RARE EARTH METAL-DOPED QUARTZ GLASS AND METHOD FOR PRODUCING THE SAME

Abstract

A method for producing rare earth metal-doped quartz glass includes the steps of (a) providing a blank of the rare earth metal-doped quartz glass, and (b) homogenizing the blank by softening the blank zone by zone in a heating zone and by twisting the softened zone along a rotation axis. Some rare earth metals, however, show a discoloration of the quartz glass, which hints at an unforeseeable and undesired change in the chemical composition or possibly at an inhomogeneous distribution of the dopants. To avoid this drawback and to provide a modified method which ensures the production of rare earth metal-doped quartz glass with reproducible properties, during homogenization according to method step (b), the blank is softened under the action of an oxidizingly acting or a neutral plasma.

Claims

1. Method for producing rare earth metal-doped quartz glass, the method comprising the steps of: (a) providing a blank of the rare earth metal-doped quartz glass; and (b) homogenizing the blank by softening the blank zone by zone in a heating zone and by twisting the softened zone along a rotation axis, wherein during homogenization according to method step (b), the blank is softened under the action of an oxidizingly acting or neutral plasma.

2. Method according to claim 1, wherein an oxygen-containing gas is supplied to the plasma.

3. Method according to claim 1, wherein a microwave atmospheric pressure plasma or an inductively coupled plasma is generated.

4. Method according to claim 1, wherein a plasma gas which is free of hydrogen or hydrogenous compounds is supplied to the plasma.

5. Method according to claim 1, wherein the homogenization of the blank includes two homogenization steps in which the blank is twisted in two directions that are perpendicular to each other.

6. Method according to claim 1, wherein a rare earth metal-doped quartz glass is generated that contains rare earth metal oxide in a concentration of 0.002 to 10 mole % and has a fluctuation in the refractive index δΔn which based on a mean refractive index difference ΔN with respect to undoped quartz glass is less than 10%, and has a bubble content represented by a total bubble cross section (TBCS) value of less than 10.

7. Method according to claim 1, wherein a rare earth metal-doped quartz glass is produced that has a mean chlorine content in the range of 300 to 3000 wt. ppm.

8. Method according to claim 2, wherein a microwave atmospheric pressure plasma or an inductively coupled plasma is generated.

9. Method according to claim 2, wherein a rare earth metal-doped quartz glass is produced that has a mean chlorine content in the range of 300 to 3000 wt. ppm.

10. Method according to claim 3, wherein a rare earth metal-doped quartz glass is produced that has a mean chlorine content in the range of 300 to 3000 wt. ppm.

11. Rare earth metal-doped quartz glass comprising rare-earth metal oxide in a concentration of 0.002 to 10 mole %, wherein the rare earth metal-doped quartz glass has a fluctuation in the refractive index δΔN which, based on a mean refractive index difference ΔN with respect to undoped quartz glass, is less than 10%, and wherein the rare earth metal-doped quartz glass has a bubble content represented by a TBCS value of less than 10.

12. Rare earth metal-doped quartz glass according to claim 11, wherein the quartz glass has a mean chlorine content in the range of 300 to 3000 wt. ppm.

13. Rare earth metal-doped quartz glass according to claim 11, wherein the quartz glass has a mean hydroxyl group content between 0.1 and 100 wt. ppm.

14. Rare earth metal-doped quartz glass according to claim 12, wherein the quartz glass has a mean hydroxyl group content between 0.1 and 100 wt. ppm.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0050] The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.

[0051] In the drawings:

[0052] FIG. 1 shows the thermo-mechanical treatment process for plasma homogenization on the basis of a schematic representation;

[0053] FIG. 2 is a diagram with a refractive index profile of an Yb-AI-F-doped quartz glass sample before and after homogenization;

[0054] FIG. 3 is a photo of a disc-shaped sample on which the two attenuation spectra of FIG. 4 have been measured;

[0055] FIG. 4 is a diagram with attenuation spectra of the partly homogenized sample shown in FIG. 2;

[0056] FIG. 5 is a diagram for comparing the WDX distribution profiles for aluminum oxide before and after plasma homogenization;

[0057] FIG. 6 is a diagram for comparing the WDX distribution profiles for ytterbium oxide before and after plasma homogenization;

[0058] FIG. 7 is a diagram for comparing the WDX distribution profiles for silicon oxide before and after plasma homogenization; and

[0059] FIG. 8 is a diagram for comparing the WDX distribution profiles for chlorine before and after plasma homogenization.

