Device for amplifying a laser beam
Patent Information
- Authority / Receiving Office
- IL · IL
- Patent Type
- Patents
- Current Assignee / Owner
- THALES SA
- Filing Date
- 2021-12-16
- Publication Date
- 2026-07-01
AI Technical Summary
Current laser amplification devices with backside-cooled thick discs suffer from geometric aliasing and temporal contrast degradation due to parasitic reflections, leading to increased optical losses and potential system unavailability, especially when using next-generation pump lasers requiring high energy and high average power.
A device with a solid amplifying medium having a front face inclined relative to a rear face, where the rear face is cooled, and a first optical return unit that returns the beam to the front face for a second pass, ensuring sub-beams of each wavelength are parallel, thus minimizing optical losses and maintaining temporal contrast.
The solution effectively compensates for chromatic spatial dispersion without introducing additional losses, maintaining high thermal extraction and temporal contrast, while reducing the need for multiple optics and potential failure points.
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Abstract
Description
[0001] DESCRIPTION
[0002] TITLE: Device for amplifying a laser beam
[0003] The present invention relates to a device for amplifying a multi-wavelength laser beam.
[0004] The field of the invention is that of solid-state laser sources for scientific, industrial, medical and military applications. More specifically, it is advantageously used for amplifying medium materials (such as a crystal) which have a relatively small thickness compared to their aperture along the propagation axis of the laser beam, typically less than 1:3.
[0005] Pumping laser technology has evolved significantly in recent years and it is now possible to have a pulsed laser source which provides an average pumping power of around a hundred Watts.
[0006] However, a number of configurations are not compatible with next-generation pump lasers for which high energy and high average power (higher repetition rate) are sought.
[0007] In the current state of the art, different solutions are used for the extraction of thermal energy in an amplifying medium by working on the form factor of the amplifying medium, typically amplifying fibers, very thin disks, slabs and so-called thick disks.
[0008] The thick disk solution is well suited to certain amplifying media such as amorphous materials (like glasses), transparent ceramics, or crystals like Ti:Sa (short for Titanium:Sapphire). It allows access to high energy levels, high average powers, and short pulse durations thanks to the material's broad amplification spectrum.
[0009] In thick disk technology, the amplifying medium (e.g., a crystal) is cooled from its rear face. Cooling is then achieved by means of a fluid, liquid or gas, or a solid. Such rear-face cooling increases the heat exchange surface. It also generates a thermal gradient in the laser propagation direction in the crystal, and also achieves high thermal extraction. The index variations related to temperature variations in the crystal are gradients predominantly oriented in the same direction as the laser beam propagation direction. However, rear-face-cooled laser amplification devices induce geometric folding of the beam due to the reflective rear face of the crystal.The output face of the crystal is then the same as the input face, which implies that the parasitic pulses (due to parasitic reflections on the front face) are found before the main pulse, consequently degrading the temporal contrast of the pulse. The temporal contrast is defined by the ratio between the intensity of the main pulse and the foot of the pulse and / or any parasitic pulses.
[0010] To avoid this degradation, it is known from patent EP 2 915 226 B to modify the air / crystal interface to separate the main pulse and the parasitic pulses. To do this, the front face of the amplifying medium is inclined relative to its rear face by a non-zero angle. Thus, after propagation in the amplifying medium, the parasitic reflections are spatially separated from the main pulse and the temporal contrast is no longer degraded by the parasitic reflections.
[0011] For short pulses (with broad spectrum), this angle produces a prismatic effect which is compensated by a compensating prism positioned on the beam path, as described in EP 2 915 226 B. Thus, when several passes through the amplification medium are made, this involves the implementation of several prisms in the amplification system.
[0012] Beyond the financial impact, the implementation of numerous optics in transmission is a source of optical loss and a potential source of failure (damage leading to unavailability of the laser and a cost of repairing the part and labor for realignment).
[0013] There is therefore a need for an amplification device that can minimize optical losses while remaining satisfactory in terms of cooling and temporal contrast.
