Optical device for shaping a light beam

Abstract

The invention relates to an optical device (DO) for shaping a light beam, which comprises an optical arrangement comprising a first converging optical element (L1) and a beam splitter (SF). The splitter is configured to receive an incident light beam produced by a source and to split this beam into a plurality of individual light beams which are angularly distributed and directed towards the optical arrangement, the plurality of beams propagating separately in space at least in a so-called "splitting" region. The optical device also comprises an optical retarder (LP) placed in the splitting region and configured to introduce distinct optical path differences into the individual light beams that pass through it.

Description

Optical device for shaping a light beam FIELD OF INVENTION

[0001] The invention relates to an optical device for shaping a pulsed light beam. Such a device is particularly useful when the light beam is, for example, a high-power laser beam, for machining, welding, brazing, cutting, ablation, drilling, surface texturing, and more generally, any materials processing. However, the invention is not limited to these high-power applications and may also find applications in other fields, such as metrology, microscopy, or medical devices. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Many laser processing equipment benefits from the ability to control the irradiance of a laser beam projected onto a work surface in which a part to be treated is positioned.

[0003] These devices can utilize a continuous or pulsed laser source, often a fiber or CO₂ laser, whose wavelength (e.g., in the infrared or ultraviolet) and power are tailored to the material being processed and the desired treatment. In some cases, its shape can be adjusted using diffractive or reflective optics, allowing, for example, a transition from the Gaussian profile typically produced by the source to a "top-hat," ring, or any other profile desirable for the application.

[0004] It is also common in these applications to provide a beam splitter ("beam splitter" according to the Anglo-Saxon terminology of the field) allowing the incident light beam to be divided into a plurality of elementary beams, which are all projected onto the part to be treated, for example to increase the speed of the process.

[0005] An optical scanning system, typically a galvanometric scanner composed of electronically controlled moving mirrors, directs elementary beams towards selected areas of the part in a controlled scanning motion.

[0006] The equipment is conventionally fitted with an f-theta lens or other focusing element optically disposed downstream of the galvanometer, in order to maintain uniform focusing of the elementary beams in the working plane and over the entire treated surface, even when these beams are collectively oriented by the optical scanning system at very different angles.

[0007] The part to be processed can be placed in the work surface, on a mobile platform or on a motorized support, allowing precise positioning and, if necessary, movement synchronized with the optical scanning system.

[0008] Some treatments require the deposition of significant energy on the part being treated. This can be achieved by using a laser source producing a femtosecond pulsed beam, each pulse of which typically has a duration between 100 fs and 10 ns and an energy greater than 10 microjoules and which can reach 1 mJ, or even 1 J.

[0009] The peak power of a pulse is extremely high due to the very short duration of each pulse. For example, a 10 µJ pulse lasting 10 femtoseconds produces a peak power of approximately 10 GW.

[0010] The inventors of this application realized that this high peak power is likely to induce nonlinear effects in beam processing optics, particularly in the f-theta or focusing optics that precede the beam projection onto the workpiece. Such nonlinear effects are described, for example, in the publication by Yu, B. et al., "Impact of Nonlinear Kerr Effect on the Focusing Performance of Optical Lens with High-Intensity Laser Incidence." Appl. Sci. 2020, 10, 1945. This document reports, in particular, that this Kerr effect is likely to affect the focusing characteristics of a lens and, consequently, the shape of the beam at its focal plane.

[0011] When such a phenomenon occurs in laser processing equipment, it can affect the accuracy of the workpiece placed in the worktable, which is obviously undesirable. These nonlinear effects are even more detrimental to a beam with shaping. For example, the multiple beams that overlap in the focusing optics material create irregular intensity distributions with interference fringes. The phase accumulated due to the Kerr effect is therefore also irregular and significantly affects beam quality.

[0012] As a further example, the energy distribution in the elementary light beams, at the level of the beam splitter, is not identical in the elementary beams at the level of the workpiece, after these have propagated through the f-theta lens.

