WO2020021196A1

WO2020021196A1 - Device for processing light radiation, comprising a multi-plane light conversion device

Info

Publication number: WO2020021196A1
Application number: PCT/FR2019/051830
Authority: WO
Inventors: Clément JACQUARD; Guillaume LABROILLE; Olivier Pinel
Original assignee: Cailabs
Priority date: 2018-07-25
Filing date: 2019-07-24
Publication date: 2020-01-30

Abstract

Device (1) for processing light radiation, comprising: - an optical input (2) for receiving an input beam from an optical source (3) and for propagating in the device (1) an input radiation, an optical output (4) for emitting an output beam having predetermined spatial parameters, an MPLC conversion device (5) which is arranged between the optical input (2) and the optical output (4) and which is configured to spatially separate, in a separation plane (7), the input radiation into useful radiation, in a target mode, which is propagated to the optical output (4) and into an interference radiation. The processing device also comprises at least one device (8) for blocking the interference radiation, which blocking device is arranged in the separation plane (7) so that it does not contribute to the output beam.

Description

DEVICE FOR PROCESSING LIGHT RADIATION COMPRISING A DEVICE FOR CONVERTING MULTIPLAN LIGHT

FIELD OF THE INVENTION

The present invention relates to an optical device for processing light radiation, for example modal filtering and / or shaping of this light beam, comprising a multiplanal light conversion device.

TECHNOLOGICAL BACKGROUND OF THE INVENTION

There are many applications where the spatial shaping of beams in free space can be relevant. Among these applications, we can cite:

• Machining, drilling, precision cutting and surface treatment of materials by laser,

• Imaging and in particular microscopy,

• Detection by LIDAR,

• Optical communications in free space.

Any device for the spatial shaping of a light beam can be sensitive to the spatial parameters of this beam. Indeed, the quality of spatial shaping of a light beam can be altered if the spatial parameters of the beam are not those which were provided during the design of the shaping device.

When the spatial parameters of a light beam are critical, it is common to perform amplitude filtering on the incident beam using one or more diaphragms and potentially an imaging optical system. This method converts a variation in position and / or direction and / or size of the incident light beam into a loss of optical power and into a deformation of this beam. Although this method has the advantage of simplicity of implementation, it only partially limits the effects of a variation in position and / or direction and / or size of the incident light beam in relation to the beam parameters. as they were supposed at the time of the design of the device. In addition, the very nature of amplitude filtering implies an initial cost in optical power.

We know from US9250454 and US2017010463 optical devices for converting multiplanar light, designated by the acronym MPLC (Multi Plane Light Conversion according to the English expression), making it possible to carry out any unitary spatial transformation of light radiation.

From a theoretical point of view, and as has been 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 actually be broken down 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 on this technology, MPLC components typically apply between 3 to 25 primary transformations.

The present invention presents a device for processing light radiation, for example for modal filtering and / or shaping of light radiation. This device, implementing an MPLC device, differs from known methods based on diaphragms in that it makes it possible to process the input beam more completely while limiting the losses of optical power.

BRIEF DESCRIPTION OF THE INVENTION

The object of the invention provides a device for processing light radiation comprising:

- an optical input to receive an input beam from an optical source and to propagate in the device an input radiation;

an optical output for emitting an output beam having predetermined spatial parameters;

an MPLC conversion device arranged between the optical input and the optical output and configured to spatially separate, in a separation plane, the input radiation into useful radiation, in a target mode, which propagates towards the optical output and in disturbance radiation; at least one device for blocking the disturbance radiation, arranged in the separation plane so that it does not contribute to the output beam.

