Laser system with filtering module and filtering process

Abstract

The invention relates to a laser system (100) comprising: - an amplifying medium (10) arranged to emit a laser beam (11) along a propagation direction (12), said laser beam (11) being a laser beam (11) having a quality factor greater than 1.00 along at least one principal axis of the laser beam (11); and - a main spatial filtering module (20) comprising a first acousto-optical modulator (30) arranged to generate an acoustic wave along a first acoustic propagation direction (31) and to form a diffracted primary beam (32) in a first plane defined by the first acoustic propagation direction (31), said diffracted primary beam (32) having two orthogonal principal axes, one of which is defined in said first plane. Figure for the abstract: Fig. 3

Description

Title of the invention: Laser system with filtering module and filtering method. Technical field of the invention

[0001] The present invention relates generally to a laser system comprising an amplifying medium and a filtering module.

[0002] It relates more particularly to spatial and angular filtering devices to improve the quality of a laser beam.

[0003] The invention finds a particularly advantageous application in laser systems emitting degraded or asymmetric laser beams.

[0004] It also relates to a method for filtering a laser beam. State of the art

[0005] Laser systems comprising a spatial filtering module are known, in which the spatial filtering module includes at least one slit aligned with a laser beam. Such systems make it possible to filter out unwanted parts of the laser beam, thereby allowing intervention on beam divergence, notably by reducing the beam quality factor. Furthermore, such systems also improve the symmetry of the laser beam, since spatial filtering allows it to approach as closely as possible the ideal shape of the laser beam, namely a Gaussian profile with rotational symmetry about an optical axis. Such systems are therefore functional but present several problems, notably the generation of edge effects, creating a diffraction pattern that can generate an unwanted spurious signal. In addition, these systems exhibit low resistance to laser powers exceeding 10 watts.Beyond this threshold, these laser systems suffer undesirable side effects, such as damage, heating, and scattered light, which result in performance losses in spatial filtering.

[0006] Laser systems using angular filtering by Bragg gratings are also known. These systems are functional but present several problems, particularly with tuning. Indeed, the diffraction grating of the Bragg grating is fixed, and therefore not easily flexible or adaptable to different optical setups. Furthermore, if other diffraction properties are desired, it is necessary to change the Bragg modulus, which implies making further adjustments to insert such a filtering module into the laser system. Such angular filtering modules are therefore difficult to implement and offer limited flexibility. In addition, these filtering modules often use materials containing impurities, causing unwanted and uncontrolled degradation of the laser beam at the output of these modules. filtering is impaired when subjected to high power levels, particularly those exceeding 10 watts. Such angular filtering modules are therefore limited by their ability to withstand high power levels, which can consequently damage the main spatial filtering module, as well as by the quality of the diffracted beam obtained at the output of these angular filtering modules at these optical power levels. Presentation of the invention

[0007] In order to overcome the aforementioned drawbacks of the prior art, the present invention proposes a laser system comprising: - an amplifying medium arranged to emit a laser beam along a propagation direction, said laser beam having a cross-section defined along two principal axes orthogonal to the propagation direction of said laser beam, said laser beam having a quality factor greater than 1.00 along at least one of the two principal axes of the laser beam; and - a main spatial filtering module comprising a first acousto-optical modulator including a first optical crystal and a first antenna arranged to generate an acoustic wave in the first optical crystal along a first acoustic propagation direction and to induce a first Bragg grating comprising at least one reflecting plane in the first optical crystal, said at least one reflecting plane being perpendicular to the first acoustic propagation direction, the first acousto-optical modulator being arranged to receive the laser beam on the first Bragg grating and to form a diffracted primary beam propagating along a propagation direction, said laser beam being incident on the first optical crystal in a first plane defined by the first acoustic propagation direction and the laser beam propagation direction,said direction of propagation of the laser beam having a non-zero angle of incidence with respect to said at least one reflecting plane of the first Bragg grating, and said diffracted primary beam having two principal axes orthogonal to its direction of propagation, of which one principal axis in the first plane, called the principal diffraction axis of the diffracted primary beam, and one principal axis of the laser beam, called the degraded principal axis of the laser beam, of said two principal axes of the laser beam having a quality factor greater than 1.00, is positioned in the first plane such that said diffracted primary beam has a quality factor along the principal axis in the first plane lower than the quality factor of the laser beam along the degraded principal axis in the first plane.

[0008] According to the invention, spatial and angular filtering is performed on the laser beam. This filtering is performed by means of the first acousto-optic modulator oriented along a principal axis of this laser beam to perform filtering along this principal axis.

[0009] Furthermore, thanks to the laser system according to this disclosure, it is possible to improve the quality factor of the laser beam along a principal axis of this laser beam so as to bring the quality factor along this axis as close as possible to the quality factor of an ideal Gaussian beam. Advantageously, this filtering also improves the symmetry of this laser beam so that the quality factors along the two principal axes of the laser beam are brought closer together. The optical properties of the laser beam are thus improved.

[0010] The diffractive properties of the main filtering module are not fixed; they are therefore flexible and adaptable to requirements. Such an arrangement is thus more easily modified according to the shape of each laser beam and is easy to implement.

[0011] Furthermore, the main spatial filtering module is adapted to operate at high laser powers. The laser system is therefore more robust and less susceptible to heating and scattering effects. The laser system according to the present disclosure is thus particularly well-suited to high laser powers. No edge or spurious beam problems are created by the main filtering module according to the invention. Consequently, the main filtering module is more precise and therefore more efficient than a fixed grating.

[0012] By beam (here laser beam or light beam), we mean a set of light rays propagating along a direction of propagation.

[0013] By direction of propagation, we mean a propagation vector.

[0014] By amplifying medium is meant any medium suitable for amplifying a beam laser. The amplifying medium can be solid, fibrous, include a two-dimensional crystalline waveguide, etc.

[0015] By diffracted beam, we mean a beam which is deviated at a determined angle with respect to an initial beam.

[0016] By initial beam, we mean a laser beam positioned upstream of an acousto-optical modulator of the main spatial filtering module.

[0017] In the following, this disclosure uses the terms "diffracted primary beam" and "diffracted secondary beam". These beams are laser beams that have been deflected by an acousto-optic modulator or respectively by at least two acousto-optic modulators arranged in series of the main spatial filtering module described in this disclosure.

[0018] Filtering refers to the blocking or passage of light rays from a laser beam. For example, spatial filtering refers to the passage or selection of the laser beam (here, light rays from the laser beam) along a given direction, therefore according to its spatial components. Angular filtering refers to a passage or a selection of part of the laser beam inclined at a given angle or direction.

[0019] In one embodiment, the quality factor of the laser beam along the degraded principal axis of the laser beam is at least 10% greater than the quality factor of the laser beam along the other principal axis of the laser beam.

[0020] In one embodiment, the quality factor of the laser beam along the degraded principal axis of the laser beam is greater than or equal to 1 and / or less than or equal to 4, and / or less than or equal to 2, preferably between 1 and 4, preferably between 1 and 2 (including any value between 1.00, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.10, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.20, 1.21, 1.22, 1.23, 1.24, 1.25, 1.26, 1.27, 1.28, 1.29, 1.30, 1.31, 1.32, 1.33, 1.34 1.35, 1.36, 1.37, 1.38, 1.39, 1.40, 1.41, 1.42, 1.43, 1.44, 1.45, 1.46, 1.47, 1.48, 1.49, 1.50, 1.51, 1.52, 1.53, 1.54, 1.55, 1.56, 1.57, 1.58, 1.59, 1.60, 1.61, 1.62, 1.63, 1.64 1.65, 1.66, 1.67, 1.68, 1.69, 1.70, 1.71, 1.72, 1.73, 1.74, 1.75, 1.76, 1.77, 1.78, 1.79, 1.80, 1.81, 1.82, 1.83, 1.84, 1.85, 1.86, 1.87, 1.88, 1.89, 1.90, 1.91, 1.92, 1.93, 1.94, 1.95, 1.96, 1.97, 1.98, 1.99, 2.00).

[0021] In one embodiment, the quality factor along the principal diffraction axis of the diffracted primary beam is less than the quality factor along the degraded principal axis of the laser beam by at least 3 percent, or at least 5 percent, or at least 10 percent.

[0022] In one embodiment, the quality factor along the principal diffraction axis of the diffracted primary beam is k percent of the quality factor along the degraded principal axis of the laser beam, with k between 3 and 50, or between 5 and 50, or between 5 and 25. Preferably, k includes any value between 3 and 50, i.e., 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50.

[0023] In one embodiment, the diffracted primary beam has a quality factor along at least one of its principal axes greater than 1.00, and the main spatial filtering module comprises a second acousto-optic modulator including a second optical crystal and a second antenna arranged to generate an acoustic wave in the second optical crystal along a second acoustic propagation direction and to induce a second Bragg grating including at least one reflecting plane in the second optical crystal, said at least one reflecting plane being perpendicular to the second acoustic propagation direction, the second acousto-optic modulator being arranged to receive the diffracted primary beam on the second Bragg grating and to form a diffracted secondary beam propagating along a propagation direction, the diffracted primary beam being incident on the second optical crystal in a defined second plane by the second acoustic propagation direction and the propagation direction of the diffracted primary beam, the propagation direction of the diffracted primary beam having a non-zero angle of incidence with respect to said at least one reflecting plane of the second Bragg grating and said diffracted secondary beam having two principal axes orthogonal to its propagation direction, a principal axis of the diffracted primary beam, called the degraded principal axis of the diffracted primary beam, among said two principal axes of the diffracted primary beam having the quality factor greater than 1.00, is positioned in the second plane so that said diffracted secondary beam has a quality factor along the principal axis in the second plane lower than said quality factor of the diffracted primary beam along the degraded principal axis in the second plane.

[0024] In one embodiment, the quality factor of the primary beam diffracted along the degraded principal axis of the primary beam diffracted is greater than or equal to 1 and / or less than or equal to 4, and / or less than or equal to 2, preferably between 1 and 4, preferably between 1 and 2 (comprising any value between 1.00, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.10, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.20, 1.21, 1.22, 1.23, 1.24, 1.25, 1.26, 1.27, 1.28, 1.29, 1.30. 1.31, 1.32, 1.33, 1.34, 1.35, 1.36, 1.37, 1.38, 1.39, 1.40, 1.41, 1.42, 1.43, 1.44, 1.45, 1.46, 1.47, 1.48, 1.49, 1.50, 1.51, 1.52, 1.53, 1.54, 1.55, 1.56, 1.57, 1.58, 1.59, 1.60, 1.61, 1.62, 1.63, 1.64, 1.65, 1.66, 1.67, 1.68, 1.69, 1.70, 1.71, 1.72, 1.73, 1.74, 1.75, 1.76, 1.77, 1.78, 1.79, 1.80, 1.81, 1.82, 1.83, 1.84, 1.85, 1.86, 1.87, 1.88, 1.89, 1.90, 1.91, 1.92, 1.93, 1.94, 1.95, 1.96, 1.97, 1.98, 1.99, 2.00).

