Laser beam distribution or recombination system

Abstract

A laser beam distribution or recombination system (LBDS) comprising a reflective optical system mounted for rotation about a principal axis (PA) parallel to an optical propagation axis (OPA), referred to as the input axis, of a laser beam (LB), said reflective optical system comprising at least two fixed mirrors arranged to reflect said laser beam along an optical propagation axis (OPA), referred to as the output axis, parallel to, but not coinciding with, said input optical propagation axis (OPA) and said principal axis (PA); said reflective optical system being configured such that its rotation about said principal axis (PA) causes a parallel translation of said output optical propagation axis (OPA) along a circular path (T). Figure for the abbreviation: Fig. 1

Description

Title of the invention: System for distributing or recombining laser beams

[0001] The invention is in the field of lasers.

[0002] It applies in particular to pulsed lasers, and more specifically to high-power (instantaneous power on the order of 1 TW or more) and short-duration (half-life of the pulses on the order of 100 fs or less) pulsed lasers. Such lasers are mainly used in fundamental scientific research, for accelerating particles, generating extreme ultraviolet (XUV) radiation, generating and studying plasmas, etc.

[0003] The invention can enable the spatial and periodic distribution of successive pulses generated by such a laser to generate a plurality of respective beams (for example, 30) consisting of lower-rate pulses that can be directed towards respective targets. Conversely, it can enable the production, from a plurality of pulsed laser sources, of a single beam of pulses at a rate multiple of that of an individual source.

[0004] High-intensity laser beams, such as those obtained by chirped pulse amplification (CPA), induce non-linear effects (e.g., the Kerr effect) when passing through a bulk optical material (glass, crystals, etc.), which degrade the quality of the transmitted beam. Consequently, a number of switching systems (such as those based on an acousto-optic or electro-optic effect in a bulk material) cannot be used.

[0005] Several systems have been developed for sequentially distributing high-power laser pulses emitted by a single source to several successive return directions. These systems usually include an optomechanical type return device, adapted to reflect the high-power laser beam to said successive return directions.

[0006] Document FR2608786, for example, describes a laser system comprising a laser device emitting a high-power pulsed laser beam towards a reflecting mirror mounted on a rotating support, here on the axis of a galvanometer whose orientation is controlled by a computer to distribute the high-power laser beam in successive directions. It is understood that any synchronization error between the laser and the galvanometer, or any inaccuracy in the signal of The control of the latter results in an error in pointing the laser beams at the output of the system.

[0007] In the case of document US2005 / 0087295, the reflector device comprises a rotating polygonal structure, formed of several reflective facets, the rotation of which allows the high-power laser beam to be distributed in several successive directions. Here too, any inaccuracy in synchronization between the reflector device and the laser results in a misalignment error, especially at high rates of rotation.

[0008] Optical switches also exist in which a laser beam is redirected by successive insertion of translationally actuated mirrors; see, for example, US patent 2002 / 0126948. Applied to the kilohertz switching of high-power laser beams with large diameters (e.g., 40 mm), this technique involves the use of bulky, expensive actuators that consume a great deal of energy and are prone to generating shocks and vibrations, resulting in misalignment errors. Furthermore, this type of switch requires all the mirrors to be repositioned to perform a new reading cycle, which severely limits the maximum achievable rate.

[0009] All the aforementioned solutions present a significant risk of misalignment, i.e., a lack of angular accuracy in the beam's reflection. This can be defined as the angular deviation between the actual reflection direction and the intended reflection direction, while the spatial accuracy of the beam's reflection is defined as the lateral spatial deviation between the actual position of the laser spot at a predefined distance and its intended position. In the frequent case where high-power laser beams from a distribution system are intended to be focused onto a target by an off-axis parabolic mirror, angular accuracy is much more important than spatial accuracy. Indeed, it is well known that an angular misalignment on a parabola will induce a decrease in illumination at its focus as well as a degradation of the spatial distribution of illumination, which can be very detrimental to applications.Consider, for example, the focusing of a 40mm diameter beam by an off-axis parabola at 90° with a focal length of f=400mm. For perfect alignment, within the framework of geometric optics, the system focuses all the rays to a single point—in reality, considering the wave nature of light, to a diffraction spot. If the beam axis is not exactly parallel to the parabola axis, geometric aberrations will prevent the beam from being focused to a single point. Even at an angle of just 0.01°, the rays will be distributed over an area on the order of the diffraction spot, and the peak illumination will decrease by a factor of approximately 2. Conversely, if the beam is laterally offset parallel to itself by an angle (Ax, Ay) along two axes Ox or Oy perpendicular to the direction of propagation Oz, it will always be [missing information]. perfectly focused by the parabola, but will have a very slightly different direction after the parabola depending on the values ​​of Ax and Ay (0x=Ax / f, 0y=Ay / f).