[0014] For this purpose, the invention relates to a device for amplifying a multi-wavelength laser beam, the device comprising: a. a solid amplifying medium having at least two planar faces from among: a front face suitable for receiving the beam to be amplified at each passage of said beam in the amplifying medium, and a reflective rear face, the front face being inclined relative to the rear face by a non-zero inclination, the rear face being suitable for being cooled, the beam received on the front face during the first passage being called the incident beam, the beam reflected by the rear face and refracted by the front face during the n-th passage being called the n-th useful beam, and b.a first optical return unit arranged on the path of the first useful beam, the first optical return unit being configured so as to return the first useful beam to the front face for a second pass in the amplifying medium so that the sub-beams of each wavelength, forming the second useful beam, are parallel to each other at the end of the second pass.
[0015] According to other advantageous aspects of the invention, the device comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations:
[0016] - the first optical return unit is configured so that the second useful beam is equivalent in terms of chromatic spatial dispersion to the beam which would have been obtained at the output of a plate with flat and parallel faces from an incident beam arriving on the front face of said plate with an angle of incidence equal to the angle of incidence of the incident beam on the amplifying medium;
[0017] - the amplifying medium is a disk whose flat faces are the front face and the rear face, said faces being inscribed in a right prism with a triangular or trapezoidal base, called the base, the first optical return unit comprising two mirrors oriented so that the path of the first useful beam between the amplifying medium and the first mirror is symmetrical, with respect to a plane of symmetry, to the path of the first useful beam between the second mirror and the amplifying medium, the plane of symmetry being a plane perpendicular, on the one hand, to a plane containing the base of the amplifying medium and, on the other hand, to a plane containing the rear face;
[0018] - the front face of the amplifying medium is suitable for receiving the incident beam and for reflecting a beam, called the first parasitic beam, from the incident beam, the first optical return unit being arranged outside the path of the first parasitic beam;
[0019] - the second useful beam has a diameter enlarged compared to the diameter of the incident beam, the amplification device comprising a second optical return unit capable of returning the second useful beam into the amplifying medium for at least a third, then a fourth pass so that the last useful beam leaving the amplifying medium, called the output beam, has a diameter substantially equal to the diameter of the incident beam and that the sub-beams of each wavelength, forming said output beam, are parallel to each other;
[0020] - the second optical return unit is configured so that the output beam is equivalent in terms of diameter and chromatic spatial dispersion to the beam which would have been obtained following the successive passage of an incident beam in a first, then a second plate with flat and parallel faces, the first plate being oriented so that the incident beam arrives on the front face of the first plate with a first angle of incidence equal to the angle of incidence of the incident beam on the amplifying medium, the second plate being oriented so as to receive the beam at the output of the first plate with a second angle of incidence equal to the opposite of the first angle of incidence;
[0021] - the second optical return unit is capable of returning the second useful beam into the amplifying medium so that the total number of passages of the beam to be amplified in the amplifying medium is a multiple of four;
[0022] - the second optical return unit is capable of returning the second useful beam into the amplifying medium so that the total number of passages of the beam to be amplified in the amplifying medium is a multiple of two and that the beam to be amplified makes an outward path and a return path, superimposed on the outward path, between the first entry of said beam into the amplifying medium and the last exit of said beam from the amplifying medium;
[0023] - at each passage through the amplifying medium, a parasitic beam is obtained directly reflected on the front face of the amplifying medium, the first return unit and the second return unit being arranged outside the path of each parasitic beam resulting from an odd passage of the beam to be amplified in the amplifying medium;
[0024] - the last useful beam at the output of the amplifying medium is called the output beam, the incident beam and the output beam being spatially offset.
[0025] Other features and advantages of the invention will become apparent upon reading the following description of embodiments of the invention, given by way of example only, and with reference to the drawings which are:
[0026] - [Fig 1] figure 1, a schematic representation seen from above of an amplification device according to a first embodiment,
[0027] - [Fig 2] figure 2, a schematic representation of an optical system equivalent in terms of chromatic spatial dispersion to the amplification device of the first embodiment,
[0028] - [Fig 3] figure 3, a schematic representation seen from above of a first example of an amplification device according to a second embodiment, the input beam and the output beam of the amplification device being superimposed in this first example and the propagation being carried out in the same plane,
[0029] - [Fig 4] figure 4, a schematic representation seen in perspective of a second example of an amplification device according to the second embodiment, the input beam and the output beam of the amplification device being offset in space in this second example, and
[0030] - [Fig 5] figure 5, a schematic representation of an optical system equivalent in terms of chromatic spatial dispersion and chromatic lateral dispersion to the amplification device of the second embodiment.
