Laser system and method for filtering a laser beam

Abstract

The invention relates to a laser system (100) comprising: - an amplifying medium (10) arranged to emit a laser beam (11) in a propagation direction (12), which laser beam (11) is a laser beam (11) that has a quality factor greater than 1.00 along at least one main axis of the laser beam (11); and - a main spatial filtering module (20) comprising a first acousto-optic modulator (30) arranged to generate an acoustic wave in 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 main axes, including one main axis that is defined in said first plane.

Description

[0001] "Laser system and method for filtering a laser beam"

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] [1] The present invention relates generally to a laser system comprising an amplifying medium and a filtering module.

[0004] [2] It relates more specifically to spatial and angular filtering devices to improve the quality of a laser beam.

[0005] [3] The invention finds a particularly advantageous application in laser systems emitting degraded or asymmetric laser beams.

[0006] [4] It also relates to a method of filtering a laser beam.

[0007] STATE OF THE ART

[0008] [5] Laser systems are known that include a spatial filtering module in which the spatial filtering module comprises at least one slit aligned with a laser beam. Such systems allow for the filtering of unwanted parts of the laser beam, thereby addressing 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 unwanted spurious signals. 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 imply performance losses in spatial filtering.

[0009] [6] Laser systems using angular filtering with Bragg gratings are also known. These systems are functional but present several problems, particularly with tuning. The diffraction grating of the Bragg grating is fixed and therefore not easily flexible or adaptable to different optical setups. Furthermore, if different diffraction properties are desired, the Bragg modulus must be changed, which requires further adjustments to incorporate such a filtering module into the laser system. Such angular filtering modules are thus difficult to implement and offer limited flexibility. In addition, these filtering modules often use materials containing impurities, causing undesired and uncontrolled degradation of the laser beam at the output of these modules when subjected to high power levels, especially above 10 watts.Such angular filtering modules are therefore limited by their ability to withstand high powers 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 powers.

[0010] PRESENTATION OF THE INVENTION

[0011] [7] In order to overcome the aforementioned drawbacks of the prior art, the present invention proposes a laser system comprising:

[0012] - 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

[0013] - 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 including 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.

[0014] [8] According to the invention, spatial and angular filtering is performed on the laser beam. This filtering is performed by means of the first acousto-optical modulator oriented along a principal axis of this laser beam to perform the filtering along this principal axis.

[0015] [9] 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 that laser beam so as to bring the quality factor along that 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.

[0016]

[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.

[0017]

[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.

[0018]

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

[0019]

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

[0020]

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

[0021]

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

[0022]

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

[0023]

[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.

[0024]

[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 the passage or selection of a portion of the laser beam inclined along a given angle or direction.

[0025]

[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.

[0026]

[0020] In one embodiment, the quality factor of the laser beam along the principal axis degraded from 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,

[0027] 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,

[0028] 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

[0029] 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,

[0030] 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).

[0031]

[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.

[0032]

[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.

[0033]

[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 second plane defined by the second direction of acoustic propagation and the direction of propagation of the diffracted primary beam, the direction of propagation 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 direction of propagation, one 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 such that said diffracted secondary beam has a quality factor along the principal axis in the second plane that is lower than said quality factor of the primary beam diffracted along the principal axis, which is degraded in the second plane.

[0034]

[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 (including any value between 1.00, 1.01,

[0035] 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,

[0036] 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

[0037] 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,

[0038] 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).

[0039]

[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.

[0040]

[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.

[0041]

[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.

[0042]

[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.

[0043]

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

[0044]

[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.

[0045]

[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.

[0046]

[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.

[0047]

[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.

[0048]

[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.

[0049]

[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 given second acousto-optical modulator having a quality factor along the principal axis of the diffracted secondary beam in the second plane associated with the given second acousto-optical modulator lower than the quality factor along the degraded principal axis of the diffracted laser beam incident on said second acousto-optical modulator in the associated second plane.

[0050]

[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.

[0051]

[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.