[0010] The invention aims to overcome, at least in part, the aforementioned drawbacks of the prior art. More particularly, it aims to enable the distribution or recombination of high-power laser beams with very high angular accuracy (low misalignment error). Advantageously, the invention also makes it possible to achieve high laser rates, on the order of several tens or hundreds of Hz, or even 1 kHz or more.

[0011] According to the invention, this objective is achieved through the use of a reflective optical system comprising at least two fixed mirrors rotating about an axis parallel to—and preferably coinciding with—the propagation axis of the input laser beam. The reflective optical system is configured to redirect the laser beam along a propagation direction parallel to that of the input beam, but laterally offset from it. Its rotation thus results in a parallel translation of the output laser beam along a circular path. By triggering the generation of laser pulses synchronously with the rotation of the reflective system (or vice versa), it is possible to periodically distribute these pulses among several output beams corresponding to respective angular positions along said circular path.Under these conditions, a synchronization fault results in a lateral shift of the output beams, but does not affect their propagation direction. However, in the systems described in documents FR2608786 and US2002 / 0126948, the rotation axis of the reflective system is perpendicular to the laser propagation direction; therefore, a synchronization fault directly affects the angular accuracy.

[0012] An object of the invention is therefore a laser beam distribution or recombination system comprising a reflective optical system mounted in rotation around an axis, called the main axis, parallel to an optical propagation axis, called the input axis, of a laser beam, said reflective optical system comprising at least two mirrors joined together and arranged to reflect said laser beam along an optical propagation axis, called the output axis, parallel to, but not coinciding with, said input optical propagation axis and said main axis; said reflective optical system being configured so that its rotation around said main axis causes a parallel translation of said output optical propagation axis along a circular trajectory.

[0013] According to particular embodiments of such a laser beam distribution or recombination system:

[0014] - Said reflective optical system may comprise a first and a second mirror, parallel to each other and forming an angle of 45° with respect to said principal axis and to said optical propagation axes of input and output, the first mirror being traversed by said principal axis and the second mirror being positioned so to receive the laser beam reflected by the first mirror, whereby the said circular trajectory is centered on the said principal axis.

[0015] - Alternatively, said reflective optical system may comprise a structure in the shape of a cube corner having three said mirrors corresponding to the respective faces of said cube corner, said structure having an axis of symmetry passing through a vertex (common to the three said faces parallel to, but not coinciding with, the principal axis. In this case, said input propagation axis may advantageously coincide with said principal axis, whereby said circular trajectory is centered on said principal axis.

[0016] - Alternatively, said reflective optical system may comprise a first and a second pair of mirrors mounted at right angles, the two said pairs of mirrors being mounted so as to face each other and being rotated 90° relative to each other around a direction of propagation of the laser beam, the main axis of the system being coincident with the optical propagation axis of the input and passing through a mirror of the first pair, whereby said circular trajectory is centered on said main axis.

[0017] - The system may also include a half-wave blade arranged on said axis of optical propagation input, mounted in rotation around the latter at an angular velocity half that of the reflective optical system.

[0018] - The system may also include a configured triggering device to synchronize the triggering of laser pulses corresponding to the passage of said output optical propagation axis through predefined angular positions of said circular trajectory. In this case, and according to different variants:

[0019] - The system may comprise a plurality of reflecting mirrors arranged to intercept the output optical propagation axis in correspondence with the said predefined angular positions of the said circular trajectory.

[0020] - Said triggering device can be adapted to trigger the amplification of a laser pulse from a train of laser pulses forming a laser beam called an oscillator beam and comprising: an optical system for extracting a fraction of said oscillator beam and separating it into a plurality of separate auxiliary beams having propagation axes parallel to each other and to the main axis, angularly spaced around the latter, and for directing them to said reflective optical system, such that the rotation of said reflective optical system causes a parallel translation of said output propagation axes along respective non-concentric and intersecting circular trajectories; and a photodetection system configured to generate trigger signals when one of the oscillator beams reflected by the reflective optical system passes through predefined angular positions, located around the main axis and corresponding to said angular positions of the circular trajectory of the output optical propagation axis.

[0021] - Said triggering device can be adapted to trigger the amplification of a laser pulse from a train of laser pulses forming a laser beam called an oscillator beam and comprising: an optical system for extracting a fraction of said oscillator beam and directing it to an auxiliary reflective optical system, mounted at the rear of the reflective optical system and rotating about said main axis, said auxiliary reflective optical system comprising a cube wedge-shaped structure having three mirrors corresponding to the respective faces of said cube wedge, said structure having an axis of symmetry passing through a vertex common to the three said faces and parallel but not coinciding with said main axis, such that the rotation of the auxiliary reflective optical system about said main axis causes a parallel translation of said oscillator beam along a circular trajectory called the auxiliary;and a photodetection system configured to generate trigger signals when the oscillator beam reflected by the auxiliary reflective optical system passes through predefined angular positions of the auxiliary circular trajectory corresponding to said angular positions of the circular trajectory of the optical propagation axis.

