HIGH-PERFORMANCE PICOSE SECOND LASER SYSTEM WITH PULSE LENS OPTICAL FIBER AND ADJUSTABLE DURATION AND USE OF SUCH A LASER SYSTEM

Description

Domaine technique

[0001] The present invention relates to the technical field of lasers for producing high peak power femtosecond (fs) or picosecond (ps) laser pulses.

[0002] More specifically, the present invention relates to the field of fiber optic lasers, which make it possible to deliver a laser pulse to the end of a waveguide that is completely flexible in all directions, without deteriorating the quality of the laser beam. Technique antérieure

[0003] Laser pulses with a duration of less than 10 ps are highly sought after for a wide variety of applications such as micromachining or eye surgery. It is known that the pulse duration is optimally less than 10 ps and a duration close to 1 ps is generally desirable. In order to cause disruption of the material by laser-induced breakdown, the peak power of the pulses generally needs to be greater than 1 MW. One of the difficulties encountered is being able to amplify ultrashort pulses (< 10 ps) up to energies greater than 1 µJ or even 10 µJ. Furthermore, it is desirable to be able to freely move the laser beam over a significant distance (greater than 1 m) and with a variable angle of incidence without modifying the geometric properties of the focal point.Since a laser beam propagates exclusively in a straight line in air, it is generally necessary to use a set of mirrors, possibly movable in space, in order to direct the laser beam to the desired location with the desired angle of incidence.

[0004] A much more flexible solution has been found with the use of waveguides, and in particular optical fibers. This allows the laser beam to be directed to any location in space and its direction to be changed very easily. This type of waveguide is widely used in optical telecommunications and in high-power lasers used in industrial applications. There are continuous-wave lasers or lasers producing long pulses, i.e., those with a duration greater than 1 ns, using optical fibers to move the laser beam over a distance of several meters or even tens of meters. These systems can be integrated into robot arms or handpieces. The end through which the beam is emitted can thus move along 6 axes.

[0005] When the pulse duration becomes less than 1 ns or if the peak power typically exceeds 300 kW, propagation over a few meters in a single-mode fiber gives rise to the appearance of non-linear effects such as the Raman effect and / or self-phase modulation which degrade the spatial and / or temporal properties of the laser pulse. In practice, it is currently impossible to propagate a laser pulse of 1 ps duration in a soft and flexible single-mode glass optical fiber with an energy greater than a few hundred nanojoules and over a distance greater than a few centimeters.

[0006] Similarly, direct amplification of ultra-short pulses, i.e. those with a duration of typically less than 10 ps, ​​is hampered by the appearance of non-linear effects that prevent the use of optical amplifiers with a thickness greater than a few millimeters. In order to amplify very short pulses, the frequency drift pulse amplification method is also known, which consists of transforming initially very short pulses (generally with a duration of less than 1 ps) by stretching them by a factor E up to several tens or even hundreds of picoseconds. Their peak power is then reduced by the same factor E, allowing their amplification without suffering from deleterious non-linear effects. These pulses are then recompressed by a dispersive system, called a compressor, generally composed of two diffraction gratings.

[0007] Frequency drift pulse amplification systems require the use of an ultra-short pulse source, a pulse stretcher, and a compressor that is very bulky and sensitive to vibration and movement. Their integration into an industrial system is complicated and requires the use of a set of mirrors to guide the beam to the part to be treated. In addition, the stretching and compression factors must be finely adjusted, which requires an adjustable stretcher and / or compressor.

[0008] To overcome this inflexibility of use, fiber lasers have been developed. In these systems, the amplifiers are composed of optical fibers doped with ions allowing the amplification of a light wave.

[0009] In order to maintain the quality of the laser beam, the fibers used, whether passive or active, must be perfectly single-mode. It is also known that despite the efforts of optical fiber manufacturers, current optical fibers with a core diameter greater than typically 15 µm are only single-mode if they are kept perfectly fixed. These fibers can be flexible and can be coiled into various geometric shapes, but once the position has been chosen to produce a good quality beam, any movement of the fiber causes a modification of the geometric properties of the beam (shape, direction, power, etc.). These instabilities are particularly sensitive for short fibers, as higher order propagation modes cannot be filtered during propagation in the fiber over a long length.

[0010] Flexible fiber lasers producing good quality beams are therefore restricted to the use of fibers with a core diameter of less than 15 µm. This limitation applies to both active amplifying fibers and passive optical fibers providing only beam propagation. We can therefore see that physics does not allow the production or propagation of laser pulses with a duration of less than 10 ps and a peak power typically greater than 100 kW over several meters in flexible waveguides allowing the quality of the beam to be maintained while moving the end of the guide in all directions.

[0011] In order to construct a laser source allowing the delivery of ultra-short pulses over long distances using optical fibers, different solutions have been proposed.

[0012] Various authors have proposed using hollow-core fibers to carry ultra-short laser pulses. These hollow-core fibers effectively avoid all the difficulties associated with non-linear effects during propagation in a solid medium but cannot be amplified and cannot be welded onto conventional glass fibers.

[0013] Frequency drift amplification has also been used to avoid non-linear effects in active or passive fibers, whether step-index or photonic structure, by temporally stretching the pulses before their propagation. In order to obtain short pulses on the final target, it is necessary to recompress these pulses and therefore to introduce a compressor after the fiber. To obtain high energies, however, it is necessary to use a very high stretching factor (> 500), which implies a very bulky compressor. Such a compressor is heavy and sensitive to vibrations or shocks. This method is therefore not compatible with a flexible 3D displacement of the beam.

[0014] Patent document EP 1 712 936 (Ramachandran et al) proposes using multimode fibers with a core diameter significantly larger than that of a single-mode fiber. The desired fundamental mode is converted into a higher-order mode before injecting it into the fiber. This system requires the use of a stretcher before injecting the pulse into the fiber and a mode converter to bring the multimode beam back to a usable shape close to that of a single-mode beam. Furthermore, it is sensitive to coupling between modes and therefore to fiber movements.

[0015] US patent document 8,948,219 proposes a solution using passively triggered lasers combined with a non-linear optical amplifier and an adjustable compressor.

[0016] US Patent 9,553,421 discloses a laser based on a picosecond source followed by a pulse transformer, a fiber amplifier, and a compressor. For those skilled in the art, since the source is fully fibered and all components are welded together, the only remaining adjustment is the compressor, which is configured to compress the pulses. Therefore, it is the compressor that adapts to the system to compress the pulses. This is also a conventional compressor, optomechanically adjustable and inherently sensitive to vibrations or shocks.

[0017] The aim of the present invention is to propose a method for producing pulses of duration less than 10 picoseconds and preferably less than 3 picoseconds, of tunable duration and high energy, i.e. greater than 100 nJ, easily orientable and movable in all directions without altering the spatial quality of the beam. Exposé de l'invention

[0018] For this purpose, the present disclosure provides a fiber optic laser system according to claim 1.

