Laser system with monolithic optical collimation and circularization device

Abstract

The invention relates to a laser system (1) comprising a solid amplifying medium (2) capable of emitting an amplified beam (4) along a propagation direction (D), the amplified beam having a first divergence angle in a first plane (Px) including the propagation direction, and a second divergence angle in a second plane including the propagation direction and distinct from the first plane, the second divergence angle being distinct from the first divergence angle; and an optical device (3). According to the invention, the optical device comprises a lens (31) arranged to refract the amplified beam into an output beam (5), the lens having a first radius of curvature in the first plane and a second radius of curvature in the second plane, the second radius of curvature being distinct from the first radius of curvature. Figure for the abstract: Fig. 1

Description

Title of the invention: Laser system with monolithic optical collimation and circularization device technical field

[0001] The present invention relates generally to the technical field of optics.

[0002] It relates more particularly to a laser system.

[0003] The invention finds a particularly advantageous application in the realization of laser systems based on a plate-amplifying medium. Technological background

[0004] A laser system (from the English acronym "light amplification by stimulated emission of radiation") classically comprises an amplifying medium, for example a solid, designed to emit a spatially and temporally coherent beam of light. Such a beam is then often also referred to as a "laser".

[0005] In the case of an amplifying medium lacking rotational symmetry about the propagation axis, the emitted light beam is often astigmatic. The light beam may then exhibit different divergence or aperture angles in different planes including the beam propagation direction. Thus, the light beam emerging from a solid amplifying medium without rotational symmetry is often elliptical and / or astigmatic.

[0006] To make this light beam circular (ellipticity close to 1), or simply stigmatic, and less divergent (collimated), laser systems include optical beam-shaping devices. Such an optical device generally requires three or four lenses that successively refract the light beam. Some lenses circularize the beam by modifying the beam's divergence in a given direction, while other lenses collimate the beam and correct the intrinsic astigmatism or astigmatism caused by the different lenses.

[0007] However, such optical beam shaping devices are expensive, complex to adjust and have a considerable size. Summary of the invention

[0008] In this context, the present invention proposes a laser system comprising: - a solid amplifying medium capable of emitting an amplified beam along a propagation direction, the amplified beam having a first divergence angle in a first plane including the propagation direction, and a second divergence angle in a second plane including the propagation direction and distinct from the first plane, the second angle of divergence being distinct from the first angle of divergence; and - an optical device comprising a lens arranged to refract the amplified beam into an outgoing beam, the lens having a first radius of curvature in the first plane and a second radius of curvature in the second plane, the second radius of curvature being distinct from the first radius of curvature.

[0009] Thus, thanks to the invention, the optical beam-shaping device is simplified. Indeed, the lens implemented by the optical device makes it possible to simultaneously modify the two divergence angles of the incident beam. Therefore, the amplified beam can be circularized and collimated by a reduced number of lenses, preferably by a single lens.

[0010] Consequently, although it has less freedom of adjustment, the optical device for shaping the laser system according to the invention is inexpensive, simple to adjust and has a small footprint.

[0011] In the laser system according to the invention, a single lens can thus replace a complex optical shaping system consisting of at least three or four lenses. The lens is then manufactured to correct the defects of a particular laser system and, although it offers fewer adjustment possibilities, it limits the risks of misalignment.

