System and method for high energy laser pulse wavefront correction by Brillouin mirror

Abstract

The present invention relates to a laser system (100) comprising a double-pass optical amplifier system (4). According to the invention, the laser system (100) comprises a laser source (1) and a time-shaping device (2) capable of generating a time-modulated source pulse (12), a Brillouin cell (6) and an optical focusing system (5) capable of focusing an amplified pulse (14) in the Brillouin cell (6) with an interaction distance of less than 500 millimeters, the pedestal (141) of the pulse focused in the Brillouin cell (6) having a surface power at the focus greater than the Brillouin threshold so as to initiate a Brillouin mirror (9), the Brillouin mirror (9) being capable of reflecting the main part (142) of the pulse to form a reflected amplified pulse (16) in the direction of the optical amplifier system (4) and forming an amplified pulse at least twice (26) with an energy greater than one joule.Figure for abstract: Fig. 1.

Description

Title of the invention: System and method for correcting the wavefront of a high-energy laser pulse using a Brillouin mirror Technical field of the invention

[0001] The present invention relates to the technical field of high energy lasers, i.e. lasers providing energy per pulse greater than one joule and preferably more than 20 joules.

[0002] It relates in particular to a pulsed laser system, delivering high-energy laser pulses having a time duration of the order of a nanosecond or greater than a nanosecond.

[0003] Such high-energy laser systems find various industrial, scientific or medical applications. State of the art

[0004] In this document, the term "high energy laser system" means a laser system capable of providing a laser beam, preferably pulsed, having an energy per pulse greater than or equal to one joule and preferably greater than or equal to 20 joules.

[0005] In the above field, it is known to manufacture and use laser systems delivering high energy laser pulses at a very limited firing rate of the order of 0.1 hertz (Hz) to 100 Hz.

[0006] It is desirable to increase the firing rate of a high energy laser system in industrial applications to increase the amount of material lasered, in basic scientific applications to generate more data, or in medical applications to laser treat more patients or treat them in a shorter time.

[0007] However, the generation of high-energy laser pulses currently encounters a physical limit linked to the thermal aberrations induced in the material of the optical amplifier(s) on the optical path of such high-energy laser pulses. During each laser shot, the optical amplifiers absorb, store, and then release energy. This energy is in principle recovered by the laser signal during its amplification, but since the efficiency is never perfect, the residue is dissipated and transformed into heat which accumulates in the amplifiers. The residual heat is evacuated by cooling systems. However, these systems only allow partial and inhomogeneous evacuation of the residual heat. Thus, for a rod amplifier immersed in water, the heat evacuation is more efficient on the part in contact with the water than inside the rod. We are therefore witnessing the creation thermal inhomogeneities or a thermal gradient in the material. Since the temperature of a material influences its optical index of refraction, optical index inhomogeneities also appear. When a laser beam propagates through a material medium with index inhomogeneities, it distorts spatially. This is the same principle as the phenomenon of mirages. An optical amplifier therefore distorts the beam more as residual heat accumulates there and therefore more as the laser firing rate increases.

[0008] The technical term characterizing the deformation of a beam is the term optical aberration. Since in our case, the origin of the aberrations is the thermal energy of the amplifiers, the aberrations are called thermal aberrations. The presence of these aberrations causes three technical problems of increasing severity depending on the amplitude of the aberrations.

[0009] For thermal aberrations of relatively low amplitude, the spatial quality of the beam degrades. A beam having a Gaussian spatial energy distribution or spatial profile may become elliptical or even exhibit local overcurrents. In particular, a beam exhibiting such aberrations is difficult to focus, which limits applications.

[0010] For thermal aberrations of moderate amplitude, the local overcurrents become so intense that they can cause damage to the optical systems or components of the laser system or the optical amplifier itself.

[0011] For thermal aberrations of large amplitude, the temperature gradients within the optical amplifier medium become very large, the material of the optical amplifier deforms until it breaks.

[0012] We therefore seek to limit aberrations as much as possible in high-energy laser systems. There are different techniques for avoiding or limiting these optical aberrations.

[0013] A first solution consists of reducing heat dissipation in the laser system. To this end, more efficient optical pumping techniques (energy storage in the laser material) have been developed. Flash lamps used at low rates are replaced by laser diodes which are more expensive but generate less thermal effects. Similarly, reducing the firing rate makes it possible to reduce optical aberrations.

[0014] Methods for removing accumulated residual heat have also been developed. The laser rods can be cooled by air or water with fluid flow along the amplifying medium. In this case, the residual heat accumulated in the center is then difficult to remove. For high-energy lasers with beams of large diameter, architectures Optical amplifiers in the form of discs are used, in which the cooling fluid flows between the discs.

[0015] Finally, residual aberration correction systems are implemented to achieve maximum beam quality. In the general case, these are deformable mirrors linked via a feedback loop to a wavefront sensor placed at the laser output. In principle, the control makes it possible to achieve very high beam quality with a relatively low response time, of the order of ten shots, if the correction to be made has a moderate amplitude, of the order of a few wavelengths. Large diameter deformable mirrors nevertheless have a significant cost.

[0016] The first two technical solutions indicated above address all the problems presented previously while the last technical solution only addresses the problems of thermal aberrations of low or moderate amplitude.

[0017] On the one hand, the most energetic laser system is the megajoule laser of the NIF (National Ignition Facility) with a rate of a few shots / day. On the other hand, to date, the maximum energy reflected by a phase conjugation mirror for a nanosecond laser pulse is 73 joules without optical breakdown according to the publication Kmetik et al., “Very high energy SBS phase conjugation and pulse compression in fluorocarbon liquids”, Advanced High power laser, Proc. SPIE, vol. 3889 (2000), pp. 818-826.

[0018] One of the aims of the invention is to propose a high or very high energy laser system, corrected for optical aberrations, which can operate at a high firing rate. Presentation of the invention

[0019] In this context, the present invention proposes a high energy nanosecond laser system greater than one joule comprising a laser source and an optical amplifier system, the laser source being capable of emitting a monochromatic source pulse having a duration greater than or equal to 1 nanosecond, the laser source having an optical frequency line width less than 1 gigahertz.

[0020] More particularly, according to the invention, a laser system is proposed comprising: - a time shaping device capable of modulating a time profile of the source pulse to generate a time-modulated source pulse, - the optical amplifier system being capable of receiving the time-modulated source pulse and forming an amplified pulse having an energy greater than or equal to 1 joule, the amplified pulse having a low-energy pulse pedestal greater than or equal to 2 mJ and up to 100 mJ and of duration ranging from 1 nanosecond to 5 ns and a high-energy main part having an energy greater than impulse pedestal, the main part being temporally consecutive to the pedestal, - an optical focusing system and a Brillouin cell, the optical focusing system being arranged on an optical path of the amplified pulse, the optical focusing system being capable of focusing the amplified pulse in the Brillouin cell with an interaction distance in the cell of less than approximately 500 millimeters, - the Brillouin cell containing a fluid having a Brillouin threshold in energy, a Brillouin line width and an optical breakdown threshold, the Brillouin line width being greater than the optical frequency line width of the laser source, - the pulse pedestal of the amplified and focused pulse in the Brillouin cell having a surface power at the focus lower than the optical breakdown threshold and higher than the Brillouin threshold so as to initiate by non-linear optical effect a Brillouin mirror in said cell, the Brillouin mirror being able to reflect the main part of the amplified pulse to form an amplified pulse reflected in the direction of the optical amplifier system, the optical amplifier system being able to receive the amplified reflected pulse to form an amplified pulse at least twice as high in energy as a joule.