[0019] The laser system is fully fibered between the injector and the compressor. In addition, all fibers used are single-mode optical fibers with a core diameter of less than 25 micrometers, and preferably less than 15 microns, or even less than 10 microns. The fibers are step-index or photonic crystal structure optical fibers comprising a solid core surrounded by a sheath of hollow capillaries. The single-mode optical fibers can be oriented and bent with a minimum bending radius of less than 50 cm and greater than 5 cm, while preserving the quality of the laser beam and the single-mode appearance. In particular, the last fiber optic amplifier is flexible along at least two rotation axes transverse to the longitudinal optical axis at the end of the last fiber optic amplifier.More specifically, the assembly formed by the compressor and the end of the last active optical fiber amplifier is flexible around at least two rotation axes transverse to the longitudinal optical axis at the end of the last optical amplifier, with a minimum radius of curvature less than 50 cm.

[0020] This fiber optic laser system structure allows generating a compressed laser pulse with a duration of less than 10 picoseconds or even 3 picoseconds, of high energy and tunable duration. The end of the last fiber optic amplifier attached to the compressor is easily orientable and movable in all directions without altering the spatial quality of the pulsed laser beam. Indeed, the compressor is non-adjustable (it is preset at the time of manufacture), which allows maintaining a small volume, while using a volume compressor capable of withstanding a high energy pulse. Advantageously, the source pulse has an energy greater than 1 nanojoule.

[0021] Other non-limiting and advantageous characteristics of the laser system according to the invention, taken individually or in all technically possible combinations, are the following: the injector further comprises a pulse selector or an optical attenuator or an electro-optical modulator or an acousto-optical modulator or any combination thereof and the energy adjustment means are adapted to adjust the energy of the source pulse by means of the pulse selector, the optical attenuator, the electro-optical modulator or the acousto-optical modulator, respectively or their combination; the electronic means for adjusting the duration of the compressed pulse comprise means for adjusting the energy of the optical amplifier system; the optical amplifier system comprises at least one pump diode adapted to generate pump radiation and the energy adjustment means of the optical amplifier system are adapted to adjust an electric current of the pump diode so as to adjust the pump radiation injected into the optical amplifier system;the injector comprises a laser oscillator selected from a gain-triggered laser diode, a solid-state laser, a phase-locked mode fiber laser, a passive mode-locked fiber laser and a semiconductor laser; the optical amplifier system is based on index-stepping optical fiber(s) and / or optical fiber(s) with a photonic crystal structure having a solid core and a guide cladding formed of capillaries arranged in a ring around the core; the optical amplifier system comprises a double-clad optical fiber; the amplifier system comprises a last passive single-mode optical fiber arranged upstream of the last flexible optical fiber amplifier; the last amplifier has a length of less than one meter or two meters;the amplifier system comprises a first flexible optical fiber amplifier arranged upstream of the last amplifier and a first spectral filter arranged downstream of the spectro-temporal shaping module and upstream of the first amplifier; the last amplifier and / or the first amplifier comprises an optical fiber having a silica-based core doped with rare earth ions chosen from the following ions: ytterbium, neodymium, erbium-ytterbium, holmium, thulium, ytterbium-thulium; the compressor comprises two fixed-pitch gratings or a dispersive volume Bragg grating with continuously variable pitch; the system comprises a flexible mechanical protective sheath arranged around the last amplifier and the passive fiber and mechanically attached to the compressor; the energy adjustment means are adapted to adjust the energy of each source pulse individually in order to vary the compressed duration of each amplified pulse individually;the duration of the compressed pulse is decreasing as a function of the increasing energy of the source pulse within a variation range of ±40% of the energy of the source pulse. ;

[0022] The invention also provides the use of a fiber optic laser system according to one of the embodiments in a micromachining device, a medical device, an ophthalmic surgery device or a skin tattoo removal device.

[0023] Of course, the various features, variants and embodiments of the invention may be combined with each other in various combinations to the extent that they are not incompatible or mutually exclusive. Brève description des dessins

[0024] The description which follows with reference to the appended drawings, given as non-limiting examples, will make it clear what the invention consists of and how it can be implemented.

[0025] On the attached drawings: Figure 1 schematically represents a fiber optic laser system according to the present disclosure, Figure 2A schematically represents an injector of the laser system according to an exemplary embodiment, Figure 2B schematically represents an injector according to a variant, Figure 3 schematically represents a fiber optic laser system according to a first exemplary embodiment, Figure 4 schematically represents a fiber optic laser system according to a variant of the first exemplary embodiment, Figure 5A illustrates a calculated example of spectral shape after the spectro-temporal shaping module of a pulse of initial duration 50 ps and initial spectral width of 0.07 nm. The spectral width of the shaping module is 0.29 nm; Figure 5B shows the calculated spectral shape of the same pulse at the output of the last optical amplifier in a system including a spectro-temporal shaping module. The initial spectral width is 0.07 nm and the spectral width after amplification is 2.43 nm; Figure 6 shows respectively the calculated autocorrelation trace corresponding to the conditions used for the figure 5B (610), and the experimental autocorrelation trace (600) recorded for initial pulse and amplifier architecture parameters identical to those used for figure (610). Figure 7 is an example of compressed pulse duration variation as a function of source pulse energy variation, Figure 8A illustrates an example of the spectral shape of an amplified pulse at the output of the last optical amplifier (for an initial pulse of 50 ps duration and 0.8 nJ initial energy). The spectral width is 3.8 nm; Figure 8B shows an autocorrelation trace of the compressed pulse of duration 0.76 ps formed from the amplified pulse illustrated in the figure 8A ; Figure 9 shows an example of a laser system configured to generate a train of pulses, the duration of each pulse in the train being modulated independently of each other; Figure 10 schematically represents a fiber optic laser system in which most of the fiber optic components are arranged inside a flexible mechanical sheath.

[0026] It should be noted that in these figures the structural and / or functional elements common to the different variants may have the same reference signs. Description détaillée

[0027] There figure 1 schematically represents a fiber optic laser system in accordance with the present disclosure for generating a compressed pulse 17 of picosecond or femtosecond duration, of high energy and adjustable in duration.

[0028] We will now describe the structure and operation of each of these components as well as the characteristics of the pulses propagating in such a system.

[0029] More specifically, the laser system comprises an injector 100, a spectro-temporal shaping module 110, an optical amplification system associated with a spectral filtering device, a compressor 170 and energy adjustment means 180.

[0030] The injector 100 produces one or more source pulses 10. The spectro-temporal shaping module 110 receives a source pulse 10 and generates a spectrally broadened pulse 11. The optical amplification system combined with the spectral filtering device receives a spectrally broadened pulse 11 and forms an amplified pulse 14. The compressor 170 receives the amplified pulse 14 and generates a compressed pulse 17.

[0031] The injector 100 produces source pulses 10 with a duration of between 30 and 100 ps. The injector 100 comprises, for example, a laser oscillator.