[0012] Other advantageous and non-limiting features of the laser system according to the invention, taken individually or in all technically possible combinations, are as follows: - the outgoing beam has two divergence angles respectively in the first plane and in the second plane, and the first radius of curvature and the second radius of curvature are determined, on the basis of the first divergence angle and the second divergence angle, so as to achieve at least one of the following criteria: a difference between the two divergence angles of the outgoing beam is less than a first threshold value, at least one of the two divergence angles of the outgoing beam is less than a second threshold value; - the first radius of curvature and the second radius of curvature are determined in such a way that the outgoing beam is less astigmatic than the amplified beam; - the amplified beam has a circular cross-section perpendicular to the direction of propagation in which the amplified beam is circular in shape, and the lens is positioned so as to intersect the circular cross-section; - the first plane is perpendicular to the second plane; - between the amplifying medium and the optical shaping device, the amplified beam is divergent in the first plane and convergent in the second plane; - the lens comprises a first optical face forming the first ray of curvature and a second optical face, opposite the first optical face, forming the second radius of curvature; - at least one of the first optical face and the second optical face extends along a cylindrical surface of revolution; - the lens comprises a first flat optical face and a second optical face, opposite to the first optical face, forming the first radius of curvature and the second radius of curvature; - the second optical face extends along a toric surface; - the first radius of curvature and the second radius of curvature are each between 1 mm and 1000 mm; - the amplified beam comprises a central wavelength, and the lens comprises optical faces whose roughness is less than a quarter of the central wavelength; - the lens is made of silica with an absorption of less than 10-5 cm-1 for a wavelength between 900 nm and 1100 nm; - the amplified beam has a Gaussian profile along a transverse direction perpendicular to the direction of propagation; - the optical shaping device consists of the lens; - the lens has a thickness, depending on the direction of propagation, of between 2 mm and 4 mm; - the lens is made of electronic-grade silica; - the lens is arranged so that the amplified beam illuminates a region of the lens with a surface area between 9 mm2 and 40,000 mm2; - the lens includes at least one of the coatings having a reflectance at normal incidence of less than 0.1% at 1030 nm; - the amplified beam is a pulsed beam whose emission duration is between 100 fs and 20 ns; - the solid amplifying medium comprises a rectangular parallelepiped crystal and the lens is positioned opposite an exit slice of the crystal.

[0013] Of course, the various features, variants, and embodiments of the invention can be combined with one another in various ways, provided they are not incompatible or mutually exclusive. Detailed description of the invention

[0014] The following description with regard to the attached drawings, given by way of non-limiting examples, will make it clear what the invention consists of and how it can be carried out.

[0015] On the attached drawings:

[0016] [Fig-1] is a schematic cross-sectional representation, in the foreground, of the laser system according to the invention;

[0017] [Fig.2] is a schematic cross-sectional representation, in a second plane, of the laser system of [Fig.1];

[0018] [Fig.3] is a schematic cross-sectional representation, in the foreground, of a part of a light beam generated by the laser system of [Fig.1];

[0019] [Fig.4] is a schematic representation of the light beam generated by the laser system propagating freely in each of the planes of figures 1 and 2 as well as in transverse planes;

[0020] [Fig.5] is a schematic representation of the light beam of [Fig.4] refracted into a beam exiting through a lens, implemented in the laser system of [Fig.1], according to a first embodiment of the invention;

[0021] [Fig.6] is a schematic perspective representation of the lens of [Fig.5];

[0022] [Fig.7] is a schematic perspective representation of a second mode of fabrication of a lens implemented in the laser system of [Fig.1].

[0023] A laser system 1 according to the invention is shown in Figures 1 and 2. As shown in [Fig. 1] or 2, the laser system 1 comprises an amplifying medium 2 and an optical device 3. The laser system 1 is referred to as a "laser" in the sense that it produces a high-intensity light beam that is spatially and temporally coherent. More specifically, the laser system 1 is of the pulsed type and based on a crystalline amplifying medium. The laser system can, for example, be used for laser cutting.

[0024] The laser system 1 is, for example, designed to generate a pulsed light beam with an energy between 10 W and 10 kW at frequencies between 50 kHz and 40 MHz. The emission time is, for example, between 100 fs and 1 ns. The power of the light beam is, for example, between 1 pJ and 10 mJ.

[0025] As shown in Figures 1 and 2, the amplifying medium 2 is capable of emitting a laser light beam, hereafter referred to as the amplified beam 4. To achieve this, the amplifying medium 2, which is solid in this case, is optically pumped to bring its constituent atoms into an excited state. Conventionally, the laser system 1 comprises an optical cavity (not shown), which includes, for example, two mirrors, within which the amplifying medium 2 is placed. Thus, a precursor beam (not shown) injected into the optical cavity passes through the amplifying medium 2 multiple times, producing, by stimulated emission, the amplified beam 4.

