EP0547212A1 - Neodymium-doped gehlenite crystal and laser using same - Google Patents

Abstract

The crystal according to the invention has the formula Ca2-xNdxAl2 + xSi1-xO7 with 0 <x 1. This crystal can be used as a laser emitter (4) optically pumped by a laser diode (6), the temperature of which is not controlled by a Peltier element component.

Description

 CRYSTAL OF GEHLENITE DOPED WITH NEODYME AND LASER USING THIS CRYSTAL DESCRIPTION

 The subject of the present invention is crystals, and more particularly neodymium doped gehlenite monocrystals. It finds an application, either in the field of microlasers for integrated optics, telecommunications by optical fibers, medicine (microsurgery, skin treatment) and for the study of semiconductors, or in the field of laser power making it possible in particular to carry out materials treatments (welds, drilling, markings, surface treatments), photochemical reactions, controlled thermonuclear fusion or polarization of the atoms of a gas such as helium. These lasers emit in the infrared being 870 and 1130nm.

 More particularly, the invention applies to lasers optically pumped by laser diode and capable of being tunable in wavelength between 1050 and 1100 nm.

The most commonly used commercial solid-state lasers use yttrium aluminum garnet Y ₃ Al ₅ O ₁₂ (YAG), doped with

Nd ³⁺ . However, this material is not without drawbacks; indeed, it exhibits crystallogenesis at a high temperature of 1900-2000 ° C., making it difficult and expensive, segregation of the dopant, narrow absorption bands, etc. Hence the search for new matrices, required on the other hand by the increasingly remarkable performances, required for new lasers: very high efficiency, miniaturization, emission at wavelengths varied.

Also, many matrices capable of being doped with Nd ³⁺ have been proposed in the literature. Among these matrices, mention may be made of compounds of the melilite type ALaG _a 3O ₇ , gehlenite AGa ₂ SiO ₇ and akermanite A MGe ₂ O ₂₊ in which A represents the lons Ca ²⁺ , Sr ^{2 + 2} or Ba and M represent the ions Be ²⁺ , Mg ²⁺ or Zn ²⁺ .

 These compounds have the great advantage over yttrium and doped aluminum garnet that they can be produced by the Czochralski method, the most commonly used in the laser industry, at temperatures close to 500 ° C lower. In addition, there is no segregation of the phosphor dopant.

 Under optical pumping by flash lamp, a certain number of these compounds exhibit the laser effect.

 The document A. Kaminskii et al. Phys. Stat.

Ground. (a), 97, 1986, p. 279-290, "Crystal structure, absorption, luminescence, properties, and stimulated emission of Ga gehlenite" describes the stimulated emission in a single crystal of Ca ₂ Ga ₂ SiO ₇ doped with neodymium ions.

The article by W. Ryba-Romanowski et al., J. Phys. Chem. Solids, 50, 1989, p. 685-692, "Relaxation of the ³ F _3/2 level of Nd ³⁺ in BaLa _1-x Nd _x Ga ₃ O _7" describes the laser emission of a single crystal of BaLa _1-x Nd _x Ga ₃ O ₇ with 0≤x≤0.2.

The article by DJ Horowitz et al., J. Appl. Phys., Vol. 43, n ° 8, August 1972, "Laser action of Nd ³⁺ in a crystal Ba ₂ ZnGe ₂ O ₇ ", p. 3527-3529 describes the laser effect of a crystal of Ba ₂ ZnGe ₂ O ₇ doped with neodymium in an amount of 2% by mole.

The article by H. Alam et al., J. Appl. Phys. 39, 1968, "Optical spectra and laser action of neodynium in a crystal Ba ₂ MgGe ₂ O ₇ ", pp. 4728-4730 describes the laser effect of a crystal of Ba ₂ MgGe ₂ O ₇ doped with neodymium at about 2 mol%.

 All of these compounds have the advantage of having a uniaxial quadratic structure while the YAG crystal is cubic; this makes it possible to obtain a polarized emission.