[0013] We therefore lose control over the shape of the elementary beams and the energy present in each of these elementary beams. SUBJECT OF THE INVENTION

[0014] One aim of the invention is to provide an optical device that at least partially addresses this problem. More specifically, one aim of the invention is to provide an optical device capable of delivering high-power energy pulses with high precision, without excessively affecting the shape of the beams at the working plane. BRIEF DESCRIPTION OF THE INVENTION

[0015] To achieve one of these goals, the object of the invention proposes an optical device for shaping a light beam comprising: an optical arrangement including a first converging optical element; a beam splitter optically disposed upstream of the first converging optical element and configured to: receive an incident light beam produced by a source; divide the incident light beam into a plurality of elementary light beams, which are angularly distributed and directed towards the optical arrangement, the plurality of beams propagating spatially separately at least in a so-called "separation" zone; an optical retarder placed in the separation zone, the optical retarder being configured to introduce distinct optical path deviations in at least some pairs of elementary light beams in order to impart a time offset between these pairs of elementary light beams.

[0016] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: the optical retarder is configured to introduce distinct optical path deviations in all pairs of elementary light beams; the optical arrangement comprises a single converging optical element and in which the optical retarder is optically disposed downstream or upstream of the first converging optical element; the optical retarder is optically disposed downstream of the first converging optical element and the optical retarder comprises a reflective optical part and a retarding blade for separating incoming and outgoing light beams; the optical arrangement comprises a second converging optical element, optically disposed upstream of the first converging optical element; the optical retarder is optically disposed between the second converging optical element and the first converging optical element;The first converging optical element and / or the second converging optical element comprises a lens; the first converging optical element and the second converging optical element are respectively made up of a first lens and a second lens; the optical retarder comprises at least one retarder plate; the optical retarder comprises a waveguide array; the optical retarder comprises at least one reflective optical component; the optical retarder comprises a diffraction grating; the optical arrangement comprises a polarization splitter; the optical retarder comprises a retarder plate for separating incoming and outgoing light beams;

[0017] According to another aspect, the invention also proposes an optical system comprising a source associated with an optical device as described above.

[0018] According to other advantageous and non-limiting features of this aspect of the invention, taken alone or in any technically feasible combination, the source is configured to produce the incident light beam in pulsed form, composed of pulses having a predetermined maximum duration and in which the distinct optical path gaps allotted by the optical retarder are chosen to temporally separate pulses present in the elementary light beams; the optical path gaps allotted by the optical retarder are chosen to be smaller, advantageously ten times smaller, than a Rayleigh distance from the input light beam; The optical system further comprises an applicative optical device disposed downstream of the optical device; the applicative optical device is an f-theta optical device;The optical system further includes a shaping device disposed between the source and the optical device.

[0019] Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the accompanying figures in which:

[0020]

[0021] Figures 1, 2, 3, 4, 5, 6 and 7 represent different embodiments of an optical device according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] For the sake of clarity, in this application we define a light beam as radiation consisting of at least one mode of the electromagnetic field, each mode forming a spatio-frequency distribution of the amplitude, phase, and polarization of the field.

[0023] For the purposes of this application, the "shape" of a light beam will be defined as the distribution of its intensity in a given transverse plane. A propagating light beam can exhibit a wide variety of shapes in different transverse planes, and its size can vary. The size of a shape is defined by a representative measure of the extent of its intensity distribution in the given transverse plane. The size of a beam can be defined as the full width at half maximum (FWHM) of its intensity for a Gaussian shape, and more generally as the second moment of area diameter "D4σ", as defined in ISO 11146.

[0024] For the purposes of this application, a change in the shape of a beam refers to any modification of its shape (its intensity distribution, as defined above) in a given transverse plane. Beam shaping therefore corresponds to any treatment aimed at giving a beam a predetermined shape in a given transverse plane.