[0025] In one embodiment, the quality factor along the principal diffraction axis of the diffracted secondary beam is less than the quality factor along the degraded principal axis of the diffracted primary beam by at least 3 percent, or at least 5 percent, or at least 10 percent.

[0026] In one embodiment, the quality factor along the principal diffraction axis of the diffracted secondary beam is k percent of the quality factor along the degraded principal axis of the diffracted primary beam, with k between 3 and 50, or between 5 and 50, or between 5 and 25. Preferably, k includes any value between 3 and 50, i.e., 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50.

[0027] In one embodiment, the acoustic wave generated by the first acousto-optic modulator and / or the second acousto-optic modulator presents(s) a time signal with a single acoustic frequency for a time period or presents(s) a time signal, for a time period, with at least two different acoustic frequencies applied sequentially one after the other.

[0028] Thus, in one embodiment, the acoustic wave emitted by the first acousto-optic modulator and / or the second acousto-optic modulator has an acoustic frequency that varies with time.

[0029] In one embodiment, the time period is between 0.5 seconds and 10 seconds.

[0030] In one embodiment, the first acousto-optic modulator is arranged to generate an active aperture or active slit in an input plane perpendicular to the first plane and / or the second acousto-optic modulator is arranged to generate an active aperture or active slit in another input plane perpendicular to the second plane.

[0031] In one embodiment, the first direction of acoustic propagation is parallel to the second direction of acoustic propagation or the first direction of acoustic propagation is orthogonal to the second direction of acoustic propagation.

[0032] In one embodiment, the first acousto-optical modulator is arranged to reduce the quality factor along the principal diffraction axis of the diffracted primary beam by at least 3.0 percent or at least 5.0 percent, or at least 10.0 percent, compared to the quality factor along the degraded axis of the laser beam and / or, respectively, the second acousto-optical modulator is arranged to reduce the quality factor along the principal diffraction axis of the diffracted secondary beam by at least 3.0 percent, or at least 5.0 percent, or at least 10 percent, compared to the quality factor along the degraded principal axis of the diffracted primary beam.

[0033] In one embodiment, the first acousto-optical modulator is arranged to diffract the laser beam to a diffraction order of norm equal to 1 and / or, respectively, the second acousto-optical modulator is arranged to diffract the diffracted primary beam to a diffraction order of norm equal to 1.

[0034] In one embodiment, the angle of incidence of the direction of propagation of the laser beam is equal to a Bragg angle of the first acousto-optic modulator and / or, respectively, the angle of incidence of the direction of propagation of the diffracted primary beam is equal to a Bragg angle of the second acousto-optic modulator.

[0035] In another embodiment, the main spatial filtering module comprises a plurality of second acousto-optical modulators arranged in series, each given second acousto-optical modulator being arranged to spatially filter a principal axis of the diffracted laser beam incident on said second acousto-optical modulator and the principal axis of the incident diffracted laser beam being positioned in the second plane associated with said given second acousto-optical modulator, the secondary beam diffracted by the second given acousto-optic modulator having a quality factor along the principal axis of the secondary beam diffracted in the second plane associated with the second given acousto-optic modulator lower than the quality factor along the degraded principal axis of the laser beam diffracted incident on said second acousto-optic modulator in the second associated plane.

[0036] Advantageously, the laser beam has an average power greater than or equal to 10 watts or greater than 1 kilowatt, preferably between 100 watts and 1 kilowatt.

[0037] Advantageously, the laser beam is a pulsed laser beam arranged to emit at least one laser pulse having a duration on the order of nanoseconds, picoseconds or femtoseconds, for example between 100 fs and 1 ns, in particular at least one pulse stretched temporally in a chirped pulse amplification (or CPA) system.

[0038] In one embodiment, the laser system according to this disclosure further includes a control circuit for said main spatial filtering module comprising at least one clock synchronized with the laser beam.

[0039] Advantageously, the laser system according to this disclosure comprises at least one secondary spatial filtering module of said laser beam positioned upstream of said primary spatial filtering module, said secondary spatial filtering module comprising at least one slot orthogonal to at least one principal axis of the laser beam, said at least one slot being arranged to spatially filter said laser beam along said at least one principal axis of the laser beam.

[0040] In one embodiment, the system further comprises an optical collimation system upstream of said main spatial filtering module and the optical collimation system having an optical axis aligned with the direction of propagation of the laser beam, said optical collimation system being arranged to reduce the quality factor of the laser beam along the principal axes of the laser beam by reducing a section of said laser beam transverse to said axis of propagation of the laser beam.

[0041] Preferably, each acousto-optic modulator comprises an optical crystal formed in at least one of the following materials: - quartz in crystalline form; - tellurium dioxide (TeO2); - quartz in amorphous form (i.e. fused silica).

[0042] In one embodiment, the amplifying medium is a solid amplifying medium.

[0043] By solid amplifying medium is meant an element or a medium comprising at least one solid element and arranged to emit or transmit a laser beam Any medium, preferably for transmitting a laser beam with a gain greater than 1. For example, a laser comprising a crystal, also known as a crystal laser, is considered a solid-state gain medium. The crystal is preferably doped, for example, with ytterbium or neodymium, to generate high-power laser beams, for example, greater than 10 watts (W), preferably greater than 100 watts. Advantageously, the gain medium has a slab-like geometry or a solid-state laser geometry with a longitudinal gain module and / or includes at least one laser diode.

[0044] In some examples, the laser beam is a degraded symmetric laser beam.

[0045] In another embodiment, the laser beam is an asymmetric laser beam.

[0046] Optionally, the asymmetric laser beam may have a disk shape in one propagation plane, and in other propagation planes, to exhibit an elliptical shape.

[0047] By asymmetric, we mean a laser beam of astigmatic shape.

[0048] The invention also relates to a filtering process comprising the following steps: - emission by a laser system, preferably comprising an amplifying medium, of a laser beam along a propagation direction, said laser beam having a cross-section defined along two principal axes orthogonal to the propagation direction of said laser beam, said laser beam having a quality factor greater than 1.00 along at least one of the two principal axes of the laser beam; and - generation of an acoustic wave along a first acoustic propagation direction in a first optical crystal of a first acousto-optic modulator so as to induce a first Bragg grating comprising at least one reflecting plane in the first optical crystal, said at least one reflecting plane being perpendicular to the first acoustic propagation direction and spatial filtering of the laser beam by means of the first acousto-optic modulator,said laser beam being incident on the first optical crystal in a first plane defined by the first acoustic propagation direction and the laser beam propagation direction, said laser beam propagation direction having a non-zero angle of incidence with respect to said at least one reflecting plane of the first Bragg grating so as to form a diffracted primary beam propagating along a propagation direction, the diffracted primary beam having two principal axes orthogonal to its propagation direction, one of which principal axis is in the first plane, one principal axis of the laser beam, called the degraded principal axis of the laser beam, among said two principal axes of the laser beam having a quality factor greater than 1.00, being positioned in the first plane so that said diffracted primary beam, presents a quality factor along the principal axis in the foreground that is lower than the quality factor of the laser beam along the principal axis that is degraded in the foreground.

[0049] In one embodiment, said diffracted primary beam has a quality factor along at least one of its principal axes greater than 1; said method further comprises a step of filtering the diffracted primary beam by means of a second acousto-optic modulator, said second acousto-optic modulator generating an acoustic wave along a second acoustic propagation direction and forming a diffracted secondary beam along a propagation direction of the diffracted secondary beam, said diffracted secondary beam being formed in a second plane defined by said second acoustic propagation direction and the propagation direction of the diffracted secondary beam, said propagation direction of the diffracted primary beam having an angle of incidence with respect to an axis of a reflecting plane generated in the second acousto-optic modulator,said axis being perpendicular to a normal to the reflecting plane oriented along the second direction of acoustic propagation, a principal axis of the diffracted primary beam, called the degraded principal axis of the diffracted primary beam, among said two principal axes of the diffracted primary beam having a quality factor greater than 1, being positioned in the second plane such that the diffracted secondary beam has a quality factor in the second plane lower than said quality factor of the diffracted primary beam along the degraded principal axis in the second plane.

[0050] In one embodiment, the acoustic wave generated by the first acousto-optic modulator or the second acousto-optic modulator presents a time signal having an acoustic frequency, said method comprising a modification of the acoustic frequency by a control device of the acousto-optic modulator.

[0051] Of course, the various features, variants, and embodiments of the invention can be combined with one another in various ways, provided they are not incompatible or mutually exclusive. Detailed description of the invention

[0052] The following description with regard to the attached drawings, given by way of non-limiting examples, will make it clear what the invention consists of and how it can be carried out.

[0053] On the attached drawings:

[0054] [Fig-1] is a schematic representation of the divergence of a non-ideal, possibly astigmatic, laser beam, compared with the divergence of an ideal symmetrical laser beam;

[0055] [Fig.2] is a schematic representation of a cross-section of an asymmetric laser beam;

[0056] [Fig.3] is a schematic representation of a first embodiment of a laser system according to the present disclosure in a foreground;

[0057] [Fig.4] is a schematic representation of the first embodiment of a laser system according to the present disclosure in a plane transverse to the plane of [Fig.3];

[0058] [Fig.5] is a schematic representation of a second embodiment of a laser system according to the present disclosure in the foreground;

[0059] [Fig.6] is a schematic representation of the second embodiment of a laser system according to the present disclosure in the second plane;

[0060] [Fig.7] is a schematic representation of a third embodiment of a laser system according to the present disclosure in the foreground;

[0061] [Fig.8] is a schematic representation of a fourth embodiment of a laser system according to the present disclosure in the foreground;

[0062] [Fig.9] is a schematic representation of a first embodiment of a spatial filtering process according to the present disclosure;

[0063] [Fig. 10] is a schematic representation of a second embodiment of a spatial filtering process according to the present disclosure;

[0064] [Fig. 11] is a schematic representation of a first acousto-optic modulator in a plane transverse to the foreground;

[0065] [Fig. 12] is a schematic representation of a time-domain signal of the acoustic wave emitted by one of the acousto-optical modulators of a main spatial filtering module of a laser system according to the present invention;

[0066] [Fig. 13] is a schematic representation of a time-domain signal of the acoustic wave emitted by one of the acousto-optical modulators of a main spatial filtering module of a laser system according to the present invention.

[0067] Definitions.