Unfortunately, all of these compounds have the disadvantage of containing a large amount of gallium, but this element is expensive. In addition, it is unstable in the 3 ⁺ oxidation state at high temperature, depending on the working atmospheres, with the possibility of sputtering.

 Furthermore, due to the volatilization of gallium, the crystals obtained by Czochralski growth exhibit insufficient qualities when it comes to achieving the large dimensions required by the power laser industry.

 In addition, these compounds have a conductiv i t t h e rm i that makes an t d i f f i c i e the ev a c u a t i on heat when used as a laser emitter in a power laser.

 In the field of microlasers, the miniaturization of solid lasers involves the use of laser diodes for optical pumping having dimensions much smaller than those of flash lamps. In addition, laser diodes are very reliable and have laser yields much higher than those of flash lamps (10 times higher).

 This high laser efficiency requires a perfect coincidence between the emission line of the diode and the maximum of the absorption band relative to the single crystal.

Due to the wavelength drift of the emission of the diode, during the rise in power, resulting from the heating of the diode, it is necessary to use a system for controlling the temperature of the diode so as to compensate for this wavelength drift .

 The diode temperature is generally controlled by a Peltier element. Unfortunately, this Peltier element has a high electrical consumption, which partly ruins the advantage of the high diode pumping efficiency.

 Only a crystal having broad and intense absorption bands in the emission domain of the diodes which makes it possible to cover their wavelength drift could make it possible to overcome this Peltier element.

Also, very recently, a compound of the melilite type SrGdGa ₃ O ₇ doped to 2% in atom by ions

Nd ³⁺ has been proposed for diode pumping (see HR Verdun et al., Tunable Solid State Lasers IV, 1990, p. 405-407, "Growth and characterization of Nd doped aluminates and galates with the mililite structure ").

 This material has the advantage of a wide line, from around 10 nm to around 810 nm, corresponding to the emission wavelength of the laser diodes generally used. However, the stimulated emission has not yet been obtained to date with this compound.

On the other hand, it contains, like the previous compounds, gallium in large quantities.

 Also, the subject of the invention is a new material, free of gallium, capable of being optically pumped by a laser diode without using any element.

Peltier for temperature control of the latter.

As a gallium-free material which can replace the YAG and. not having its drawbacks of crystallogenesis and segregation of the activating dopant, lanthaneneodymium magnesium aluminates are known, called LNAs with the chemical formula La _1-x Nd _x MgAl ₁₁ O ₁₉ with 0 <x≤1 and in particular with x = 0.1.

 These aluminates have been the subject of patents

FR-A-2 448 134 and EP-A-O 043 776 and from the publication by L.D. Schearer et al., IEEE Journal of Quantum Electronics, vol. QE-22, n ° 5 of May 1986, p. 713-717, "LNA: a new CW Nd laser tunable around 1,05 and 1,08μm". The single crystals of these aluminates have optical properties comparable to those possessed by neodymium-doped YAG.

 But here again the manufacture of these aluminates in the form of large single crystals by the Czochralski method along the optical axis which would be optimum for the laser properties poses a problem and leads to crystals of insufficient quality as soon as it is a question of achieve the large dimensions required for power lasers.

 In the field of microlasers, the LNA can be optically pumped by laser diode provided that a Peltier element for controlling the temperature of the diode, such as YAG, is used.

 The subject of the invention is therefore a gehlenite crystal doped with neodymium, which can be used as a laser transmitter, making it possible to remedy in particular the various drawbacks mentioned above.

 In particular, this compound can be produced in monocrystalline form of large dimensions, free of bubbles and defects, by the Czochralski method. This gehlenite single crystal can therefore be used in the power laser industry.

In addition, it can be used in the field of microlasers with diode pumping without using a Peltier element. Its energy efficiency as well as its energy cost are therefore much lower than those of known crystal microlasers.

More specifically, the subject of the invention is a neodymium-doped gehlenite crystal of formula Ca _2-x Nd _x Al _{2 + x} Si _1-x O ₇ with 0 <x≤1.