[0052]

[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.

[0053]

[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.

[0054]

[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.

[0055]

[0041] Preferably, each acousto-optic modulator comprises an optical crystal formed in at least one of the following materials:

[0056] - quartz in crystalline form;

[0057] - tellurium dioxide (TeOz);

[0058] - quartz in amorphous form (i.e. fused silica).

[0059]

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

[0060]

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

[0061]

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

[0062]

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

[0063]

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

[0064]

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

[0065]

[0048] The invention also relates to a filtering process comprising the following steps:

[0066] - 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

[0067] - 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 principal axis being in the first plane, and 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 such that said diffracted primary beam has a quality factor along the principal axis in the first plane that is lower than the quality factor of the laser beam along the degraded principal axis in the first plane.

[0068]

[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.

[0069]

[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.

[0070]

[0051] Of course, the different features, variants and embodiments of the invention can be combined with each other in various ways insofar as they are not incompatible or mutually exclusive.

[0071] DETAILED DESCRIPTION OF THE INVENTION

[0072]

[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.

[0073]

[0053] On the attached drawings:

[0074]

[0054] Figure 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;

[0075]

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

[0076]

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

[0077]

[0057] Figure 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 Figure 3;

[0078]

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

[0079]

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

[0080]

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

[0081]

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

[0082]

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

[0083]

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

[0084]

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

[0085]

[0065] Figure 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;

[0086]

[0066] Figure 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.

[0087]

[0067] Definitions.

[0088]

[0068] The quality of a laser beam is often described by its beam propagation factor, also called the laser beam quality factor, and denoted M 2 .

[0089]

[0069] This quality factor M 2 is expressed according to the following formula 1:

[0090]

[0070] [Math 1]

[0091] M 2 = - a

[0092]

[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 zo along an optical axis Z along which the laser beam propagates (also called direction of propagation of the laser beam) and ® corresponding to the actual divergence angle of a laser beam 11 for the same diameter wo (see figure 1) at the same given position zo along the optical axis Z.

[0093]

[0072] More generally, the quality factor M 2 is defined according to ISO 11146-1 and the following formula:

[0094]

[0073] [Math 2]

[0095] , 2 Â 0 = M 2 - n:w0

[0096]

[0074] With X corresponding to the wavelength of the laser beam, M 2The quality factor, w0, is the minimum cross-section of the laser beam at a given position z0. According to the Math 2 formula, the divergence of a laser beam is a function of its quality factor M. 2 , its minimum cross-section w0 and its wavelength X. Thus, according to this formula Math 2, the quality factor is proportional to the actual divergence angle ® of the laser beam. Consequently, the higher a laser beam's divergence angle ® compared to the divergence angle α of a "perfect" Gaussian beam (with the same minimum cross-section), the higher the quality factor M 2 of this laser beam will be raised. In the following, the minimum cross-section of the laser beam is obtained in a plane in which the luminous intensity of the laser beam is greater than or equal to half the maximum luminous intensity of the laser beam, or to the maximum luminous intensity of the laser beam divided by e 2 .

[0097]

[0075] A "perfect" or "ideal" laser beam has a transverse intensity distribution with a Gaussian profile. A "perfect" or "ideal" laser beam is considered to have a quality factor M 2 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 M 2 The intensity of this beam is high. Thus, the quality of a laser beam can be assessed or described by the quality factor M. 2 of this laser beam.

[0098]

[0076] The quality factor M 2 is related to the actual divergence of the laser beam relative to the divergence of an ideal laser beam (i.e., M 2 = 1).

[0099]

[0077] Generally, a laser beam is considered symmetrical when it exhibits a similar divergence along its two principal axes Xi, Y. However, this divergence can be greater than that of an ideal Gaussian beam, that is, a Gaussian beam having a quality factor M 2 equal to 1.00.

[0100]

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

[0101]

[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 asymmetric shape with respect to the propagation axis. These beams are called asymmetric laser beams. Asymmetric laser beams include elliptical and astigmatic laser beams. Generally, they are defined by ISO 11146-2.

