Alignment device for beam-folding optical system

Abstract

The invention relates to the alignment device (10) for aligning a laser beam folding optical system, the folding optical system comprising a first (E1) and a second (E2) set of N mirrors, the alignment device (10) comprising: a first (DO1), a second (DO2) and a third (DO3) observation device, a first removable partially reflective device (DPR1), configured to be positioned, during alignment, in a plane including the interaction point (PI), a second partially reflective device (DPR2) configured to generate a transmitted fraction (TF) and a reflected fraction (RF), a first fraction, chosen from the transmitted fraction and the reflected fraction, being directed towards the second observation device, and the second fraction being directed towards the third observation device (DO3),The third observation device is configured to acquire a third image (Im3) of the interaction point (PI), and a selective shuttering device (DOS) is configured to selectively shutter one or more chosen converging beams. Figure 3,

Description

Title of the invention: Alignment device for a beam-folding optical system. FIELD OF THE INVENTION

[0001] The present invention relates to an alignment device for adjusting a laser beam folding optical system, in which the laser beam makes several round trips, a set of beams focusing at a point. PRIOR TECHNOLOGY

[0002] A laser beam folding optical system in which several beams focus to a single point in space finds application in numerous fields. With this type of system, the reflected optical path is centralized to form a high-photon-density interactive region (IR). An interaction target, such as a gas, liquid, solid, plasma, particle beam, or electron beam, is introduced into the interactive region to induce optical interactions such as optical excitation, optical ionization, photolysis, optical dissociation, photosynthesis, optical generation, and optical analysis.

[0003] For example, the interaction between a nearly collinear FLA laser beam and a nearly collinear FPC charged particle beam makes it possible, in certain processes, to produce a third beam of particles of interest Fint in the direction of the charged particle beam. In this case, the laser beam and the charged particle beam form an angle of a few degrees, as illustrated [Fig. 1].

[0004] The interaction zone between the two beams FLA and FPC is as small as possible, in order to increase the probability of collision between the particles constituting the two beams: photons for the laser beam, and charged particles for the second beam. They are therefore propagated in such a way as to obtain a "cold" (focusing spot) at their intersection. The size of this neck is generally on the order of a few tens of micrometers.

[0005] An example of application is the realization of an X source by inverse Compton effect, the charged particles then being electrons.

[0006] When the interaction efficiency is low (low interaction cross-section), the beams are practically unchanged after they meet. To improve the production efficiency of the third Fint beam thus produced, folding optical systems are used.

[0007] The folding system 20 to which the alignment device according to the invention applies is illustrated [Fig. 2]. The principle is to fold the path of the laser beam using concave mirrors so that it passes through a certain number times through the same interaction zone when their direction is opposite to that of the beam of charged particles: the outward paths pass through the interaction point PI, the return paths pass around the periphery of the interaction point.

[0008] The folding system 20 comprises a first set E1 of N indexed mirrors i M1(i), i ranging from 1 to N, arranged in an annular fashion, and a second set E2 of N indexed mirrors j M2(j), j ranging from 1 to N, arranged in an annular fashion opposite the first set. The mirrors are arranged in an annular fashion around a common axis called OA. In the example in [Fig. 2], N = 7. The mirrors are typically concave, and the two sets preferably form a symmetrical structure with a point of symmetry PI, located in a plane PPI perpendicular to the axis OA and equidistant from the two sets. The incident laser beam FI is configured to pass through PI, thus forming an initial laser spot Tini in the plane PPI, and to impact a first mirror M1(1) of the first set, which is configured to reflect this beam towards a first mirror M2(1) of the second set.This mirror M2(l) is configured to reflect this ray along a path that passes through PI again, forming a first laser spot TL(1) in the plane PPI, which then impacts a second mirror M1(2) of the first set, which is configured to reflect this ray towards a second mirror M2(2) of the second set. This mirror M2(2) is configured to reflect this ray along a path that passes through PI again, forming a second laser spot TL(2) in the plane PPI, which then impacts a third mirror M1(3) of the first set, and so on until the last mirror of the second set M2(N).