[0021] Thus, the combination of the temporal shaping of the amplified pulse incident on the Brillouin cell for the first time and the short interaction distance in the Brillouin cell makes it possible to obtain a Brillouin mirror having a very high reflection coefficient, i.e. without significant loss for the main part of the pulse, to invert the wavefront of the pulse so as to cancel the optical aberrations during the next passage in the optical amplifier system and to conserve the temporal shape of the main part of the pulse, i.e. without pulse compression unlike the prior art.

[0022] In a first embodiment, the optical focusing system and the Brillouin cell are arranged on an optical path of the amplified pulse at the output of the optical amplifier system.

[0023] In a second embodiment, the laser system comprises a mirror arranged on an optical path of the amplified pulse at the output of the optical amplifier system, and a quarter-wave plate arranged between the optical amplifier system and the mirror.

[0024] Other non-limiting and advantageous characteristics of the system according to the invention, taken individually or in all technically possible combinations, are the following: - the time shaping device comprises an electro-optical modulator, a pulsed laser diode with distributed feedback, a semi-optical optical amplifier driver, an acousto-optic modulator and / or a Pockels cell; - the time shaping device is arranged between the laser source and the optical amplifier system, the time shaping device being capable of receiving the source pulse; - the laser system comprises an optical beam splitter arranged on an optical path between the laser source and the optical amplifier system, the optical beam splitter being able to transmit the source pulse towards the optical amplifier system, the optical beam splitter being able to receive and direct the amplified pulse at least twice towards an optical output; - the Brillouin cell has an entrance porthole; - the inlet porthole has at least one convex face and / or one concave face; - the focusing optical system has a numerical aperture greater than F / 5 where F represents a focal length of the focusing optical system; - the Brillouin cell (6) comprises a liquid based on fluorocarbon, perfluorocarbon, carbon tetrachloride, titanium tetrachloride or a gas chosen from nitrogen, sulfur hexafluoride, argon or xenon; - the optical amplifier system comprises several optical amplifiers arranged in series between the time shaping device and the Brillouin cell.

[0025] The invention also relates to a method for generating a laser pulse with energy greater than one joule comprising the following steps: - emission of a monochromatic source pulse of duration greater than 1 nanosecond and having an optical frequency line width less than 1 gigahertz; - shaping the time profile of the source pulse to generate a time-modulated source pulse, - optical amplification of the temporally modulated source pulse to form an amplified pulse having an energy greater than or equal to 1 joule, the amplified pulse having a low energy pulse pedestal greater than or equal to 2 mJ and up to 100 mJ and of duration ranging from 1 nanosecond to 5 ns and a high energy main part having an energy greater than the pulse pedestal, the main part being temporally consecutive to the pulse pedestal, - focusing the amplified pulse in a Brillouin cell with an interaction distance in the cell of less than 500 millimeters, the Brillouin cell having a Brillouin energy threshold, a Brillouin line width and an optical breakdown threshold, the Brillouin line width being greater than the optical frequency line width of the laser source, the pedestal of the amplified and focused pulse in the Brillouin cell having a surface power at the focus less than the optical breakdown threshold and greater than the Brillouin threshold so as to initiate by non-linear optical effect a Brillouin mirror in said cell, - reflection on the Brillouin mirror of the main part of the amplified pulse to form an amplified pulse reflected in the direction of the optical amplifier system, - optical amplification of the amplified pulse reflected by the optical amplifier system to form an amplified pulse at least twice as high in energy as a joule.

[0026] Of course, the various features, variants and embodiments of the invention may be combined with each other in various combinations provided that they are not incompatible or mutually exclusive. Brief description of the drawings

[0027] Furthermore, various other characteristics of the invention emerge from the appended description given with reference to the drawings which illustrate non-limiting forms of embodiment of the invention and where:

[0028] [Fig-1] is a schematic view of a high energy laser system according to a first embodiment of the invention;

[0029] [Fig.2] is an example of a time profile of the optical power of a time-modulated and once-amplified pulse incident on a Brillouin cell of the laser system of the invention (dashed curve), with superimposed the time profile of the optical power of a pulse reflected by the Brillouin cell (solid curve);

[0030] [Fig.3] shows an example of conservation of the time profile of a pulse upon reflection on a Brillouin cell of a laser system of the invention: the dashed curve represents the time profile of optical power of an incident pulse and the solid line curve represents the time profile of optical power of a pulse reflected by the Brillouin cell;

[0031] [Fig.4] shows, in the same example as in [Fig.3], the dashed curve representing the temporal profile of optical power of a pulse incident on a Brillouin cell of a laser system of the invention and a solid line curve representing the reflection efficiency of the Brillouin cell as a function of time;

[0032] [Fig.5] is a graph showing the ratio between the maximum power of a reflected pulse and the maximum pump power of an incident pulse on a Brillouin cell of a laser system of the invention, as a function of the focal length of the focusing optical system, and in inserts shows the time profiles of the incident and reflected pulses for different focal lengths; [Fig.6] is a schematic view of a high energy laser system according to a second embodiment of the invention; [Fig.7] is a flowchart illustrating the steps of the process of this di- popularization.

[0033] It should be noted that in these figures the structural and / or functional elements common to the different variants may have the same references. Detailed description

[0034] [Fig.l] schematically represents a high-energy laser system 100 according to a first embodiment of the invention. The laser system 100 here comprises a laser source 1, a time-shaping device 2, an optical beam splitter 3 based on a polarizer making it possible to avoid a return of the amplified pulse at least twice in the laser system, an optical amplifier system 4 and a phase conjugation mirror device 8.

[0035] The laser source 1 is a source capable of emitting a beam of source pulses, each source pulse 11 having a duration greater than or equal to one nanosecond. The laser source 1 is a monochromatic source, that is to say that the beam of source pulses has an optical frequency line width of less than 1 gigahertz (GHz) and preferably 200 megahertz (MHz). The laser source 1 emits, for example, the beam of source pulses at a wavelength in the ultraviolet (UV), visible or near infrared (NIR) range, for example of approximately 1 micrometer (pm). The laser source 1 is for example based on a distributed feedback laser diode or DFB (for Distributed Feedback Laser) emitting a pulse of wavelength 1064 nm, spectral width 100 kHz, temporally shaped by an electro-optical modulator and subsequently amplified up to millijoule levels by a neodymium-YAG (Nd:YAG) regenerative amplifier.Advantageously, the source pulse beam is polarized, for example linearly. The laser source 1 emits the source pulses at a given firing rate or repetition rate, generally between 1 Hz and several hundred hertz. Alternatively, the laser source 1 emits a source pulse on demand.

[0036] The temporal shape of each source pulse 11 at the output of the laser source 1 can be arbitrary. In the absence of the temporal shaping device 2, the temporal profile of the source pulse 11 is a temporal profile, for example rectangular, Gaussian, trapezoidal, increasing exponential, or any combination of temporal profiles previously indicated.

