GB2098790A - Material for laser device manufacture - Google Patents

Abstract

Material for a laser device consists of a compound of the elements erbium, thulium, holmium, lithium, lutecium and fluorine of stoichiometric composition given by the formula: Li Lu1-x-y-z Erx Tmy HozF4 ; x+y+z<1 Preferably x = 0.5 +/- 0.1, 0.025<y<0.13, 0.003<z<0.017. Melt material is formed by compounding fluorides in a heated inert ambient, or ambient of hydrogen fluoride, to form an ingot, zone- refining the ingot and selecting clear polycrystalline material from the ingot. Single crystal material is ski process during which the crystal is maintained at a temperature in excess of 400 DEG C during growth, and then rapidly cooled at a rate of not less than 2 DEG C/min. A rod of the laser material is optically pumped by a Xenon flash lamp.

Description

SPECIFICATION Material for laser device manufacture The present invention concerns material for laser device manufacture, and in particular a material comprising active ions of the elements erbium, thulium, and holmium, compounded in suitable proportion in a host lattice of lithium, fluorine and a diluent element.

Compounds based on the scheelite-structured lithium yttrium fluoride have been used as laser host lattices for various rare-earth ions but predominant interest has recently centred upon compounds containing holmium (Ho3+) ions which emit stimulated radiation in the eye-safe region of the electromagnetic spectrum at 2.06 ym wavelength. These compounds usually contain erbium (Er3+) ions and thulium (Tm3+) ions in order to assist the transfer of energy into the holmium (Ho3+) ion states which otherwise absorb radiation only weakly. This has given rise to a range of compositions of the type LiYo.5xy Er0.5Tm Ho F4, which are frequently referred to as ass-YLF.

Single crystals of ass-YLF have been the subject of extensive study and have been grown by top-seeded solution growth (see for example; Appl Phys Lett 19(1971)119), as well as by the Stockbarger technique (see for example J Crystal Growth 30(1975)21) and by the Czochralski techniques (see for example; Crystal Growth and Materials (North Holland, Amsterdam 1976) p706; Mat Res Bull 10(1975)501, and J Crystal Growth 39(1977)38). Of these, the Czochralski technique has been the most widely adopted, due largely to the difficulties in controlling crystal orientation in the Stockbarger technique and the slow growth rates endemic to solution growth techniques.

Two main approaches have been used in the application of the Czochralski technique to ass-YLF growth, both aiming to control the principal contaminant in fluorides, namely water vapour. The first approach has been to purify the components and to grow the crystals in a controlled neutral atmosphere, whilst the second approach has been to use a reactive atmosphere containing hydrogen fluoride HF, or other volatile fluorides, to prevent water vapour combining with the constituents.

Whilst the material ass-YLF has favourable opto-electronic properties that make it suitable as a laser material medium, its preparation for use in laser devices is beset with problems. In crystal form, it is comparatively difficult to polish since it has a relatively poor mechanical hardness.

When ass-YLF is grown by the Czochralski technique, there is a tendency for crystallites to form during growth, and this tendency is particularly pronounced during the seed-on and neck growth phases of the pulling operation ie during the initial growth. The formation of crystallites weakens the neck and can lead to abrupt failure. Even if failure does not occur, the crystallites can promote the formation of sub-grain structure during subsequent growth of the crystal and this can often lead to lattice misorientation Crystal orientation differing by as much 15 from the orientation of the seed crystal has been observed, in extreme cases, the misorientation could be even worse. All these effects limit the useful yield of laser material.

The invention is intended to provide an alternative material with comparable opto-electronic properties, a material that is mechanically harder and thus better suited to polishing preparation.

It is also intended to provide a material which too can be prepared in single crystal form by Czochralski growth, but with better yield.

Material in accordance with the present invention is characterised by a stoichiometric composition including the element lutetium, a composition given by the formula: Li Lu1 ~x~y~zErxTmyHozF4; x+y+z < 1 Hereinafter this material will be referred to as "aP-LuLF".

In the material "aP-LuLF" where the rare earth elements erbium, thulium and holmium are compounded in suitable proportion, the proportion of holmium necessarily lies within a range of values: zmin < z < zq. At the lower limit of the range z min-0.001, the proportion of holmium ions ensures just adequate provision of emission centres needed to produce effective laser action. At the upper limit, zq, the proportion of holmium is restricted to avoid substantial loss of laser gain due to self absorption by holmium-holmium ion interaction which tends to quench the radiation emitted. Up to 0.13 proportion of holmium in ass-YLF is found useful, and it is expected that this proportion will also be suitable in ass-LuLF, and perhaps marginally higher.

These limits will in any particular case, depend on the proportions of the other components; such is well known for the similar ass-YLF system. The optimum proportion 'x' of Erbium is around 0.5, and the optimum proportion 'y' of thulium is in the range 0.025 < y < 0.13 when holmium is provided in proportion in the range 0.003 < z < 0.017.

Single crystal rods of ass-LuLF can be grown by the Czochralski technique. However present attempts to produce single crystals directly from nominally pure, commercially available component fluorides (LiF, Lu F3, ErF3, Tm F3, and HoF3) using the Czochralski techniques in a controlled netral atmosphere have yielded crystals of poor quality and clarity. This approach has been modified to overcome this problem.