[0031] In the remainder of the description, a propagation direction z is defined, represented in the figures by a z axis and corresponding to the propagation direction of the laser beam. A first transverse direction is further defined, perpendicular to the propagation direction, and represented in the figures by an x axis, such that the plane (xOz) corresponds to a top view of the amplification device 10. A second transverse direction y is also defined, perpendicular to the propagation direction z and to the first transverse direction x. The second transverse direction y is represented in the figures by a y axis and is such that the plane (yOz) corresponds to a side view of the amplification device 10. Those skilled in the art will understand that the notations used for such axes are arbitrary, and could be replaced by other notations.
[0032] In the remainder of the description, the term "chromatic spatial dispersion" is understood to mean the angular dispersion of a beam due to variations in the deflection angle as a function of the wavelengths in a diopter. The term "chromatic lateral dispersion" is understood to mean the broadening of the diameter of a beam as a function of the wavelengths (pupil shift) following passage through two diopters whose interfaces are parallel (plates with parallel faces).
[0033] A first embodiment of an amplification device 10 is illustrated in FIG. 1.
[0034] The amplification device 10 is configured to amplify a laser beam, in particular a multi-wavelength pulsed laser beam. The beam to be amplified is, for example, an infrared beam.
[0035] The beam to be amplified has, for example, an average power greater than 10 Watts (W).
[0036] The amplification device 10 according to the first embodiment comprises an amplifying medium M and a first optical return unit 18.
[0037] The medium M is a solid medium. The medium M is, for example, a crystal such as Titanium-doped sapphire, or Yb:YAG, Yb:CaF2 or a polymer, a ceramic or a glass or any other material in the solid state.
[0038] The medium M has a refractive index n. Preferably, the following relationship is verified:
[0039] Where v is the constringency of the amplifying medium M. This aims to preserve the multi-wavelength character of the beam Fs at the output of the amplification device 10.
[0040] The medium M has at least two flat faces including a front face 20 suitable for receiving the beam to be amplified each time said beam passes through the amplifying medium M and a reflective rear face 22.
[0041] The front face 20 is inclined relative to the rear face 22 by a non-zero inclination p (angle). In the following, P' denotes the projection of the inclination p onto the plane (xOz) and P” the projection of the inclination p onto the plane (yOz). In an exemplary implementation, the amplifying medium M is a disk whose faces (front 20 and rear 22) are inscribed in a right prism with a triangular or trapezoidal base, called base 24. The base 24 of the prism, and therefore the inclination p, is entirely contained in a plane perpendicular to, on the one hand, a plane P22 containing the rear face 22, and on the other hand, to a plane perpendicular to the plane (yOz).
[0042] The front face 20 of the amplifying medium M is suitable for receiving the beam to be amplified at each passage of said beam in the amplifying medium M and for, on the one hand, reflecting a parasitic beam (direct reflection) and, on the other hand, refracting a useful beam after such a beam has been reflected by the rear face 22. The beam received on the front face 20 during the first passage is called the incident beam Fi. The beam reflected by the rear face 22 and refracted by the front face 20 during the n-th passage is called the n-th useful beam F Un The useful beam at the output of the amplifying medium M during the last pass is also called the output beam F s The beam directly reflected by the front face 20 from the beam to be amplified during the nth pass is called the nth parasitic beam F Pn .
[0043] Advantageously, the front face 20 is anti-reflective treated.
[0044] The rear face 22 of the amplifying medium M is capable of reflecting, at each passage, the beam to be amplified, after its passage through the front face 20 of the amplifying medium M, to form the corresponding useful beam.
[0045] The rear face 22 is suitable for being cooled by a cooling device, which is, for example, included in the amplification device 10. The cooling is represented in FIG. 1 by an arrow attached to the rear face 22.