[0025] In the implementations presented below, the optical device DO is associated with a source S producing an incident light beam FI in pulsed form. This can be a femtosecond laser source capable of producing a train of very short pulses, with a maximum duration, for example, between 1 fs and 100 fs. More generally, the pulses of the laser source S can have a maximum duration between 1 fs and 100 ns. The energy contained in such a pulse is typically greater than 1 microjoule and can reach 10 mJ or more. The repetition frequency of the pulses in the pulse train can range from 1 Hz to 10 GHz. The emission wavelength of the source S is naturally chosen according to the intended application and can range from 200 nm to 3000 nm.

[0026] The incident light beam FI can also exhibit any polarization, usually linear or elliptical polarization.

[0027] The incident light beam FI produced by the source S can have any shape suitable for the application, for example a Gaussian, "top hat" or ring shape, and it can be foreseen that this source S is associated with a shaping device configured to provide an incident light beam FI of the chosen shape.

[0028] Lare represents a method of implementing a DO optical device particularly suited to an application aimed at depositing a very large amount of energy on a part using a working beam composed of a plurality of elementary light beams.

[0029] The DO optical device in this implementation is associated with an f-theta DFT optical device, located downstream of the DO optical device. As is well known, and indeed mentioned in the introduction to this application, such an f-theta DFT optical device allows for controlled scanning of the part being processed by the working beam. It comprises a galvanometer scanner and an f-theta lens system located downstream of the scanner.

[0030] The galvanometric scanner of the f-theta optical device (DFT) typically consists of electronically controlled moving mirrors that allow modification of the angle of incidence of the elementary beams produced by the optical device (DO). The "f-theta" lens system, located downstream of the galvanometric scanner, is designed to linearly convert the angle of incidence of the elementary light beams to a focusing position of these beams onto a flat image plane (the FT working plane) in which the part to be processed resides, without affecting the shape of the beams in this plane.

[0031] As can be seen in the figure, the DO optical device is positioned between the source S and the f-theta DFT optical device, and the assembly forms an optical system that can be integrated into processing equipment (machining, cutting, drilling, etc.) for a workpiece, for example, a metal part. It can naturally be assumed that this system includes other devices besides those explicitly mentioned in relation to the description of the implementation method.

[0032] The optical DO device has an optical input port and an optical output port, allowing it to be coupled to the source S and the f-theta DFT optical device, respectively, for example via fiber optics or free-space propagation. The optical device typically includes, arranged between the input and output ports, a beam splitter SF, an optical arrangement, and an optical retarder LP.

[0033] The optical arrangement comprises a first converging optical element, here a first lens L1, with a first focal length f1, and a second converging optical element, here a second lens L2, with a second focal length f2. In the illustration, the two lenses L1 and L2 are optically separated by a distance corresponding to the sum of the first focal length f1 and the second focal length f2, thus forming an afocal device. In other words, the distance separating the first lens L1 and the second lens L2 is such that the object focus of the first lens L1 coincides with the image focus of the second lens L2.

[0034] The optical arrangement has an entrance plane PE positioned optically upstream of the second lens L2 and, in the case shown in the figure, at a distance corresponding to the first focal length f2 of this lens L2, that is, positioned at an object focus of the second lens L2. The optical arrangement also has an exit plane PS positioned optically downstream of the first lens L1 and, in the case shown in the figure, at a distance corresponding to the second focal length f1 of this lens L1, that is, positioned at an image focus of the first lens L1.

[0035] Advantageously, the beam splitter SF is disposed at the PE inlet plane and a rear focal plane of the f-theta DFT optical device is disposed at the PS outlet plane of the optical device, for example to be coincident with this plane.

[0036] The optical arrangement also presents a Fourier plane PF optically positioned between the first lens L1 and the second lens L2. This Fourier plane PF is separated by the first focal length f1 of lens L1. Advantageously, in the configuration shown in the figure, it is separated by the second focal length f2 of the second lens. In other words, in this advantageous configuration, the distance separating the first lens L1 and the second lens L2 is such that an object focus of the first lens L1 coincides with an image focus of the second lens L2, and the Fourier plane PS is located at these foci.

[0037] Preferably, for simplicity of implementation, the first lens L1 and the second lens L2 are identical and therefore have the same focal length. In this case, the Fourier plane PF is placed midway between the two lenses L1 and L2, and the optical arrangement has a magnification of -1, so as not to affect the shape of the incident light beam FI.

