FR3048137A1 - LASER AMPLIFICATION SYSTEM AND METHOD FOR CORRECTING AN ASYMMETRIC CROSS-PRESSURE TRANSVERSAL PROFILE IN A LASER-ACTIVATED MEDIUM OF A SOLID BODY - Google Patents

Abstract

To improve a laser amplification system comprising a solid body with a laser activity medium, a first source of pumping radiation, designed to generate a first pumping radiation field that travels at least once, in particular several times said solid body, for the purpose of exciting said laser-active medium, in a manner to improve the quality of the beams in a solid-state laser, it is proposed that it include less a second source of pumping radiation, designed to generate at least a second pumping radiation field which travels at least once, in particular several times the solid body, for the excitation of the laser activity medium, the first pumping radiation field and the second field pumping radiation being oriented to substantially compensate or compensate, in a transverse plane defined by the solid body, the sum of all the first transverse members of said first pump radiation field incident on said solid body, and the sum of all second transverse components of said second pump radiation field incident on said solid body.

Description

LASER AMPLIFICATION SYSTEM AND METHOD FOR CORRECTING AN ASYMMETRICAL RADIATION PRESSURE TRANSVERSAL PROFILE IN A LASER ACTIVITY MEDIUM OF A SOLID BODY

The present invention relates to a laser amplification system comprising a solid body provided with a laser activity medium, a first source of pump radiation, designed to generate a first pump radiation field that travels at least once, in particular several times said solid body, for the purpose of exciting said laser activity medium. The invention also relates to a method of correcting an asymmetric transverse profile of radiation pressure on a solid body of a laser amplification system, having a laser activity medium, which profile of radiation pressure is generated. by a first pumping radiation field which travels through said solid body.

A laser amplification system of the type described in the introduction, devoted to the formation of a solid-state laser, is for example known from WO 01/57970 A1 or US 2001/0040909 A1. such that the pumping radiation field generated by a pump radiation source, for example a battery of diode lasers, can traverse the solid body several times in order to excite, as optimally as possible, the laser activity material. For this purpose, as described in WO 01/57970 A1 or US 2001/0040909 A1, the pumping radiation field is reproduced in the focused state on the solid body and, after double passing through said solid body, it is retrodévié to said solid body by means of a deflector unit. Said pumping radiation field travels twice said solid body after each incidence, since it is reflected on a posterior face of said solid body which bears a highly reflective coating and that, therefore, taking into account the law of reflection, it leaves said solid body by describing the same angle as that according to which it came to incide on said solid body.

A problem affecting this procedure is the radiation pressure that the pumping radiation field exerts on the laser-active medium in the solid body as a result of the multiple superposition of the individual pumping transits and the individual pressure forces. of radiation emanating from absorption and emission processes. To achieve sufficient amplification of the laser amplification system, it operates in particular with pumping power densities of the order of several kilowatts per square centimeter. Correspondingly, the solid body, on which the pumping radiation field is reproduced several times with focusing, is solicited by a very high surface power of the order of megawatts per square centimeter. Whereas the pumping radiation field incites, on the solid body, not perpendicularly to a transverse plane defined by said solid body, but at the oblique angle, that is to say by describing an angle, a parallel force transverse plane, that is to say a transverse force, also acts on the solid body, in the laser activity medium of said solid body, in addition to the radiation pressure exerted perpendicularly by said pumping radiation field, more to absorption and emission processes. The radiation pressure forces emanating from said absorption and emission processes are attributable to the bonded charges in the laser activity material and generate, in addition to the longitudinal component, a transverse force under the effect of the magnetic force of Lorentz. . This governs an asymmetric deformation of the solid body, with depths of the order of a nanometer (see, for example, M. Mansuripur: "Radiation pressure and the linear momentum of the electromagnetic fleld", Optics Express, volume 12, No. 22, 2004, p. 5375). It should then be observed that the radiation pressure exerted on the solid body depends not only on the pumping radiation field and the power of the latter, but also on the concentration of the laser activity material in said body. solid. The higher a concentration of the laser activity material, in the solid body, the greater the effect of the transverse force. Since the power of the pump radiation field is substantially lower during each passage of the latter, because of the absorption of the solid body, the transverse force respectively operating decreases with each passage. This results in asymmetrical deformation of the solid body, implying astigmatism or more pronounced deformation. This deformation has repercussions on the quality of the laser modes forming in the resonator of a solid-state laser in which said solid body is integrated. Thus, ultimately, the known laser amplification system preferably allows the excitation, in the solid-state laser, of laser modes whose radiation profiles are not symmetrical in rotation.

Accordingly, an object of the present invention is to improve the quality of beams in a solid-state laser, particularly in a disk laser.

According to the invention, in a laser amplification system of the type described in the introduction, this object is achieved by the fact that it includes at least one second source of pumping radiation, designed to generate at least a second radiation field pump which travels at least once, in particular several times the solid body, for excitation of the laser activity medium, the first pump radiation field and the second pump radiation field being oriented so as to compensate for or substantially compensating, in a transverse plane defined by the solid body, the sum of all the first transverse components of said first pumping radiation field inciding on said solid body, and the sum of all the second transverse components of said second field of pumping radiation coming to incide on said solid body.

