Monolithic telescope, a thin disk laser, use thereof for compensation of aberration and method for compensation of the off-axis aberration

Abstract

The present invention relates to a monolithic telescope (22) for compensating off-axis aberrations in a thin disk laser gain module comprising a parabolic mirror (15) and a gain medium. The monolithic telescope (22) features two spaced-apart, cylindrical, concave surfaces in the direction of the pump source, and convex in the direction of the parabolic mirror (15), each surface having distinct radii (R1 and R2, where R1 > R2). This configuration enables the telescope to shape and direct a pump beam toward the parabolic mirror (15), compensating for aberrations introduced by the mirror and ensuring a circular, evenly distributed pump spot on the thin disk gain medium. The invention also includes a method of using the monolithic telescope (22) in a thin disk laser setup to achieve multiple amplifications of the pump beam, enhancing the efficiency of the laser output.

Description

Monolithic telescope, a thin disk laser, use thereof for compensation of aberration and method for compensation of the off-axis aberrationTechnical field

[0001] The present invention pertains to an optical element designed to address and compensate off-axis aberrations of a parabolic mirror in an optical system, in particular in a thin disk laser comprising an active medium. More particularly, the present invention relates to a monolithic telescope compensating off axis aberration of a parabolic mirror in a thin disk laser amplifier.

[0002] The second aspect of the present invention relates to a laser system for amplification of a pump beam in a laser head having off-axis parabolic mirror configured to direct the pump beam from a pump beam source to a thin disk.

[0003] The third aspect of the present invention relates to the use of the monolithic telescope for compensation of a parabolic mirror in a thin disk laser amplifier.

[0004] The forth aspect of the present invention relates to a method for compensation the off-axis aberration in a thin disk laser medium.State of the art

[0005] High-power solid-state lasers generally depend on the availability of optical pump beams with high optical power and brightness, i.e., the required pump power needs to be made available in as small space as possible.

[0006] It is a general technical desire to optically couple a fibre to a thin disk active medium.This is generally achieved by a system of optical elements, such as lenses, parabolic mirrors, optical fibres etc. However, these optical elements are causing off-axis aberrations. In particular, a laser head, with the meaning of gain module (or laser module, pump chamber) comprises a mounted gain medium and assemblies for directing the pump beam to the thin disk. The assemblies for directing the pump beam to the gain medium can be a parabolic mirror, as described in Martin Smrz, Jiri Muzik, Denisa Stepankova, Hana Turcicova, Ondfej Novak, Michal Chyla, Petr Hauschwitz, Jan Brajer, Jan Kubat, Filip Todorov, and Tomas Mocek, "Picosecond thin-disk laser platform PERLA for multi-beam micromachining, " OSA Continuum 4, 940-952 (2021), https:AdQixxg / 10, 1364 / OSACA18293. However, the use of parabolic mirror in the laser head often introduces off-axis aberration.

[0007] Off-axis aberration refers to optical aberrations that occur away from the optical axis of an optical system. The optical axis is an imaginary line passing through the centre of the optical components, such as lenses or mirrors. When light rays deviate from this axis, various types of aberrations can occur, affecting the quality of the image formed by the optical system. Considering the parabolic mirror used in the laser head, the off-axis aberration introduces elliptical pump spot, which is undesired during amplification. In a prior art, the off-axis aberration is usually compensated by means of adaptive optics, a cylindrical lenses telescope or by an anamorphic prism pair.

[0008] Telescopes, also called as laser beam expanders, are known to a skilled person in the art. They are of two types: reflective and refractive. An example of refractive telescope according to state of the art is shown in Fig. 1. Fig. 1 shows a coupling system 10 between laser diode stack 11 providing a diverging pump beam and an amplifier 14. The telescope 10 typically consists of two main components: a first cylindrical lens 12 and a second cylindrical lens 13. The first lens 12 is the primary component responsible for converting the diverging pump beam emitted by a laser diode stack 11 into a parallel pump beam. The use of the telescope requires high precision alignment.