[0038] We can naturally choose the first lens L1 and the second lens L2 so that they have different focal lengths, and therefore have a magnification different from -1, given by the formula -f2 / f1.

[0039] Continuing the description, the beam splitter SF is associated with the source S and receives the incident light beam FI to produce a plurality of elementary light beams, angularly distributed and directed towards the second lens L2. Each elementary light beam replicates the incident light beam, thus presenting the same pulse train, but each pulse has reduced energy compared to a pulse in the input light beam by a factor corresponding approximately to the number of replications, assuming the energy is distributed uniformly among each elementary light beam. It can naturally be assumed that the beam splitter SF is configured to distribute the energy of a pulse from the incident light beam according to any desired distribution among the elementary light beams, without this distribution necessarily being uniform.

[0040] As an illustration, the SF beam splitter can be formed from a diffraction grating ("beam splitter grating" according to the established Anglo-Saxon expression), and it can in particular be an optical part having a surface engraved or covered with periodic patterns acting in transmission or reflection like a diffraction grating.

[0041] More generally, the SF beam splitter can be formed of at least one diffractive optical element (DOE for "Diffractive Optical Element" according to the Anglo-Saxon expression).

[0042] It can also be implemented by an MPLC device (Multiplane Light Converter). For the sake of completeness, it should be noted that in such an MPLC device, incident light undergoes a succession of reflections and / or transmissions, each reflection and / or transmission being followed by propagation of the radiation in free space. At least some of the optical components on which the reflections and / or transmissions occur, and which guide the propagation of the incident radiation, have a microstructured surface that modifies the incident light, in this case to separate it into elementary light beams. Further details can be found in the document "Programmable Unitary Spatial Mode Manipulation," Morizuret Al, J. Opt. Soc. Am. A / Vol. 27, No. 11 / November 2010; N.Fontaine et al, (ECOC, 2017), “Design of High Order Mode-Multiplexers using Multiplane Light Conversion”; US9250454 and US2017010463 the theoretical foundations and practical implementation examples of an MPLC device.

[0043] Regardless of how the SF beam splitter is implemented, it can be configured to decompose the incident light beam into any number of elementary light beams, but typically between 2 and 30, preferably between 4 and 8 or between 4 and 15 elementary light beams.

[0044] Advantageously, and as previously presented, the beam splitter SF is arranged in the entrance plane PE of the optical arrangement. Consequently, the elementary light beams propagate along parallel propagation axes in a portion between the second lens L2 and the first lens L1.

[0045] The beam splitter SF (characterized by the separation angles of the elementary light beams) and the second lens L2 (characterized by its focal length) are chosen so that the elementary light beams propagate spatially separately from each other, at least within a region, called the "separation zone," located between the second lens L2 and the first lens L1 of the optical arrangement. In other words, the beam splitter SF and the second lens L2 are configured so that, at least within the separation zone between the two lenses L1 and L2, the propagation axes of the elementary light beams are separated from each other by a distance greater than their sizes. It is thus assumed that the elementary light beams do not spatially overlap within this zone.In the implementation mode of the, this separation zone extends on either side of the image focus of the second lens L2, and therefore includes the Fourier plane PF of the optical arrangement.

[0046] The first lens L1, in combination with the second lens L2, allows imaging of the beam splitter SF at the level of the exit plane PS of the optical device.

[0047] According to the invention, the elementary light beams produced by the optical device DO are each composed of pulse trains that are time-shifted from one beam to the next, as will be explained later in this description. The pulses of the elementary beams, at the output plane PS, each have an energy identical or close to that of the pulses of the elementary light beams produced by the beam splitter SF. However, due to the time shift of the pulses from one elementary beam to another, the power density and / or peak power of the elementary light beams taken collectively at a given instant in the output plane PS (or in any plane located downstream of the optical arrangement) is significantly lower. Also due to this time shift, the elementary light beams do not interfere, and the spatial distribution of the power density is much more regular.