 The crystallogenesis of this crystal according to the Czochralski method is well controlled and can be carried out without risk because there is no volatilization of the constituents. It is carried out at temperatures lower than those used for YAG (around 1600 ° C.) and there is no segregation of the neodymium dopant. The fusion of this compound is congruent, hence the possibility of developing large crystals which can be used as a laser emitter in a power laser.

Compared to its gallium counterpart Ca ₂ Ga ₂ SiO ₇ : Nd), the crystal of the invention has better chemical inertness as well as higher hardness, thus giving it better mechanical strength. It contains aluminum, not gallium, which is much cheaper. In addition, it must have a higher thermal conductivity than that of the gallium counterpart. Indeed, in the expression of the thermal conductivity K = Cvl, C which is the heat capacity per unit of volume increases if the molecular mass decreases; v represents the speed of the phonons in the crystal and l the mean free path of the phonons.

Crystallographically, the mesh of the gehlenite of the invention is smaller than that of the gallium counterparts, which leads to modifications of the crystal field and therefore of the optical properties themselves. In particular, the crystallographic structure is more disordered than that of BaLaGa O ₇ : Nd. Indeed, if in the latter the site of Nd ^{3 + 3} is surrounded only by Ga ³⁺ ions, in Ca ₂ Al ₂ SiO ₇ : Nd ₃₊ , this same site is surrounded by both Al ions and Si ⁴⁺ ions.

 It is possible to accentuate this disorder by making substitutions on the three types of cationic sites.

 The compound of the invention is intended to be used as a laser emitter emitting in the infrared, both in power lasers and in microlasers.

Also, the subject of the invention is a laser comprising a laser cavity containing as a light emitter a single crystal, means for amplifying the light coming from the single crystal, means for extracting the light from the laser cavity and means pumping, characterized in that the single crystal is a neodymium doped gehlenite of formula Ca _2-x Nd _x Al _{2 + x} Si _1-x O ₇ with 0 <x <1.

 In particular, x is chosen so that

0 <x≤0.3. A v an t a g eu s e x is chosen so that 0.01≤x≤0.2 and better still from 0.025 to 0.05. Preferably x is 0.04.

The disorder of the structure is reflected in particular by a widening of the emission bands. The transition ⁴ F _3/2 ╌> ⁴ I _11/2 , which is at the origin of the main laser emission, presents an important contribution from 1.05um until around 1.10μm, which is remarkable if we consider that for the YAG the laser wavelength is 1.064um. It can be used to tune the laser emission as in the LNA.

Thus, the crystal of the invention has a wide range of tunability in wavelength, currently the largest existing for a neodymium doped laser crystal, with the exception of glasses.

 The lifetime of the excited state is of the order of 280 μs for x = 0.02; for x = 0.1 it is of the order of 160μs. For x = 0.2, the lifetime is still 30 μs, a high value for such a doping rate.

The great advantage of the material of the invention for the envisaged laser application is that it is suitable for pumping by laser diode. Indeed, the optical absorption spectrum of Nd ³⁺ in this compound has the same appearance as those of other laser matrices activated with Nd ³⁺ but with the particularity that here, it is the band around 800nm ( ⁴ I _{9 / 2} ╌> ⁴ F _5/2 , ² H _9/2 ) which is the most intense while in most of the other cases, it is the hypersensitive transition (towards

580nm) which is the most intense.

 This result is particularly remarkable because this wavelength (around 800nm) corresponds exactly to the emission range of laser diodes.

In addition, this absorption band is very wide as a result of the disorder between Ca ²⁺ and Nd ³⁺ and between Al ³⁺ and Si ⁴⁺ , and completely covers the emission domain of the diode. Also, even if there is a wavelength drift of the latter, the absorption will be sufficient.

Nd ³⁺ sites have very low symmetry

(Cs); they therefore favor a high intensity of the absorption bands.

