Laser system adapted to distribute a high-power pulsed laser beam along several successive directions

Abstract

The invention relates to a laser system 1 adapted to emit a pulsed laser beam of power FL and to distribute it along several successive reflection directions, comprising: a laser power device 10; a reflection device 20 adapted to reflect and distribute the laser beam of power FL along the successive reflection directions, and comprising at least one reflection mirror; and a control device (30) adapted to provide a trigger signal sd(t) to a pulse selector (12) enabling the transmission of the laser beam of power FL, which is then received by the reflection mirror (21j) in a reference position Pmr,ref.i, which reflects it along the corresponding reflection direction (Dr,i). Figure for the abstract: Fig. 1

Description

Title of the invention: Laser system adapted to distribute a pulsed laser beam of power in several successive directions Technical field

[0001] The field of the invention is that of power laser systems adapted to emit a power pulsed laser beam, and to distribute it in several successive return directions. STATE OF THE PRIOR ART

[0002] There are laser systems that allow a high-power pulsed laser beam to be emitted from a single laser source and then distributed sequentially in several successive return directions. Such laser systems can be used for various applications, for example in 3D radiography, product marking, substrate processing, among others.

[0003] These laser systems usually comprise a power laser device emitting a pulsed laser beam, and a return device, of the opto-mechanical type, adapted to reflect the power laser beam in different successive return directions. The return device comprises at least one return mirror, and a mechanical positioning device to which the return mirror is assembled. This return device is adapted to position the return mirror, at a given return frequency, in several successive reference positions respectively associated with the desired return directions. Thus, when the return mirror is positioned in a reference position, it receives the power laser beam and reflects it in the corresponding return direction.

[0004] By way of example, document FR2608786A1 describes a laser system comprising a laser device emitting a pulsed power laser beam towards a deflecting device. This comprises a deflecting mirror mounted on a rotating support, here on the axis of a galvanometer whose orientation is controlled by a computer. In this example, the power laser beam is distributed successively towards different optical fibers.

[0005] Document US2005 / 0087295A1 also describes an example of a laser system comprising a laser device emitting a pulsed power laser beam towards a return device. Here, the return device comprises a rotating polygonal structure (formed of several reflective facets). The rotation of this polygonal mirror makes it possible to distribute the power laser beam in several successive directions. In this document, the objective is to distribute the laser beam of power on different areas of the surface of a substrate in order to perforate it.

[0006] However, in these examples, the laser system is not able to ensure effective synchronization between the emission of the power laser pulses and the angular position of the deflection mirror, so that such laser systems cannot exhibit good performance in terms of spatial and / or angular deflection precision, in particular at high laser rates, for example of the order of a few kilohertz. The angular deflection precision can be defined as the angular deviation between the effective deflection direction relative to the desired deflection direction, and the spatial deflection precision is defined as the spatial deviation between the effective position of the laser spot at a predefined distance and its desired position.

[0007] Note that there are also laser systems for controlling the direction of return of the power laser beam. Documents EP0390662A1 and WO93 / 06641A1 describe examples of such laser systems. However, these laser systems are not configured to distribute the power laser beam in several successive directions at a high laser rate. In addition, the direction of return of the power laser beam does not depend on the position of a moving mirror, but results from the acousto-optic deflection of the injection laser beam (of low power).

[0008] Finally, note that there are optical switches operating on the principle of insertable mirrors. The optical beams are then redirected by successive insertion of insertable mirrors by translation, as described for example in document US2002 / 0126948A1. Applied to the kilohertz switching of high-power laser beams of large diameters (e.g. 40 mm), this technique involves the use of bulky, expensive actuators, which consume a lot of energy, and are likely to generate shocks and vibrations. In addition, this type of switch requires all the mirrors to be put back in place to perform a new reading cycle. This switching technique (generally based on MEMS) is used in optical telecommunications but is not suitable for switching high-power beams. Statement of the invention

[0009] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose a laser system adapted to emit a pulsed laser beam of power FL, and to distribute it in several successive return directions, with good performance in terms of angular return precision A0r and / or spatial return precision ADr, with laser rates fL which can be high.

[0010] For this, the object of the invention is a laser system adapted to emit a pulsed laser beam of FL power and to distribute it in several directions of successive returns {D,} , ;NDr, with NDr>l, including: - a power laser device, adapted to emit the power laser beam Fl at a laser rate fL; - a return device, adapted to reflect and distribute the laser beam of power FL along the successive return directions {Dr>i}, comprising: at least one return mirror {2^}^! ;Nmr with Nini> I; and a first mechanical positioning device, to which the return mirror(s) {21j} are assembled, adapted to continuously move the return mirror(s) {21j} to position it or them successively, at a return frequency fr, in reference positions {bi ;NDr associated respectively with the return directions {Dri}, so that a return mirror positioned in a reference position receives the laser beam of power FL and reflects it along the corresponding return direction.

[0011] According to the invention, the power laser device comprises a pulse selector, adapted to transmit, at the laser rate fL, an incident pulsed laser beam Fo, the transmitted laser beam being intended to form the power laser beam FL.

[0012] In addition, the laser system comprises a servo-control device, adapted to provide a trigger signal sd(t) to the pulse selector authorizing the transmission of the laser beam of power FL, comprising: - a control laser source, adapted to emit a control laser beam Fc - a control device, comprising: at least one control mirror {41k] k=i; Nmc with Nmc>l; and a second mechanical positioning device, to which the control mirror(s) {4lk] are assembled, adapted to continuously move in rotation the control mirror(s) {41k] to position it or them successively, at the return frequency fr, in at least one threshold position {Pmc>th,m}m=i;NPth, with Npth>l, so that a control mirror positioned in a threshold position receives the control laser beam Fc and reflects it in the same predefined return threshold direction; - a matrix photodetector, adapted to receive the control laser beam Fc reflected by a control mirror during rotation, and to emit in response a detection signal spd(t), representative of the displacement of the control mirror, passing through a threshold value associated with the threshold position(s) {P mc,th,m} 5 - an electronic triggering device, adapted to emit the trigger signal sd(t) when the detection signal spd(t) reaches said threshold value spdjth, the threshold position(s) {Pmc>th,m} being predefined so that the laser beam of power FL, transmitted by the pulse selector in response to the trigger signal sd(t), is received by a deflecting mirror at a reference position P^efi, and therefore reflected in the corresponding deflecting direction.

[0013] Some preferred but non-limiting aspects of this laser system are as follows.

[0014] The power laser device may comprise a passive mode-locking injection laser source, suitable for generating the pulsed laser beam Fo.

[0015] The electronic triggering device may comprise: a multiple pulse inhibiting electronic circuit adapted to emit the trigger signal sd(t) when the detection signal spd(t) reaches said threshold value spd>th; and a delay generator adapted to transmit the trigger signal sd(t) to the pulse selector, with a predefined delay so that the trigger signal sd(t) is received by the pulse selector in phase with a laser pulse of the pulsed laser beam Fo to be selected.

[0016] The power laser device may comprise: an optical amplifier, located downstream of the pulse selector, and comprising a pulse-pumped regenerative amplifier.

[0017] The servo-control device may comprise an optoelectronic gain servo-control circuit, adapted to emit a pumping signal sp(t) authorizing transmission of a pumping pulse to the regenerative amplifier, such that the pumping pulse is received by the regenerative amplifier in phase advance upon reception by the latter of a laser pulse transmitted by the pulse selector in response to the trigger signal sd(t), with a predefined advance such that the regenerative amplifier has an optimal gain when it receives the laser pulse transmitted by the pulse selector.

[0018] The laser rate fL can be equal to the return frequency fr.

[0019] The return threshold direction can be coaxial with an optical axis of the control laser beam Fc then incident on the control mirror {41j} and coming from the control laser source.

[0020] The deflection device may comprise several deflection mirrors {21j}, with Nmr>l, assembled to a first rotating support, which is a cylinder with a circular or polygonal section, on the surface of which the deflection mirrors {21j} are distributed.

[0021] The control device may comprise a rotating cylinder, having an external surface intended to be illuminated by the control laser beam Fc, the external surface being reflective to the control laser beam Fc and traversed by non-reflective zones distributed regularly along the external surface.

[0022] The control device may comprise several control mirrors {41k], with N mc>l, assembled to a second rotating support, which is a cylinder with a circular or polygonal section, on the surface of which the control mirrors {41j are distributed }•

[0023] The control device may comprise at least one fixed mirror {43i]i=i;Nmf with Nmf >1, the control mirrors {41k] being optically coupled to the fixed mirror(s) {43J, so that a control laser beam Fc is first reflected by a control mirror 41k in the direction of the fixed mirror 43i, which reflects it back towards the same control mirror 41k.

[0024] The fixed mirror(s) {43J may be fixed to an angular adjustment support, adapted to modify an angular inclination of the fixed mirror(s) {43J with respect to a displacement axis of the control mirror(s) {41k}, so that the control laser beam Fc is reflected by a control mirror 41k, when it occupies a threshold position Pmc>th>m, in phase advance with the reflection of the laser beam of power Fp by a return mirror 21j while it occupies a reference position P^efj.

[0025] The control mirrors {41k] may be distinct from the return mirrors {21j}, the control device comprising a single fixed mirror {43i} optically coupled to each of the control mirrors {41k}.

[0026] The control mirrors {41k] can be confused with the deflection mirrors {21j}, the deflection mirrors {21j} forming the control mirrors {41k}, the laser beam of power FL being incident on the deflection mirrors {21j} along a first optical axis (Ar), and the control laser beam Fc being incident on the control mirrors {41k] along a second optical axis distinct from the first optical axis. The control device can then comprise several fixed mirrors {43i}, optically coupled respectively to the control mirrors {41k}.