[0009] US2007177260A1 discloses a telescope for a laser beam. Fig. 4 of the document shows a telescope with monolithic lens having a predetermined length and an input radius which, with the index of refraction of the material of the lens, and the wavelength at which the lens is to be operated, defines the singlet lens structure. It has been found that such a lens meets and exceeds the anti-aberration requirement especially for the application in which a pumping laser is to pump an optical parametric oscillator. The telescope has only two reflective surfaces. The surfaces are convex in the direction of the pumping laser. The single telescope can be used for magnification or demagnification for focusing the energy from a laser pump source into a nonlinear crystal of an optical parametric oscillator without affecting efficiency. However, only single pass through the active medium is disclosed.

[0010] Another monolithic cylindrical lens is disclosed in US5973853. FIG. 7 of the disclosure shows an isometric view of a monolithic cylindrical lens improving the alignment of the laser beam, in particular a monolithic cylindrical lens suitable for optical coupling arrangement. The monolithic lens replaces a conventional collimating lens and is capable to transform an elliptical beam output from a laser source into an essentially symmetrical beam suitable for coupling into a single mode fibre. However, themonolithic lens is provided with input and output cylindrical surfaces, wherein each surface has protrusion.

[0011] A laser head generates laser radiation by utilizing a laser diode as the pump source and an active medium to amplify the light. An example of such a laser head according to the state of the art is shown in Fig. 2. Fig. 2 in particular shows an example of a laser head 14 a laser diode stack 11 , which emits high-intensity light, which is used to excite the atoms or molecules in the active medium 17, resulting in stimulated emission and the generation of coherent laser beam 18. The laser diode stack 11 serves as the pump source. It is a compact semiconductor device that emits intense light when an electric current passes through it. The active medium 17 is the material within the laser head that undergoes stimulated emission to produce laser light. The laser head 14 includes an optical cavity, which is formed by two mirrors 15 and 16. One mirror is highly reflective mirror 16, while the other mirror is a parabolic mirror 15 provided with a hole for coupling out a laser beam 18. The mirrors 15 and 16 are precisely aligned to ensure feedback of the laser light and establish the conditions for stimulated emission and laser oscillation. However, as mentioned above, the parabolic mirror may introduce aberrations.

[0012] A technical problem that can arise in the coupling between a laser diode stack 11 and the active medium 17 is known as poor coupling efficiency. This problem occurs when there is a suboptimal transfer of pump light from the laser diode stack 11 to the active medium 17, resulting in reduced overall laser performance. Example aberration introduced by parabolic mirror in a laser head is shown in Fig. 3. Sub figure A) shows a cross-section of a laser beam emitted from a laser diode stack 11. The cross-section reveals a sharp edge and a top flat beam intensity. In the sub figure B), the observer can see divergence and so called off-axis alignment resulting in non-uniform intensity distribution. In sub figure C), the observer see non-homogenous beam profile incident at active medium. It is desired that a pump beam with sharp edges and circular spot is incident at the gain medium.Summary of the Invention

[0013] The first aspect of the present invention is a monolithic telescope for compensation off-axis aberrations of a thin disk laser gain module as defined by claim 1.

[0014] The monolithic telescope according to the present invention is suitable for compensation off-axis aberration introduced by a parabolic mirror in a thin disk lasergain module. The thin disk laser gain module implicitly comprises at least a source of pump beam, so called a pump source, which is directed to a laser head. The laser head comprises a parabolic mirror, which reflects the pump beam onto a thin disk gain module. The thin disk gain module comprises a gain medium, which can be a crystal of Yb:YAG or Yb:Lu2Os used to amplify the pump beam. Optionally and advantageously, the thin disk gain module may comprises a heat sink for thermal distribution of heat dissipated on the thin disk gain medium. The pump beam is partially reflected back to the parabolic mirror. Due to the sequences of the reflection from the parabolic mirror and planar mirror as substantially described in the prior art, the multiple pass through the gain medium is achieved. The inventors found that parabolic mirror requires a precise alignment. If not, an off axis aberration is introduced, which effectively provides non-circular shape on the gain medium. The inventors found that the compensation of the off axis aberration introduced by the parabolic mirror can be compensated by a monolithic telescope. The monolithic telescope is cylindrical and has two spaced-apart surfaces. Both surfaces are radiused in the same direction. The both curved surfaces are continuous and do not contain any additional protrusions or recesses. When the telescope is used for compensation the off-axis aberration, the telescope is positioned so that the surfaces, which are concave, are facing to the pump source. The surfaces, which are convex, are facing to the parabolic mirror. The monolithic telescope emits the pump beam toward the parabolic mirror. The present invention at least partially compensate the off-axis aberration introduced by the parabolic mirror.

