System for distributing or recombining laser beams

Abstract

The invention relates to a system for distributing or recombining laser beams (SDL), comprising a reflective optical system (SOR2) which is mounted so as to rotate about an axis referred to as the main axis (APR) and parallel to an optical propagation axis (APE), referred to as the input axis, of a laser beam (FL), the reflective optical system comprising a cube corner-shaped structure having three mirrors (F1, F2, F3) which correspond to respective faces and are arranged to return the laser beam along an optical propagation axis (APS) referred to as the output axis and parallel to, but not coincident with, the input optical propagation axis (APE) and the main axis (APR); the reflective optical system being configured so that its rotation about the main axis (APR) leads to a parallel translation of the output optical propagation axis (APS) along a circular trajectory (T).

Description

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

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

[0002] It applies particularly 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 allows for 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 allows for obtaining, from a plurality of pulsed laser sources, a single beam of pulses at a rate multiple of that of an individual source.

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

[0005] Several systems have been developed to sequentially distribute high-power laser pulses emitted from a single source to several successive reflection directions. These systems usually include an optomechanical reflection device adapted to reflect the high-power laser beam back to these successive reflection directions.

[0006] Document FR2608786, for example, describes a laser system comprising a laser device emitting a pulsed laser beam of power 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 laser beam of power towards successive directions. It is understood that any synchronization error between the laser and the galvanometer, or any inaccuracy in the control signal 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, whose rotation 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 frame rates.

[0008] Optical switches also exist in which a laser beam is redirected by successively inserting mirrors actuated in translation; see, for example, US2002 / 0126948. When applied to 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 throughput.

[0009] All these solutions present a significant risk of misalignment, meaning a lack of angular accuracy in the beam's reflection. This angular accuracy can be defined as the difference between the actual reflection direction and the intended reflection direction, while the spatial accuracy is defined as the lateral spatial difference between the actual position of the laser spot at a predefined distance and its intended position. In the common 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 lead to a decrease in illumination at its focal point as well as a degradation of the spatial distribution of illumination, which can be very detrimental to applications.For example, consider the focusing of a 40mm diameter beam by an off-axis parabola at 90° with a focal length of f=400 mm. For perfect alignment, within the framework of geometric optics, the... The system focuses all 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 axis of the parabola, 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 roughly the size of the diffraction spot, resulting in a decrease in peak illumination of approximately a factor of 2. However, if the beam is laterally deflected parallel to itself by an angle (Ax, Ay) along two axes Ox or Oy perpendicular to the direction of propagation Oz, it will still be perfectly focused by the parabola, but will have a very slightly different direction after the parabola depending on the values ​​of Ax and Ay (0 X = Ax / f, 0 y = Ay / f).

[0010] US patent 3,924,937 describes a laser beam combining system using a reflective optical system comprising a first and a second reflecting surface that are fixed and parallel to each other (they are, in fact, the reflective faces of the same prism). The system is mounted for rotation about an axis passing through the first surface and inclined relative to it. Several pulsed laser beams to be recombined propagate along optical axes parallel to the system's axis of rotation and arranged around it; these laser beams are intercepted in turn by the second surface and reflected back to the first surface, which returns them along the main axis. If the pulsed laser beams are synchronized with the rotation of the reflective optical system, a single output pulsed beam is obtained at a rate that is a multiple of the input beam rate.One advantage of this system is its low sensitivity to synchronization errors. The system could also be used to distribute a single pulsed beam among several lower-rate output beams; in this case, a synchronization error would result in a lateral shift of the output beams, without affecting their angular direction.

[0011] One drawback of this system, however, is the size of the motor that drives the reflective system, which necessitates a relatively large distance between the two reflecting surfaces. Another drawback is that if the laser beams must propagate in a vacuum (which is often necessary in the case of intense pulses), the The motor itself must be inside a vacuum chamber, which poses problems of overheating and available power.