The improvement of the known laser amplification system, proposed in accordance with the invention, makes it possible in particular to prevent, in the transverse plane, an asymmetrical deformation of the solid body coating, for example, in the form of a disc in a body solid, due to the radiation pressure exerted by the first pumping radiation field and the transverse forces thus operating which are also presented as transverse components of the radiation pressure forces developed by said first pumping radiation field, since the transverse force, acting on the solid body, is compensated for by soliciting said solid body by at least a second pumping radiation field. If, for example, two fields of pumping radiation are generated, the latter are oriented, in particular, towards the solid body, so as to govern a compensation of the respectively resulting transverse components. In particular, this can be achieved because the transverse components of the radiation pressure forces, acting on the solid body under the action of said pumping radiation fields, point in mutually opposite directions. Thus, the transverse forces applied to said solid body by said pumping radiation fields cancel each other out, so that no deformation of said solid body exhibiting rotational asymmetry occurs. This also prevents undesirable deformation of the laser modes that form in the resonator of the solid-state laser, which deformation therefore has negative effects on a beam quality of laser radiation generated by said solid-state laser. In this way, it is possible to form radiation laser modes with full rotational symmetry in cross section. If more than two pump radiation fields are used, the latter are oriented in such a way that the pressure forces, acting on the solid body due to said pump radiation fields, add up in the transverse plane, tending towards zero. In the presence of two pumping radiation fields, these are preferably directed towards the solid body, with a 180 ° shift relative to each other in the transverse plane, said offset having an angle of 120 ° in the case of three fields of pumping radiation. In the presence of more than two pumping radiation fields, the angle is then preset between said pumping radiation fields, in the transverse plane, corresponding to the number of said pumping radiation fields.

In principle, it is also possible to dispense with focusing systems, also referred to as "focusing devices" hereinafter, so that the pump radiation fields travel only once or twice the solid body. Twice, only when a posterior surface of said solid body is provided with a highly reflective coating, such that said pumping radiation fields are reflected to the posterior face of said solid body after they have traveled the latter once and that they then travel through the solid body once more. Nevertheless, the efficiency of such a laser system is not particularly high since only a very small portion of the power of the pump radiation fields can be absorbed by the solid body when said pumping radiation fields only go once or twice to the solid body. This is why it is advantageous for the laser amplification system to be designed in such a way that the pumping radiation fields travel the solid body repeatedly and, more specifically, at least three times in particular.

It is advantageous for the laser amplification system to include a first focusing device which generates a plurality of different branches of the first pump radiation field penetrating the solid body and then converts at least one branch out of the solid body into the one of the branches penetrating into said solid body and differing from said outgoing branch; and that said laser amplification system includes at least a second focusing device which generates a plurality of different branches of the second pump radiation field, penetrating the solid body, and then converts at least one branch, exiting the solid body, into the one of the branches penetrating into said solid body and differing from said outgoing branch. The mode of operation of such focusing devices is described in detail in WO 01/57970 A1 or US 2001/0040909 A1, to the full scope of which reference is made in this regard. Said focusing devices provide, to the pump radiation fields, the possibility of traversing the solid body several times in order to obtain the strongest possible excitation of the laser activity medium in said solid body and, consequently, a factor of high amplification of the solid-state laser.

For the formation of a solid-body laser, it is expedient that said solid body is made in the form of a disk in a solid body. This can in particular be coupled to a heat sink to prevent heat build-up in the solid body. The structural organization of the laser amplification system becomes particularly simple when it includes a first pump radiation source and a second pump radiation source. Thus, when only two sources of pumping radiation are used, these can be arranged, as already mentioned, in such a way that the pumping radiation fields generated by said sources are oriented with transverse components of the pressure forces. acting on said solid body, which point in mutually opposite directions. The pumping radiation fields are therefore directed to the solid body, in the transverse plane, with a shift of 180 °. If said pumping radiation fields do not incide on said solid body by describing the same angle, relative to the transverse plane, it may result in slightly different transverse forces for an identical pumping power of said pumping radiation sources. Said forces may, for example, be regulated by power matching of a pumping radiation field to thereby generate an upper or lower transverse force which then compensates, or best compensates for, the transverse component of the pressure force due to the radiation pressure and applied to said solid body by the other pumping radiation field.

According to another preferred embodiment of the invention, it may be provided that the first focusing device includes at least a first deflector unit, for the purpose of retrodeviation of the first pump radiation field to the solid body after passing through said first focusing device. solid body; the at least one second focusing device includes at least one second deflector unit for the purpose of retrodeviation of the at least one second pump radiation field, to said solid body after passing through said solid body; and said at least one first baffle unit and said at least one second baffle unit direct said first pump radiation field and said at least one second pump radiation field toward said solid body to compensate or offset substantially, in a transverse plane defined by the solid body, the sum of all the first transverse components of said first deviated pumping radiation field, inciding on said solid body, and the sum of all the second transverse components of said at least one second deviated pumping radiation field inciding on said solid body. As with the first incidence of pumping radiation fields on the solid body, the realization of the focusing devices has the effect of ensuring that the pressure forces of the pumping radiation fields, acting on the solid body parallel to the transverse plane , are also compensated after single or multiple deviation of said fields. This is thus embodied, for any number of deviations of the pumping radiation fields, by corresponding arrangement and embodiment of the focusing devices for the at least two pumping radiation fields.

It is advantageous for the first focusing device to include at least two first deflector units and for the at least one second focusing device to include at least two second deflector units. This allows, in particular, that the pumping radiation fields generated by the pumping radiation sources radiate at least eight, in particular sixteen times through the solid body, so that said solid body can extract energy from said fields. of pump radiation, in all eight times and in particular sixteen times, by absorption in the laser activity medium. An amplification factor of the laser amplification system can thus be increased in a desired manner.

The laser amplification system can be made particularly simple when the at least one first baffle device comprises two reflective surfaces extending at an angle to each other and when the at least one second baffle unit comprises two reflective surfaces extending at an angle to each other. An angle, between said reflective surfaces of the deflector units, can measure 90 ° in particular. A pumping radiation field that has once radiated through the solid body, has been reflected to the posterior face of said solid body and then radiated a second time through said solid body can thus simply be backward to said solid body and be optionally focused on it.