According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination:

- The processing device comprises a transmission device for shaping the useful radiation and forming the output beam; the processing device comprises a plurality of blocking devices respectively disposed in a plurality of separation planes in which part of the disturbing radiation is spatially isolated;

the blocking device comprises an absorbing, diffusing or reflecting optical element;

the blocking device is integrated at least in part into the MPLC conversion device;

the blocking device comprises a detection element for collecting at least part of the disturbing radiation;

the transmission device comprises, arranged downstream of the conversion device, a diffractive optical element, a spatial phase modulator, an optical system comprising at least one lens, an axicon, a beam splitter and / or a second conversion device MPLC.

- the transmission device is integrated at least in part into the MPLC conversion device.

- The transmission device comprises at least one non-spherical and non-planar optical element, and preferably between 1 and 3 non-spherical and non-planar optical elements.

- the non-spherical and non-planar optical element is a reflective optical element. - The processing device also includes a control device between the optical input and the MPLC conversion device.

- the control device is an inactive optical element;

- the MPLC conversion device is configured to transform a base of input modes into a base of output modes, the bases of input and output modes being with separable spatial variables.

BRIEF DESCRIPTION OF THE FIGURES

Other advantages and particular features of the invention will appear on reading the detailed description of implementations and embodiments which are in no way limitative, with reference to the appended drawings in which:

FIG. 1 represents a first example of implementation of a processing device according to the invention,

FIG. 2 is a graph representing the variation of the overlap between the mode of the output beam and the target mode and of the optical power in the output beam as a function of the direction of the input beam,

FIG. 3 represents a second example of implementation of a processing device according to the invention,

FIG. 4 illustrates an example of modal decomposition of an input beam according to the invention,

• Figure 5 shows a modal filtering device according to the invention. DETAILED DESCRIPTION OF THE INVENTION

In this description, “mode” designates a transverse vector mode of the electromagnetic field, that is to say the data of the spatio-frequency amplitude and phase distributions associated with the two transverse spatial components of the mode.

A mode base is a set of orthonormal modes of the electromagnetic field. Consequently, the modification or the transformation of a mode base designates the spatio-frequency modification or transformation of at least one of the modes of the mode base.

The overlap between two modes is defined as the square module of the integral in a transverse plane of the scalar product between the two modes.

By light beam is meant an electromagnetic field consisting of a mode or of a superposition of several modes.

The spatial parameters of the light beam denote the scalar parameters defining the amplitude and phase distributions associated with the electromagnetic field.

As an example of spatial parameters of a light beam, the following parameters can be cited:

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

the position of the light beam, defined as the position of the center of gravity of the beam intensity distribution 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.

It will be considered in the present description that the light beams are polarized in a single direction. However, the principles set out are entirely applicable to a light beam having more than one direction of polarization. Thus, the spatial phase shift applied to a light beam by a phase mask in the context of the description may be expressed more generally as a spatio-frequency phase shift, that is to say modifying the phase shift according to the space variables in a transverse plane and according to the optical frequency.

For the sake of completeness, the principles of operation of a multiplanal light conversion device, more commonly known by the English terms “Multi-Plane Light Converter” (MPLC), are recalled. In such a device, an incident light beam undergoes a succession of reflections and / or transmissions, each reflection and / or transmission being followed by propagation of the beam in free space. At least some of the optical parts on which the reflections and / or transmissions take place, generally at distinct successive 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 it an optical function predetermined. There is 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 the modified light radiation are different from one another.

By “microstructured surface” it is meant that the surface of the optical part can have a relief, for example in the form of “pixels” whose dimensions are between a few microns to a few hundred microns. Each pixel has an elevation, with respect to an average plane defining the surface in question, of at most a few microns or at least a few thousandths of a micron. An optical part having such a microstructured surface forms a phase mask introducing local phase shifts within the transverse section of the light beam which is reflected or transmitted therein.

There is shown in Figure 1 an example of implementation of a processing device 1 according to the invention.