[0009] Each concave mirror in the first set M1(i) is oriented such that an incident laser beam is reflected to a corresponding concave mirror M2(j=i) in the second set of mirrors. Each concave mirror in the second set M2(j=i) is oriented such that an incident ray from a corresponding concave mirror M1(i) in the first set is reflected to a concave mirror M1(i+1) adjacent to the corresponding concave mirror in the first set M1(i). This has the effect of sequentially shifting the reflected beams in a circumferential direction around the mirror sets. This type of optical cavity system is also called an azimuthal shift system.

[0010] The folding system 20 is thus configured to generate, from the incident laser beam FI, a plurality of N beams reflected by the mirrors of the first set, called first reflected beams FRI, and a plurality of N beams FR2 reflected by the mirrors of the second set, called second reflected beams FR2. The incident beam FI and the second reflected beams FR2 form a plurality of N+l beams that converge at the interaction point PI, and are called converging beams FC.

[0011] The laser spots formed by the FR2 reflected beams in the PPI plane are denoted TL2(j) and the laser spots formed by the FRI reflected beams in the PPI plane are denoted TL1(i) (i and j corresponding to the mirror from which the spot originates). The subscripts i and j respect the order of impact of the beams on the mirrors. The set of TL2 spots forms an interaction zone with Tini. When the system is properly adjusted, in the PPI plane all the TL2(j) and TLini laser spots coincide at point PI; typically, the spots with radii of 30-50 pm are superimposed such that the centers of these spots are contained within a circle of 5-10 pm maximum. Similarly, when the system is properly adjusted, the TL1(i) laser spots are located on the periphery of this point (see [Fig. 2]), typically a few millimeters from PL

[0012] It should also be noted that since the mirrors are concave, for optimal adjustment the laser spots must also correspond to the neck (“waist” in English), i.e. the focal point of the beams reflected by the associated concave mirror: the geometric characteristics of the incoming laser beam are adapted to the radius of curvature of the concave mirrors and to the distance between the two plates supporting these mirrors, so as to precisely obtain a pinch of the beam (neck) at the level of the interaction zone at each pass.

[0013] Such a folding system, as well as various applications of this system, is described for example in document US6487003.

[0014] The operation of the system 20 requires very precise alignment of each of the mirrors, independently of the others. The necessary accuracy of the orientation of the mirror faces is on the order of 10 prad, in order to obtain good superposition of all the folds at the interaction zone. It is necessary to successively adjust the orientation of the mirrors in order to obtain superimposed focusing saddles and correct positioning of the laser beam impacts on each mirror. The procedure is as follows: • Using the focusing and orientation adjustments upstream of the device, we ensure that the initial laser beam arrives at the first mirror Ml(l) at the expected position with a focusing neck at the center of the device. • The first mirror Ml(l) is oriented so that the return laser beam is well positioned on the 2nd mirror M2(l). • The 2nd mirror M2(l) is oriented using the adjustment screws so that the laser beam is correctly positioned on the 3rd mirror Ml(2) and has a new neck superimposed on the first neck. • The 3rd mirror M1(2) is oriented using the adjustment screws so that the laser beam is correctly positioned on the fourth mirror M2(2). • And so on.

[0015] Manually aligning the device as described is a difficult and tedious step and constitutes a significant drawback for this type of system. Indeed, it takes several hours or even several consecutive days to achieve perfect alignment of the device, and the level of expertise required is high in optics, lasers, and diffraction.

[0016] Furthermore, the mechanical design of the system must allow the mirrors to be viewed with the naked eye and their orientation to be adjusted directly by hand. This implies, on the one hand, the use of large viewing windows or transparent panels, which are sometimes incompatible with the vacuum requirements of this type of experiment, and on the other hand, that the alignments must be performed in open air at atmospheric pressure. When the system must be placed under vacuum, this induces mechanical stresses that misalign the device, making its use practically impossible.

[0017] Observation along the axis must also be done through the exit window of the beam of interest, which is often opaque to visible light (for example, Beryllium when the beam of interest is a 10 keV X-ray beam): it is then necessary to remove it for alignment, which has the same disadvantage as the open-air adjustment described above.

[0018] Once adjusted, the device is subject to drifts over several hours or days. It is impossible to compensate for these drifts using the method described above, because once under vacuum, the user no longer has access to the mirror display. In this case, a complete adjustment is necessary.