[0022] - Said triggering device may include a Michelson interferometer having a movable reflector formed by a cube wedge-shaped structure having three mirrors corresponding to three respective faces of said cube wedge and an axis of symmetry passing through a vertex common to the three said faces, said structure being mounted in rotation about said principal axis in a manner fixed to said reflective optical system, the axis of symmetry being parallel to said principal axis and laterally offset from the latter, said triggering device being configured to generate a trigger signal from an interferometric signal from said Michelson interferometer when an auxiliary laser beam is incident on said movable reflector along a propagation direction forming a non-zero angle with said axis of symmetry.

[0023] Another object of the invention is a laser system comprising: a pulsed laser source; and a laser beam distribution system as defined above; wherein the pulsed laser source is configured to emit a pulsed laser beam called input propagating towards the distribution system along said input optical propagation axis; whereby the pulses of said input pulsed laser beam are distributed among a plurality of lower rate output pulsed beams propagating along optical propagation axes parallel to each other.

[0024] Yet another object of the invention is a laser system comprising: a plurality of pulsed laser sources; and a laser beam recombination system as defined above; wherein the pulsed laser sources are configured to emit respective pulsed laser beams called input beams propagating towards the recombination system along the optical propagation axis of the latter when it is in correspondence with said angular positions of said circular trajectory; whereby the pulses of said input pulsed laser beams are recombined into a higher rate output pulsed beam.

[0025] Such a laser system may include at least one laser oscillator having a rate greater than or equal to 1 MHz and at least one frequency-drift amplifier operating at a rate less than or equal to 1 kHz.

[0026] Yet another object of the invention is the use of a laser beam distribution system as defined above to distribute pulses from a so-called input pulsed laser beam between a plurality of so-called output pulsed beams at a lower rate propagating along parallel optical propagation axes.

[0027] Yet another object of the invention is the use of a laser beam recombination system as defined above to recombine pulses from a plurality of so-called input pulsed laser beams into a so-called output pulsed beam at a higher rate.

[0028] Other features, details and advantages of the invention will become apparent from the description given with reference to the accompanying drawings provided by way of example, which represent, respectively:

[0029] [Fig.1], the functional diagram of a laser system comprising a beam distribution system according to one aspect of the invention;

[0030] [Fig.2], the functional diagram of a laser system comprising a beam recombination system according to another aspect of the invention;

[0031] [Fig.3], a detailed view of a laser beam distribution or recombination system according to a first embodiment of the invention;

[0032] [Fig.4], a detailed view of a laser beam distribution or recombination system according to a second embodiment of the invention;

[0033] [Fig. 5A], a diagram illustrating the operation of a first variant of a laser system according to said second embodiment of the invention;

[0034] [Fig.5B] and [Fig.5C], diagrams illustrating the operation of a second variant of a laser system according to said second embodiment of the invention;

[0035] [Fig.6], a detailed view of a laser beam distribution or recombination system according to a third embodiment of the invention;

[0036] [Fig.7], diagrams illustrating the operation of a first variant of a laser pulse triggering device of a system according to an embodiment of the invention;

[0037] [Fig.8A] and [Fig.8B], a second variant of a triggering device laser pulses from a system according to an embodiment of the invention; and

[0038] [Fig.9], a third variant of a laser pulse triggering device of a system according to an embodiment of the invention.

[0039] Fig. 1 illustrates schematically the structure and operation of a high-power laser system comprising a beam distribution system according to a first aspect of the invention.

[0040] The laser system includes a laser oscillator OL, for example a mode-locked Ti:Sapphire oscillator emitting a pulse train having an energy on the order of a few nanojoules (nJ), a duration of a few tens of femtoseconds (fs) and a rate of a few MHz.

[0041] The system also includes a CPA amplifier (acronym for "Chirped Pulse Amplification," i.e., frequency-drift pulse amplification) which: - Selects pulses to amplify, at a much lower rate than that of the oscillator - typically from a few Hz to a few kHz; - Stretches them, up to durations of several picoseconds (ps), by introducing strong dispersion, for example using diffraction gratings; - Amplifies them, generally by multiple passes through a laser amplifying medium, up to an energy that can reach several hundred millijoules, or even several joules; stretching the pulses limits their peak power and thus avoids undesirable non-linear effects, or even damage to the amplifying medium; and - Compresses them, until they return to durations in the femtosecond range, by introducing a dispersion opposite to that used for stretching. The amplified pulses thus obtained can have peak powers on the order of terawatts (TW), or even petawatts (PW) and can be used to generate plasmas, accelerate particles or energetic radiation, induce nuclear fusion reactions etc.