The values of the oscillator forces calculated for a refractive index n = 1.77 are. particularly high; largest (7,35.10 ^-6) corresponds to the transition ⁴ I _9/2 ╌> ⁴ F _5/2, ² _9/2 H to 800nm.

Other features and benefits of the invention will emerge more clearly from the description which follows, given purely by way of illustration and without limitation, with reference to the appended drawings, in which:

 FIGS. 1 and 2 show schematically the fluorescence spectrum at 300K of a gehlenite crystal according to the invention,

 FIG. 3 gives the variations in the fluorescence intensity as a function of the neodymium content,

 FIG. 4 gives part of the absorption spectrum of a crystal according to the invention,

 FIG. 5 gives, by way of comparison, part of the absorption spectrum of a monocrystal of LNA doped with neodymium,

 FIG. 6 gives the laser power emitted as a function of the pump power for a gehlenite crystal with x = 0.02, and

 - Figure 7 schematically shows a power laser according to the invention, optically pumped by a laser diode.

 To make a neodymium doped gehlenite crystal of formula (I):

(I) Ca _2-x Nd _x Al _{2 + x} Si _1-x O ₇ ,

high-purity commercial powders, calcium carbonate, neodymium and aluminum oxides are used, as well as silica which is weighed in the desired proportions. These powders are mixed for several hours using a mechanical stirrer which can be compressed into a cylinder. Sintering for 20 hours at 1450 ° C. is carried out to obtain the doped calcium aluminosilicate intended to form the molten bath.

The starting materials used are in the form of a powder with a particle size of 1 to 10 μm and have a purity greater than 99.99% in order to obtain as high a yield as possible for laser emission.

 The sintered mixture is then placed in an iridium crucible and brought to a temperature of 1600 ° C. corresponding to the melting temperature of the mixture. The drawing is carried out under an argon or nitrogen atmosphere from a seed having the desired orientation.

Generally the drawing is done along the axis c of the crystal.

 The drawing speed varies from 0.5 to 1 mm per hour and the rotation speed varies around 40 rpm.

 Of course, any other method of crystallogenesis using a molten bath such as the Bridgmann method, of the floating zone, of

Kyropoulos or self-crucible can be used.

 A perfectly monocrystalline sample can be isolated from the crystal obtained according to the Czochralski method, by cleavage or by cutting and polishing, so as to obtain two strictly parallel faces; this sample can then be placed in a laser cavity as shown in Figure 5 and be optically pumped by a laser diode emitting at around 800nm.

In FIGS. 1 and 2, the variations in the fluorescence intensity I _f (in arbitrary units) are represented diagrammatically as a function of the wavelength expressed in nanometers, at 300K for a gehlenite of the invention. Figure 1 relates to the optical transition ⁴ F _3/2 ╌> ⁴ I _11/2 the most interesting from the laser point of view and Figure 2 corresponds to the transition ⁴ F _3/2 ╌> ⁴ I _13/2 ; this last transition is of interest for the transmission of information by optical fibers. It can be seen from these curves that the laser emission takes place over a wide band, from 1050 to 1100nm for the ⁴ F _3/2 transition ╌> ⁴ I _11/2 and from 1300 to 1430nm for the ⁴ F ₃ transition _{/ 2} ╌> ⁴ I _13/2 .

 Thus, the crystals of the invention have a broad wavelength tunability as well as a laser emission with a longer wavelength than that of most other laser crystals doped with neodymium. In particular, the neodymium-doped LNA emits at 1054nm, 1083nm and 1320nm with a wavelength tunability of a few nm (less than 10nm) and the neodymium-doped YAG emits at 1064nm with a tunability of less than 0 , 6 nm.

These fluorescence spectra were obtained using an excitation wavelength of 577 nm, corresponding to the transition ⁴ I _9/2 ╌> ⁴ G _5/2 , ² G _7/2 in absorption of the crystal, and a composition x in neodymium of 0.02. Note however that the general appearance of this spectrum is valid regardless of the amount of neodymium; only the absorption intensity may be slightly different.