[0012] US patent 4,154,507 describes a laser beam combining or distribution system using two V-shaped mirrors arranged face to face and rotating around the same axis. As in US patent 3,924,937, a synchronization error would result in a lateral shift of the output beams without affecting their angular direction. However, this system has several drawbacks, including a high number of reflections leading to significant sensitivity to the surface quality of the mirrors, a mechanical footprint incompatible with processing large-diameter laser beams, and the need to use an auxiliary flat mirror or a perforated V-shaped mirror.

[0013] The invention aims to overcome, at least in part, the aforementioned drawbacks of the prior art. More specifically, it aims to provide a system for distributing or combining high-power laser beams that achieves low misalignment error and has a reduced mechanical footprint. 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.

[0014] 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 principal, parallel to an optical propagation axis, called input, of a laser beam, said reflective optical system comprising at least two mirrors joined and arranged to reflect said laser beam along an optical propagation axis, called output, parallel to, but not coinciding with, said input optical propagation axis and said principal axis;said reflective optical system being configured so that its rotation around said principal axis causes a parallel translation of said output optical propagation axis along a circular trajectory, characterized in that said reflective optical system comprises a cube-shaped structure having three said mirrors corresponding to the respective faces of said cube-shaped structure, 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, and in that said input propagation axis coincides with said principal axis, whereby said circular trajectory is centered on said principal axis.

[0015] - 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° 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.

[0016] According to different modes of implementation:

[0017] - The system can be arranged in a vacuum chamber, with a drive motor for said reflective optical system arranged outside said chamber.

[0018] - The system may also include a half-wave plate arranged on said input optical propagation axis, mounted to rotate around the latter at half the angular velocity of the reflective optical system.

[0019] - The system may also include a triggering device configured 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:

[0020] - The system may include a plurality of reflecting mirrors arranged to intercept the output optical propagation axis in correspondence to said predefined angular positions of said circular trajectory.

[0021] - Said triggering device may be adapted to trigger the amplification of a laser pulse from a train of laser pulses forming a so-called oscillator laser beam and comprise: 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 towards 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 triggering signals when one of the oscillator beams reflected by the reflective optical system pass 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.

[0022] - 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 comprise: 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 an 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 path corresponding to said angular positions of the circular path of the optical propagation axis.

[0023] - 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 offset laterally with respect to 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.

[0024] 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 the input beam 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 parallel optical propagation axes.

[0025] 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.

[0026] Such a laser system may include at least one laser oscillator with 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.

[0027] 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.

[0028] 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.

[0029] Other features, details and advantages of the invention will become apparent from the description provided with reference to the accompanying drawings given by way of example, which represent, respectively:

[0030] [Fig.1], the functional diagram of a laser system including a beam distribution system;

[0031] [Fig. 2], the functional diagram of a laser system including a beam recombination system;

[0032] [Fig. 3], a detailed view of a laser beam distribution or recombination system not covered by the invention;

[0033] [Fig. 4], a detailed view of a laser beam distribution or recombination system according to an embodiment of the invention;

[0034] [Fig. 5A], a diagram illustrating the operation of a first variant of a laser system according to said embodiment of the invention;

[0035] [Fig. 5B] and [Fig. 5C], diagrams illustrating the operation of a second variant of a laser system according to said embodiment of the invention;

[0036] [Fig. 6], a detailed view of another laser beam distribution or recombination system;

[0037] [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;

[0038] [Fig. 8A] and [Fig. 8B], a second variant of a laser pulse triggering device of a system according to an embodiment of the invention; and

[0039] [Fig. 9], a third variant of a laser pulse triggering device of a system according to an embodiment of the invention.

[0040] Figure 1 schematically illustrates the structure and operation of a high-power laser system with a beam distribution system.

[0041] The laser system includes a laser oscillator OL, for example a mode-locked Ti:Sapphire oscillator emitting a pulse train with 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.

[0042] The system also includes a CPA amplifier (acronym for "Chirped Pulse Amplification," that is, amplification of pulses with drift frequency) which: - Selects pulses to amplify, at a rate much lower 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.