[0068] The quality of a laser beam is often described by its beam propagation factor, also called the laser beam quality factor, and denoted M2-

[0069] This quality factor M2 is expressed according to the following formula 1:

[0070] [Math.l] M2 = %

[0071] With a corresponding to the divergence angle (also called divergence) of an ideal Gaussian laser beam 15 of minimum cross-section w0 (also called minimum diameter w0 or size known in English as "waist") at a position z0 along an optical axis Z along which the laser beam propagates (also called direction of propagation of the laser beam) and Q corresponding to the actual divergence angle of a laser beam 11 for the same diameter w0 (see [Fig. 1]) at the same given position z0 along the optical axis Z.

[0072] More generally, the quality factor M2 is defined according to ISO 11146-1 and the following formula:

[0073] [Math.2] 0 =

[0074] With 1 corresponding to the wavelength of the laser beam, M2 the quality factor, and w0 the minimum cross-section of the laser beam at a given position z0. According to the formula Math 2, the divergence of a laser beam is a function of its quality factor M2, its minimum cross-section w0, and its wavelength 1. Thus, according to this formula Math 2, the quality factor is proportional to the actual divergence angle Q of the laser beam. Consequently, the higher the divergence angle Q of a laser beam compared to the divergence angle α of a "perfect" Gaussian beam (with the same minimum cross-section), the higher the quality factor M2 of that laser beam will be.

[0075] A "perfect" or "ideal" laser beam has a Gaussian profile transverse intensity distribution. A "perfect" or "ideal" laser beam is considered to have a quality factor M2 equal to 1. According to this principle, the more a laser beam's transverse intensity distribution deviates from the "ideal" Gaussian profile, the higher the quality factor M2 of that beam. Thus, the quality of a laser beam can be assessed or described by the quality factor M2 of that laser beam.

[0076] The quality factor M2 is related to the actual divergence of the laser beam compared to the divergence of an ideal laser beam (i.e. M2 = 1).

[0077] In general, a laser beam is considered symmetrical when it exhibits a similar divergence along its two principal axes Xi5 Y;. However, this divergence may be greater than that of an ideal Gaussian beam, i.e. a Gaussian beam having a quality factor M2 equal to 1.00.

[0078] A laser beam exhibiting a quality factor greater than 1.00 is here referred to as a degraded laser beam.

[0079] Furthermore, the divergence of a laser beam can vary along two directions transverse to the propagation axis of that laser beam, particularly for laser beams having an asymmetrical shape with respect to the propagation axis. These beams These are called asymmetric laser beams. Asymmetric laser beams include elliptical and astigmatic laser beams. Generally, they are defined by the ISO 11146-2 standard.

[0080] An asymmetric laser beam is evaluated along two axes, called principal axes, which are typically denoted X and Y, for beam propagation along the Z-axis (see [Fig. 2]) (also called the optical axis Z), where i corresponds to the index of the system under consideration, in an orthonormal XYZ coordinate system. The principal axes are defined as the axis along which the laser beam exhibits maximum intensity. For asymmetric laser beams, the parameters of formula Math 2, such as the divergence angle Q, the quality factor M2, and the minimum cross-section Wo, are then evaluated along the two principal axes X and Y of the asymmetric laser beam in order to describe the propagation of an asymmetric beam along the two principal axes X and Y.

[0081] Figure 2 illustrates a cross-section of an asymmetric laser beam 11 at a given position z0 along the Z-axis of laser beam propagation. In this example, the asymmetric laser beam is an astigmatic laser beam 11 that has a cross-section defined along a first principal axis, denoted Xi5, and a second principal axis, denoted Yj. The first principal axis Xj and the second principal axis Yj are orthogonal to the direction of laser beam propagation. Furthermore, this beam has, in a plane transverse to the propagation axis, a size along the first principal axis Xi that differs from its size along the second principal axis Yi. This size can vary along the laser beam propagation axis. According to this example, the asymmetric laser beam has an elliptical shape.

[0082] In the following, the laser beam can be a symmetrical but degraded laser beam or an asymmetrical laser beam as described above.

[0083] Device

[0084] Figures 3 and 4 illustrate a first embodiment of a laser system 100 according to the present disclosure.

[0085] In [Fig.3], the elements of the laser system 100 are represented in a first transverse plane (XZ) defining the laser system with Z corresponding to the optical axis of the laser system 100. Conversely, [Fig.4] illustrates the laser system 100 in a second transverse plane (YZ).

[0086] In this disclosure, particularly in Figures 3-8, laser beams are schematically represented by a single light ray. Thus, it is understood that the propagation vector of the laser beam corresponds to the propagation vector of the light ray in the illustrated laser beam. Similarly, each plane or incident plane described below is associated with a light ray of the incident laser beam under consideration.

[0087] The laser system 100 illustrated in Figures 3 and 4 comprises an amplifying medium 10 arranged to emit a laser beam 11 along a propagation direction 12 and a main spatial filtering module 20 comprising a first acousto-optical modulator 30 arranged to spatially and angularly filter the laser beam 11. The first acousto-optical modulator 30 is preferably positioned at the minimum cross-section (i.e. at the "waist" w0) of the laser beam 11. The first acousto-optical modulator 30 is electronically controlled by a control circuit 70.

[0088] In one embodiment, the amplifying medium 10 is a solid amplifying medium 10.

[0089] In particular, the solid amplifying medium 10 emitting the laser beam 11 has a solid-state geometry with a longitudinal amplification modulus, as described in the document "Innoslab amplifiers", IEEE Journal of selected topics in quantum electronics, vol. 21, No. 1, January / February 2015.

[0090] In one variant, the amplifying medium 10 may have a plate-shaped geometry, also known as solid-state laser geometry, from the English "Solid-State Bulk Lasers (slab)" as described in the document "Reduced Thermal Focusing and Birefringence in Zig-Zag Slab geometry Crystalline Lasers", IEEE Journal of quantum Electronics, vol. QE-19, No.9, September 1983.

[0091] The amplifying media 10 illustrated in the last two cited documents make it possible, in particular, to obtain a pulsed laser beam 11 with a power greater than 10 W, preferably greater than 100 W. However, the laser beams obtained from such solid amplifying media produce an asymmetric laser beam, as will be explained below. In practice, the divergence along the principal axes of an asymmetric laser beam is 30% greater than the divergence along the principal axes of a perfect Gaussian beam.

[0092] Of course, in another variant, the amplifying medium 10 of the laser system 100 illustrated in figures 3 and 4 may include at least one laser diode for emitting or amplifying the laser beam 11.

[0093] The amplifying medium 10 emits the laser beam 11 along a propagation direction 12.

[0094] In this example, the laser beam 11 has a wavelength of 1 pm and is a pulsed laser beam 11 with pulses, for example, of duration on the order of picoseconds. The laser beam 11 also has a cross-section (diameter of the laser beam 11 at the waist) between 50.00 micrometers (pm) and 5.00 millimeters, preferably between 1.00 millimeters and 2.00 millimeters (mm) for a laser beam 11 with a power greater than 10 watts. In particular, in In this example, the laser beam 11 emitted by the amplifying medium 10 is 2.00 mm in diameter.

[0095] The laser beam 11 emitted by the amplifying medium 10 also has a power greater than or equal to 10 watts, preferably between 100 Watts and 1 kilowatt (here 100 Watts).

[0096] As explained above, the laser beam 11 illustrated in Figures 3 and 4 is an asymmetric laser beam 11 as illustrated in [Fig.2].

[0097] The laser beam 11 is in particular an astigmatic laser beam 11 which has a cross-section defined along a first principal axis, denoted Xa, and a second principal axis, denoted Ya. The first principal axis Xa and the second principal axis Ya are orthogonal to the propagation direction 12 of the laser beam 11.

[0098] In this example, the laser beam 11 has a quality factor greater than 1.00 along at least one of its two principal axes Xa, Ya. For example, the quality factor MXa2 along the principal axis Xa is 1.50, while the quality factor MY a2 is, for example, 1.10. Thus, in this case, the quality factor MXa2 (here called the degraded principal axis of the laser beam 11) oriented along the principal axis XA differs here by 20% to 30% (here by 26.7%) from the quality factor MY a2 oriented along the other principal axis (here Ya) of the laser beam 11. Here, the degraded principal axis of the laser beam is the principal axis of the laser beam 11 exhibiting the highest quality factor M2. As will be explained later, this main degraded axis of the laser beam 11 corresponds to the main axis of the laser beam which will be filtered by the first acousto-optical modulator 30.

[0099] Of course, in another example, the laser beam 11 can be a degraded laser beam 11 comprising a quality factor greater than 1.00 and equivalent along its two principal axes Xa, Ya, for example a quality factor of 1.5 along its two principal axes Xa, Ya.

[0100] In the laser system 100, the propagation direction 12 of the laser beam 11 lies in the XZ plane but is not aligned along the Z axis. The laser beam 11 thus strikes the first acousto-optic modulator 30 at an angle of incidence. The angle of incidence is greater than 0. It can therefore be between 0 (exclusive) and 75 degrees. As an example, the angle of incidence on the first acousto-optic modulator is 0.35 degrees.

[0101] The first acousto-optic modulator 30 comprises a first optical crystal 39 and a first high-frequency or radio frequency (RF) antenna 34. In a known manner, the first antenna 34 is arranged to generate an acoustic wave in the first optical crystal 39 along a first acoustic propagation direction 31 defined by the normal to the first antenna 34 emitting the RF radiation. The first acousto-optic modulator 30 thus generates, via the first antenna, a first diffractive Bragg grating 35 in the first optical crystal 39. The first Bragg grating 35 can be schematically represented by a series of reflecting planes 36 arranged perpendicular to the first direction of acoustic propagation, i.e. to the normal to the first antenna 34 emitting the RF radiation.

[0102] As illustrated in [Fig. 12], the acoustic background generated by the first acousto-optic modulator 30 presents a single-frequency time-domain signal over a given duration or time period Tp (corresponding to the emission time of the acoustic background). Here, the time-domain signal of the acoustic background is constant in amplitude and phase.

[0103] Following this example, the first acousto-optic modulator 30 comprises a first quartz optical crystal 39. Typically, the crystal of the first acousto-optic modulator 30 has a cross-section larger than the cross-section of the laser beam 11, for example, 4 mm in this case. Thus, the size of the crystal of the first acousto-optic modulator 30 is adapted to the diameter of the laser beam 11.

[0104] The first acousto-optic modulator 30 is arranged to generate a first diffractive Bragg grating 35 in the crystal of the first acousto-optic modulator 30 by means of acoustic background generated by the first antenna 34. Here, the first Bragg grating 35 comprises a plurality of parallel reflecting planes 36. Each reflecting plane 36 of the first Bragg grating 35 is spaced from another reflecting plane 36 of the first Bragg grating 35 by a distance dnl equal to the acoustic wavelength of the first acousto-optic modulator 30. The first antenna 34 of the first acousto-optic modulator 30 can be a high-frequency or radio-frequency (RF) antenna. The first antenna 34 of the first acousto-optical modulator 30 is arranged to emit acoustic background between 1 MHz and 260 MHz, preferably at 250 MHz.Following this example, the first Bragg network 35 of the first acousto-optic modulator 30 depends on the acoustic background frequency generated by the first acousto-optic modulator 30. The acoustic background emission can be adjusted manually or automatically by a control circuit.