[0037] The time shaping device 2 is for example based on an optical modulator external to the source or a modulation system integrated into the laser source. However, the time shaping device is usually integrated into the laser source. The laser source 1 is generally a laser source which emits a continuous laser signal passing through a time modulation device allowing, on demand, to absorb or attenuate the continuous laser signal so as to emit nothing or to act as a transparent medium to transmit the continuous laser signal or to amplify the continuous laser signal to transmit an amplified laser signal. The behavior of the time modulation device is electronically controllable in real time via a voltage or current setpoint with modulation speeds depending on the component and a response time usually in the time range 100 ps - 30 ns. The amplitude of the absorption or amplification also depends on the electronic setpoint.Such a time modulation device can be based on an electro-optical modulator (with a response time of the order of 100 ps, ​​controlled via a voltage setpoint controlling the state of constructive or destructive interference at the output of the EO modulator, on a pulsed laser diode (with a response time of the order of 500 ps, ​​controlled via a current setpoint), a semiconductor optical amplifier (with a response time of the order of 1 ns and a current setpoint controlling the absorption below the threshold and the amplification above the laser threshold), an acousto-optical modulator (with a response time of the order of 10 ns and a voltage setpoint controlling the transmission on the optical axis) or any combination of these individual solutions.Usually the output energy of these systems is very low (nJ) and the pulses must therefore be subsequently amplified using preamplifiers such as a series of amplifiers, a single multi-pass amplifier, a regenerative amplifier or any combination of these techniques. Another possible alternative is to produce a laser source with a relatively high energy level (via a Q-switch cavity for example) and then to carry out the modulation at an intermediate energy level via a Pockels cell (with a response time of the order of 2 ns, via a voltage setpoint controlling the polarization state) for example. This last solution is usually more expensive, more complex and much less efficient than the direct modulation of a continuous laser.

[0038] Whether or not it is integrated into the laser source, the time-shaping device 2 is configured to modulate the time profile of each source pulse so as to generate a time-modulated source pulse 12. In particular, the time-shaping device 2 forms a pedestal in front of each pulse, i.e. located temporally before the main part of each pulse. In the case where the time-shaping device 2 is based on a semiconductor optical amplifier, this time-shaping is obtained by varying the absorption of the semiconductor optical amplifier. For example, the semiconductor optical amplifier is configured to vary its transmission by a factor of 1 / 100,000 to 1 or any intermediate value with a transition speed corresponding to a transition time of the order of a nanosecond.

[0039] More precisely, the time shaping device 2 is configured so as to that each time-modulated source pulse 12 has a pedestal having a low energy and a duration greater than or equal to 1 nanosecond (ns), for example at least 3 ns, up to 5 ns, and so that the rest of the time profile of the modulated source pulse, i.e. the main part of the modulated source pulse has a high energy over a duration of at least 3 ns and then a decreasing energy in the later part or falling edge of the pulse. The main focus here is on the pedestal of the pulse, i.e. the part temporally prior to the main part of the pulse. The pedestal starts at a zero power level, since it is the start of the pulse and remains at a stable power level for the duration of the preparation. The main part of the pulse is at a power level higher than the power level of the pedestal.However, the temporal shape of the main part of the pulse can be arbitrary. The operation of the laser system according to the present disclosure has been successfully tested with a main part having a power or energy temporal profile having a linear rising edge, or a square-shaped temporal profile, or a Gaussian, exponential temporal profile, or a temporal profile composed of a serial combination of at least two of these shapes.

[0040] In particular, the temporal shape of the main part of the pulse may have a decreasing energy over time, as long as the power flux density at the focus does not fall below the threshold for initiation of the Brillouin effect in the Brillouin cell, as described in detail below. In this way, each temporally modulated pulse 12 has a low energy pedestal and a high energy main part. For example, the low energy of the pedestal is of the order of 1% and the high energy of the main part of the order of 99% of the total energy of the pulse. However, the energy ratio between the pedestal and the main part of the pulse depends on the total energy of the pulse.Indeed, the Brillouin threshold corresponds to a determined surface power (of the order of 100 MW / cm2), for a given focal surface (i.e. a given focusing and diameter of the incident beam), and a given duration of the pedestal, this corresponds to a determined energy (of the order of 2 mJ to 100 mJ). The energy ratio between the pedestal and the main part of each pulse depends on the energy of the main part to be reflected. Assuming a pedestal energy of 100 mJ, for a main part of the pulse having an energy of 1 J, the energy ratio is 10%. For a pulse with a total energy of 100 J, the energy ratio between the pedestal and the main part of the pulse is 0.1%.Moreover, the energy ratio is not necessarily equal to the power ratio; for the same duration and power of the pulse pedestal, the energy, duration and temporal shape of the main part of the pulse can vary. The same pedestal for preparing the effect. nonlinear can lead to efficient reflection of different temporal shapes of the main part of the pulse.

[0041] In [Fig. 1], the wavefront of a temporally modulated pulse 12 is also shown schematically in cross-sectional view, which is here flat, i.e. devoid of optical aberrations.

[0042] In the example of [Fig.l], the optical beam splitter 3 receives the time-modulated source pulse 12. The optical beam splitter 3 is for example based on a polarizing component to obtain polarization separation. For example, the polarization of the incident pulse 12 is linear horizontal on the outward journey and passes through the polarizing component and is then rotated by 90 degrees on the outward journey in the optical amplifier system 4 (for example via a double-pass quarter-wave plate or a Faraday rotator) and the doubly amplified pulse 26, also referred to hereinafter as the twice-amplified pulse 26, is then reflected by the optical beam splitter 3 in the return direction. The optical beam splitter 3 receives the time-modulated source pulse 12 and transmits each pulse, without modifying its duration or its temporal profile, towards the optical amplifier system 4.As schematically illustrated in [Fig.l], at the output of the optical beam splitter 3, the wavefront of a time-modulated pulse 12 remains flat.

[0043] The optical amplifier system 4 comprises a single optical amplifier or a plurality of optical amplifiers arranged in series or in cascade. By way of non-limiting example, the optical amplifier system 4 comprises one or more amplifiers) with rod, disk, pumped by flash lamp, by an assembly of laser diodes or lasers and any combination of the amplifiers previously mentioned. The amplifier materials are in particular cooled by liquid or gas or solid. The amplifier materials are in particular made of gain materials of the neodymium-doped glass type (Nd:Glass) or of the crystalline type such as Nd:YAG, Nd:YLF, titanium-sapphire (Ti:SA) or ceramics thereof. The gain medium(s) may be of variable length (ranging from 1 mm to 1000 mm) and of variable diameter (ranging from 1 mm to 1000 mm).

[0044] The optical amplifier system 4 is capable of operating in double pass, that is to say that it is capable of amplifying pulses propagating along the X axis in one direction and also pulses propagating along the X axis in the opposite direction. Here, in the forward direction, the optical amplifier system 4 receives the time-modulated source pulse 12 and amplifies it while generally preserving the shape of the pulse. However, the time profile of the pulse may be distorted during amplification. Usually the initial modulation of the pulse takes this distortion into account and pre-compensates the effects of the amplification on the time profile. optical amplifier system 4 thus forms an amplified pulse 14 having a total energy greater than or equal to 1 joule, for example 1 joule, 10 joules, 50 joules or 100 joules. The amplified pulse 14, which has been amplified once in the optical amplifier system 4, propagates along the optical axis X in the forward direction towards the phase conjugation mirror device 8.

[0045] In [Fig. 1], the wavefront of an amplified pulse 14 propagating at the output of the optical amplifier system and incident on the phase conjugation mirror device 8 is schematically represented in sectional view. The wavefront of the amplified pulse 14 has optical aberrations induced by the optical amplifier system 4 through thermal inhomogeneities.