[0043] The amplified pulsed laser beam FL, propagating along an input optical propagation axis APE, is fed into a laser beam distribution system SDL. The SDL's function is to distribute the laser pulses among a plurality of lower-rate output pulsed beams FLS1, ... FLSN, propagating along distinct optical propagation axes. Specifically, the propagation axes of the output pulsed beams are parallel to each other and arranged symmetrically around a principal axis APR. Alternatively, an output optical propagation axis APS can be considered to undergo a parallel translation around a circular trajectory T, advantageously (but not necessarily) centered on the principal axis APR.

[0044] The movement of the output APS propagation axis along the trajectory T is synchronous with the input pulse train of the SDL distribution system, such that the laser pulses exit the latter at predefined angular positions along the trajectory of the APS axis, thus forming the output pulse beams. Advantageously, this is achieved by controlling the CPA amplifier with the SDL laser beam distribution system. For example, when the output APS propagation axis 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 axis APS to form one of the output pulse beams FLSi (i=1 - N).

[0045] The SDL laser beam distribution system includes a reflective optical system mounted for rotation around the main optical axis. The invention relates to the implementation of such a reflective optical system.

[0046] 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]. 1] by swapping 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.

[0047] An input propagation axis of the SRL system (not shown), parallel to the main APR axis, undergoes parallel translation along a circular path (also not shown) centered on the APR axis. When the input propagation axis coincides with the propagation axis of the pulsed beam output from one of the optical amplifiers CPAi (i=1 - 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. The SRL redirects the pulsed pulse along an output axis to form a single output pulsed beam, FLS.

[0048] Figure 3 illustrates an embodiment of the laser beam distribution system of Figure 1 or the laser beam recombination system of Figure 2, comprising a two-mirror SOR1 reflective optical system, with mirrors M1 and M2. This system is not part of the invention, but is similar to that disclosed in US patent 3,924,937.

[0049] The two mirrors M1 and M2 of the SOR1 reflective optical system are plane mirrors, parallel to each other and forming a 45° angle with respect to the principal axis APR, which passes through mirror M1—preferably at its center. Advantageously, an imaginary line 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.

[0050] The two mirrors M1 and M2 are joined together, for example by a rigid structure (not shown), and are rotated around the main axis APR by a motor, for example an electric one, MENT. 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 may also be present in systems according to the embodiments of [Fig. 4] and of the [Fig. 6].

[0051] 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 translation lateral to the output laser beam which, as explained above, is generally less serious than a pointing error.

[0052] In the example in [Fig. 3], the input propagation axis APE coincides with the main axis of the APR system. This is not essential, and the APE axis can be offset from the APR axis—while remaining parallel to it—provided that the offset is small enough to ensure the reflection of the laser beam by both mirrors M1, M2 regardless of the angular position of the rotating SOR1 system.

[0053] If 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 for multilayer dielectric mirrors, which can be advantageous due to their higher reflection coefficient and improved flux resistance. In this case, to maintain linear polarization, a rotating half-wave plate (LD) can be positioned upstream of mirror M1. This plate rotates 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 mirror M1 remains constant during the rotation of the SOR1 system. (A zero-order half-wave plate (LD) with a thickness of a few tens of micrometers is preferred; consequently, the nonlinear phase shift it might introduce remains negligible.)These considerations also apply to the embodiments of [Fig. 4] and [Fig. 6].

[0054] It has been observed that there is a minimum permissible distance between mirrors M1 and M2, preventing the APS axes from intersecting the MENT drive motor. However, the greater the distance between the mirrors, the higher the torque required from the motor and the greater the linear speed of M2, which can cause aerodynamic problems during operation in open air. In the case of vacuum operation, preferred for applications with intense laser beams, it is difficult to avoid placing the MENT motor inside the sealed enclosure, which complicates its implementation.