[0102]

[0080] An asymmetric laser beam is evaluated along two axes, called principal axes, which are typically named Xi and Y, for beam propagation along the Z-axis (see Figure 2) (also called the optical Z-axis), with i corresponding to the index of the system under consideration, in an orthonormal XYZ coordinate system. In the present disclosure, the principal axes delimit a plane in which the laser beam exhibits maximum intensity. For asymmetric laser beams, the parameters of the Math 2 formula, such as the divergence angle θ, and the quality factor M 2 and the minimum section w0 are then evaluated along the two principal axes and Y of the asymmetric laser beam in order to describe the propagation of an asymmetric beam along the two principal axes and Y.

[0103]

[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 X, and a second principal axis, denoted Yi. The first principal axis X and the second principal axis Y 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.

[0104]

[0082] Thus, in the present disclosure, the principal axes correspond to two orthogonal axes, each oriented perpendicular to the direction of propagation of the laser beam. For a laser beam having a minimal Gaussian symmetric cross-section (at minimum diameter w0), the principal axes correspond to two orthogonal axes oriented perpendicular to the direction of propagation of the laser beam, and along which the widths of the laser beam defined along these principal axes can be equal but minimal. It is thus understood that the dimensions (i.e., width) of the laser beam oriented along the principal axes Xi, Yi are minimal compared to other cross-sections of the laser beam obtained at other positions.For a symmetrical laser with a top hat shape (square or rectangular, also known as a "top hat"), the principal axes Xi and Yi, at their minimum cross-section, also define one of the minimum widths of the laser beam. For the square case, the width of the laser beam at its minimum cross-section is identical along both principal axes Xi and Yi. The principal axes Xi and Yi are the two orthogonal axes passing through the center of the square and parallel to its sides (e.g., each principal axis is parallel to two sides of the square). They define the minimum width of the laser beam at its minimum cross-section. For the rectangular case, the principal axes are the two orthogonal axes that pass through the center of the rectangle and are parallel to its sides.At the minimum cross-section, one of these principal axes defines the minimum beam width (at the waist, the shortest dimension of the rectangle), and the other principal axis defines the minimum beam length (at the waist, the longest dimension of the rectangle). In the case of an asymmetric beam, they each represent a direction along which the diameter (or width) of the laser beam defined along that axis is maximum or minimum. Thus, in the case of an elliptical beam, one of the principal axes of such a beam defines, at the minimum cross-section, the axis along which the diameter of the laser beam is maximum, and the other principal axis defines the axis along which the diameter of the laser beam is minimum.

[0105]

[0083] In the following, the laser beam can be a symmetrical but degraded laser beam or an asymmetrical laser beam as described above.

[0106]

[0084] Device

[0107]

[0085] Figures 3 and 4 illustrate a first embodiment of a laser system 100 according to the present disclosure.

[0108]

[0086] In Figure 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, Figure 4 illustrates the laser system 100 in a second transverse plane (YZ).

[0109]

[0087] 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 of 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.

[0110]

[0088] 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" wo) of the laser beam 11. The first acousto-optical modulator 30 is electronically controlled by a control circuit 70.

[0111]

[0089] In one embodiment, the amplifying medium 10 is a solid amplifying medium 10.

[0090] 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.

[0112]

[0091] 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.

[0113]

[0092] 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.

[0114]

[0093] 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.

[0115]

[0094] The amplifying medium 10 emits the laser beam 11 along a propagation direction 12.

[0116]

[0095] 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 this example, the laser beam 11 emitted by the amplifying medium 10 is 2.00 mm in diameter.

[0117]

[0096] 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).

[0118]

[0097] As explained above, the laser beam 11 illustrated in Figures 3 and 4 is an asymmetric laser beam 11 as illustrated in Figure 2.

[0119]

[0098] 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.