[0046] [Fig. 2] shows an example of a time profile in optical power of an amplified pulse 14. Each amplified pulse 14 has a total duration of at least one nanosecond, for example ranging from 1 nanosecond to 2 ns, 3 ns, 5 ns, 10 ns, 17 ns or 100 ns or more. The duration of the pulse is generally taken "at the foot" and depends on the relative level of the plateau of the pedestal compared to the peak. In the case of the reflection of a 100 J pulse and a preparation pedestal at 100 mJ, the level of the plateau is at least 1 / 1000 of the level of the main part of the pulse and the criterion is then 0.1% of the peak. For an 1 J pulse, this same criterion will correspond to 10% of the peak. The duration of the amplified pulse corresponds to the duration over which the power flux density at the focus is greater than the power flux density of the Brillouin threshold. For example, the amplified pulse in [Fig.2] starts at time t0 and ends at time t4.Each amplified pulse 14 has a low energy pedestal 141 between time t0 and time t2, a high energy main part 142 between time t2 and time t4. The pulse pedestal 141 has a duration ranging from 1 nanosecond to 2 ns, 3 ns, 5 ns, 10 ns. The optical power, denoted P, of the pedestal 141 of the amplified pulse 14 is increasing from time t0 until time ti where the optical power P reaches a power plateau Pb In other words, the pedestal 141 comprises a first rising edge between time t0 and time 0 during which the power increases from a zero power Po until reaching the power Pi Between time 0 and time t2, the pedestal 141 has a power greater than a threshold for initiation of the Brillouin effect in the Brillouin cell. In the example of [Fig.2], the pulse pedestal 141 has a duration of 2 ns and an energy of 10 mJ.In this example, the total incident energy of the amplified pulse 14, between times t0 and t4 is 100 J. The main part 142 has an energy greater than the maximum energy of the pulse pedestal 141. The main part 142 continuously follows the pulse pedestal 141. In this example, the optical power P of the main part 142 of the amplified pulse 14 increases continuously from time t2 until time t3 where the optical power P reaches a maximum P3. Then, the power . optical power P of the amplified pulse 14 decreases continuously from time t3 until time t4 where the optical power P4 is reached. In another example, the main part of the pulse consists of a descending ramp while remaining above the threshold for initiation of the Brillouin effect in the Brillouin cell. The main part of the pulse is followed by a falling edge which is indistinguishable in the example of [Fig.2]. The time defining the end of the main part corresponds to the beginning of the falling edge, which is defined by a criterion according to which the beginning of the falling edge is the last instant of the pulse such that the power flux density at the focus is greater than or equal to the power flux density of the Brillouin threshold and the first instant from which the nonlinear effect and therefore the Brillouin mirror dissipates.

[0047] Each amplified pulse 14 from the optical amplifier system 4 and which has been amplified once in the forward direction propagates along the optical axis X towards the phase conjugation mirror device 8.

[0048] The phase conjugation mirror device 8 comprises a Brillouin cell 6 and an optical focusing system 25.

[0049] The Brillouin cell 6 is generally cylindrical or parallelepipedal in shape and circular or rectangular in cross-section. The Brillouin cell 6 extends longitudinally along the X axis. The Brillouin cell 6 comprises an inlet porthole 7 at one end of the cylinder and, optionally, an outlet porthole 17 at the opposite end. The porthole 7 is transparent to the wavelength of the amplified pulses 14. In the example shown schematically in [Fig. 1], the inlet porthole 7 comprises a plate with flat and parallel faces. The Brillouin cell 6 has a total length, between the inlet porthole 7 and the outlet porthole 17, of between 200 mm and 1000 mm, for example approximately 500 mm.

[0050] The porthole 17 allows the transmission of the residue from the pulse pedestal used to initiate the nonlinear effect. Although almost all of the energy is reflected, the total incident pulse is so energetic that the residual transmission 27 by the Brillouin mirror would be enough to degrade the metal of the output surface and contaminate the fluid in which the effect takes place. In addition, the porthole is an additional safety feature in case the nonlinear effect does not trigger correctly and a more significant portion of the energy is transmitted. Although this latter case has never been observed in practice, the introduction of a porthole 17 on the output side provides a safety margin that is considered reasonable most of the time.

[0051] The residual energy potentially transmitted by the output porthole 17 can also serve as a diagnosis of the proper functioning of the Brillouin mirror. For example, an energy detected in abnormally high power transmission can be an indicator of a malfunction. The Brillouin cell 6 and in particular the input porthole 7 have dimensions transverse to the X axis adapted to the dimensions transverse of the amplified pulse beam 14. The Brillouin cell 6 has an opening, for example circular, with a transverse diameter of between 10 mm and 500 mm, for example approximately 100 mm. In particular, the higher the energy of the incident beam, the wider the porthole. The entrance porthole 7 is for example made of silica, with a diameter approximately equal to 127 mm and a thickness approximately equal to 10 mm.

[0052] The Brillouin cell 6 comprises a solid envelope filled with a fluid capable of generating a non-linear optical effect of stimulated Brillouin scattering or SBS (Stimulated Brillouin Scattering). For example, the fluid comprises a liquid based on fluorocarbon (for example C6Fi4, C8Fi8) or perfluorocarbon (HT70, C12F24O3), and / or carbon tetrachloride, titanium tetrachloride. The liquid is generally used pure. The liquid can also be a mixture as long as it remains homogeneous. Alternatively, the fluid comprises a gas chosen, by way of non-limiting examples, from: dinitrogen (N2), sulfur hexafluoride (SF6), argon (Ar) and xenon (Xe).

[0053] The Brillouin cell has non-linear optical properties defined in particular by the following parameters: a Brillouin threshold optical power density, generally reduced to an energy in a given optical configuration, a Brillouin line width and an optical breakdown threshold in energy / optical power density.

[0054] The Brillouin threshold determines the minimum energy capable of initiating a Brillouin mirror 9 in the cell 6 by non-linear optical effect of stimulated Brillouin scattering. The optical breakdown threshold determines the maximum energy density / surface power of the light pulses beyond which the fluid of the Brillouin cell ionizes, which generally leads to the deterioration or even destruction of the Brillouin cell.

[0055] The Brillouin linewidth corresponds to the maximum linewidth of the pulse that can be reflected efficiently on the Brillouin mirror. The Brillouin linewidth is proportional to the inverse of the reaction time of the fluid, also called lifetime, which is generally of the order of 0.2 ns to 2 ns for liquids and 0.1 ns to 20 ns typically for gases (this value depends mainly on the pressure). The Brillouin linewidth is here greater than the optical frequency linewidth of the laser source or the amplified pulse. The Brillouin linewidth is imposed by the available fluids.A fluid with as high a Brillouin linewidth as possible is preferably chosen to reduce the initiation time and increase the cell performance, for example a Brillouin linewidth of the order of 200 MHz, to be less than the source linewidth of the order of 1 GHz.

[0056] The values ​​of the Brillouin threshold and optical breakdown threshold parameters depend on the chosen fluid. The optical breakdown threshold is of the order of 100 GW / cm2. The breakdown threshold and Brillouin threshold values ​​are given in peak power flux density because the activation of the mirror effect a priori prevents the pulse energy from fully reaching the focus. In our example of a pulse amplified 14 times with an energy of 100 J, the threshold values ​​are as follows: Brillouin threshold of 100 MW / cm2 (which corresponds to a pedestal with an energy of 2 mJ for 10 ns under the focusing conditions described below) and optical breakdown threshold of 100 GW / cm2, with a Brillouin line width of 600 MHz.

[0057] The optical focusing system 25 comprises the input porthole 7 and / or a converging lens 5 arranged between the optical amplifier system 4 and the Brillouin cell. Alternatively or additionally, the input porthole 7 has at least one concave face. In certain embodiments, the optical focusing system 25 comprises only the input porthole 7 in the form of a converging lens.

