Processes for enhancing fluorescence of tunable titanium-doped oxide laser crystals

Abstract: The Figure of Merit of titanium-doped oxide crystals suitable for use as a tunable laser is enhanced by treating the article at a temperature of about 50°C to 80°C below the melting point of the crystal in an atmosphere containing at least about 20 volume percent hydrogen.

Description: The present invention relates generally to the field of lasers i.e. to a process for improving the lasing efficiency of tunable titanium-doped oxide laser crystals. As used herein, the term "titanium-doped laser crystal" is intended to include oxide monocrystalline materials, that is, an oxide material comprised of a single crystal or two or more single crystals grown together longitudinally but separated by a relatively small angle (usually within 4°, preferably within 1°, determined with respect to the axes of neighboring single crystals) grain boundary having a minor amount of titanium incorporated into the crystalline framework to provide for lasing. Without wishing to be bound by theory, in order to provide lasing capability to the crystalline material, the titanium must be present in the Ti⁺³ oxidation state with an outer electronic shell configuration of 3D¹, i.e., the 3d electronic shell contains one unpaired electron. The most stable natural configuration of the titanium ion is, however, Ti⁺⁴ with an electronic configuration of 3d⁰ (i.e., no unpaired electron in the d shell). Accordingly, an equilibrium exists between the titanium ions in the +3 and +4 states during the formation of the laser crystal. The ratio between the Ti⁺³ and Ti⁺⁴ ions is dependent on the electron affinity of the other constituents present in the crystal forming mixture. If the electron affinity of other constituents is high, the portion of Ti⁺³ ions in the mixture will remain high. Conversely, if any of the other crystal forming constituents have low electron affinity, the amount of Ti⁺³ ions available will be low. Generally, there are always Ti⁺⁴ ions present in the molten mixture of constituents during the growth of the titanium-doped crystals which are caused by the presence of oxygen or reactive oxygen containing species. These Ti⁺⁴ ions are incorporated into the growing crystals in conjunction with the Ti⁺³ ions. Still not wishing to be bound by theory, when the laser crystal containing the Ti⁺³ ions is subjected to a pumping light, the sole electron in the 3d electronic shell advantageously absorbs particular wavelengths of this pumping light such that it becomes energized and is shifted to an outer electronic shell, such as the 4f shell. When the electron eventually becomes deenergized, it returns to its original 3d¹ electronic shell configuration with, however, a concomitant release of light energy. By the use of mirrors at the ends of the laser crystals (a technique which is well known to those skilled in the art), it is possible to control and amplify this light emitting phenomenon so as to produce the desired laser light. However, concurrently with this desirable absorption of light energy which activates the single electron from the 3d¹ shell to the higher electronic shells, there is also an unwanted simultaneous absorption of the laser light energy produced by the deactivation of the energized electron from the outer electronic shell to its starting 3d¹ position. Still not wishing to be bound by theory, this undesirable absorption of laser light is believed to be caused by the Ti⁺⁴ ions which are believed to be in the form of Ti⁺³:Ti⁺⁴ couples. These couples cause an optical absorption band at longer wavelengths than the absorption band of the Ti⁺³ ions which activate the 3d¹ electrons. Because the absorption band caused by the presence of Ti⁺⁴ in the crystal is of longer wavelength than that of the Ti⁺³ ions, it frequently overlaps the lasing wavelength of the Ti⁺³ ion and therefore manifests itself as an optical loss which decreases the energy of the lasing beam. In numerous cases, depending upon the titanium concentration, optical loses reach levels that effectively prevent the crystals from lasing at all. One useful parameter in reporting the lasing efficiency of the titanium-doped laser is referred to as the "Figure of Merit". The Figure of Merit is the ratio of the transmission (in percent per centimeter) of a crystal at the wavelength at which the unwanted absorption of the laser light occurs relative to the transmission (in percent per centimeter) of the crystal at a wavelength in which the desirable absorption occurs in which the Ti⁺³ ion is activated. Typically, the Figure of Merit for the titanium-doped sapphire crystal is 30 to 50. For other oxide monocrystalline materials, their Figure of Merit is typically in the range of from 1 to 30. The higher the Figure of Merit, the more lasing capability the crystal has. Tunable solid state laser materials have been