[0019] Another weakness is that the device must be aligned with a visible laser to perform the adjustments. However, to optimize the production of the beam of interest in certain applications, it is preferable to use a laser beam with a longer wavelength, generally in the infrared. It is then no longer possible to use the alignment method employed until now because the laser is no longer visible to the person performing the alignment.

[0020] It is desirable to be able to perform the alignment in both the infrared (typically at 1064 nm) and the visible (typically 532 nm) ranges. Furthermore, it is desirable to be able to perform the alignment remotely and in a vacuum, and to be able to implement an automated alignment system. To achieve this, it is necessary to be able to simultaneously observe the position of the laser at the mirrors and at the interaction point, in a non-destructive manner. The most obvious way to observe the position of the laser at these different points is to use cameras positioned facing the mirrors, off-axis to avoid obstructing the path of the laser beam, but this can result in imaging defects. Moreover, in practice, given the size and limited space available, it is It is difficult to place cameras in a vacuum to properly image the different points of interest.

[0021] One object of the present invention is to overcome the aforementioned drawbacks by providing an improved alignment system for the folding optical system 20. DESCRIPTION OF THE INVENTION

[0022] The present invention relates to an alignment device for aligning an optical system for folding a laser beam,

[0023] the folding optical system comprising a first and a second set of N mirrors each arranged in an annular manner, and being configured to generate, from an incident laser beam, a plurality of N first beams reflected by the mirrors of the first set and a plurality of N second beams reflected by the mirrors of the second set, the incident beam and the second reflected beams forming a plurality of N+l so-called convergent beams, converging at a point called the interaction point,

[0024] the alignment device comprising:

[0025] • a first, a second and a third observation device,

[0026] • a first partially removable reflective device, configured to be positioned, during alignment, in a plane including the interaction point and configured for:

[0027] to reflect a fraction of the plurality of first beams towards the first observation device, the first observation device being configured to produce a first image of the first set of mirrors,

[0028] to reflect a fraction of the plurality of converging beams towards the second observation device, the second observation device being configured to produce a second image of the second set of mirrors,

[0029] • a second partially reflective device configured to generate, from of said fraction of the plurality of converging beams, a transmitted fraction and a reflected fraction, a first fraction, chosen from the transmitted fraction and the reflected fraction, being directed towards the second observation device, and the second fraction being directed towards the third observation device, the third observation device being configured to produce a third image of the interaction point,

[0030] • a selective shuttering device disposed in the path of the second fraction at a place where the converging beams of the second fraction are spatially separated, and configured to selectively block one or more chosen converging beams.

[0031] According to one embodiment, the first partially reflective device comprises a partially reflective parallel-sided blade. Preferably, the blade has a thickness of less than 150 µm.

[0032] According to one embodiment, the selective shuttering device is further configured to selectively attenuate said one or more selected converging beams.

[0033] According to one embodiment, the selective shuttering device comprises at least one liquid crystal cell, a first polarizer and a second polarizer.

[0034] According to one embodiment, the alignment device according to the invention comprises a plurality of N+l liquid crystal cells, each forming a single pixel, arranged along at least two planes and in such a way that at least one area of ​​each liquid crystal cell is crossed by a single converging beam.

[0035] According to one embodiment, the selective shuttering device consists of a single matrix liquid crystal cell.

[0036] According to one embodiment, at least one liquid crystal cell is configured to block light having at least two distinct wavelengths.

[0037] According to another aspect, the invention relates to an optical system for folding a laser beam having an alignment capability, referred to as an aligned folding system, comprising:

[0038] • an optical folding system comprising a first and a second set of N mirrors each arranged in an annular fashion, and configured to generate, from an incident laser beam, a plurality of N first beams reflected by the mirrors of the first set and a plurality of N second beams reflected by the mirrors of the second set, the incident beam and the second reflected beams forming a plurality of N+l so-called convergent beams, converging at a point called the interaction point (PI),

[0039] • an alignment device (10) according to the invention.

[0040] According to one embodiment, the first assembly, the second assembly and the first partially reflective device are arranged in a vacuum chamber (EV).

[0041] According to one embodiment, the second partially reflective device, the first observation device, the second observation device, the third observation device and the selective shuttering device are located outside the vacuum enclosure, the vacuum enclosure comprising a first and a second window.