It is advantageous for the first focusing device to comprise collimating and focussing elements, which provide an intermediate focusing of the first pumping radiation field directed away from the solid body and which, in the direction of said solid body, focus said first field of view. pumping radiation subjected to intermediate focusing; and that the at least one second focusing device comprises collimating and focussing elements which provide an intermediate focusing of the at least one second pumping radiation field directed away from the solid body and which focus, in the direction of said solid body, said at least a second pumping radiation field, subjected to intermediate focusing. With the aid of such focusing devices, it is possible to prevent an expansion of the pumping radiation field by intermediate focusing. The focusing elements then allow, for their part, a renewed solicitation or a repeated solicitation of the solid body by the pumping radiation fields in a region of the particularly small surface, in order to reach, in the laser activity medium, a optimal excitation of it. The structural organization of the laser amplification system becomes particularly simple when the first focusing device, and the at least one second focusing device include a common focusing element. It can then be, in particular, a common parabolic mirror.

In addition, it is favorable that the first focusing device, and the at least one second focusing device are respectively provided with at least one collimating lens. Such a structural arrangement makes it possible to thwart an enlargement of the pumping radiation fields.

A particularly compact structural arrangement of the laser amplification system may, for example, be obtained in that the at least one first deflector unit and the at least one second deflector unit annularly surround the solid body. A rectangular arrangement of several deflector units may also be provided alternatively. It is advantageous that the at least one first deflector unit and the at least one second deflector unit define a double ring or multiple ring structure surrounding the solid body. This makes it possible to form a particularly compact laser amplification system with a pumping module including the focusing devices.

The laser amplification system preferably comprises second focusing devices of one, two, three, four, five or more. The number of said focusing devices preferably corresponds to the number of pumping radiation sources. Therefore, if, for example, three second sources of pumping radiation are used, three focusing devices are preferably provided for the latter, just as an own focusing device is provided for the first radiation source of pumping.

Preferably, a posterior surface of the solid body carries a highly reflective coating. If such a solid body is biased by a pumping radiation field, it is reflected on the rear face of said solid body, so that pumping radiation, generated by a pumping radiation source, can radiate two once through said solid body.

Particularly good excitation efficiencies may in particular be achieved by the fact that the first source of pump radiation, and the at least one second source of pump radiation are in the form of a laser. They may, in particular, be made in the form of a diode laser or a fiber laser. A battery of diode lasers may in particular be provided as a source of laser radiation.

The solid body presents itself favorably as a laser-active crystal, in particular an yttrium-aluminum garnet crystal. It can, for example, be doped with ytterbium or holmium as a material with laser activity. The choice of the solid body and the laser activity material enclosed by the latter makes it possible to pre-establish, as desired, the wavelength of the solid-state laser.

For the formation of a solid-body laser, it is favorable for said solid body to be integrated in a resonator defining a resonance radiation field which traverses said solid body. Said resonator may include, in particular, two highly reflective elements between which said resonance radiation field is formed.

The resonator can be realized in a particularly simple manner when the posterior face of the solid, highly reflective body forms an end mirror of said resonator.

Furthermore, it is favorable that the resonator includes a decoupling mirror, and that the resonance radiation field extends between the end mirror and said decoupling mirror. Said decoupling mirror may, in particular, be partially transparent or actively switchable in order to decouple, from said resonator, laser radiation having a wavelength pre-established by the laser activity material.

According to the invention, moreover, the object fixed in the preamble is reached, in a process of the type described in the introduction, in that at least a second pumping radiation field is directed towards the solid body; and that the first pumping radiation field, and the at least one second pumping radiation field are oriented relative to one another so as to compensate or substantially compensate, in a transverse plane defined by the solid body , the sum of all first transverse components of said first pump radiation field inciding on said solid body, and the sum of all second transverse components of said at least one second pump radiation field incident on said solid body. The "cross-sectional components" include the parts of the radiation pressure forces acting on the solid body, in the transverse plane, under the action of the pumping radiation fields. As already described in the introduction, the proposed method respectively makes it possible to avoid, or prevent a deformation of the solid body exhibiting asymmetry of rotation, by compensation of radiation pressure forces which act in said transverse plane and are caused by the pressure radiation generated by said pumping radiation fields. In particular, this makes it possible to produce, in the solid-state laser, laser mode radiation profiles that are optimally provided with rotational symmetry.

It is expedient that the first pumping radiation field, and the at least one second pumping radiation field, are oriented relative to each other so as to compensate or substantially compensate in pairs, in the transverse plane, the first transverse components of said first pump radiation field inciding on the solid body, and the second transverse components of said at least one second pump radiation field inciding on said solid body. This possibility is particularly particularly advantageous when an even number of pump radiation fields is used for the excitation of said solid body.

In order to achieve the highest possible amplification, it is advantageous for the first pumping radiation field, and the at least one second pumping radiation field to be deflected at least once towards the solid body, to cause at least one at least a doubling of the number of passages by said solid body. The frequency of occurrence of the passages of said fields of pumping radiation, by said solid body, is all the higher as a deflection is frequently made possible, so that said body can take exciter energy from said radiation fields pumping, by absorption, during each passage through said solid body.

Preferably, the first pump radiation field, and the at least one second pump radiation field are at least respectively focused towards the solid body. This makes it possible concretely to obtain, in the latter, an energy density as high as possible in order to excite said solid body. It is advantageous for the first and the at least one second pump radiation field to be deflected several times in order to multiply the number of passes by the solid body. An amplification factor of the laser amplification system can thus be increased in a desired manner.

For excitation and generation of laser radiation, it is favorable for a solid-state disk to be used as a solid body. This allows, in particular, to form a disk laser solid body.