known to the art since the early 1960's and Ti:Al₂O₃, for example, was disclosed by P. F. Moulton (Laser Focus, May 1983) as a tunable laser material having an effective fluorescence tuning range of 750 to 900 nm. The absorption spectra range for Ti:Al₂O₃ has been given as extending up to about 650 nm; however, it has been discovered that unless special precautions are taken in the course of processing titanium-doped sapphire, Ti:Al₂O₃, the absorption spectra, while reaching a minimum value at about 650 nm, extends over the entire lasing (fluorescence) range with the undesirable result that lasing efficiency of the tunable Ti:Al₂O₃ material is significantly reduced. Moreover, only relatively small amounts of titanium have been found useful in preparing a crystalline material suitable for use as laser crystals. Even at concentrations of titanium of one percent in the melt, Ti:Al₂O₃ crystals, for example, prepared by conventional techniques have an absorption of light in the range of 650 to 1100 nm of over 12 percent per centimeter. Often the undesirable absorption peaks at about 850 nm, in the mid-range of the tunable band. Accordingly, it is desired to improve the lasing efficiency of tunable titanium-doped crystal lasing materials. One approach to enhancing the lasing efficiency of tunable titanium-doped sapphire lasing material is disclosed in my US - A - 4,711,696, incorporated herein by reference. The disclosed process involves enhancing the fluoresence of titanium-doped sapphire crystal by the process conditions used in the manufacture of the crystal. In general, the process conditions involve providing precursor mixtures for the crystal under a carbon monoxide-containing atmosphere which is being heated toward melt temperatures. Advantageously, the carbon monoxide is present before the mixture is liquified. Often, the carbon monoxide atmosphere is replaced by a hydrogen-containing atmosphere at the melt temperature and during cooling. Carbon monoxide can degrade at the temperature of the melt, i.e., above 1600°C to 2050°C and higher. The foregoing steps are conducted in a non-reactive ambient atmosphere, e.g., nitrogen, argon or other inert gases, with the reducing gas being provided in the ambient gas atmosphere. The non-reactive atmosphere preferably contains 5 ppm to 1% by volume hydrogen; however greater concentrations of hydrogen may be used, e.g., 5 ppm to 20 volume percent but are usually avoided in view of the safety concerns. The crystals prepared by the aforementioned technique do have enhanced and desirable fluorescent characteristics. The processes described in my patent application have yielded titanium-doped sapphire crystals having Figures of Merit of 100 or more, and crystals having Figures of Merit of 60 to 80 are facilely produced. While the crystals produced using the aforementioned process are a considerable improvement over those previously available, a need has been evidenced for titanium-doped laser crystals having even greater Figures of Merit, e.g., 120 and more. The US-A- 4,587,035 discloses a vacuum annealing treatment of titanium-doped laser crystals to enhance lasing efficiency. The DE-A-2 030 345 discloses a method for enhancing the lasing efficiency of doped lasing monocrystals in particular doped YAG crystals by subjecting the crystals to either a reducing or an oxidising atmosphere in a certain temperature regime. In some instances even inert atmospheres shall be employed. Family member GB-A-1 314 854 teaches that YAG crystals will exhibit improved light emissive properties upon heating the monocrystal in an oxidising atmosphere under certain specified conditions. This invention is directed to processes for enhancing the Figure of Merit of titanium-doped oxide laser crystals, such that their Figures of Merit increase by at least 10 % typically at least 50 % and more typically by at least 100 % or more. The problem underlying the present invention is to provide a method to enhance the Figure of Merit of a Ti-doped oxide crystal suitable as a laser material, whereby the Figure of Merit is the ratio of the transmission (in percent per centimeter) of a crystal at the wavelength at which the unwanted absorption of the laser light occurs relative to the transmission (in percent per centimeter) of the crystal at a wavelength in which the desirable absorption occurs in which the Ti⁺³ ion is activated. This problem is solved by the method as described in claim 1. Further embodiments of the invention are described in dependent claims 2 to 7. Claim 8 concerns a tuneable titanium-doped oxide laser crystal obtainable by the processes of the invention. In accordance with the processes of the invention, a titanium-doped laser crystal as defined in claim 1 is maintained at a temperature of at least 1750°C to a temperature below which the crystal looses its crystallinity, e.g., a temperature of 50° to 80°C below the