[0042] The following description presents several embodiments of the device of the invention: these examples are not limiting to the scope of the invention. These embodiments present both the essential features of the invention and additional features related to the embodiments considered.

[0043] The invention will be better understood and other features, objectives and advantages thereof will become apparent from the following detailed description and with reference to the accompanying drawings given by way of non-limiting examples and in which:

[0044] The [Fig.1] already cited illustrates a schematic diagram of the interaction of a laser beam (beam of photons) with a beam of almost collinear charged particles, allowing the generation of a third beam of particles of interest.

[0045] Figure 2, already cited, illustrates a folding system to which the device applies alignment according to the invention.

[0046] Figure 3 illustrates an alignment device according to the invention.

[0047] Figure 4 illustrates an example of the implementation of the alignment device according to the invention.

[0048] Fig. 5 illustrates an image of the interaction point and its neighborhood produced by the third observation device in a situation where the alignment is not correct.

[0049] Figure 6 illustrates an image of the interaction point and its neighborhood, produced by the third observation device, comprising 8 spots and for which the identification of the different spots was carried out using the alignment device according to the invention.

[0050] Figure 7 illustrates an example of a DOS selective shuttering device comprising eight single-pixel liquid crystal cells, arranged in two planes, four cells per plane, and in such a way that at least one area of ​​each cell is crossed by a single beam.

[0051] Fig. 8 illustrates the selective shuttering operated by the selective shuttering device illustrated in Fig. 7.

[0052] Figure 9 illustrates one embodiment of the aligned folding optical system including a vacuum chamber. DETAILED DESCRIPTION OF THE INVENTION

[0053] The invention relates to an alignment device 10 for aligning a folding optical system 20 of a laser beam as described above.

[0054] The folding optical system 20 comprises a first set El of N mirrors and a second set E2 of N mirrors, the mirrors of each set being arranged annularly around a common axis, the two sets being arranged opposite each other. The two sets El and E2 are also referred to as plates. Preferably, the mirrors are concave. The folding optical system 20 is configured to generate, from an incident laser beam FI, a plurality of N first beams FRI reflected by the mirrors of the first set, and a plurality of N second beams FR2 reflected by the mirrors of the second set. The incident beam FI and the second reflected beams FR2 form a plurality of N+l so-called convergent bundles FC, converging at a point called the interaction point PI.

[0055] The alignment device 10 according to the invention is illustrated [Fig.3].

[0056] It comprises a first observation device DO1, a second observation device DO2 and a third observation device DO3.

[0057] The device 10 according to the invention also includes a first partially reflective and removable device DPR1, configured to be positioned, during alignment, in a plane including the interaction point PI. Removable means that the device DPR1 is configured to be positioned in the plane including the point PI to perform the alignment, and then to be moved so as to no longer be located in the path of the laser beams once the alignment has been achieved.

[0058] According to one embodiment, the first partially reflective device DPR1 is associated with a remotely controllable motorization (pneumatic, electric, etc.) and is configured to move in translation between a position on the path of the laser beams for the alignment of the system 20, and a position outside the path of the laser beams when the system 20 is in operation.

[0059] The first partially reflective device DPR1, positioned for alignment, is configured to reflect a fraction FPFR of the plurality of first reflected beams FRI towards the first observation device DO1. The first observation device DO1 is configured to produce an image, called the first image Iml, of the first set of mirrors EL

[0060] The first partially reflecting device DPR1 is also configured to reflect a fraction FPFC of the plurality of converging beams FC, towards the second observation device DO2. The second observation device DO2 is configured to produce an image, called the second image Im2, of the second set of mirrors E2.

[0061] According to one embodiment, the first partially reflective device comprises a partially reflective plate with parallel faces. Preferably, the plate has a thickness of less than 150 µm, so as not to cause a significant shift in the paths of the laser beams. Preferably, the plate has one face treated with an antireflective coating. The plate may have no coating on the other face, and in this case, the standard reflection of this component (typically about 4%) is used. The plate may have a coating on this other face to exhibit a reflection other than 4%. Typically, the plate is inclined at an angle of about 45° with respect to the axis OA (not shown in [Fig. 3]).