[0026] The amplifying medium 2 is here parallelepiped-shaped, for example rectangular. The inlet and outlet faces of the amplifying medium can also be angled, i.e., non-parallel, so as to avoid feedback into the amplifying medium 2. More specifically, the amplifying medium 2 has a plate-like shape, the beam The precursor is injected through a slice of the plate, perpendicular to the plate's thickness, i.e., its smallest dimension. For example, the amplifying medium 2 has a width between 5 mm and 30 mm, a length between 5 mm and 30 mm, and a thickness between 0.3 mm and 2 mm.

[0027] The amplifying medium 2 is, for example, made of neodymium-doped yttrium aluminum garnet (Nd:YAG) or ytterbium-doped yttrium aluminum garnet (Yb:YAG). The amplified beam 4 has a central wavelength, whose intensity is at its maximum, which is, for example, between 1000 nm and 1100 nm. The central wavelength depends on the material of which the amplifying medium 2 is made. Thus, for example, for an amplifying medium made of Yb:YAG, the central wavelength is 1030 nm and for an amplifying medium made of Nb:YAG, the central wavelength is 1064 nm.

[0028] The amplified beam 4 is emitted by the amplifying medium 2 along a propagation direction D corresponding here to the Z-axis of an orthonormal XYZ frame. The incident beam 4 has a width that is defined along a direction perpendicular to the propagation direction D, corresponding for example to the X-axis or the Y-axis of the orthonormal XYZ frame, as follows: - a segment for which, at the central wavelength, the intensity is greater than half the maximum intensity, such a segment corresponds to a full width at half-maximum (commonly called "full width at half-maximum" in English); - a segment for which, at the central wavelength, the intensity is greater than the maximum intensity divided by e2; or - a segment for which the energy is greater than 86% of the total energy of the incident beam 4.

[0029] Subsequently, the width of the amplified beam 4 is defined as the full width at half height.

[0030] The geometry of the amplifying medium 2 gives astigmatism to the amplified beam 4. In an amplifying medium 2 whose output slice 21 is rectangular, the divergence along the small width of the slice is mainly guided by the gain of the amplifying medium and the divergence along the large width is mainly guided by the radii of curvature of the mirrors forming the optical cavity.

[0031] Here, the amplified beam 4 more specifically exhibits a first divergence angle Ax in a first plane Px which includes the propagation direction D, and a second divergence angle in a second plane Py which also includes the propagation direction D and which is distinct from the first plane Px. The mathematical term "include" here means that the propagation direction D is encompassed, that is, extends, into both the first plane Px and the second plane Py. Here, the first divergence angle Ax is distinct from the second divergence angle Ay.

[0032] Subsequently, as shown in [Fig. 1], a first width Lx of the incident beam 4 is defined as the width of the incident beam 4 along a direction perpendicular to the propagation direction D and contained in the first plane Px. Similarly, as shown in [Fig. 2], a second width Ly of the incident beam 4 is defined as the width of the incident beam 4 along a direction perpendicular to the propagation direction D and contained in the second plane Py.

[0033] Each divergence angle Ax, Ay represents a variation in the width of the amplified beam 4 along the propagation direction D in its respective plane Px, Py. As shown in [Fig. 3], the first divergence angle Ax represents the variation in the first width Lx, and the second divergence angle Ay represents the variation in the second width Ly. Each divergence angle Ax, Ay is defined, for example, in accordance with ISO 11146.

[0034] More specifically, as shown in [Fig. 3], the first divergence angle Ax is defined here, in the first plane Px, as half the angle between a first contour Fx of the amplified beam 4 and the propagation direction D, measured at a first size Tx of the incident beam 4, that is, at the focal point of the amplified beam 4 in the first plane Px, where the first width Lx is minimal. As shown in [Fig. 3], the first contour Fx represents the variation of the first width Lx in the first plane Px.

[0035] The second divergence angle Ay is here defined analogously in the second plane Py as the half-angle between a second contour Fy of the amplified beam 4 and the propagation direction D, measured at a second size Ty of the incident beam 4, that is to say at the focal point of the amplified beam 4 in the second plane Py, where the second width Ly is minimum, the second contour Fy representing the variation of the second width Ly in the second plane Py.