Such a device 1 has an optical input 2 for receiving an input light beam from an optical source 3 and for propagating in the device 1 an input radiation, and 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 single passage allowing the propagation in free space of the beams, but one could provide that this input and this output is one and / or the other formed by a connector or an optical stage, making it possible for example to couple the device 1 to at least one input and / or output optical fiber. The processing device 1 comprises a multi-plane MPLC light conversion device 5 which makes it possible to provide modal filtering of the input beam, that is to say to separate and / or extract from the input beam unwanted modal components. The output beam therefore corresponds, at a minimum, to the filtered input beam of undesired modal components. The output beam has parameters chosen in advance (predetermined) and these may be different from the parameters of the input beam. The MPLC device 5 can possibly be supplemented by any element for shaping the beam 6, also designated by the term of transmission device 6, once the filtering has been carried out.

In general, an MPLC makes it possible to transform a base of 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 an assumed (or nominal) input light beam. The base of output modes of the MPLC is determined by the light beam desired at the output of the device and by the nature of the means of extraction of the optical power of the undesired modal components. The mode of the desired light radiation (or “useful”) at the output of the MPLC device 5 will be designated as the target mode.

As an example, we can follow the following procedure to determine the base of MPLC input modes, when the assumed input beam is single mode:

Define a supposed input mode, the mode as close as possible to the mode of the input beam which will actually be used, • construct a family of modes from the assumed input mode of which at least one of the spatial parameters is varied,

• build a mode base from the mode family using a decomposition into singular values,

• determine a first mode of the base, like the mode of the base having a 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 by means of a multimode fiber, the input base of the MPLC 5 can be constructed from the modes of the source rather than by the method described upper. The assumed input mode is then the modal component of the assumed input beam having the most optical power.

In all cases, the input mode base is composed 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)

(https: // www. osapublishing. org / ol / abstract. cfm? uri = ol-31 -10 - 1537), only within the limits of small variations of a physical parameter of interest, (for example, but not exclusively, the direction and / or the position of a light beam), part of the energy of the light beam is transferred to the higher order mode (s). So, in the case of a supposed Gaussian input mode, we could decide to use a Hermite-Gauss entry base, the first mode of which is the supposed Gaussian entry mode.

To determine the base of output modes of the MPLC 5, a base is chosen for which there is a plane, called separation plane in the present description, in which the output modes other than the target mode are sufficiently spatially disjoint from the target mode so that these output modes can be blocked, for example by an absorbing or diffusing optical element, or deviated by a reflecting optical element, it being understood that the target mode undergoes an arbitrarily low energy loss. For example, we 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 5 of a device 1 conforming to the present description is configured to transform the base of input modes into the base of output mode while ensuring in particular that the assumed input mode is transformed into the target mode

The blocking, diffusing or reflecting optical element (or more generally the blocking device 8) can be integrated into the device 1 of the invention. It can in particular be integrated into the MPLC 5, or physically separated from it, as is the case in the schematic representation of FIG. 1. The MPLC 5 can in particular be accompanied by a detection element making it possible to collect the modes of MPLC 5 output associated with the disturbance modes of the incident light beam with a view to their total or partial detection.

In the case of a particular input beam, corresponding to a supposed input beam of which at least one of the parameters space would have been modified, the MPLC 5 makes a projection, in the mathematical sense of the term, of this particular beam on the basis of its input modes. The optical power in each input mode of the MPLC 5 is transferred to the associated output mode. In the absence of a variation of at least one of the spatial parameters, the entire optical power of the particular input light beam (which then corresponds to the assumed input beam) is transferred by the MPLC to the mode target. If the spatial parameters vary, the share of optical power of the particular input light beam in the assumed input mode, designated as the useful power, is transferred by the MPLC 5 from the assumed input mode to the target mode. in the form of useful radiation. The share of optical power of the particular input light beam in the other modes of the base, designated as the non-useful power, is transferred by the MPLC 5 to the output modes, orthogonal to the target mode in the form of disturbance radiation. . In other words, the MPLC is configured to spatially separate, in a separation plane 7, the input radiation into useful radiation, in a target mode, which propagates towards the optical output 4 and into disturbance radiation.