[0099] 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. By way of example, the quality factor Mxa 2 along the principal axis Xa is 1.50 while the quality factor My a 2 is, for example, 1.10. Thus, in this case, the quality factor Mxa 2(here called the degraded main axis of the laser beam 11) oriented along the main axis XA differs here from 20% to 30% (here from 26.7%) of the quality factor M Ya 2 oriented along the other principal axis (here Ya) of the laser beam 11. Here, the principal axis of the laser beam 11 exhibiting the quality factor M is called the degraded principal axis of the laser beam. 2 the largest. As will be explained later, this degraded main 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.

[0120]

[0100] 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.

[0121]

[0101] 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.

[0122]

[0102] 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 acoustic propagation direction, i.e., to the normal to the first antenna 34 emitting the RF radiation.

[0123]

[0103] As illustrated in Figure 12, the acoustic wave generated by the first acousto-optic modulator 30 presents a single-frequency time signal over a given duration or time period Tp (corresponding to the emission time of the acoustic wave). Here, the time signal of the acoustic wave is constant in amplitude and phase.

[0124]

[0104] Following this example, the first acousto-optic modulator 30 comprises a first optical crystal 39 made of quartz. 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 here 4 mm. Thus, the size of the crystal of the first acousto-optic modulator 30 is adapted to the diameter of the laser beam 11.

[0125]

[0105] 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 the acoustic wave 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 dn1 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 the acoustic wave 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 frequency of the acoustic wave generated by the first acousto-optic modulator 30. The emission of the acoustic wave can be adjusted manually or automatically by a control circuit.

[0126]

[0106] Each reflecting plane 36 is formed in the first acousto-optic modulator 30 (here in its crystal) when the acoustic wave is generated in the first acousto-optic modulator 30.

[0127]

[0107] 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 n1 to a reflector 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. In particular, here, the angle of incidence is defined as the angle (of one of the light rays of the laser beam 11) between a reflector 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 n1 of the reflecting plane 36 (considered) is oriented along the acoustic propagation direction 31 of the first acousto-optic modulator 30.

[0128]

[0108] 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.

[0109] 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.

[0129]

[0110] 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.

[0130]

[0111] 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 81 in the first plane. The diffraction angle 61 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.

[0131]

[0112] 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.

[0132]

[0113] 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.

[0133]

[0114] 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.

[0134]

[0115] 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.

[0135]

[0116] In the example illustrated in Figures 3 and 4, the diffracted primary beam 32 exhibits a quality factor 2 xb along the principal diffraction axis (here, the first principal axis Xb of the diffracted primary beam 32) equals 1.21, then the quality factor M 2 x a along the principal degraded axis (here, the first principal axis Xa of the laser beam 11) is 1.50. Therefore, the quality factor M 2 xb 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.

[0136]

[0117] 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 angle 0 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 0 of the laser beam 11 oriented along the degraded principal axis of the laser beam 11 positioned in the foreground.

[0137]

[0118] Following this example, the first acousto-optical modulator 30 makes it possible to reduce the quality factor Mxb 2 oriented along the principal diffraction axis Xb by at least 3%, in particular at least 15% in this example, compared to the quality factor Mxa 2 of the laser beam 11 oriented along the degraded main axis Xa.

[0138]

[0119] 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 51 in Figure 3 and determined by the following formula:

[0139]

[0120] [Math 3]

[0140]

[0121] with St 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), X the wavelength of the laser beam 11 and A 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 A = — .

[0141]

[0122] 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.

[0142]

[0123] 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 Figure 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.

[0143]

[0124] 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).

[0144]

[0125] Preferably, the propagation direction 12 of the laser beam 11 can arrive at the first acousto-optical modulator 30 inclined at the 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 ôi.

[0145]

[0126] The Bragg angle is given by the following relation:

[0146]

[0127] [Math 4]

[0147]

[0128] with i the index associated with the acousto-optic modulator i, Â ncorresponding 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 A 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 -, A 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.

[0148]

[0129] 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):

[0149]

[0130] [Math 5]

[0150]

[0131] with p 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.