[0058] The optical focusing system 25 is arranged so as to focus the amplified pulse 14 coming from the optical amplifier system 4 inside the fluid 61 of the Brillouin cell 6, at an interaction distance, denoted d, from the entrance porthole 7. According to the present disclosure, the interaction distance d is less than or equal to 500 mm, and preferably less than 400 mm, 300 mm, 200 mm, 150 mm or even 100 mm. The interaction distance d depends on the position and the focal length of the optical focusing system 25, the optical index of refraction of the fluid 61 inside the Brillouin cell and the wavelength of the amplified pulse 14.

[0059] For example, a focusing system 25 is used comprising a converging lens 5 having a focal length of 340 mm placed at a distance of 135 mm from the entrance porthole 7, which has flat and parallel faces. Given the transverse size of the beam of amplified pulses 14, which is generally greater than 60 mm, the numerical aperture of the focusing system 25 is for example greater than F / 5 for an optical system having a focal length F of 300 mm. This large numerical aperture makes it possible to preserve the temporal profile of the amplified pulse for a high energy beam, i.e. having an energy per pulse greater than 20 joules.

[0060] Advantageously, the lens 5 is placed as close as possible to the entrance porthole 7. The lens-porthole distance is minimal to avoid a beam that is too small on the entrance porthole which would risk being damaged if the beam being focused is too small and the fluence perceived by the porthole is greater than the break-up fluence (LIDT) of the porthole.

[0061] Alternatively, the entrance porthole forms the focusing optics. This configuration makes it possible to avoid the appearance of parasitic foci between the lens and the entrance porthole, in particular on the reflected beam.

[0062] The fluid 61 is adapted to receive the amplified pulse 14 of high energy focused at the interaction distance d. Before the arrival of the amplified pulse 14, the Brillouin cell is transparent. The Brillouin cell 6 first receives the pedestal 141 of the amplified pulse 14 from time t0 until time t2. Between time ti and time t2, the pedestal 141 of the amplified pulse 14 which is focused in the Brillouin cell has an energy El greater than the Brillouin threshold. The pedestal 141 of the amplified pulse 14 generates by stimulated Brillouin scattering a network of acoustic waves represented schematically in [Fig.l] by a set of lines transverse to the X axis. This acoustic network in the fluid forms a Brillouin mirror also called a phase conjugation mirror.The maximum power PI of the pedestal 141 of the amplified pulse 14 is limited so that the energy density at the focus is lower than the optical breakdown threshold of the fluid of the Brillouin cell. The duration of the pedestal 141 of the amplified pulse 14 is between one nanosecond and a few nanoseconds, for example from 1 ns to 10 ns, for example 5 ns, which allows the establishment of the Brillouin mirror in the cell 6. The Brillouin mirror extends longitudinally between the focus and the entrance porthole 7. Thus the main part 142 of the pulse is mainly reflected upstream of the focus which makes it possible to limit the energy density at the focus.

[0063] Between time t2 and time t4, the Brillouin cell 6 then receives the main part 142 of high energy of the amplified pulse 14 which is focused on the Brillouin mirror 9. Between time t2 and time t4, the main part 142 of the amplified pulse 14 which is focused in the Brillouin cell has an energy greater than the Brillouin threshold, which makes it possible to maintain the Brillouin mirror 9 in the cell. The “Brillouin mirror” effect being initiated by the pedestal 141 of the pulse, the incident beam, during the main part of the pulse, is mainly reflected in an area close to the entrance porthole 7 where the diameter of the beam is larger and where the energy density remains lower than the breakdown threshold of the fluid regardless of the peak power P3 of the pulse.Indeed, the optical focusing system and the input porthole have damage thresholds lower than the breakdown threshold of the fluid near the porthole. Thus, the maximum power P3 of the main part 142 of the incident amplified pulse 14 is not limited by the breakdown threshold. The Brillouin mirror 9 operates as a phase conjugation mirror. The high-energy main part 142 of the amplified pulse 14 is reflected on the Brillouin mirror 9 to form a reflected amplified pulse 16. In addition, a reflectivity rate R of 99% is measured on high-energy amplified pulses 14, for example 100 J or even 122 J.

[0064] Modulating the source pulse to form a pulse front having a low energy plateau allows to initiate the non-linear optical effect generating the Brillouin mirror, before the arrival of the main, high energy part of the incident pulse. The Brillouin cell is transparent during the first few nanoseconds, before the arrival of the main, high energy part of the pulse, then becomes totally reflective for the entire duration of the main, high energy part of the pulse. The Brillouin cell becomes transparent again after the main part of the pulse, during or after the falling edge. This modulation of the temporal profile of the incident pulse makes it possible to maximize the reflection efficiency, denoted R, of the Brillouin mirror and to obtain a stable reflected amplified beam of very good spatial quality.The modulation of the temporal shape of the incident pulse combined with the small interaction distance makes it possible to avoid the transparency of the Brillouin cell during the main part of the pulse, which would be likely to ionize the fluid beyond the focusing point inside the Brillouin cell. In addition, the activation of the mirror effect by the pedestal 141 allows an efficient conservation of the temporal shape of the main part 142 which can then be of arbitrary shape, as long as the Brillouin mirror is maintained.

[0065] The reflected amplified pulse 16 propagates along the X axis in the opposite direction to the amplified pulse 14 incident on the Brillouin cell. Furthermore, as shown schematically in [Fig.l], the reflected amplified pulse 16 has a wavefront which is exactly the opposite of the wavefront of the incident amplified pulse 14. The reflected amplified pulse leaves the Brillouin cell and is collimated via the focusing optical system 25. Each reflected amplified pulse 16 is received by the optical amplifier system 4 which amplifies it for the second time. The wavefront of the reflected amplified pulse 16 is inverted with respect to the wavefront induced by the optical aberrations of the optical amplifier system 4. This produces a doubly amplified pulse 26 propagating from the optical amplifier system 4 towards the optical beam splitter 3.In addition, the doubly amplified pulse 26 has a wavefront which is corrected for the aberrations induced by the optical amplifier system 4. Indeed, the optical aberrations of thermal origin of the optical amplifier system 4 do not have time to evolve between the passage of the incident amplified pulse 14 and the passage of the reflected amplified pulse 16, which are separated by only a few nanoseconds.

[0066] The optical beam splitter 3 directs the twice amplified pulse 26 along the Y axis, transverse to the X axis, towards an output of the laser system 100.

[0067] The laser system of the invention uses a phase conjugation mirror device, which is a passive device. This device adapts to the shape of the wavefront of each pulse to invert it perfectly. In addition, the Brillouin mirror makes it possible to correct optical aberrations over a large amplitude dynamic range that can range up to 100 wavelengths on the wavefront of the amplified beam.

[0068] More generally, the present disclosure applies to an optical amplifier system adapted to allow an even number 2N of round-trip passes and a single number of reflections on a phase conjugation mirror device between a round-trip pass and a return pass. The pulse after each round-trip pass being perfectly corrected for optical aberrations is easily reflected on a conventional mirror.

[0069] [Fig. 6] schematically represents a high-energy laser system 200 according to a second embodiment of the invention. The laser system 100 here comprises a laser source 1 including a time-shaping device 2, an optical beam splitter 3 based on a polarizer, a Faraday rotator 31, a half-wave plate 32, a second optical beam splitter 33 based on a polarizer, an optical amplifier system 4, a quarter-wave plate 34, a reflecting mirror 35 and a phase-conjugation mirror device 8. Unlike the first embodiment, the pulse is here amplified four times by the optical amplifier system 4. The reflecting mirror 35 is a conventional plane mirror, for example made of gold or aluminum.