[0036] Since the amplified beam 4 is astigmatic, its first size and its second size are spatially separated, they are for example separated by 1 mm at 10000 mm along the propagation direction D.

[0037] Due to its astigmatism, the amplified beam 4 has a section, perpendicular to the direction of propagation D, which is generally elliptical between the amplifying medium 2 and the optical device 3.

[0038] Here, as shown in [Fig. 4], the first plane Px and the second plane Py are defined such that they are perpendicular to each other. As shown in Figures 1 and 2, the first plane Px corresponds here to the XZ plane of the orthonormal coordinate system XYZ and the second plane Py corresponds to the YZ plane of the orthonormal coordinate system XYZ.

[0039] The first plane Px and the second plane Py are more particularly defined so as to correspond to the major axis and minor axis of the elliptical section of the incisive beam 4 in a transverse plane T1, T2, T3 perpendicular to the direction of pro- page D.

[0040] In the example illustrated in [Fig. 4], the amplified beam 4 is elliptical with its major axis lying in the second plane Py at the level of a first transverse plane Tl, then elliptical with its major axis lying in the first plane Px at the level of a second transverse plane T2 and a third transverse plane T3. The first transverse plane Tl corresponds here to that of the output slice 21 of the amplifier medium 2.

[0041] Here, perpendicular to the direction of propagation D, that is to say along transverse directions perpendicular to the direction of propagation D, for example along the X and Y axes of the orthonormal frame XYZ, the amplified beam 4 generally presents a Gaussian intensity profile at the central wavelength.

[0042] The optical device 3 is adapted to shape the amplified beam 4 in the sense that it allows modification of geometric characteristics of the amplified beam 4.

[0043] As shown in Figures 1 and 2, the optical device 3 comprises a lens 31 arranged along the propagation direction D. The lens 31 is positioned opposite the output edge 21 of the amplifying medium 2, through which the amplified beam 4 is emitted. Thus, the lens 31 refracts the amplified beam 4 into an output beam 5. The output beam 5 also has a divergence angle in the first plane Px, called the principal divergence angle, and a divergence angle in the second plane Px, called the secondary divergence angle. The divergence angles of the output beam 5 are defined in the same way as those of the infrasonic beam 4.

[0044] Remarkably, lens 31 has two different radii of curvature in two distinct planes. In other words, lens 31 is a bifocal lens. Each of the radii of curvature Rx, Ry is associated with a strictly positive, i.e., non-zero, curvature. Lens 31 is oriented to present a first radius of curvature Rx in the first plane Px and a second radius of curvature Ry in the second plane Py.

[0045] By determining the first radius of curvature Rx and the second radius of curvature Ry on the basis of the first divergence angle Ax and the second divergence angle Ay, the lens 31 is adapted to circularize or collimate the outgoing beam 5. Preferably the lens 31 is adapted to circularize and collimate the outgoing beam 5.

[0046] The effect of lens 31 on the amplified beam 4 is shown in [Fig. 5], in comparison with [Fig. 4], which illustrates free propagation of the amplified beam 4. In [Fig. 4], i.e., without lens 31, the amplified beam 4 is elliptical in the second cross-section T2 and in the third cross-section T3 and diverges since the first width Lx and the second width Ly increase between the second cross-section T2 and the third cross-section T3. In [Fig. 5], the outgoing beam 5 is circular, as shown in the second cross-section T2 and in the third cross-section T3. The beam The outgoing beam 5 is therefore also stigmatic. Furthermore, the outgoing beam 5 is collimated since its diameter is approximately equal in the Rayleigh zone, for example here in the second transverse section T2 and in the third transverse section T3.

[0047] Thus, the outgoing beam 5 can be shaped solely by means of the lens 31. The lens 31 is therefore preferably specifically designed with respect to the shape of the amplified beam 4. The adjustment of the optical device 3 is simple since the latter here consists of a single optical element: the lens 31. On the other hand, the design of the lens 31 depends on the amplified beam 4 and therefore on the amplifying medium 2.