[0151]

[0132] According to this arrangement, the laser beam 11 arriving at an angle of incidence equal to the Bragg angle AON 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 AOH of the first acousto-optic modulator 30.

[0152]

[0133] Thus, the first acousto-optical modulator 30, due to the dependence of the diffraction efficiency as described in formula Math 5, 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 a strong divergence (M 2 > 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 strong divergence (M 2 > 2) are diffracted with a diffraction efficiency of less than about 50%.

[0153]

[0134] In Figure 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.

[0154]

[0135] It will be described how spatial filtering is carried out in the first plane using Figure 11 which illustrates the first acousto-optical modulator 30 in an XY view plane of the laser system.

[0155]

[0136] 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.

[0156]

[0137] 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 Figure 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.

[0157]

[0138] 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-optic modulator 30. This control circuit 70 includes a clock 71 arranged to synchronize the first acousto-optic modulator 30 with the laser beam 11, in particular to synchronize the first acousto-optic modulator 30 to the pulses of the laser beam 11. By way of example, the clock of the control circuit 70 is generated by a main synchronization module 72. In addition, the control circuit 70 may include a secondary synchronization module 73 used if the first acousto-optic modulator 30 performs time or amplitude modulation of the laser beam 11. This control circuit 70 also optionally includes control electronics 74 for 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. Furthermore, 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.

[0158]

[0139] A variant of the laser system 100 will now be described using Figure 13.

[0159]

[0140] In this variant, the first acousto-optic modulator 30 generates an acoustic wave that presents a time-domain signal with two different frequencies applied sequentially, for example within the same time period Tp. (Here, by time period, we mean a given or determined duration). In this example, two time periods Tp are illustrated.

[0160]

[0141] 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.

[0161]

[0142] 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.

[0162]

[0143] In this case, for a given time period Tp, the time signal of the acoustic wave can have two secondary time periods Ts1, Ts2, for each secondary time period, the time signal of the acoustic wave has the same frequency.

[0163]

[0144] Each frequency of a secondary time period Ts1, Ts2 allows obtaining a Bragg grating 35 with reflecting planes spaced by a proper distance dn1. 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-optic modulator 30).

[0164]

[0145] In this example, the time signal of the acoustic wave at the first frequency is generated during a first secondary time period Ts1 and the time signal of the acoustic wave 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.

[0165]

[0146] The first secondary time period Ts1 and the second secondary time period Ts2 may be identical or different.

[0166]

[0147] Here, for each time period Tp, the time signal of the acoustic wave presents:

[0167] - a first frequency of 68MHz for a duration corresponding to the first secondary time period Ts1 and

[0168] - a second frequency of 100MHz for a duration corresponding to the second secondary time period Ts2.

[0169]

[0148] The first secondary time period Ts1 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.

[0170]

[0149] In practice, the control module generates the acoustic wave for several time periods Tp, which allows for an acoustic wave with two alternating frequencies.

[0171]

[0150] Preferably, the first and second secondary time periods Ts1, Ts2 last less than 5 seconds, preferably less than 2 seconds, which allows for rapid alternation of different propagation directions.

[0172]

[0151] In practice, the frequency of the acoustic wave's time-domain 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 frequency of the acoustic wave, including modifying it. For example, it can adjust the frequency as well as the emission time of the acoustic wave at that 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 wave's time-domain signal, and can also adjust the repetition of the time period Tp.

[0173]

[0152] 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.

[0174]

[0153] 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.

[0175]

[0154] Figures 5 and 6 illustrate a second embodiment of a laser system 200 according to this disclosure.

[0176]

[0155] 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.

[0177]

[0156] In this example, the laser beam 11 has a quality factor M Ya 2 along its second principal axis Ya of 1.10. Therefore, the diffracted primary beam 32 has a quality factor M 2 Y b of the same order of magnitude along its second principal axis Yb.

[0178]

[0157] 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.

[0179]

[0158] 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.

[0180]

[0159] 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 acousto-optic modulator 40 (here in its crystal) 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.