The description of preferred embodiments of the invention, provided hereinafter, serves to comment on the invention in more detail in support of the drawings. On these: Figure 1 is a schematic illustration of the structural organization of a disk laser (prior art); FIG. 2 is a diagrammatic representation of transverse pressure forces acting on the disk of said disk laser during 8 round trips in solid body; Figure 3 shows a schematic three-dimensional arrangement of a solid-state disk laser provided with two deflector units; FIG. 4 is a schematic illustration of an interferometric device, aimed at highlighting a deformation of the disk in solid body stressed by the pump laser radiation; FIG. 5 shows an interferogram of a spot spot distribution on a solid disk that is not precisely regulated; FIG. 6 is a representation of a laser mode obtained by a resonator I provided with a Ho: YAG disc; FIG. 7a shows an interferogram recorded using the device according to FIG. 4, for a pumping power of 0 watt; FIG. 7b illustrates an interferogram similar to that of FIG. 7a, for a pumping power of 15 watts; FIG. 7c represents an interferogram similar to that of FIG. 7a, for a pumping power of 40 watts; Fig. 8a shows a laser mode profile involving a precisely tuned pumping module, and a pumping power of 15 watts; FIG. 8b illustrates the laser mode profile similar to that of FIG. 8a, for a pumping power of 30 watts; FIG. 8c represents a laser mode profile similar to that of FIG. 8a, for a pumping power of 40 watts; Fig. 9 is a schematic illustration of the simulation of deflection of a glass plate resulting from vertical loading by pumping laser radiation; Figure 10 is a schematic representation of a disk in solid non-deformed body, prior to the solicitation by laser pumping radiation; Fig. 11 is a schematic illustration of the deformation of a thin solid body by the radiation pressure of a laser radiation field; Fig. 12 is a schematic representation of the radiation pressure forces acting on a thin solid-state disk in the laser-powered pumped medium thereof; Fig. 13 is a schematic illustration of the operating radiation pressure p of the pump laser radiation, involving retroreflection of said pump laser radiation onto the highly reflective back surface of the solid-state disk, knowing that p = 2 l / c, I being intensity, and c the speed of light; Fig. 14 is a schematic representation of absorbed photonic power for 10% absorption of pumping radiation, within the disc, during each pass; Fig. 15 is a schematic illustration of a transverse compensation of the radiation pressure forces acting on the solid-state disk, when using two pumping fields focused on the solid body with 180 ° mutual offset; Fig. 16 is a diagrammatic representation of compensating transverse radiation pressure forces involving two sources of pump radiation and deflector units integrated into a double ring structure; Fig. 17 is a schematic illustration of the double ring structure of the deflector units, in accordance with Fig. 16; Fig. 18 is a schematic representation of radiation pressure forces acting in the transverse plane, in a double ring structure involving, respectively, 2 times 12 radiation passes through the solid-state disc; Fig. 19 is a schematic illustration of a variant of transversely acting radiation pressure forces in the presence of deflection units defining a double ring structure and assigned to two pumping radiation sources, with a total number of Twice 12 passages of radiation by the disk in solid body; Fig. 20 is a schematic representation of a dual ring structure of deflection units, and transverse forces of radiation pressure, in 2 x 14 passages of radiation by the solid-state disk; Fig. 21 is a schematic illustration of operative radiation pressure forces in a double ring structure of the deflector units, involving a total number of four pumping radiation sources, and 2 x 12 radiation passages by the solid-state disk; Fig. 22 is a schematic representation of the structural organization of the disc laser, with a view over the reflective surface of the parabolic mirror; Fig. 23 is a schematic illustration of the operating radiation forces in a dual ring structure of the disk laser deflector units shown schematically in Fig. 22, having three deflector units for each ring of said double ring structure.

Figure 1 shows schematically the structural organization of a disk laser, known from the prior art. It includes a thin disk 12 in a solid body which is disposed on a cooling plate 14, and whose rear face 16 carries a highly reflective coating.

A resonator 18 of the disk laser 10 is defined, on the one hand, by the rear face 16 which forms an end mirror 20 and, on the other hand, by a decoupling mirror 22. The resonator 18 defines a field 24 of resonance radiation.

Through the decoupling mirror 22, a laser radiation 26 can be decoupled in continuous or pulsed mode, depending on the choice of the material of the solid-state disk and the laser-active material enclosed by the latter.

A source 28 of pumping radiation generates a pumping radiation field 30 which, via a focusing system 32, is reproduced on the solid-state disc 12 and passes therethrough several times, as appropriate, after multiple deflection.

Figure 2 shows schematically the type of radiation pressure forces acting on the solid-state disc 12 in a transverse plane 34 defined by the latter. In this schematic illustration, it is assumed that said disk 12 solid body, represented by a dotted line, is in the center of the annular structure shown.

The pumping radiation field 30 incites, in the position (J), on the parabolic mirror 38 of the focusing system 32. From there, said pumping radiation field 30 is deflected and comes to incite, in the focused state, on the disc 12 in solid body. A transverse force 36a acts in said transverse plane 34. After having traversed said disk twice in solid body, said pumping radiation field 30 again incites on said parabolic mirror, in the position ®, under the action of the face posterior 16 with highly reflective coating. To be able to focus again said pump radiation field 30 on said disk 12 in solid body, it is again diverted to said parabolic mirror 38, in the position (3), via a first deflector unit 40 provided with two reflective surfaces 46 and 48 oriented, with respect to a plane of symmetry 42, by mutually describing an angle 44 of 90 °. From there, the radiation field 30 again traverses the disk twice in solid body, a transverse force 36b then being applied to said disk 12 in solid body. Said pump radiation field 30 again incites on said parabolic mirror 38 in the @ position.

To once again down-focus the pump radiation field to the solid-state disk 12, the beam reflected by the parabolic mirror 38 is deflected by a second deflector unit 50 including two reflective surfaces 54 and 56 similarly inclined, one compared to the other, at an angle 52 of 90 °. Said second deflecting unit 50 defines, between the two reflective surfaces 54 and 56, a plane of symmetry 58 inclined at an angle 60 with respect to the plane of symmetry 42.

By means of the second deflector unit 50, the pumping radiation field is backscored to the parabolic mirror, more precisely to the position (5). From there, said pumping radiation field again traverses twice the disc 12 in solid body, to which it applies the transverse force 36c.

Then, the pumping radiation field again incites on the parabolic mirror 38, more precisely in the position (f). The first deflector unit 40 again returns said pump radiation field 30 to said parabolic mirror 38, in the position (7) more precisely. From there, said pumping radiation field 30 is again focused towards the disk 12 in solid body and traverses the latter twice, the transverse force 36d then being applied to said disk 12 as a solid body in the transverse plane 34.