[0048] The first radius of curvature Rx and the second radius of curvature Ry can be determined so as to optimize the circularity of the outgoing beam 5, that is to say so as to make a section of the outgoing beam 5 circular in a plane perpendicular to the direction of propagation D. The circularity of a laser beam is defined here according to the ISO11146 standard. Thus a beam is considered circular when its ellipticity is greater than 87%.

[0049] The first radius of curvature Rx and the second radius of curvature Ry are therefore determined so as to minimize the difference between the principal divergence angle and the secondary divergence angle of the outgoing beam 5. In practice, the radii of curvature Rx, Ry of the lens 31 are then determined such that the difference between the principal divergence angle and the secondary divergence angle is less than a first threshold value. The first threshold value is, for example, less than 0.1 mrad.

[0050] The first radius of curvature Rx and the second radius of curvature Ry can also be determined so as to minimize the divergence of the outgoing beam 5, i.e., to minimize the broadening of the outgoing beam 5. In other words, the first radius of curvature Rx and the second radius of curvature Ry are determined so as to minimize the principal divergence angle or the secondary divergence angle. Preferably, the radii of curvature Rx, Ry of the lens 31 are determined so as to minimize both the principal divergence angle and the secondary divergence angle. In practice, the radii of curvature Rx, Ry of the lens 31 are then determined such that the principal divergence angle and / or the secondary divergence angle is less than a second threshold value. The second threshold value is, for example, between 0.1 prad and 2 mrad.

[0051] Of course, the value of the radii of curvature Rx, Ry also depends on the optical index of the lens 31. The design of the lens 31 can therefore be carried out in terms of focal lengths which are then converted into radii of curvature, for example according to the following formula: R=f-(nl) where R is the radius of curvature, / the desired focal length and n the optical index of the lens.

[0052] Furthermore, thanks to the lens 31, the outgoing beam 5 is less astigmatic than the amplified beam 4.

[0053] As shown in [Fig. 4], although the amplified beam 4 is astigmatic, it has a circular cross-section S, perpendicular to the propagation direction D, in which the amplified beam 4 is circular. Before and after this circular cross-section S, the incident beam 4 is elliptical.

[0054] As can be seen in [Fig. 5], remarkably, lens 31 is positioned so as to intersect the circularity section S. This improves the combined collimation and circularization effect of lens 31. Lens 31 is here designed to operate within the circularity section S.

[0055] In the example illustrated in Figures 1, 2, and 5, between the amplifying medium 2 and the lens 31, the amplified beam 4 is divergent in the first plane Px and convergent in the second plane Py. Such an amplified beam is typically generated by slab laser systems. In these systems, the smaller width of the slab edge lies in the first plane Px, and the larger width of the slab edge lies in the second plane Py.

[0056] As shown in [Fig. 4], at the output of the amplifying medium 2, the first width Lx is increasing while the second width Ly is decreasing along the propagation direction D. As schematically represented in [Fig. 4], the circularity section S then corresponds to the plane, perpendicular to the propagation direction D, in which the first width Lx is equal to the second width Ly. Here, the amplified beam 4 is therefore circular in the circularity section S. Upstream of the circularity section S, the incident beam 4 is elliptical with its major axis along the second plane Py, and downstream of the circularity section S, the incident beam 4 is elliptical with its major axis along the first plane Px.

[0057] Therefore, as illustrated in [Fig. 5], to circularize and collimate the outgoing beam 5, the first radius of curvature Rx is associated with a positive focal length, in the sense that the associated image focus is located downstream of the lens 31 along the direction of propagation, i.e. on the side of the outgoing beam 5. Conversely, the second radius of curvature Ry is associated with a negative focal length, in the sense that the associated image focus is located upstream of the lens 31 on the direction of propagation, i.e. on the side of the amplified beam 4.

[0058] Thus, thanks to the lens 31 oriented to present the first radius of curvature Rx in the first plane Px and the second radius of curvature Ry in the second plane Py, and positioned in the circularity section S, the outgoing beam 5 is circular and collimated. In [Fig. 5], the diameter of the outgoing beam 5 is thus globally constant up to the third transverse plane T3, in the Rayleigh zone.