[0105] Each reflecting plane 36 is formed in the first acousto-optic modulator 30 (here in its crystal) when acoustic background is generated in the first acousto-optic modulator 30.

[0106] A first plane of incidence (located in the XZ plane) is defined by the propagation vector 12 of the laser beam 11 and a normal ni to a reflecting plane 36 of the first acousto-optic modulator 30 along the acoustic propagation direction 31. One of the principal axes of the laser beam 11 is positioned in the first plane of incidence, hereafter referred to as the first plane; in particular, here, the principal axis Xa of the laser beam 11, corresponding to the degraded principal axis of the laser beam 11. The angle of incidence of the laser beam 11 is defined in the first plane. Here, in particular, the angle The angle of incidence is defined as the angle (of one of the light rays of the laser beam 11) between a reflecting plane 36, on which the light ray arrives, and the propagation direction 12 of the light ray (corresponding to the propagation direction 12 of the laser beam 11). The normal ni of the reflecting plane 36 (considered) is oriented along the acoustic propagation direction 31 of the first acousto-optic modulator 30.

[0107] In the illustrated example, the first acousto-optical modulator 30 is also arranged to form a diffracted primary beam 32 propagating along a propagation direction 33 of the diffracted primary beam 32. In this example, the diffracted primary beam 32 is a light beam formed in a first plane. The first plane here is the XZ plane.

[0108] Thus, it is understood that the propagation direction 33 of the diffracted primary beam 31 is also contained in the first plane XZ. Similarly, it is understood that the reflecting planes 36 of the Bragg grating 35 are perpendicular to the first plane.

[0109] It is understood that each ray of the laser beam 11 diffracted by the first acousto-optic modulator 30 defines an incident plane. In practice, as many foregrounds can be defined as there are light rays passing through the first acousto-optic modulator along the Y-axis of the laser system 100.

[0110] The diffracted primary beam 32 is a laser beam deflected angularly with respect to the laser beam 11 incident on the first acousto-optic modulator 30, via the first Bragg grating 35. Thus, the propagation direction 33 of the diffracted primary beam 32 has a diffraction angle dl in the first plane. The diffraction angle dl for a ray of the diffracted primary beam corresponds to the angle formed between the reflecting plane 36 and the propagation direction 33 of the diffracted primary beam 32 of the considered diffracted primary beam ray.

[0111] In this example, the diffracted primary beam 32 has two principal axes, denoted Xb for its first principal axis, and Yb for its second principal axis. The two principal axes Xb, Yb of the diffracted primary beam 32 are orthogonal to the propagation direction 33 of the diffracted primary beam 32.

[0112] In this disclosure, a principal axis of the diffracted primary beam 32 is located in the foreground. It is also understood that it is contained in the plane of incidence. This principal axis of the diffracted primary beam 32 (positioned in the foreground) is called the principal diffraction axis of the diffracted primary beam 32. As illustrated in Figures 3 and 4, the principal diffraction axis of the diffracted primary beam 32 is the first principal axis Xb of the diffracted primary beam 32.

[0113] Thus, in this configuration, the degraded principal axis of the laser beam 11 (i.e. principal axis Xa of the laser beam 11), and the principal diffraction axis of the diffracted primary beam 32 (i.e. principal axis Xb of the diffracted primary beam 32) are therefore positioned in the first plane relative to the first acousto-optic modulator 30 and the incident laser beam 11. It is thus understood that the principal axis of the laser beam 11 which is filtered by the first acousto-optic modulator corresponds to the principal axis of the laser beam which is positioned in the first plane.

[0114] Of course, in a variant of the laser system 100, the degraded principal axis of the laser beam 11 can be the second principal axis Ya of the laser beam 11 and the diffraction principal axis of the diffracted primary beam 32 can be the second principal axis denoted Yb of the diffracted primary beam 32.

[0115] In the example illustrated in Figures 3 and 4, the diffracted primary beam 32 has a quality factor M2xb along the principal diffraction axis (here the first principal axis Xb of the diffracted primary beam 32) equal to 1.21, while the quality factor M2Xa along the degraded principal axis (here the first principal axis Xa of the laser beam 11) is 1.50. Therefore, the quality factor M2xb along the first principal axis of the diffracted primary beam Xb is less than the quality factor of the laser beam 11 along the first principal axis Xa of the laser beam 11.

[0116] Thus, according to this configuration, the first acousto-optical modulator 30 is arranged to spatially filter the laser beam 11 along the degraded principal axis positioned in the foreground, thereby reducing the divergence Q of the laser beam 11 at the output of the first acousto-optical modulator 30 in a direction given by the chosen principal axis. Such an arrangement allows the main spatial filtering module 20 to improve, in particular to reduce, the divergence angle Q of the laser beam 11 oriented along the degraded principal axis of the laser beam 11 positioned in the foreground.

[0117] Following this example, the first acousto-optical modulator 30 makes it possible to reduce the quality factor Mxb2 oriented along the principal diffraction axis Xb by at least 3%, in particular by at least 15% in this example, compared to the quality factor MXa2 of the laser beam 11 oriented along the degraded principal axis Xa.

[0118] Preferably, the first acousto-optical modulator 30 is arranged to diffract the laser beam 11 according to a grating law, defined by the first Bragg grating 35, in order to obtain the diffracted primary beam 32 deviated according to the diffraction angle, denoted dl on [Fig.3] and determined by the following formula:

[0119] [Math.3] \ ftl -À = TÂ

[0120] with di corresponding to the diffraction angle obtained at the output of an acousto-optic modulator i (corresponding, in this example, to the first acousto-optic modulator 30), m the diffraction order of the grating of the acousto-optic modulator i (here of the first Bragg grating 35 of the first acousto-optic modulator 30), 1 the length wave of the laser beam 11 and L corresponding to the acoustic wavelength of the acousto-optic modulator i which is a function of the acoustic frequency Fac and the acoustic velocity Vac in the crystal of the acousto-optic modulator i and expressed according to the ratio _ v<*.

[0121] Following this principle, the diffractive properties of the first acousto-optic modulator 30 are not fixed; they can be modified according to the acoustic wave emitted by the antenna 34 of the first acousto-optic modulator 30. This makes it possible to propose a main spatial filtering module 20 that is easy to implement and adapt in a laser system 100. The diffraction properties of the first Bragg grating 35 in the system 100 depend on a modulation of the optical index of the crystal of the first acousto-optic modulator 30 generated by its acoustic wave. For example, for a radio frequency (RF) wave with an acoustic frequency Fac of 68 MHz and an acoustic velocity Vac of 5750 m / s, the acoustic wavelength is 85 pm.

[0122] In a preferred configuration, the first acousto-optic modulator 30 can be configured to diffract the laser beam 11 according to an order of norm 1. Following this example, the first acousto-optic modulator 30 diffracts the laser beam 11 to the order +1 or -1 of the diffractive grating (i.e., first Bragg grating 35) obtained in the crystal of the first acousto-optic modulator 30. In particular, in the example illustrated in [Fig. 3], the first acousto-optic modulator 30 is arranged to diffract the laser beam 11 according to the order 1 of the diffractive grating (i.e., first Bragg grating 35) formed in the crystal of the first acousto-optic modulator 30.

[0123] Diffracting the laser beam 11 according to the +1 or -1 order of the Bragg grating 35 of the first acousto-optic modulator 30 makes it possible to obtain more efficient diffraction by the Bragg grating 35 of the first acousto-optic modulator compared to diffraction according to higher or lower orders. This notably improves the filtering properties of the first acousto-optic modulator 30 while limiting optical losses. In this case, the +1 or -1 order of the Bragg grating 35 of the first acousto-optic modulator 30 is arranged to efficiently recover at least 70%, preferably at least 80% or even 90%, of the power of the incident beam (here the laser beam 11).

[0124] Preferably, the propagation direction 12 of the laser beam 11 can arrive at the first acousto-optical modulator 30 inclined at an angle of incidence equal to a Bragg angle, denoted AOI1, and given by the first acousto-optical modulator 30. Thus, according to this embodiment, the angle of incidence (i.e., Bragg angle) denoted AOI1 is equal to the diffraction angle db

[0125] The Bragg angle is given by the following relation:

[0126]

[0127]

[0128]

[0129]

[0130]

[0131]

[0132] [Math.4] i sxv Foc AF«c AUli - ty — T — 2 A with i the index associated with the acousto-optic modulator i, ln corresponding to the wavelength of the laser beam propagating in the crystal of the acousto-optic modulator i (here the first acousto-optic modulator 30) and being a function of the wavelength 1 of the laser beam 11 propagating in the vacuum and of the refractive index n of the crystal of the acousto-optic modulator i and can be written as the ratio 4, L corresponding to the acoustic wavelength of the acousto-optic modulator i which is a function of the acoustic frequency Fac and the acoustic velocity Vac in the crystal of the acousto-optic modulator i. The angle of incidence has an effect on the diffraction efficiency. Indeed, the dependence on the diffraction efficiency is given by the following formula based on the intensity I of the beam diffracted by the acousto-optic modulator considered (here, the first acousto-optic modulator 30): [Math.5] with b corresponding to an angular deviation from the Bragg angle associated with the acousto-optic modulator considered, L corresponding to an interaction length between the optical and acoustic wave in the optical crystal (following a direction orthogonal to the direction of acoustic propagation of the modulator considered), Fac and Vac being respectively the acoustic frequency Fac and the acoustic velocity Vac in the crystal of the acousto-optic modulator, I the intensity of the diffracted beam being in this example the intensity of the primary diffracted beam 32. According to this arrangement, the laser beam 11 arriving at an angle of incidence equal to the Bragg angle AOIi of the first acousto-optic modulator 30 is diffracted more effectively by the first acousto-optic modulator 30 compared to a laser beam arriving at the first acousto-optic modulator 30 at a smaller, zero, or larger angle of incidence. In particular, the further the angle of incidence deviates from the Bragg angle associated with the first acousto-optic modulator 30, the less effective the diffraction performed by the first acousto-optic modulator 30. Thus, the first acousto-optic modulator 30 is arranged to achieve maximum diffraction efficiency when the laser beam 11 is inclined at an angle of incidence equal to the Bragg angle AOL of the first acousto-optic modulator 30. Thus, the first acousto-optic modulator 30, due to the dependence of the diffraction efficiency as described in the Math 5 formula, diffracted Gaussian beams better than "non-Gaussian beams", further improving the Quality of the diffracted primary beam 32. To this end, the portions of the laser beam 11 exhibiting high divergence (M2 > 2) are less diffracted by the first acousto-optic modulator 30 and undergo significant losses through filtering in the first acousto-optic modulator 30 (angular filtering). Typically, for an RF wave with an acoustic frequency of 68 MHz and a laser beam 11 with a diameter (waist) of 2.00 mm, the portions of the laser beam 11 exhibiting high divergence (M2 > 2) are diffracted with a diffraction efficiency of less than approximately 50%.

