EP4097535A1

EP4097535A1 - Device for processing a light beam via a multi-plane converter with a view to forming it into a predetermined shape

Info

Publication number: EP4097535A1
Application number: EP21705597.9A
Authority: EP
Inventors: Clément JACQUARD
Original assignee: Cailabs SAS
Current assignee: Cailabs SAS
Priority date: 2020-01-29
Filing date: 2021-01-25
Publication date: 2022-12-07
Also published as: WO2021152245A1; FR3106668A1; FR3106668B1; US20230141335A1

Abstract

The present invention relates to a device (1) for processing an input light beam comprising at least one optical pulse having an original duration, the processing device (1) aiming to form the input light beam into a predetermined shape. The device comprises an optical input (2); a stretching device (9), with a view to temporally elongating the duration of the optical pulse and thus transmitting a temporally stretched emission; a compressing device (10), with a view to at least partially restoring the original duration of the optical pulse; and an optical output (4). The processing device also comprises a shaping device (6), comprising at least one multi-plane converter placed upstream of the compressing device (10), which is configured to process the temporally stretched emission with a view to forming the output beam into the predetermined shape.

Description

DEVICE FOR PROCESSING A LIGHT BEAM VIA A MULTI-PLANE CONVERTER TO CONFORM IT TO A PREDETERMINED SHAPE

FIELD OF THE INVENTION

The present invention relates to an optical device for processing a light beam comprising a multi-plane converter, the optical device aiming to shape this beam and conform it to a predetermined shape. TECHNOLOGICAL BACKGROUND OF THE INVENTION

There are many applications where the spatial shaping of a beam which propagates in free space, ie shaping this beam so that it has a predetermined spatial shape, may be relevant. Among these applications, we can mention:

• Machining, drilling, precision cutting and surface treatment, ablation or functionalization of materials by laser, typically a pulsed laser,

• Imaging and in particular microscopy,

• Coupling in functional optical media such as waveguides, amplifying or converting media,

• Detection by LIDAR,

• Optical communications in free space.

This spatial shaping of the light beam can be implemented by multiple means, for example by using optical elements such as elements. imaging optics, diffractive elements, phase modulators, non-spherical and non-planar optical elements, such as aspherical or free-form optical elements (from the translation of the English expression “freeform optics”). In this regard, reference may be made to the document “Tailored laser beam shaping for efficient and accurate microstructuring”, Applied Physics A, Material Sicence & Processing, of 01/10/2018. Also known from US9250454 and US2017010463 are optical devices for converting multi-plane light (and more succinctly "multi-plane converter" in the remainder of this description), designated by the acronym MPLC (for "Multi Plane Light Conversion" according to the Anglo-Saxon expression), making it possible to achieve any unitary spatial transformation of light radiation.

From a theoretical point of view, and as established in “Programmable unitary spatial mode manipulation”, Morizur et Al, J. Opt. Soc. Am. A / Vol. 21, No. 11 / November 2010, a unitary spatial transformation can effectively be decomposed into a succession of primary transformations, each primary transformation affecting the transverse phase profile of light radiation. In practice, and without this forming any limitation of this technology, a multi-plane converter typically applies between 3 to 25 primary transformations.

As documented in patent application FR3076357, the implementation of a multi-plane converter requires the arrangement between them with great precision of optical parts whose tolerances are very tight. For example, when the converter is implemented in the form of a multi-pass cavity comprising a reflecting phase plate as described in the aforementioned application, this phase plate must have as great a flatness as possible, and the arrangement of this blade screw- with respect to the other optical parts of the converter be of the order of a micrometer and of a microradian. The assembly must also be very robust for its use in an industrial environment, and be able to withstand thermal and mechanical stresses (vibrations or turbulence of the surrounding air) of high intensities. Forming a more compact converter, using optical parts of smaller dimensions and arranged closer to each other, can help to solve this general problem. However, this search for compactness is limited by the size of the microstructured zones on which the beam is reflected and transformed, as well as by the propagation distance of the beam in free space necessary to complete the optical transformation. To remove this limitation according to both the transverse and the longitudinal dimension, it is necessary to reduce the size of the beam.

Reducing the size of the beam leads to densifying the spatial distribution of its energy. This increase in density can be amplified locally when the shaping of the beam, as is the case in a multi-plane converter, is based on an iterative modification of its spatial profile, which during these iterations affects the spatial distribution of his energy. The fluence, that is to say the surface energy density, on the optical parts which make up the converter can become locally very important and exceed the damage limits of these parts.

