with other parts of the laser amplifier device 100. The first reflector 14, could for example be provided at the upper surface of the heat spreader 8, or at the lower surface of the pump-focussing element 50, or as a self-standing part without physical contact with either the pump-focussing element 50 or the heat spreader 8.
In some embodiments, the amplification element 1 may advantageously comprise several layers. One example of this is presented in
Very often, the laser signal 3 which is amplified in a solid-state amplifier consists in a nearly Gaussian beam, and it is important for the intended applications that the amplified signal still conserves a nearly Gaussian profile. For example, in laser machining applications it is advantageous to have a Gaussian laser beam, since it can be more tightly focussed than beams presenting other transversal modes.
Theoretically, a Gaussian beam will propagate undistorted through a transparent medium only in the absence of any boundaries. In practice, material boundaries separated from the beam axis by a distance few times larger than the 1/e2 beam width will produce a negligible distortion of said beam. There is thus an interest in providing a gain medium layer 61 sufficiently thick, to avoid significant edge distortion of the signal beam 3 at the boundaries of the gain medium. A thick gain medium layer 61 however is in contradiction with the goal to maximize the power efficiency of the amplifier. Indeed, if a large gain medium is provided, where the laser signal can propagate without interacting with the boundaries, all the pump power absorbed by the gain medium far from the optical axis is basically lost for the amplification purposes.
The composite structure of the amplification element 1 presented in
Advantageously, the surrounding layers 60, 62 could feature a gradient concentration of doping ions (e.g., Yb3+) which at the interface with the gain medium layer 61 equals the doping concentration of the gain medium layer 61. The concentration of the doping ions can then progressively decay to zero from said interface with the gain medium layer 61 into the bulk of the surrounding layers 60, 62.
In another aspect, the layers 60, 61, 62 in the example of
Another important advantage of a composite structure of the amplification element 1 as presented in
Typical dimensions of a structured amplification element 1 according to the example of
The gain medium layer 61 preferably presents a thickness between 100 μm and 3 mm; more preferably, between 200 μm and 300 μm. The surrounding layers 60, 62 preferably present a thickness between 200 μm and 1 mm, more preferably between 200 μm and 300 μm.
A large, collimated pump beam 12 is directed towards the heat spreader 8 at an angle such that, upon reaching the first reflector 14, the collimated pump light is transmitted into the amplification element 1 with a predetermined angle of entrance (θ) which is, in this example, slightly larger than the transmission threshold value AOITh. In
In this embodiment, the amplification element 1 presents a small wedge, defining an angle α between the first and second reflectors 14, 16, which in practice can be comprised between 1° and 10°, preferably between 1° and 5°. When the pump ray 70 is reflected on the wedged plane of the second reflector 16, it returns towards the first reflector 14 following a trajectory 71 which defines a new, other AOI (β) on the first reflector 14 which in the present example is lower than the angle of entrance θ. Provided that this new other AOI β is lower than the reflector angular threshold AOITh, the reflected beam 71 will be efficiently reflected back through the amplification element 1. Further reflections of the beam will impinge on the first reflector 14 at increasingly lower AOI and will therefore be also reflected, creating an optical trap effect.
The reflection-tilting effect of the wedge may even result at some point in a backwards lateral propagation of the rays (not represented in the figure), where the AOI starts increasingly growing in the opposite direction until the reflected rays could eventually escape from the wedge trap when the AOI exceeds again the reflector angular threshold AOITh. Obviously, the system can be dimensioned to ensure that substantially all the pump energy has been absorbed in the gain medium 61 before backward reflections can escape the wedge trap.
In another aspect of this configuration, the face of the heat spreader 8 where the pump light 12 initially impinges, can be advantageously provided with an anti-reflection coating 34 to increase the efficiency of pump transmission.
As can be understood from the previous examples of
The strategy of angular trapping of the multiple reflections of the pump, generally requires arranging the incoming pump beam 12 at a defined angle with respect to the laser amplification device 100. This may possibly influence design aspects of the system mounting, which can be adapted by the skilled person as needed. For illustration purposes in
Conveniently, the arrays of apertures 20 and 20′ can be laterally shifted at intercalated positions as indicated in
In the example of
Many embodiments similar to the example of
The thermal contact of the amplification element 1 to a heat sink 11 (
Contacting the transparent heat spreader 8 with the amplification element 1, on the other hand, requires a pump transparent interface. This can be done with a thin layer of transparent adhesive or via bonding techniques, such as hydroxide catalysis bonding. In some cases, and depending on the materials, the deposition of an additional layer on the amplification element 1 and/or on the heat spreader 8 might be required to ensure the bonding. The nature of this additional layer depends on the materials of the amplification element 1 and the heat spreader 8. For example, the bonding of a diamond heat spreader 8 and a YAG-based amplification element 1 can be mediated by a thin (typically 1 μm) layer of a transparent oxide material (SiO2, TaO2, etc) deposited on one or the two substrates that are then bonded with, for instance, hydroxide catalysis bonding.
The bonding of multiple layers 60, 61, 62 constituting in some embodiments the amplification element 1, may be done by standard composite crystals bonding techniques, for example, thermal diffusion bonding of Yb:YAG (gain medium layer 61) with YAG (surrounding layers 60, 62).
| Number | Date | Country | Kind |
|---|---|---|---|
| 21199345.6 | Sep 2021 | EP | regional |