[0015] In a preferred embodiment, the monolithic telescope is provided with the radiused surfaces having two different radius. The first radiused surface is having a first radius Ri and the second surface is having a second radius R2. The first radius R1 is larger than R2, R1 > R2. Even more preferably, the first surface is closer to the pump source while the second surface is closer to the parabolic mirror. In other word, the pump beam emitted from the pump source is incident onto the first radiused surface having the first radius. The pump beam is emitted from the second radiused surface having the second radius toward the parabolic mirror.

[0016] In a preferred embodiment, the ratio of the first and the second radius is equal to the ratio of length of semi-major axis and a length of semi-minor axis of an elliptical spot of the pump beam measured on the thin disk laser gain medium without the monolithic telescope. The preferred embodiment exactly compensate the off-axis aberration ofthe parabolic mirror. A nearly ideal circular pump spot is achieved on a thin disk laser module.

[0017] According to the second aspect of the present invention, a thin disk laser is provided.The thin disk laser comprises a pump source configured to emit a pump beam toward the monolithic telescope according to anyone of the previous embodiment. The thin disk laser comprises a laser head having a parabolic mirror reflecting the pump beam to a gain module as substantially described above.

[0018] The pump source can be a laser diode, a bar or a stack 11 of laser diodes, another laser beam coupled with an optical fibre. In a preferred embodiment, the pump source is a fibre coupled laser diode pump source.

[0019] As mentioned above, the third aspect of the present invention is the use of the monolithic telescope according to any of the previous embodiment for compensation of aberration introduced by a parabolic mirror. More preferably, the use of the monolithic telescope for compensation of aberration introduced by parabolic mirror in a laser head.

[0020] In a fourth aspect of the present invention, a method for compensation off-axis aberration of a parabolic mirror is provided. The method comprises the steps:

[0021] A method for compensation off-axis aberration of a parabolic mirror and providing a circular pump spot on a thin disk laser gain module comprising:a. generating a pump beam;b. directing the pump beam to first concave curved surface, with respect to the pump source, of a cylindrical monolithic telescope, wherein the first concave curved surface is radiused with a firs radius (Ri);c. refracting the pump beam in the monolithic telescope;d. ejecting the pump beam from a second concave curved surface, with respect to the pump source, of the monolithic telescope, wherein the second concave curved surface is radiused with a second radius (R2); and wherein the first radius (R1) is different from the second radius (R2); ande. directing the refracted pump beam to a the parabolic mirror of a laser head comprising a thin disk for laser beam amplification; andf. multiple reflecting of the pump beam between the thin disk and the parabolic mirror, hence multiple amplification of the pump beam; andg. ejecting a laser beam.

[0022] In a preferred embodiment, the first radius Ri is lager than the second radius R2.Brief description of drawings

[0023] Fig. 1 represents a scheme of state-of-the-art telescope configuration.Fig. 2 represents a scheme of state-of-the-art fibre coupled laser diodes to a laser head.Fig. 3 represents a stat of the art cross section profile of a pump beam at: A) fibre end; B) pump spot after a single pass; and C) pump spot superimposing.Fig. 4 represents a monolithic cylindrical telescope according to the present invention.Fig. 5 represents a schematic drawing of the present invention in top view.Fig. 6 represents a schematic drawing of the present invention in side view.Fig. 7 represents resulted cross section profile of a pump beam at: A) fibre end; B) pump spot after a single pass; and C) pump spot superimposing, according to the present invention.Fig. 8 represents a single pass of the pump beam.Detailed description of preferred embodiments

[0024] Fig. 4 illustrates a monolithic cylindrical telescope that serves as a cylindrical telescope for compensation off-axis aberration introduced by a parabolic mirror 15 in a thin disk laser. The telescope is manufactured from a single material body, which can be a suitable transparent material chosen for its optical properties, such as glass or a specialized optical polymer.