[0048] It is understood that the time delay, and therefore the optical path length, sufficient to prevent two elementary light beams from interfering as defined in this application, depends on the pulse duration. The longer a pulse lasts, the greater the time delay must be to prevent such harmful interference. If the pulses of some elementary light beams overlap in time, their partial overlap must reduce the maximum intensity created by their interference. Thus, for a pulse with a duration (defined as the full width at half maximum of the pulse's temporal expansion) of 10 fs, an optical path length of at least 3 micrometers was sought to significantly reduce the temporal overlap of pulses.For a pulse duration of 200 fs, an optical path length of at least 90 micrometers should be chosen, and for a pulse duration of 500 fs, an optical path length of at least 150 micrometers is sufficient to reduce harmful interference. The time delay imparted to two elementary light beams is therefore distinct from a simple phase shift between these two pulses.

[0049] This avoids or limits the introduction of nonlinear effects in the f-theta DFT optical device located downstream of the DO optical device, or in any other application optical device associated with the DO optical device. This time spreading is achieved without substantially affecting the shape of the incident light beam FI; this shape is therefore preserved in the elementary light beams produced by the DO optical device.

[0050] To enable these advantageous characteristics of the DO optical device, and continuing the description of its implementation, it includes an optical retarder LP positioned in the region where the elementary light beams are spatially separated. Preferably, the optical retarder is positioned at or near the Fourier plane PF.

[0051] In the embodiment illustrated in the figure, the LP optical retarder is formed by a plurality of retarder plates that intercept some of the elementary light beams to increase the lengths of the optical paths along which they propagate. As is readily apparent to those skilled in the art, the increase in the optical path length traveled by an elementary light beam is related to the thickness of the retarder plate through which that elementary light beam passes and to the refractive index of the material composing that plate.

[0052] The delay blades of the LP optical retarder are configured to introduce distinct optical path deviations to the elementary light beams. For the avoidance of doubt, the "optical path deviation" is defined as the difference in length of the optical paths traveled by two distinct elementary light beams during their propagation between the first lens L1 and the second lens L2. To achieve this effect, in the implementation shown in Figure 1, in which the SP beam splitter produces 5 elementary light beams, 4 LP delay blades are assembled in a stepped block and arranged at the level of the Fourier plane PF, such that a first elementary light beam intercepts all 4 delay blades, a second elementary light beam intercepts only 3 delay blades, a third light beam intercepts only 2 blades, and so on.The fifth and final elementary light beam does not intercept any of the delay blades of the LP optical retarder.

[0053] The optical retarder LP, when operating in transmission, is made of a material transparent at the wavelength of the incident light beam. The material also has a different refractive index than the optical device DO medium in which the elementary light beams propagate. An optical retarder can thus, by way of illustration, be made of glass or quartz, and the propagation medium be air, but the invention is by no means limited to these materials and this medium.

[0054] To introduce distinct optical path lengths, one can use a plurality of delay plates arranged in a stepped block, as provided in [the example], or a single, monolithic delay plate shaped to provide the same stepped profile. Any other arrangement or configuration leading to distinct optical path lengths is, of course, conceivable.

[0055] One approach is to collect elementary light beams in a waveguide array, such as a fiber optic array (known as a "fiber array" in English terminology). The waveguides in the array are chosen to have lengths that impart the desired optical path deviations. In such a configuration, each optical fiber in the array is preferably selected so that one of its eigenmodes corresponds to the mode of the elementary light beam that the fiber is intended to propagate. This propagation process introduces the optical path deviation while preserving the shape of the guided elementary light beam.

[0056] One can also consider implementing the LP optical retarder via a transmission diffraction grating. Advantageously, the grating can be a blazed grating optimized for a given diffraction order, as shown in Figure 1. As is well known, the maximum optical power of such a grating is concentrated in the desired diffraction order, while the residual power in other orders (especially the zero order) is minimized. When elementary light beams are transmitted to an order other than the zero order, the diffraction grating introduces distinct optical path deviations. In Figure 1, the elementary light beams are directed in a direction that maximizes efficiency, this direction forming an angle of attack B with respect to a normal to the grating.