[0046] The first optical return unit 18 is arranged on the path of the first useful beam Fui. The first optical return unit 18 is configured so as to return the first useful beam Fui to the front face 20 for a second pass through the amplifying medium M so that the sub-beams of each wavelength, forming the second useful beam Fus, are parallel to each other at the end of the second pass. In FIG. 1, only two sub-beams are shown so as not to overload the figure. The first optical return unit 18 thus makes it possible to compensate for the chromatic spatial dispersion induced by the prismatic effect resulting from the inclination p between the front face 20 and the rear face 22 of the amplifying medium M.
[0047] Advantageously, as illustrated in FIG. 2, the first optical return unit 18 is configured so that the second useful beam Fus is equivalent in terms of chromatic spatial dispersion to the beam which would have been obtained at the output of a plate with flat and parallel faces L1 from an incident beam arriving on the front face of said plate with an angle of incidence equal to the angle of incidence 0 of the incident beam Fi on the amplifying medium M.
[0048] In the example illustrated by Figure 1, the first optical return unit 18 comprises two mirrors M1, M2 oriented so that the path of the first useful beam Fui between the amplifying medium M and the first mirror M1 is symmetrical, with respect to a plane of symmetry P H, to the path of the first useful beam Fui between the second mirror M2 and the amplifying medium M. The plane of symmetry PH is a plane perpendicular, on the one hand, to a plane P24 containing the base 24 of the amplifying medium M and, on the other hand, to a plane P22 containing the rear face 22.
[0049] Thus, after a first reflection on the mirror M1 and a second reflection on the mirror M2, the image of the beam to be amplified is returned upon arriving at the front face 20 of the amplifying medium M. The amplifying medium M itself then plays the role of the compensation prism of the state of the art. In the particular configuration of figure 1, the plane of symmetry PH is a plane perpendicular to the plane (xOz) (plane of the sheet in figure 1) and perpendicular to a plane P 22 containing the back face 22, and the image is flipped in the x direction.
[0050] Advantageously, the first optical return unit 18 does not include a prism.
[0051] Advantageously, the first optical return unit 18 is arranged outside the path of the first parasitic beam F P i.
[0052] The operation of the amplification device 10 according to the first embodiment will now be described.
[0053] Initially, the beam (pulse) to be amplified Fi has a diameter arrives on the front face 20 of the amplifying medium M with an angle of incidence 0 which breaks down into an angle O x in the plane (xOz) and an angle O y in the plane (yOz).
[0054] The useful beam (main pulse) is reflected by the rear face 22, the parasitic beam F Pi by the front face 20. The parasitic beam, also called parasitic pulses, is deflected on this front face 20 by an angle 2© x in the plane (xOz) and 2© yin the plane (yOz). The useful beam is deflected at the exit by an angle 2(© x + P'.(n-1 ))= 2(© x + p.(n-1 )) in the plane (xOz) and an angle 2(© y + P”.(n-1 ))= 2©y in the plane (yOz).
[0055] Since this is a multi-wavelength laser source, the angle p formed by the faces 20 and 22 produces a prismatic effect. Thus, after the first passage through the amplifying medium M, the wavelengths of the first useful beam Fui are angularly separated.
[0056] The first optical return unit 18 arranged after the separation of the first useful beam F ui and the first parasitic beam F P i, on the path of the first useful beam Fui allows to correct the chromatic spatial dispersion.
[0057] In particular, in the particular example of Figure 1, © y =0, P'= p and P”=0, which means that all propagation takes place in the plane (xOz).
[0058] It should be noted that at the output of the amplifying medium M, the spectral components of the second useful beam F U 2 form a spot of diameter <t> + A <t>. Note that A <t>includes the increase in diameter introduced by the divergence of the beam during the first passage in the amplifying medium M, then that introduced by the divergence of the beam between its exit from the amplifying medium M and its second entry into the amplifying medium M. We find the same diameter <t> + A <t>after the second pass through the amplifying medium M. To preserve the multi-wavelength character of the output beam, the broadening A <t>of the diameter of the second useful beam F U 2 must be small in front <t>. This is the case when n »
[0059] Indeed, now, (2Q X + p. n)) implies that A <P « <P.