[0027] The laser beam of power FL may have a power at least equal to 10 GW and / or in which the laser rate fL is at least equal to 50 Hz. Brief description of the drawings

[0028] Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which:

[0029] [Fig.l] is a schematic and partial view of a laser system according to one embodiment, adapted to emit a pulsed laser beam of power and to distribute it in several successive return directions;

[0030] [Fig.2A] is a longitudinal, schematic and partial view of a return device and a control device of a laser system according to another embodiment, in which the return mirrors are fixed on a rotating cylindrical support, and in which the control mirrors are all coupled to the same fixed mirror;

[0031] [Fig.2B] is a longitudinal, schematic and partial view of a device for return and a control device of a laser system according to a variant of the embodiment of [Fig.2A], in which the return mirrors play the role of control mirrors, and are respectively coupled to fixed mirrors;

[0032] [Fig.3A] is a schematic and partial view, in cross-section, of a return device and a control device of a laser system according to another embodiment, in which the return mirrors are fixed on a rotating polygonal support, and in which the control mirrors are all coupled to the same fixed mirror;

[0033] [Fig.3B] is a schematic and partial view, in cross-section, of a return device and a control device of a laser system according to a variant of the embodiment of [Fig.3A], in which the return mirrors play the role of control mirrors, and are respectively coupled to fixed mirrors;

[0034] [Fig.4A] is a schematic and partial view, in top view, of a matrix photodetector, here a four-quadrant photodiode;

[0035] [Fig.4B] illustrates the movement of a laser spot of a control laser beam on the photodiode of [Fig.4A], and also illustrates an example of a detection signal s Pd(t) delivered by the photodiode;

[0036] [Fig.5A] illustrates an example of a multiple pulse inhibiting electronic circuit, adapted to deliver a trigger signal sd(t) when the detection signal spd(t) of the photodiode reaches a predefined threshold value;

[0037] [Fig.5B] is an example of a timing diagram illustrating the operation of the electronic inhibitor circuit of [Fig.5A];

[0038] [Fig.6] illustrates a laser system according to an embodiment where the return device comprises a rotating polygonal support;

[0039] [Fig.7A] and [Fig.7B] are schematic and partial views, in cross-section, of a deflection mirror and a control mirror fixed to a rotating cylindrical support, at different instants of rotation, in the case of [Fig.7A] where the fixed mirror does not have an angular adjustment for compensating for the time delay of electronic origin, and in the case of [Fig.7B] where the fixed mirror has such an angular adjustment;

[0040] [Fig.8] illustrates a laser system according to another embodiment, in which the control laser beam Fc is emitted by an auxiliary laser source separate from the injection laser source.

[0041] DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0042] In the figures and in the remainder of the description, the same references represent identical or similar elements. In addition, the different elements are not shown to scale so as to enhance the clarity of the figures. Furthermore, the different embodiments and variants are not mutually exclusive and can be combined with each other. Generally speaking, the terms "substantially", "approximately", "of the order of" mean to within 10%, and preferably to within 5%. On the other hand, concerning the angular and spatial precision of the return, these terms mean to within 50%. Furthermore, the terms "between ... and..." and equivalents mean that the limits are included, unless otherwise stated.

[0043] [Fig.l] is a schematic and partial view of a laser system 1 according to an embodiment, adapted to emit and distribute a pulsed laser beam of power Fl, along several successive return directions {D,}, ;NDr, with NDr>l, periodically at a scanning frequency fsq, with good performance in terms of angular return precision A0r and / or spatial return precision ADr, and for a high laser rate fL.

[0044] The laser beam of power FL is formed from a train of laser pulses having a high power PL (peak power), for example at least equal to 10 GW. The pulse duration may be low, for example of the order of a femtosecond or ten femtoseconds. The laser rate fL may be at least equal to 50 Hz (in the case for example of a local oscillator clocked at 10 MHz and for 30 return directions), and may be of the order of a few tens of hertz to a few kilohertz. Furthermore, the angular return precision A0r may be equal to approximately ±15 prad, and the spatial return precision ADr may be equal to approximately ±5 pm.

[0045] The distribution sequence, i.e. the scanning by the laser beam of power FL of all the NDr return directions {Dr>i}, is carried out at the scanning frequency fsq. ​​In addition, the reflection of the laser beam of power FL along each of the return directions Dri, one after the other, is carried out at the return frequency fr, which is therefore defined by the relation fr = NDrx fsq. ​​For example, for a number NDr of return directions {Dri} equal to 30, and a scanning frequency fsq of 5 Hz, the return frequency fr is equal to 150 Hz. Preferably, the laser rate fL is equal to the return frequency fr, so that, for each return direction Dri, a single laser pulse of the laser beam of power FL is reflected by a return mirror 21j.

[0046] The laser system 1 comprises at least: a power laser device 10 for emitting the laser beam of power FL at the laser rate fL; a return device 20 for distributing the laser beam of power FL along the successive return directions {Dri}; and a servo device 30 for servo-controlling the transmission of the laser beam of power FL to the return frequency fr defined by the return device 20.

[0047] As described in detail below, the servo-control device 30 is thus adapted to emit a trigger signal sd(t) allowing the selection of a laser pulse Fo of the local oscillation (injection laser source 11), which is called in English pulse picking, at the moment when the deflecting mirror is in the appropriate position to reflect the laser beam of FL power in the desired deflecting direction while respecting the conditions in terms of spatial and / or angular deflecting precision.

[0048] Furthermore, in particular when the local oscillator (injection laser source 11) operates in passive mode locking mode, therefore autonomously, the servo device 30 is advantageously adapted so that the laser pulse selection trigger signal sd(t) is synchronized (i.e. in phase) with the laser pulse of the local oscillator to be sampled, which makes it possible to sample it without altering it. It is then preferably centered on the laser pulse to be sampled, in particular when a single laser pulse is selected at the laser rate fL. The error rate (PER, for Pulse Error Rate in English) is then optimized, which is particularly important in particular in the context of tomographic reconstruction.

[0049] The power laser device 10 is adapted to emit the pulsed laser beam of power FL at the laser rate fL. It comprises an injection laser source 11 (called a local oscillator), a pulse selector 12, and preferably an optical amplifier 13.

[0050] The injection laser source 11 (local oscillator) is adapted to emit a pulsed injection laser beam Fo, which has an average power Po, which can be equal to approximately 500 mW for a peak power of approximately 400 kW, and a repetition frequency f0 (pulse repetition rate, in English). For example, the laser source 11 is a mode-locked local oscillator, for example a femtosecond laser. Mode locking can be carried out passively (PML for passive mode-locking, in English) by the use of semiconductor saturable absorber mirrors (SESAM for SEmiconductor Saturable Absorber Mirror, in English) or by Kerr effect lens (Kerr Lens Mode Locking, in English) at the focal point in the laser crystal. The laser source 11 may be a titanium-sapphire (Ti-Sa) crystal, neodymium or ytterbium doped crystal or glass laser source emitting at a frequency of a few tens to hundreds of MHz.As an example, this is a Ti-Sa laser emitting at a wavelength of 800 nm with a repetition frequency f0 of 80 MHz.

[0051] The pulse selector 12 (pulse picker, in English) is arranged downstream of the injection laser source 11. It is adapted to receive the injection laser beam Fo, and to select, in response to a trigger signal sd(t), at least one laser pulse, preferably a single pulse, at a frequency fL (laser rate) lower than f0. At the output, the injection laser beam Fo therefore has a frequency fL and a power Po. As described later with reference to [Fig. 6], the pulse selector 12 can be formed of a polarization modulator (called Pockels cell) 12.1 followed by a polarization separator 12.2 (for example a separator cube which will separate the two light contributions according to their vertical or horizontal polarization state). rizontal).

[0052] The optical amplifier 13 is adapted to receive the injection laser beam Fo transmitted by the pulse selector 12 and to amplify it so that it has the desired PL power. The optical amplification can be carried out by frequency drift (CPA for Chirped Pulse Amplification in English). Thus, the received laser pulse is temporally stretched, amplified, then temporally compressed. The optical amplifier 13 then comprises a pulse time stretcher 13.1 (stretcher in English), a regenerative amplifier 13.2 (and / or multi- or single-pass amplifiers), and a pulse time compressor 13.3. The laser pulse can then have a pulse width of the order of a few femtoseconds to a few tens of femtoseconds and a peak PL power of the order of 10 to 100 TW, or even more. The transmitted laser beam then forms the laser beam of power FL. It is transmitted towards the return device 20.

[0053] The return device 20 is adapted to reflect and distribute the laser beam of power FL along the successive return directions {Dr>i}. It comprises: at least one return mirror {21,(,-, ;Nmr with Nmr> 1; and a mechanical positioning device 22. It receives the laser beam of power along the same incident optical axis AL.

[0054] The deflection device 20 may comprise a single deflection mirror 21j (Nmr = 1) or several deflection mirrors {21j}, in which case the number Nmr of deflection mirrors is preferably equal to the number NDr of deflection directions {Dri}. The deflection mirror(s) {21j} are assembled to the mechanical positioning device 22, which ensures their movement to position them successively, at a deflection frequency fr (here equal to the laser rate fL), in reference positions {bi;NDr* The reference positions {Pmr,ref,i} are respectively associated with the deflection directions {D ri}. In other words, the reference position Pmr>ref,i is associated with the deflection direction Dr b the reference position Pmr>refj2 is associated with the deflection direction Dr2, and so on.By reference position P^efj associated with the return direction Dri, we mean that a return mirror 21j, when positioned in a reference position Pmr>refji, receives the laser beam of power FL and reflects it along the corresponding return direction Dri.