The share of optical power in the output modes of the MPLC 5 orthogonal to the target mode, that is to say the disturbing 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 for total or partial detection.

The MPLC 5 thus constitutes a passive modal filter where, within the limit of small variations in spatial parameters, the optical power of the light beam which is outside the assumed input mode is fully extracted. The device 1 thus provides a device 8 for blocking the disturbing radiation, arranged in the separation plane, so that this radiation does not propagate towards the optical output 4 of the device and that it does not contribute to the output beam. As we have seen, this blocking device 8 can comprise an optical blocking, diffusing and / or reflective element.

In a particular implementation, the optical power in the target mode at the output of the MPLC 5, that is to say the useful radiation, can be directed, within the processing device 1 to another optical element 6 , designated by the term of transmission device 6 in the present description, forming a light beam. This optical element 6 can be, for example, but not exclusively:

Another MPLC device,

• a diffractive optical element (DOE for Diffractive Optical Element according to the Anglo-Saxon expression),

• a spatial phase modulator (SLM for Spatial Light Modulator according to the Anglo-Saxon expression),

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

• an axicon,

• 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”).

In a particularly advantageous embodiment, the shaping of the beam produced by the transmission device 6 can be implemented at least in part by another MPLC. In this case, it is possible to design an MPLC implementing successively or simultaneously the modal filtering function of a light beam and the function of shaping of this same light beam, or part of this shaping.

The target mode (constituting the useful radiation) and the other modes (constituting the disturbing 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 one can successively isolate a part of the other modes contributing to the disturbance radiation. In this case, a plurality of blocking devices 8 will be provided which will be disposed respectively in the separation planes, in order to block the part of the spatially isolated disturbing radiation in each plane.

The interest of the invention is demonstrated in the following paragraphs by detailing a concrete implementation of the modal filtering of a light beam and, possibly, of its spatial shaping.

Example 1

We want to stabilize an input light beam whose transverse spatial profile is Gaussian with a diameter of 1 mm and which is likely to experience a variation in its direction of propagation. The mode of this light beam, without variation of the direction of propagation, defines the supposed input mode of the MPLC 5.

We decide here to use a Hermite-Gauss base of only three modes as the input base. There are two higher order modes associated with beam direction variation input, one for each direction orthogonal to the plane transverse to the propagation of the beam.

The output modes of the MPLC 5 are determined by the light beam desired at output 2 of the device 1. For reasons of simplicity, but without limiting the generality of the device, a light beam at output is chosen here consisting of an identical target mode in the input mode, ie a light beam whose transverse spatial profile is Gaussian with a diameter of 1 mm. In other words, the device 1 of this example does not include a transmission device 6 for shaping the output beam.

For the two output modes complementary to the target mode, that is to say the modes of the MPLC output base associated with the higher order modes of the input base, two modes identical to the target mode are chosen but spatially separated by 1.5 mm (150% of the diameter of the target mode), the three output modes being arranged at the corners of an isosceles triangle. This arbitrary but common value guarantees that the overlap between the output modes of the MPLC is less than 0.02% of their energy.

Once the input and output bases of the MPLC 5 have been determined, the phase masks of the MPLC can be digitally generated using the method described for example in US9250454. We choose to use here 5 phase masks. The phase plate implementing these phase masks can be manufactured, for example, from photolithography techniques. Once the phase plate is manufactured, it is assembled with the other optical components of an MPLC, for example with a mirror which faces it to form a multi-passage cavity, in which the incident radiation is propagated to operate the chosen transformation. The chosen distance between the phase plate and the mirror can be here of the order of 20 cm.