[0181]

[0160] 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 41.

[0182]

[0161] 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 AOL and AOH (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 makes it possible to propose a laser system 200 that is easier to implement and also less expensive.In the case where the first Bragg network 35 of the first acousto-optic modulator 30 is different from the second Bragg network 45 of the second acousto-optic modulator 40, then the arrangement of the filtering module 20 may be longer and more difficult to adjust but nevertheless allows to propose a filtering module 20 which adapts to different implementation constraints or overall assembly of the laser system 300.

[0183]

[0162] 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).

[0184]

[0163] In this example, the second acousto-optical modulator 40 is preferably positioned at the minimum cross-section (i.e. at the "waist" wo) of the diffracted primary beam 32.

[0185]

[0164] 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-optic modulator 40 in order to control it. The control module 75 of the other control circuit 70 is arranged to control and regulate the acoustic wave emitted by the second acousto-optic modulator 40.

[0186]

[0165] 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.

[0187]

[0166] 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.

[0188]

[0167] 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.

[0189]

[0168] According to this arrangement, the principal diffraction axis Yc of the diffracted secondary beam 42 has a quality factor M2 Yc lower audit quality factor M 2 Y b along the principal degraded axis Yb of said diffracted primary beam 32 in the second plane. For example, the quality factor M 2 Yc is 1.00 while the quality factor M 2 Yb is 1.10. The second acousto-optic modulator 40 reduces the quality factor M Yc 2 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 M Y b 2 of the diffracted primary beam 32 oriented along its degraded principal axis (second principal axis Yb of the diffracted primary beam 32).

[0190]

[0169] As before, in this example, the second acousto-optical 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. Spatial filtering is achieved via an active aperture as described above for the first acousto-optical modulator. Similarly, angular filtering is performed on the portions of the diffracted primary beam 32 positioned in the second plane that exhibit strong divergence.

[0191]

[0170] As with the first acousto-optic modulator 30, the second acousto-optic modulator 40 is preferably arranged to diffract the diffracted primary beam 32 according to a grating law function of the second Bragg grating 45 in order to obtain the diffracted secondary beam 42 deviated at a diffraction angle, denoted ô2 in Figure 5, and determined by the formula Math 4 related to the second acousto-optic modulator 40.

[0171] 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 S2 under which the principal diffraction axis Yc of the diffracted secondary beam 42 is inclined is also equal to the Bragg angle AOI2.

[0192]

[0172] 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 Figure 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.

[0193]

[0173] 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. Therefore, 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 θ 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.

[0194]

[0174] 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.

[0195]

[0175] Figure 7 illustrates a third embodiment of a laser system 300 according to this disclosure.

[0196]

[0176] The laser system 300 illustrated in Figure 7 includes all the elements of the laser system 100 illustrated in Figures 5 and 6. Therefore, only the differences with the laser system 100 illustrated in Figures 5 and 6 will be described. Thus, in this example, the filtering module comprises 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.

[0197]

[0177] 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.

[0198]

[0178] 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.

[0199]

[0179] As a result, the laser system 300 makes it possible to obtain a quality factor MXc 2 along the first principal axis Xc of the diffracted secondary beam 42 lower than the quality factor M X b 2 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 © of the laser beam 11 along the first principal axis Xa and along the first principal axis Xb of the diffracted primary beam.

[0200]

[0180] Of course, in a variant of the laser system 300, the 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.

[0201]

[0181] Figure 8 illustrates a fourth embodiment of a 400 laser system according to this disclosure.

[0202]

[0182] The laser system 400 illustrated in Figure 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.

[0203]

[0183] 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.

[0204]

[0184] 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 M 2along 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 filtering 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 M 2Typically, 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 Figure 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 Figure 8, the first slit 51 is aligned with the first principal axis Xa of the laser beam 11.

[0205]

[0185] 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 @ 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.