[0055] The mechanical positioning device 22 ensures the continuous movement of the deflection mirror(s) {21j}. By continuous movement, it is meant that the speed of movement of the deflection mirror(s) {21j} is non-zero. It can be constant or variable. It comprises at least one support on which the deflection mirror(s) {21j} are assembled, the support being set in motion by a motor (not shown). The movement of the deflection mirror(s) {21j} can be of the rotary or translational type.

[0056] Figures 2A and 2B are schematic and partial views, in longitudinal view, of a return device 20 as well as a control device 40 of the servo device 30, according to alternative embodiments. Here, the return device 20 comprises Nn„ return mirrors 21j with Nmr > 1, for example equal to 30, like the number NDr of return directions {Dri}. These [Fig.2A] and 2B are obviously schematic and only half the length of the cylinder is shown here. The number of mirrors 21j illustrated in the figures is different from 30 for the sake of clarity.

[0057] The mechanical positioning device 22 comprises a rotary support 23 on which all the deflecting mirrors {21j} are fixed. The rotary support 23 is a cylinder with a circular section whose axis of rotation is here, for illustration purposes, parallel to the incident optical axis AL of the power laser beam FL and the incident optical axis Ac of the control laser beam Fc. The cylindrical support 23 is rotating at an angular speed cosq = 2irxfsq. In this example where fsq = 5Hz, the angular speed cosq is equal to approximately 31 rad / s.

[0058] The reflecting mirrors {21j} are distributed at different longitudinal and angular positions along the longitudinal axis x of the cylindrical support 23. A position Pmrj(t) is thus defined for each reflecting mirror 21j. The position Pmrjj is defined here in the cylindrical frame (r,0,x), where the origin O of the longitudinal axis x is here located at one end of the cylinder. Generally speaking, a position of a reflecting mirror corresponds to the position of the geometric center of the reflecting mirror (of the reflecting surface) in the cylindrical frame (r,0,x), as well as to the angular orientation of the normal to the reflecting mirror in this same frame. Preferably, for the rotating cylindrical support 23 and the deflecting mirrors {21j} to form a rotating system with balanced moment of inertia, the number Nmr of deflecting mirrors {21j} is even, and the mirrors are distributed along the azimuthal axis 0 in a regular manner.Calibrated weights 24 are fixed to the cylindrical support 23 opposite each mirror 21j in order to balance the rotational moment of inertia and to guarantee rotation without imbalances, which improves the performance of the laser system 1 in terms of optical return.

[0059] Thus, each deflection mirror 21j has a position Pmrj(t) which varies over time periodically at a frequency fsq, which corresponds to the scanning frequency fsq of the laser system 1. At the deflection frequency fr (here equal to the laser rate fL), each deflection mirror 21j comes to occupy its reference position Pmr>refji. Also, it receives the laser beam of power FL (which is incident along the optical axis Ar) and reflects it along the associated deflection direction Dri.

[0060] In this example, the reference positions {Pmr,ref,j} are identical in terms of angular orientation, and differ from each other only in the longitudinal position. Also, the return directions {Dr>i} are contained in the plane (r,x) and are parallel to each other. Alternatively, the reference positions {Pmr>refj} may differ from each other, so that the return directions {Dri} may not be contained in the same return plane and / or may not be parallel to each other. Note also that, in this example, the return mirrors {21j} are positioned longitudinally in a regular manner, but non-regular positioning is of course possible.

[0061] The deflection mirrors {21j} have dimensions allowing the laser beam of power FL to be completely reflected. For example, this has a diameter of a few tens of millimeters, for example between 40 and 50 mm. The deflection mirrors {21j} may here have a diameter of 60 mm. The longitudinal period between two deflection mirrors {21j} may be 40 mm, and for 30 mirrors, the cylindrical support preferably has a length of approximately 1.2 m.

[0062] Figures 3A and 3B are schematic and partial cross-sectional views of a deflection device 20 and a control device 40 of a servo-control device according to alternative embodiments. Here, the deflection device 20 comprises Nm deflection mirrors 21j with Nmr > 1, for example equal to 10, as the number NDr of deflection directions {Dr>i}.

[0063] The mechanical positioning device 22 comprises a rotary support 23 (not illustrated in [Fig.3A]) on which all the deflecting mirrors {21j} are fixed. The rotary support 23 is a cylindrical structure with a polygonal section (hereinafter called a polygonal support) whose rotation axis is here, for illustration purposes, orthogonal to the incident optical axis AL of the power laser beam FL and the incident optical axis Ac of the control laser beam Fc.

[0064] In [Fig.3A], it is considered that all the mirrors {41k] of the control device 40 have the same angle between, on the one hand, the normal to the reflecting surface, and on the other hand, the axis passing through the center O of rotation and the geometric center of each mirror {41k]. The return mirrors {21j} are located on different faces of the polygonal support 23, and have different angular orientations from each other. In other words, each return mirror {21j} has a different angle between the normal to the reflecting surface and the axis passing through the center O of rotation and the geometric center of the return mirror. Thus, the mirror 211 has a normal to the plane of angular incidence of 0° relative to the axis passing through the center of rotation, then the mirror 212 has an angular incidence of 0.5°, the third mirror an angular incidence of 1°, and so on.

[0065] Preferably, so that the rotating polygonal support 23 and the deflecting mirrors {21j} form a rotating system with balanced moment of inertia, the number Nm of deflecting mirrors {2lj} is even, and the mirrors are distributed along the azimuthal axis 0 in a regular manner. In this case, all the beams are contained in the plane of the polygonal support 23 (i.e. reflected in a plane perpendicular to the axis of rotation of the polygon 23). It is also possible to proceed in the opposite manner, i.e. to assemble all the reflecting mirrors {21j} at the same angle and to modify the angular orientations of the mirrors {41k} according to the same progression (0.5°, 1°, etc.).

[0066] It is also possible to orient the normals to the plane of incidence of each mirror {21j} according to a combination of two different angles (a first angle in the plane contained in the polygon 23 and a second angle in a perpendicular plane). In this case, only the mirrors {21j} are oriented according to this combination and the mirrors {41k] are all oriented in a similar manner, in cooperation with the single fixed mirror 43 which has the function of returning the beam Fc in substantially the same direction as the incident direction.

[0067] [Fig.3B] describes another variant of a rotating polygonal support 23 which only comprises the deflection mirrors {21j}, also used as trigger mirrors {41k] for synchronization. There are then as many fixed mirrors {43J as there are deflection mirrors {21j}, these fixed mirrors being arranged on a spherical cradle 44 in cooperation with the deflection mirrors {43i} to deflect the control beam F c in substantially the same direction as the incident direction. According to this variant, the normals of the deflection mirrors {21j} are oriented in a plane perpendicular to the plane of the polygonal support 23 (for example, the first mirror has an incidence of 0°, the second mirror has an incidence of 0.5° and so on). The deflection mirrors {43i} are then placed along a circular generatrix on the spherical support 44.The center of the sphere is located at the intersection between the control beam Fc and the line formed by the intersection between the mid-plane of the polygon 23 and the surface of the mirror. The mirrors {43i} are distributed along the circular generator, at adjustable positions along the path of the reflected beam (see dotted arrow in [Fig.3B]). The position of each mirror {43i} along this generator also makes it possible to adjust the phase of the trigger signal sd(t) and thus the emission angle of the return direction Dri.

[0068] Here also, at the return frequency fr (here equal to the laser rate fL), each return mirror {21j} comes to occupy its corresponding reference position P^refj. The control beam Fc is then returned along the incident axis Ac when the mirror 41k is placed in correspondence with the fixed mirror 43b, that is to say when the beam reflected by the mirror 43i takes the same path in the opposite direction along the incident axis Ac. Also, it receives the laser beam of power FL and reflects it along the associated return direction Dri. The positions {Pmr,ref,j} are identical in terms of positions of the geometric center of each return mirror, but differ from each other by the angular orientation of the reflecting surface. Also, the return directions {Dri] have the same origin but are not parallel to each other.

[0069] The servo device 30 is adapted to provide a trigger signal sd (t) to the pulse selector 12 thus authorizing the transmission of the laser beam of FL power. It comprises: a control laser source 31; a control device 40 (comprising at least one control mirror {41k} and a mechanical positioning device 42), a matrix photodetector 33, and an electronic triggering device 50.

[0070] The control laser source 31 is adapted to emit a control laser beam Fc. The beam Fc can come from the injection laser source 11 (local oscillator) or from an auxiliary laser source 31.

[0071] In the case where the control laser beam Fc comes from the injection laser source 11, it is transmitted towards the control device 40. This same laser beam Fc can be used to resynchronize the trigger signal sd(t). It has the same repetition frequency as the repetition frequency f0 of the injection laser beam Fo> and the phase of the trigger signal sd(t), generated by the electronic inhibitor circuit 51 of the electronic trigger device 50, is then adjustable by a time delay generator 55. Note that the injection laser source 11 therefore provides, depending on the trigger signal sd(t), the injection laser beam Fo or the control laser beam Fc.

[0072] According to a variant (see [Fig. 8]), the control laser beam Fc comes from an auxiliary laser source 31 which preferably operates in continuous mode. As described in detail below, the control laser beam Fc is transmitted towards a rotary encoder 46 secured to the rotary support 23 to provide a trigger signal sd(t) when the rotation angle of the rotary encoder 46 crosses one or more angular thresholds defined by construction. Such an encoder can be optical, as illustrated in [Fig. 8], or magnetic.