[0032] The energy adjustment means 180 make it possible to adjust the energy of the source pulse 10 emitted by the injector 100. Since the energy of the pulse injected into the optical amplifier system is adjustable, the energy of the amplified pulse is indirectly adjustable. Consequently, the injector 100 combined with the energy adjustment means 180 makes it possible to generate a source pulse 10 of adjustable energy. The energy of the source pulse at the output of the injector is generally in a range between 0.1 nJ and 2 nJ. The duration of the source pulse is for example determined by the spectral width of the filter contained in the cavity of the laser oscillator. For a given injector, the duration of the source pulses 10 is therefore generally constant as a function of time, from one pulse to the next. The duration of the source pulse, denoted d 0 , is here between 30 ps and 100 ps. The energy setting of a source pulse 10 generally does not affect its duration.The source pulse 10 is emitted in a spectral band B 0 around an emission wavelength, for example 1030 nm, the spectral band B 0 having a width between 0.03 nm and 0.3 nm.

[0033] The spectro-temporal shaping module 110 is arranged between the injector 100 and the optical amplification system. The spectro-temporal shaping module 110 consists of a passive single-mode fiber with a length of between 100 m and 1000 m. This fiber is polarization-preserving and has a core with a diameter of between 5 and 12 µm. Examples of fibers useful for this application are the PM980 (Nufern, USA or YOFC, China) or the HI1060 (Corning, USA). The spectro-temporal shaping module 110 receives a source pulse 10. The propagation of the source pulse in the passive optical fiber induces a self-modulation effect of the phase of the pulse which depends on the light intensity of the source pulse and which produces a spectral broadening proportional to the peak power of the injected pulses and to the length of the fiber.During its propagation in the fiber, the pulse undergoes the combined effects of group velocity dispersion (variation of the index with the wavelength) and self-phase modulation (variation of the index with the instantaneous power). The spectral and temporal phase of the pulse is then modified and takes on a shape which depends on the parameters of the fiber and the pulse used. In particular the frequency drift (derivative function of the phase with respect to the frequency) depends on these parameters. In this way, the spectro-temporal shaping module 110 fulfills a phase adapter function upstream of the optical amplifier system.

[0034] In the system illustrated on the figure 1 , the energy adjustment means 180 make it possible to adjust the energy of the source pulse emitted by the injector 100 and thus to vary the frequency drift of the pulse coming from the spectro-temporal shaping module 110. In other words, the spectro-temporal shaping module 110 makes it possible to generate a spectrally broadened pulse 11, having a frequency drift controlled as a function of the energy of the source pulse. In other words, the spectral bandwidth of the spectrally broadened pulse 11 is adjustable as a function of the energy of the source pulse. The spectrally broadened pulse 11 has a spectral bandwidth which extends around the source wavelength and which is 5 to 50 times wider than the spectral width B0 of the source pulse. The spectral bandwidth of the spectrally broadened pulse 11 is greater than 0.3 nm.This spectrally broadened pulse 11 is injected into the optical amplification system and the associated spectral filtering device.

[0035] The optical amplification system consists of single-mode optical fiber(s). The optical amplification system comprises at least one last optical amplifier 150. Alternatively, the optical amplification system comprises at least one first optical amplifier 130 and the last optical amplifier 150 arranged in series. According to other variants (not illustrated), the optical amplification system comprises more than two optical fiber amplifiers arranged in series to form successive amplification stages.

[0036] In addition, a section of passive optical fiber, with a fiber length of less than 10m, can be inserted between two amplification stages. In particular, a single-mode passive optical fiber can be inserted upstream of the last optical fiber amplifier 150 to allow the length of flexible optical fiber to be extended.

[0037] Each fiber optic amplifier consists of an active optical fiber. According to one embodiment, an active optical fiber is a double-clad optical fiber having a core doped with rare earth ions, for example ytterbium ions. The double-clad structure allows optical pumping by injecting the radiation from one or more laser diodes into the first cladding, called the pump cladding. The active optical fibers are preferably polarization-maintaining. These fibers are well known to those skilled in the art and are not described here. They may be step-index or have a photonic crystal structure. The pump radiation may be guided by step-index between the core and a solid cladding surrounding the core or by a ring of very thin capillaries (air clad) surrounding the core. The absorption of the pump radiation injected into the pump cladding is typically between 3 dB / m and 20 dB / m at the pump wavelength.

[0038] Alternatively, the energy adjustment means 180 also make it possible to directly adjust the energy of the amplified pulse at the optical amplifier system, for example by adjusting the current of a pump diode for at least one optical fiber amplifier of the amplification system. The amplifier being composed of at least one optical fiber, the adjustment of the energy of the amplified pulse induces a variation in the phase modulation when passing through this amplifier and all the fibers located downstream of this amplifier and consequently a modification of the duration of the compressed pulse.

[0039] A spectral filter 120, respectively 140 is arranged upstream of each optical amplifier 130, respectively 150. More precisely, the last spectral filter 140 is arranged upstream of the last optical amplifier 150. In a variant comprising several amplifiers, a first spectral filter 120 is arranged upstream of the first optical amplifier 130, also called optical pre-amplifier. In other words, each optical amplifier is associated with a spectral filter arranged upstream. Generally, the spectral filter is adjacent to the optical amplifier with which it is associated. A spectral filter is composed for example of a glass plate supporting a multi-dielectric treatment whose transmission is maximum around the wavelength of the source pulse and very low outside this zone.This filter is advantageously placed in a component comprising an input for a fiber followed by a free-space collimation lens, itself followed by the free-space filter followed by a second refocusing lens in an output fiber. It is also possible to use a fiber Bragg grating or possibly a solid Bragg grating placed in a free-space component. The last spectral filter 140, respectively the first spectral filter 120, is generally a bandpass filter. More precisely, the spectral filter 120, respectively 140, is chosen to transmit a spectral band of approximately 50 nm and less than 100 nm, centered on the wavelength of the source pulse, for example 1030 nm, while strongly attenuating the generation of a parasitic Raman signal in each optical amplifier 130, respectively 150.This spectral band can be voluntarily reduced to 8 nm or even 4 nm around the central wavelength of the source pulse.

[0040] The spectral filter 120 receives a spectrally broadened pulse 11 with adjustable frequency drift depending on the energy of the source pulse and transmits a spectrally filtered pulse 12 to the first optical amplifier 130. The first optical amplifier 130 amplifies the filtered pulse 12 and generates a pre-amplified pulse 13. The spectral filter 140 receives the pre-amplified pulse 13 and transmits a spectrally filtered pulse 14 to the last optical amplifier 150. The last optical amplifier 150 amplifies the filtered pulse 14 and generates an amplified pulse 15.

[0041] The spectral filter 120, respectively 140, thus makes it possible to limit a power transfer towards a Raman line likely to appear in the optical fiber of the spectro-temporal shaping module or in the optical fiber amplifiers 130, 150, by stimulated Raman effect, this Raman line being likely to grow as it propagates and is amplified. In this way, the Raman radiation is strongly attenuated or even eliminated after each stage in order to avoid the progressive growth of the Raman radiation, while accumulating during the different propagation or amplification stages a spectral broadening by self-phase modulation.