[0206]

[0186] 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, making it possible to shape the laser beam 11 before its entry into the first acousto-optical modulator 30. Thus, according to 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 inlet of the optical collimation system 60 (i.e. the cross-section of the laser beam will therefore be reduced at the output of the optical collimation system 60).Thus, in this example, the primary spatial filter module 20 is positioned at the minimum cross-section of the laser beam 11 obtained after the collimation module 60, while the secondary spatial filter module 50 is positioned at the minimum cross-section wo (at the "waist") of the laser beam 11. For example, the collimation optical system 60 is a hybrid afocal system. The collimation system 60 can comprise two cylindrical telescopes or a spherical telescope combined with a cylindrical telescope.

[0207]

[0187] 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.

[0208]

[0188] Process

[0209]

[0189] Figure 9 illustrates a first embodiment of a spatial and angular filtering method 1000 according to this disclosure.

[0210]

[0190] Preferably, the laser systems 100, 200, 300, 400 are each arranged to implement the process 1000.

[0211]

[0191] 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.

[0212]

[0192] The laser beam 11 can be a degraded symmetric laser beam.

[0213]

[0193] In the example illustrated in Figure 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.

[0214]

[0194] In this example, the cross-section 132 of the laser beam 11 comprises a main emission lobe 11a and two secondary emission lobes 11xa oriented along the first principal axis Xa of the laser beam 11. Each secondary emission lobe 11xa illustrates in this example a parasitic residue of the laser beam 11 signifying a quality factor Mx a 2 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.

[0215]

[0195] 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.

[0216]

[0196] 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.

[0217]

[0197] 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.

[0218]

[0198] 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 has no 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 M X b 2 along the principal diffraction axis (its first principal axis Xb) less than the quality factor M Xa 2 along the degraded main axis (its first main axis Xa) of the laser beam 11.

[0219]

[0199] 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 in Figure 13, the quality factor M 2The 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 therefore makes it possible to obtain a more symmetrical laser beam at the output of the main spatial filtering module 20, while limiting the power losses of the laser beam 11 that could be caused by the main spatial filtering module 20.

[0220]

[0200] 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.

[0221]

[0201] 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.

[0222]

[0202] The 1100 filtering process illustrated in Figure 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 Figure 9 will be described.

[0223]

[0203] Following this example, the laser beam 11 obtained by the emission step 1002 has a cross-section 132 with two secondary lobes 11ya 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 Figure 10 is therefore strongly divergent on its two principal axes Xa, Ya.

[0224]

[0204] 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 Figure 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 M Xa 2 oriented along the first principal axis Xa of the laser beam 11 has been improved.

[0225]

[0205] The process 1100 then includes 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-optical modulator 40 is arranged to receive the diffracted primary beam 32 on the second Bragg grating 45 and to form in the filtering stage 1102 a diffracted secondary beam 42 in the second plane defined by said second acoustic propagation direction 41 and by the propagation direction of the diffracted primary beam 32.

[0226]

[0206] 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.

[0227]

[0207] 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.

[0228]

[0208] Thus, the principal diffraction axis of the diffracted secondary beam 42 has a quality factor (here MYC 2 ) lower than the quality factor (here Myb 2 ) along the degraded principal axis of said diffracted primary beam 32. The diffracted secondary beam 42 thus presents, at the output of the second acousto-optic modulator 40, a more symmetrical cross-section 142. Following the example in Figure 10, the quality factor M2 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 a very good quality and good symmetry of the diffracted secondary beam 42. In addition, the diffracted secondary beam 42 has a power less than 20% lower than the power of the laser beam 11.

[0229]

[0209] 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.

[0230]

[0210] Variants

[0231]

[0211] 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.