Although this does not appear on the curves, the crystal of the invention also emits between 870 and 930nm, which corresponds to the transition ⁴ F _3/2 ╌> 4I _9/2 .

In Table I below, the lifetime of the excited state ⁴ F _{3/2 is given} as a function of the quantity of Nd ³⁺ ions for four of the gehlenite crystals according to the invention.

In this table, x indicates the quantity of Nd ³⁺ ions, the two middle columns give the short and long lifetime of the laser effect and the last column gives the fluorescence intensity in arbitrary units. These values have been established for a crystal 1mm thick. From this table I, it can be seen that the decrease in the life time and in the fluorescence intensity for x = 0.20 is linked to the self-extinction phenomenon. Above x = 0.30, the life time and the fluorescence intensity are insufficient to use this compound as a laser emitter.

 Compound number 2 is. the one with the best laser properties.

 FIG. 3 gives the variations in the fluorescence intensity If (in arbitrary units) as a function of the doping rate x in neodymium in crystals of formula (I). The curve of Figure 3 was determined experimentally.

 In this curve, it is clear that the maximum intensity is for x≤0.05 and in particular between 0.025 and 0.05 and more precisely at x = 0.04.

 In FIG. 4, the absorption spectrum of a gehlenite crystal of the invention has been represented and in FIG. 5, the absorption spectrum of an LNA crystal from p o a to n eo d ym e. These are the optical optics (D.O.) as a function of the wavelength expressed in nanometers.

 In these figures, the absorption transitions have been shown.

From FIG. 4, it can be seen that the absorption spectrum of the Nd ³⁺ doped gehlenite according to the invention has an intense and wide absorption band around 800 nm. The peaks A and B correspond respectively to absorption wavelengths of 797.1 nm and 806.7 nm. It is in this wavelength range that the laser diodes emit.

The crystals of the invention also have an absorption band around 590 nm (hypersensitive domain), peak C of the absorption curve, like the most other neodymium doped materials, but this absorption band, unlike other neodymium doped compounds, is much less intense than that around 800nm.

 This is in particular illustrated by FIG. 5 where the maximum absorption is around 580 nm, peak D of the absorption curve, while the absorption intensity around 800 nm, peak E of curve 4 , is much weaker than that at 580 nm.

 As an indication, in Table II, the oscillator forces of the absorption transitions are given below. These oscillator forces are among the highest observed for known compounds doped with neodymium.

 Due to the wide absorption band around 800-805nm and the high absorption intensity, it is not necessary to use a diode temperature control system of the Peltier component type. Indeed, the optical pumping can be ensured between 790 and 820nm and thus cover the wavelength drift of the diode.

 Also, the crystals of the invention prove to be perfectly well suited for optical pumping by laser diode.

Figure 6 gives the variations of the laser emission power (in mW) versus pump power (mW) of a crystal composition of Ca _{1 gehlenite, 98} Nd _0.02 Al _2.02 Si _{0 , 98} O ₇ . These curves established experimentally clearly prove that the crystals of the invention exhibit the laser effect.

It appears from these results, given on a qualitative basis, that the crystal tested corresponding to x = 0.02 leads to a differential yield of the order of 40%. However, this crystal was not optimized. Indeed, its crystalline quality was poor and its rate of doping did not correspond to the optimal Nd ^{3+ content} determined on the curve of FIG. 3.

It is therefore certain that with a crystal of better crystalline quality and with an Nd ^{3+ content} of between 0.03 and 0.05, the laser yield of the material will increase considerably.

 In Figure 7, there is shown schematically a power laser operating continuously, using as laser transmitter a crystal of the invention.

 This laser comprises a laser cavity 2 containing a bar 4 of the compound number 2, arranged perpendicular to the longitudinal axis 3 of the laser, the axis c of the bar being coincident with the axis 3 of the laser. The laser emission is located in the infrared

(see figure 4).