melting point of the particular crystal, in an atmosphere containing sufficient hydrogen for a time sufficient to increase the Figure of Merit of the crystal. Suitable titanium-doped oxide host crystals include titanium-doped spinel (Ti:MgAlO₄); titanium-doped yttrium aluminate (Ti:YAlO₃); titanium-doped yttrium garnate (Ti:Y₃Al₅O₁₂); titanium-doped lanthanum magnesium aluminate (Ti:LaMgAl₁₁O₁₉); titanium-doped gadolinium scandate (Ti:GdScO₃); titanium-doped scandium borate (Ti:ScBO₃). When the crystal is treated by the process of the present invention, i.e., heat treated in an inert atmosphere containing hydrogen, the undesirable (parasitic) long wavelength absorption is essentially eliminated producing a titanium-doped lasing oxide crystal with substantially improved Figures of Merit. The increase in the Figure of Merit for a titanium-doped oxide lasing crystal by the present invention is believed to be caused by reducing the amount of Ti⁺⁴ ions in the crystal to Ti⁺³ ions as well as by breaking the Ti⁺³ :Ti⁺⁴ couples that are present. The enhanced crystals provided by the present invention are particularly useful as tunable laser materials having desirable fluorescent characteristics. For Ti:MgAlO₄, Ti:ScBO₃ and Ti:GdScO₃, for example, their Figures of Merit may increase from 1 to 10 to at least 100 or 150, or more. For Ti:YAlO₃ and Ti:Gd₃Sc₂AlO₁₂, for example, their Figures of Merit generally increase from 10 to 30 to at least 100 to 200. Other crystals, after being treated by the method of the present invention also exhibit similar remarkable improvements in their Figures of Merit. A titanium-doped oxide crystal is frequently made from a melt containing from 0.02 to 2.0, often 0.02 to 1.0 atomic percent of titanium. The melt is prepared by heating a mixture of high purity TiO₂ (less than 50 ppm of Cr, Fe, Si, Ca) and high purity laser host material containing less than 100 ppm of impurities such as Si, Cr, Fe and Mg, at an appropriate melt temperature for that specific host crystal material A titanium-doped lasing crystal can be formed from the melt, e.g., by the well known Czochralski technique and the crystal is cooled to room temperature. In accordance with the process disclosed in my earlier patent application, the crystal can be formed in the presence of a non-reactive ambient atmosphere containing, e.g., 5 ppm to 1 percent by volume of reducing gas. The reducing gas can be hydrogen; the carbon monoxide used can also be used in accordance with my earlier invention as the reducing gas at temperatures between 800°C and 1600°C, being replaced by hydrogen for the higher temperature for crystal formation, e.g, 2050°C. The replacement of carbon monoxide at these high temperatures is required to avoid carbon contamination due to decomposition of carbon monoxide. The processes of this invention may be employed to treat the titanium-doped lasing oxide crystals as defined in claim 1 in any convenient form. For instance, the crystal boule as made from the melt may be treated in accordance with this invention as well as any crystal article obtained therefrom such as the crystal in the form of laser rods. Often, the crystal has as its smallest mean cross-sectional thickness of less than 10 cm, preferably less than 5 cm. Larger crystals can be used but the treatment may have to be conducted for longer periods of time. The crystal article to be treated is heated in a hydrogen-containing atmosphere to a temperature of at least 1750°C. Although lower temperatures can be used, the rate that the Figure of Merit of the crystal is improved is generally too slow to be commercially feasible. The temperature of the crystal article should not be heated to temperatures at which it begins to become noticeably plastic. Once a melt occurs, the generation of oxygen or oxygen-containing species that adversely affect the titanium oxidation state may result. The respective melting points of the crystals will dictate the maximum allowable temperature to which they may be heat treated. Subjecting the titanium-doped oxide crystal to as close as the maximum allowable temperature will generally reduce the time required to provide a given improvement in the Figure of Merit. Generally, the maximum temperature will be 50° to 80°C below the melting point of the particular crystal, and preferably about 50°C below the melting point. The rate of heating and cooling of the crystal article should be sufficiently slow to avoid inducing undue stress in the article. The rate will therefore depend upon the size and configuration of the article as well as the thermal shock resistance of the particular crystal material. Usually, the heating and cooling rates are less than 50°C change per hour. The amount of hydrogen present in the atmosphere will have an effect on the rate that the Figure of Merit improves. Generally, the hydrogen is present in an amount of at least 20 percent up to 