[0057] In general terms, then, and according to the invention, the optical retarder is configured to introduce distinct optical path deviations to the elementary light beams that intercept it. The introduction of these optical path deviations leads to a time shift in the pulses of one elementary light beam relative to the pulses of another elementary light beam. This time shift, also called the delay, is determined as the product of the speed of light in its medium and the optical path deviation between these two elementary light beams.

[0058] It is noted that by placing the LP optical retarder in the separation zone in which the elementary beams are spatially separated, it is possible to act, individually, on the pulses of each elementary beam.

[0059] Advantageously, and as shown in the figures, the LP optical delay device provides distinct and carefully chosen optical path lengths to temporally separate the pulses present in the elementary light beams. The delays introduced by the LP optical delay device are preferably chosen to be greater than the maximum duration of a pulse. In other words, in such a configuration, a pulse from one elementary light beam does not temporally overlap a pulse from another elementary light beam. The pulses of the elementary light beams are therefore separated in time, because the delays introduced by the LP optical delay device exceed the maximum duration of a pulse.

[0060] In some cases, the optical path lengths imposed by the optical retarder LP are chosen to be smaller, advantageously ten times smaller, than the Rayleigh distance of the incident light beam. Recall that this Rayleigh distance is the distance over which a focused light beam maintains a relatively small cross-sectional area, generally defined as the point where the beam's cross-sectional area doubles its beam size. When the optical path lengths are much smaller than the Rayleigh distance, the elementary light beams all focus approximately in the same plane, behind the first lens L1. This limits the introduction of optical aberrations that could alter the shape of the elementary light beams.

[0061] In other cases, it can be advantageous to choose the optical path lengths allocated by the LP optical retarder so that they are on the order of magnitude of the Rayleigh distance or greater. This allows the focal plane of the elementary light beams, or some of them, to be modified. This can be useful for treating a part with a curved surface, or for modifying the shape of the beams at the treated surface of the part. This shape modification may affect some, but not all, of the elementary light beams.

[0062] It is noted that this question of change in the shape of elementary beams only arises when their propagation takes place in free space, and therefore does not apply to an LP optical retarder implemented by waveguides, such as optical fibers, as has been proposed previously.

[0063] Figures 2 and 3 depict alternative embodiments that make the DO optical device more compact. These figures clearly show the beam splitter SF, the first lens L1, and the second lens L2 of the optical arrangement, just as in the first embodiment. However, in these two embodiments, the optical retarder LP is reflective rather than transmissive, as was the case in the first embodiment.

[0064] In this diagram, the optical retarder comprises a plurality of reflective optical elements arranged along the propagation axes of the elementary light beams to impart distinct optical path deviations. These reflective optical elements have reflective surfaces oriented to direct the elementary light beams towards the second lens L2.

[0065] The optical retarder LP comprises a monolithic reflective optical element with a stepped profile. This embodiment also includes a polarization splitter SP, one of whose principal faces is not parallel to the principal plane of the first lens L1, here positioned at 45° to this principal plane (but, more generally, whose orientation can depend on the design of the polarization splitter SP). The polarization splitter SP reflects the elementary beams from the second lens L2 (which have a first polarization) towards the optical retarder LP.The elementary beams reflected by the optical retarder, after their double passage through a retarder plate L4 intended to separate the light beams entering and exiting (such as a quarter-wave retarder plate) of the optical retarder LP, have a second polarization, different from the first (more precisely orthogonal to the first), so that they are transmitted by the polarization separator SP to propagate towards the first lens L1.

[0066] It would also be possible, particularly within the framework of the implementations shown in Figures 2 and 3, to form the LP optical retarder as a reflective diffraction grating. Advantageously, this could be a ladder grating designed for a given, non-zero diffraction order, as already discussed in a previous paragraph. When the elementary light beams are reflected to a diffraction order other than zero, the diffraction grating introduces distinct optical path deviations.