In the separation plane where the output modes of the MPLC 5 are spatially separated, a blocking device 8 is placed formed by a diaphragm with a diameter of 1.5 mm (ie 150% of the diameter of the target mode) centered on the target mode. The value chosen for the diaphragm diameter ensures that approximately more than 98.9% of the energy of the target mode is transmitted while less than approximately 0.1% of the energy of the other output modes of the MPLC is transmitted .

To evaluate the performance of the device 1, a variation in the direction of the input light beam of the device is introduced into a digital simulation of this device and two physical quantities of interest are evaluated after the diaphragm.

• The overlap between the output light beam mode and the target mode,

• The optical power in the output light beam.

The results are presented in FIG. 2. The following observations are made:

The overlap between the mode of the output light beam and the target mode remains constant over an angular range of the particular non-zero input beam,

• The optical power in the output light beam drops when the angular difference increases.

These two observations demonstrate that the device according to the invention makes it possible to maintain a constant overlap between the mode of the output light beam and the target mode over a wide range of variation of a parameter of the input light beam, at the price of 'a loss of optical power on the output beam. This loss of optical power corresponds exactly to the optical power which has left the mode to be stabilized to populate the disturbance modes and constitutes a fundamental limit which cannot be surpassed without an active device.

The ability of a device according to the present invention to passively maintain the constancy of the mode of the output light beam constitutes an important innovation in the field of modal light beam filtering.

In the example which has just been presented, a Gaussian target mode has been chosen, but it is entirely possible to envisage a target mode whose intensity corresponds, after propagation in free space or not, to for example:

a flat or square flat beam (“Fiat Top”);

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

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

a ring-shaped beam.

To facilitate the shaping of the useful radiation coming from the MPLC device 5 and to form a beam according to one of the forms given in example above, one can supplement the MPLC device 5 with a transmission device 6 comprising at least one element. non-spherical and non-planar optics, and preferably between 1 and 3 of these elements. By “non-spherical and non-planar optical element”, is meant an optical, transmissive or reflective element, the surfaces of which are neither spherical nor planar. It may for example be an aspherical optical element (which generally has a symmetry of revolution around an axis perpendicular to its mean plane) or of a free-form optical element (which does not have symmetry of revolution or of translation around an axis perpendicular to its mean plane).

In a very general manner, a small number of such non-spherical and non-planar optical elements, from 1 to 3, makes it possible to shape the useful radiation into a wide variety of beam shape, in particular those given as an example above, and which are of particular interest in applications for machining, drilling, precision cutting and surface treatment of materials by laser.

By combining the MPLC device 5 with a transmission device 6 configured with such optical elements, the design of the MPLC device 5, and in particular the number of phase masks, is simplified by carrying out the “shaping” part of the useful radiation by a more conventional optical system.

Advantageously, these optical elements will be reflective because they have a lower chromatic dispersion and a higher power withstand than a transmissive optical element, which is particularly important in applications using power or pulse lasers whose duration is around ten femtoseconds.

A first use case of a device according to the invention is the stabilization of a beam of which at least one of the spatial parameters is not constant over time. The device according to the invention then makes it possible to produce a beam whose spatial parameters are stable in the time and whose optical power fluctuates with variations in these parameters.

A second use case of a device according to the invention is that in which there is no input beam whose spatial parameters are strictly those of the assumed input beam, that is to say which has was used during the design of the device. This makes it possible to produce a beam whose spatial parameters are fixed at design and do not depend on absolute knowledge of the input beam. In other words, the device makes it possible to be robust to a statistical variability of the input beam which can vary from one implementation to another or from one optical source 3 to another.

The 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 by respectively maximizing and / or minimizing respectively the useful power and / or the non-useful power. One can use a control device 9 active (motorized mirrors, or an acousto-optical device for example) or non-active (mirrors with opto-mechanical mount) disposed upstream of the MPLC device. 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 the useful powers and / or not useful. Such a configuration is shown in FIG. 3, making it possible, via the control device 9, to modify the input beam to for example seek to reduce the energy of the disturbance radiation collected at the blocking device 8.