[0062] The outward and return paths are not reflected in the same direction by DPR1. Indeed, the FRI and FC beams propagate in opposite directions, the fractions FPFR and FPFCs are sent in two different directions, for example "up" for one and "down" for the other.

[0063] The presence of the blade allows, in association with DO1 and DO2, to visualize the two sets of mirrors respectively El and E2, and also to visualize the impact of the associated laser beam on each of the mirrors.

[0064] The alignment device 10 according to the invention also includes a second partially reflective device DPR2 configured to generate, from the fraction of the plurality of converging beams FPFC, a transmitted fraction FT and a reflected fraction FR. A first fraction, designated FPFC1, chosen from the transmitted and reflected fractions, is directed to the second observation device DO2, and the second fraction, designated FPRC2, is directed to the third observation device DO3. The third observation device is configured to produce an image, designated Im3, of the interaction point PI and thus of an area around this point. In [Fig. 3], by way of example, the device DO2 receives the transmitted fraction, but it could just as easily receive the reflected fraction. Typically, the device DPR2 is a semi-reflective plate.

[0065] The alignment device also includes a selective shuttering device (DOS) disposed in the path of the second fraction FPRC2 at a location where the converging beams of the second fraction are spatially separated. This DOS device is configured to selectively shutter one or more selected converging beams. Typically, selective shuttering is performed via a control signal (SC) that drives the DOS device, but manually controlled selective shuttering is also possible.

[0066] Typically, the observation devices DO1, DO2 and DO3 are cameras comprising an imaging optic and a detector.

[0067] The DPR2 device thus separates the light beams from the interaction point PL into two parts. One part allows imaging of the mirrors of the second assembly E2 (which are then illuminated by additional light also reflected onto DPR2) on DO2, as well as the positions of the laser beams on these mirrors, for example using a CCD camera or other device. The other part allows imaging of the interaction point and its vicinity at DPR1 (typically a partially reflective plate) on DO3, for example using another CCD camera or other device.

[0068] With the device 10 according to the invention, we thus simultaneously image on the one hand the mirrors of the two platforms and the points of impact of the laser beam on these mirrors, via DO1 and DO2, and on the other hand the point of interaction PI and its neighborhood via DO3.

[0069] Figure 4 illustrates an example of an implementation of the device 10 according to the invention, with DPR1 and DPR2 being blades, and DO1, DO2, and DO3 being cameras. Once reflected off DPR1, the various beams that converge towards PI or pass through its The diverging neighborhood is again reached, and optics L1, L2, and L3 redirect the beams into the camera field, while simultaneously ensuring optical conjugation between the observation planes and the CCD camera sensors. For clarity, in [Fig. 4], only the first FRI beams are shown as dashed lines in A (left), and only the converging FC beams are shown as solid lines in B (right). The incident beam FI is introduced into the device with an insertion mirror MIS. The FC "outgoing" beams pass through the interaction point PI (the laser spots TL2(j) and TL1 coincide at point PI), and the FRI "returning" beams pass around the periphery of the interaction point.

[0070] The images Iml and Im2 of the mirrors, onto which the laser beam impacts are superimposed, ensure that each mirror is indeed impacted by a laser beam, and in the correct location, typically identified beforehand on the mirror image. However, even when this is verified, it does not mean that the FC beams converge at PI. Figure 5 illustrates an image Im3 of the interaction point PI and its neighborhood in this situation. Different laser spots are visible, but they do not converge at a single point, and it is not known which spot corresponds to which mirror. There is poor superposition, and it is impossible to determine which mirror is misaligned.

[0071] The use of the DOS selective shuttering system combined with the DO3 observation device eliminates ambiguity regarding the origin of the beams when observing the focal points of the TL2(j)+TLini laser beams that are to be superimposed, that is, when associating each TL2(j) spot in the interaction plane with its originating mirror M2(j). This mirror / laser spot identification is essential for correcting alignment errors. To achieve alignment, all mirrors are equipped with fine orientation adjustments along two planes (for example, adjustment screws or a piezoelectric device).

[0072] The DOS device allows the transmission of any number of beams to be completely or partially blocked in order to determine the individual contribution of each beam to the measurement, without altering the beam path in the system 20. The DOS makes it possible to identify the origin of each spot, this identification occurring in several successive steps by blocking / unblocking the various FC beams incident on the DOS device. When the system is not yet aligned, the identification of the different spots makes it easy and independent to identify, remotely and without the use of complex mechanical parts, which beams are misaligned.