[0059] As shown in Figures 1 and 2, the lens 31 comprises two opposing optical faces. More specifically, the lens 31 comprises a first face The lens 31 consists of an optical face 32 illuminated by the amplified beam 4 and a second optical face 33 from which the outgoing beam 5 is emitted. In other words, the first optical face 32 is oriented towards the amplifying medium 2, and the second optical face 33 is oriented in the opposite direction to the amplifying medium 2. Here, the optical faces 32 and 33 are arranged perpendicular to the propagation direction D. The lens 31 is arranged so that the amplified beam 4 illuminates an area of ​​the first optical face 32 ranging from 0.2 mm² to 40,000 mm², for example, from 9 mm² to 10,000 mm². Advantageously, the lens 31 has small optical faces 32 and 33, for example, ranging from 0.2 mm² to 100 mm², making it less expensive and more compact.

[0060] The lens 31, for example, has a thickness between 2 mm and 4 mm. The thickness of the lens 31 can correspond to its dimension along the propagation direction D or to the smallest distance between the first optical face 32 or the second optical face 33.

[0061] The lens 31 also includes a peripheral edge 34 connecting the optical faces 32, 33. The peripheral edge 34 may, for example, have a square profile perpendicular to the propagation of the amplified beam 4, as shown in Figures 5 and 6, or a circular profile.

[0062] In a first embodiment represented in figures 5 and 6, each optical face 32, 33 forms respectively one of the radii of curvature Rx, Ry.

[0063] Thus, here, the first optical face 32 forms the first radius of curvature Rx and the second optical face 33 forms the second radius of curvature Ry. This means that the intersection between the first optical face 32 and the first plane Px defines an arc of a circle whose radius of curvature is equal to the first radius of curvature Rx. Similarly, this means that the intersection between the second optical face 32 and the second plane Py defines an arc of a circle whose radius of curvature is equal to the second radius of curvature Ry.

[0064] Of course, equivalently, the first optical face 32 can form the second radius of curvature Ry and the second optical face 33 can form the first radius of curvature Rx.

[0065] Advantageously, in this first embodiment, the lens 31 can be manufactured simply, at a lower cost, and with high precision. The radii of curvature Rx, Ry thus designed have a tolerance of less than 1%.

[0066] Indeed, as can be seen in [Fig. 6], each optical face 32, 33 extends here along a cylindrical surface of revolution. In other words, the first optical face 32 corresponds to a portion of the cylindrical face of a cylinder of revolution whose radius is equal to the first radius of curvature Rx. Similarly, the second optical face 33 corresponds to a portion of the cylindrical face of a cylinder of revolution whose radius is equal to the second radius of curvature Ry.

[0067] Here, since the first plane Px is perpendicular to the second plane Py, the optical faces 32, 33 extend along cylindrical surfaces of revolution whose axes are oriented orthogonally to each other. In other words, the first optical face 32 corresponds to a portion of the cylindrical face of a cylinder of revolution whose axis lies in the second plane Py. Similarly, the second optical face 32 corresponds to a portion of the cylindrical face of a cylinder of revolution whose axis lies in the first plane Px.

[0068] The aforementioned cylindrical face parts depend here on the shape of the peripheral edge 34, they are therefore, for example, square or circular.

[0069] As an alternative to this first embodiment, one of the optical faces can extend along a cylindrical surface of revolution while the other optical face extends along a spherical surface.

[0070] In this first embodiment, to shape the amplified beam 4 shown in Figures 1 and 2 (which is divergent in the first plane Px and convergent in the second plane Py), the first optical face 32 is convex and the second optical face 33 is concave. Equivalently, of course, when the first optical face 32 forms the second radius of curvature Ry and the second optical face 33 forms the first radius of curvature Rx, the first optical face 32 is concave and the second optical face is convex.

[0071] In this first embodiment, the lens 31 has a mean plane PM located midway between the optical faces 32 and 33. This mean plane PM is, for example, the plane best fitted to the optical faces 32 and 33 by a first-order regression. Preferably, the mean plane PM of the lens 31 coincides with the circularity section S of the incident beam 4 as illustrated in [Fig. 5]. This improves the combined collimation and circularization effect of the lens 3.