Example 2

In a second application example, modal filtering can be combined with specific shaping of the useful radiation by the transmission device 6 to implement a beam splitter device. Such a device produces an output beam consisting of a coherent superposition of single-mode light beams whose amplitude lobes are disjoint in a plane of space. In a particularly advantageous configuration, these single-mode light beams each correspond to copies of a reference light beam, and differ only in their position and possibly their direction of propagation. The reference light beam consists of the target mode whose transverse intensity distribution, once the mode possibly propagated in free space in a new space plane, can be that of a beam with a flat square or circular plate, or a Bessel beam or a Gaussian beam. This device 1 according to the invention in which the transmission device 6 comprises a beam splitter therefore implements an optical beam separation function robust to a variation of the spatial parameters of the input beam.

Example 3

A light beam can be used to machine a part with very high precision. It is not always possible to mechanically position the beam in space with this very high precision. The manufacturing tolerances of the components of the machining equipment, the fins and the parasitic movements (vibrations) that the equipment can undergo can introduce shifts in the position occupied by the laser source and therefore affect the shape of the beam which attacks the workpiece.

A processing device 1 according to the invention makes it possible to solve this problem. The MPLC 5 of this device 1 can be designed by choosing bases of input / output modes with separable spatial variables. Such a breakdown is described in “Fabrication and Characterization of a Mode- selective 45-Mode Spatial Multiplexer based on Multi-Plane Light Conversion”, Bade et al, Optical Fiber Communication Conférence Postdeadline Papers, OSA Technical Digest, Optical Society of America, 2018 , paper Th4B .3, pp. 1-3 and French patent application FR1851664. These documents demonstrate that such a decomposition allows a reduction in the number of phase masks constituting the MPLC conversion device.

It is considered in this example that the source 3 is supposed to emit an input light beam whose shape is Gaussian, centered at the point (0, 0) of a Cartesian coordinate system of a first transverse plane PI to the input beam. As has been specified, the exact position of the input beam in this first plane is likely to be imperfect or to vary over time, with respect to its expected position, centered on the reference frame of the plane. Whatever the exact position of this beam at a determined instant, and as shown in FIG. 4, the shape E (x, y) of the beam in the first plane PI can be decomposed modally in the form of a development limit :

E (x, y) = ko Eoo (x, y) + ki Eoi (x, y) + k2 Eo2 (x, y) + k3 Eio (x, y) + k4 En (x, y) + k ₅ Ei2 (x, y) + k6 E2o (x, y) + k ₇ E2i (x, y) + ks E22 (x, y) where Eoo is a Gaussian mode centered in (0,0), Eoi in E22 the first 8 Hermite Gauss modes HG01 to HG22 also centered in (0,0), and kO in k8 the modal decomposition coefficients of limited development.

The modes Eoo to E22 form the first family of modes with separable variables. They are all spatially superimposed.

A second family of modes is also defined, in a second transverse plane P2 forming the separation plane 7 disposed downstream of the conversion and also provided with a Cartesian coordinate system (x, y). This second family is composed of a main mode E'oo (x, y) "top hat", and of eight disturbance modes E'oi (x, y) to E'22 (x, y). These 9 modes are spatially separated and arranged in the form of a square in the second plane. They do form a family with separable variables, as shown in Figure 5.

To solve the problem posed, the MPLC 5 conversion device is configured so that the modal transformation of the input radiation leads to transform each mode Eq (x, y) of the foreground in the mode E'q (x, y) of the second plan, that is to say transform each mode of the first family into the same index mode of the second family. In this way, whatever the exact position of the input light beam, the part corresponding to the first Gaussian mode Eoo (x, y) is transformed into the “top hat” mode E'oo (x, y) · The others parts of the input radiation, projecting into the other modes of the first family, are transformed into the other modes of the second family which are all spatially separated from the first mode E'oo (x, y) · We can thus place a mask M (forming the blocking device 8) at the output of the MPLC conversion device, after the transformation carried out by the phase masks and at the level of the separation plane 7, aimed at blocking the propagation of this part of the transformed radiation, so that only the part corresponding to the first mode E'oo (x, y) propagates, as is clearly visible in FIG. 5, in the transverse representation of a third plane P3 downstream of the mask M.