[0073] The control device 40 is adapted to reflect a control laser beam Fc along the same predefined return optical direction Drth, by at least one control mirror {41k] when the latter or these are positioned successively in one or more threshold positions {Pmc>th,k}- H comprises: at least one control mirror {41k] k=i;Nmc with Nmc>l; and a mechanical positioning device 42. It receives the control laser beam Fc which is incident along the optical axis Ac.

[0074] The control device 40 may comprise a single control mirror (Nmc = 1) or several control mirrors {41k], in which case the number Nmc is preferably equal to the number NDr of return directions. The control mirror(s) {41k] are assembled to the mechanical positioning device 42, which ensures their continuous rotary movement to position them successively, at the return frequency fr (here equal to the laser rate fL), in at least one threshold position {PmCjth,m}m=i ;NPth, with Npth>L The threshold positions {Pn^refj} are respectively associated with the same optical return direction Dr>th of the control laser beam Fc, which is then transmitted to the matrix photodetector.

[0075] In other words, the threshold position Pmc>th,i is associated with the return threshold direction Dcth, the threshold position Pmc>th>2 also, and so on. By threshold position Pmc>th>m associated with the same return threshold direction Dcth, it is meant that a control mirror 41k, when positioned in a threshold position Pmc>th>m, receives the control laser beam Fc and reflects it along the same return threshold direction Dc th, which is preferably coaxial with the incident optical axis Ac of the control laser beam Fc (optical alignment).

[0076] The mechanical positioning device 42 ensures the continuous rotary movement of the control mirror(s) {41k]. It comprises at least one support on which the deflecting mirror(s) {21j} are assembled, the support being set in rotary motion by a motor capable of rotating at a constant or slowly variable angular rotation speed. De facto, the motor can, optionally, be controlled in rotation speed (in fact, the electronic triggering device 50 (and more precisely the electronic inhibitor circuit 51) adapts to the actual rotation speed of the rotary support 23). Preferably, in the case where the movement of the deflecting mirrors {21j} is rotary, like that of the control mirrors {41k], the latter are fixed to the same support as that of the deflecting mirrors {21j}.

[0077] In the embodiment of FIGS. 2A and 2B, and in that of [Fig. 3A] and 3B, the control device comprises Nmc control mirrors 41k with Nmr > 1, for example equal to the number Nmc of return mirrors {21j}.

[0078] In the case of [Fig.2A] and 2B, the mechanical positioning device 42 comprises a conical support fixed to one end of the cylindrical support 23, rotatable along the axis of rotation of the cylindrical support. All the control mirrors {41k] are fixed to the conical support of the device 42, and are distributed along the azimuthal axis 0, preferably in a regular manner. Here also, the position Pmc>k(t) corresponds to the position of the geometric center of the control mirror (of its reflecting surface) in the cylindrical frame (r,0,x), as well as to the angular orientation of the control mirror {4lk} in this same frame. Preferably, so that the moment of inertia is balanced, as for the deflecting mirrors {21j}, the number Nmc of control mirrors {4lk} is even and they are distributed along the azimuthal axis 0 in a regular manner.

[0079] Thus, each control mirror 41k has a position Pmc>k(t) which varies periodically over time at a frequency fsq. ​​At the return frequency fr (here equal to the laser rate fL), each control mirror 41k occupies a threshold position Pmc>th>m. Also, it receives the control laser beam Fc, and reflects it along the same return threshold direction DCjth, which is preferably coaxial with the incident optical axis Ac.

[0080] The control mirrors {41k] have dimensions allowing the control laser beam Fc to be fully reflected. For example, this has a diameter of a few millimeters, for example equal to 2mm. The control mirrors {41k} can here have a diameter of 5mm to 10mm (fixed by a mount, for example, 12.7mm in diameter, or inch).

[0081] The control device 40 preferably comprises one or more fixed mirrors {43i}i=i ;Nmf, optically coupled, respectively, to the control mirror(s) {41k}. Also, when the control laser beam Fc is received by a control mirror 41k along the incident optical axis Ac, the latter reflects it in the direction of the fixed mirror 43i, which reflects the beam in the direction of the same control mirror 41k which returns it in a direction which is coaxial, or not, with the incident optical axis Ac depending on the position of the control mirror 41k during rotation. The fixed mirror 43i coupled to the control mirror 41k makes it possible to ensure the angular selectivity of the triggering.

[0082] The fixed mirror(s) {43i} are preferably assembled to a goniometric support 44, or equivalent (Gimbal mount, for example), the angular position of which can be modified relative to an axis of rotation of the goniometer passing through the point of contact of the control beam Fc with the control mirror 41k. As described in detail below, this makes it possible to generate a trigger signal sd(t) in phase advance of the movement of the deflection mirror 21j, thus making it possible to compensate for any time delays in triggering of electronic and optical origins (propagation of the laser beams). This phase advance of mechanical origin makes it possible to obtain optimal operation of the control at the highest laser rates (kHz), i.e. a selection of a laser pulse (preferably single) from the injection laser source 11 (local oscillator) satisfying the desired conditions in terms of spatial and / or angular precision of deflection.

[0083] With reference to [Fig.2A], the control device 40 here comprises a single fixed mirror 43 optically coupled to the control mirrors {41k]. In this example, there is only one threshold position Pmc>th for all the control mirrors {41k]. Also, the control laser beam Fc is reflected a first time in the direction of the fixed mirror 43 by the control mirror 41k during rotation, which reflects it in the direction of the same control mirror 41k, which reflects it a second time. The fixed mirror 43i is fixed to a goniometric support 44, the axis of rotation of which is coaxial with the incident optical axis Ac.

[0084] During the rotary movement of the control mirror 41k, the control laser beam Fc is returned, after the 3 reflection steps, in a direction (here not coaxial with the incident optical axis Ac) different from the return threshold direction Dr>th, except when it occupies the threshold position Pmc>th. Indeed, when this is the case, the control laser beam Fc is returned in the return threshold direction DCjth, which is here coaxial with the incident optical axis Ac. The control laser beam Fc then has an optical alignment of its return optical path (in the direction Dr>th) with its forward optical path (along the incident optical axis Ar).

[0085] With reference to [Fig.2B], the control device 40 comprises several fixed mirrors {43J optically coupled respectively to the control mirrors {41k}, with N mf = Nmc. There are therefore here as many fixed mirrors {43J as there are control mirrors {41k}, and there are then as many threshold positions {Pmc>th,m} as there are control mirrors {41k}: Npth = Nmc. The fixed mirrors 43i are all fixed to the same goniometric support 44, the axis of rotation of which is coaxial with the incident optical axis Ac.

[0086] In this example, the deflection mirrors {21j} are also the control mirrors {41k]. Also, while a deflection mirror ensures the reflection of the laser beam of power FL in a deflection direction Dri, another deflection mirror ensures the function of control mirror, and reflects the control laser beam Fc in the direction of the corresponding fixed mirror 43!.

[0087] With reference to [Fig.3A], the mechanical positioning device 42 comprises a polygonal support fixed to the polygonal support 22 of the deflecting mirrors, and having an axis of rotation coaxial with that of the polygonal support 22. The control mirrors {41k] are arranged on the facets of the polygonal support 42. The control device 40 comprises a single fixed mirror {43i} optically coupled to the control mirrors {41k]. Also, the control mirrors {41k] have a single threshold position Pmc>th. This configuration is similar to that of [Fig.2A]. The fixed mirror 43i is fixed on a goniometric support 44, the axis of rotation of which is here orthogonal to the incident optical axis Ac and passes through the threshold position Pmc>th. The center of rotation of the goniometric support is located at the intersection between the control beam Fc and the surface of the mirror 41k when the latter occupies the threshold position.

[0088] With reference to [Fig.3B], the control device 40 comprises several fixed mirrors {43J optically coupled respectively to the control mirrors {41k}, with N mf = Nmc. Here, the deflecting mirrors {21j} act as control mirrors {41k}, so that the mechanical positioning device 22 is also that of the control device. There are therefore as many threshold positions {Pmc>th,m} as there are control mirrors {41k}. This configuration is similar to that of [Fig.2B]. The fixed mirrors 43i are all fixed to the same goniometric support 44.

[0089] Let us note here that the configurations of [Fig.2B] and [Fig.3B], where the return mirrors {21j} also perform the function of control mirrors {41k}, are advantageous, insofar as the adjustment constraints are transferred to the fixed mirrors {43i}, the mount of which is not subject to vibrations and accelerations, which makes it possible to improve the reliability of the laser system 1 as well as its performance in terms of spatial and / or angular return precision.

[0090] The servo device 30 comprises a matrix photodetector 33. It is adapted to receive the control laser beam Fc reflected by a control mirror 41k during rotation, and to emit in response a detection signal spd(t) representative of the displacement of the control mirror 41k. The detection signal spd(t) passes through a threshold value spd>th associated with the threshold position(s) {Pmc>th,m}- More precisely, the detection signal spd(t) depends on the position of the laser spot of the control laser beam Fc on the sensitive surface of the matrix photodetector 33, this position being a function of the position Pmc>k(t) of the control mirror 41k, during rotation, while it reflects the control laser beam Fc.