[0060] Thus, the amplification device 10 according to the first embodiment makes it possible to compensate for the chromatic spatial dispersion induced by the inclination p of the amplifying medium M without, however, introducing additional losses. On the contrary, the compensation is carried out by an additional passage in the amplifying medium M itself which does not introduce losses, but on the contrary more gain.
[0061] The amplification device 10 according to the first embodiment therefore makes it possible to minimize optical losses while remaining satisfactory in terms of cooling, gain and temporal contrast.
[0062] According to a second embodiment as seen in Figures 3 and 4, the elements identical to the amplification device 10 according to the first embodiment described with reference to Figure 1 are not repeated. Only the differences are highlighted.
[0063] In the second embodiment, in addition to the elements of the first amplification device 10, the amplification device 10 comprises a second optical return unit 30 capable of returning the second useful beam Fus into the amplifying medium M for at least a third, then a fourth pass so that the last useful beam at the output of the amplifying medium M, called output beam F s , has a diameter substantially equal to the diameter of the incident beam Fi and that the sub-beams of each wavelength, forming said output beam Fs, are parallel to each other. Thus, the optical assembly formed by the first optical return unit 18 and the second optical return unit 30 makes it possible to compensate for both the chromatic spatial dispersion and the chromatic lateral dispersion.
[0064] Advantageously, as illustrated in FIG. 5, the second optical return unit 30 is configured so that the output beam F s is equivalent in terms of diameter (chromatic lateral dispersion) and chromatic spatial dispersion to the beam which would have been obtained following the successive passage of an incident beam in a first, then a second plate with flat and parallel faces L1 and L2, the first plate L1 being oriented so that the incident beam arrives on the front face of the first plate with a first angle of incidence ©1 equal to the angle of incidence 0 of the incident beam on the amplifying medium M, the second plate L2 being oriented so as to receive the beam at the output of the first plate L1 with a second angle of incidence 02 equal to the opposite of the first angle of incidence Oi.
[0065] Advantageously, the second optical return unit 30 is capable of returning the second useful beam Fus into the amplifying medium M so that the total number of passages of the beam to be amplified in the amplifying medium M is a multiple of four.
[0066] In addition or as a variant, the second optical return unit is capable of returning the second useful beam Fu2 into the amplifying medium M so that the total number of passages of the beam to be amplified in the amplifying medium M is a multiple of two and the beam to be amplified makes an outward path and a return path, superimposed on the outward path, between the first entry of said beam into the amplifying medium M and the last exit of said beam from the amplifying medium M. According to the principle of reverse return of the light, this makes it possible to compensate for the chromatic lateral dispersion of the beam Fs at the output of the amplification device 10.
[0067] In the examples of Figures 3 and 4, the beam to be amplified makes four passes through the amplifying medium M. More precisely, in the example of Figure 3, the second optical return unit 30 comprises a mirror MAR (plane mirror) capable of making the beam to be amplified make a round trip in the amplification device 10. As the beam to be amplified already makes two passes via the first optical unit 18, the total number of passes through the amplifying medium M is four.
[0068] In the example illustrated by Figure 4, the second optical return unit 30 comprises four mirrors M3, M4, M5 and M6. The mirrors M3 and M4 are suitable for returning the second useful beam Fus into the amplifying medium M for a third passage through the amplifying medium M. The mirrors M5 and M6 are suitable for returning the third useful beam Fus into the amplifying medium M for a fourth passage through the amplifying medium M. The total number of passages through the amplifying medium M is thus four. In particular, in this example, PH designates a horizontal plane of symmetry for the mirrors M1, M2 of the amplification device 10 and Pv a vertical plane of symmetry for the mirrors M1, M2 with respect to the mirrors M5, M6. The beam to be amplified enters through the quadrant 1) of the medium M, the second passage is made through the quadrant 2), the third passage through the quadrant 3), and the fourth passage through the quadrant 4).
[0069] Advantageously, the second optical return unit 30 comprises at least one mirror.
[0070] Advantageously, the second optical return unit 30 does not comprise a prism.
[0071] Advantageously, the first return unit 18 and the second return unit 30 are arranged outside the path of each parasitic beam resulting from an odd passage in the amplifying medium M. Thus, in the examples of figures 3 and 4, the first parasitic beam F Pi and the third parasitic beam F P3 are not returned to the amplifying medium M and are thus eliminated.