[0055] As illustrated in [Fig. 4], the invention avoids these difficulties by using a reflective optic SOR2 based on a cube corner-shaped structure, comprising three mirrors F1, F2, and F3 corresponding to their respective, perpendicular faces of a cube corner or half-cube. In a known manner In itself (see [Rafalowski]), a light ray incident on a corner of a reflecting cube 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 FS – propagates along the axis APS, parallel to APS but laterally offset.

[0056] The SOR2 cube-shaped structure is driven by rotation around the principal axis APR, which is parallel to, but does not necessarily coincide with, the axis of symmetry ASY. This axis corresponds to an internal diagonal of the cube passing through the vertex S common to the three faces F1, F2, and F3. The drive motor is not shown, but it is understood to be located at the rear of the cube-shaped structure. Therefore, it cannot intercept the laser beams and thus presents no space constraints. In the case of operation in a vacuum, it can easily be arranged outside a sealed enclosure EV, of which only a drive shaft for the cube-shaped structure needs to pass through the wall.

[0057] In [Fig. 5A], the circle ESOR2 represents the envelope defined by the rotation of the reflecting optical system SOR2 around the principal axis APR. The vertex S describes a circle centered on the principal axis APR, with a radius equal to the distance d between said principal axis APR and the axis of symmetry ASY 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.

[0058] 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 V² the beam diameter, for example, 60 mm) mounted on a rigid frame, which could have, for example, a lattice structure. This solution is clearly Less expensive and easier to manufacture than a full cube corner, it has large reflective faces (for example, a 50 cm edge) with 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 (for example, 30), the use of lasers with a higher repetition rate.

[0059] Figure 5C illustrates in more detail the operation of the embodiment of Figure 5B. 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 this 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 SOR2(t4) system 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 ESOR2 envelope is defined by a guide cylinder that ensures the absence of vibrations that could induce yaw and roll movements in the SOR2 system. Such movements would result in lateral translations of the output laser beam FLS, 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 wedge reflective optical system, but can also be used in conjunction with a 2 or 4 mirror system as described above.

[0060] It is easy to see that, as in the case of the two-mirror system in [Fig. 3], a lack of synchronization between the rotation of said system optics and the rate of laser pulses, results in a lateral translation of the output laser beam.

[0061] It is noted that, in the 3-mirror embodiment of [Fig. 4] to [Fig. 5C], according to the invention, the laser beam is reflected in a direction opposite to the initial one ("U-shaped configuration"), while in the 2-mirror embodiment of [Fig. 3], which is prior art, the laser beam retains its direction of propagation ("Z-shaped configuration").

[0062] Figure 6 illustrates another embodiment in which the reflective optical system, SOR3, comprises four mirrors. More specifically, it comprises two pairs of mirrors, E11, E12 and E21, E22, mounted at right angles. Unlike the case in US patent 4,154,507, the two right angles are arranged facing each other and rotated 90° relative to each other about a line DP, which corresponds to the direction of propagation of a laser beam between the two right angles. This results in a simpler mechanical structure and fewer reflections of the laser beams.

[0063] The brackets E11 / E21 and E21 / E22 are mounted so that they are fixed together and can be rotated around an axis APR passing through the mirror E11 of the first bracket, which is not directly opposite a mirror of the second bracket.

[0064] The laser beam FL is incident on mirror E11 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 E12, then by the first mirror E21 of the second right-angle mirror along the propagation direction DP, and then by the second mirror of said second right-angle mirror 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.

[0065] As in the other embodiments described above, a yaw or roll movement of the SOR1 optical system, or a synchronization failure The relationship between the rotation of said optical system and the rate of laser pulses results in a lateral translation of the output laser beam.

[0066] As explained above, the periodic distribution of input laser pulses between output beams corresponding to fixed angular positions along a circular path requires synchronization between these laser pulses and the rotation of the reflecting optical system. In practice, this 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.

[0067] A first option, the principle of which is illustrated in [Fig. 7], consists of creating 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, three—having propagation axes parallel to each other and to the main axis APR, angularly spaced around the latter—for example, 120° apart in the case of three beams. This optical system can be simply implemented using beam splitters and fixed mirrors.