[0070] In this second embodiment, the pulse is first amplified in a double pass during a first round trip by the optical amplifier system 4 with reflection on the reflecting mirror 35. Then, the doubly amplified pulse is directed towards the phase conjugation mirror device 8 which reflects it while inverting the wavefront of the twice amplified pulse. Finally, the pulse reflected by the phase conjugation mirror device 8 is returned to the optical amplifier system 4 for a second round trip with a new reflection on the reflecting mirror 35. This gives a pulse amplified four times and corrected for the optical aberrations induced during the first round trip in the optical amplifier system 4.

[0071] More precisely, the source 1 and the time shaping device 2 are configured to generate a time-modulated source pulse 12, in a manner analogous to the first embodiment. In the example of [Fig. 2], the time-modulated source pulse 12 is linearly polarized. The optical beam splitter 3 receives the time-modulated source pulse 12 and transmits it to the Faraday rotator 31 followed by the half-wave plate 32. The Faraday rotator associated with the half-wave plate 32 induces a rotation of the polarization state of the incident light beam by magneto-optical effect. This property, known as the Faraday effect, makes it possible to obtain a non-reciprocity in the polarization obtained at the output of such a polarization rotator, according to the direction of passage of the light. At the output of the wave plate 31, the time-modulated source pulse 12 is for example of polarization, noted p, here parallel to the plane of [Fig.6]. The second optical beam splitter 33 transmits the time-modulated source pulse 12 to the optical amplifier system 4 which amplifies it a first time in the forward direction. The pulse amplified a first time is of polarization p at the output of the optical amplifier system 4. The quarter-wave plate rotates the polarization of the light. The mirror 35 reflects the amplified pulse a first time and sends it back to the optical amplifier system 4 which amplifies it a second time in the return direction and forms a doubly amplified pulse 24. At the output of the first round trip through the optical amplifier system 4, the doubly amplified pulse 24 has a wavefront which presents optical aberrations induced by the optical amplifier system 4. In addition, the quarter-wave plate 34, which is crossed on the outward and return journey, rotates the polarization of the light.The doubly amplified pulse 24 is of s polarization. The second optical beam splitter 33 reflects the doubly amplified pulse 24 toward the phase conjugate mirror device 8. The doubly amplified pulse 24 is incident on the phase conjugate mirror device 8. The phase conjugate mirror device 8 operates in a manner analogous to that described in connection with [Fig.l]. The doubly amplified pulse 24 has temporal, energy, and power characteristics analogous to the amplified pulse 14 of the first embodiment. The doubly amplified pulse 24 is incident on the phase conjugate mirror device 8. The phase conjugate mirror device 8 reflects the main portion of the doubly amplified pulse 24 by inverting the wavefront so as to form a reflected doubly amplified pulse 36 of s polarization.The second optical beam splitter 33 reflects the doubly reflected pulse 36 to the optical amplifier system 4 which amplifies it a third time in the forward direction. The triple-amplified pulse is of s polarization at the output of the optical amplifier system 4. The quarter-wave plate rotates the polarization of the light. The mirror 35 reflects the triple-amplified pulse and sends it back to the optical amplifier system 4 which amplifies it a fourth time in the return direction and forms a quadruple-amplified pulse 46. At the output of the second round trip through the optical amplifier system 4, the quadruple-amplified pulse 46 has a wavefront corrected for optical aberrations. In addition, the quarter-wave plate 34, which is crossed on the outward and return journey, rotates the polarization of the light. The quadruple-amplified pulse 46 is of p polarization.The second optical beam splitter 33 transmits the four times amplified pulse 46 to the quarter-wave plate 32, the Faraday rotator 31 and then the optical beam splitter 3. The optical beam splitter 3 directs the four times amplified pulse 46 along the Y axis, transverse to the X axis, to an output of the laser system 200.

[0072] [Fig.2] shows a solid line curve representing the optical power time profile of the reflected amplified pulse 16. The solid line curve is superimposed on the dashed curve of the incident amplified pulse 14. On the one hand, it is noted that the pedestal of the incident amplified pulse on the Brillouin cell is not found in the reflected amplified pulse 16. The energy corresponding to the pedestal 141 of the incident amplified pulse 14 has been consumed to form the Brillouin mirror. On the one hand, it is observed that the time profile of the reflected amplified pulse 16 exactly reproduces the optical power time profile of the main part 142 of the incident amplified pulse 14. In summary, the laser system of the present disclosure allows the temporal profile of the main high energy portion 142 of the incident amplified pulse 14 to be maintained.The energy contrast between the low-energy pedestal 141 and the high-energy main part 142 of the incident amplified pulse 14 is here greater than 103. In the example of [Fig.2], the total energy of the amplified pulse 14 incident on the Brillouin cell is 100 J, the total energy of the reflected amplified pulse 16 is practically 100 J and the total energy of the doubly amplified pulse 26 is approximately 1200 J. Of course, these values ​​depend on the type of amplifier and its operating regime.

[0073] The system of the present disclosure finds applications for the generation of high energy pulses. The self-compensation of all the optical aberrations generated by the optical amplifier system 4 at each amplified pulse, i.e. in real time, makes it possible to generate a doubly amplified pulse of high energy, without risk of deterioration of the optical amplifier system. For example, doubly amplified pulses 26 are obtained having an energy greater than or equal to 20 joules, in particular 100 J or even 1 kJ while retaining the temporal profile of the incident pulse. In addition, each doubly amplified pulse has a profile corrected for optical aberrations. Finally, the doubly amplified pulse 26 has excellent spatial quality. In one example, an integral correction of spherical aberration is obtained over an amplitude dynamic of 26 PV wavelengths.In another example, perfect wavefront correction is obtained for amplitude aberrations of the order of four PV wavelengths, without local overcurrent.

[0074] The system of the present disclosure allows the Brillouin mirror to be initiated with the pedestal of the amplified pulse without risk of damage to the Brillouin cell. In addition, the combination of the small interaction distance and the temporal shape of the time-modulated pulse allows the temporal shape of the main part of the amplified pulse to be preserved. Unlike prior art Brillouin cell-based systems, the amplified pulse reflected is not temporally compressed in the Brillouin cell.

[0075] Figures 3-4 show another example of a time profile of an amplified pulse incident on the Brillouin cell of the present disclosure. In [Fig. 3], the dashed curve represents the optical power time profile of an incident amplified pulse 14 and the solid curve represents the optical power time profile of an amplified pulse reflected 16 by the Brillouin cell. In [Fig. 4], the dashed curve represents the same optical power time profile of an incident amplified pulse 14 as in [Fig. 3] and the solid curve represents, or reflection coefficient, of the Brillouin cell as a function of time.In these figures 3-4, the pedestal 141 of the incident amplified pulse 14 is located between the instant t ~ 5 ns and the instant t « 9 ns, the main part 142 of the incident amplified pulse 14 is located between the instant t « 9 ns and the instant t « 22 ns and the falling edge 143 of the incident amplified pulse 14 is located between the instant t « 22 ns and the instant t ~ 30 ns. Similarly, the pedestal 161 of the reflected amplified pulse 16 is located between the instant t ~ 5 ns and the instant t « 9 ns, the main part 162 of the reflected amplified pulse 16 is located between the instant t « 9 ns and the instant t « 22 ns and the falling edge 163 of the reflected amplified pulse 16 is located between the instant t « 22 ns and the instant t ~ 30 ns. [Fig. 3] shows the almost perfect conservation of the time profile of the amplified pulse, in the main part and on the falling edge, upon reflection on the Brillouin cell of the present disclosure.In the present curve, the total energy of the main part of the incident pulse is 122 J. The pedestal is here almost completely masked by the electronic noise of the photodiode used. It is also observed that the energy of the pedestal 161 of the reflected amplified pulse 16 is lower than that of the pedestal 141 of the incident amplified pulse 14. This is explained by the fact that the energy of the pedestal 141 of the incident amplified pulse 14 is consumed to generate the Brillouin mirror of the phase conjugation mirror.