[0091] Figures 4A and 4B schematically and partially illustrate a matrix photodetector 33 according to one embodiment. Here, it is a four-quadrant photodiode (called photodiode 4Q in the remainder of the document), comprising 4 identical sensitive zones, placed symmetrically two by two. The detection zones are denoted Zmn, with m and n ranging from 1 to 2, where m is the index of the rows and n that of the columns, and each provide an electrical detection signal denoted Vmn. Of course, this choice is in no way limiting and other types of matrix detectors can be used.

[0092] With reference to [Fig.4A], in the case of a laser spot of the control laser beam F c with a diameter of the order of 5 mm (after propagation in free space over a distance of several meters), the quadrants may have a quadri-circular or square shape with a width of the order of 2.5 to 5 mm. They may be electrically separated from an intermediate non-photosensitive zone of a few tens of microns in width. It should be remembered that the diameter of a laser spot can be defined from the contour where the intensity is equal to 1 / e2 of its maximum value.

[0093] Here, the laser spot moves on the active surface of the 4Q photodiode 33 along a horizontal axis. Also, the detection signal spd(t) can then be equal to (Vi2(t)+V22(t) - (Vn(t)+V2i(t)). For this, the photodiode 33 comprises a 4-way electronic adder-subtractor circuit, receiving as inputs the values ​​Vu, Vi2, V2i and V22 and providing as output spd = (Vi2+V22) - (Vn+V2i).

[0094] Similarly, the laser spot can move on the active surface of the photodiode 4Q 33 along a vertical axis at a much lower speed, due to the possible mechanical evolution of the reflection surfaces and the possible fluctuation of angular pointing of the control laser beam Fc, both linked to thermomechanical fluctuations. A second signal equal to [Vn(t)+Vi2(t)] - [V2i(t)+V22(t)] can then be used to control the verticality of the laser spot. This signal is provided by a second adder-subtractor circuit receiving as inputs the values ​​Vu, V[2, V2i and V22 and providing as output the signal [Vn(t)+Vi2(t)] - [(V2i(t)+V22(t)]. This low-pass filtered signal is then communicated to the input of a positioning plate which can be actuated by a piezoelectric ceramic, for example, allowing automatic centered adjustment of the laser spot.

[0095] With reference to [Fig.4B], the optical assembly of the servo device 30 has here been previously adjusted so that the laser spot is positioned at the center of the sensitive surface of the photodiode 4Q 33. Thus, when it is reflected by a control mirror 41k during rotation, the laser spot travels along the sensitive surface of the photodiode 33 along a given axis, here the horizontal axis.

[0096] Initially, the position Pmc>k(t) of the control mirror 41k is upstream of the corresponding threshold position Pmc>th>m. The control laser beam Fc is therefore reflected by the control device 40 and is returned in a direction different from the return threshold direction Dcth. The laser spot moves over the sensitive zones Zn and Z2B so that the detection signal spd(t) has a constant value, here negative (below the detection threshold).

[0097] Then, the position Pmc>k(t) approaches the threshold position Pmc>th>m. The laser spot begins to illuminate the sensitive zones Zn and Z22, so that the detection signal s pd(t) increases progressively. It increases linearly when the control laser beam Fc emits continuously, or in steps when the laser emits in a pulsed manner (i.e. local oscillator). When the control mirror 41k occupies the threshold position Pmc>th,m, the control laser beam Fc is reflected along the return threshold direction DCjth, so that the laser spot is located in a position such that the value of the detection signal spd(t) is equal to the threshold value spdjth. Note here that the threshold value s pdjth is non-zero (addition of a constant), so as to avoid untimely triggering of the pulse selector by the noise of the electronic components when the laser spot does not illuminate the photodiode 4Q 33.

[0098] Finally, the position Pmc>k(t) of the control mirror 41k moves away from the corresponding threshold position P mc,th,m. The laser spot leaves the sensitive zones Zn and Z2b and only illuminates the sensitive zones Zn and Z22. The detection signal spd(t) then has a constant value, here positive (above the detection threshold).

[0099] The servo-control device 30 comprises an electronic triggering device 50. This electronic device 50 comprises a multiple pulse inhibiting electronic circuit 51 and preferably a delay generator 55. The electronic inhibiting circuit 51 is adapted to emit the triggering signal sd(t) when the detection signal spd(t) reaches said threshold value spd>th, that is to say when a control mirror 41k occupies the corresponding threshold position Pmc>th>m, and therefore reflects the control laser beam in the return threshold direction Dc th. Also, the pulse selector 12, in response to the triggering signal sd(t), transmits the laser beam of power FL which is then received by a deflecting mirror 21j in a reference position Pmrjef.i, and is therefore reflected in the corresponding deflecting direction Dri.

[0100] [Fig.5A] illustrates a diagram of an electronic inhibitor circuit 51 according to one embodiment. It is given here for illustrative purposes, it being understood that other confi gurations are possible. [Fig.5B] is a timing diagram illustrating the operation of the electronic inhibitor circuit 51 of [Fig.5A]. Note here that the electronic components of the electronic circuit 51 have unaffected response times. These response times are illustrated in [Fig.5B] purely for illustrative purposes: each response time can be more or less significant and include a certain tolerance (notion of temporal jitter).

[0101] With reference to [Fig.5A], the electronic inhibitor circuit 51 makes it possible to inhibit successive laser pulses emitted by the injection laser source 11 (local oscillator) in order to select, at the laser rate fL, preferably only one. It firstly comprises a comparator 52 connected to the photodiode 4Q 33. It therefore receives the detection signal spd(t), compares the received value with the threshold value spd>th, and provides in return a logic signal sc(t), for example 0 when spd(t) < spd>th and 1 when spd(t) > s sp>th. The comparator 52 is chosen to have a very short response time, for example of the order of 2ns.

[0102] The output of comparator 52 is connected to the input IET>a of an AND logic gate. The other input IETjb of the AND logic gate is connected to the output of an XOR (exclusive OR) logic gate. This XOR logic gate has an input IxoR,a connected to a monostable circuit 53 (also called a monostable flip-flop) and another input connected to the output of the AND logic gate via a delay line 54. Furthermore, the monostable circuit 53 can be triggered on a rising edge coming from the output of the AND logic gate (also via the delay line 54) and is armed to logic state 1 by default, awaiting the trigger signal sd(t). The output of the AND logic gate thus forms the trigger signal sd(t), which is transmitted to the pulse selector 12 (here to the modulator 12.1 via an amplifier 12.3).

[0103] The electronic triggering device 50 advantageously comprises a programmable delay generator 55, connected between the output of the AND logic gate and the pulse selector 12. The phase of the trigger signal sd(t) can thus be adjusted by the delay generator 55, so that the trigger signal sd(t) is received by the pulse selector 12 in phase with the laser pulse (to be sampled) from the injection laser source 11. Also, the signal sd(t) is resynchronized with the laser pulses generated by the injection laser source 11. This can be translated by the fact that the laser pulse to be sampled is centered on the signal sd(t). Thus, the laser pulse can be selected without alteration and transmitted to the optical amplifier 13. The applied delay can be predefined, or even be set in real time to be a function of the rotation speed of the rotating support 23 (and therefore of the laser rate fL).

[0104] With reference to [Fig.5B], for t < ti>0, the detection signal spd(t) has a value lower than the threshold value spdjth. The comparator 52 then provides a signal sc(t) whose value is 0, which is delivered to the input IET a of the AND gate. Furthermore, the Monostable circuit 53 provides a signal sm(t) whose value is 1. This value is delivered to the input IXOR>a of the XOR gate. However, whatever the value delivered by the XOR gate, the AND gate delivers the value 0, since the input IETa is at 0. The output at 0 of the AND gate is looped back to the input IXORjb of the XOR gate. The inputs being at 1 and 0, the XOR gate delivers the value 1. Also, as soon as the comparator 52 provides the value 1, the AND gate can switch to the value 1, thus forming the trigger signal sd(t).

[0105] At time t = ti>0, the detection signal spd(t) reaches the threshold value spd>th. Also, the comparator 52 switches to logic state 1 at time t^i, which causes the AND gate to switch to logic state 1. The rising edge of the trigger signal sd(t) is therefore generated.

[0106] With a certain delay Air predefined by the delay line 54, the logic value 1 from the AND gate is delivered to the input IXOR,b of the XOR gate. The latter then finds itself with its two inputs in logic state 1, which causes it to change to logic state 0 from the instant ti,3. Also, from the instant ti.4, the AND gate changes to logic state 0, which corresponds to the falling edge of the trigger signal sd(t). Also, the time width of the trigger signal sd(t) transmitted to the pulse selector 12 is therefore essentially defined by the delay A^ of the delay line 54 (and by the response times of the electronic components).

[0107] In parallel, the rising edge delivered by the AND gate from time ti,2 is also delivered with the delay Ak to the monostable circuit 53, which also goes to logic state 0 from time ti 3. Also, the two inputs of the XOR gate are at logic state 0, so that the output of the XOR gate goes to state 0.

[0108] Also, the IETb input of the AND gate is forced to state 0, which makes it possible to block all changes of logic state which would occur at the IET>a input, due for example to voltage oscillations at the output of the photodiode 33. This reduces the risks of untimely triggering which would not be associated with the threshold position Pmc>th>m of the control mirror 41k. The untimely triggering of multiple pulses can be due to power fluctuations (pulse-to-pulse repeatability) of the local oscillator (cf. voltage oscillations at the output of photodiode 33), in particular at low laser rate.

[0109] The monostable circuit 53 remains in state 0 for a predefined duration Am, until the instant ti>6, the time for the laser spot of the control laser beam Fc to leave the sensitive surface of the photodiode 33, which is the case here at the instant ti>5. Then, the monostable circuit 53 returns to the logic state 1, which also resets the XOR gate. We are then in the initial configuration. The electronic inhibitor circuit 51 is again able to detect the passage of the threshold value in the detection signal spd(t) and to emit in response the trigger signal sd(t).