[0025] The monolithic cylindrical telescope features two curved surfaces, both of which are radiused in the same direction. These curved surfaces affects the pump beam emitted from a pump source. Radius in the same direction means that both curved surfaces are concave with respect to the incident, resp. refracted optical beam. In therespective figure, the monolithic telescope 22 has the surfaces with radiuses Ri and R2. A pump beam is incident on the first surface having the radius R1, then it propagates through the body of the telescope, and the pump beam leaves the monolithic telescope 22 at the second surface having the second radius R2.

[0026] The monolithic cylindrical telescope as shown in Fig. 4 is a segment of a cylindrical ring or a partial cylinder with curved lateral surfaces. Geometrically, it can be described as a portion of a cylinder, with its curved outer and inner surfaces suggesting a circular cross-section. The curvature of these surfaces is defined by a radii R1 and R2, which determines the extent of the curvature. The monolithic telescope 22 possesses a specific thickness between its outer and inner surfaces, representing the distance separating these two curved faces. Viewed from the side, the monolithic telescope 22 resembles a semicircular cross-section, as if it were a cylindrical ring cut along its central axis. This configuration provides symmetry along the cylindrical axis.

[0027] The both curved surfaces are continuous and do not contain any additional protrusions or recesses. As it can be seen from Fig.4, the first surface being radiused R1 is a cylindrical surface. The first surface does not contain any planar surface nor additional protrusions or recesses. The first surface intends to be an incident surface for a pump optical beam. The second surface is intended as an output surface of the pump beam.

[0028] In an example, the monolithic cylindrical telescope can have a total length of 6.5 mm, and the center thickness is 5 mm. The telescope has a height and width of 10 mm, resulting in a magnification of 0.85. The dimensions mentioned here are provided as examples to demonstrate the concept, and the actual sizes may vary depending on the requirements of the laser system.

[0029] The radiused surfaces are configured with different curvatures, defined by their respective radii, R1 and R2, with R1 being greater than R2. This differentiation in curvature enables the telescope to control the divergence and focusing properties of the pump beam as it travels through the gain medium and is reflected by the parabolic mirror 15. The radiused surfaces thus function together to manage the beam's trajectory and focus, allowing for optimal alignment with the parabolic mirror's 15 reflective surface.

[0030] The monolithic telescope 22 is formed from a single piece material. Suitable material can be crown glass, flint glass, polycarbonate, CR-39 (Columbia Resin 39), acrylic(PMMA), fused silica, and high-performance proprietary glasses such as Zeiss glass. These materials have been chosen for their specific optical properties, including high transparency, low dispersion, and impact resistance, which are critical for enhancing the performance and durability of the lens. The use of these materials allows for the production of lenses that not only provide superior image quality but also meet the demanding requirements of various applications, including eyewear, photographic equipment, and scientific instruments.