The pumping radiation field then incites on a retroreflector 62 and travels the focusing system 32, in the reverse direction, until it leaves said focusing system 32, after reflection, in the position ® of the parabolic mirror 38.

The transverse forces 36e, 36f, 36g and 36h, respectively developed during the return and return of the field of pumping radiation traveling through the disc 12 in solid body, are shown in FIG. 2, knowing that the transverse forces 36a and 36b 36b and 36g, 36c and 36f, as well as 36d and 36e are reversed in respective pairs.

The magnitude of the transverse forces nevertheless decreases successively, starting with the transverse force 36a. This is because radiation energy is taken from the pump radiation field, by absorption, during each passage through the solid-state disc 12, and the radiation pressure acting on said disc 12 solid body, is therefore diminished.

If we proceed to a vectorial addition of the transverse forces 36a to 36h shown, we obtain a resultant transverse force vector, schematically drawn in FIG. 2. This results, in this way, in a resulting total force which acts in the transverse plane 34 and which echoes on the disk 12 in solid body, under the effect of the transverse magnetic force of Lorentz generated by the field of pumping radiation and emanating from the bonded charge carriers, in the active material laser, from which it follows, undesirably, that said solid-state disc 12 deforms asymmetrically and that, as a result, a radiation profile of the laser modes of the solid-state laser similarly incurs asymmetrical deformation .

Figure 3 illustrates again, schematically and in three dimensions, the structural organization of the disk laser. The references used coincide with the references mentioned in FIG. 1, and are completed in accordance with the commentary relating to FIG. 2.

FIG. 4 is a schematic representation of an interferometer device 66 for illuminating deformation of the solid-state disc 12 caused by the pump radiation field.

Laser radiation 70 scattered by a measurement laser 68 is directed, via a beam splitter, to the solid-state disk 12 on which the pump radiation field 30 simultaneously engages. The laser radiation 70 retroreflected by said solid-body disc 12 again incites on the beam splitter 72 and superimposes itself, as laser radiation 70b, on the laser radiation 70a directly inciting, from said beam splitter 72, on an optical detector in the form of a camera chip.

FIG. 7a illustrates an interferogram recorded by the interferometric device 66 and having a pumping power of 0 watts, that is to say that the disk 12 in solid body is not biased by the pumping radiation field 30. In this case, said solid-body disk 12 has a thickness of 300 μm.

Figure 7b shows an interferogram of the solid-state disk 12, the pump radiation field having a power of 15 watts. The longitudinal pressure of the laser pumping radiation gives rise to a crater 76 oriented perpendicularly to the transverse plane 34. This crater 76 is formed reversibly and disappears again, therefore, after disconnection of the source 28 of pumping radiation.

FIG. 7c shows an interferogram in which the pump radiation field has a power of 40 watts, a focal point of said pump radiation field measuring 2 mm on the disk 12 in solid body. A width 78 of the interferogram corresponds to 10 mm in FIG. 7c. The power density of said pump radiation field is 2 kW / cm 2 at said focal point, and the radius of curvature of the constrained disk can be estimated to be about one meter.

Fig. 8a shows the profile of the laser mode of the disk laser in the presence of a precisely tuned pump module, as illustrated in the interferograms of Figs. 7a to 7c. The power of the pump radiation field is 10 watts in the laser mode profile of Fig. 8a, 30 watts in the laser mode profile shown in Fig. 8b, and 40 watts in the laser mode profile shown. in Figure 8c. The three laser mode profiles all show, in FIGS. 8a to 8c, a rotation symmetry difference caused by the uncompensated transverse forces acting in the solid-state disc 12. In this regard, reference is made to the comment above relating to FIG. 2. Other effects, resulting in a rotation symmetry deviation of the profile of the laser mode, notably reside in an asymmetric distribution of the pumping spots. in a disk 12 in solid body not precisely adjusted, as shown by way of example in the interferogram of Figure 5. Such adjustment errors, or manufacturing inaccuracies affecting the deflector units 40 and 50, can in particular result in other rotational symmetry deviations from the profile of the laser mode.

FIG. 6 illustrates, by way of example, a laser mode obtained by a resonator I provided with a solid-body disk in the form of a yttrium garnet crystal and aluminum (YAG) doped with holmium, 300 μm thick, pumped by a thulium doped fiber laser and a transfer fiber with a pumping power of 40 watts. Maximum laser power from the laser beam emitted from the disk laser is 8 watts in this case. A rotation symmetry deviation of the profile of the laser mode is clearly highlighted.

FIG. 9 shows, by way of example, the manner in which a glass plate 80 flexes over an area of 100 mm 2, in the longitudinal direction, in the presence of a total power of 1 MW resulting from a pumping power of 10 kW and an occurrence of 20 passes through said glass plate. In this case, a radiation pressure p can be estimated at about 10 -3 N / mm 2. A thickness of the glass plate measuring 100 μm was admitted, and a modulus of elasticity reduced with consideration of the temperature of a few hundred degrees Celsius prevailing inside the solid body. Taking into account the temperature of a few hundred degrees Celsius prevailing inside the glass plate 80, this results in a reduced modulus of elasticity governing an easier deflection.

FIG. 9 represents the deflection of the glass plate 80, supported by the support points 82 spaced apart by 10 mm, as a function of a distance from a median axis of said glass plate 80. In the center of the point The focal point of the pumping radiation field 30 on said glass plate 80 results in a sag of about 0.1 μm due to the prevailing radiation pressure.