[0042] The spectral filter 120, respectively 140, makes it possible to accumulate the self-phase modulation by eliminating the power of the Raman line at each pass of a filter. The competition between self-phase modulation and generation of a Raman line being eliminated, it is thus possible to achieve a spectral broadening of the amplified pulse 15 by self-phase modulation by a factor greater than 50 or even 100 compared to the spectral width of the source 100 without being limited by the growth of the Raman effect.

[0043] Non-linear optical effects, including self-phase modulation and the Raman effect, are proportional to the product of the length traversed by the local light intensity. These non-linear optical effects are therefore particularly important in the final part of the amplifier system in which the energy of the pulses is maximum. The power produced by the amplifier(s) is likely to be limited by the appearance of Raman radiation. According to the present disclosure, on the one hand, the length of the active fiber of the last amplifier 150 is limited, and, on the other hand, any passive optical fiber after the last amplifier 150 is eliminated. This arrangement makes it possible to limit the length of fiber traversed by the amplified pulse. Flexible offset cannot be achieved by using an amplifier housed in the main housing of the laser followed by a section of passive fiber which would ensure flexible transport of the pulses.In this case, the Raman effect induced by a maximum energy pulse over lengths of the order of 2 to 3 m would be unacceptable. In an exemplary embodiment, the last fiber section comprises a passive optical fiber section followed by the last optical amplifier 150 consisting of an active fiber section. The passive fiber and the active fiber have core diameters that are close or even identical, for example two fibers with a core diameter of 10 µm or a passive fiber of 10 µm and an active fiber of 12 µm or a passive fiber of 10 µm and an active fiber of 14 µm and are both double-clad structures. The active fiber of the last optical amplifier 150 has a core diameter of between 9 µm and 20 µm, and advantageously between 10 µm and 15 µm. The active fiber of the last optical amplifier 150 has a length of less than 2m and preferably less than 1m.In one embodiment, for example for a last optical amplifier 150 having a length of 1 m, the passive fiber has a length of approximately 3 m. The active fiber of the last optical amplifier 150 has a core doped with ytterbium ions.

[0044] The passive fiber of the last section upstream of the last optical amplifier 150 makes it possible to increase the laser-matter interaction volume in which the distal end of the last optical amplifier 150 can be moved, without modifying the properties of the amplified laser pulse.

[0045] At the output of the last optical amplifier 150, the amplified pulse 15 has an energy greater than or equal to approximately 1 microjoule (µJ). In addition, this energy is adjustable as a function of the energy of the source pulse. In a complementary manner, the energy of the amplified pulse 15 is further adjustable, for example when the energy adjustment means 180 make it possible to adjust the pump current of at least one optical amplifier.

[0046] The compressor 170 is disposed downstream of the optical amplification system. More specifically, the compressor is attached to the distal end of the last fiber optic amplifier 150. For example, the active fiber of the last amplifier may be equipped with an FC-APC or equivalent type connector which is connected to the mechanical part supporting the compressor, or be fixed in a collimator which makes it possible to produce a collimated beam, said collimator being attached to the compressor support.

[0047] The compressor 170 is massive and non-adjustable. In other words, the spectral phase shift introduced by the compressor is fixed. Since the compressor 170 has a predetermined dispersion of approximately 5 to 30 ps / nm, it does not include any adjustment means and has a reduced footprint. In a particular embodiment, the compressor has typical dimensions of 5 mm x 5 mm x 7 mm and is included in a mechanical module which has, for example, a volume of 2 cm x 2 cm x 2 cm on each side. The compressor 170 allows compression by a factor typically of 20 to 50 for an initial pulse of 50 ps. Since the compressor is not adjustable, it is stable with respect to external disturbances, for example vibrations. In an exemplary embodiment, the compressor 170 is composed of a fixed set of two diffraction gratings in series on the optical path according to the conventional compression scheme.According to a particularly advantageous variant, the compressor 170 is composed of a massive Bragg grating with variable pitch (or CVBG according to the English terminology for Chirped Volume Bragg Grating, produced for example by the company OPTIGRATE, Florida, USA). More precisely, a Bragg grating with variable pitch has a drift of the diffraction pitch along its propagation axis. A Bragg grating with variable pitch is known to be able to produce a spectral phase shift very close to a parabola and therefore a quasi-linear frequency drift.

[0048] The compressor 170 receives the amplified pulse 15 at the output of the last amplifier 150 and forms a high-energy compressed pulse 17. The energy, respectively the peak power of the compressed pulse 17 is of the order of 1 µJ to 30 µJ, respectively 500 kW to 30 MW. The duration of the compressed pulse 17 is less than 10 ps, ​​or even less than 3 ps, or even less than 1 ps, i.e. in the femtosecond range. More precisely, the duration of the compressed pulse 17 is adjustable as a function of the energy of the source pulse.

[0049] The adjustment of the compressed pulse duration 17 is of crucial importance in certain applications, for example in eye surgery, particularly in cataract surgery. Here, an increase of the order of 20% to 25% of the energy of the source pulse makes it possible to vary the duration of the compressed pulse from 2 ps to 1 ps (see Figure 6 ), without any other adjustment of the laser system (no adjustment in phase or dispersion of the compressor in particular). However, a reduction in the pulse duration by a factor of 2 corresponds to an increase in peak power by a factor of 2. Controlling the energy of the source pulse and therefore of the amplified pulse makes it possible to very finely control the peak power delivered at the output of the compressor. This control is carried out here entirely and solely via an electronic control system and does not require any optical or opto-mechanical adjustment. Consequently, according to the present disclosure, the pulse duration adjustment can be carried out more quickly than with a mechanical or opto-mechanical system. This makes it possible to adjust the duration of each pulse individually and therefore to optimize the laser-matter interaction, for example.

[0050] There figure 2A represents a first example embodiment of an injector in a laser system according to the present disclosure. The injector 100 here comprises a laser oscillator 200, an optical source preamplifier 220 and a pulse selector 230. The laser oscillator 200 generates at least one initial pulse 1 of duration d 0 and spectral band B 0 . The optical source preamplifier 220 makes it possible to increase the intensity / energy of the initial pulse 1 and to generate a preamplified initial pulse 2, without modifying its duration or its spectral band. More precisely, the energy adjustment means 180 make it possible to adjust the intensity / energy of the preamplified initial pulse 2. The pulse selector 230 makes it possible to modify the rate of the laser source and to select one or more source pulses 10 also called input pulses from among the preamplified initial pulses 2.In one example, the laser oscillator 200 generates initial pulses 1 at a rate generally between 1 MHz and 100 MHz. The pulse selector 230 makes it possible to reduce the rate of the source pulses 10 or to select a source pulse 10 on demand. The pulse selector is for example an acousto-optic modulator or an electro-optic modulator. In addition to its role of adjusting the rate of the source, it possibly makes it possible to modulate the energy of the pulses 10.