[0133] In [Fig. 4], it can be seen that light rays from the laser beam 11 are not deflected by the first acousto-optic modulator and remain in the YZ plane. It follows that portions of the laser beam 11 contained in a plane transverse to the first plane and collinear with the reflecting plane 36 of the Bragg grating 36 are transmitted by the first acousto-optic modulator 30 without being deflected. Thus, the first acousto-optic modulator 30 modifies the divergence of the laser beam 11 oriented in the first plane; spatial filtering is therefore performed on the portions of the laser beam contained within the first plane. It is also understood that angular filtering is only performed on the portions of the laser beam contained within the first plane.

[0134] It will be described how spatial filtering is achieved in the foreground using [Fig. 11] which illustrates the first acousto-optical modulator 30 in an XY view plane of the laser system.

[0135] To achieve spatial filtering, the first acousto-optic modulator 30 is arranged to generate a Bragg grating 35 having an active aperture 38 in the optical crystal of the first acousto-optic modulator 30, the active aperture being arranged in a plane transverse to the foreground to receive the laser beam 11. The active aperture 38 has a first dimension H1 oriented along the first acoustic propagation direction 31 and a second dimension H2 oriented along an axis perpendicular to the first acoustic propagation direction 31.

[0136] Preferably, the second dimension H2 of the active aperture 38 is modifiable according to the acoustic wave emitted by the antenna 34 of the first acousto-optic modulator 30 in order to perform spatial filtering along the principal axis positioned in the first plane. In the example illustrated in [Fig. 11], the laser beam 11 has a cross-section adapted to the active aperture 38 of the first acousto-optic modulator 30. Typically, the cross-section of the active aperture is 2 mm to 6 mm. The first dimension defines the number of first diffraction planes that can be considered. Each ray of the laser beam 11 arriving at the active aperture along the Y-axis is associated with a first plane.

[0137] Optionally, the laser system 100 includes a control circuit 70 for the main spatial filtering module 20. In this example, the control circuit 70 is connected to the first acousto-optical modulator 30. This control circuit 70 includes a clock 71 arranged to synchronize the first acousto-optical modulator 30 with the laser beam 11, specifically to synchronize the first acousto-optical modulator 30 to the pulses of the laser beam 11. For example, the clock of the control circuit 70 is generated by a primary synchronization module 72. In addition, the control circuit 70 may include a secondary synchronization module 73 used if the first acousto-optical modulator 30 performs time or amplitude modulation of the laser beam 11. This control circuit 70 also optionally includes control electronics 74 for the time or amplitude modulation of the laser beam 11. The control electronics 74 are arranged to vary the power of the acoustic wave between 0 and 100% of its maximum or nominal value.In addition, the control circuit 70 may include a control module 75 arranged to control and adjust the acoustic wave emitted by the first acousto-optic modulator 30, particularly when no time or amplitude modulation is performed.

[0138] A variant of the laser system 100 will now be described using [Fig. 13].

[0139] In this variant, the first acousto-optical modulator 30 generates a wave An acoustic signal that presents a time-domain signal with two different frequencies applied sequentially, for example, within the same time period Tp. (Here, a time period is understood to be a given or determined duration). In this example, two time periods Tp are illustrated.

[0140] Such an acoustic wave makes it possible to generate several acoustic frequencies from the same acousto-optic modulator during the time period Tp. Here the time signal sequentially presents two distinct (i.e., different) frequencies.

[0141] It can be seen that for each portion of the time signal having the same frequency, the time signal has an amplitude and a phase which does not vary over time.

[0142] In this case, for a given time period Tp, the time signal of the acoustic wave can have two secondary time periods Tsl, Ts2, for each secondary time period, the time signal of the acoustic wave has the same frequency.

[0143] Each frequency of a secondary time period Ts1, Ts2 allows obtaining a Bragg grating 35 with reflecting planes spaced at a proper distance dnl. It is understood that such an arrangement makes it possible to obtain a diffracted primary beam that can propagate in two different directions of propagation (since the direction of propagation of the diffracted primary beam is related to the diffraction of the laser beam on the reflecting planes formed in the first acousto-optical modulator 30).

[0144] In this example, the acoustic ground time signal at the first frequency is generated during a first secondary time period Tsl and the acoustic wave time signal at the second frequency is generated during a second time period Ts2, the time period Tp being a function of the combination of the first and second secondary time periods.

[0145] The first secondary time period Tsl and the second secondary time period Ts2 may be identical or different.

[0146] Here, for each time period Tp, the time signal of the acoustic wave presents: - a first frequency of 68MHz for a duration corresponding to the first secondary time period Tsl and - a second frequency of 100MHz for a duration corresponding to the second secondary time period Ts2.

[0147] The first secondary time period Tsl and / or the second secondary time period Ts2 is worth at least one quarter of the time period, preferably at least half of the time period.

[0148] In practice, the control module generates acoustic background for several time periods Tp, which allows for an acoustic wave with two alternating frequencies.

[0149] Preferably, the first and second secondary time periods Tsl, Ts2 last less than 5 seconds, preferably less than 2 seconds, which allows for rapid alternation of different propagation directions.

[0150] In practice, the frequency of the acoustic ground time signal is controlled by the control circuit 70 of the first acousto-optic modulator 30. Typically, the control circuit 70 is configured to control the acoustic ground frequency, including modifying it. For example, it can adjust the frequency as well as the emission time of the acoustic wave at this given frequency. Thus, here, the control circuit 70 controls the duration of the time period Tp, the duration of the first and second secondary time periods Ts1, Ts2, as well as the first and second frequencies of the acoustic ground time signal, and can also adjust the repetition of the time period Tp.

[0151] The advantage of this arrangement is that it allows for time-based control of the acoustic waves emitted by the acousto-optic modulator, thus enabling temporal control of the different propagation directions of the diffracted primary beam. Spatial filtering is then transformed into spatiotemporal filtering.

[0152] Of course, in one variant, for the repetition of time period Tp, the two acoustic frequencies generated in this time period Tp may be different from the first preceding time period Tp. It is also understood that the time periods Tp may be different, that is to say, correspond to different durations. The same applies to the secondary time periods Ts1 and Ts2.

[0153] Figures 5 and 6 illustrate a second embodiment of a laser system 200 according to this disclosure.

[0154] The laser system 200 illustrated in Figures 5 and 6 includes all the elements of the laser system 100 illustrated in Figures 3 and 4. Thus, only the differences with the laser system 100 illustrated in Figures 3 and 4 will be described.

[0155] In this example, the laser beam 11 has a quality factor MYa2 along its second principal axis Ya of 1.10. Therefore, the diffracted primary beam 32 has a quality factor M2Yb of the same order of magnitude along its second principal axis Yb.

[0156] The filtering device 20 of the laser system 200 illustrated in Figures 5 and 6 includes a second acousto-optical modulator 40 arranged to generate an acoustic wave along a second acoustic propagation direction 41 and to form a diffracted secondary beam 42 propagating along a propagation direction 43.

[0157] The second acousto-optic modulator 40 comprises a second optical crystal 49 and a second high-frequency or radio frequency (RF) antenna 44. In a known manner, the second antenna 44 is arranged to generate an acoustic wave in the second optical crystal 49 along a second acoustic propagation direction 41 defined by the normal to the second antenna 44 emitting the RF radiation. Following this example, the second acousto-optic modulator 40 comprises a second quartz optical crystal 49.

[0158] The second acousto-optic modulator 40 thus generates, via the second antenna, a second diffractive Bragg grating 45 in the second optical crystal 49. As before, the second Bragg grating 45 can be schematically represented by a series of parallel reflecting planes 46 arranged perpendicular to the second direction of acoustic propagation, i.e., to the normal to the second antenna 44 emitting the RF radiation. Each reflecting plane 46 of the second Bragg grating 45 is spaced from another reflecting plane 46 of the second Bragg grating 45 by a distance dn2 equal to the acoustic wavelength of the second acousto-optic modulator 40. The second Bragg grating 45 has an active aperture in the second optical crystal, the active aperture being arranged in a plane transverse to the second plane to receive the diffracted primary beam 32. Each reflecting plane 46 is formed in the second The second acousto-optic modulator 40 (here in its crystal) is shown when the acoustic wave is generated in the second acousto-optic modulator 40. The second antenna 44 of the second acousto-optic modulator 40 can be a high-frequency or radio frequency (RF) antenna. Thus, following this example, the second Bragg grating 45 of the second acousto-optic modulator 40 depends on the frequency of the acoustic wave generated by the second acousto-optic modulator 40.

[0159] The second acoustic modulator 40 is arranged to receive the diffracted primary beam 32 and form a diffracted secondary beam 42 propagating along a propagation direction 43. Similarly, the considered ray of the diffracted secondary beam is formed in a second plane, here YZ, defined by the propagation vector 43 of this light ray and the acoustic propagation direction 4L.

[0160] The second acousto-optic modulator 40 may be similar to or different from the first acousto-optic modulator 30. In particular, in one embodiment, the first Bragg grating 35 obtained in the first acousto-optic modulator 30 may be similar to the second Bragg grating 45 obtained in the second acousto-optic modulator 40. If the first Bragg grating 35 is similar to the second Bragg grating 45, then the Bragg angles AOI1 and AOI2 (corresponding to the Bragg angle of the second acousto-optic modulator 40) are equal.Combining the first Bragg grating 35 with an identical second Bragg grating 45 allows for a laser system 200 that is easier to implement and also less expensive. If the first Bragg grating 35 of the first acousto-optic modulator 30 differs from the second Bragg grating 45 of the second acousto-optic modulator 40, then the arrangement of the filter module 20 may be longer and more difficult to adjust, but it still allows for a filter module 20 that adapts to different implementation or overall assembly constraints of the laser system 300.

[0161] According to this embodiment, the second acoustic propagation direction 41 is orthogonal to the first acoustic propagation direction 31. Thus, the first plane, here XZ, is perpendicular to the second plane, here YZ. Such an arrangement makes it possible to reduce the divergence of the laser beam 11 oriented along its other principal axis (i.e., the second principal axis Ya).

[0162] In this example, the second acousto-optical modulator 40 is preferably positioned at the minimum cross-section (i.e. at the "waist" w0) of the diffracted primary beam 32.

[0163] Furthermore, the laser system 200 includes, in this example, another control circuit 70, identical to the control circuit 70 of the laser system 200. This other control circuit 70 is connected to the second acousto-optical modulator 40 in order to control it. The control module 75 of the other control circuit 70 is arranged to control and adjust the acoustic background emitted by the second acousto-optical modulator 40.