[0110] Thus, the laser system 1 is effectively adapted to distribute the laser beam im power pulse FL following several successive return directions {Dr>i}, with the expected spatial return precision ADr and / or angular return precision A0r, with a laser rate fL which can be high. The laser rate fL is controlled by the return frequency fr (here equal to the laser rate fL) of the return device 20. In fact, the power laser device 10, and more precisely the pulse selector 12, is controlled by the return device 20 by means of the control device 30, and more precisely by means of the control device 40 which operates at the same return frequency fr (preferably equal to the laser rate fL) as the return device 20.Also, the return device 20 has a 'master' character while the power laser device 10 has a 'slave' character; it is the return device 20 which conditions the return frequency fr, to which the power laser device 10 is set by means of the servo device 30. Also, the trigger signal sd(t) is generated so that the power laser pulse is selected and transmitted at the moment when the return mirror 21j is in the appropriate position to reflect it in the corresponding return direction. In addition, by the advantageous presence of the delay generator 55, the servo device 30 makes it possible to ensure that the trigger signal sd(t) is in phase with the laser pulse generated by the injection laser source 11, so as to collect it without alteration and then communicate it to the optical amplifier 13 at the desired moment.Furthermore, it should be noted that the laser system 1 is particularly simple to produce and compact. It consumes little energy, which makes it compatible with the requirements of industrial operation.

[0111] [Fig.6] is a schematic and partial view of a laser system 1 according to an embodiment where the return device 20 and the control device 40 are similar to those of [Fig.3A], and where the servo device 30 comprises an optoelectronic gain servo circuit 60.

[0112] The regenerative amplifier 13.2 is here optically pumped in a pulsed manner, so as to optimize thermal resistance and electrical consumption. A gain trigger signal (hereinafter called: pumping signal sp(t)) can then be supplied to the regenerative amplifier 13.2 by the optoelectronic gain control circuit 60. This comprises a 4Q photodiode 61 (here a 4Q photodiode), a comparator 62, a monostable circuit 63, and an amplifier 64. Other electrical diagrams are obviously possible.

[0113] The transmission of the pump signal sp(t) occurs in phase advance to establish the gain of the amplifier before emission of the laser pulse by the injection laser source 11 (local oscillator). For a titanium-sapphire crystal laser, the pump signal sp(t) is emitted in advance by a duration of the order of approximately 3 ps. Note that since most Ytterbium lasers are pumped by a continuously emitting laser, this circuit Optoelectronic gain control 60 may not be used with this type of laser.

[0114] The photodiode 4Q 61 detects the control laser beam Fc once reflected by the control mirror 41k, and provides a detection signal spd2(t) which is representative of the position of the control mirror 41k during rotation.

[0115] Then, a comparator 62 detects the instant when the detection signal spd2(t) reaches the threshold value spd>th corresponding to the optical alignment of the control laser beam Fc between the forward and return paths. It then supplies the pumping signal sp(t) to a monostable circuit 63 then to a control amplifier 64. The rising edge of the pumping signal sp(t) causes the monostable circuit 63 to switch to a logic state which prevents the transmission to the amplifier 64 of new unwanted trigger signals. This ensures that only one pumping command is communicated during a laser period.

[0116] The pump signal sp(t) is received by the regenerative amplifier 13.2 in phase advance, so that the gain is maximum when the laser pulse is received. For this, the position of the photodiode 4Q 61 on the optical path of the control laser beam Fc is predefined to receive the laser spot of the control beam Fc before the photodiode 4Q 33 (here about 3 ps earlier).

[0117] The operation of the laser system 1 is now described with reference to [Fig. 6].

[0118] It is considered here that the laser system 1 successively distributes the laser beam of power FL along 30 return directions {Dr>i}, by means of 30 return mirrors {21j}, at the scanning frequency fsq of 5 Hz, with a return frequency fr of 150 Hz. The servo device also comprises 30 control mirrors {41 k], which are all coupled to the same fixed mirror 43b. It is recalled that it is considered here that the laser rate fL is equal to the return frequency fr, so that a single power laser pulse is reflected for each return direction Dri.

[0119] The injection laser source 11 emits the injection laser beam Fo, with a repetition frequency f0, for example 80 MHz, and a power Po. The injection laser beam Fo is received by the pulse selector 12.1, 12.2. This is formed by an electro-optical polarization modulator 12.1 (Pockels cell) followed by a polarization splitter prism 12.2. The polarization modulator 12.1 operates as a voltage-controllable half-wave plate: in response to the trigger signal sd(t), it switches the polarization of the light (which remains rectilinear) by 90°.

[0120] Note here that, due to the high power of the laser beam FL, part of the propagation of the laser beams takes place in an enclosure placed under secondary vacuum. The vacuum enclosure is not shown in the figure, for the sake of clarity. It is delimited by the portholes 3. It includes the pulse time compressor 13.3, the return device 20 and the control device 40.

[0121] In the absence of the trigger signal sd(t) at the input of the polarization modulator 12.1, the polarization of the laser beam Fo remains unchanged. After transmission by the polarization modulator 12.1, the laser beam Fo is reflected by the polarization splitter 12.2. The transmitted laser beam therefore forms the control laser beam Fc. It propagates in the direction of the control splitter plate 46, passes through the window 3 to enter the vacuum chamber. The window 3 can be treated with an antireflection coating at the wavelength of the laser or oriented at the so-called Brewster incidence (55.5° for a silica window at the wavelength of 800 nm (Ti-Sa)) for the polarization of the laser, as shown in [Fig.6]. Then, it is reflected by the reflector 5 which transmits it along the optical axis Ac onto the control mirror 41k.

[0122] The control laser beam Fc is reflected by the control mirror 41k during rotation, the fixed mirror 43b and then again by the control mirror 41k during rotation, and is finally detected by the photodiode 4Q 33 as well as by another matrix photodetector 61, here also a photodiode 4Q. The photodiode 4Q 33 delivers a detection signal spd(t) whose value is a function of the position of the control mirror 41k during rotation.

[0123] When the position of the control mirror 41k reaches the threshold position, there is an optical alignment between the incident optical axis Ac and the return threshold direction Dr>th. The detection signal spd(t) then reaches the threshold value spdjth, which is detected by the electronic inhibitor circuit 50 which delivers the trigger signal sd(t).

[0124] In this example, a programmable delay generator 55 is used to adjust the delay between the electrical signals and resynchronize the trigger signal sd(t) to the laser pulses generated by the injection laser source 11 (local oscillator), before communicating it to the amplifier 12.3 whose output is connected to the modulator 12.1. Such a delay generator 55 can be a commercial element (e.g. the DG535 generator from Stanford Research Systems, Inc.) or a programmed component of the CPLD (Complex Programmable Logic Devices) or FPGA (Field Programmable Gate Array) type. The delay generator 55 receives the trigger signal sd(t) at the input To, then delivers it to the output A with a predefined delay with respect to the input To. This time delay can be adjusted during the calibration of the optoelectronic chain (for example using an oscilloscope).An example of the pre-calibration phase is detailed later.

[0125] Furthermore, output B of delay generator 55 delivers another electrical pulse used to activate the logic gate located in regenerative amplifier 13.2 to input the laser pulse transmitted by pulse selector 12.1, 12.2. The time delay A - B can be of the order of a few nanoseconds. Finally, output C delivers another electrical pulse used to activate again the logic gate located in regenerative amplifier 13.2 to output the laser pulse once amplified. The time delay B - C can be of the order of 50-100 ns.

[0126] When the polarization modulator 12.1 receives the trigger signal sd(t), it switches the rectilinear polarization of the incident light by 90°, so that it is then transmitted (and not reflected) by the polarization separator 12.2. It is then transmitted towards the optical amplifier here of the CPA type formed by the elements 13.1, 13.2, 13.3. It is thus stretched in the time domain by the pulse time stretcher 13.1, amplified in the regenerative amplifier 13.2, and finally compressed in the time domain by the pulse time compressor 13.3. Note that the pulse time compressor 13.3 is placed in the vacuum enclosure. The laser beam FL thus amplified is then directed along the optical axis Ar onto the deflecting mirror 2lj. This then occupies the reference position Pmr>refji so that it is reflected along the associated return direction Dri.The angular pointing can then be precise (of the order of a few tens of microradians). In addition, the error rate in the transmission of the pulses can be very low (103 or less) taking into account the adjustment of the temporal phase by the delay generator 55. The guarantee of a low error rate is an important performance criterion of the laser system 1 with switching of the power laser beam for the purpose of image reconstruction by 3D tomography.

[0127] Let us note here that if the injection laser source 11 and the regenerative amplifier 13.2 are based on Ti-Sa technology, which is characterized by high pulse energies (of the order of the Joule), the laser rate fL and the return frequency fr are preferably of the order of 100 to 150 Hz, and the return mirrors {21j} are preferably rotating at an angular speed of between 3.3 and 5 revolutions / s for 30 return mirrors. On the other hand, if Yb technology is used, which is characterized by lower pulse energies (rnJ) but a higher repetition rate (> kHz), the laser rate fL and the return frequency fr can reach a laser rate fL of the order of a few kilohertz, and the 30 return mirrors can be rotated at a rotation speed reaching a few tens of revolutions / s to approximately 200 revolutions / s (12,000 rpm).For comparison, turbo-molecular pumps rotate at rotation speeds between 30,000 and 70,000 rpm. This is possible with Yb laser technology because the mirrors are then smaller (and therefore have lower inertia) than for beams from Ti-Sa lasers.