[0076] In [Fig.4], we note that the percentage of reflection efficiency, noted R, of the Brillouin cell goes from approximately 0% over the duration of the pedestal to practically 100% over the duration of the main part of the amplified pulse, then drops abruptly to approximately 0% during the falling edge of the pulse. Here, a filter has been applied to remove the bias introduced by the photodetector and place the reflectivity at 0% for power values ​​lower than 10% of the maximum power. [Fig.4] also shows the reversibility of the Brillouin mirror, which appears and disappears with a very short response time, of the order of a nanosecond or less.

[0077] Other examples of measurements have been carried out for an incident amplified pulse in a lower total energy range, of the order of 2 J. In this case, we also observe a very good conservation of the time profile of the pulses. amplified reflected, for any time profile, for example trapezoidal, linearly increasing (ramp), exponential, square or combination of these last forms measurements recently extended up to energies of the order of 50 J.

[0078] [Fig.5] shows numerical simulation curves. The dashed curve in [Fig.5] represents a maximum overcurrent obtained during the reflection of an amplified pulse with a Gaussian time profile and an energy of 100 joules on a Brillouin cell 6, as a function of the focal length F of the focusing optical system 25, on which the interaction distance d linearly depends, according to the following formula: d = [f - (lens / cell entrance distance)] *n, where f represents the focal length of the lens 5 and n the optical refractive index of the fluid 61. In the example of an incident amplified pulse with an energy of 122 J, the applied formula becomes: d = [f - 138 mm] * 1.27. In the following simulations illustrated in [Fig.5]: d = f * 1.27 The solid line curve is the integral of the energy of the incident pulse between t0 and ti=t0 + 2dn / c divided by the lifetime of the Brillouin cell fluid.tl-tO here represents the time required for the pulse to make a round trip between the cell entrance window and the focus of the lens and the solid line curve represents the energy included in this time recompressed by the cell over a time equal to the Brillouin lifetime. The dashed curve in [Fig.5] represents the ratio of the peak powers of the reflected and incident amplified pulse, i.e. the compression efficiency of the cell, as a function of the focal length F of the focusing optical system.

[0079] According to the present disclosure, the duration of the pulse pedestal is greater than 1 ns, and preferably of the order of 2 ns or 3 ns, is much greater than 2.nd / c, so that the induced Brillouin grating has a very high efficiency on the main part of the pulse.

[0080] In [Fig.5] the temporal shape of the incident pulse is Gaussian, which allows the choice of focal length to be decorrelated from the choice of the pedestal level. In the simulation [Fig.5] presented, the pedestal is not present and the beginning of the Gaussian shape therefore serves as a pedestal. It is the beginning of the Gaussian which is more or less compressed by the Brillouin cell. This also illustrates the interest of the preparation of the nonlinear effect by the pedestal: without preparation by a well-defined pedestal, the beginning (the first 2nd / c nanoseconds) of the pulse is temporally compressed over a time equal to the lifetime of the liquid whereas if a pedestal is introduced, it is this pedestal which undergoes the compression and the main part of the pulse remains unchanged. The pedestal must also therefore be at least 2nd / c duration (generally less strict condition than the other condition of 3 liquid life times).

[0081] In [Fig.5] we have also shown vertical dashed lines which delimit different operating regimes. Finally, the time profile simulations of the incident amplified pulse 14 and the reflected amplified pulse 16 for different values ​​of the focal length F are shown in inserts.

[0082] For the focal length F of about 50 mm, we observe the almost perfect conservation of the time profile of the pulse, except for the pulse pedestal, which is consumed for the generation of the Brillouin mirror. For the focal length F of about 350 mm, we observe a slight deformation of the time profile of the pulse, with a significant narrowing of the pedestal of the reflected pulse compared to the pedestal of the incident pulse.

[0083] For the focal length F of 1000 mm, a strong compression of the reflected pulse is observed compared to the incident pulse, with an overintensity of the maximum in the main part of the pulse and a secondary peak.

[0084] For the focal length F of approximately 1500 mm, we observe both a strong compression of the reflected pulse compared to the incident pulse, with a strong overintensity of the maximum in the main part of the pulse.

[0085] For a focal length less than approximately 60 mm (first vertical dashed line), which here corresponds to an interaction distance d of 72 mm, no compression of the reflected pulse is observed with respect to the incident pulse: in this case, the conservation of the time profile is limited by the initiation time of the Brillouin mirror.

[0086] Between the focal length of 60 mm and 400 mm, we are in a zone of sharpening or thinning of the pulse pedestal: in this case, the conservation of the temporal profile is limited by the initiation time of the mirror followed by the compression of the pulse over a time 2nd / c.

[0087] From the focal length of 400 mm (second vertical dashed line), which here corresponds to an interaction distance d of 330 mm, we are in a pulse compression zone: the compression over a time 2nd / c = 4.3 ns becomes comparable to the pulse time, here 10 ns, which is not desirable because the reflected pulse then has an overcurrent in the main part of the pulse likely to damage the optical amplifier system during the second pass. Up to a focal length F of approximately 500 mm, which here corresponds to an interaction distance d of 635 mm, we observe that the overcurrent curve remains equal to 1, which reflects the absence of compression of the pulse during reflection. This desired effect of no compression is a priori achievable regardless of the propagation distance by adding a pedestal of sufficient duration of at least 2nd / c.However, in practice, the shorter the interaction distance, the easier it is to preserve the time profile. The interaction distance depends weakly on the liquid because its index can slightly vary d for a fixed focal length. On the other hand, the pulse compression is compressed over a proportional duration . to the liquid lifetime. Poor initiation of the nonlinear effect causes compression inversely proportional to the liquid lifetime.

[0088] The laser system of the present disclosure makes it possible to reflect a high-energy laser pulse, of more than 20 joules or even very high energy, 100 J, 122 J onto the Brillouin cell, which makes it possible to obtain at the output of the double pass in the amplifier 4 a laser pulse having an energy almost equal to twice the incident pulse, which was not previously achieved. The temporal shaping of the source pulse and the use of an optical focusing system operating at a short interaction distance, less than 400 mm and preferably less than 300 mm, in the Brillouin cell, makes it possible to generate a Brillouin mirror with very high reflection efficiency, without compression and without over-intensity of the reflected pulse relative to the incident pulse. In addition, the laser system has the advantage of being very compact.

[0089] The firing rate of the laser source can be adjusted for example between 0.1 Hz and 1000 Hz.

[0090] [Fig.7] shows a flowchart illustrating the steps of the method of the present di popularization.

[0091] The first step is the step 10 of emitting a monochromatic source pulse 11 of duration greater than or equal to 1 nanosecond and having an optical frequency line width less than 1 gigahertz.

[0092] The following step or simultaneous with step 10 is the step 20 of shaping the temporal profile of the source pulse to generate a temporally modulated source pulse 12.