[0073] The use of the DOS selective shutter system combined with the DO3 observation device also makes it possible to achieve alignment, remotely and outside the path of the beams in the system 20, via the fine adjustments of the mirrors. Preferably, the DOS (Direct Oscillation System) is used to retain only two spots: one already aligned and the spot to be aligned. The originating mirror (from the second set) identified for the spot to be aligned is then adjusted to superimpose this spot onto the already aligned spot. All the spots are thus successively aligned, respecting, of course, the order of impact of the laser beams, since the adjustment of a mirror M2(j) affects the paths following this reflection. The indices i and j were chosen to respect the order of impact of the beams on the mirrors, and therefore the adjustment of mirror M2(j) affects not only the position of the spot TL2(j) but also the positions of all spots with an index strictly greater than j.

[0074] Thus, TL2(1) is first aligned with TLini. To do this, only TLini and TL2(1) are viewed, and if they are not superimposed, mirror M2(1) is moved so as to shift TL2(1) until TL2(1) is superimposed on TLini. Then, TL2(1) is turned off, leaving only TLini and TL2(2) visible, and mirror M2(2) is moved, which will have no effect on TLini or TL2(1). TL2(2) is then superimposed on TLini. And so on until each spot is superimposed on TLini.

[0075] Fig. 6 illustrates an Im3 image comprising 8 spots (N=7) in which the identification of the different spots has been carried out, the spots TLini, TL2(1), TL2(2), TL2(3) are superimposed, the spots TL(4) to TL(7) are to be adjusted.

[0076] The alignment device 10 according to the invention thus allows the simultaneous and unambiguous measurement of the position of the laser beams of the system 20, at the point of interaction and at the mirrors of the two platforms E1 and E2, in a non-destructive manner. It allows the identification of the different laser spots in the interaction zone and the implementation of a simplified alignment process. This alignment is performed remotely and can be automated. It has a limited cost because it uses conventional optical components. It also allows alignment outside the visible spectral range, using cameras and a DOS device compatible with the wavelength of the incident beam. Finally, the observation device DO3 can be replaced by another measurement and diagnostic device for characterizing laser beams, for example, a device measuring the pulse duration, energy, spectrum, etc.Thanks to the two devices DPR1 and DPR2, possibly associated with a variable tilt mirror which individually directs each beam towards an associated measurement line, the measurement is moved away from system 20.

[0077] In the event of drift in the alignment setting, the device 10 makes it easy to identify the mirror which has deteriorated the alignment and makes it much easier to correct this realignment, compared to what is done with a system 20 without device 10 according to the invention.

[0078] According to one embodiment, the selective shuttering device DOS is further configured to selectively attenuate the chosen converging beam(s). This allows for better adjustment of the superposition of two spots. Indeed, as reflections occur, the initial beam decreases in intensity, so the different laser spots are not of equal intensity. Selective attenuation makes it possible to bring the intensity of the two spots to be aligned to the same level for superposition.

[0079] The shuttering can be carried out via transmission (for example with a liquid crystal cell or with a set of mechanical shutters) or via reflection (for example with a digital micromirror device, called DMD for "Digital Micromirrors Device", manufactured by Texas Instruments, associated with the objective of the observation device).

[0080] According to one embodiment, the selective shuttering device DOS comprises at least one liquid crystal cell CCL, a first polarizer PI (input polarizer) and a second polarizer P2 (output polarizer). The liquid crystal cell is driven by an electrical control signal.

[0081] According to one example, the DOS device consists of a single matrix liquid crystal cell between two polarizers.

[0082] According to another example, the selective shuttering device DOS comprises a plurality of N+l liquid crystal cells, each forming a unique pixel, arranged along at least two planes and such that at least one area of ​​each liquid crystal cell is traversed by a single convergent beam FC of the fraction FPFC directed towards DO3. In this case, each elementary cell is controlled independently.