[0128] The adjustments are made in the atmosphere via access points made at at least two places in the enclosure, on the one hand at ground level and on the other hand, in the upper part of the gantry. The adjustment procedure is carried out statically (engine stopped) but the laser still operates thanks to the electronic inhibitor circuit 51, which allows the adjustments to be made in optimal laser operation.

[0129] The rotating mechanical device 22 is then disengaged from the motor control and then locked in position for each of the angular orientations corresponding to each of the target sights (directions of return). The injection laser source 11 (local oscillator) is then put into operation, as is the optical amplifier 13, at reduced power.

[0130] For the current angular position, the collection of the laser beam of power FL by the targets is then ensured by adjusting the normal of the mirrors {21j} using the 3-point mounts and by conventional positioning elements (micrometric screw displacement stage). Then, the same procedure is carried out with the trigger mirrors {41k] and the fixed mirror(s) {43i}. This operation is repeated for all the targets (return directions). Then, once the enclosure is closed and the pumping is carried out, the mechanical device 22 is set in motion by the motor at the rate prescribed for optimal operation of the laser system 1.

[0131] The outputs of the different signals are then observed using a fast oscilloscope (High Definition Oscilloscope - HDO) and the time delay durations are adjusted on the delay generator 55 in order, on the one hand, to symmetrize the trigger signal sd(t) of the laser pulse on the laser signal observed at the output of the CPA chain 13 (with the aim of minimizing the error rate in the generation of pulses) and optimizing the amplification in the regenerative amplifier 13.2.

[0132] Figures 7A and 7B are schematic and partial views of a deflection mirror 21j and a control mirror 41k rotating around the same axis of rotation, according to an embodiment similar to that of [Fig.2B]. The mirrors are rotating clockwise around the same axis of rotation. These figures illustrate several successive instants of rotation. In the case of [Fig.7A], the goniometer (not shown) on which the fixed mirror 43i is fixed is not adjusted so as to compensate for the electronic time delay. On the other hand, this is the case in [Fig.7B].

[0133] Indeed, a time delay Ah exists between the instant t0 when the control laser beam Fc is received by the control mirror 41k (while the latter occupies the threshold position Pmc>th ) then by the matrix photodetector 33, and the instant ti of the transmission of a laser pulse by the pulse selector and its reflection by the return mirror 21j (while it occupies the reference position P^ef) - This delay Ah comes essentially from the response times of the electronic circuit of the matrix photodetector 33, of the electronic trigger circuit 50, and of the electronic control circuit of the pulse selector 12.

[0134] Thus, during this time delay Atr, the position of the rotating deflection mirror 21j passes from the position Pmrj(t0) to the position Pmrj(ti=t0+Ah). In the case of a rotary displacement at the angular speed cosq of 2irxfsq with fsq = 5 Hz, this is equal to approximately 31 rad / s. The angular difference between the positions Pmrj(t0) and Pmrj(ti), for a time delay Ah of 30 ns for example, is of the order of 0.9 prad, which can still respect the angular precision condition of return A0r, for example here from one to a few tens of microradians. In other words, the positions Pmrj(t0) and Pmrj(ti) still belong to the reference position Pmr>ref and guarantee that the laser pulse is reflected by the return mirror 21j along the return direction Dri with the desired precision. Another way of presenting the situation is to calculate that the angular offset of the return mirror 21j, which is rotating at 31 rad / s, between two successive laser pulses at the repetition frequency f0 of 80 MHz, is equal to 0.39 prad (31rad / s / 80MHz). For a condition of angular precision of return A0r at 10 prad, it appears that 25 successive laser pulses (10 / 0.39) are compatible with this condition.At such a rate, it is therefore not necessary to impose a mechanical phase advance as described previously (on page 13).

[0135] Thus, with reference to [Fig.7A], the fixed mirror 43; is orthogonal to the vertical axis. Before the instant to, the control laser beam Fc is received by the control mirror 41k (dotted line) in the direction of the fixed mirror 43i in a first direction, then is reflected by the latter (solid line) in the direction of the mirror 41k in another direction. The optical alignment of the control laser beam Fc in the threshold position P mc>th is obtained at the instant to. The return mirror 21j is located vertically to the control mirror 41k and the fixed mirror 43p. However, it is at the instant ti that it receives the laser pulse to reflect it in a return direction. The reference position Pmr>ref is then passed. The condition on the spatial precision of return can however still be verified, depending on the return frequency fr and therefore the laser rate fL.

[0136] In certain cases, this time delay Ah may lead to no longer respecting the desired condition on the spatial return precision ADr, (or the angular return precision A0r) in particular when the laser rate fL is higher, as is the case for example for fL equal to 6kHz. The angular speed cosq is 2irx200 Hz, or equal to approximately 1250 rad / s. The angular difference between the positions Pmrj(t0) and Pmr>j(ti), for a time delay Ah of 30 ns for example, is then of the order of 37 prad, which is here greater than the desired angular return precision A0r of 10 prad.

[0137] In this case, the threshold position Pmc>th of the control mirror 41k is adapted by adjusting the goniometer (or a Gimbal mount) to compensate for this time delay. More precisely, the threshold position Pmc>th of the control mirror 41k is configured to be reached by the control laser beam at time t0: Pmc>k(t0) = Pmc.*, so that the position Pmrj(ti) of the deflection mirror 21j corresponds to the reference position Pmr>ref. In other words, the control mirror 41k reaches its threshold position Pmc>th in phase advance, before the deflection mirror 21j reaches its reference position Pmr>ref. This phase advance of the control mirror 41k with respect to the deflection mirror 21j makes it possible to compensate for the time delay Atr, so that the laser beam of power FL is reflected by the deflection mirror 21j (which actually occupies the reference position P mrjef,i) along the deflection direction Dri with the desired angular deflection precision A0r, even though the deflection frequency fr and therefore the laser rate fL can be high.

[0138] Thus, with reference to [Fig.7B], the fixed mirror 43; is inclined relative to the vertical axis, due to an adjustment of the goniometer (not shown). The time shift here corresponds to t0 - hb For a return frequency fr of 6kHz (corresponding to a rotation of 1250 rad / s), an inclination of the fixed mirror of 37 prad, in the counterclockwise direction, between the normal to the reflecting surface and the vertical axis, makes it possible to obtain a phase advance of 30 ns. This inclination corresponds to a shift of 30 pm on a goniometric radius of 80 cm. Note that this shift can be pre-set mechanically, for example by means of a micrometric screw, or adjustable in real time, for example by means of a piezoelectric actuator.Also, the control laser beam Fc is received by the control mirror 41k at the threshold position Pmc>th at time ti, and the power laser beam FL is then received by the deflection mirror 21j at time t0 while it precisely occupies the reference position Pmr>ref (here located vertically above the control mirror 41k). The spatial precision of the deflection is then particularly high, even though the deflection frequency fr and therefore the laser rate fL are high.

[0139] Note here that the laser system 1 can also be used with a return frequency fr and therefore a variable laser rate fL. For this, the phase advance of the triggering of the pulse selector 12 can be adjusted by means of a presetting of the goniometric support, which can possibly be adjusted by a delay applied by the programmable delay generator 55. Advantage is thus taken of the fact that the return device 20 has a 'master' character and that the power laser device 10 has a 'slave' character due to the servo device 30.

[0140] Thus, we first determine the goniometric angle ô allowing to generate a time advance Atg to compensate for the time delay Ah of optical and electronic origin: ô = conom x Atg, where conom is the nominal angular speed at which the goniometric angle ô has been preset. We can then program in the generator 55 a delay to be applied Atsup which is a function of the effective angular speed coeff to be measured, by the following relation: Atsup = Atg + ôx(l / coeff - l / conom). Thus, it is possible to compensate for the time delay Ah, even though the effective angular speed is different from the nominal angular speed which was used to set the goniometric angle ô.

[0141] [Fig. 8] is a schematic and partial view of a laser system 1 according to an alternative embodiment, which differs from that of [Fig. 6] essentially in that, on the one hand, the control laser beam Fc is emitted by a control laser source 31 distinct from the injection laser source 11, and on the other hand, the control device 40 comprises a rotary encoder 46 secured to the rotary support 23. Note that the control laser source 31 can obviously be used in the context of the variants of [Fig.2A], 2B, 3A and 3B. [Fig.8] also includes a front view of the rotary encoder 46, located here on the upstream face of the rotary support 23.

[0142] The control laser source 31 is adapted to emit the control laser beam Fc, which therefore does not come from the injection laser source 11 (local oscillator). It is transmitted towards the control device 40, and here passes through a semi-reflecting plate 4, a focusing lens, and a porthole 3 (oriented here at the Brewster incidence of 55°). The laser beam Fc is preferably a continuous beam.

[0143] The control device 40 comprises a rotary encoder 46, here an optical encoder, assembled to the rotary support 23 of the return device 20, so that the encoder 46 is rotating when the support 23 is rotating. In this example, the return device 20 is identical or similar to that of [Fig.2A] and 2B, and the rotary encoder 46 is located on the upstream face of the rotary support 23.

