FR2911012A1 - Intracavity laser source for e.g. military application, has pumping unit including thulium doped crystal in form of thin strip, to transversely pump holmium doped yttrium aluminum garnet crystal, where unit pumps thulium crystal - Google Patents

Abstract

The source has a pumping unit (2) with a thulium doped crystal in the form of a parallelepiped thin strip (3), to transversely pump a holmium doped crystal (4) i.e. holmium doped yttrium aluminum garnet (YAG) crystal. The thulium crystal is chosen from thulium doped Yttrium orthoaluminate (YALO), thulium doped YAG, thulium doped Potassium gadolinium tungstate and thulium doped yttrium orthovanadate (YVO4) crystals. The crystals are arranged in a laser cavity (5). The unit pumps the thulium crystal. The strip has a main side (10) covered by low refractive index material i.e. sapphire.

Description

The invention relates in particular to the field of lasers and more

  particularly for object a laser source capable of emitting at ocular safety wavelengths and high energy. The problem of eye safety of laser sources is crucial when it comes to high energy laser sources. It is undoubtedly in the field of industrial and military applications, where such sources are implemented, that the risks are the most important. However, for economic and strategic reasons, we always favor sources for which congestion and yield are the most favorable, rather than taking into account ocular risk. On an industrial site where the sources are established in a fixed way, one can protect the personnel. It is quite different for military applications. The choice of the 1.315 μm oxygen iodine laser as a high energy source in the military field is at the acceptable limit when it is intended to operate at high altitude so that this type of risk does not really exist. Moreover, with the development of pump diodes whose efficiency is expected to reach 70 to 80%, the Nd: YAG laser emitting at 1.06 μm has become of considerable importance to the point that this type of source has been selected as a high-energy source. for military applications. It is true that this laser has benefited from many technological advances that have enhanced its potential as a high-energy laser. In this respect, patent application 2003/0138021, which describes a laser high energy laser source which uses an active material of Nd: YAG type in the form of a thin structure in the form of a thin blade so as to be able to to evacuate more easily the part of the pumping energy transformed into heat. This thin blade is sometimes referred to as Slab. This thin blade is of parallelepipedal shape, length L of width 2b and thickness h. It thus comprises two main faces of large section 2b.L and four secondary faces of smaller section, including two longitudinal faces of section L.h and two transverse faces of section 2b.h.

  With this thin blade whose thickness is small, namely between 0.4 and 1 mm and preferably 0.7 mm, the 3 functions for obtaining the laser emission are distributed along the three axes of space: this thin plate is pumped by a radiation P generated by laser diode arrays and penetrating inside by one or two longitudinal faces of section Lh and in a direction x, the laser emission takes place along a perpendicular y-axis at x and located in the plane of the blade, and across the cross-sectional surfaces of section 2b.h. the heat dissipated by the thin plate, due to the absorption, by this blade, of the pump beam P, is removed by conduction, with known cooling means, through the main faces along an axis z perpendicular to x and there.

  By means of reflective deposits or blades with a lower index, for example using sapphire, placed on both sides on both sides, a laser emission and pumping light guidance is obtained. The length of the thin slice in which the laser emission takes place is the parameter which makes it possible to extrapolate towards the high powers.

  The essential problem that remains is related to the particular geometry of the emitted laser beam, namely a beam whose section is a slot with very different divergences in both directions and which has an inhomogeneity of the intensity profile due to the d absorption in the direction of the width of the slot.

  In addition to their intrinsic drawbacks, these high energy sources have the disadvantage of not transmitting at a wavelength with an eye safety, ie located above about 1.5 μm. Moreover, the article by Mackenzie et al. Entitled intra-cavity sidepumped Ho: YAG laser which describes a low energy intracavity laser source comprising first pumping means constituted by diode capable of emitting at the wavelength of 0.792pm and able to pump longitudinally second pumping means. , comprising a first crystal doped with thulium and constituted by a thin plate Tm: YLF, themselves able to pump transversely a second crystal doped with holmium Ho: YAG, the first and second crystals being arranged in a same cavity. By longitudinal pumping means that the direction of the pump beam of the crystal is parallel to the laser beam emitted by this crystal. By transverse pumping means that the direction of the pump beam is perpendicular to the laser beam emitted by this crystal.