[0031] Fig. 5 shows a part of a of a think disk laser being pumped by a pump beam emitted from an optical fiber 11. Both of the above-mentioned surfaces are concave in the direction of the pumping beam emitted from the optical fiber 11. This embodiment focuses on optimizing the control and manipulation of the pumping beam to enhance the performance of laser systems, in particular to correct the off-axis aberration introduced by a parabolic mirror 15 in a thin disk laser head 17. The optical fiber 11, which serves as the source of the pumping laser beam, emits the pump beam toward the monolithic telescope 22. The optical fiber 11 can be equipped with a collimating lens 21. The monolithic telescope 22 is modulating the pump beam and emits the pump beam onto the parabolic mirror 15. The laser head 14 further comprises a thin laser disk module 17, which comprises a thin disk gain medium and preferably a heat sink for thermal distribution of the heat dissipated on the thin disk. The pump beam is multiple reflected by the parabolic mirror 15 and a diffraction mirror 16 back to the thin disk gain medium 17. The cylindrical monolithic telescope 22 is characterized by the specific orientation of its radiused surfaces. When the telescope is in use within the laser gain module, the surfaces exhibit a dual orientation based on their interaction with the pump source and the parabolic mirror 15. The surfaces facing the pump source are concave. This inward curvature helps to collect and direct the incoming pump beam towards the center of the telescope, ensuring that the beam enters the telescope at the correct angle and with the desired level of focus. In contrast, the surfaces facing the parabolic mirror 15 are convex. This outward curvature facilitates the emission of the pump beam towards the parabolic mirror 15. By focusing the beam in the desired direction, the convex surfaces ensure that the beam is appropriately expanded or focused as it exits the telescope and moves toward the mirror. This careful control over the beam's focus and direction mitigates off-axis aberrations and aligns the beam with the gain medium’s optical path. The monolithic telescope 22 is specifically designed to interact with a parabolic mirror 15, which plays a pivotal role in the laser gain module. The telescope’s cylindrical shape and radiused surfaces work together to emit the pump beam towards the parabolic mirror 15 with optimalalignment. The mirror reflects and focuses the beam back through the gain medium, where it undergoes amplification. By ensuring that the beam follows the intended optical path, the telescope reduces aberrations that would otherwise distort the laser output. The different radii of the two radiused surfaces, Ri and R2, further enhance this functionality. The larger radius R1 on the first surface allows for a more gradual focusing of the beam as it enters the telescope, while the smaller radius R2 on the second surface facilitates a tighter focus as the beam exits towards the parabolic mirror 15. This configuration enables fine-tuned control over the beam’s divergence and focus, tailored to the specific requirements of the laser gain module. The monolithic structure of the telescope offers several key advantages in high-precision laser applications. By integrating both radiused surfaces into a single, solid piece, the telescope eliminates the need for complex alignment procedures, ensuring stable and accurate positioning of the optical components. This stability is essential for maintaining beam quality over time, particularly in high-power laser systems where slight misalignments can lead to significant aberrations and reduced performance. By this, a stable laser beam 18 can be achieved.

[0032] Fig. 6 represent the schematic drawing of the same embodiment as shown in Fig. 5 in side view.

[0033] The embodiment’s objective is to achieve a final circular pumping beam. The monolithic cylindrical telescope, in conjunction with the aspheric lens collimator 21, effectively manipulates the pumping laser beam to shape it into a circular profile. This circular pumping beam is then directed towards the laser head 14, specifically targeting the laser active medium 17.

[0034] Unlike conventional cylindrical telescopes constructed using two separate cylindrical lenses, which often require complex configurations and meticulous alignment, the monolithic cylindrical telescope of the present invention offers a simplified and more efficient solution. By utilizing a single solid body with two concave surfaces of different radii, the monolithic cylindrical telescope can easily correct the optical aberration and achieve the desired circular pump spot shape.

[0035] In an embodiment, Figs. 7A, 7B, and 7C illustrate the cross-sectional profile of the optical beam as it propagates through different stages of the thin disk laser according to the present invention. Figure 7A shows the cross-section of the optical beam at the output of the optical fiber. At this stage, the beam profile is primarily determined by the characteristics of the fiber itself, displaying a certain divergence and intensitydistribution as it exits the fiber. Figure 7B illustrates the cross-section of the beam at the output of the monolithic telescope 22 after a single reflection on the parabolic mirror 15, as depicted in Figure 8. This stage demonstrates how the monolithic telescope, in conjunction with the parabolic mirror 15, reshapes and redirects the beam, adjusting its focus and alignment to meet the requirements of the system. The beam profile here reflects the initial compensation for off-axis aberrations introduced by the telescope design. Figure 7C shows the cross-section of the beam on a screen after multiple reflections between the thin disk and the parabolic mirror 15. At this stage, the beam has undergone several interactions with the gain medium and mirror, resulting in a refined profile suitable for laser amplification. The repeated reflections enable precise alignment and beam shaping, producing a stable, uniform output ideal for high-power laser applications. These figures collectively demonstrate the progression of the beam's cross-sectional profile as it moves from the fiber through the monolithic telescope, reflecting off the parabolic mirror 15, and finally, interacting with the gain medium. This progression highlights the effectiveness of the monolithic telescope 22 and parabolic mirror 15 setup in achieving optimal beam quality and stability.