[0083] Figure 7 illustrates an example of such a DOS selective shuttering device, for the case N=7, configured to selectively shutter 8 beams (a fraction of the seven beams reflected by the seven mirrors of the second set plus the incident beam). The eight CCL cells are arranged in two planes. In A, four cells are shown arranged in a first plane and spaced apart; in B, four cells are arranged in a second plane at 45° to the cells in the first plane. In C, the arrangement of all 8 cells together is shown. The elementary CCL cells are low-cost, single-pixel liquid crystal cells, typically square (a few centimeters on each side). The diameter of the laser beam (millimeters) is small compared to the size of the CCL cells, and the individual beams are all close to the axis. It is therefore not possible to use more than four of these cells in the same plane.Since the beams are small in diameter, the arrangement along two planes, as illustrated in C, allows each beam to pass through a single cell in one plane, then into the empty space between two other cells in the other plane. D illustrates the arrangement of the 8 CCL cells associated with the different laser beams passing through them. The zones... Textured areas 71 illustrate a fraction of the areas of each cell where the beam passes through only one cell, in which the laser beams to be blocked are positioned. If a single polarizer is to be used for all cells (input and output), they must all have the same alignment with respect to the polarizer. In the example of [Fig. 7], the cells are of the "controlled birefringence" type, and the orientation of the liquid crystals is illustrated by direction 72. The orientation of the liquid crystals within the cells in [Fig. 7] B is at 45° to the cell edges, while the liquid crystals within the cells in [Fig. 7] A are oriented parallel to the cell edges. The arrangement of the CCL liquid crystal cells in [Fig.[7] is such that each laser beam passes through both polarizers and a single liquid crystal cell, each laser beam passing through a different cell. Varying the voltage in each liquid crystal cell allows the transmission to be varied and thus the incident intensity on the DO3 sensor to be adapted independently for each laser beam. Since the laser beams are close to each other, the area used (through which the laser passes) corresponds to the corner of the cell. It is necessary that the edges of each cell be thin, i.e., that the effective area corresponds to the dimensions of the entire cell, except for a fringe 10-20 µm thick at the edge.

[0084] Fig. 8 illustrates the selective shuttering operated by the DOS device illustrated in Fig. 7 at D: at A all cells are in shuttering except one, only one associated laser spot is detected by DO3; at B all cells are in open mode except one, at C all cells are in open mode, all laser spots are visible.

[0085] The advantage of the arrangement illustrated [Fig. 7] is that it uses very low-cost single-pixel liquid crystal cells. The liquid crystal cells can be held in a support that allows the assembly to be secured and the cells to be positioned precisely. This support can be easily manufactured using a low-cost additive manufacturing method.

[0086] At the liquid crystal cell level, several configurations are possible. For example, the cells can be birefringent controlled or TN (for Twisted Nematic), with the two associated polarizers being parallel or perpendicular. The cell type combined with the polarizer configuration makes it possible to obtain voltage-controlled shuttering and therefore maximum transmission without voltage, or vice versa. Liquid crystal cells also allow for controlled beam attenuation.

[0087] According to one embodiment, at least one liquid crystal cell is configured to block light having at least two distinct wavelengths. This allows alignment to be performed with a laser beam of wavelength XI or with a laser beam of wavelength / .2 as required of the application. For this to work, the observation devices DO1 and DO2 (for visualizing beam impacts on mirrors) and DO3 (for visualizing the interaction zone around point PI) must have optics and detectors adapted to operate at XI or X2. For example, XI is in the visible spectrum and X2 in the infrared. For example, XI = 532 nm and X2 = 1.064 pm.

[0088] According to another aspect, the invention relates to an optical system for folding a laser beam having an alignment capability 30, referred to as an aligned folding system, comprising:

[0089] • an optical folding system 20 comprising a first set of N mirrors and a second set of N mirrors E2, the mirrors of each set being arranged in an annular fashion, and being configured to generate, from an incident laser beam FI, a plurality of N first beams FRI reflected by the mirrors of the first set and a plurality of N second beams FR2 reflected by the mirrors of the second set, the incident beam and the second reflected beams forming a plurality of N+1 so-called convergent beams FC, converging at a point called the interaction point PI, and

[0090] • an alignment device 10 as described above.