[0232]

[0212] By way of non-limitation, the first acousto-optic modulator 30 and / or the second acousto-optic modulator 40 comprise a quartz crystal, thereby enabling the first and / or second acousto-optic modulator to withstand high-power laser beams, in particular laser powers exceeding 10 watts, preferably exceeding 100 watts. In particular, acousto-optic modulators comprising this type of crystal are widely used standard acousto-optic modulators. The quartz crystal is particularly used in acousto-optic modulators when combined with high-power lasers (i.e.(with a power output greater than or equal to 10 W) emitting a wavelength around 1 pm and doped with ytterbium (Yb) or neodymium (Nd), since the quartz crystal (crystalline form) exhibits very good optical, thermal, and mechanical properties compared to other crystals used 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 a variation, Yb or Nd doping can be combined with other matrices (CaF2, silica, vanadate-YVO4, etc.). For example, but not limited to, for a laser emitting a laser beam at a wavelength around 1 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 of the second acousto-optic modulator 40 can be made of TeO2 or fused silica (amorphous form of quartz).Acousto-optical modulators made of fused silica or TeO2 are typically less expensive. However, they are less suitable for use 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 in the laser system described in this disclosure is selected based on the laser system, specifically the wavelength emitted by the laser system and the power of the flux emitted by that laser system.

[0233]

[0213] 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 be obtained with extended diffractive properties and therefore very modulable, thus allowing a wide choice of filtering parameters.

[0234] [2

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

[0235]

[0215] According to this disclosure, the main spatial filtering module may comprise a plurality of second acousto-optical modulators arranged in series and oriented as in the example of device 200 or 300, each being arranged to spatially and angularly filter the upstream (incident) beam. In this embodiment, each given second acousto-optical modulator 40 is 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 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.

[0236]

[0216] Although in the present disclosure the first acousto-optic modulator 30 and the second acousto-optic modulator 40 are used to perform spatial filtering of the laser beam 11, these acousto-optic modulators can also be arranged to perform other properties known of acousto-optic modulators such as performing spatial and temporal modulation of the laser beam 11, or being used as an optical deflector.

[0237]

[0217] Although not illustrated, in the case of a symmetrical laser beam, for example of the "top hat" type, the graduated principal axis of the laser beam corresponds to the axis defining the minimum width of the laser beam at the minimum cross-section or the axis defining the minimum length (of the rectangle at the minimum cross-section). This principal axis is (approximately) parallel to two parallel edges of the minimum cross-section of the laser beam. It is therefore understood that the other principal axis of the laser beam (other than the one named graduated) defines either a minimum width equal to the minimum width defined along the graduated principal axis or a different width from that defined along the graduated principal axis, this different width being able to be less if the graduated principal axis defines the maximum length or greater if the graduated principal axis defines the minimum width of the laser beam.

Claims

DEMANDS 1. Laser system (100, 200, 300, 400) comprising: - 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) including 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) including 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 a 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 lower than the quality factor of the diffracted primary beam (32) along the degraded principal axis 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 claims 1 to 5, in which 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 claims 1 to 6 taken in dependence on claim 2, wherein the main spatial filtering module (20) comprises a plurality of second acousto-optical modulators (40) arranged in series, each given second acousto-optical modulator (40) being arranged to spatially filter a principal axis of the diffracted laser beam incident on said second acousto-optical modulator (40) and the principal axis of the incident diffracted laser beam being positioned in the second plane associated with said given second acousto-optical modulator (40),the secondary beam diffracted by the second given 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 given acousto-optic modulator 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 claim 7, 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 claims 1 to 8, 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 claims 1 to 9, 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 claims 1 to 10, 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 claims 1 to 11 taken in dependence on claim 2, in which 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 with at least two different acoustic frequencies applied sequentially.

13. System (100, 200, 300, 400) according to any one of claims 1 to 12, wherein the laser beam (11) is an asymmetric laser beam.

14. Method (1300) for spatial filtering of 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 in the first plane, one principal axis of the laser beam (11), called the degraded principal axis of the laser beam (11), of said two principal axes of the laser beam (11) having a quality factor greater than 1, is positioned in the first plane such that said diffracted primary beam (32) has a quality factor along the principal axis in the first plane less than laser beam quality factor (11) along the principal axis degraded in the foreground.

15. A method (1100) according to claim 14, 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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