[0093] The next step is the first optical amplification step 30 which is applied to the time-modulated source pulse 12 to form an amplified pulse 14 having an energy greater than or equal to 1 joule, for example 2 joules, 10 joules, 50 joules, or 100 joules. The amplified pulse 14 has a pedestal 141 of low energy and duration ranging from 1 nanosecond to 5 ns and a main part 142 of high energy having an energy greater than the pedestal 141, the main part 142 being temporally consecutive to the pedestal 141 of the pulse.

[0094] The next step is the step of focusing 40 the amplified pulse 14 in a Brillouin cell 6 with an interaction distance in the cell less than 500 millimeters, or 400 mm, or even 300 mm. The Brillouin cell 6 is configured to have a Brillouin threshold in energy, a Brillouin linewidth and an optical breakdown threshold in energy / power density, the Brillouin linewidth being greater than the optical frequency linewidth of the laser source. The pedestal 141 of each amplified and focused pulse in the Brillouin cell 6 has an energy density less than the optical breakdown threshold and an energy greater than the Brillouin threshold so as to initiate by non-linear optical effect a Brillouin mirror 9 in said cell.

[0095] The next step is the reflection step 50 on the Brillouin mirror 9 of the main part 142 of the amplified pulse to form a reflected amplified pulse 16 in the direction of the optical amplifier system 4.

[0096] The next step is the second optical amplification step 60 by the optical amplifier system 4 which is applied to the reflected amplified pulse 16 to form a doubly amplified pulse 26 of high energy, without time compression of said doubly amplified pulse.

[0097] The Brillouin mirror which allows phase conjugation during the reflection of the pulse allows the self-compensation of all the optical aberrations generated during the first passage of the pulse by the optical amplifier system. This provides a real-time correction, at each pulse, which is instantaneous and perfectly corrects the optical aberrations induced by the optical amplifier system on the second passage.

[0098] The invention makes it possible to manufacture a high-energy laser system with an energy per pulse ranging from 1 joule to several hundred joules or even of the order of a kilojoule. Such a high-energy laser system finds in particular industrial applications in various material treatments, for example of metals by laser shock peening process ("laser shock peening" in English terminology), scientific applications such as shock generation, the study of matter under high pressure in astrophysics or even nuclear fusion. Such laser systems also find applications as a pump laser for high-energy femtosecond lasers (petawatt or PW class) or even as a pump laser for nuclear fusion lasers. The latter also have various scientific or medical applications and are more recently used for laser particle acceleration.This latter application in particular makes proton therapy more accessible by replacing a conventional particle accelerator with a high-energy laser system, which is more compact and less expensive.

Claims

Claims

1. Laser system (100) with energy greater than one joule comprising a laser source (1) and an optical amplifier system (4), the laser source (1) being capable of emitting a monochromatic source pulse (11) having a duration greater than 1 nanosecond, the laser source (1) having an optical frequency line width less than 1 gigahertz, characterized in that the laser system (100) comprises: - a time shaping device (2) capable of modulating a time profile of the source pulse to generate a time modulated source pulse (12), - the optical amplifier system (4) being capable of receiving the time-modulated source pulse (12) and of forming an amplified pulse (14, 24) having an energy greater than or equal to 1 joule, the amplified pulse (14, 24) having a pulse pedestal (141) of low energy greater than or equal to 2 mJ and up to 100 mJ and of duration ranging from 1 nanosecond to 5 ns and a main part (142) of high energy having an energy greater than the pulse pedestal (141), the main part (142) being temporally consecutive to the pulse pedestal (141), - an optical focusing system (5) and a Brillouin cell (6), the optical focusing system (5) being arranged on an optical path of the amplified pulse (14, 24), the optical focusing system (5) being capable of focusing the amplified pulse (14, 24) in the Brillouin cell (6) with an interaction distance in the cell of less than 500 millimeters, - the Brillouin cell (6) having a Brillouin energy threshold, a Brillouin line width and an optical breakdown threshold, the Brillouin line width being greater than the optical frequency line width of the laser source, - the pulse pedestal (141) of the amplified and focused pulse in the Brillouin cell (6) having a surface power at the focus lower than the optical breakdown threshold and higher than the Brillouin threshold so as to initiate by non-linear optical effect a Brillouin mirror (9) in said cell, the Brillouin mirror (9) being able to reflect the main part (142) of the amplified pulse to form a reflected amplified pulse (16, 36) in the direction of the optical amplifier system (4), the optical amplifier system (4) being able to receiving the reflected amplified pulse (16, 36) to form an amplified pulse at least twice (26, 46) of energy greater than one joule.

2. Laser system (100) according to claim 1 wherein the optical focusing system (5) and the Brillouin cell (6) are arranged on an optical path of the amplified pulse (16) at the output of the optical amplifier system (4).

3. A laser system (100) according to claim 1 comprising a mirror (35) disposed on an optical path of the amplified pulse (16) at the output of the optical amplifier system (4), and a quarter-wave plate (34) disposed between the optical amplifier system (4) and the mirror (35).

4. Laser system (100) according to one of claims 1 to 3 wherein the temporal shaping device comprises an electro-optical modulator, a pulsed laser diode with distributed feedback, a semiconductor optical amplifier, an acousto-optical modulator and / or a Pockels cell.

5. Laser system (100) according to one of claims 1 to 4 in which the Brillouin cell (6) comprises an entrance porthole (7).

6. Laser system (100) according to claim 5 wherein the entrance porthole (7) has at least one convex face and / or one concave face.

7. A laser system (100) according to one of claims 1 to 6 wherein the focusing optical system has a numerical aperture greater than F / 5 where F represents a focal length of the focusing optical system.

8. Laser system (100) according to one of claims 1 to 7 in which the Brillouin cell (6) comprises a liquid based on fluorocarbon, perfluorocarbon, carbon tetrachloride, titanium tetrachloride or a gas chosen from nitrogen, sulfur hexafluoride, argon or xenon.

9. Laser system (100) according to one of claims 1 to 8 in which the optical amplifier system (4) comprises several optical amplifiers arranged in series between the time shaping device (2) and the Brillouin cell (6).

10. Method for generating a laser pulse with energy greater than one joule comprising the following steps: - emission (10) of a monochromatic source pulse (11) with a duration greater than 1 nanosecond and having an optical frequency line width less than 1 gigahertz; - shaping (20) the temporal profile of the source pulse to generate a temporally modulated source pulse (12), - optically amplifying (30) the temporally modulated source pulse to form an amplified pulse (14, 24) having an energy greater than or equal to 1 joule, the amplified pulse having a pulse pedestal (141) of low energy greater than or equal to 2 mJ and up to 100 mJ and of duration ranging from 1 nanosecond to 5 ns and a main part (142) of high energy having an energy greater than the pulse pedestal (141), the main part (142) being temporally consecutive to the pulse pedestal (141), - focusing (40) of the amplified pulse (14, 24) in a Brillouin cell (6) with an interaction distance in the cell of less than 500 millimeters, the Brillouin cell (6) having a Brillouin energy threshold, a Brillouin line width and an optical breakdown threshold, the Brillouin line width being greater than the optical frequency line width of the laser source, the pedestal (141) of the amplified and focused pulse in the Brillouin cell (6) having a surface power at the focus less than the optical breakdown threshold and greater than the Brillouin threshold so as to initiate by non-linear optical effect a Brillouin mirror (9) in said cell, - reflection (50) on the Brillouin mirror (9) of the main part (142) of the amplified pulse to form a reflected amplified pulse (16, 36) in the direction of the optical amplifier system (4), - optical amplification (60) of the reflected amplified pulse (16, 36) by the optical amplifier system (4) to form an amplified pulse at least twice (26, 46) of energy greater than the joule.

Images