[0036] Fig. 8 represents an embodiment showing a single pass of the pump beam as mentioned in connection to Fig. 7B.

[0037] In an another embodiment, a method for compensation off-axis aberration of a parabolic mirror 15 is described herein. The method is providing a circular pump spot on a thin disk laser gain module. The method is in particular suitable for compensating off-axis aberration of a parabolic mirror 15 and creating a circular pump spot on a thin disk laser gain module. The method aims to enhance the precision and efficiency of the pumping process by generating a circular and evenly distributed pump spot on the thin disk, thereby achieving optimal laser amplification. In an embodiment, the method comprises the following steps: generating a pump beam followed by directing the pump beam to the first concave curved surface of a cylindrical monolithic telescope, preferably the telescope as described in any of the above mentioned embodiment. The method continues by a step of refraction of the pump beam in the monolithic telescope 22 followed by ejecting the pump beam from a second concave curved surface of the monolithic telescope. The method continues by a step directing the refracted pump beam to the parabolic mirror 15 of the laser head and multiple reflection of the pump beam between the thin disk and the parabolic mirror 15. The method step is finished by a step of ejecting the amplified laser beam 18.

[0038] In some embodiment, the method begins by generating a pump beam, which serves as the source of energy that will be used to amplify the pump beam within the thin disk laser gain module. The pump beam can be produced by a high-power diode laser or another suitable pumping source designed to deliver a specific wavelength and beam profile suited to the gain medium in the thin disk laser.

[0039] The generated pump beam is then directed toward the first concave curved surface of a cylindrical monolithic telescope. This monolithic telescope 22 is designed with two concave surfaces that help to shape and control the pump beam before it is directed to the thin disk. The concave surfaces are positioned with respect to the pump source. Hence, the convex surfaces are with respect to the laser head as schematically illustrated on Fig. 5 or Fig. 8B by bulging part of the telescope 22.

[0040] The first concave surface, with respect to the pump source, has a first radius Ri. The curvature of this surface is chosen based on the desired focusing characteristics and is tailored to ensure the pump beam is appropriately compressed or expanded as it passes through this surface.

[0041] As the pump beam enters the cylindrical monolithic telescope, it undergoes refraction.The refractive index of the telescope material and the curvature of the first concave surface work together to modify the beam's properties, altering its trajectory and focusing characteristics. This refraction compensates potential aberration introduces by the parabolic mirror 15 and prepare the pump beam for an optimized reflection and amplification process later in the thin disk laser.

[0042] The refracted pump beam exits the telescope through a second concave curved surface, which is also shaped to influence the beam's properties further. This second surface has a different radius of curvature R2 from the first surface R1. The difference between R1 and R2 is critical, as it creates a beam with the correct divergence and alignment needed to counteract off-axis aberrations when the beam interacts with the parabolic mirror 15.

[0043] The second surface's radius R2 is selected to adjust the beam’s divergence to ensure that the resulting spot on the thin disk is circular. The difference in radii (R1 R2) is an intentional design choice to control beam focusing asymmetrically, correcting off-axis aberrations that could otherwise lead to beam distortions or uneven pumping.

[0044] After exiting the monolithic telescope, the pump beam is directed to a parabolic mirror 15 integrated into the laser head. This parabolic mirror 15 reflects the pump beamtoward the thin disk gain medium. The parabolic mirror 15 is positioned and shaped to produce a focused reflection of the pump beam directly onto the thin disk, ensuring an even distribution across the disk’s surface.