The effects exerted by the obliquity, precisely describing an angle 83 with respect to a normal 84 extending perpendicularly to the transverse plane, are illustrated by way of example in FIG. 10. The disk 12 in solid body, consisting of a YAG crystal, typically has a thickness 86 in a range of 100 to 300 μm. The localized heating of the laser activity material of the solid body 12 generates by absorption, within the pumped volume, a temperature of a few hundred degrees Celsius and has a relatively constant temperature distribution, rotationally symmetrical. Therefore, heating of said solid body is not responsible for an asymmetrical deformation of the disc 12 in solid body. An upper face 88 of said disk 12 in solid body diffuses heat which can be measured using a thermal camera and is about 150 ° C, while the cooled rear face 16 has, on the other hand, a temperature about 50 ° C under the action of the contacting material.

The effects exerted by the pump radiation field 30, coming to incide on the disc 12 in solid body, are represented by way of example in FIG. 11. It has formed, on the upper face 88 of said disk 12 in the body solid, a recess 90 substantially configured in a hollow sphere, also referred to as "crater" insinuating, likewise, in the material establishing a contact and attributable, precisely, to the longitudinal radiation pressure parallel to the normal 84.

The contact of the disk 12 in solid body is introduced on the cooling plate 14 by a layer 94 of solder or epoxy adhesive.

Figure 12 shows schematically the forces of the radiation pressure acting on the solid-state disk 12 via the pump radiation field. The operating force 98 of the radiation pressure can be divided vectorially into the transverse force 36, also referred to as a "transverse component" parallel to the transverse plane 34, and a longitudinal force 96 parallel to the normal 84. Said pressure forces of radiation emanate from the charges in the laser activity material by means of the Lorentz magnetic force: n + i K = V ε, n being the refractive index, K the absorption coefficient, and ε the dielectric constant complex. The transverse force 36 Ft is calculated using Ft ~ sin0 V (ε-sin20) Ε2, 0 being an angle, and E the power of the laser field. This results in a p = 1 / c for the radiation pressure resulting from the absorption, I corresponding to the intensity absorbed by the material.

The cooling plate 14 may in particular be made of copper or diamond, and have a thickness 102 of about 1 to 2 mm. A thickness 104 of the layer 94 is in the micrometer range, depending on the contact initiation process. The disk 12 in solid body has a thickness of about 0.1 to 0.3 mm.

The transverse force 36, acting in the manner illustrated in FIG. 12, governs a deformation of the recess 90 which deviates from the symmetry of rotation and then involves a deformation of the laser mode profile of the field 24 of the ideally symmetrical resonance radiation. in rotation.

Reflection of unabsorbed photons from the pumping radiation field 30 to the posterior face 16 of the solid-body disc 12 provided with a highly reflective coating results in a double pulse transmission. This is shown diagrammatically in FIG. 13. Taking into account the fact that the radiation pressure p corresponds to the energy density u of the electromagnetic wave, this gives a radiation pressure that effectively reigns in the range of approximately 10'3 N / mm2. Said radiation pressure p is calculated as follows: p = 2 l / c, where I is the intensity, and c is the speed of light.

FIG. 14 illustrates, by way of example, the drop in the power of the pump radiation field for a 10% absorption of said pump radiation field, inside the solid-state disc 12, at each passage through it. After one passage, therefore, the pumping power drops to 90% in the pumping radiation field and, after ten passes, it represents only about 35% of the power initially supplied by the pumping radiation source. . The effect exerted by this absorption of the pumping radiation in the disk 12 in solid body, has already been explained in support of Figure 2. An asymmetric radiation pressure acting on said disk 12 in solid body, in the plane transversal 34, results in astigmatism.

The solution of the problem now described in detail, residing in the asymmetric deformation of the solid-state disk 12 by the radiation pressure exerted by the pumping radiation field 30, is discussed below in conjunction with FIG. example.

A second source 28 'of pumping radiation is arranged such that the field 30' of pumping radiation, generated by the latter, comes to incide on the disk 12 in solid body at 180 degrees in the opposite direction. The radiation pressure forces 98 and 98 'of the pump radiation fields 30 and 30', acting on said solid-state disk 12, are therefore added as illustrated, such that the transverse forces 36 and 36 ' the opposite direction cancel each other out, and the longitudinal pressure forces 96 and 96 'add up vectorially. Admittedly, the radiation pressure force directed perpendicularly to the transverse plane 34 is thereby increased, but the transverse force 36, acting on the solid-state disk 12 solely because of the pumping radiation field 30, is compensated by the transverse force 36 'directed in the opposite direction. This makes it possible to effectively prevent deformation of said disk 12 in a solid body. Astigmatism occurring in the presence of a single source of pumping radiation can thus be corrected effectively.

FIG. 16 schematically shows, in a manner analogous to FIG. 2, an exemplary embodiment according to the invention. It comprises a double ring structure 106 which includes an inner ring 108, including the first and second deflecting edges 40 and 50, and an outer ring including deflector units 40 'and 50'. The latter are correspondingly marked in FIG. 17 in the schematic, non-true-to-projection illustration of a part of the focusing system 32. The inner ring 108 is therefore shown diagrammatically in FIG. , in accordance with the arrangement illustrated in Figure 2 and described above. The outer ring, including the coupling of the radiation field 30 ', is arranged in such a way that the transverse forces 36, which the radiation field 30 applies to the disk 12 in solid body, are vectorially compensated, up to zero, by transverse forces 36 'pointing directly opposite and applied to said disk in solid body by said radiation field 30'.

Each of the radiation fields 30 and 30 'runs through the respective ring 108 or 110, which gives a total of 2 times 8, that is to say a total number of 16 passages of said radiation fields 30 and 30' by the disc 12 in solid body.

Figure 18 schematically illustrates another alternative for a dual ring structure 106. It again includes, for the inner ring 108 and the outer ring 110, two deflector units 40 and 50, respectively 40 'and 50'.

The passage of the pump radiation fields 30 and 30 ', by the focussing systems 32 and 32', proceeds in a manner analogous to that explained in detail in the context of FIG. 2. FIG. however, only the path of the radiation fields 30 and 30 'to the retroreflectors. The transverse forces, acting during the return of said radiation fields 30 and 30 'traversing the focusing systems 32 and 32', are not shown in FIG. 18 in order to provide a good overview. The double ring illustrated structure allows, for each field of radiation, 2 times 12, that is to say a total number of 24 passes through the disc 12 solid body.