[0042] The amplified pulsed laser beam FL, propagating along an input optical propagation axis APE, is supplied to the input of a laser beam distribution system SDL whose function is to distribute the laser pulses among a plurality of lower-rate output pulsed beams FLS1, ... FLSN, propagating along distinct optical propagation axes. More particularly, within the scope of the invention, the propagation axes of the pulsed beams of The outputs are parallel to each other and arranged symmetrically around a principal axis called the APR. Alternatively, one can consider that an optical propagation axis of the output APS undergoes a parallel translation around a circular trajectory T, advantageously (but not necessarily) centered on the principal axis APR.

[0043] The displacement of the output propagation axis APS along the trajectory T is synchronous with the pulse train at the input of the SDL distribution system, such that the laser pulses exit the latter at predefined angular positions of the trajectory of the APS axis, thereby forming said output pulse beams. Advantageously, this is achieved by controlling the CPA amplifier with the SDL laser beam distribution system.For example, when the output propagation axis APS passes through a predefined angular position of the circular trajectory T, a trigger device DD emits a trigger signal SD which is transmitted to the amplifier CPA and causes the selection of a pulse from the pulse train emitted by the laser oscillator OL which is thus amplified and transmitted to the distribution system SDL which redirects it along the APS axis to form one of the output pulse beams FLSi (i=l - N).

[0044] According to the present invention, the SDL laser beam distribution system comprises a reflective optical system mounted for rotation about the main optical axis. Different embodiments of said reflective optical system, comprising two, three or four mirrors, are illustrated in [Fig.3], [Fig.4] and [Fig.6], respectively.

[0045] In the case of [Fig. 1], the SDL system is used to distribute the pulses of an input pulsed laser beam into a plurality of output pulsed laser beams whose rate is a fraction of that of the input beam. Conversely, this same system can be used to combine several input pulsed beams into a single output pulsed beam with a multiple rate. In the example of [Fig. 2], four laser oscillators OL1, OL2, OL3, and OL4 are associated with respective amplifiers CPA1, CPA2, CPA3, and CPA4. The pulsed laser beams at the output of these amplifiers are arranged symmetrically around an axis, which advantageously coincides with the main axis APR, and arrive at the input of the laser beam recombination system SRL, which can be identical to the SDL distribution system of [Fig.1] by means of a permutation of the input and output.Alternatively, the sources of the pulsed laser beams at the input of the SRL recombination system can use a common laser oscillator, or even share a first CPA amplification stage.

[0046] An input propagation axis of the SRL system (not shown), parallel to the main axis APR, undergoes parallel translation along a circular path (also not shown) centered on the axis APR. When the input propagation axis coincides with the propagation axis of the pulsed beam output from one of the optical amplifiers CP Ai (i=l - 4), the trigger device DD generates the trigger signal SD. This triggers the selection of a pulse from the pulse train emitted by the corresponding laser oscillator OLi, which is amplified and transmitted to the SRL recombination system, which redirects it along an output axis to form a single output pulsed beam, FLS.

[0047] Fig. 3 illustrates a first embodiment of the laser beam distribution system of Fig. 1 or of the laser beam recombination system of Fig. 2, comprising a two-mirror SOR1 reflective optical system, M1 and M2.

[0048] The two mirrors M1, M2 of the SOR1 reflective optical system are plane mirrors, parallel to each other and forming an angle of 45° with respect to the principal axis APR, which passes through mirror M1 – preferably at its center. Advantageously, an imaginary segment joining the centers of the two mirrors (or, more generally, two points on their respective reflecting surfaces) is perpendicular to said axis APR. In this way, an input beam FL propagating along a propagation axis APE parallel to the principal axis APR and incident on mirror M1 is reflected by this mirror perpendicular to its initial propagation axis; it is then incident on mirror M2, which reflects it along an output propagation axis APS parallel to the input propagation axis APE, and therefore to the principal axis APR.

[0049] The two mirrors M1, M2 are joined together, for example by means of a rigid structure (not shown), and are rotated about the main axis APR. Under these conditions, the rotation of the reflective optical system SOR1 causes the parallel translation of the output propagation axis APS along the circular trajectory T. Optionally, reflecting mirrors MR (only one is shown in the figure) are arranged to intercept the output propagation axis APS at respective angular positions of said circular trajectory; as explained above, the triggering device DD ensures that a laser pulse is emitted at each of these angular positions. The reflecting mirrors MR allow the various pulsed output laser beams FLS to be directed towards their respective targets.Such mirrors can also be present in systems according to the embodiments of [Fig.4] and [Fig.6].

[0050] It is possible to demonstrate that a yaw or roll motion of the SOR1 optical system, or a synchronization failure between the rotation of said optical system and the laser pulse rate, results in a lateral translation of the beam laser output which, as explained above, is generally less serious than a pointing error.