[0144] The rotary encoder 46 is formed of a reflective surface 46.1, reflective at the wavelength of the control laser beam Fc, and which extends along the circumference of the upstream face of the rotary support 23. Non-reflecting zones 46.2, here holes, are regularly distributed along the reflective surface 46.1, along the circumference of the upstream face of the rotary support 23, at a rate of 1 hole 46.2 for 1 return direction Dri. Each hole 46.2 is separated from the neighboring holes by the reflective surface 46.1. The holes 46.2 are positioned along a circumferential line which is traversed by the laser beam Fc as the support 23 rotates. For example, the holes 46.2 may have a diameter of 50 to 100 pm for a laser spot of the laser beam Fc with a diameter of approximately 30 pm. The non-reflecting areas 46.2 may be holes or lines. The holes 46.2 can have a diameter of 50 pm to 100 pm, or even more. The lines can have a width of the same order. Triggering by a succession of lines is probably more reliable than by a succession of holes (independence vs. axial displacement of the device).

[0145] In operation, the laser beam Fc is reflected by the reflective surface 46.2 of the rotary encoder 46, then is returned to a photodiode 33 (for example of the PIN type). The electric current generated by the photodiode 33 is here transformed into electric voltage by a transimpedance circuit 56.

[0146] When a hole 46.2 arrives in correspondence with the laser beam Fc, the latter is no longer reflected, and the optical power received by the photodiode 33 decreases sharply, then falling below a predefined threshold. The electrical signal, called here sc(t), is here reformatted into positive logic by an inverting amplifier 57: it then corresponds to a rising edge (leading edge, in English).

[0147] This electrical signal sc(t) is here communicated to the trigger input of the laser pumping circuit to regenerate the gain of the regenerative amplifier 13.2. In parallel, this same electrical signal sc(t) is communicated to a delay generator 58 which imposes a predefined time delay, for example of the order of 3 ps for a Ti-Sa laser, the time for the population inversion to be effective.

[0148] After this delay, the electrical signal sc(t) therefore appears at the input of an AND logic cell to arm the synchronization. The first laser pulse generated by the injection laser source 11 is detected by a second photodiode 59, which then switches the logic output of the AND cell (here to 1) and triggers the emission of the trigger signal sd(t), and therefore the sampling of the laser pulse according to the timing diagram adapted to the electronic circuit (taking into account the optical and electronic propagation time delays), as described in [Fig.5B]. The following laser pulses are blocked by the electronic inhibitor circuit 51. Finally, the trigger signal sd(t) is shifted again by the delay generator 55, to open the amplification cell of the regenerative amplifier 13.2, as described previously.

[0149] This optoelectronic assembly therefore allows selection without alteration of a laser pulse emitted by the injection laser source 11 when the rotary encoder 46 is in the angular position adapted to a return of the laser beam of power FL in the desired return direction Dri. In this example, the laser source 31 is placed outside the vacuum enclosure to facilitate its adjustment and maintenance, as well as its thermalization.

[0150] Particular embodiments have just been described. Different variants and modifications will appear to those skilled in the art. Thus, an optical type rotary encoder 46 has been described, but magnetic type encoders can also be used.

Claims

Claims

1. Laser system (1), adapted to emit a pulsed laser beam of power FL, and to distribute it in several successive return directions {D,}, ;NDr, with NDr>l, comprising: • a power laser device (10), adapted to emit the power laser beam FL at a laser rate fL; • a return device (20), adapted to reflect and distribute the laser beam of power FL according to the successive return directions {Dr>i}, comprising: • at least one return mirror {21,(,-, ;Nmr with Nmr> 1; • a first mechanical positioning device (22), to which the deflection mirror(s) {21j} are assembled, adapted to continuously move the deflection mirror(s) {21j} to position it or them successively, at a deflection frequency fr, in reference positions {bi, NDr associated respectively with the deflection directions {Dri}, so that a deflection mirror (21j) positioned in a reference position (P^efj) receives the laser beam of power FL and reflects it in the corresponding deflection direction (Dr>i); • characterized in that the power laser device (10) comprises a pulse selector (12), adapted to transmit, at the laser rate fL, an incident pulsed laser beam Fo, the transmitted laser beam being intended to form the power laser beam FL; • and in that it comprises a servo-control device (30), adapted to supply a trigger signal sd(t) to the pulse selector (12) authorizing the transmission of the laser beam of power FL, comprising: • a control laser source (31), adapted to emit a control laser beam Fc; • a control device (40), comprising: • at least one control mirror {41 k}k_, ;Nmc with Nmc>l ; • a second mechanical positioning device (42), to which the mirror(s) of

2. control {41k} are assembled, adapted to continuously move in rotation the control mirror(s) {41k} to position it or them successively, at the return frequency fr, in at least one threshold position {P mc,th,m}ml;Npth, UVCC Npdl> 1, of SOl'tC qU UU control mirror (41k) positioned in a threshold position (Pmc>th,m) receives the control laser beam Fc and reflects it in the same predefined return threshold direction (Dc>th); • a matrix photodetector (33), adapted to receive the control laser beam Fc reflected by a control mirror (41k) during rotation, and to emit in response a detection signal spd(t), representative of the displacement of the control mirror (41k), passing through a threshold value associated with the threshold position(s) {P mc,th,m} 5 • an electronic triggering device (50), adapted to emit the triggering signal sd(t) when the detection signal spd(t) reaches said threshold value spdjth, the threshold position(s) {Pmc>th,m} being predefined so that the laser beam of power FL, transmitted by the pulse selector (12) in response to the triggering signal sd(t), is received by a deflection mirror (21j) at a reference position Pmrjref.i, and therefore reflected in the corresponding deflection direction (Dr>i). Laser system (1) according to claim 1, wherein: • the power laser device (10) comprises an injection laser source (11) with passive mode locking, suitable for generating the pulsed laser beam Fo; • the electronic triggering device (50) comprises a multiple pulse inhibiting electronic circuit (51) adapted to emit the triggering signal sd(t) when the detection signal spd(t) reaches said threshold value spd>th, and a delay generator (55) adapted to transmit the trigger signal sd(t) to the pulse selector (12), with a predefined delay so that the trigger signal sd(t) is received by the pulse selector (12) in phase with a laser pulse of the pulsed laser beam Fo to be selected.

3. Laser system (1) according to claim 1 or 2, wherein: • the power laser device (10) comprises: an optical amplifier (13.1, 13.2, 13.3), located downstream of the pulse selector (12), and comprising a regenerative amplifier (13.2) pumped in a pulsed manner, • the servo device (30) comprises an optoelectronic gain servo circuit (60), adapted to emit a pumping signal sp(t) authorizing transmission of a pumping pulse to the regenerative amplifier (13.2), so that the pumping pulse is received by the regenerative amplifier (13.2) in phase advance upon reception by the latter of a laser pulse transmitted by the pulse selector (12) in response to the trigger signal sd(t), with a predefined advance so that the regenerative amplifier (13.2) has an optimal gain when it receives the laser pulse transmitted by the pulse selector (12).

4. Laser system (1) according to any one of claims 1 to 3, wherein the laser rate fL is equal to the return frequency fr.

5. Laser system (1) according to any one of claims 1 to 4, in which the return threshold direction (DCjth) is coaxial with an optical axis (Ac) of the control laser beam Fc then incident on the control mirror {41j} and coming from the control laser source (31).

6. Laser system (1) according to any one of claims 1 to 5, wherein the deflection device (20) comprises several deflection mirrors {21j}, with Nmr>l, assembled to a first rotary support (23), which is a cylinder with a circular or polygonal section, on the surface of which the deflection mirrors {21j} are distributed.

7. A laser system (1) according to claim 6, wherein the control device (40) comprises a rotating cylinder (46), having a surface external surface (46.1) intended to be illuminated by the control laser beam Fc, the external surface (46.1) being reflective to the control laser beam Fc and traversed by non-reflective zones (46.2) distributed regularly along the external surface (46.1).

8. Laser system (1) according to claim 6, wherein the control device (40) comprises several control mirrors {41k], with Nmc>l, assembled to a second rotating support (42), which is a cylinder with a circular or polygonal section, on the surface of which the control mirrors {41j} are distributed.

9. Laser system (1) according to claim 8, wherein the control device (40) comprises at least one fixed mirror {43i]i=i ;Nmf with Nmf>l, the control mirrors {41k] being optically coupled to the fixed mirror(s) {43i}, so that a control laser beam Fc is first reflected by a control mirror 41k towards the fixed mirror 43i, which reflects it back towards the same control mirror 41k.

10. Laser system (1) according to claim 9, in which the fixed mirror(s) {431} are fixed to an angular adjustment support (44), adapted to modify an angular inclination of the fixed mirror(s) {43J with respect to a displacement axis of the control mirror(s) {41k}, so that the control laser beam Fc is reflected by a control mirror 41k, when it occupies a threshold position Pmc>th>m, in phase advance with the reflection of the laser beam of power Fp by a deflection mirror 21j while it occupies a reference position P^efj-

11. Laser system (1) according to claim 9 or 10, in which the control mirrors {41k] are distinct from the return mirrors {21j}, the control device (40) comprising a single fixed mirror {43i} optically coupled to each of the control mirrors {41k].

12. Laser system (1) according to claim 9 or 10, in which the control mirrors {41k] are merged with the return mirrors {21j}, the return mirrors {21j} forming the control mirrors {41k}, the laser beam of power FL being incident on the return mirrors {21j} along a first optical axis (Ar), and the control laser beam Fc being incident on the control mirrors {41k] along a second optical axis (Ac) distinct from the first optical axis, and in which the control device (40) comprising several fixed mirrors {43i}, optically coupled respectively to the control mirrors {41k].

13. A laser system (1) according to any one of claims 1 to 12, wherein the laser beam of power FL has a power at less than 10 GW and / or in which the laser rate fL is at least 50 Hz.

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