[0164] As with the first acousto-optic modulator 30, the propagation direction 33 of the diffracted primary beam 32 has a defined angle of incidence in the second plane. Here, in particular, the angle of incidence is defined as the angle (of one of the light rays of the diffracted primary beam 32) between an axis 47 of a reflecting plane 46 on which the light ray arrives and the propagation direction of the diffracted primary beam 32. Here, the axis 47 is orthogonal to the normal n2 of the reflecting plane 46 (considered), which is oriented along the acoustic propagation direction 41 of the second acousto-optic modulator 40. Advantageously, this angle of incidence is equal to the Bragg angle of the second acousto-optic modulator 40, denoted AOI2.

[0165] In this example, the other principal axis Yb of the diffracted primary beam 32 is positioned in the second plane. It is thus referred to hereafter as the degraded principal axis Yb of the diffracted primary beam 32.

[0166] The diffracted secondary beam 42 has two orthogonal principal axes, denoted respectively Xc for its first principal axis and Yc for its second principal axis. The two principal axes Xc, Yc of the diffracted secondary beam 42 are orthogonal to the propagation direction 43 of the diffracted secondary beam 42. Here, the principal axis Yc (called the principal diffraction axis of the diffracted secondary beam 42) is positioned in the second plane relative to the second acousto-optic modulator 40 and to the first diffracted beam incident on this second acousto-optic modulator 40.

[0167] According to this arrangement, the principal diffraction axis Yc of the diffracted secondary beam 42 has a quality factor M2Yc lower than the quality factor M2Yb along the degraded principal axis Yb of the diffracted primary beam 32 in the second plane. For example, the quality factor M2Yc is 1.00 while the quality factor M2Yb is 1.10. The second acousto-optic modulator 40 reduces the quality factor MYc2 oriented along its principal diffraction axis (second principal axis Yc of the diffracted secondary beam 42) by at least 3% compared to the quality factor MYb2 of the diffracted primary beam 32 oriented along its degraded principal axis (second principal axis Yb of the diffracted primary beam 32).

[0168] As before, in this example, the second acousto-optic modulator 40 is therefore arranged to spatially filter the diffracted primary beam 32 along the degraded principal axis Yb of the diffracted primary beam 32 positioned in the second plane. The spatial filtering is achieved via an active aperture as described above for the first acousto-optic modulator. Similarly, the filtering angular is performed on the portions of the diffracted primary beam 32 positioned in the second plane which exhibit a strong divergence.

[0169] As with the first acousto-optic modulator 30, the second acousto-optic modulator 40 is preferably arranged to diffract the primary diffracted beam 32 according to a lattice law function of the second Bragg grating 45 in order to obtain the secondary diffracted beam 42 deviated according to a diffraction angle, noted d2 on the [Fig.5], and determined by the formula Math 4 related to the second acousto-optic modulator 40.

[0170] Since the angle of incidence of the diffracted primary beam 32 contained in the second plane is equal to the Bragg angle AOI2 associated with the second acousto-optic modulator 40, the diffraction angle d2 under which the principal diffraction axis Yc of the diffracted secondary beam 42 is inclined is also equal to the Bragg angle AOI2.

[0171] Preferably, the second acousto-optic modulator 40 is arranged to diffract the diffracted primary beam 32 according to orders of magnitude 1. In the example illustrated in [Fig. 5], the second acousto-optic modulator 40 diffracts the diffracted primary beam 32 according to the first order of the second Bragg grating 45 formed in the crystal of the second acousto-optic modulator 40, thereby improving the filtering efficiency. Indeed, the filtering properties are improved with diffraction according to the first or -1 order of the grating of the crystal of the acousto-optic modulator considered.

[0172] Since the first acoustic propagation direction 31 and the second acoustic propagation direction 41 are orthogonal to each other, the components of the diffracted primary beam 32 positioned in the first plane are not spatially filtered by the second acousto-optic modulator. Consequently, the diffracted primary beam 32 has a component along its principal diffraction axis (first principal axis Xb) that is not deviated by the second acousto-optic modulator 40. The divergence angle Q of the diffracted primary beam 32 defined along its principal diffraction axis Xb is preserved (i.e., remains unchanged) by the second acousto-optic modulator 40.

[0173] Thus, in the example illustrated in figures 5 and 6, the filtering module 20 is arranged to first filter the laser beam 11 along its principal axis Xa and then along the principal axis Yb of the diffracted primary beam 32. Of course, in other embodiments, these types of filtering can be reversed.

[0174] Figure 7 illustrates a third embodiment of a 300 laser system according to the present disclosure.

[0175] The laser system 300 illustrated in [Fig. 7] comprises all the elements of the laser system 100 illustrated in Figures 5 and 6. Thus, only the differences with the laser system 100 illustrated in figures 5 and 6 will be described. Thus, in this example, the filtering module includes a first acousto-optical modulator 30 similar to that illustrated in figures 3 and 4 and a second acousto-optical modulator comprising the same elements as the second acousto-optical modulator described in figures 5 and 6 except that its orientation is different.

[0176] Indeed, in this embodiment, the first acoustic propagation direction 31 is parallel to the second acoustic propagation direction 41. Consequently, the first plane is parallel to the second plane.

[0177] Thus, according to this embodiment, the first principal axis Xb of the diffracted secondary beam 32 is positioned in the second plane. The first principal axis Xb of the diffracted secondary beam 32 therefore corresponds in this example to the degraded principal axis of the diffracted primary beam 32. Consequently, the first principal axis Xc of the diffracted secondary beam 42 is positioned in the second plane. In this way, the first modulator is arranged to spatially and angularly filter the components of the laser beam positioned in the first plane, and the second acousto-optic modulator is arranged to spatially and angularly filter the components of the diffracted primary beam 32 oriented in the first plane. Thus, filtering along the first principal axis Xa of the laser beam 11 is performed in two successive steps.

[0178] As a result, the laser system 300 makes it possible to obtain a quality factor MXc2 along the first principal axis Xc of the diffracted secondary beam 42 lower than the quality factor Mxb2 along the first principal axis Xb of the diffracted primary beam 32 since the first acousto-optic modulator 30 and the second acousto-optic modulator 40 are respectively arranged to decrease the divergence angle Q of the laser beam 11 along the first principal axis Xa and along the first principal axis Xb of the diffracted primary beam.

[0179] Of course, in a variant of the laser system 300, successive spatial (and angular) filtering can be carried out along the second principal axis Ya of the laser beam 11 by positioning the second principal axis Ya of the laser beam 11 in the first plane and by positioning the second principal axis Yb of the diffracted primary beam 32 in the second plane.

[0180] Figure 8 illustrates a fourth embodiment of a 400 laser system according to this disclosure.

[0181] The laser system 400 illustrated in [Fig.8] includes all the elements of the laser system 100 illustrated in Figures 3 and 4. Thus, only the differences with the laser system 100 illustrated in Figures 3 and 4 will be described.

[0182] The laser system 400 further includes a secondary spatial filtering module 50 positioned between the amplifying medium 10 and the main spatial filtering module 20.

[0183] By way of non-limitation, the secondary spatial filtering module 50 comprises at least one first slot 51 aligned with the first principal axis Xa or the second principal axis Ya of the laser beam 11. Such a secondary spatial filtering module 50 makes it possible to improve, upstream of the main spatial filtering module 20, the quality factor M2 along one of the principal axes Xa, Ya of the laser beam 11, while limiting power losses related to the main spatial filtering module 20. Indeed, the first and / or each second acousto-optical modulator of the main spatial filtering module 20 can induce a beam power loss of the order of 10 to 100 watts or at least 10% compared to the laser beam 11 entering the main spatial filtering module 20, whereas the secondary spatial filtering module 50 has a less significant impact on the transmitted power of the laser beam 11.The secondary spatial filter module 50 can cause optical losses (here on the amount of power of the laser beam 11 transmitted through the slit) which depend on the type of slit used, its position, and the spatial distribution of the quality factor M2. Typically, in the example considered, the secondary spatial filter module 50 can induce a loss of less than 9% of the power of the laser beam 11 entering the secondary spatial filter module 50. As an example, the secondary spatial filter module 50 illustrated in [Fig. 12] is identical to the spatial filter module described in the document "Innoslab amplifiers", IEEE Journal of selected topics in quantum electronics, vol. 21, No. 1, January / February 2015. Furthermore, in the example in [Fig. 8], the first slit 51 is aligned with the first principal axis Xa of the laser beam 11.

[0184] In another embodiment, the secondary spatial filtering module 50 comprises two slits 51, 52, referred to as the first slit 51 and the second slit 52, each of the two slits being aligned with one of the principal axes (i.e., the first principal axis Xa or the second principal axis Ya) of the laser beam 11. Such an arrangement makes it possible to improve the overall divergence of the laser beam 11, i.e., the divergence angle Q along the first principal axis Xa and along the second principal axis Ya upstream of the main spatial filtering module 20. Furthermore, such an arrangement makes it possible to further limit the power losses of the laser beam 11 because such a configuration can make it possible to avoid using a second acousto-optical modulator 40 in the main spatial filtering module 20.

[0185] Optionally, the laser system 400 further comprises an optical collimation system 60. The optical collimation system 60 is positioned upstream of said main spatial filtering module 20, in particular in the example considered between the secondary spatial filtering module 50 and the main spatial filtering module 20. The optical collimation system 60 is arranged to collimate said laser beam 11 at the inlet of said main spatial filtering module 20, enabling shaping The laser beam 11 before its entry into the first acousto-optic modulator 30. Thus, following this example, the divergence along its two principal axes Xa, Ya at the output of the collimation system will be less than the divergence along the two principal axes of the laser beam at the input of the collimation optical system 60 (i.e., the cross-section of the laser beam will therefore be reduced at the output of the collimation optical system 60). Thus, in this example, the main spatial filtering module 20 is positioned at the minimum cross-section of the laser beam 11 obtained after the collimation module 60, while the secondary spatial filtering module 50 is positioned at the minimum cross-section w0 (i.e., at the "waist") of the laser beam 11. For example, the collimation optical system 60 is a hybrid afocal system.The 60 collimation system can include two cylindrical telescopes or a spherical telescope combined with a cylindrical telescope.

[0186] Of course, the secondary spatial filtering module 50 and collimation system 60 are fully combinable with the other laser systems 100, 200, 300 described in this disclosure.

[0187] Process

[0188] Figure 9 illustrates a first embodiment of a spatial and angular filtering process 1000 according to the present disclosure.

[0189] Preferably, the laser systems 100, 200, 300, 400 are each arranged to implement the process 1000.

[0190] Following this example, the filtering process 1000 includes an emission step 1002 of the laser beam 11 by the amplifying medium 10 along the propagation direction 12 of the laser beam 11.