In further accordance with the invention there is provided a preparatory process wherein the components lithium fluoride (LiF), lutetium fluoride (Lu F3), erbium fluoride (ErF3), thulium fluoride (Tm F3) and holmium fluoride (how3) all of high purity, are mixed in appropriate proportion and compounded by heating in an inert atmosphere, or in an atmosphere containing hydrogen fluoride or other volatile fluoride, to form an ingot of aP-LuLF containing less than 1 ppma of dissipative impurity (eg other rare earth elements, or transition elements, that would otherwise quench laser activity) and less than 10 ppma total impurity; the ingot is zone-refined to form polycrystalline material, and the clear portion of this polycrystalline material is selected.

In order to avoid undue loss of lithium fluoride during the above process, it is desirable to maintain an ambient pressure of 1.7 atmospheres, or above.

Single crystal material, suitable for use in a laser device, may then be grown using the Czochralski technique taking as starting material the selected material produced by this preparatory process. An ambient 1.7 atmospheres pressure is again desirable.

Some crystals produced in this way have exhibited small particle scattering when viewed in collimated light.

In further accordance with the invention there is provided an improved Czochralski process wherein the pulled crystal of ass-LuLF is maintained at a temperature above 400"C during growth, and is cooled to room temperature at the rate of 2"C min-1. It is found that this in-situ annealing and relatively rapid cooling avoid the scattering problem referred to above.

One way of carrying out the invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1 is an illustrative drawing of apparatus used during the preparatory process, Figure 2 is an illustrative drawing of apparatus used during crystal growth, and, Figure 3 is a perspective drawing of a laser device including a single crystal laser medium.

As shown in the apparatus of Fig. 1, the material ass-LuLF is prepared from the component fluorides Lu F3, ErF3, TmF3 and HoF3 obtained as crystalline powders heat treated in an hydros HF and supplied by Rare Earth Products Limited, England, whilst the LiF used is supplied by British Drug Houses Limited in clear polycrystalline lumps produced by the Stockbarger process.

These components 1 are crushed and mixed together in appropriate proportion, loaded into a vitreous carbon boat 3 and introduced into the interior of a platinum tube furnace 5. This furnace 5 is mounted via a cradle support 7 onto a carriage 9 traversed by a lead screw mechanism 11. Initially a solenoid induction heater is located about the furnace 5 and the charge is melted to form an ingot. The solenoid is replaced by a co-axial single turn concentrator coil 1 3 and power is coupled directly into the platinum furnace to produce a localised hot zone 1 5 at one end of the ingot. The screw mechanism 11 is operated to drive the ingot through the concentrator 1 3. Throughout the zone-refining process an inert atmosphere of argon gas at a pressure above 1.7 atmospheres is maintained.The argon used is first purified by means of a rare gas purification unit to reduce the total impurity content to below 1 vol ppm, as determined by gas chromotography. A single pass at a traverse rate of 0.6 cm h-1 is proven sufficient to yield a polycrystalline bar of satisfactory quality.

Material is chosen from the clear portion of this zone-refined bar and used as starting material for crystal growth using the apparatus shown in Fig. 2.

This starting material 21 is melted in a platinum crucible 23 heated by means of an rf induction coil 25. The crucible 23 and coil 25 are contained in a double-walled stainless steel chamber 27 and the crucible is mounted on a platinum pedestal 29 above a weighing cell 31 outside the chamber 27. A seed crystal 33 held at one end of a pull rod 35 is lowered into the melt and the temperature and rod rotation and pull rates, adjusted to start the seed-on phase of crystal growth. A neck and flare out-portion of crystal 37 is then grown under manual control before automatic pull control is engaged. Subsequently the crystal diameter is controlled automatically in response to signals derived from the weighing cell 31. Further details of this technique are given in J Crystal Growth 38 (1977) 38 and also in UK Patent Specification No 1,434,527.A pulling speed of 0.1 cm h-' and crystal rotation rate of 10 rpm proves satisfactory.

During the above process the material is maintained in an inert ambient of argon gas. To avoid LiF losses this gas is held at a pressure above 1.7 atmospheres. At this pressure the loss in weight of a standard 100 g charge is found to be less than 0.0019 over a period of 60h, the normal growth cycle time.

To avoid solid state precipitation in the crystal 37, the crystal is annealed in situ and after growth it is rapidly cooled at a rate of 2"C min-' (this may be compared with normal cooling rates --0.5"C min-' used for other refractory materials). The apparatus thus includes a platinum shield after-heater 39 to maintain the ambient temperature experienced by the growing crystal above 400"C (M pt 840"C).

Using the above processes ass-LuLF single crystals of composition: LiLu0.434Er0.5Tm0.055Ho0.011 F4 have been grown and have been compared with crystals of ass-YLF of similar composition (ie Y0.434 instead of Lu0.434). During the initial process-zone refining-the yield of ass-LuLF clear polycrystalline material proved to be significantly better, a yield of 80-90% of clear usable LuLF material compared with a yield of 50-60% clear usable ass-YLF. Also during Czochralski growth, no seeding problem has been encountered during ass-LuLF growth, whilst in the case of ass-YLF polycrystalline nucleation tends to occur, decreasing useful yield.