This problem arises in particular when the light beam is formed by a train of very short pulses, from a few femtoseconds to a few tens of nanoseconds, leading to pulsed fluences which may exceed the ablation threshold of the materials making up the parts. optical, this threshold typically being of the order of a fraction of J / cm ^A 2 for femtosecond pulses to a few J / cm ^A 2 for nanosecond pulses. Choosing more robust materials to form these parts or to provide a protective coating can be very expensive to implement, if not impossible.

An aim of the present invention is to remedy this problem at least in part. More specifically, an aim of the invention is to provide a device for shaping a light beam comprising a multi-plane converter, the beam being formed of at least one optical pulse and the converter being able to be produced in a compact manner, so as to facilitate its manufacture and improve its robustness during use, while limiting its cost.

BRIEF DESCRIPTION OF THE INVENTION

The object of the invention proposes a device for processing an input light beam comprising at least one optical pulse having an original duration, the processing device aiming to shape the input light beam according to a predetermined shape.

According to the invention, the treatment device comprises and comprising:

- an optical input to receive the input light beam;

a stretcher device for temporally lengthening the duration of the optical pulse by spectral spreading of the input light beam and thus propagating a temporally stretched radiation; a compressor device for at least partially restoring the original duration of the optical pulse;

an optical output, arranged downstream of the compressor device, to propagate an output beam; a shaping device, comprising at least one multi-plane converter arranged upstream of the compressor device, configured to process the temporally stretched radiation with a view to conforming the output beam to the predetermined shape.

The lengthening of the duration of the optical pulse makes it possible to reduce the size of the beam and to provide a compact multi-plane converter, which facilitates its manufacture and improves its robustness. This effect is all the more marked as the propagation distances necessary for carrying out the optical transformation, and therefore the longitudinal dimension of the converter, change quadratically with the size of the beam. According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination:

• the shaping device further comprises at least one diffractive optical element, a spatial phase modulator, an optical system comprising at least one lens, an axicon or a non-spherical and non-planar optical element; · The non-spherical and non-planar optical element is a reflective optical element;

• the multi-plane converter is configured to spatially separate, in a separation plane, the temporally stretched radiation into a radiation useful, in a target mode, and in a disturbance radiation, and the shaping device also comprises at least one device for blocking the disturbing radiation, arranged in the separation plane so that it does not contribute to the beam of disturbance. exit ;

• the processing device comprises a plurality of blocking devices arranged respectively in a plurality of separation planes in which part of the disturbing radiation is spatially isolated;

• the blocking device comprises at least one absorbing, diffusing or reflecting optical element;

• the processing device comprises at least one optical amplifier arranged between the stretching device and the compressor device;

• the multi-plane converter is placed downstream of an optical amplifier;

• the multi-plane converter is placed upstream of an optical amplifier;

• the optical amplifier is integrated into the multi-plane converter; · The shaping device comprises a first part disposed upstream of the compressor device, and a second part disposed downstream of this device.

According to another aspect, the object of the invention proposes an optical system comprising a source emitting a beam input light comprising at least one optical pulse having an original duration and a processing device as described above. Advantageously, the original duration of the optical pulse is between 1 femtosecond and several nanoseconds, and preferably less than 1 picosecond. BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and particularities of the invention will become apparent on reading the detailed description of implementations and embodiments which are in no way limiting, with regard to the appended drawings in which:

• Figure 1 shows an example of application of a processing device according to the invention;

• Figure 2 schematically shows a processing device according to the invention;

• Figure 3 shows a particular example of the processing device. DETAILED DESCRIPTION OF THE INVENTION

Definitions

For the sake of clarity, in the present application, a light radiation or a light beam is defined as radiation formed from at least one mode of the electromagnetic field, each mode forming a spatio-frequency distribution of the amplitude, of the phase, and the polarization of the field. Consequently, the modification or transformation of the phase of the light radiation means the modification or the spatio-frequency transformation of each of the modes of radiation.

For the sake of simplicity, in the present description, it will be considered that the radiations and / or beams are polarized in a single direction. However, the principles exposed are entirely applicable to radiation or a beam exhibiting more than one direction of polarization.

The term “shape” of a radiation or of a beam will denote the transverse distribution of the amplitude and the phase of the mode or the combination of the transverse distributions of amplitude and phase of the modes composing this radiation. It is therefore the spatial shape of the radiation or of the beam.

The term “spatial parameters of a light beam”, or more simply “parameter of the light beam”, denotes the scalar parameters defining the amplitude and phase distributions associated with the electromagnetic field. The following parameters can be cited as examples of such spatial parameters:

• the direction of propagation of the light beam, defined by means of the mean linear phase of the associated electromagnetic field;

• the position of the light beam, defined as the position of the center of gravity of the intensity distribution of the beam in a plane perpendicular to the direction of propagation of the beam;

• the horizontal or vertical size of the light beam, defined as the standard deviation of the horizontal or vertical marginal intensity distribution (as well defined by the international ISO standard); • the ellipticity of the beam;

• the divergence of the beam, possibly anisotropic.