[0045] Compensation of off-axis aberration: The parabolic mirror 15 can cause off-axis aberration, where parts of the pump beam could focus unevenly due to the mirror’s shape. However, the cylindrical monolithic telescope’s design, particularly the differing radii of its concave surfaces, compensates for this aberration, helping to maintain a uniform and circular spot on the thin disk.

[0046] The pump beam is reflected multiple times between the thin disk and the parabolic mirror 15. Each reflection allows the pump beam to interact with the gain medium in the thin disk, transferring energy to the laser-active material and amplifying the beam with each pass.

[0047] As the pump beam repeatedly strikes the thin disk, it continues to undergo amplification, increasing the overall energy and power of the resulting laser output. This repeated amplification is made possible due to the highly reflective nature of the thin disk’s back surface and the precise alignment provided by the parabolic mirror 15.

[0048] After multiple reflections and successful amplification within the laser head, the fully amplified laser beam 18 is ejected.

[0049] This method provides a way to focus and amplify a pump beam on a thin disk laser gain module. By compensating for off-axis aberrations and generating a circular, even pump spot, the method is well-suited for applications requiring high-power, stable laser beam 18.

Claims

Claims1. A monolithic telescope (22) for compensation off-axis aberrations of a thin disk laser gain module comprising a parabolic mirror (15) and a gain medium, wherein• the monolithic telescope (22) is having two spaced-apart surfaces radiused in the same direction; characterized in that• the monolithic telescope (22) is cylindrical; and wherein, when the telescope is usedo the surfaces are concave in the direction of the pump source;o the surfaces are convex in the direction of the parabolic mirror (15);wherein• the monolithic telescope (22) is configured to emit the pump beam toward the parabolic mirror (15).

2. The monolithic telescope (22) according to claim 1, wherein a first radiused surface is provided with a first radius (Ri) and a second radiused surface is provided with a second radius (R2); and wherein R1 > R2.

3. The monolithic telescope (22) according to claim 1, wherein the pump beam emitted from the pump source is incident onto the first radiused surface having the first radius (R1); and the pump beam is emitted from the second radiused surface having the second radius (R2) toward the parabolic mirror (15).

4. The monolithic telescope (22) according to claim 2 or 3, wherein Ri / R2=a / b, wherein a is a length of semi-major axis and b is a length of semi-minor axis of an elliptical spot of the pump beam measured on the thin disk laser gain medium without the monolithic telescope.

5. A thin disk laser comprising— a pump source emitting a pump beam toward the monolithic telescope (22) according to anyone of the preceding claim; and— a laser head comprising a parabolic mirror (15) reflecting the pump beam to a gain medium.

6. The thin disk laser according to claim 5, wherein the pump source is a fibre coupled laser diode pump source.

7. Use of a monolithic telescope (22) according to anyone of the claims 1 - 4 for compensation of aberration introduced by a parabolic mirror (15).

8. Use of the monolithic telescope (22) according to claim 7 for compensation of aberration introduced by parabolic mirror (15) in a laser head.

9. A method for compensation off-axis aberration of a parabolic mirror (15) and providing a circular pump spot on a thin disk laser gain module comprising:— generating a pump beam;— directing the pump beam to first concave curved surface, with respect to the pump source, of a cylindrical monolithic telescope (22), wherein the first concave curved surface is radiused with a firs radius (Ri);— refracting the pump beam in the monolithic telescope;— ejecting the pump beam from a second concave curved surface, with respect to the pump source, of the monolithic telescope, wherein the second concave curved surface is radiused with a second radius (R2); and wherein the first radius (R1) is different from the second radius (R2); and— directing the refracted pump beam to a the parabolic mirror (15) of a laser head comprising a thin disk for laser beam (18) amplification; and— multiple reflecting of the pump beam between the thin disk and the parabolic mirror (15), hence multiple amplification of the pump beam; and— ejecting a laser beam (18).

10. The method according to claim 9, wherein the first radius (R1) is lager than the second radius (R2).

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