[0067] In the case of the implementation mode of the, the reflective diffraction grating can be put in a so-called "Littrow" configuration in which the elementary light beams are reflected back on themselves.

[0068] This presents an implementation method that does not require a second lens L2. The first lens L1 (or more generally, the first converging optical element) is traversed twice by the elementary light rays. As in the previous example, a polarization splitter SP is provided, positioned here between the beam splitter SF and the first lens L1. The optical retarder LP, in this implementation, consists of at least one reflective optical component and includes a retarder plate L4 designed to separate the incoming and outgoing light beams, for example, a quarter-wave plate "lambda / 4".

[0069] One could naturally plan to integrate these two elements together, for example by structuring the reflective surface of the reflective optical part in the form of a meta surface allowing the polarization to be rotated during reflection.

[0070] This represents yet another implementation method which, like that of the previous one, requires only a single converging optical element that, in this case, is intercepted only once by the elementary light beams. In the configuration of the previous one, the first converging optical element L1 is optically positioned between the beam splitter SF and the optical retarder LP. In this implementation, the beam splitter SF is not placed at the focal point of the first lens L1. Note that the optical retarder LP can alternatively be placed optically upstream of the first lens L1, provided that there is a separation zone in which the elementary light beams do not overlap.

[0071] Such a configuration is illustrated in Figure 1. This figure also illustrates an optical system comprising a shaping device M, here placed between the source S and the optical device DO. This shaping device M can be a simple lens, as is the case in Figure 1, allowing the incident beam FI to be focused before it intercepts the optical device. But in a general case, it can be any device that modifies the shape of the incident beam produced by the source S. In particular, it can be an MPLC device configured to give a specific shape to the beam it produces. It can also be configured to perform modal filtering of the incident light radiation, as described, for example, in document US12164151.It can also consist of at least one diffractive optical element (DOE), a spatial light modulator (SLM), an imaging optical system, or one comprising at least one lens, an axicon, and at least one non-spherical, non-planar transmissive or reflective optical element, such as an aspheric or freeform optical element. These elements can, of course, also be combined to create a shaping device M that allows the light beam produced by the source to be shaped as desired.

[0072] Such a shaping device can be combined with DO optical devices of all the presented implementation modes.

[0073] In all the implementation modes presented, the DO optical device allows for good temporal separation of the pulses of the elementary light beams (the introduced delays advantageously exceeding the duration of the pulse), which makes it possible to reduce the instantaneous or peak power of the elementary light beams, taken in combination, compared to a prior art device.

[0074] The energy distribution present in the incident light beam into the elementary light beams, achieved by the SF beam splitter, is found to be similar or identical in the elementary beams at the workpiece, after they have propagated through the f-theta optical device. This energy distribution is in all cases less affected than in prior art systems.

[0075] Of course the invention is not limited to the modes of implementation described and alternative embodiments can be made without departing from the scope of the invention as defined by the claims.

[0076] Thus, although configurations have been illustrated in which the optical retarder LP introduces distinct optical path deviations for every pair of elementary light beams, such a characteristic is not essential. It is sufficient that distinct optical path deviations be introduced in at least one pair of elementary light beams, but not necessarily all. For example, one could consider decomposing the incident light beam FI into 9 elementary light beams and imposing distinct optical paths on 3 "beam bunches," each composed of 3 elementary light beams, with the optical paths traveled by the elementary light beams in each bunch being identical.

[0077] Furthermore, the optical device is not necessarily associated with the f-theta optical device shown in the illustration. More generally, it can be associated with an application optical device located downstream of the output plane PS of the optical device DO. This application device includes at least one optical component (for example, a lens) which, when the combined energy of the elementary light beams passing through it is excessive, can affect the shape of these light beams and / or the energy distribution between them.

[0078] It has been proposed to form the LP optical retarder in the form of a reflective or transmissive retarder plate, a waveguide array, a diffractive array; it is understood that an optical retarder according to the invention can, if necessary, combine these solutions.

[0079] Similarly, the optical device can find applications other than treating a part with a high-power incident laser beam. It can be applied in other fields, such as metrology or microscopy, where the incident laser beam may have a relatively lower power, but where preserving the shape of the elementary beams can be more critical.