[0051] Thus, by adjusting the energy of the source pulse 1 using the energy adjustment means 180, it is possible to adjust the duration of the compressed pulse 17. The evolution of the duration of the compressed pulse 17 at the output of the compressor 170 is observed on the figure 7 .

[0052] In the case where the injector 100 comprises an optical source preamplifier 220 and a pulse selector 230, the optical source preamplifier 220 can be arranged either upstream or downstream of the pulse selector 230.

[0053] If the energy of the initial pulse 1 from the oscillator is less than approximately 100 pJ, this source pulse 1 is injected into the optical source preamplifier 220 in order to bring the energy of the preamplified initial pulse 2 to a value greater than 100 pJ and preferably to a value greater than 1000 pJ or 1 nJ. The gain of the optical source preamplifier 220 is electronically adjustable using the energy adjustment means 180, which makes it possible to adjust the energy of the source pulse 10 at the output of the injector 100. The optical source preamplifier 220 takes the form of a doped fiber pumped by one or more laser diodes. Examples of doping materials for active optical fibers include ytterbium (Yb), neodymium (Nd), erbium (Er), erbium-ytterbium co-doping (Er / Yb), holmium (Ho), thulium (Tm), Yb,Tm co-doping, but not limited to this list. The source optical preamplifier 220 is pumped by one or more laser diodes.Adjusting the gain of the optical source preamplifier can thus be done by adjusting the parameters of the pump diode(s), such as their supply current.

[0054] The rate of the oscillator 200 is typically 10 to 100 MHz but may be less than 10 MHz or even less than 1 MHz without being less than the final rate of production of the compressed pulses 17. If the rate of the oscillator 200 does not correspond to that desired for the compressed pulses 17 at the output of the system, the initial pulse 1 or, respectively, the preamplified initial pulse 2 is injected into a pulse selector 230 which reduces the rate and selects the source pulses 10 intended to be amplified. This pulse selector 230 is for example an acousto-optic modulator whose inputs and outputs are fibered. It may also take the form of an electro-optic modulator or any other element fast enough to select the pulses. The 230 pulse selector is optional if the 200 oscillator operates directly at the desired rate at the system output.This is usually the case for a triggered or modulated laser diode oscillator.

[0055] In another embodiment, illustrated in the figure 2B , the injector comprises a laser oscillator 200, an optical source preamplifier 220, a pulse selector 230 and an optical attenuator 260. The energy of the source pulses 10 is finely adjusted using a component which can take the form of an attenuator or a controllable modulator. The optical attenuator 260 can be a separate element or be included in the same optical modulator 270 as the pulse selector 230. An acousto-optic modulator makes it possible, for example, to select the pulses one by one and at the same time to adjust the energy of these pulses at the output of the modulator. In this way, a train of initial pulses with an energy of the order of 1 nJ at a rate of 20 MHz can be transformed into a train of 10 source pulses at a rate of, for example, 250 kHz and with an energy adjustable between 0.01 nJ and 0.5 nJ. The rate of the 10 source pulses can also be chosen independently from among the sub-multiples of 20 MHz.The advantage of a fast modulator such as an acousto-optic or electro-optic modulator is that it allows the energy of the source pulses 10 to be modulated one by one and to produce pulse trains of adjustable energy for each source pulse 10.

[0056] The injector 100 is controlled by a control unit 180 which makes it possible to modulate the energy of each source pulse 10 independently. The energy of the source 10 is modulated, which will have the effect of slightly modifying the energy of the amplified 15 and compressed 17 pulses, but above all of modifying their compressed duration. The injector 100 is then capable of producing a train of source pulses 10 at a maximum rate which is the rate, denoted F0, of the laser oscillator 200 but which can be any rate that is a sub-multiple (F0 / n) of this initial rate, n being an integer, each source pulse 10 having an adjustable energy. The injector 100 can also produce packets of N source pulses 10 at the rate F0 or at a lower rate F0 / n, these packets being generated periodically or non-periodically. Generally, the injector 100 produces a train of source pulses 10 at the rate desired by the user and the energy of which is adjustable.

[0057] There figure 3 schematically represents a fiber optic laser system according to a first exemplary embodiment. This fiber optic laser system comprises an injector 100 as described in connection with the figure 2A ou 2B , a fiber optic spectro-temporal shaping module 110, a series of doped fiber optical amplifiers 130, 150 and a compressor 170. This fiber optic laser system allows the production of high energy ultra-short pulses 17. The series of doped fiber optical amplifiers 130, 150 allows the pulse energy to be increased by a factor of 1000, or even 10,000 or 100,000 or 1,000,000 or more. Examples of doping materials for optical fibers include Yb, Nd, Er, Er / Yb, Ho, Tm, Yb / Tm ions but are not limited to this list. The compressor 170 allows the duration of the amplified pulses 15 to be compressed by a factor greater than 10.

[0058] In order to reduce the duration of the compressed pulses 17 at the output of the fiber optic laser system, the present disclosure uses self-phase modulation and a spectro-temporal shaping module 110. The self-phase modulation phenomenon creates a spectral broadening accompanied by a modulation of the phase. To obtain maximum compression of the amplified pulse, it is necessary that the frequency drift associated with the phase modulation created in all the elements located between the injector 100 and the compressor 170, including the optical amplifiers 130, 150, corresponds precisely to the frequency drift induced in the compressor 170 to the nearest sign. The positive drift due to the self-phase modulation is compensated by a negative drift induced by the compressor.To solve this problem, the present disclosure uses a spectro-temporal shaping module 110 which makes it possible to adjust the frequency drift of the amplified pulse 15 at the input of the compressor 170. The spectro-temporal shaping module 110 is composed of a passive single-mode optical fiber with polarization preservation whose length in meters is between 6 and 12 times the duration in picoseconds of the source pulse 10 incident on this fiber. As described in connection with the . figures 2A et 2B , the energy of the source pulse 10 injected into the fiber 110 is adjusted using the energy adjustment means 180. The diameter of the core of this fiber 110 is between 5 and 10 µm and generally between 5 and 7 µm. The spectrum of the spectrally broadened pulse 12 at the output of this fiber 110 is broadened by a factor of between 5 and 30. A spectral broadening factor greater than 30 is harmful because it is likely to induce the appearance of stimulated Raman scattering which will distort the pulse.

[0059] The profile of the spectrally broadened pulse 12 is only slightly modified in this propagation. Its duration remains less than 1.5 times the duration of the source pulse 10 and the parabolic quality factor introduced by Pierrot is greater than 0.13. The spectrally broadened pulse 12 is therefore not considered to be quasi-parabolic. Nevertheless, it can be shown using a fiber propagation simulation code that when this spectro-temporal shaping module 110 is followed by a non-linear optical amplifier in which the pulse spectrum will again be broadened by a factor of 3 to 20, the final frequency drift after amplification is linear over a significant portion of the amplified pulse 15, which significantly improves the quality of the compressed pulses 17.