The pulsed laser performance of the two crystal materials ass-YLF and ass-LuLF has also been measured. In each case a laser rod of length 27 mm and diameter 3 mm were cut from a single crystal having an axis lying along the [100] growth direction. The end faces of the rods were polished flat to within 0.061tm (ie 1/o wavelength of mercury green light) and parallel to better than 10 arc sec. As shown in Fig. 3, each rod 41 was placed in a resonant cavity formed by two external optical flat dielectric mirrors 43, 45, separated by 20 cm, one 43 of nominal reflectivity of 100%, the other 45 standardised at 85% at the laser wavelength (2.06m).

In each case the rod 41 was located at the focus of an electroformed, aluminium faced, copper elliptical cylinder reflector 47 and was energised by means of Xenon flash lamp 49 located at the other focus of the reflector 47. The lamp had a bore of 3 mm and length 27 mm between electrodes, its volume matching that of the rod 41 and was filled with Xenon to 1500 Torr. The elliptical cross-section of the reflector 47 was approx 14 mm major axis, 1 1.5 mm minor axis. In each case the laser rod 41 was mounted by means of open ended metal ferrules 51, 53 held each in an aperture in a reflective stainless steel end-plate 55, 57. Each rod was then tested by pulsing the lamp. Here the lamp was energised by discharging a 1 00F storage capacitor through it.A 30 H coil of negligeable resistance was placed in series with the lamp to prevent too rapid a build-up of current, the inductance value chosen so that the circuit inductance, capacitance and arc resistance together, gave critical (or near critical) damping. The results for the threshold energy E1-ie the energy dissipated in the lamp at the point where the laser fires, and slope efficiency (gradient of input energy vs output energy curve) are given below: Crystal Composition Threshold Energy Slope Efficiecny (Joules) (%) Li Y0.434Er0.5Tm0.055Ho0.011F4 13.0 1.3 LiLuO434ErO5TmO055HooO11F4 1 2.1 1 .3 LiLu0434Er05Tm0055Ho0011 F4 12.3 1.2 It is apparent that the threshold energies and slope efficiencies of ass-LuLF and ass-YLF are very similar.

ass-LuLF compositions within the ranges x = 0.5, 0.025 < y < 0.13 and 0.003 < Z < 0.017, at least, are considered suitable for laser application. However, those familiar with the many ass-YLF compositions already found useful in laser applications will appreciate that the invention is not restricted to just these ranges, and that other values of the proportions x, y, z will also be suitable.

Claims (10)

1. Material for laser device manufacture being a compound of the elements erbium, thulium, holmium, lithium, lutecium and fluorine of stoichiometric composition given by the formula: Li Lu1~x~y-z Erx Tmy Hoz F4; x + y + z < 1

2. Material as claimed in claim 1 having a content x of erbium of 0.5 i 0.1 and a content y and z of thulium and holmium, respectively, lying in the ranges: 0.025 < y < 0.13; 0.003 < z < 0.017

3. A single crystal rod for use in a laser, the rod being of material as claimed in either claims 1 or 2 above.

4. A preparatory process for the manufacture of laser device material wherein high purity fluorides of erbium, thulium, holmium, lithium and lutecium are mixed and compounded by heating in an inert atmosphere, or in an atmosphere containing hydrogen fluoride or other volatile fluoride, thereby to form an ingot of material of stoichiometric composition given by the formula: Li Lu1-x-y-z Erx Tmy HozF4 ; x + y + z < 1 containing less than 1 ppma total impurity; the ingot is zone-refined to form polycrystalline material; and, a clear portion of polycrystalline material is selected from the refined ingot.

5. A process as claimed in preceding claim 4 wherein the fluorides are heated and compounded by heating in an ambient of at least 1.7 Atmospheres pressure.

6. Selected material produced by the processes claimed in either preceding claims 4 or 5.

7. A process for producing single crystal material of stoichiometric composition given by the formula: LiLu1-x-y-z Erx Tmy HozF4 ; x + y + z < 1 comprising the steps of forming a melt of selected material produced by the process claimed in either preceding claims 4 or 5; lowering a seed crystal into the melt, raising the seed crystal at a controlled rate and rotating the same to pull a single crystal from the melt; maintaining the crystal at a temperature in excess of 400 C during crystal growth; and rapidly cooling the grown crystal at a rate of not less than 2 C min-'.

8. A process for producing single crystal material of stoichiometric composition given by the formula: Li Lu1-x-y-z Erx Tmy Hoz F4; x + y + z < 1 performed substantially as described hereinbefore with reference to the accompanying drawings Figs. 1 and 2.

9. Single crystal material grown by the process as claimed in either claims 7 or 8 above.

10. A laser device incorporating a single crystal rod of stoichiometric composition given by the formula: Li Lul~x~y-z Erx Tmy Ho F4; x + y + z < 1