As described above, the use of a decomposition into separable variables makes it possible to provide a limited number of phase masks of the MPLC conversion device to implement the modal transformation which has just been described. One can for example configure the MPLC so that it comprises five phase masks, that is to say five reflections or transmissions of the input radiation imposing a spatial phase shift of the radiation. And we search numerically for the profiles of each of the phase masks leading to precisely transforming the first family of modes into the mode of the same pair of indices of the second family of modes. This research can be posed as a numerical optimization problem (a detailed description of which will be found in the cited state of the art), and the separability of the variables of the first and second family of modes makes it possible to effectively converge this problem towards a solution with a good degree of optimality, even when the number of phase masks is reduced, five in the example taken.

Of course, the invention is not limited to the mode of implementation described and it is possible to make variant embodiments without departing from the scope of the invention as defined by the claims.

There can in particular exist a multitude of transverse planes in which the useful radiation and the radiation of disturbance 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 composing the blocking device 8 in a plurality of such planes.

In the case where the input beam consists of femtosecond pulses (from 10 to 900 fs), the processing device 1 becomes particularly relevant because of:

its compatibility with light beams of wide optical spectra,

its compatibility with beams whose energies per pulse are important,

the robustness it provides to changes in the spatial parameters of the input beam,

the small number of optical elements that make up the device ensures the stability of the spatial parameters of the beams produced.

Claims

1. Device for processing (1) a light radiation comprising:

an optical input (2) for receiving an input beam from an optical source (3) and for propagating in the device (1) an input radiation; - an optical output (4) for emitting an output beam;

- an MPLC conversion device (5) arranged between the optical input (2) and the optical output (4) and configured to spatially separate, in a separation plane (7), the input radiation into useful radiation, in a target mode, which propagates towards the optical output (4) and in a disturbing radiation;

- At least one blocking device (8) of the disturbance radiation, arranged in the separation plane (7) so that it does not contribute to the output beam.

2. Processing device (1) according to the preceding claim comprising a transmission device (6) for shaping the useful radiation and forming the output beam.

3. Treatment device (1) according to one of the preceding claims comprising a plurality of blocking devices (8) respectively disposed in a plurality of separation planes (7) in which part of the disturbance radiation is spatially isolated.

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

5. Processing device (1) according to one of the preceding claims, in which the blocking device (8) is integrated at least in part into the MPLC conversion device (5).

6. Treatment device (1) according to one of the preceding claims wherein the blocking device (8) comprises a detection element for collecting at least part of the disturbance radiation.

7. Processing device (1) according to one of claims 2 to 6 in which the transmission device (6) comprises, arranged downstream of the conversion device (5), a diffractive optical element, a spatial phase modulator, an optical system comprising at least one lens, an axicon, a beam splitter and / or a second MPLC conversion device.

8. Processing device (1) according to one of claims 2 to 7 wherein the transmission device (6) is integrated at least partially in the MPLC conversion device (5).

9. Processing device (1) according to one of claims 2 to 7 in which the transmission device (6) comprises at least one non-spherical and non-planar optical element, and preferably between 1 and 3 non-spherical optical elements and no plans.

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

11. Processing device (1) according to one of the preceding claims also comprising a control device (9) between the optical input (2) and the MPLC conversion device (5).

12. Processing device (1) according to the preceding claim wherein the control device (9) is an inactive optical element.

13. Processing device (1) according to one of the preceding claims, in which the MPLC conversion device (5) is configured to transform a base of input modes into a base of output modes, the bases of input and output being with separable spatial variables.