[0060] The spectrally broadened pulse beam 12 from the spectro-temporal shaping module 110 is coupled into one or more components that provide isolation between the amplification stages and spectral filtering of wavelengths outside a spectral band close to that of the source 100. In particular, the filters 120, 140 have very low transmission at the wavelength of the Raman frequency produced by the radiation from the source 100. This value is easily calculated from known parameters such as the Raman shift of fused silica (450 cm -1 < ). For example, for a laser oscillator emitting at a wavelength of 1030 nm, the filter 120, 140 is chosen to have high transmission around 1030 nm, for example between 1025 nm and 1035 nm, and very low transmission around 1080 nm.An example of a filter is a transmission filter having a transmission greater than 90% in the 1026 - 1034 nm band and a transmission less than 1% in the 1050 nm - 1150 nm band.

[0061] After passing through a first spectral filter 120 and an optical isolator (not shown), the spectrally broadened pulse 12 propagates in a passive fiber 330 which guides the pulse to the input of the first active fiber amplifier 130. In a particular mode of use, the spectral filter 120 and the optical isolator are included in the same fiber component which provides both the filtering and isolation functions. The passive fiber 330 and the active fiber 130 have an identical or very close diameter and typically between 5 and 11 µm. The active fiber of the first amplifier 130 has a length between 1 m and 5 m and preferably between 1 m and 2 m. The active fiber of the first amplifier 130 is pumped by one or more laser diodes 322. The pump radiation is injected into the pump cladding of the active fiber of the first amplifier 130, for example using a pump combiner 321.The gain of the first amplifier 130 is typically from 10 to 1000, and advantageously between 10 and 100.

[0062] A second assembly comprising an optical isolator and a last spectral filter 140 is arranged at the output of the first amplifier 130. The last spectral filter 140 operates on the same principle as the first spectral filter 120. The spectrally broadened pulse 14 from this optical isolator and last spectral filter 140 assembly is transmitted to a passive optical fiber 360 through another pump combiner 351. The pump combiner 351 makes it possible to inject into the passive fiber 360 the pump radiation of one or more pump diodes 352. The passive fiber 360 is for example a polarization-preserving double-clad passive fiber whose core and cladding diameters are substantially identical to those of the active fiber of the last amplifier 150 to which it is welded.

[0063] The length of the passive fiber 360 is between 1 and 10m and preferably between 1 and 3m in order to limit non-linear effects in this fiber. The passive fiber 360 has a core having a core diameter of approximately 10 to 20 µm. The active fiber of the last amplifier 150 is a double-clad fiber whose core is doped with rare earth ions. In our example, the active fiber of the last amplifier 150 is an ytterbium-doped fiber whose core diameter is 12.5 µm. The absorption of the fiber for a pump injected into the cladding with a diameter typically 105 µm is of the order of 8 dB / m at 976 nm. The gain of this active fiber 150 is between 100 and 3000 and preferably between 300 and 1000. The energy of an amplified pulse 15 from the last amplifier 150 exceeds 1 µJ and preferably lies between 10 µJ and 40 µJ.The energy per pulse is limited by the appearance of Raman radiation linked to the very strong non-linear optical effects in the last amplifier 150. In order to limit these effects, it is appropriate to limit the length of the active fiber of the last optical amplifier 150 as much as possible. A length of 0.7 m is a good example of optimization. The assembly consisting of the passive fiber 360 and the active fiber 150 is advantageously protected by a flexible metal or plastic sheath which covers in particular the weld between the fibers 360 and 150. This sheath is flexible and makes it possible to position the end of the active fiber of the last amplifier 150 in all directions and all positions over a volume of at least 2 m in diameter.

[0064] The radiation of the amplified pulse 15 from the last active fiber amplifier 150 is incident on the compressor 170 whose frequency drift is fixed and not adjustable. An example of a compressor of this type is sold by OPTIGRATE (Oviedo, USA) under the reference CBG-1030-XX. The dispersion of this compressor is typically between 7 and 50 ps / nm and preferably less than 30 ps / nm. The compressor 170 is adapted to support the transmission of compressed pulses 17 of duration less than 3 ps with energies greater than 1 µJ. These operating parameters are incompatible with a fiber Bragg grating compressor. The compressor 170 is fixedly attached to the active fiber of the last amplifier 150. The compressor 170 has dimensions compatible with the easy movement of the end of the fiber to which it is attached in all directions and positions.For example, its size is less than 50x50x50 mm and in our example less than 25 x 25 x 25 mm. The beam diameter in the compressor 170 is chosen to allow propagation without damage in the components of the system of the targeted application which will host the laser source, for example 3 mm. A smaller or larger diameter is of course possible without altering the performance of the present invention.

[0065] There figure 4 illustrates an interesting variant of the fiber optic laser system of the figure 3 . According to this variant, the spectro-temporal shaping module 110 is used in reflection. The output of the injector 100 is coupled into an optical circulator 411 well known to those skilled in the art. The optical circulator 411 directs the source pulse 10 towards a passive optical fiber 410 of half the length of the passive fiber of the spectro-temporal shaping module 110 described in connection with the figure 3 . In practice, the passive optical fiber 410 is a polarization-preserving single-mode fiber, the length of which in meters is between 3 and 5 times the duration of the incident pulse on this fiber in picoseconds. A fiber Bragg mirror 412 with a spectral width between 0.1 and 3 nm centered at the wavelength of the source pulse is welded to the end of this passive optical fiber 410. The fiber Bragg mirror 412 returns the laser pulse for a second pass in the passive optical fiber 410 and also eliminates any radiation outside a spectral band a few nanometers wide, centered on the laser wavelength. The fiber Bragg mirror 412 eliminates in particular the Raman radiation resulting from propagation in the fiber 410 before reflection on this Bragg mirror.On the other hand, it reflects the pulse broadened spectrally by the self-phase modulation undergone during propagation in the 410 fiber before reflection on this Bragg mirror without reducing the spectral width of the pulse. This variant makes it possible to accumulate the desired effects of spectral broadening by self-phase modulation without suffering the associated deleterious effects such as the Raman effect. Furthermore, it makes it possible to reduce by half the length of the 410 optical fiber used. The rest of the amplification and compression chain is similar to that described in connection with the . figure 3 .

[0066] An example of using a fiber optic laser system as described in connection with the figures shown in the figures 1 à 4 is detailed below.

[0067] The laser oscillator 200 generates initial pulses 1 having a duration of 50 ps at a rate of 20 MHz. A fixed-gain optical source preamplifier 220 amplifies the energy of the preamplified pulses up to 2 nJ. A fiber acousto-optic modulator 270 is used to reduce the rate to 250 kHz and adjust the energy of the source pulses 10 within a window between 0.2 nJ and 0.8 nJ.

[0068] The source pulses 10 are injected into a spectro-temporal shaping module 110 consisting of a 400 m long single-mode fiber and then into an optical amplifier system (as illustrated in the figure 3 ). The total optical fiber length of the optical amplifier system is 10 m, including 2 m of active fibers. The pulses are amplified to an energy of 16 µJ at the output of the optical amplifier system and then recompressed in the 170 compressor with dispersive volume Bragg grating.