[0051] In the example of [Fig. 3], the input propagation axis APE coincides with the principal axis of the APR system. This is not essential, and the APE axis can be offset from the APR axis—while remaining parallel to the latter—provided that the offset is small enough to ensure reflection of the laser beam by the two mirrors M1, M2 regardless of the angular position of the rotating SOR1 system.

[0052] If the mirrors M1 and M2 are metallic, the system preserves the polarization state—linear, circular, or elliptical—of the incoming laser beam. However, this is not necessarily true in the case of multilayer dielectric mirrors, which can be advantageous due to their higher reflection coefficient and improved flux resistance. In this case, to preserve linear polarization, a rotating half-wave plate LD can be arranged upstream of the mirror M1, rotating at half the angular velocity of the reflective optical system. This ensures that the orientation of the polarization plane relative to the plane of incidence of the beam on the mirror M1 remains constant during the rotation of the SOR1 system (preferably, a zero-order half-wave plate LD, with a thickness of a few tens of micrometers, is used; consequently, the nonlinear phase shift it may introduce remains negligible).These considerations also apply to the embodiments of [Fig.4] and [Fig.6].

[0053] In the embodiment of [Fig. 4], the SOR2 reflective optical system is based on a cube-corner-shaped structure comprising three mirrors Fl, F2, and F3 corresponding to mutually perpendicular faces of a cube corner or half-cube. In a manner known per se (see [Rafalowski]), a light ray incident on a reflecting cube corner is reflected along an optical propagation axis parallel to itself after three reflections. In the figure, this is illustrated for two distinct light rays, one of which corresponds to a laser beam FL from the CPA amplifier of [Fig. 1], initially propagating along the input propagation axis APE; after reflection by the optical system, the beam—designated by the reference numeral FS—propagates along the axis APS, parallel to APS but laterally offset.

[0054] The SOR2 cube-shaped structure is actuated in rotation around the main axis APR which is parallel to, but does not necessarily coincide with, the axis of symmetry ASY, corresponding to an internal diagonal of the cube passing through the vertex S common to the three faces Fl, F2 and F3.

[0055] In [Fig. 5A], the circle ESOR2 represents the envelope defined by the rotation of the reflecting optical system EOR2 around the principal axis APR. The vertex S describes a circle centered on the principal axis APR and whose radius is equal to the distance d between the principal axis APR and the symmetry axis AS Y of the cube corner. The FLS beam, for its part, successively describes positions aligned on a circle T of radius 2d. The figure shows discrete positions, corresponding to the arrival times of the laser pulses.

[0056] Figure 5B is similar to Figure 5A, but corresponds to the case where the principal axis APR coincides with the propagation axis APE. In this case, the circle described by vertex S and the trajectory T are concentric. This embodiment is preferred because of its smaller size, and especially because the position of the laser beam remains fixed on the three mirrors. On the one hand, this minimizes the risk of a beam intercepting an edge of the cube corner during its rotation; on the other hand, it avoids the need for fully reflective faces, requiring only small mirrors (diameter on the order of the beam diameter, for example 60 mm) fixed to a rigid frame, which could, for example, have a lattice structure.This solution is significantly less expensive and easier to implement than a full cube corner with large reflective faces (e.g., a 50 cm edge) and adequate optical quality. Furthermore, the use of small mirrors allows for a significant reduction in mass and much easier mechanical balancing of the system, thus eliminating mechanical vibrations. This last point enables higher rotation speeds and therefore, for the same number of targets (e.g., 30), the use of lasers with a higher repetition rate.

[0057] Figure 5C illustrates in more detail the operation of the embodiment of Figure 5C. In this figure, the reflecting optical system SOR2, ​​which has a circular shape when viewed from the front, is shown in four positions corresponding to times t1, t3, t3, and t4. At time t1, the vertex S(t1) of the optical system SOR2(t1) is located directly above the point of intersection between the cube corner and the principal axis of rotation APR, which coincides with the point of incidence of the laser beam FL. The reflected laser beam, FLS(t1), intercepts the cube corner at a point symmetrical to said point of incidence with respect to S(t1), which is therefore located at the top of the circular trajectory T. At time t2, the system SOR2(t2) has rotated by almost 90°. The point FLS(t2) has rotated by the same angle (it is the same point, in the frame of reference of the reflecting optical system) and is therefore at the right end of T; S(t2) is also in its rightmost position.Then, at time t3, SOR2(t3) has rotated 180° from its initial position; FLS(t3) is at the lower end of T and S(t3) is also at its lowest position. Finally, at time t4, the system SOR2(t4) has rotated almost 270° from its initial position; FLS(t4) is at the left end of T and S(t4) is also at its leftmost position. The envelope ESOR2 is... materialized by a guide cylinder that ensures the absence of vibrations that could induce yaw and roll movements of the SOR2 system. Such movements would result in lateral translations of the FLS output laser beam which, although less damaging than variations in its propagation direction, must nevertheless be minimized. Such a guide cylinder is described here with reference to the embodiment using a cube-shaped reflective optical system, but can be used in all embodiments of the invention.