For the sake of completeness, we recall the operating principles of a multi-plane converter. In such a device, an incident light beam undergoes a succession of reflections and / or transmissions, each reflection and / or shaping being followed by propagation of the beam in free space. At least some of the optical parts on which the reflections and / or the transmissions take place, generally in successive distinct locations, and which guide the propagation of the incident beam, have a microstructured surface which modifies the phase of the incident light beam to impart to it a predetermined optical function. There are thus generally provided at least 4 reflections and / or transmissions, such as for example 8, 10, 12, 14, or even at least 20 reflections and / or transmissions. Advantageously, the shape of the incident light radiation and of the modified light radiation are different from each other. One will also find in the documents “Programmable unitary spatial mode manipulation”, Morizur et 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 examples of practical implementation of such a multi-plane converter.

The term “microstructured surface” means that the surface of the optical part may have a relief, for example in the form of “pixels” whose dimensions are between a few microns to a few hundred microns. The relief or each pixel of this relief has a variable elevation with respect to a mean plane defining the surface in question, ranging from a few hundred nanometers at most to a few hundred microns at most, in absolute value. An optical part having such a microstructured surface forms a phase mask introducing local phase shifts within the cross section of the light beam which is reflected thereon or which is transmitted therein.

Application

By way of introduction to the present description, and by way of illustration of the needs which have led to the present invention, FIG. 1 shows an example of application of a treatment device 1 according to the invention in the field of machining and surface treatment.

Such a device 1 has an optical input 2 for receiving an input light beam from an optical source 3, for example a pulsed laser source. The input beam comprises at least one optical pulse having a determined duration. Conventionally, the input beam is formed of a train of pulses of substantially identical durations, the pulses of the train having a determined repetition frequency, but the invention is in no way limited to this conventional configuration. A processing device 1 in accordance with the invention is of very particular interest when the source is configured to generate an input beam having pulses the determined duration of which is very short, for example of the order of magnitude of femtoseconds (from Ifs to lps). The repetition frequency of these pulses is typically between a few kHz and 1 MHz, for example of the order of 100 kHz. The energy of an optical pulse generated by the source, in particular when the beam input has been amplified beforehand, can reach values of the order of micro Joule to several millijoules.

The size of the beam at the optical input 2 of the device has a value preferably less than 180 microns, preferably less than 130 microns, or even 100 microns. The beam can be treated in this way using compact devices. To be complete, it should be remembered that the optical spectrum of such an ultrashort pulse is not single-frequency, as may be the case for a continuous beam, and this spectrum therefore has a certain spectral width.

The processing device 1 also has an optical output 4 for emitting an output light beam. In the representation of FIG. 1, the optical input and output 2, 4 consist of a simple passage allowing the beams to propagate in free space, but provision could be made for this input and this output to be one and / or the other formed of a connector or an optical stage, for example making it possible to couple the processing device 1 to at least one input and / or output optical fiber. As will be detailed in the remainder of this description, the main function of the processing device 1 is to conform the output light beam to a predetermined shape. In the example shown in FIG. 1, the input beam has a Gaussian shape, symbolized by a circle in this figure, and the output beam has a so-called “top hat” shape, symbolized by a square. The shape of the output light beam is generally different from that of the input light beam, but this is not necessarily always the case. Indeed, the processing device 1 can seek to make stable and invariant the shape of an input beam, when this is capable of varying over time, without necessarily seeking to modify the nominal shape of this beam. These variations can correspond to an instability of the source or to a relative displacement of the source with respect to the other elements of the system, this displacement being able for example to be caused by vibrations or another type of mechanical interaction with the system environment. These variations lead to varying a parameter of the beam emitted by the source, and the processing device 1 can seek to compensate for this variation.

In the application example of FIG. 1, the treatment device is connected to a scanning device S making it possible to orient the output beam in a controlled manner, to direct it towards a part which must be treated and to scan the part thereof. area. For this purpose, the scanning device S, symbolized here by two mirrors, can comprise a plurality of movable mirrors whose orientation is controlled by a control device, not shown, so that the output beam follows a predetermined path. The scanning device S can include other optical parts, such as lenses, in order to image the output light beam on the part to be treated.

Depending on the nature of the treatment of the part, it may be preferable to project on this part a beam having a specific shape rather than another. The present invention therefore provides for providing a processing device for implementing this controlled shaping of the beam.