[0068] The three auxiliary beams are directed onto the SOR2 reflective optical system, which includes a rotating cube corner. Since the auxiliary beams are incident on the cube corner at points eccentric to its axis of rotation, they are reflected, each undergoing a parallel translation along a respective, non-concentric, and intersecting circular path, 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; therefore, there will 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 position. reflective 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.

[0069] 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 is used to generate the trigger signal. The auxiliary laser beam extracted from the splitter SF1 passes through a second, polarizing splitter SF2, then is reflected by a mirror M1 and focused by a lens LF 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.

[0070] The two reflective optical systems SOR2 and SOA are mounted back-to-back and rotated around 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 LF lens 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]) with 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 LQ plate 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.

[0071] As suggested by [Fig. 8B], it is possible to fine-tune the timing of the trigger pulses by rotating the MSF mask around its axis.

[0072] Figure 9 illustrates yet another embodiment of a DD triggering device that can be used in a system according to the invention.

[0073] The device in [Fig. 9] is based on a Michelson interferometer receiving at its input an auxiliary laser beam (ALB), preferably a continuous-wave laser with a long coherence length, for example, a He-Ne laser. A beam splitter (BSB) divides the ALB into two components, one propagating in a first arm of fixed length, delimited by a fixed reflector RF1, 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 arm's length. The movable reflector RM is a wedge-shaped cube structure mounted for rotation around 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, or even coincides with, the reflective optical system SOR1 / SOR2 / SOR3.The rotation of the RM reflector around the APR axis generates a periodic variation in the optical path difference between the two components which, after reflection, 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 as.

[0074] S(t)ocsin(P).sin(œ.t)

[0075] Where O is the angular velocity of rotation of RM and P is 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 œ.t and thus generate the trigger signal at the desired time.

[0076] References

[0077] [Rafalowski] “Determination 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

[0078] [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 (LBS, RLS) comprising a reflective optical system (ROS2) mounted for rotation about an axis, called the main axis (MAS), parallel to an optical propagation axis (APA), called the input axis, of a laser beam (LB), said reflective optical system comprising at least two mirrors (F1, F2, F3) joined together and arranged to reflect said laser beam along an optical propagation axis (APA), called the output axis, parallel to, but not coinciding with, said input optical propagation axis (APA) and said main axis (MAS); said reflective optical system being configured so that its rotation about said main axis (MAS) causes a parallel translation of said output optical propagation axis (APA) along a circular path (T);characterized in that 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), and in that said input propagation axis coincides with said principal axis, whereby said circular trajectory (T) is centered on said principal axis.

2. System according to claim 1 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.

3. System according to claim 1, arranged in a vacuum chamber (EV), a drive motor (MENT) of said reflective optical system (SOR2) being arranged outside said chamber.

4. 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.

5. System according to claim 4 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.

6. System according to any one of claims 4 or 5, 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 (TO1, 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.; 7. A system according to claim 4 or 5, wherein said triggering device is adapted to trigger the amplification of a laser pulse from a laser pulse train 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 principal 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 to, but not coinciding with, said principal axis, such that the rotation of the optical system auxiliary reflective around said main axis causes a parallel translation of said oscillator beam along a circular path called auxiliary (TA); and a photodetection system (MO, Ml, 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.

8. System according to any one of claims 4 or 5 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 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 (FLA) is incident on said movable reflector along a propagation direction forming a non-zero angle with said axis of symmetry.

9. 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.

10. Laser system comprising: a plurality of pulsed laser sources (OL1 - OL4; CPA1 - CP A4); and a laser beam recombination system (LBS) according to any one of claims 4 to 8; 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). 11 Laser system according to any one of claims 9 or 10, 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.

12. Use of a laser beam distribution system according to any one of claims 1 to 8 for distributing 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.

13. Use of a laser beam recombination system according to any one of claims 4 to 8 to recombine pulses from a plurality of so-called input pulsed laser beams into a so-called output pulsed beam at a higher rate.

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