  Such a laser source does not provide a high energy laser source since the authors give an output power of 15W. In addition, such a source exhibits variations in the wavelength emitted by the thin plate and therefore the risks of emission at a wavelength that is not the maximum absorption wavelength of the Ho: YAG which further decreases the power of the laser source. Also known are the achievements of Mackenzie et al. Proposed in the article entitled 15 Watt diode side pumped Tm: YAG laser waveguide at 2pm Electronics Letters, July 5, 2001, vol.37, No. 14 concerning an eye-safe source suitable for emit at a wavelength of 2.01 μm. However, these embodiments make it possible to obtain laser sources of low energy, namely less than one hundred Watt, and in no way make it possible to obtain a high energy laser source, that is to say a power greater than kW . In addition, the absorption coefficient of the Ho: YAG pumping radiation at 1.91 μm is about 10 times lower than that of Nd: YAG, which leads to a beam geometry that is still far less suitable than that of Nd: YAG. in a slab configuration to produce high power. Since the pumping power densities are very different, the diad architecture of the Nd: YAG slab is not suitable for Ho: YAG, especially since the power of the available 1.91 μm diodes is relatively low, namely less than 20 W per unit. The object of the invention is to provide a laser source with ocular safety and high energy and also allowing the obtaining of weakly divergent beams that it will be easy to transport remotely.

  The solution provided is, according to a first embodiment, an intracavity laser source comprising first pumping means capable of pumping second pumping means, comprising a first crystal doped with thulium and constituted by a thin blade, themselves capable of to be pumped transversely a second crystal doped with Ho: YAG holmium, the first and second crystals being arranged in the same cavity and characterized in that the first crystal is in Tm: YAG and in that the first pumping means are able to pump transversely this first crystal.

  In addition to the aforementioned advantages, a source according to the invention makes it possible to obtain a pumping beam whose geometry is particularly well adapted to the use of a second bar-shaped crystal. The absorption cross-section as a function of the wavelength for a thulium doped crystal Tm: YAG is for example given in FIG. 7. A number of peaks at 765, 779, 781 are observed, 786, 793, 798 and 804nm. Also, according to a characteristic making it possible to promote high energy emission, the first pumping means are capable of emitting radiation at a wavelength different from those concerned by the absorption peaks of said FIG. 7 in this case. at a wavelength at which the absorption cross-section is less than 1.5 × 10 -2 cm 2, the latter value being shown in dashed lines in this FIG. 7. According to a particular characteristic enabling very high powers to be obtained, a laser source comprises first pumping means capable of pumping second pumping means comprising said first crystal doped with thulium and themselves capable of pumping a second crystal doped with holmium, the first and second crystals being disposed in According to one particular characteristic, the thin plate has a thickness e less than 1 cm, and preferably less than 1 mm. and a length and a width, respectively L and 2b, greater than 1 cm. According to another characteristic, the thin plate comprises two main faces and at least three secondary faces, the first pumping means being arranged facing at least one of said secondary faces. According to a particular characteristic, the blade has a parallelepiped shape. According to another characteristic, the first crystal is constituted by one of the following crystals: YAG, YALO, YVO4 or KGW doped with thulium. According to an additional feature, the laser comprises means for cooling the main faces of the thin blade.

  According to another characteristic, the first pumping means comprise an assembly consisting of fiber laser emitters, these emitters preferably being constituted by diodes capable of emitting radiation, for example of 0.8 μm.

  According to a particular characteristic, the main faces are covered by a lower refractive index material to obtain a guiding effect, for example sapphire. According to a particular characteristic, the second crystal is arranged facing one of the lateral faces, preferably adjacent to one of the faces facing the first pumping means. According to another characteristic, the thin plate is capable of emitting laser radiation in a direction perpendicular to the radiation emitted by the first pumping means.

  According to another characteristic, the dopant concentration inside the thin plate and between a first plane formed by a secondary face disposed facing the first pumping means and a second plane of the parallel blade in the foreground and containing the center of the blade, varies, this concentration being preferably lower in the foreground than in the background.

  According to one particular characteristic, the concentration N (x) dopant varies between the first and second planes according to the following formula: ab N (x) = 1 i 1 x 'where T b, T being the residual transmission, and o- the section effective absorption.

  Other advantages and features of the present invention will become apparent in the description of various alternative embodiments of the invention, with reference to the appended figures in which: FIG. 1 schematizes the general means constituting an intracavity pumping laser according to a variant embodiment of the invention. - Figure 2 shows a diagram of a thin blade used in the context of this embodiment of the invention. - Figure 3 shows a diagram of a longitudinal section of radiation guiding and cooling means of a thin blade. FIG. 4 shows a diagram of a longitudinal section of means for guiding radiation and for cooling a thin plate according to a second variant embodiment. FIG. 5 shows a diagram of means for shaping radiation from first pumping means according to a second variant embodiment. FIGS. 6a and 6b show the dopant concentration inside the thin plate, respectively when it is pumped on a longitudinal face and on two longitudinal faces. Fig. 7 shows a diagram of the absorption cross-section of Tm: YAG as a function of the pumping wavelength of this crystal.