[0091] According to an embodiment of the aligned folding optical system 30 illustrated [Fig.9], the first assembly El, the second assembly E2 and the first partially reflective device DPR1 are arranged in a vacuum chamber EV. In the non-limiting example of [Fig. 9], the incident beam FI is introduced into the chamber by an insertion mirror MIS located outside the chamber and an insertion window HIS. A vacuum chamber is required for applications involving the interaction between a photon flux and a charged particle flux (not shown in [Fig. 9]).

[0092] Preferably, the second partially reflective device DPTR2, the first observation device DO1, the second observation device DO2, the third observation device DO3, and the selective shuttering device DOS are located outside the vacuum chamber, the vacuum chamber comprising a first H1 and a second H2 window, as illustrated [Fig. 9]. Alignment is thus performed remotely with these elements located outside the EV chamber.

Claims

Demands

1. Alignment device (10) for aligning a laser beam folding optical system, the folding optical system comprising a first (E1) and a second (E2) set of N mirrors, each arranged in an annular fashion, and configured to generate, from an incident laser beam (IF), a plurality of N first beams (F1) reflected by the mirrors of the first set and a plurality of N second beams (FR2) reflected by the mirrors of the second set, the incident beam and the reflected second beams forming a plurality of N+1 so-called convergent beams (CF), converging at a point called the interaction point (IP), the alignment device (10) comprising: a first (DO1), a second (DO2) and a third (DO3) observation device, a first partially reflective device (DPR1) removable, configured to be positioned, during alignment, in a plane including the interaction point (PI) and configured to: • reflect a fraction (FPFR) of the plurality of first beams towards the first observation device (DO1), the first observation device being configured to produce a first image (Iml) of the first set of mirrors (El), • reflect a fraction (FPFC) of the plurality of converging beams, towards the second observation device (DO2), the second observation device being configured to produce a second image (Im2) of the second set of mirrors (E2), a second partially reflecting device (DPR2) configured to generate, from said fraction of the plurality of converging beams, a transmitted fraction (TF) and a reflected fraction (RF), a first fraction, chosen from the transmitted fraction and the reflected fraction, being directed towards the second observation device, and the second fraction being directed towards the third observation device (DO3), the third observation device being configured to make a third image (Im3) of the interaction point (PI), • a selective shuttering device (DOS) disposed on the path of the second fraction at a place where the converging beams of the second fraction are spatially separated, and configured to selectively shutter one or more chosen converging beams.

2. Alignment device according to claim 1 in which the first partially reflective device comprises a partially reflective parallel-sided blade.

3. Alignment device according to the preceding claim in which said blade has a thickness of less than 150 pm.

4. Alignment device according to any one of the preceding claims wherein the selective shuttering device is further configured to selectively attenuate said one or more selected converging beams.

5. Alignment device according to any one of the preceding claims wherein said selective shuttering device comprises at least one liquid crystal cell (LCC), a first polarizer (PI) and a second polarizer (P2).

6. Alignment device according to the preceding claim comprising a plurality of N+l liquid crystal cells each forming a single pixel, arranged in at least two planes and such that at least one area of ​​each liquid crystal cell is traversed by a single converging beam.

7. Alignment device according to claim 5 wherein the selective shuttering device consists of a single matrix liquid crystal cell.

8. Alignment device according to any one of claims 5 to 7 wherein at least one liquid crystal cell is configured to block light having at least two distinct wavelengths.

9. An optical system for folding a laser beam having an alignment capability (30), referred to as an aligned folding system, comprising:

10.

11. • an optical folding system (20) comprising a first (E1) and a second (E2) set of N mirrors each arranged in an annular manner, and being configured to generate, from an incident laser beam (FI), a plurality of N first beams (FRI) reflected by the mirrors of the first set and a plurality of N second beams (FR2) reflected by the mirrors of the second set, the incident beam and the second reflected beams forming a plurality of N+1 so-called convergent beams (FC), converging at a point called the interaction point (PI), • an alignment device (10) according to any one of claims 1 to 8. Aligned folding optical system (30) according to the preceding claim in which the first assembly, the second assembly and the first partially reflective device (DPR1) are arranged in a vacuum chamber (EV). Optical folding system aligned according to the preceding claim wherein the second partially reflective device, the first observation device (DO1), the second observation device (DO2), the third observation device (DO3) and the selective shuttering device (DOS) are located outside the vacuum chamber, the vacuum chamber comprising a first (H1) and a second (H2) window.

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