Treatment device With reference to FIG. 2, a description will now be given of the general architecture of an embodiment of the processing device 1. In addition to the optical input 2 and the optical output 4 which have already been described, such a device comprises, downstream of the optical input 2, a stretcher device 9 for temporally lengthening the duration of the optical pulses of the input beam. Such a stretching device tends to introduce a controlled delay (phase shift) between the different optical frequencies of the input beam in order to provide and propagate radiation, referred to in the remainder of this description as “temporally stretched radiation”. This temporally stretched radiation therefore exhibits pulses the duration of which can be up to 100 or 1000 times greater than the original duration of the pulses of the input beam. Consequently, the peak power of the pulses composing the temporally stretched radiation is very low. The shape of the input beam is not particularly affected by the stretcher device 9, so that an input beam having for example a Gaussian shape will lead to forming a temporally stretched radiation which also has an essentially Gaussian shape. It is noted that in general, the temporal spread of the "stretched" pulse must be less than the repetition period of this pulse (when the beam coming from the source 3 is composed of such a repetition of pulses ), to avoid temporally superimposing a plurality of pulses after this treatment. There are many ways of making such a stretching device 9. FIG. 2 shows an implementation based on a pair of diffraction gratings, a pair of lenses and a plane mirror. This arrangement, as is well known per se (see in particular document US Pat. No. 4,92931), makes it possible to introduce a time-spreading of the pulses without introduce spatial transformation. Alternative arrangements are possible by replacing, for example, the second half of the device by a plane mirror so as to effect a folding. One could of course consider other implementations to achieve this temporal spreading of the pulses composing the input beam, for example based on optical prisms or fiber or volume Bragg gratings. Continuing the description of FIG. 2, the processing device 1 also comprises a compressor device 10, arranged downstream of the stretching device 9 and upstream of the optical output 4. This device aims to restore (at least in part) the original duration. of the optical pulse, by performing an inverse treatment to that of the stretcher device, that is to say by eliminating at least part of the controlled delay introduced between the different optical frequencies of the original pulse. It provides an output beam, comprising a train of pulses whose duration is similar to the duration of the pulses of the input beam, or even reduced compared to the original duration of the pulses of the input beam. This output beam is directed towards the optical output 4 of the processing device 1.

Similar to the stretching device 9, an exemplary implementation of the compressor device can comprise two diffraction gratings placed parallel to one another and facing each other and a mirror arranged so that the beam makes a path. round trip optic between these two networks.

Other modes of implementation are possible, in particular based on optical prisms or fiber or volume Bragg gratings. The radiation propagating in the treatment device 1, between the stretching device 9 and the compressor device 10, is therefore formed of pulses whose duration is much greater than that of the original pulses of the input beam and whose peak power is much less. This radiation can therefore be treated, in order to be shaped, by conventional optical parts, of relatively small and inexpensive dimensions. It is therefore possible, to a certain extent, to reduce the size of the input beam or to preserve a beam of reasonable size, which makes it possible to give a compact shape to the shaping device 6, in particular when the latter comprises a multi converter. plan, as will be detailed in a later section of this application.

Continuing the description of the example shown in FIG. 1, arranged at least in part between the stretching device 9 and the compressor device 10, a treatment device in accordance with the present description comprises a shaping device 6 for treating the radiation temporally stretched from the stretcher device 9 and produce radiation, designated in the remainder of this description by “output radiation” having a predetermined shape. This output radiation is then propagated to and processed by the compressor device 10, as described above, without affecting the shape of this radiation, so that the shaping device 6 has the effect of shaping the output beam. of the treatment device in the predetermined shape.

In the context of the present description, and with reference to FIG. 3, the shaping device 6 comprises a multi-plane converter 5, arranged upstream of the compressor device 10. This multi-plane converter 5 may have been configured to put in place. forms the beam so that the output beam has the determined shape and / or to stabilize the input beam, by modal filtering. It is possible to rely on the references provided in an earlier passage of this description to configure the multi-plane converter so that it shapes the temporally spread radiation into radiation of a different shape, contributing to the determined shape of the output beam. But in addition to this multi-plane converter 5, the shaping device can also include an arrangement 11 of optical elements chosen from the following list to assist the transformations carried out by this converter 5:

• a diffractive optical element (“DOE” for “Diffractive Optical Element” according to the English expression);

• a spatial phase modulator (“SLM” for “Spatial Light Modulator” according to the English expression),

• an optical system imaging or comprising at least one lens;

• an axicon;

• a spherical mirror;

• at least one optical, transmissive or reflective, non-spherical and non-planar element, such as an aspherical or free-form optical element (from the translation of the English expression “freeform optics”).

Advantageously, these optical elements will be of the reflective type, since they have a lower chromatic dispersion and a higher power handling than an optical element of the transmissive type.