[0058] It is easy to realize that, as in the case of the two-mirror system of [Fig.3], a lack of synchronization between the rotation of said optical system and the rate of laser pulses results in a lateral translation of the output laser beam.

[0059] It is noted that, in the 3-mirror embodiment of [Fig. 4] to [Fig. 5C], the laser beam is reflected in a direction opposite to its initial direction ("C configuration"), whereas in the 2-mirror embodiment of [Fig. 3] the laser beam retains its direction of propagation ("Z configuration"). Either configuration may be advantageous depending on the context in which the invention is implemented.

[0060] Figure 6 illustrates another embodiment in which the reflective optical system, SOR3, comprises four mirrors. More particularly, it comprises two pairs of mirrors, E11, E12 and E21, E22, mounted at right angles. The two right angles are arranged facing each other and rotated 90° relative to each other with respect to a line DP which corresponds to the direction of propagation of a laser beam between the two right angles.

[0061] The brackets El 1 / E21 and E21 / E22 are mounted so as to be fixed together and to be able to be actuated in rotation around an axis APR passing through the mirror Eli of the first bracket, which is not directly opposite a mirror of the second bracket.

[0062] The laser beam FL is incident on mirror Eli along a propagation axis that coincides with the rotation axis APR (this is not essential; the two axes can be slightly offset from each other, provided they are at least approximately parallel). It is reflected by mirror El2, then by the first mirror E21 of the second right-angle along the propagation direction DP, and then by the second mirror of said second right-angle E22, which reflects it a final time. The beam reflected by E22 is designated FLS. The general configuration is "Z-shaped," as in the embodiment of [Fig. 3]. It is readily understood that the rotation of the four-mirror system around the axis APR induces a translation of the FLS beam along a circular trajectory.

[0063] As in the other embodiments of the invention described above, a yaw or roll movement of the SOR1 optical system, or a defect in Synchronization between the rotation of said optical system and the rate of laser pulses results in a lateral translation of the output laser beam.

[0064] As explained above, the periodic distribution of the input laser pulses between output beams corresponding to respective fixed angular positions along a circular path requires synchronization between said laser pulses and the rotation of the reflecting optical system. In practice, synchronization can be achieved by appropriately selecting the pulses to be amplified from a high-frequency pulse train directly from a laser oscillator (see the description in [Fig. 1]), depending on the rotation of the reflecting optical system. This can be done in several different ways.

[0065] A first option, the principle of which is illustrated in [Fig. 7], consists of implementing a triggering device that uses the laser oscillator beam from a laser chain and the cube-wedge system described previously with reference to Figures [Fig. 4], [Fig. 5A], [Fig. 5B], [Fig. 5C]. In such a device, an auxiliary optical system extracts a fraction of the FLO oscillator beam and splits it into a plurality of separate auxiliary beams—for example, 3—having propagation axes parallel to each other and to the main axis APR, spaced angularly around the latter—for example, 120° apart in the case of 3 beams. This optical system can be implemented simply using beam splitters and fixed mirrors.

[0066] The three auxiliary beams are directed onto the SOR2 reflective optical system comprising a rotating cube corner. Since the auxiliary beams are incident on the cube corner at points eccentric with respect to its axis of rotation, they are reflected, each undergoing a parallel translation along a respective, non-concentric, and intersecting circular trajectory, T1, T2, T3. These reflected beams are then focused onto a plate, for example, made of glass, on which a mask allows the focused beam to be reflected only at selected positions (see [Fig. 8B]). The oscillator beam consists of a high-frequency pulse train, for example, 80 MHz; there will therefore always be an oscillator pulse corresponding to a reflective position of the mask.The reflected beams are extracted, for example by a Faraday isolator, and directed to a photodetector which generates the SD trigger signal corresponding to each passage of an auxiliary beam through a reflective position of the mask. Alternatively, it is also possible to send the pulses selected by the reflective mask directly to the input of the CPA amplifier.

[0067] Figure 8A illustrates another embodiment of a DD triggering device that can be used in a system according to the invention. As in the previous embodiment, a beam splitter SF1 extracts a portion of the FLO oscillator beam, which will be used to generate the trigger signal. The beam The auxiliary laser beam extracted from the SF1 splitter passes through a second, polarizing splitter, SF2, then is reflected by an MI mirror and focused by an LF lens to be directed to an auxiliary reflective optical system, SOA, mounted behind the reflective optical system, SOR2 (a 2- or 4-mirror system, as described above, could also be used, but the cube-wedge system is more easily compatible with the space constraints). The auxiliary beam, assumed to be linearly polarized, also passes through a quarter-wave plate, LQ, which makes its polarization circular.