[0072] When the incident beam Fi and the output beam F s are superimposed, the amplification device 10 comprises, for example, an optical separation assembly 40 for separating the two beams. In the example illustrated by FIG. 3, the optical separation assembly 40 comprises a polarizing cube 42 and a quarter-wave plate 44, as well as a mirror MO.
[0073] Preferably, the incident beam Fi and the output beam F s are spatially offset (not superimposed). Such a configuration is illustrated in particular in Figure 4. Such a configuration makes it possible to dispense with an optical assembly for separating the incident beam Fi and the output beam F s .
[0074] During operation of the amplification device 10 according to the second embodiment, in addition to the operation described for the first embodiment, the second useful beam Fus is returned, via the second optical unit 30, to the front face 20 of the amplifying medium M, for a third passage in said medium M. This gives a third parasitic reflection F P3 and a third useful beam F U3 The third useful beam F U3 is in turn returned, via the second optical unit 30, to the front face 20 of the amplifying medium M, for a fourth passage in said medium M. This gives a fourth parasitic reflection F P4 and a fourth useful beam F U4 . It will be noted that in Figure 2, for the sake of clarity, the parasitic reflections F P2 , F P3 and F P4 were not represented.
[0075] The configuration of the second optical unit 30 relative to the first optical unit 18 and to the amplifying medium M thus makes it possible to compensate for the chromatic lateral dispersion of the amplified output beam of the amplification device 10.
[0076] By choosing a suitable arrangement of the first return unit 18 and the second return unit 30, the parasitic beams generated during odd passages in the amplifying medium M (first and third passages in particular) are not returned to the amplifying medium M and are ejected.
[0077] The parasitic beams generated during even passages in the amplifying medium M are also easily dissociable from the useful beam. Indeed, in the case of a two-dimensional assembly (figure 3), the parasitic pulses generated during even passages take the same optical path as the useful pulses but are delayed compared to the main pulse. Such pulses will, therefore, be at the origin of a "post pulse" after the useful beam (pulse). As these parasitic pulses arrive after the useful beam, they do not reduce the contrast and it is easy to filter them. In the case of a three-dimensional assembly (figure 4), the parasitic pulses generated during even passages take the same optical path as the useful pulses but in the opposite direction, and therefore do not affect the contrast.
[0078] Thus, the amplification device 10 according to the second embodiment makes it possible, in addition to the advantages of the first embodiment, to compensate for the chromatic lateral dispersion induced during the first two crossings of the amplifying medium M, and this without introducing additional losses. On the contrary, the compensation is carried out by additional passages in the amplifying medium M which thus introduce an amplification gain while remaining satisfactory in terms of cooling and temporal contrast.
[0079] The person skilled in the art will understand that the examples of figures 3 and 4 illustrate only four passages in the amplifying medium M. Nevertheless, the second embodiment is generalized to a greater number of passages in the amplifying medium M with the condition either that this number is a multiple of four (a multiple of two but not of four would amount only to compensating for the chromatic spatial dispersion but not the chromatic lateral dispersion), or that this number is a multiple of two but that the beam to be amplified makes a round trip in the amplification device 10. The person skilled in the art will understand that the embodiments and characteristics of the examples described previously are capable of being combined with each other when such a combination is compatible.< / t> < / t> < / t> < / t> < / t> < / t> < / t>
Claims
DEMANDS 1. A device (10) for amplifying a multi-wavelength laser beam, the device (10) comprising: a. a solid amplifying medium (M) having at least two flat faces among: a front face (20) adapted to receive the beam to be amplified on each pass of said beam through the amplifying medium (M), and a reflective rear face (22), the front face (20) being inclined relative to the rear face (22) at a non-zero angle (P), the rear face (22) being adapted to be cooled, the beam received on the front face (20) during the first pass being called the incident beam (Fi), the beam reflected by the rear face (22) and refracted by the front face (20) during the nth pass being called the nth useful beam (F Un), and b. a first optical return unit (18) disposed on the path of the first useful beam (Fui), the first optical return unit (18) being configured so as to return the first useful beam (Fui) to the front face (20) for a second pass in the amplifying medium (M) so that the sub-beams of each wavelength, forming the second useful beam (Fus), are parallel to each other at the end of the second pass.