The multi-plane converter 5, possibly assisted by the optical elements of the arrangement 11 of the shaping device 6, are configured and arranged so that the output beam has the predetermined shape. This shape is chosen according to the needs of the application and may correspond, at the output of the processing device 1 or after propagation of the output beam in free space or guided to, for example:

• a square or rectangular “Fiat Top” beam;

• a circular flat top beam (“Fiat Top”);

• a Bessel beam;

• a line-shaped plate beam (product of a flat plate in a first direction and a Gaussian in a direction perpendicular to the first);

• a ring-shaped beam;

• or any other form of beam relevant to the process concerned.

To facilitate the shaping of the output radiation and to form a beam according to one of the shapes given as an example above, provision can in particular be made to include in the shaping device 6 at least one non-spherical optical element and non-planar, and preferably between 1 and 3 of these elements. The term “non-spherical and non-planar optical element” denotes an optical, transmissive or reflective element, the surfaces of which are neither spherical nor plane. It can for example be an aspherical optical element (which generally has a symmetry of revolution about an axis perpendicular to its mean plane) or a free-form optical element (which does not present a symmetry of revolution. or translation around an axis perpendicular to its mean plane).

A small number of such non-spherical and non-planar optical elements, from 1 to 3, allows output radiation to be shaped into a wide variety of beam shapes, particularly those exemplified above, which find a particular interest in machining, drilling, precision cutting and surface treatment of materials by laser applications. This is most particularly the case when the input beam of the device 1 is single-mode.

Multi-plane modal filter converter

Advantageously, the shaping device 6 comprises a multi-plane converter 5 (MPLC) which makes it possible to ensure modal filtering of the temporally stretched radiation, that is to say to separate and / or extract from this radiation unwanted spatial modal components. These components can come from a variation over time in the shape of the beam coming from the source 3, or more generally coming from the optical elements arranged upstream of the multi-plane converter, which it is therefore desired to stabilize.

Advantageously, this MPLC is placed upstream of a possible arrangement of optical elements 11 aimed at shaping the output beam, or at completing its shaping, as illustrated in FIG. 3.

In general, an MPLC makes it possible to transform a base of transverse spatial input modes into a base of output modes. To design an MPLC, it is therefore necessary to define these two bases. To implement modal filtering, the input base is determined from the assumed (or nominal) incident light radiation at the MPLC and knowledge of its possible variations. The basis of the MPLC output modes is determined by the desired light radiation at the output of this device and by the nature of the means for extracting the optical power of the components. unwanted modals. The desired (or “useful”) radiation mode at the output of the MPLC device 5 will be designated as the target mode. By way of example, the following procedure can be followed to determine the base of input modes of the MPLC 5, when the nominal temporally stretched radiation is single-mode:

• define as nominal input mode, the mode as close as possible to the time-stretched radiation mode that will actually be used;

• construct a family of modes from the nominal input mode of which at least one of the spatial parameters is varied;

• to build a base of modes starting from the family of modes by using a decomposition in singular values;

• determine a first mode of the base, such as the mode of the base having maximum overlap with the assumed input mode; · Complete the first mode with the modes obtained during the decomposition into singular values then apply a process of orthonormalization of these modes to form the input base. If the input beam is produced by a multimode optical source 3, for example through a multimode fiber, the input basis of the MPLC 5 can be constructed from the modes of the source rather than by the described method. upper. The nominal input mode is then the modal component of the beam comprising the most optical power.

In all cases, the input mode base is made up of modes which are not entirely spatially disjoint. We know from the document “Quantum measurements of spatial conjugate variables: displacement and tilt of a Gaussian beam”, V. Delaubert, Optics Letters, Vol. 31, Issue 10, pp. 1537-1539, (2006), that within the limit of small variations of a physical parameter of interest (for example, but not exclusively, the direction and / or the position of a light beam), a part of the energy of the light beam is transferred to the higher order mode (s). Thus, in the case of an assumed Gaussian input mode, one could decide to use a Hermite-Gauss input basis whose first mode is the assumed Gaussian input mode.

To determine the base of output modes of the MPLC 5 modal filtering device, a base is chosen for which there is a plane, called a separation plane in the present description, in which the output modes other than the target mode are sufficiently disjointed. spatially of the target mode so that these output modes can be blocked, for example by an absorbing or diffusing optical element, or deflected by a reflecting optical element, it being understood that the target mode undergoes an arbitrarily low energy loss. For example, one could use Gaussian output modes, sufficiently spatially separated so that their overlap is zero two by two and distributed linearly in the separation plane, in a triangle or a rectangle.

The MPLC modal filtering device 5 of a shaping device 6 conforming to the present description is configured to transform the base of input modes into the base of output mode by ensuring in particular that the assumed input mode is transformed into target mode.