[0069] There figure 5A represents an example of a spectrum of a spectrally broadened pulse 11 at the output of the spectro-temporal shaping module 110. The spectrum is here calculated by a simulation code using the input data of the experiment, in particular for an input energy in the module of 0.2 nJ. The spectral width at the output of module 110 is 0.29 nm. The figure 5B represents the calculated spectrum of the compressed pulse 17 at the output of the compressor 170. This image shows the typical modulations of spectral broadening by self-phase modulation. The spectral width is then 2.43 nm. The figure 6 shows the autocorrelation function of the compressed pulse measured at the system output (curve 600) and respectively of the spectrally broadened pulse calculated with our code (curve 610). The pulse duration is 1.2 ps.

[0070] The optimum compression expressed as giving the compressed pulse 17 of shortest duration is obtained by adjusting the diffraction efficiency of the acousto-optic modulator 270 of an injector 100 as described in connection with the figure 2B , which makes it possible to adjust the energy of the source pulse 10 injected into the spectro-temporal shaping module 110.

[0071] The duration of the compressed pulses 17 can also be tuned by modifying the diffraction efficiency of the acousto-optic modulator 270, which makes it possible to adjust the energy of the source pulse 10 injected into the spectro-temporal shaping module 110.

[0072] There figure 7 represents a calculated example of compressed pulse duration D 17 as a function of the energy of the source pulse 10 injected into the spectro-temporal shaping module 110. The energy adjustment of the source pulse 10 between 0.2 nJ and 0.25 nJ, i.e. 25% variation, makes it possible to precisely adjust the duration of the compressed pulse between approximately 2.5 ps and 1 ps (more than 150% variation).

[0073] It is observed that the energy adjustment of the source pulse 10 can be obtained in an equivalent manner by modifying the gain of the optical source preamplifier 220 in an injector 100 as described in connection with the figure 2A .

[0074] In an exemplary embodiment, the energy of the source pulse 10 injected into the spectro-temporal shaping module 110 is increased to 0.8 nJ, producing, at the output of this module, a spectrum with a width equal to 0.87 nm. The gain of the source optical pre-amplifier 220 is reduced to maintain the same incident energy on the last optical amplifier 150 and the gain of this last optical amplifier 150 is adjusted to optimize the compression. figures 8A-8B show the spectrum ( fig.8A ) and the autocorrelation function ( fig.8B ) of the compressed pulse 17. The spectrum of the fig. 8A is strongly broadened (bandwidth ~3.8 nm) and has a different shape from that of the figure 5B . It remains highly modulated but the extreme frequency peaks of the Fig.5B are greatly reduced in favor of the central frequencies which correspond to these areas where the frequency drift is linear and therefore easily compensated by the compressor. The autocorrelation function corresponds to a notably shorter pulse duration of approximately 0.76 ps, with an extremely reduced pedestal compared to the figures 5A and respectively 5B. It is also observed here that the choice of the parameters for using the spectro-temporal shaping module 110 influences the quality and shape of the compressed pulse 17. Adjusting the energy of the source pulse 10 at the input of the spectro-temporal shaping module makes it possible to very precisely adjust the duration of the compressed pulse 17.

[0075] In a particular embodiment presented on the figure 9 , the injector 100 produces a train of source pulse packets of variable amplitude or energy E1, E2, E3, E4, the energy of each source pulse 10 being chosen by the user. At the output of the injector 10, the source beam consists for example, and without this being limited to this example, of a periodic train at the rate F1 of pulse packets at the rate F2, each pulse of a packet having a variable energy E1, E2, E3, E4 and not necessarily constant. This train of energy-modulated pulses produces, after passing through the spectro-temporal shaping module 110 and the entire amplifier chain and compressor 170, a train of compressed pulses 17 of duration D1, D2, D3, D4 which varies as a function of the energy modulation of the source pulses imposed by the injector 100. In addition, each compressed pulse 17 has an energy E11, E12, E13, E14 which varies as a function of the energy of the source pulses E1, E2, E3, E4 respectively.

[0076] There Figure 9 schematically shows an example of a laser system for producing a pulse duration modulated train according to the present disclosure.

[0077] Adjusting the energy of an incident pulse in the last amplifier 150 modifies the amount of self-phase modulation induced in this last amplifier 150. In order to maintain the minimum compressed duration, it is then necessary to adjust the amplification factor of the last amplifier, which makes it possible to adjust the energy of the compressed pulse 17 at the output of the compressor. Thus, the present disclosure proposes a method for slightly modifying the energy of the compressed pulse 17 while maintaining the minimum duration permitted by the phase and amplitude characteristics of the amplified pulse, for example by modifying the gain of the first optical amplifier 130, for example by decreasing this gain, and compensating for this variation by modifying the gain of the last optical amplifier 150, in this case by increasing it.Since self-phase modulation is not created at exactly the same level in the amplification chain, it can be shown theoretically and observed experimentally that the final energy giving the optimum compression is obtained for a greater energy when the gain of the pre-amplifier is reduced and that of the final amplifier increased.

[0078] In order to allow easy use of the invention, the final segment consisting of a passive fiber 360 and the last fiber amplifier 150 is inserted into a flexible sheath 160. The sheath 160 is metallic or made of polymer or a combination of materials suitable for providing protection against shocks and traction on the fibers. This sheath has a typical diameter of 1 to 9 mm and a minimum bending radius of less than 50 cm. Preferably, the diameter of the sheath 160 is less than 6 mm and the minimum bending radius less than 10 cm. The sheath 160 is securely attached, on one side, to the body 1001 of the laser (comprising the injector 100, the spectro-temporal shaping module 110 and possibly one or more other amplification and spectral filtering stages) and, on the other side, to the mechanical body 190 supporting the compressor 17.The optical fibers can move freely in the sheath 160 including the weld between the passive fiber section 360 and the active fiber section 150. The distal end of the last optical amplifier 150 is fixed to the mechanical body surrounding the compressor 17 (see . figure 10 For example).

[0079] The system described here is usable in many applications that require ultra-short and intense laser pulses. In particular, all micromachining applications benefit from the flexibility due to the optical fiber flexibility of the latest amplifier for the delivery of the laser beam at any point of a scene and at any incidence while maintaining the ability to focus at the diffraction limit pulses having a duration of less than 3 ps.