  FIG. 1 schematizes the general means constituting an eye-safe and high-power laser source according to an alternative embodiment of the invention. It comprises first pump means 1 capable of pumping second pump means 2 comprising a first crystal doped with thulium and in the form of a thin plate 3, these second means 2 being themselves able to pump a second crystal 4 doped with holmium, the first and second crystals being disposed in the same cavity 5. The thulium doping is preferably greater than or equal to 2% or even higher while the second crystal is doped with the holmium with a rate preferably less than 1%, for example 0.30 / 0. The second pumping means consist of a laser cavity 5, called the first cavity in the following, and in which is placed the first crystal 2 in the form of a thin plate 3 and which is delimited by two mirrors 6 and 7 each comprising, cavity side a coating respectively 8 and 9, reflecting the laser emission wavelength of this first crystal, that is to say at a wavelength of about 1.9 .mu.m. As shown in FIG. 2, the thin plate 3 is of parallelepipedal shape, of length L, of width 2b and of thickness h. It thus comprises two main faces 10 of section 2b.L and four secondary faces of smaller section, including two longitudinal faces 11 of section L.h and two transverse faces 12 of section 2b.h.

  This thin blade 3 is a structure of thin solid laser material, a few tenths to 1 mm, in which the 3 functions for obtaining the laser emission are distributed along the three axes of space: - this thin blade 3 is pumped by a radiation P through one or two longitudinal faces 11 of section L. h and in a direction x, the laser emission takes place along an axis y perpendicular to x and located in the plane of the blade, and through transverse surfaces 12 of section 2b.h. the heat dissipated by the thin plate 3, due to the absorption, by this blade, of the pump beam P, is removed by conduction, with known means not shown, through the main faces along an axis z perpendicular to x and y. The thin blade 3 must guide the laser emission over the entire length crossing and guide the pumping light over the entire path 2b. For this we can place this blade between two other blades, 41, 42 of lower refractive index, for example in sapphire, as shown in Figure 3 thus giving rise to a 0.47 numerical aperture waveguide, and placed cooling means 43 of the blade 3, in this case water-cooled copper plates 44, 45, above the sapphire blades 41 and 42. Another possibility, shown in FIG. 4, consists in coating the thin plate by layers 46,47 totally reflecting for the pump and the laser light emitted by the thulium-doped plate and placing cooling means 44,45 above each of said reflecting layers 46,47. The first pumping means are constituted by laser emitters 13, in this case diodes, coupled to at least one of the longitudinal faces by optical fibers 14. These assemblies are known as fibered diodes 15 obtaining pump beams P with a pump power density of the order of a kilowatt per cm 2 and an overall pump power of several kW. These diodes emit, in this embodiment, a wavelength of about 0.800pm. Fixing means 40 are able to hold the end 17 of the fibers facing the longitudinal face 11 to which they are coupled. Optionally, as shown in FIG. 5, the means for coupling the diodes with the longitudinal face 11 may comprise an optic 16, for example a collimation optics disposed between the end 17 of the fiber diodes and the said longitudinal face 11. The second crystal 4 is disposed in a second cavity 18 delimited on the one hand by a first mirror 19 having, on the side of this cavity, a coating 20 reflecting the emission wavelengths of the second crystal 4, in this case about 2.1 pm and a second output mirror 21 having a cavity-side face 18, covered with a highly reflective coating 22 at 2.1 μm and having a reflectivity of the order of 95% at this same wavelength. This second crystal 4 is in the form of a bar of length substantially equal to the width 2b of the thin plate 3 and disposed facing one of the transverse faces 12 of the blade and inside said first cavity 5 and at a distance of such distance from this transverse face that the whole of the radiation 25 exiting penetrates into the crystal 4, despite the strong two-dimensional divergence of this radiation. Thus, the Ho: YAG bar is pumped in transverse mode directly by the source Tm: YAG without the use of optical system to expand the pump beam which simplifies its implementation and improves the compactness of the source. The first and second crystals used are YAG (garnet of aluminum and yttrium). However any other suitable crystal or crystal combination could be used such as YSAG (scandium and yttrium aluminum garnet), as YSGG (gallium garnet of scandium and yttrium), YGG (gallium garnet and d yttrium), GGG (gallium garnet and gadolinium garnet), GSGG (gallium garnet of scandium and gadolinium), GSAG (aluminum garnet of gallium and gadolinium), LLGG (gallium garnet, lutetium and lanthanum), YAP (aluminum and yttrium perovskite), YLF (lithium fluoride and yttrium), LuLF (lithium fluoride and lutetium), YVO4 (yttrium vanadate) ... The operation of this laser device is the following. The pumping beams P emitted by the laser diodes 13 are conveyed by the coupling means, in this case the fibers 14, and penetrate inside the thin plate 3, therefore of the first doped crystal Tm: YAG, by the one and the other of its longitudinal faces 11. These pump beams are absorbed almost entirely by this first crystal 3 which then starts to laser by emitting a laser beam 25 whose wavelength is 2.01 pm. This beam 25 then passes through the second crystal 4 in which it is partially absorbed. The unabsorbed portion is reflected by the coating 8 of the mirror 6 delimiting on one side the first cavity, and in the direction of the second crystal 4 in which it is again partially absorbed. The portion absorbed by the second crystal 4 of the beam coming from the first crystal causes the second crystal 4 to emit a beam 26 at a wavelength of approximately 2.1 μm. Leaving the second crystal, 90 to 95% of this beam 26 is reflected by the coating 22 of the second mirror 21 while 5 to 10% of the beam passes through it; This part of the referenced beam 27 can then be used, in known manner, outside the cavity. Advantageously, the thulium doping, inside the constituent thin section of the first crystal, is gradually and progressively distributed in the direction of the pumping beam over the distance 2b in the blade if it is pumped by a single face, or the distance b if we pump by the two opposite faces. FIGS. 6a and 6b show, respectively, the proposed Thulium doping as a function of the x direction from one of the longitudinal faces and respectively, when the pumping takes place on one or both longitudinal faces (11). Theoretically, doping should vary according to the law; 6b N (x) = 1, where b is the residual transmission (eg 10%), and the absorption cross-section. With this doping distribution law, substantially the same pumping power density is deposited throughout the blade. With Tm: YAG it would typically be 30-40 mm and the N-doping would range from 3 to 4% (put plausible values corresponding to Thulium). To obtain a sufficient pumping power density, optical fibers carrying the pumping light are distributed over the entire length L of one or both opposing faces reserved for pumping. The relationship between the laser power P and the pumping power density Dp is written as follows: P, = n 2 h.L. Dp 5 i1 being the optical efficiency of the laser (z 0.4 to 0.6 for Tm: YAG). The heat power density to be evacuated per unit area of the slab is:

dQ aDp (1-) .h dS 1n (0.9)

  a = absorption coefficient for 1.91 pm. This value of dQ / dS is substantially the same as for Nd: YAG because the products Ddp of Tm: YAG and Nd: YAG are very close.

Claims (9)

claims   1 intracavity laser source comprising first pumping means (2) capable of pumping second pumping means, comprising a first crystal (3) doped with thulium and constituted by a thin blade, themselves capable of pumping transversely a second crystal (4) doped with holmium, the first and second crystals (3; 4) being arranged in the same cavity (5) and characterized in that the first crystal is in Tm: YAG and in that the first means of pumping (2) are able to pump transversely this first crystal (3). 2 laser source according to claim 1, characterized in that the first pumping means (2) are able to emit pumping radiation of Tm: YAG at a wavelength at which the absorption cross section is less than 1 10-21 cm2. 3 laser source according to any one of claims 1 and 2, characterized in that the thin plate (3) has a thickness e less than 1 mm and a length and a width, respectively L and 2b, greater than 1 cm. 4 laser source according to any one of claims 2 and 3, characterized in that the thin plate comprises two main faces (10) and at least three secondary faces (11, 12), the first pumping means (1) being arranged facing at least one of said secondary faces (11, 12). 5. Laser source according to any one of claims 1 to 4, characterized in that the first crystal (3) is constituted by one of the following crystals: YAG, YALO, YVO4 or KGW doped with thulium. Laser source according to any one of claims 2 to 5, characterized in that the first pumping means (1) comprise an assembly (15) consisting of fiber laser emitters, these emitters preferably being constituted by diodes (13). capable of emitting at a wavelength of 0.800 μm. Laser source according to any one of claims 4 to 6, characterized in that the main faces (10) are covered by a material (41, 42) of lower refractive index which makes it possible to obtain a guiding effect. for example sapphire. 8 laser source according to any one of claims 1 to 7, characterized in that the second crystal (4) is disposed opposite one of the facessecondaires (11,12), preferably adjacent to one of the facing faces with the first pumping means. 9 laser source according to any one of claims 1 to 8, characterized in that the dopant concentration inside the thin blade (3) and between a first plane formed by a secondary face (11, 12) facing the first pumping means (1) and a second plane of the blade parallel to the first plane and containing the center of the blade, varies and, preferably, the concentration is lower in the foreground than in the second plane. A laser source according to claim 9, characterized in that the dopant concentration N (x) varies between the first and second planes according to the following formula: 6b N (x) = 1 (1 x \ uT b, T being the transmission residual, and 6 the absorption cross section.