The optical blocking, diffusing or reflecting element (or more generally the blocking device 8) can be integrated into the shaping device 6. It can in particular be integrated into the MPLC 5, or physically separated from the latter, as is the case in the schematic representation of FIG. 3. The MPLC 5 can in particular be accompanied by a detection element making it possible to collect the output modes of the MPLC 5 associated with the disturbance modes of the incident light beam with a view to their total or partial detection. There can be a multitude of transverse planes in which the useful radiation and the disturbing radiation are spatially separated. In this case, it is possible to place the blocking device 8 in at least one of these planes, or to distribute the optical elements making up the blocking device 8 in a plurality of such planes.

In the case of a particular temporally stretched radiation, corresponding to a nominal temporally stretched radiation of which at least one of the spatial parameters would have been modified, the modal filtering device MPLC 5 performs a projection, in the mathematical sense of the term, of this particular radiation based on its input modes. Optical power in each input mode of the MPLC 5 is transferred to the associated output mode. In the absence of a change in at least one of the spatial parameters, all of the optical power of the particular input light radiation is transferred by the MPLC 5 to the target mode. In case of variation of the parameters of the temporally stretched radiation vis-à-vis the nominal parameters of this radiation, the part of optical power of the particular temporally stretched radiation in the nominal input mode, designated as the useful power, is transferred by the MPLC 5 from nominal input mode to target mode as wanted radiation. The optical power share of the temporally stretched radiation particular in Other input modes of the base, referred to as unnecessary power, is transferred by the MPLC 5 to the output modes, orthogonal to the target mode, as disturbance radiation. In other words, the MPLC is configured to spatially separate, in a separation plane 7, the temporally stretched radiation into useful radiation, in a target mode which propagates to the shaping elements of the arrangement 11, and in a disturbance radiation.

The part of optical power in the output modes of the MPLC 5 orthogonal to the target mode, that is to say the disturbance radiation, can then be blocked by the blocking device 8, formed for example of an absorbing optical element , diffusing, or by any optical collection element with a view to total or partial detection.

The MPLC 5 thus constitutes a passive modal filter where, within the limit of small variations of the spatial parameters of the temporally stretched radiation, the optical power of the light radiation which is outside the nominal input mode is completely extracted.

The target mode (constituting the useful radiation) and the other modes (constituting the disturbance radiation) may not all be separable in a single separation plane. In this situation, it is possible to provide a plurality of separation planes, distant from each other by a propagation space, in which it is possible to successively isolate some of the other modes contributing to the disturbance radiation. In this case, a plurality of blocking devices 8 will be provided, which will be placed respectively in the separation planes, in order to block the part of the disturbance radiation which is spatially isolated in each plane. The MPLC 5 modal filtering device makes it possible to separate the useful power from the non-useful power, that is to say the useful radiation from the disturbing radiation. It may be particularly advantageous to optimize the spatial parameters of the input light beam or of the temporally stretched radiation by maximizing and / or minimizing respectively the useful power and / or the non-useful power. It is possible to use an active control device (motorized mirrors, or an acousto-optical device for example) or non-active (mirrors with optomechanical mount) arranged upstream of the MPLC device 5, for example integrated into the stretching device 9 or arranged between the source 3 and the processing device 1. In the case of the use of active alignment devices, their parameters are adjusted by means of a feedback loop of which at least one of the input parameters would be the total or partial measurements of useful and / or non-useful powers. As has already been stated, the multi-plane converter 5 can both seek to shape, at least in part, the temporally stretched radiation and modally filter this radiation. This is the case in particular when the multiplane converter has been configured so that the target mode of the output base has a form different from the nominal input mode of the input base. In this case, it is possible to design a single multiplane converter implementing successively or simultaneously the function of modal filtering of the temporally stretched radiation and the function of shaping this same radiation, or part of this shaping.

Optical amplifier According to a particular embodiment, provision can be made to place an optical amplifier between the stretching device 9 and the compressor device 10. It is then taken advantage of the fact that the radiation propagating between these two devices has a relatively low peak power in order to be injected into an optical gain medium. The optical amplifier can be implemented by means of any known technique, for example a doped fiber amplifier, or an amplifier in free space where the gain medium is a transparent optical element.

It may for example be a regenerative cavity and a multipass amplifier.

Several variants of this mode of implementation are possible.

Thus, provision can be made to place the optical amplifier directly downstream of the multi-plane converter 5. The latter therefore processes a temporally stretched radiation coming from the stretching device 9 which therefore has a relatively low peak power, which makes it possible to limit its bulk. .

Alternatively, it is also possible to envisage placing the optical amplifier upstream of the multi-plane converter.