100 percent by volume of the atmosphere. Most preferably, the hydrogen is present in at least 40, e.g., 50 to 100, preferably 75 to 100, volume percent of the atmosphere. The atmosphere may contain inert components such as nitrogen or argon. The treatment may be conducted at any convenient pressure from subatmospheric to superatmospheric pressure, e.g., 0.001 to 1000 bar. Preferably, the treatment is at a low superatmospheric pressure due to safety considerations, although high total pressures will favor advantageously higher partial pressures of hydrogen. The pressures may often be from 0.01 to 10 bar gauge. The concentration of hydrogen and the partial pressure of hydrogen will affect the rate of improvement of the Figure of Merit. In general, the higher the hydrogen concentration, the faster the rate of improvement. At hydrogen concentrations below about 40 volume percent, the rate of improvement can be unduly slow for commercial operations. The time of the treatment will depend upon the desired Figure of Merit sought for the crystal as well as the temperature and hydrogen concentrations used. Thus, the treatment may be for as little as one hour to weeks or more. Preferably, the time is sufficient to increase the Figure of Merit by at least 10 percent, and most preferably, the crystal, after treatment, has a Figure of Merit of at least 100, say, at least 120, and most preferably, at least 150 or 200. At Figures of Merit above 1000, analytical imprecision can often limit any accuracy in determining the Figure of Merit. The processes of this invention can be used with any titanium-doped oxide crystal having any initial Figure of merit value. Usually, the crystals have an initial Figure of Merit of 1 to 50, typically 1 to 10, prior to treatment. The processes of this invention provide a convenient means for upgrading the fluorescing efficiency of a titanium-doped oxide crystal. Particularly meritorious features of the processes are its efficacy and the ability to upgrade existing titanium-doped oxide articles. The following examples are provided to further illustrate the invention. EXAMPLE Various titanium doped crystals in the form of cylindrical rods are treated in an annealing furnace. The Figure of Merit of each of the crystals is determined before and after the treatment. Table I is directed to crystals according to the invention. Table I sets forth the treatment conditions and results for the crystals according to the invention. For the examples shown in Table I, the Ti⁺³ content in the melt is about 1% atomic weight. However, due to differences in the Distribution Coefficient for the respective crystals, the amount of Ti⁺³ that ultimately ends up in the crystal is generally 5 to 10 times lower than the amount present in the melt, i.e., in a range of from 0.1% to 0.2% atomic weight. The crystals exhibit lower laser thresholds after treatment.

Claims: A method to enhance the Figure of Merit of a Ti doped oxide crystal, selected from the group consisting of Ti:MgAlO₄, Ti:YA10₃, Ti:Y₃Al₅0₁₂, Ti:LaMgAl₁₁0₁₉, Ti:GdSc0₃, Ti:ScB0₃, and Ti: Gd₃Sc₂Al₃ 0₁₂, suitable as a laser material wherein the Figure of Merit is the ratio of the transmission of a crystal at the wavelength at which unwanted absorption of the laser light occurs relative to the transmission of the crystal at a wavelength in which the desirable absorption occurs in which the Ti⁺³ ion is activated, comprising maintaining the crystal at a temperature of 50° C to 80° C below the melting point of the crystal in an inert atmosphere containing sufficient hydrogen for a sufficient time. The method of claim 1, wherein hydrogen is present in the atmosphere in an amount of at least 20 volume percent. The method of any one of the preceding claims, wherein hydrogen is present in the atmosphere in an amount of at least 40 volume percent. The method of any one of the preceding claims, wherein the crystal is maintained at a temperature of at least 1750° C. The method of any one of the preceding claims, wherein the crystal contains from 0.02 to 1.0 atomic percent titanium. The method of any one of the preceding claims, wherein the Figure of Merit of the crystal is improved by at least 10 percent. The method of any one of the preceding claims, wherein the Figure of Merit of the crystal is enhanced to at least 100. A tunable titanium-doped oxide laser crystal selected from the group consisting of Ti:mgAlO₄, Ti:YAlO₃, Ti:Y₃Al₅O₁₂, Ti:LaMgAl₁₁O₁₉, Ti:GdScO₃, Ti:ScBO₃ and Ti:Gd₃Sc₂Al₃O₁₂, obtainable by a process as claimed in any one of the preceding claims having from 0.02 to 1.0 atomic percent of titanium and a Figure of Merit of at least 100.

Inventors: KOKTA MILAN RASTISLAV, KOKTA, MILAN RASTISLAV
Applicants: UNION CARBIDE CHEM PLASTIC, UNION CARBIDE CHEMICALS AND PLASTICS COMPANY INC. (A NEW YORK CORPORATION)
Application year: 1989
Main classification: H01S3
IPC codes: C09K11, C30B33, H01S3