[0191] The laser beam 11 can be a degraded symmetric laser beam.

[0192] In the example illustrated in [Fig.9], the emitted laser beam 11 is an asymmetric laser beam 11 having a cross section 132 to the direction of propagation of the laser beam 11 and defined along the first principal axis Xa and the second principal axis Ya which are orthogonal to the direction of propagation 12 of the laser beam 11.

[0193] In this example, the cross-section 132 of the laser beam 11 comprises a main emission lobe 1la and two secondary emission lobes 1Ixa oriented along the first principal axis Xa of the laser beam 11. Each secondary emission lobe 1Ixa illustrates in this example a parasitic residue of the laser beam 11 signifying a quality factor MXa2 greater than 1.00 along the first principal axis Xa of the laser beam 11. The cross-section 132 of the laser beam 11 does not include, or only very weakly includes, a secondary emission lobe oriented on the second principal axis Yb of the laser beam 11.

[0194] The process 1000 comprises, after the emission step 1002, a filtering step 1004 of the laser beam by the filtering module 20. A first acousto-optic modulator 30 is arranged and configured to generate, via a first antenna, an acoustic wave following a first acoustic propagation direction 31 in a first optical crystal so as to induce a first Bragg grating 35. As before, the laser beam 11 arrives at the first Bragg grating 35 of the first acousto-optic modulator 30 with an incidence as described above. The main spatial filtering module 20 spatially and angularly filters the laser beam 11 emitted in the filtering step 1004 via the first acousto-optic modulator 30.The first acousto-optic modulator is arranged to receive the laser beam 11 on the first Bragg grating 35 and to form in the filtering stage 1004 the diffracted primary beam 32 in the first plane defined by the first acoustic propagation direction 31 and by the propagation direction 12 of the laser beam 11 incident on the first acousto-optic modulator. As before, this diffracted primary beam has a diffraction angle that can be equal to the angle of incidence.

[0195] The first principal axis of the laser beam 11 is positioned in the first plane. The degraded principal axis of the laser beam 11 therefore corresponds to the first principal axis Xa of the laser beam 11.

[0196] In this example, the principal axis of the diffracted primary beam 32 (i.e. principal axis is positioned in said first plane) corresponds to the first principal axis of the diffracted primary beam 32.

[0197] In this embodiment, the diffracted primary beam 32 has a cross-section 134 comprising a single principal emission lobe 32a. The diffracted primary beam 32 does not have any secondary lobe aligned along the first principal axis Xb of the diffracted primary beam 32. Thus, the diffracted primary beam 32 has a quality factor Mxb2 along the principal diffraction axis (its first principal axis Xb) that is lower than the quality factor MXa2 along the degraded principal axis (its first principal axis Xa) of the laser beam 11.

[0198] Furthermore, no secondary lobe appeared along the second principal axis Yb of the diffracted primary beam 32. Consequently, the first acousto-optic modulator 30 did not degrade the diffracted primary beam 32 along its second principal axis Yb. Thus, following the example of [Fig. 13], the quality factor M2 of the diffracted primary beam 32 along the first principal axis is closer to 1, which improves the symmetry of the diffracted primary beam 32. The diffracted primary beam 32 thus ideally presents a symmetrical cross-section 134. The process 1000 thus makes it possible to obtain a more symmetrical laser beam at the output of the main spatial filtering module 20, while limiting losses of laser beam power 11 which could be generated by the main spatial filtering module 20.

[0199] Optionally, the method 1000 may include a first preliminary measurement step 1006 prior to the filtering step 1004 configured to determine the first principal axis Xa and / or the second principal axis Ya of the laser beam 11. Such a step allows the first acousto-optic modulator 30 to be oriented with respect to one of the principal axes in order to perform filtering on that axis. The first preliminary step 1006 may be carried out by simulation or by measuring the cross-section of the laser beam 11 at different given positions along the propagation direction 12 of the laser beam 11.

[0200] Figure 10 illustrates a second embodiment of a filtering process 1100 according to this disclosure. The process 1100 is implemented by the laser system 200.

[0201] The 1100 filtering process illustrated in [Fig. 10] includes all the elements of the 1000 spatial filtering process illustrated in Figure 9. Thus, only the differences with the 1000 spatial filtering process illustrated in [Fig. 9] will be described.

[0202] Following this example, the laser beam 11 obtained by the emission step 1002 has a cross-section 132 with two secondary lobes 11a oriented along the second principal axis Ya of the laser beam 11 in addition to the two secondary lobes 11xa oriented along the first principal axis Xa. The laser beam 11 of the example in [Fig. 10] is therefore strongly divergent on its two principal axes Xa, Ya.

[0203] Following the filtering step 1004 of the laser beam 11 along its first principal axis Xa, the cross section 134 of the diffracted primary beam 32 illustrated in [Fig. 10] is improved along the first principal axis Xb but still includes two strong secondary lobes 32yb oriented along the second principal axis Yb of the diffracted primary beam 32. Thus, only the quality factor MXa2 oriented along the first principal axis Xa of the laser beam 11 has been improved.

[0204] The method 1100 then comprises a filtering step 1102 of the diffracted primary beam 32 via the second acousto-optic modulator 40. In this embodiment, the second acousto-optic modulator 40 is arranged and configured to generate, via a second antenna 44, an acoustic wave oriented along the second acoustic propagation direction 41 in a second optical crystal so as to induce a second Bragg grating 45. As before, the diffracted primary beam 32 arrives at the second Bragg grating 45 of the second acousto-optic modulator 40 with an incidence as described above. The second acousto-optic modulator 40 is arranged to receive the diffracted primary beam 32 on the second Bragg grating 45 and to form, in the filtering step 1102, a diffracted secondary beam 42 in the second plane defined by said grating second direction of acoustic propagation 41 and by the direction of propagation of the diffracted primary beam 32.

[0205] In this example, the second acoustic propagation direction 41 is perpendicular to the first acoustic propagation direction 31. The degraded axis of the degraded primary beam corresponds to the second principal axis Yb of the primary beam 32 and the principal diffraction axis of the diffracted secondary beam 42 therefore corresponds to the second principal axis Yc of the diffracted secondary beam 42.

[0206] Following this example, the diffracted secondary beam 42 has a cross-section 142 comprising a single main emission lobe 42a. The diffracted secondary beam 42 thus has no secondary lobe (or a very weak one compared to the main emission lobe 42a) aligned along the first principal axis Xc of the diffracted secondary beam 42, as well as no secondary lobe (or a very weak one compared to the main emission lobe 42a) aligned along the principal axis Yc.

[0207] Thus, the principal diffraction axis of the diffracted secondary beam 42 has a quality factor (here MYc2) lower than the quality factor (here MYb2) along the degraded principal axis of the diffracted primary beam 32. The diffracted secondary beam 42 therefore has, at the output of the second acousto-optic modulator 40, a more symmetrical cross-section 142. Following the example of [Fig. 10], the quality factor M2 of the diffracted secondary beam 42 along the first principal axis Xc and the second principal axis Yc are close to 1 and almost equal to within 10%, highlighting the very good quality and symmetry of the diffracted secondary beam 42. Furthermore, the diffracted secondary beam 42 has a power that is less than 20% lower than the power of the laser beam 11.

[0208] Optionally, the method 1100 may include a second preliminary measurement step 1104 prior to the filtering step 1102 configured to determine the first principal axis Xb and / or the second principal axis Yb of the diffracted primary beam 32. Such a step allows the second acousto-optical modulator 40 in the filtering module to be aligned to perform spatial filtering along the desired principal axis (here on the second principal axis Yb of the diffracted primary beam 32). The second preliminary step 1104 may be performed by simulation or by measuring the cross-section of the diffracted primary beam 32 at various given positions along the propagation direction 33 of the diffracted laser beam 32.

[0209] Variants

[0210] The present invention is in no way limited to the embodiments described and represented, but a person skilled in the art will be able to make any variation in accordance with the invention.

[0211] By way of non-limiting agreement, the first acousto-optic modulator 30 and / or the second acousto-optic modulator 40 comprise a quartz crystal, allowing Thus, the first and / or second acousto-optic modulators must be able to withstand high-power laser beams, particularly laser powers exceeding 10 watts, and preferably exceeding 100 watts. Acousto-optic modulators incorporating this type of crystal are widely used standard acousto-optic modulators. Quartz crystals are particularly well-suited for use in acousto-optic modulators when combined with high-power lasers (i.e., 10 W or higher) emitting a wavelength around 1 pm and doped with ytterbium (Yb) or neodymium (Nd), as quartz crystals (in their crystalline form) exhibit very good optical, thermal, and mechanical properties compared to other crystals found in acousto-optic modulators at this wavelength.Thus, using an acousto-optic modulator incorporating a quartz crystal is a very good price / performance compromise. In one variant, Yb or Nd doping can be combined with other matrices (CaF2, silica, vanadate-YVO4, etc.). For example, and not exhaustively, for a laser emitting a laser beam at a wavelength around del pm, the doping could be Yb:YAG, Nd:YAG, Yb:glass, Nd:glass, Nd:YVO4, etc. Of course, in other embodiments, the crystal of the first acousto-optic modulator 30 and / or the second acousto-optic modulator 40 can be made of TeO2 or fused silica (an amorphous form of quartz). Acousto-optic modulators made of fused silica or TeO2 are typically less expensive. However, they are less suitable for working with high laser powers (i.e. those greater than or equal to 10 W for fused silica crystals or less than 1 W for TeO2) because they are less thermally stable.Other materials may be more suitable for emerging lasers emitting in the mid-infrared range at 1.5 pm (Erbium (Er)-doped) or around 2 pm (Thulium (Tm) or Holmium (Ho)-doped). Thus, each acousto-optical modulator of the laser system described in this disclosure is chosen based on the laser system, specifically the wavelength emitted by the laser system and the power of the flux emitted by such a laser system.

[0212] By way of example, the radio frequency antenna of each acousto-optic modulator preferably emits a wave in the frequency range between 1 MHz and 10 GHz, thus allowing acousto-optic modulators to have extended diffractive properties and therefore be very modulable, thus allowing a wide choice of filtering parameters.

[0213] Preferably, the crystal of each acousto-optic modulator is chosen according to the acoustic wavelength range emitted by the radio frequency antenna of the acousto-optic modulator in question. For example, for acoustic waves For acoustic waves emitted between 100 MHz and 1000 MHz, the preferred crystal for the acousto-optic modulator is TeO2. For acoustic waves emitted between 1 MHz and 100 MHz, the preferred crystal for the acousto-optic modulator is quartz or fused silica.