FIG. 19 represents another alternative arrangement relating to the passage of the radiation fields 30 and 30 'through the double ring structure 106, each of said radiation fields then traveling twice 12 times, that is to say, in total , 24 times the disc in solid body. Again, only transverse forces relating to the forward path to the retroreflectors are illustrated and are symbolized schematically by the radiation positions ® relating to the two rings 108 and 110. The fields 30 and 30 'of pumping radiation, generated by the sources 28 and 28 'of pumping radiation, are coupled on the parabolic mirror 38 in the positions (Γ).

FIG. 20 shows, in a similar manner, an exemplary embodiment of a double ring structure 106 including, for each of the two radiation fields 30 and 30 ', twice 14, that is to say a total number of 28 passes through the disk 12 in solid body. Again, only the transverse forces operating in the forward path of said radiation fields 30 and 30 'to the retroreflectors 62 and 62' are illustrated.

FIG. 21 represents, by way of example, another option targeting the compensation of transverse forces developed by the fields 30 and 30 'of pumping radiation. The schematic illustration shows, in this case, a structural organization having a total number of four focusing systems which are traversed, in a similar manner to that described above, by a total number of four fields of pumping radiation. These are generated by four sources of pumping radiation, for example with a respective pumping power of 10 kW. Each double ring allows, for each radiation field, 2 times 12 radiation passes through the solid body 12, or 24 passages in all.

In principle, it is possible to make any number of rings, insofar as this is geometrically compatible by taking into account effective diameters of the pumping radiation fields on the parabolic mirror 38, as well as effective dimensions of the deflector units. Likewise, more than two deflecting imitates can be provided for each annular structure.

An exemplary arrangement comprising three deflector units, symbolically illustrated by the three arrows 112, is shown in association with the disk laser of FIG. 22. The illustration also includes in this case, for example, a lens collimator 116 assigned to the collimation of the pumping radiation field 30 generated by the source 28 of pumping radiation.

A double ring associated structure, having two focusing systems 32 and 32 ', is shown schematically in FIG. 23 showing, in each case, three planes of symmetry 114 and three planes of symmetry 114' of the three deflector units, as well as transverse forces acting on the solid-state disk 12 during passage through said focusing systems 32 and 32 ', towards the retroreflectors 62 and 62'.

As already mentioned, the laser amplification systems described can, in principle, be graduated at will provided that this is not pre-established in any other way by purely spatial restrictions, as to the disposition of the deflector units, more precisely so much as regards the number of said deflecting units, than the number of focusing systems. Arrangements provided with an odd number of rings can also be envisaged in particular, the odd-numbered pumping radiation fields then having to be focused on the solid-state disk 12, with a mutual offset suitable for involving compensation, or a substantial compensation of the respective transverse components of the radiation pressure forces which are applied to said solid-state disk 12 by said pumping radiation fields.

The laser amplification systems described are suitable, in particular, for use in medical techniques and in the machining of materials, because they make it possible to produce laser beams having a high power and, at the same time, a high quality said beams.

List of numerical references 10 disk laser 12 solid-state disk 14 cooling plate 16 posterior face 18 resonator 20 end mirror 22 decoupling mirror 24 resonance radiation field 26 laser radiation 28 pumping radiation source 30, 30 ' pumping radiation field 32, 32 'focusing device 34 transverse plane 36, 36' transverse component 38 parabolic mirror 40, 40 'first deflector unit 42, 42' plane of symmetry 44.44 'angle 46, 46' reflecting surface 48 , 48 'reflecting surface 50, 50' second deflecting unit 52,52 'angle 54, 54' reflecting surface 56, 56 'reflecting surface 58, 58' plane of symmetry 60,60 'angle 62,62' retroreflector 66 interferometric device 68 laser measurement 70 laser radiation 72 beam splitter 74 deflector 76 crater 78 width 80 glass plate 82 support points 83 angle 84 normal 86 thickness 88 face su upper 90 recess 94 layer 96, 96 'longitudinal force 98, 98' radiation pressure force 100 pumping focal point 102 thickness 104 thickness 106 double ring structure 108 inner ring 110 outer ring 112 arrow 114 plane of reflection 116 collimating lens

Claims (20)