5. This particularly advantageous configuration, when the conversion device 5 of the shaping device 6 performs modal filtering, makes it possible to eliminate any instabilities of the radiation induced by the optical amplifier. Although the energy of the temporally stretched radiation processed by the multi-plane converter is greater than the configuration of the previous variant, advantage is taken of the temporal spreading of the radiation to preserve the compactness of the shaping device.

6. In another alternative, it is possible to exploit the presence of an optical amplifier in the processing device 1 to limit the energy of the input beam supplied by the source 3. In this case, it is then possible to provide for placing the multi-plane converter. 5, between optical input 2 and stretching device 9.

It is also possible to envisage mixing the preceding variants, by placing the multi-plane converter 5 between two optical amplifiers, a first optical amplifier being arranged upstream of the converter 5 (before or after the stretching device 9) and a second optical amplifier being arranged in parallel. downstream of converter 5, upstream of compression device 10.

Finally, it is also possible to provide a single device combining the beam shaping function with the amplification function. For example, this single device can be formed from a gain medium of a multipass cavity, through which the radiation passes multiple times. At least one of the reflecting surfaces of the cavity can be provided with a plurality of microstructured zones implementing a multi-plane converter aiming to shape the radiation in the cavity. To simplify the practical realization of this unique device, a limited number of microstructured zones (2 to 5) can be arranged on at least one of the reflecting surfaces to intercept the radiation during its last reflections, before it s' extracted from the cavity.

Alternatively, it is possible to form a single device based on a multi-pass cavity architecture as in the previous example, but in which at least one of the reflecting surfaces would be formed of an optical element. aspherical, non-planar or free-form. This shape would be designed and configured to shape the radiation and help give the output beam its predetermined shape.

In the case where the input beam consists of short pulses, for example between 1 femtosecond and several nanoseconds (such as 5 ns), and preferably less than 1 picosecond, the processing device 1 becomes particularly relevant by:

• its compatibility with light beams of wide optical spectra;

• its compatibility with beams whose energies per pulse are high, while preserving a relatively small transverse dimension of the beam, which makes it possible to form a compact device;

• the robustness it provides to changes in the spatial parameters of the input beam when a modal filtering device is provided;

• the small number of optical elements that make up the device ensures the stability of the spatial parameters of the beams produced, and the ease of manufacture of the device.

Example

An example of implementation making the advantages of the invention very apparent is described below.

A pulsed laser source emits an input light beam having a Gaussian shape and whose carrier frequency is in the visible or near infrared. This source delivers pulses with a duration of the order of a hundred femtoseconds with a repetition rate of the order of a few tens to a few hundred megahertz and energies per pulse of the order of ten nanojoules. In some cases, it may be advantageous to lower the repetition rate to ten or a hundred kilohertz using an electro-optical or acousto-optical device that selects one pulse out of a hundred or a thousand, for example. .

This input beam is introduced into the optical input 2 of a processing device 1 by free propagation. The pulses contained in the beam are stretched temporally using a stretcher device 9 similar to the stretcher device shown in FIG. 3 and composed of standard components, such as holographic diffraction gratings at 600 lines per millimeter and lenses of common focal lengths, for example one hundred or two hundred millimeters. A temporally stretched radiation is then commonly obtained having pulses the duration of which is of the order of ten to one hundred picoseconds, that is to say 100 to 1000 times longer than their original durations. The pulses are then amplified using an optical amplifier in free space such as a regenerative cavity or a multipass amplifier, or the successive combination of these two solutions. The pulses thus obtained then contain an energy of a few tens of microjoules to a few tens of millijoules depending on the gain of each amplifier stage.

Following this optical amplification, a spatial shaping of the amplified radiation is carried out using a shaping device 6 in accordance with the device 6 described. in connection with the description of FIGS. 2 and 3. The shaping device 6 comprises in particular here a multi-plane conversion device 5 configured to stabilize the parameters of the radiation coming from the optical amplification stage, the propagation of the radiation of disturbance then being blocked by an absorbent element. The useful radiation supplied by the multiplane conversion device exhibits, at the end of this treatment, a stable and Gaussian form. This radiation is then shaped into a flat square plate by an arrangement formed of 2 free-form reflective optics, the respective shapes of which have been designed to precisely effect this transformation. Assuming that the radiation incident to the shaper 6 is composed of 100 picosecond pulses each containing 1 millijoule with a repetition rate of 100 kilohertz, this radiation then has an average optical power of 100 watts. In the case of Gaussian incident radiation, and for a maximum fluence tolerated by the parts making up the shaping device 6 of 10J / cm ² for the pulse durations concerned, a size of the radiation incident to the shaping device 160 micron shape 6 prevents damage. For such a beam size, the multi-plane conversion device 5 can then be contained in a volume of the order of 250cm ³ .