[0080] For example, an industrial robot may be equipped with the laser system of the present disclosure. The injector 100, the spectro-temporal shaping module 110, any preamplifiers and the pump coupler 351, as well as the pump laser diodes and all of the electronic power supply and control boards are included in a source box 1001 as illustrated in the figure 10. An active transport module 1002 comprises a passive fiber section 360 and a short active fiber section forming the last optical amplifier 150 in a flexible mechanical sheath 160 securely attached on one side to the source housing 1001 and the other to the compression module 1003. The active transport module 1001 has a minimum radius of curvature in any direction of 10 cm, possibly 3 cm. The compression module 1003 comprises the compressor 17 and its mechanical support body 190. The compression module 1003 makes it possible to compress the duration of a source pulse 10 typically between 30 and 100 ps to a compressed pulse duration 17 of less than 3 ps. The compression module 1003 is for example attached to the end of an arm of the robot which makes it possible to move it in all positions and directions around the part to be treated.The quality of the beam from the compressor being limited by diffraction, a focal spot diameter of less than 5 µm is obtained over a field of more than 1 m 2< and at a variable working distance which can reach more than 1 m while maintaining a focal spot with a diameter of less than 100 µm.

[0081] The invention finds applications in the micromachining of flat screens or solar cell panels several meters on each side without having to move the panel or screen relative to the laser, only the fiber end connected to the compressor moving.

[0082] The invention is particularly suitable for the manufacture of large industrial molds with micro-structures.

[0083] Finally, this laser system allows the design of medical devices for tissue ablation, such as flap cutting in myopia corrective surgery, laser-assisted cataract surgery, skin tattoo removal or any other medical or dermatological treatment device using short pulses. In particular, it is possible to design a handpiece, held by the surgeon, which focuses pulses of duration less than 3 ps on spots of less than 5 µm regardless of the surgeon's hand movements.

[0084] The use of this laser system is obviously not limited to the examples cited above.

[0085] Of course, various other modifications may be made to the invention within the scope of the appended claims.

Claims

1. An optical-fibre laser system comprising an injector (100) suitable for generating at least one source pulse (10) of duration between 30 and 100 picoseconds and having a source spectral bandwidth, a spectro-temporal shaping module (110) arranged at the output of the injector (100), the spectro-temporal shaping module (110) comprising a passive single-mode optical fibre having a length between 100 metres and 1 kilometre, the passive single-mode optical fibre having a physical length in metres that is between 6 and 12 times the duration of the source pulse in picoseconds, the spectro-temporal shaping module (110) being configured to receive the source pulse (10) and to generate a spectrally broadened pulse (11) having a spectral bandwidth 5 to 50 times broader than the source spectral bandwidth and a duration of less than 1.5 times the source pulse (10) duration, an optical amplifier system (130, 150) with flexible single-mode optical-fibre(s), the optical amplifier system (130, 150) having a flexible optical fibre length longer than one metre, the optical amplifier system comprising at least one last optical fibre amplifier (150), the last amplifier (150) having a length of less than ten metres, at least one last spectral filter (140) arranged downstream from the spectro-temporal shaping module (110) and upstream from the last optical fibre amplifier (150), the last spectral filter (140) having a spectral bandwidth of less than 50 nm and higher than or equal to the spectral bandwidth of the spectrally broadened light pulse, in such a way as to limit the generation of a Raman signal in the last optical-fibre amplifier (150), the last amplifier (150) being suitable for receiving a filtered pulse (14) from the spectral filter (120) and for generating a controlled chirped amplified pulse (15) at one end (155) of the last amplifier (150), the end (155) of the last optical-fibre amplifier (150) being flexible about at least two axes of rotation transverse to the optical axis, with a minimum bending radius of less than 50 cm, the amplified pulse (15) having an energy of more than 1 microjoule, a volume compressor (170) attached to the end (155) of the last optical-fibre amplifier (150), the compressor (170) having a predetermined almost-linear chirp, the compressor (170) being suitable for receiving the amplified pulse (15) and forming a compressed pulse (17) of less than 3 picoseconds duration, and electronic means for adjusting the compressed pulse duration, said electronic duration adjustment means being consisted of energy adjustment means (180) suitable for adjusting the energy of the source pulse (10) and / or of the amplified pulse (15), the duration of the compressed pulse (17) being tunable based on the energy setting.

2. The fibre laser system according to claim 1, wherein the injector (100) further includes a pulse selector (230) or an optical attenuator (260) or an electro-optic modulator or an acousto-optic modulator (270) or any combination of these elements and wherein the energy adjustment means (180) are suitable for adjusting the energy of the source pulse (10) by means of the pulse selector (230), the optical attenuator (260), the electro-optic modulator or the acousto-optic modulator (270), respectively, or the combination thereof.

3. The laser system according to claim 1 or 2, wherein the electronic means for adjusting the duration of the compressed pulse comprise means for adjusting the energy of the optical amplifier system (130, 150).

4. The laser system according to claim 3, wherein the optical amplifier system (130, 150) comprises at least one pump diode suitable for generating a pump radiation and wherein the means for adjusting the energy of the optical amplifier system (130, 150) are suitable for adjusting an electrical current of the pump diode in such a way as to adjust the pump radiation injected into the optical amplifier system (130, 150).

5. The fibre laser system according to any one of claims 1 to 4, wherein the optical amplifier system (130, 150) is based on step-index optical fibre(s) and / or photonic-crystal-structure optical fibre(s) having a solid core and a guiding cladding formed of capillaries arranged in a ring around the core.

6. The fibre laser system according to any one of claims 1 to 5, wherein the optical amplifier system (130, 150) includes a double-cladding optical fibre.

7. The fibre laser system according to any one of claims 1 to 6, wherein the amplifier system includes a last passive single-mode optical fibre (360) arranged upstream from the last flexible optical-fibre amplifier (150).

8. The fibre laser system according to any one of claims 1 to 7, wherein the last amplifier (150) has a length of less than one metre or two metres.

9. The fibre laser system according to any one of claims 1 to 8, wherein the amplifier system includes a first flexible optical-fibre amplifier (130) arranged upstream from the last amplifier (150) and a first spectral filter (120) arranged downstream from the spectro-temporal shaping module (110) and upstream from the first amplifier (130).

10. The fibre laser system according to any one of claims 1 to 9, wherein the last amplifier (150) and / or the first amplifier (130) includes an optical fibre having a core based on silica doped with rare earth ions, chosen among the following ions: ytterbium, neodymium, erbium-ytterbium, holmium, thulium, ytterbium-thulium.

11. The fibre laser system according to any one of claims 1 to 10, wherein the compressor (17) includes two fixed-pitch gratings or a dispersive volume Bragg grating with continuously variable pitch.

12. The fibre laser system according to any one of claims 1 to 11, comprising a flexible protective mechanical cladding (160) arranged around the last amplifier (150) and the passive fibre (360) and mechanically attached to the compressor (170).

13. The fibre laser system according to any one of claims 1 to 12, wherein the energy adjustment means (180) are suitable for adjusting the energy of each source pulse (10) individually in order to vary the compressed duration of each individually amplified pulse (17).

14. The fibre laser system according to any one of claims 1 to 13, wherein the duration of the compressed pulse decreases as a function of the increasing energy of the source pulse in a variation range of ±40% of the source pulse energy.

15. The use of an optical-fibre laser system according to any one of claims 1 to 14 in a micro-machining device, a medical device, an ophthalmic surgery device or a skin tattoo removal device.