[0068] The two reflective optical systems SOR2 and SOA are mounted back-to-back and rotated about the same principal axis APR. As explained above with reference to the "main" optical system SOR2, ​​the auxiliary beam is reflected with a lateral offset, and the rotation of the auxiliary optical system causes its parallel translation along a circular path TA. The lens LF has a focal length adapted to focus the reflected auxiliary beam onto a plane mirror MO at normal incidence carrying an opaque mask MSF (see [Fig. 8B]) having slits corresponding to predefined angular positions. When it passes through these predefined angular positions, the auxiliary beam is reflected and travels the same path in reverse to the beam splitter SF2. The second pass through the plate LQ makes the beam linearly polarized in a plane perpendicular to that of the incident beam.Also, the retro-reflected beam is deflected by the second separator SF2 (which it had passed through on the outward journey) which directs it towards a photodetector SD which generates the trigger signal SD.

[0069] As suggested by [Fig.8B], it is possible to finely adjust the timing of the trigger pulses by rotating the MSF mask around its axis.

[0070] Figure 9 illustrates yet another embodiment of a DD triggering device that can be used in a system according to the invention.

[0071] The device in [Fig. 9] is based on a Michelson interferometer receiving at its input an auxiliary laser beam FLA, which is preferably a continuous-wave laser with a long coherence length, for example, a He-Ne laser. A beam splitter LS separates the auxiliary laser beam into two components, one propagating in a first arm of fixed length, delimited by a fixed reflector RFI, and the other in a second arm of variable length, delimited by a fixed reflector RF2 but including, along its optical path, a movable reflector RM responsible for varying the length of the arm. The movable reflector RM is a wedge-shaped structure mounted for rotation about the main axis APR and having an axis of symmetry AS parallel to said main axis but offset from it. The movable reflector is integral with the reflective optical system SOR1 / SOR2 / SOR3, or even coincides with it.The rotation of the SR reflector around the APR axis generates a periodic variation in the optical path difference between the two components which, after . reflections are recombined by the LS plate and interfere with the PD photodetector. It is possible to demonstrate (see [Haschberger]) that the interference signal can be written

[0072] S(t)oosin([3).sin(cot)

[0073] Where co is the angular velocity of rotation of RM and [3 the angle between the propagation direction of the auxiliary beam and the axis APR. By counting the interference fringes (i.e. the number of times the signal passes through a maximum or a minimum) it is possible to precisely determine the rotation angle cot and thus generate the trigger signal at the desired time. References

[0074] [Rafalowski] “Détermination of the working area for the corner-cube mirror System with variable angle of incident beam for interferometric application” Optica Applicata. VoL XXV, No. 2, 1995

[0075] [Haschberger3] P. Haschberger and V. Tank “Optimization of a Michelson interferometer with a rotating retroreflector in optical design, spectral resolution, and optical throughput” J. Opt. Soc. Am. A / Vol. 10, No. 11 / November 1993

Claims

Demands

1. Laser beam distribution or recombination system (LBDS, LBS) comprising a reflective optical system (ROS1, ROS2, ROS3) mounted to rotate about an axis, called principal (PPA), parallel to an optical propagation axis (OPA), called input, of a laser beam (LB), said reflective optical system comprising at least two mirrors (M1, M2; Fl, F2, F3; Eli, E12, E21, E22) joined together and arranged to reflect said laser beam along an optical propagation axis (OPA), called output, parallel to, but not coinciding with, said input optical propagation axis (OPA) and said principal axis (LBDS); said reflective optical system being configured so that its rotation about said principal axis (OPA) causes a parallel translation of said output optical propagation axis (OPA) along a circular path (T).

2. System according to claim 1 wherein said reflective optical system (SOR1) comprises a first (M1) and a second (M2) mirror, parallel to each other and forming an angle of 45° with respect to said principal axis (APR) and to said optical propagation axes of input (APE) and output (APS), the first mirror (M1) being traversed by said principal axis (APR) and the second mirror (M2) being positioned so as to receive the laser beam reflected by the first mirror, thereby said circular trajectory (T) is centered on said principal axis.

3. System according to claim 1 in which said reflective optical system (SOR2) comprises a cube wedge-shaped structure having three said mirrors (F1, F2, F3) corresponding to respective faces of said cube wedge, said structure having an axis of symmetry (ASY) passing through a vertex (S) common to the three said faces parallel to, but not coinciding with, the principal axis (APR).

4. System according to claim 3 in which said input propagation axis coincides with said principal axis, whereby said circular trajectory (T) is centered on said principal axis.

5. A system according to claim 1, wherein said reflective optical system (SOR3) comprises a first (Eli, E12) and a second (E21, E22) pair of mirrors mounted at right angles, the two pairs of mirrors being mounted so as to face each other and being rotated 90° to each other about a direction of propagation (DP) of the laser beam, the main axis of the system being coincident with the optical propagation axis of the input and passing through a mirror (El 1) of the first pair, whereby said circular trajectory (T) is centered on said main axis.