[0080] Although some implementation methods suggest advantageously positioning the elements of the DO optical device at very specific relative positions, it should be understood that these advantageous positions are not to be interpreted strictly. A person skilled in the art may adjust the relative positioning of the components of the DO optical device to obtain the desired result. The proposed advantageous positioning, however, constitutes a reference configuration that can be used as a starting point for this fine-tuning.

[0081] Finally, to limit harmful interference between two adjacent elementary beams due to their potential partial overlap, their polarization can be modified, for example, so that they are at 90° to each other. Therefore, for the purposes of this description, it is not necessary for all elementary beams to have the same polarization.

Claims

Optical device (OD) for shaping a light beam comprising: an optical arrangement including a first converging optical element (L1); a beam splitter (BS) optically disposed upstream of the first converging optical element (L1) and configured to: receive an incident light beam produced by a source; divide the incident light beam into a plurality of elementary light beams, which are angularly distributed and directed towards the optical arrangement, the plurality of beams propagating spatially separately at least in a so-called "separation" zone; an optical retarder (OP) placed in the separation zone, the optical retarder (OP) being configured to introduce distinct optical path deviations in at least some pairs of elementary light beams in order to impart a time offset between these pairs of elementary light beams. Optical device (DO) according to the preceding claim in which the optical retarder (LP) is configured to introduce distinct optical path deviations in all pairs of elementary light beams. Optical device (OD) according to any one of the preceding claims wherein the optical arrangement comprises a single converging optical element (L1) and wherein the optical retarder (LP) is optically disposed downstream or upstream of the first converging optical element (L1). Optical device (OD) according to the preceding claim in which the optical retarder (LP) is optically disposed downstream of the first converging optical element (L1) and the optical retarder (LP) comprises a reflective optical part and a retarder blade (L4) for separating incoming and outgoing light beams. Optical device (OD) according to claim 1 or 2 in which the optical arrangement comprises a second converging optical element (L2), optically disposed upstream of the first converging optical element (L1). Optical device (OD) according to claim 5, wherein the optical retarder (LP) is optically arranged between the second converging optical element (L2) and the first converging optical element (L1). Optical device (OD) according to any one of claims 5 to 6, wherein the first converging optical element (L1) and / or the second converging optical element (L2) comprises a lens. Optical device (OD) according to the preceding claim, wherein the first converging optical element (L1) and the second converging optical element (L2) are respectively made up of a first lens and a second lens. Optical device (OD) according to any one of the preceding claims wherein the optical retarder (OP) comprises at least one retarder blade. Optical device (OD) according to any one of the preceding claims wherein the optical retarder (OP) comprises a waveguide array. Optical device (OD) according to any one of the preceding claims wherein the optical retarder (OP) comprises at least one reflective optical part. Optical device (OD) according to any one of the preceding claims wherein the optical retarder (OP) comprises a diffraction grating. Optical device (OD) according to any one of the preceding claims wherein the optical arrangement includes a polarization splitter (PS). Optical device (DO) according to the preceding claim in which the optical retarder (LP) comprises a retarder blade (L4) for separating incoming and outgoing light beams. Optical system comprising a source associated with an optical device (OD) according to one of the preceding claims. Optical system according to the preceding claim wherein the source is configured to produce the incident light beam in pulsed form, composed of pulses having a predetermined maximum duration and wherein the distinct optical path gaps allotted by the optical retarder are chosen to temporally separate pulses present in the elementary light beams. Optical system according to any one of claims 15 to 16 wherein the optical path gaps allocated by the optical retarder are chosen to be smaller, advantageously ten times smaller, than a Rayleigh distance from the input light beam. Optical system according to any one of claims 15 to 17 further comprising an optical application device disposed downstream of the optical device (OD). Optical system according to the preceding claim in which the application optical device is an f-theta optical device. Optical system according to any one of claims 15 to 19 further comprising a shaping device (M) disposed between the source (S) and the optical device (DO).

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