[0072] In a second embodiment represented in [Fig.7], one of the optical faces 32, 33 is flat and the other optical face 32, 33 forms the first radius of curvature Rx and the second radius of curvature Ry.

[0073] In the example illustrated in [Fig. 7], the first optical face 32 is planar and the second optical face 33 forms both the first radius of curvature Rx and the second radius of curvature Ry. This means that the intersection between the second optical face 33 and the first plane Px defines a circular arc whose radius of curvature is equal to the first radius of curvature Rx, and that the intersection between the second optical face 33 and the second plane Py defines a circular arc whose radius of curvature is equal to the second radius of curvature Ry.

[0074] Advantageously, in this second embodiment, the lens 31 is positioned so that the second optical face 33 intersects the circularity section S. Preferably, the lens 31 is placed so that the circularity section S coincides with a mean plane of the second optical face 33. The mean plane of the second optical face 33 is, for example, the plane tangent to the second optical face 33 at the center of the second optical face 33 or the plane best fitted to the second optical face 33 by a first-order regression.

[0075] Thus, in this second embodiment, the lens 31 generates almost no astigmatism since the two radii of curvature Rx, Ry are coplanar.

[0076] Here, as illustrated in [Fig. 7], the second optical face 33 extends along a toroidal surface. The second optical face 33 then corresponds, for example, to a portion of a surface generated by the rotation of a circle whose radius is equal to the first radius of curvature Rx around a straight line located at a distance equal to the second radius of curvature Ry. The aforementioned portion depends here on the shape of the peripheral edge 34; it is, for example, square or circular.

[0077] In this second embodiment, to shape the amplified beam 4 shown in Figures 1 and 2 (which is divergent in the first plane Px and convergent in the second plane Py), the second optical face 33 is therefore both convex and concave. More specifically, the second optical face 33 is convex in the first plane Px and concave in the second plane Py. The second optical face 33 then extends along a surface portion of an open torus located opposite the axis of rotation of the torus.

[0078] The first optical face 32 is preferably perpendicular to the direction of propagation D.

[0079] Regardless of the embodiment, the lens 31 is here made of silica. The lens 31 can also be made of another optical glass such as flint or crown glass. The lens 31 is here made from electronic-grade silica (SiO2). This reduces inclusions that could be present in the lens 31 and contribute to its heating when illuminated by the amplified beam 4. The OH ion content of the lens is preferably low, for example less than 1000 ppm, so that the lens absorbs little infrared radiation, thus limiting its heating.

[0080] Here, the lens 31 is made of silica exhibiting an absorption of less than 10⁵ cm⁻¹ ppm for wavelengths between 900 nm and 1100 nm. Since the infrared range is a preferred operating range for laser systems, it is advantageous for the lens 31 to have low absorption in this range. Thus, heating of the lens 31 is significantly limited when the amplified beam 4 is within the aforementioned wavelength range.

[0081] The lens 31 is here manufactured by computer numerical control machining, also called "CNC" machining, which makes it possible to produce complex optical surfaces, For example, a toroidal surface such as that of the second embodiment, with high precision. Computer numerical control (CNC) machining allows, in particular, the shaping of spherical, aspherical, or freeform surfaces. After machining, the optical faces 32, 33 are polished so that their roughness is less than one-quarter of the central wavelength. According to MIL-PRF-13830B, the optical faces 32, 33 are polished so that the scratch and dig are between 10 and 20.

[0082] The lens 31 can also be treated by applying coatings to its optical surfaces 32, 33. The lens 31 may, for example, include one of the following coatings: an anti-reflective coating, or a nanostructured coating. Preferably, the anti-reflective coating has a reflectance at normal incidence of less than 0.1% at 1030 nm. The coatings are applied after polishing.