6. System according to any one of the preceding claims also comprising a half-wave plate (LD) arranged on said input optical propagation axis (APE), mounted to rotate about the latter at an angular velocity half that of the reflective optical system.

7. System according to any one of the preceding claims comprising a triggering device (DD) configured to synchronize the triggering of laser pulses in correspondence with the passage of said output optical propagation axis (APS) through predefined angular positions of said circular trajectory.

8. System according to claim 7 comprising, a plurality of reflecting mirrors (RM) arranged to intercept the output optical propagation axis in correspondence of said predefined angular positions of said circular trajectory.

9. A system according to any one of claims 7 or 8 when they depend on claim 4, wherein said triggering device is adapted to trigger the amplification of a laser pulse from a train of laser pulses forming a laser beam called an oscillator beam (FLO) and comprises: - an optical system for extracting a fraction of said oscillator beam and separating it into a plurality of separate auxiliary beams having propagation axes parallel to each other and to the main axis, angularly spaced around the latter, and for directing them to said reflective optical system (SOR2), such that the rotation of said reflective optical system (SOR2) causes a parallel translation of said output propagation axes along respective non-concentric and intersecting circular trajectories (TOI, TO2, TO3);- a photodetection system configured to generate trigger signals when one of the oscillator beams reflected by the reflective optical system passes through predefined angular positions (PAi), located around the main axis and corresponding to said angular positions of the circular trajectory of the output optical propagation axis.;

10.

11. A system according to any one of claims 7 or 8, wherein said triggering device is adapted to trigger the amplification of a laser pulse from a train of laser pulses forming a laser beam called an oscillator beam (FLO), and comprises: - an optical system (SF1) for extracting a fraction of said oscillator beam and directing it to an auxiliary reflective optical system (SOA), mounted at the rear of the reflective optical system (SOR2) and rotating about said main axis (APR), said auxiliary reflective optical system (SOA) comprising a cube-shaped structure having three mirrors corresponding to respective faces of said cube-shaped structure, said structure having an axis of symmetry (ASA) passing through a vertex common to the three said faces and parallel but not coinciding with said main axis,such that the rotation of the auxiliary reflective optical system around said principal axis causes a parallel translation of said oscillator beam along a circular trajectory called auxiliary (TA); and, - a photodetection system (MO, MI, SF2, PD) configured to generate trigger signals (SD) when the oscillator beam reflected by the auxiliary reflective optical system passes through predefined angular positions of the auxiliary circular path corresponding to said angular positions of the circular path of the optical propagation axis. A system according to any one of claims 7 or 8, wherein said triggering device comprises a Michelson interferometer having a movable reflector (RM) formed by a cube wedge-shaped structure having three mirrors corresponding to three respective faces of said cube wedge and an axis of symmetry (AS) passing through a vertex common to the three said faces, said structure being mounted for rotation about said principal axis (APR) in a manner fixed to said reflective optical system, the axis of symmetry (AS) being parallel to said principal axis and offset laterally from the latter, said triggering device being configured to generate a trigger signal from an interferometric signal from said Michelson interferometer when an auxiliary laser beam (FLA) is incident on said movable reflector along a propagation direction forming a non-zero angle with said axis of symmetry.

12. A laser system comprising: - a pulsed laser source (OL, CPA); and - a laser beam distribution system (SDL) according to any one of the preceding claims; wherein the pulsed laser source is configured to emit a pulsed laser beam called input (FL) propagating towards the distribution system (SDL) along said input optical propagation axis (APE); whereby the pulses of said input pulsed laser beam are distributed among a plurality of lower rate output pulsed beams (FLS1, ... FLSN) propagating along parallel optical propagation axes.

13. Laser system comprising: - a plurality of pulsed laser sources (OL1 - OL4; CPA1 - CPA4); and - a laser beam recombination system (LBS) according to any one of claims 7 to 11; wherein the pulsed laser sources are configured to emit respective pulsed laser beams (FL1 - FL4) said to be input propagating towards the recombination system (LBS) along the optical propagation axis of the latter when it is in correspondence with said angular positions of said circular trajectory; whereby the pulses of said input pulsed laser beams are recombined into a higher rate output pulsed beam (FLS).

14. A laser system according to any one of claims 12 or 13, comprising at least one laser oscillator having a rate greater than or equal to 1 MHz and at least one frequency-drift amplifier operating at a rate less than or equal to 1 kHz.

15. Use of a laser beam distribution system according to any one of claims 1 to 11 for distributing pulses from a pulsed laser beam, referred to as the input beam, between a plurality of pulsed beams, referred to as the output beams, at a lower rate of propagation along parallel optical propagation axes between pi iv

16. CUA. Use of a laser beam recombination system according to any one of claims 7 to 11 for recombining pulses from a plurality of pulsed laser beams called input into a pulsed beam called output at a higher rate.

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