2. Amplification device (10) according to claim 1, wherein the first optical return unit (18) is configured so that the second useful beam (Fus) is equivalent in terms of chromatic spatial dispersion to the beam that would have been obtained at the output of a plate with flat and parallel faces from an incident beam arriving on the front face of said plate with an angle of incidence equal to the angle of incidence (0) of the incident beam (Fi) on the amplifying medium (M).
3. Amplification device (10) according to claim 1 or 2, wherein the amplifying medium (M) is a disk whose flat faces are the front face (20) and the rear face (22), said faces (20, 22) being inscribed in a right prism with a triangular or trapezoidal base, referred to as the base (24), the first optical reflecting unit (18) comprising two mirrors (M1, M2) oriented such that the path of the first useful beam (Fui) between the amplifying medium (M) and the first mirror (M1) is symmetrical with respect to a plane of symmetry (P H ), to the path of the first useful beam (Fui) between the second mirror (M2) and the amplifying medium (M), the plane of symmetry (PH) being a plane perpendicular, on the one hand, to a plan (P 24 ) containing the base (24) of the amplifying medium (M) and, on the other hand, to a plane (P22) containing the rear face (22).
4. An amplification device (10) according to any one of claims 1 to 3, wherein the front face (20) of the amplifying medium (M) is adapted to receive the incident beam (Fi) and to reflect a beam, called the first parasitic beam (FRI), from the incident beam (Fi), the first optical reflection unit (18) being disposed outside the path of the first parasitic beam (FRI). P i).
5. Amplification device (10) according to any one of claims 1 to 4, wherein the second useful beam (Fus) has an enlarged diameter ( <t> + A <t>) compared to the diameter ( <t>) of the incident beam (Fi), the amplification device (10) comprising a second optical reflection unit (30) suitable for reflecting the second useful beam (Fus) back into the amplifying medium (M) for at least a third, then a fourth pass so that the last useful beam exiting the amplifying medium (M), called the output beam (Fs), has a diameter substantially equal to the diameter ( <t>) of the incident beam (Fi) and that the sub-beams of each wavelength, forming said output beam (F s ), are parallel to each other.
6. Amplification device (10) according to claim 5, wherein the second optical return unit (30) is configured such that the output beam (F s ) is equivalent in terms of diameter and chromatic spatial dispersion to the beam which would have been obtained following the successive passage of an incident beam through a first, then a second plate with flat and parallel faces, the first plate being oriented so that the incident beam arrives on the front face of the first plate with a first angle of incidence (0i) equal to the angle of incidence (0) of the incident beam on the amplifying medium (M), the second plate being oriented so as to receive the beam at the exit of the first plate with a second angle of incidence (O2) equal to the opposite of the first angle of incidence (O1).
7. Amplification device (10) according to claim 5 or 6, wherein the second optical return unit (30) is adapted to return the second useful beam (Fus) into the amplifying medium (M) such that the total number of passes of the beam to be amplified into the amplifying medium (M) is a multiple of four. 15 8. Amplification device (10) according to any one of claims 5 to 7, wherein the second optical return unit (30) is adapted to return the second useful beam (Fus) into the amplifying medium (M) such that the total number of passes of the beam to be amplified into the amplifying medium (M) is a multiple of two and that the beam to be amplified makes a forward path and a return path, superimposed on the forward path, between the first entry of said beam into the amplifying medium (M) and the last exit of said beam from the amplifying medium (M).
9. Amplification device (10) according to any one of claims 5 to 8, wherein at each pass through the amplifying medium (M), a parasitic beam is obtained directly reflected onto the front face (20) of the amplifying medium (M), the first reflection unit (18) and the second reflection unit (30) being arranged outside the path of each parasitic beam resulting from an odd pass of the beam to be amplified through the amplifying medium (M).
10. Amplification device (10) according to any one of claims 1 to 9, wherein the last useful beam at the output of the amplifying medium (M) is called the output beam (F s ), the incident beam (Fi) and the output beam (F s ) being spatially offset.< / t> < / t> < / t> < / t>