-REVENDICATIONS- A laser amplification system comprising a solid body (12) having a laser activity medium, a first source (28) of pumping radiation, configured to generate a first field (30) of pump radiation that travels through the least once, in particular several times said solid body (12), for the purpose of exciting said laser activity medium, characterized by at least a second source (28 ') of pumping radiation, designed to generate at least a second field (30 ') of pumping radiation which travels at least once, in particular several times the solid body (12), for the excitation of the laser activity medium, the first field (30) of pumping radiation and the second pump radiation field (30 ') being oriented to compensate or substantially compensate, in a transverse plane (34) defined by the solid body (12), the sum of all first transverse components (36) of said first field ( 30) of pump radiation incident on said solid body (12), and the sum of all second transverse components (36 ') of said second pump radiation field (30') incident on said solid body (12). A laser amplification system according to claim 1, characterized by a first focusing device (32) which generates a plurality of different branches of the first pump radiation field (30), penetrating the solid body (12), and then converts at least one branch, coming out of the solid body (12), at one of the branches penetrating into said solid body (12) and different from said outgoing branch; and by at least a second focusing device (32 ') which generates a plurality of different branches of the second field (30') of pumping radiation, penetrating the solid body (12), and then converts at least one branch, coming out of the body solid (12), at one of the branches penetrating into said solid body (12) and differing from said outgoing branch. Laser amplification system according to one of the preceding claims, characterized in that the solid body (12) is formed as a disk (12) into a solid body. Laser amplification system according to one of the preceding claims, characterized by a first source (28) of pumping radiation and a second source (28 ') of pumping radiation. 5. laser amplification system according to claim 4, characterized in that the first source (28) of pump radiation and the second source (28 ') of pumping radiation, and / or the first focusing system (32). ) and the second focusing system (32 ') are oriented such that said at least one first transverse component (36) in the first field (30) of pump radiation directed towards the solid body (12) ), and said at least one second transverse component (36 ') in the second field (30') of pumping radiation directed to said solid body (12), point to said solid body (12) with 180 ° mutual offset. Laser amplification system according to one of claims 2 to 5, characterized in that the first focusing device (32) includes at least one first deflector unit (40, 50), for the purpose of retrodeviation of the first field (30) of pumping radiation to the solid body (12) after passing through said solid body (12); the at least one second focusing device (32 ') includes at least one second deflecting unit (40', 50 ') for the retrodeviation of said at least one second pump radiation field (30') to said body solid (12) after passing through said solid body (12); and said at least one first deflector unit (40, 50) and said at least one second deflector unit (40 ', 50') orient said first pump radiation field (30) and said at least one second field (30 '). ) of pump radiation, towards said solid body (12), so as to compensate or substantially compensate, in a transverse plane (34) defined by the solid body (12), the sum of all the first transverse components (36). ) of said first deflected pumping radiation field (30), inciding on said solid body (12), and the sum of all second transverse components (36 ') of said at least one second pumping radiation field (30') deviated coming to incide on said solid body (12). Laser amplification system according to claim 6, characterized in that the first focusing device (32) includes at least two first deflector units (40, 50) and said at least one second focusing device (32 '). ) includes at least two seconds deflector units (40 ', 50'); and / or said at least one first baffle device (40, 50) comprises two reflective surfaces (46, 48, 54, 56) extending at an angle (44) with respect to each other and that said at least one second baffle unit (40 ', 50') comprises two reflective surfaces (46 ', 48', 54 ', 56') extending at an angle (52) relative to the other . 8. laser amplification system according to one of claims 2 to 7, characterized in that the first focusing device (32) comprises collimators and focusing elements, which provide an intermediate focus of the first field (30) of radiation pumping directed away from the solid body (12) and focussing, in the direction of said solid body (12), said first field (30) of pumping radiation subjected to intermediate focusing; and said at least one second focusing device (32 ') has collimating and focussing elements which provide intermediate focusing of said at least one second pump radiation field (30') directed away from the solid body (12) and which focus, in the direction of said solid body (12), said at least one second field (30 ') of pumping radiation, subjected to intermediate focusing. 9. laser amplification system according to claim 8, characterized in that the first focusing device (32), and said at least one second focusing device (32 ') include a common focusing element, in particular a parabolic mirror (38) common; and / or that said first focusing device (32), and said at least one second focusing device (32 ') are respectively provided with at least one collimating lens (116). 10. Laser amplification system according to one of claims 6 to 9, characterized in that said at least one first deflector unit (40, 50) and said at least one second deflector unit (40 ', 50') surround annularly the solid body (12). Laser amplification system according to one of claims 6 to 10, characterized in that said at least one first deflector unit (40, 50) and said at least one second deflector unit (40 ', 50') define a double ring or multiple ring structure (106) surrounding the solid body (12). Laser amplification system according to one of claims 2 to 11, characterized by second focusing devices (32 ') of one, two, three, four, five or more. 13. Laser amplification system according to one of the preceding claims, characterized in that a rear face (16) of the solid body (12) carries a highly reflective coating; and / or that the first pump radiation source (28), and said at least one second pump radiation source (28 ') are in the form of a diode laser or a fiber laser; and / or that the solid body (12) is a laser-active crystal, in particular an yttrium-aluminum garnet crystal; and / or that said solid body (12) contains ytterbium or holmium, and is especially doped with ytterbium or with the holmium. 14. Laser amplification system according to one of the preceding claims, characterized in that the solid body (12) is integrated in a resonator (18) defining a field (24) of resonance radiation which traverses said solid body ( 12). 15. laser amplification system according to claim 14, characterized in that the rear face (16) of the solid body (12), highly reflective, forms an end mirror (20) of the resonator (18); and / or said resonator (18) includes a decoupling mirror (22), and the resonance radiation field (24) extends between said end mirror (20) and said decoupling mirror (22). 16. A method of correcting an asymmetric cross-sectional profile of radiation pressure on a solid body (12) of a laser amplification system, having a laser-active medium, which profile of radiation pressure is generated by a first field Pumping radiation (30) which flows through said solid body (12), characterized in that at least a second pumping radiation field (30 ') is directed to the solid body (12); and that the first field (30) of pumping radiation, and said at least one second field (30 ') of pumping radiation are oriented relative to each other so as to compensate or substantially compensate, in a transverse plane (34) defined by the solid body (12), the sum of all first transverse components (36) of said first pump radiation field (30) incident on said solid body (12), and the sum of all the second transverse components (36 ') of said at least one second pump radiation field (30') incident on said solid body (12). 17. Method according to claim 16, characterized in that the first field (30) of pumping radiation, and said at least one second field (30 ') of pumping radiation are oriented relative to each other to substantially compensate or compensate in pairs, in the transverse plane (34), the first transverse components (36) of said first field (30) of pumping radiation coming to incide on the solid body (12), and the second components transverse (36 ') of said at least one second field (30') of pumping radiation coming to incide on said solid body (12). 18. A method according to claim 17, characterized in that the first field (30) of pumping radiation, and said at least one second field (30 ') of pumping radiation are deflected at least once towards the solid body. (12) to cause at least a doubling of the number of passages by said solid body (12); and / or the first pump radiation field (30), and said at least one second pump radiation field (30 ') is at least respectively focused towards said solid body (12). 19. Method according to one of claims 16 to 18, characterized in that the first (30), and said at least one second field (30 ') of pumping radiation are deflected several times in order to multiply the number of passes by the solid body (12). 20. Method according to one of claims 16 to 19, characterized in that a disk (12) in a solid body is used as a solid body (12).