[0214] According to this disclosure, the main spatial filtering module may comprise a plurality of second acousto-optic modulators arranged in series and oriented as in the example of device 200 or 300, each arranged to spatially and angularly filter the upstream (incident) beam. In this embodiment, each given second acousto-optic modulator 40 is arranged to spatially filter a principal axis of the diffracted laser beam incident on said second acousto-optic modulator, and the principal axis of the incident diffracted laser beam being positioned in the second plane associated with said given second acousto-optic modulator.The secondary beam diffracted by the second given acousto-optic modulator has in this case a quality factor along the principal diffraction axis of the diffracted secondary beam 42 of the second given acousto-optic modulator lower than the quality factor along the degraded principal axis of the diffracted laser beam incident on said second acousto-optic modulator.

[0215] Although in the present disclosure the first acousto-optical modulator 30 and the second acousto-optical modulator 40 are used to perform spatial filtering of the laser beam 11, these acousto-optical modulators can also be arranged to perform other properties known of acousto-optical modulators such as performing spatial and temporal modulation of the laser beam 11, or being used as an optical deflector.

Claims

1. Demands Laser system (100, 200, 300, 400) including: - an amplifying medium (10) arranged to emit a laser beam (11) along a propagation direction (12), said laser beam (11) having a cross-section defined along two principal axes orthogonal to the propagation direction (12) of said laser beam (11), said laser beam (11) having a quality factor greater than 1.00 along at least one of the two principal axes of the laser beam (11); and - a main spatial filtering module (20) comprising a first acousto-optical modulator (30) comprising a first optical crystal (39) and a first antenna (34) arranged to generate an acoustic wave in the first optical crystal (39) along a first acoustic propagation direction (31) and to induce a first Bragg grating (35) comprising at least one reflecting plane (36) in the first optical crystal (39), said at least one reflecting plane (36) being perpendicular to the first acoustic propagation direction (31), the first acousto-optical modulator (30) being arranged to receive the laser beam (11) on the first Bragg grating (35) and to form a diffracted primary beam (32) propagating along a propagation direction (33),said laser beam (11) being incident on the first optical crystal (39) in a first plane defined by the first acoustic propagation direction (31) and the propagation direction (12) of the laser beam (11), said propagation direction (12) of the laser beam (11) having a non-zero angle of incidence with respect to said at least one reflecting plane (36) of the first Bragg grating, and said diffracted primary beam (32) having two principal axes orthogonal to its propagation direction (33), one of which principal axis is in the first plane, one principal axis of the laser beam (11), called the degraded principal axis of the laser beam (11), among said two principal axes of the laser beam (11) having the quality factor greater than 1.00,is positioned in the foreground such that said diffracted primary beam (32) has a quality factor along the principal axis in the foreground that is lower than the quality factor of the laser beam (11) along the principal axis degraded in the foreground.

2. System (200) according to claim 1, wherein the main spatial filtering module (20) comprises a second acousto-optic modulator (40) comprising a second optical crystal (49) and a second antenna (44) arranged to generate an acoustic wave in the second optical crystal (49) along a second acoustic propagation direction (41) and to induce a second Bragg grating (45) comprising at least one reflecting plane (46) in the second optical crystal (49), said at least one reflecting plane (46) being perpendicular to the second acoustic propagation direction (41), the second acousto-optic modulator (40) being arranged to receive the diffracted primary beam (32) on the second Bragg grating (45) and to form a diffracted secondary beam (42) propagating along a propagation direction (43),the diffracted primary beam (32) being incident on the second optical crystal (49) in a second plane defined by the second acoustic propagation direction (41) and the propagation direction (33) of the diffracted primary beam (32), the propagation direction (33) of the diffracted primary beam (32) having a non-zero angle of incidence with respect to said at least one reflecting plane (46) of the second Bragg grating (45), and said diffracted secondary beam (42) having two principal axes orthogonal to its propagation direction (43), the diffracted primary beam (32) having a quality factor along at least one of its two principal axes greater than 1, and a principal axis of the diffracted primary beam (31), called the degraded principal axis of the diffracted primary beam (31), among said two principal axes of the diffracted primary beam (31) having the quality factor greater than 1,is positioned in the second plane such that the diffracted secondary beam (42) has a quality factor along the principal axis in the second plane that is lower than the quality factor of the diffracted primary beam (32) along the principal axis, which is degraded in the second plane.

3. System (200, 300, 400) according to any one of claims 1 or 2, wherein the first acoustic propagation direction (31) is parallel to the second acoustic propagation direction (41) or the first acoustic propagation direction (31) is orthogonal to the second acoustic propagation direction (41).

4. System (100, 200, 300, 400) according to any one of claims 1 to 3, wherein the first acousto-optic modulator (30) is arranged to reduce the quality factor along the principal diffraction axis of the diffracted primary beam (32) by at least 3 percent relative to the quality factor along the degraded axis of the laser beam (11) and / or, respectively, the second acousto-optic modulator (40) is arranged to reduce the quality factor along the principal diffraction axis of the diffracted secondary beam (42) by at least 3 percent relative to the quality factor along the degraded principal axis of the diffracted primary beam (32).

5. System (100, 200, 300, 400) according to any one of claims 1 to 4, wherein the first acousto-optic modulator (30) is arranged to diffract the laser beam (11) to a diffraction order of norm equal to 1 and / or, respectively, the second acousto-optic modulator (40) being arranged to diffract the diffracted primary beam (32) to a diffraction order of norm equal to 1.

6. System (100, 200, 300, 400) according to any one of the preceding claims, wherein the angle of incidence of the propagation direction (12) of the laser beam (11) is equal to a Bragg angle of the first acousto-optic modulator (30) and / or, respectively, the angle of incidence of the propagation direction (33) of the diffracted primary beam (32) is equal to a Bragg angle of the second acousto-optic modulator (40).

7. A system according to any one of the preceding claims taken in dependence on claim 2, wherein the main spatial filtering module (20) comprises a plurality of second acousto-optic modulators (40) arranged in series, each given second acousto-optic modulator (40) being arranged to spatially filter a principal axis of the diffracted laser beam incident on said second acousto-optic modulator (40), and the principal axis of the incident diffracted laser beam being positioned in the second plane associated with said given second acousto-optic modulator (40), the secondary beam diffracted by the given second acousto-optic modulator having a quality factor along the principal axis of the diffracted secondary beam (42) in the second plane associated with the second acousto-optic modulator. given optics lower than the quality factor along the principal axis of the diffracted laser beam incident on said second acousto-optic modulator in the second associated plane.

8. System (100, 200, 300, 400) according to the preceding claim, further comprising a control circuit (70) of said main spatial filtering module (20) comprising at least one clock (71) synchronized with the laser beam (11).

9. System (400) according to any one of the preceding claims, comprising at least one secondary spatial filtering module (50) of said laser beam (11) positioned upstream of said primary spatial filtering module (20), said secondary spatial filtering module comprising at least one slot orthogonal to at least one principal axis of the laser beam (11), said at least one slot being arranged to spatially filter said laser beam (11) along said at least one principal axis of the laser beam (11).

10. System (400) according to any one of the preceding claims, wherein the system (400) further comprises an optical collimation system (60) upstream of said main spatial filtering module (20) and the optical collimation system (60) having an optical axis aligned with the propagation direction (12) of the laser beam (11), said optical collimation system (60) being arranged to reduce the quality factor of the laser beam (11) along the principal axes of the laser beam (11) by reducing a section of said laser beam (11) transverse to the propagation direction (12) of the laser beam (11).

11. System (100, 200, 300, 400) according to any one of the preceding claims, wherein the laser beam (11) has a power greater than or equal to 10 watts, preferably between 100 watts and 1 kilowatt.

12. System (100, 200, 300, 400) according to any one of the preceding claims taken in dependence on claim 2, wherein Acoustic background generated by the first acousto-optic modulator and / or the second acousto-optic modulator presents(s) a time signal with a single acoustic frequency for a time period or presents(s) a time signal with at least two different acoustic frequencies applied sequentially.

13. System (100, 200, 300, 400) according to any one of the preceding claims, wherein the laser beam (11) is an asymmetric laser beam.

14. A method (1300) for spatially filtering a laser beam comprising the following steps: - emission (1002) by a laser system (100, 200, 300, 400) of a laser beam (11) along a propagation direction (12), said laser beam (11) having a cross-section defined along two principal axes orthogonal to the propagation direction (12) of said laser beam (11), said laser beam (11) having a quality factor greater than 1 along at least one of the two principal axes of the laser beam (11); and - generation of an acoustic wave along a first acoustic propagation direction (31) in a first optical crystal of a first acousto-optic modulator (30) so as to induce a first Bragg grating (35) comprising at least one reflecting plane (36) in the first optical crystal (39),said at least one reflecting plane (36) being perpendicular to the first acoustic propagation direction (31); - spatial filtering (1004) of the laser beam (11) by means of the first acousto-optic modulator (30), said laser beam (11) being incident on the first optical crystal (39) in a first plane defined by the first acoustic propagation direction (31) and the propagation direction (12) of the laser beam (11), said propagation direction (12) of the laser beam (11) having a non-zero angle of incidence with respect to said at least one reflecting plane (36) of the first Bragg grating (35) so as to form a diffracted primary beam (32) propagating along a propagation direction (33), the diffracted primary beam (32) having two principal axes orthogonal to its propagation direction (33), one of which is a principal axis in the first plane, and the other a principal axis of the laser beam (11), called the degraded principal axis of the laser beam (11),of the two principal axes of the laser beam (11) having a quality factor greater than 1, is positioned in the first plane such that the diffracted primary beam (32) has a quality factor along the principal axis in the first plane lower than the quality factor of the laser beam (11) along the principal axis degraded in the first plane.

15. Method (1100) according to the preceding claim, wherein said diffracted primary beam (32) has a quality factor along at least one of its principal axes greater than 1, said method (1100) further comprising a filtering step (1102) of the diffracted primary beam (32) by means of a second acousto-optic modulator (40), said second acousto-optic modulator (40) generating an acoustic wave along a second acoustic propagation direction (41) and forming a diffracted secondary beam (42) along a propagation direction (43) of the diffracted secondary beam (42), said diffracted secondary beam (42) being formed in a second plane defined by said second acoustic propagation direction (41) and the propagation direction (43) of the diffracted secondary beam (42),said propagation direction (33) of the diffracted primary beam (32) having an angle of incidence with respect to an axis (47) of a reflecting plane (46) generated in the second acousto-optic modulator (40), said axis (47) being perpendicular to a normal of the reflecting plane (46) oriented along the second acoustic propagation direction (41)„ a principal axis of the diffracted primary beam (31), called the degraded principal axis of the diffracted primary beam (31), among said two principal axes of the diffracted primary beam (31) having a quality factor greater than 1, is positioned in the second plane such that the diffracted secondary beam (42) has a quality factor in the second plane lower than said quality factor of the diffracted primary beam (32) along the degraded principal axis in the second plane.,

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