The useful part of the radiation emitted by the shaping device 6 propagates and the pulses it contains are temporally compressed using a compressor device 10 similar to that described in relation to the description of FIG. 2. It is here composed of diffraction gratings similar to those of the stretcher device 9 described above. The pulses of radiation emitted by the compressor device 10 then reach a duration of the order of a hundred femtoseconds, 500 femtoseconds for example. This radiation propagates towards the optical output to form the output beam, the shape of which is a square flat plate and the parameters of which are very stable.

Under the same working hypotheses (i.e. Gaussian-shaped incident radiation containing 500 femtosecond pulses of 1 milli joule each and a repetition rate of 100 kilohertz), the use of a setting device in shape 6 of the state of the art would have required that the radiation incident to the shaping device 6 has a minimum size of 1100 micrometers in order to avoid any damage to the optical parts. The multi-plane converter 5 of this counterexample would then have occupied a volume of the order of 5000cm ³ , twenty times greater than the multi-plane converter 5 of a shaping device 6 in accordance with the invention.

Of course, the invention is not limited to the embodiment described and to its variants. It is possible to provide alternative embodiments without departing from the scope of the invention as defined by the claims.

The processing device 1 can provide optical elements other than those which have just been described. They may in particular be elements, such as the mirrors shown in FIG. 2, making it possible to guide the propagation of the optical radiation passing through the device 1 so that it is processed successively by the different devices that compose it (stretching device, compressor, transmission, amplifier (s)).

Furthermore, provision can be made for the processing device 1 to include other optical elements aimed at putting into forms the bundle, for example disposed downstream of the compressor device 10. The shaping device 6 then comprises a first part disposed upstream of the compressor device 10, and a second part disposed downstream of this compressor device 10. Care will be taken in this case to choose the optical elements of the second part of the shaping device 6 so that they are compatible with the energy delivered by the output beam.

Claims

1. Processing device (1) of an input light beam comprising at least one optical pulse having an original duration, the processing device (1) aiming to shape the input light beam according to a predetermined shape and comprising: an optical input (2) for receiving the input light beam; - a stretcher device (9) for temporally lengthening the duration of the optical pulse by spectral spreading of the input light beam and thus propagating a temporally stretched radiation;

- a compressor device (10) for at least partially restoring the original duration of the optical pulse;

- an optical output (4), arranged downstream of the compressor device (10), for propagating an output beam;

- a shaping device (6), comprising at least one multi-plane converter (5) arranged upstream of the compressor device (10), configured to process the temporally stretched radiation with a view to conforming the output beam to the predetermined shape .

2. Processing device (1) according to the preceding claim wherein the shaping device (6) further comprises at least one diffractive optical element, a spatial phase modulator, an optical system comprising at least one lens, an axicon or a non-spherical and non-planar optical element.

3. Processing device (1) according to the preceding claim wherein the non-spherical and non-planar optical element is a reflective optical element.

4. Processing device (1) according to one of the preceding claims wherein the multi-plane converter (5) is configured to spatially separate, in a separation plane (7), the temporally stretched radiation into useful radiation, in a target mode, and in a disturbance radiation, and the shaping device (6) also comprises at least one blocking device (8) of the disturbing radiation, arranged in the separation plane (7) so that it does not not contribute to the output beam.

5. Treatment device (1) according to the preceding claim comprising a plurality of blocking devices (8) respectively arranged in a plurality of separation planes (7) in which a part of the disturbing radiation is spatially isolated.

6. Treatment device (1) according to one of the two preceding claims wherein the at least one blocking device (8) comprises an absorbing, diffusing or reflecting optical element.

7. Treatment device (1) according to one of the preceding claims comprising at least one optical amplifier arranged between the stretching device.

(9) and the compressor device (10).

8. Processing device (1) according to the preceding claim wherein the multi-plane converter (5) is disposed downstream of an optical amplifier.

9. Processing device (1) according to claim 7 or 8 wherein the multi-plane converter (5) is arranged upstream of an optical amplifier.

10. Processing device (1) according to claim 7 wherein the optical amplifier is integrated into the multi-plane converter (5).

11. Treatment device (1) according to one of the preceding claims wherein the shaping device (6) comprises a first part disposed upstream of the compressor device (10), and a second part disposed downstream of this device. (10).

12. Optical system comprising:

- a source (3) emitting an input light beam comprising at least one optical pulse having an original duration; - a processing device (1) according to one of the preceding claims.

13. Optical system according to the preceding claim in which the original duration of the optical pulse is between 1 femtosecond and several nanoseconds, and preferably less than 1 picosecond.