[0083] The present invention is in no way limited to the embodiments described and illustrated, but those skilled in the art will be able to make any variation therein in accordance with the invention. For example, the amplified beam can be divergent (between the amplifying medium and the optical device) both in the first plane and in the second plane. This is the case, for example, when the amplifying medium corresponds to that of a laser diode. For such an amplified beam, the lens is then provided to have two positive focal lengths. When the divergence of such an amplified beam is not the same in the first or second plane, it also presents a circular cross-section at which the lens is preferably placed. When the amplifying medium corresponds to that of a laser diode, the radii of curvature are, for example, between 1 mm and 1000 mm.

Claims

Demands

1. Laser system (1) comprising: - a solid amplifying medium (2) capable of emitting an amplified beam (4) along a propagation direction (D), the amplified beam (4) having a first divergence angle (Ax) in a first plane (Px) including the propagation direction (D), and a second divergence angle in a second plane (Py) including the propagation direction (D) and distinct from the first plane (Px), the second divergence angle being distinct from the first divergence angle (Ax);and - an optical device (3) comprising a lens (31) arranged to refract the amplified beam (4) into an output beam (5), the lens (31) having a first radius of curvature (Rx) in the first plane (Px) and a second radius of curvature (Ry) in the second plane (Py), the second radius of curvature (Ry) being distinct from the first radius of curvature (Rx), characterized in that, between the amplifying medium (2) and the optical shaping device (3), the amplified beam (4) is divergent in the first plane (Px) and convergent in the second plane (Py).

2. Laser system (1) according to claim 1, wherein the outgoing beam (5) has two divergence angles respectively in the first plane (Px) and in the second plane (Py), and wherein the first radius of curvature (Rx) and the second radius of curvature (Ry) are determined, on the basis of the first divergence angle (Ax) and the second divergence angle, so as to achieve at least one of the following criteria: - a difference between the two divergence angles of the outgoing beam (5) is less than a first threshold value; - at least one of the two divergence angles of the outgoing beam (5) is less than a second threshold value.

3. Laser system (1) according to claim 1 or 2, wherein the first radius of curvature (Rx) and the second radius of curvature (Ry) are determined such that the outgoing beam (5) is less astigmatic than the amplified beam (4).

4. A laser system (1) according to any one of claims 1 to 3, wherein the amplified beam (4) has a circular cross-section (S) perpendicular to the direction of propagation (D), in which the amplified beam (4) is circular in shape, and in which the lens (31) is po- positioned so as to intersect the circularity section (S).

5. Laser system (1) according to any one of claims 1 to 4, wherein the first plane (Px) is perpendicular to the second plane (Py).

6. Laser system (1) according to any one of claims 1 to 5, wherein the lens (31) comprises: - a first optical face (32) forming the first radius of curvature (Rx); and - a second optical face (33), opposite to the first optical face (32), forming the second radius of curvature (Ry).

7. Laser system (1) according to claim 6, wherein at least one of the first optical face (32) and the second optical face (33) extends along a cylindrical surface of revolution.

8. Laser system (1) according to any one of claims 1 to 5, wherein the lens (31) comprises: - a first flat optical face (32); and - a second optical face (33), opposite the first optical face (32), forming the first radius of curvature (Rx) and the second radius of curvature (Ry).

9. Laser system (1) according to claim 8, wherein the second optical face (33) extends along a toroidal surface.

10. Laser system (1) according to any one of claims 1 to 9, wherein the first radius of curvature (Rx) and the second radius of curvature (Ry) are each between 1 mm and 1000 mm.

11. Laser system (1) according to any one of claims 1 to 10, wherein the amplified beam (4) comprises a central wavelength, and wherein the lens (31) comprises optical faces (32, 33) whose roughness is less than one-quarter of the central wavelength.

12. Laser system (1) according to any one of claims 1 to 11, wherein the lens (31) is made of a silica having an absorption of less than 105 cm 1 for a wavelength between 900 nm and 1100 nm.

13. Laser system (1) according to any one of claims 1 to 12, wherein the amplified beam (4) has a Gaussian profile along a transverse direction perpendicular to the propagation direction (D).

14. Laser system (1) according to any one of claims 1 to 13, wherein the optical shaping device (3) is made up of the lens (31).

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