 A monochromatic light source 6, such as a laser diode or a strip of laser diodes, makes it possible to irradiate the gehlenite bar 4, via a convergence lens 7, in order to ensure the optical pumping of the bar 4. A device 5 of circulation of distilled water around the bar 4 ensures its cooling. However, no temperature control of the diode is provided.

 For a high laser yield, it is preferable that the electric field of the pump light emitted by the diode is perpendicular to the axis c of the crystal.

 The laser cavity 2 also consists of a converging lens 8 transforming the light emitted by the gehlenite bar 4 into a parallel beam of light which is sent to an exit mirror 10.

After reflection on this mirror 10, the light beam again crosses the converging lens 8 and the amplifying medium or bar 4. The beam amplified laser is then reflected by a dichroic input mirror 12 near which is placed the bar 4; this mirror 12 is transparent to the light emitted by the monochromatic source 6 and opaque to that emitted by the single crystal of gehlenite 4.

 The sufficiently amplified laser beam in the cavity 2 is then sent to the outside of the laser cavity, via the mirror 10, which is partially transparent to the light emitted by the gehlenite single crystal 4.

 Wavelength tunability can be obtained using a wavelength selection system 14 interposed between the converging lens 8 and the output mirror 10 of the laser cavity 2, of the angle prism type Brewster filter or Lyot filter formed by several blades of birefringent material.

 In addition, a solid standard 15 of the blade type with parallel faces can be interposed between the converging lens 8 and the Lyot filter 14 to fix the emission wavelength.

 Diode 6 has the advantage of being extremely small, considerably reducing the total dimensions of the crystal laser; it emits at a wavelength around 800nm. However, the absorption spectrum of Figure 3 highlights a wide and intense absorption band around 800-805nm.

 In addition, the laser diodes have an excellent efficiency of around 50% and the laser conversion is around 30 to 40%, which corresponds to a laser effect efficiency of at least 20% from electric current.

The single crystals of the invention can be used in all applications currently using a YAG type laser transmitter. In particular, these single crystals can be used in lasers intended for cutting or marking materials as well as for welding.

In addition to YAG type applications, these oxides have their own applications. They are particularly suitable for pumping by laser diodes and therefore for making miniaturized devices (military applications, scientific research, medical applications). In addition, their particular emission wavelengths and the tunability of the latter can be used to advantage in optical telecommunications or for the polarization of certain atoms by optical pumping.

Claims

1. Neodymium doped gehlenite crystal of formula Ca _2-x Nd _x Al _{2 + x} Si _1-x O ₇ with 0 <x≤1.

 2. Crystal according to claim 1, characterized in that x is chosen so that 0 <x≤0.3.

 3. Crystal according to claim 1 or 2, characterized in that x is chosen so that 0.01≤x≤0.2.

 4. Crystal according to claim 1, characterized in that x is chosen so that 0.025≤x≤0.05.

 5. Crystal according to claim 1, characterized in that x is 0.04.

6. Laser essentially comprising a laser cavity (2) containing as a light emitter a single crystal (4), amplification means (10, 12) of the light coming from the single crystal, extraction means (10) light outside the laser cavity and pumping means (6, 7), characterized in that the single crystal is a neodymium doped gehlenite of formula Ca _2-x Nd _x Al _{2 + x} Si _1-x O ₇ with 0 <x≤1.

 7. Laser according to claim 6, tunable in wavelength in the infrared, characterized in that it comprises tunability means (14).

 8. Laser according to claim 6, characterized in that the pumping means consist of at least one laser diode.

 9. Laser according to claim 8, characterized in that the laser diode is a diode emitting around 800-805nm.

10. Laser according to any one of claims 6 to 9, characterized in that x is chosen so that 0 <x≤0.3.

11. Laser according to any one of claims 6 to 9, characterized in that x is chosen so that 0.01≤x≤0.2.

 12. The laser as claimed in claim 6, characterized in that x is chosen so that 0.025≤x≤0.05.

 13. Laser according to claim 6, characterized in that x is equal to 0.04.