DETAILED DESCRIPTION OF THE INVENTION

[0060] Definitions and Measuring Methods

[0061] Individual method steps and terms of the above description as well as measuring methods will now be defined in a supplementary manner. The definitions are part of the description of the present invention. The statements made in the description are governing in case of an inconsistency in the contents between one of the following definitions and the remaining description.

[0062] Quartz Glass

[0063] Quartz glass means, herein, a glass with a high silicic-acid content and with an SiO.sub.2 proportion of at least 90 mole %.

[0064] Granulates

[0065] A distinction can be made between build-up granulation and press granulation and, in terms of the technical processes, between wet, dry and freeze granulation methods. Known methods are roll granulation in a pan granulator, spray granulation, centrifugal atomization, fluidized-bed granulation, granulation methods using a granulating mill, compaction, roller presses, briquetting, flake production, or extrusion.

[0066] Discrete, rather large agglomerates, herein called “SiO.sub.2 granulate particles” or “granulate particles” for short, are formed during granulation by agglomerations of the SiO.sub.2 primary particles. In their entirety, the SiO.sub.2 granulate particles form a “SiO.sub.2 granulate”.

[0067] Purification

[0068] The granulate or a compact made from the granulate is normally purified prior to sintering. The main impurities are residual water (OH groups), carbonaceous compounds, transition metals, alkali metals and alkaline earth metals that derive from the feed material or are introduced by the processing operation. A low impurity content can already be achieved by using pure feed materials and corresponding equipment and processing under cleanroom conditions. To satisfy even higher demands made on purity, the granulate or the compact may be treated at a high temperature (up to 1200° C.) in a chlorine-containing and oxygen-containing atmosphere. Residual water evaporates in this process, organic materials react to form CO and CO.sub.2, and many metals (such as for instance iron and copper) can be converted into volatile, chlorine-containing compounds.

[0069] Sintering/Densifying and Vitrifying/Melting

[0070] Here, “sintering” or “densifying” designates a process step in which a SiO.sub.2 granulate is treated at an elevated temperature of more than 1100° C. either in a dynamic furnace (for instance in a rotary furnace) or in a static furnace. The specific surface area (BET) is here decreasing, whereas the bulk density and the mean particle size can increase due to agglomerations of granulate particles.

[0071] During “vitrifying” or “melting”, the pre-densified, sintered SiO.sub.2 granulate is vitrified while forming a quartz glass body.

[0072] Vacuum/Negative Pressure

[0073] The gas-pressure sintering process may include a negative pressure phase in which the intermediate product is heated under “vacuum.” The negative pressure is indicated as an absolute gas pressure. Vacuum means an absolute gas pressure of less than 2 mbar.

[0074] Measurement of the Concentration of Hydroxyl Groups (OH Groups)

[0075] The measurement is carried out by way of the method presented by D. M. Dodd and D. B. Fraser, “Optical determination of OH in fused silica,” Journal of Applied Physics, Vol. 37(1966), p. 3911.

[0076] Measurement of Radial Concentration Profiles of Components and Determination of the Mean Value of the Chlorine Concentration

[0077] The measurement of concentration profiles for components of the rare earth metal-doped quartz glass, particularly of the rare earth metals contained therein and of chlorine, is carried out by way of a wavelength-dispersive X-ray fluorescence analysis (XRF) in combination with an electron probe micro analysis (EPMA) on measurement samples over a length of 2 mm at a measurement interval of 0.01 mm. The measurement value at the measurement length =1 mm is positioned as exactly as possible in the center of the measurement sample. The mean value of the chlorine concentration follows as an arithmetic mean of all measured values.

[0078] Measurement of the Fluctuation of the Refractive Index (δΔN)

[0079] The measurement of the refractive index profile is performed by way of a commercial profile analyzer “P104” of the company York Technology Ltd. The usual operating wavelength range of this device is 632.8 nm. The mean refractive index ΔN is determined from the refractive index profile as the refractive index difference with respect to undoped quartz glass. To minimize edge effects in the determination of the maximum refractive-index fluctuation δΔN and the measure for the refractive index fluctuation δΔN/ΔN, the refractive index profile is evaluated over a measurement length of ⅓×r to ½×r, where r=radius of the cylindrical measurement sample (measured from the zero point in the sample center to the outside). The measure for the refractive index fluctuation δΔN/ΔN then follows as a maximum refractive-index difference δΔN over the measurement length, based on the mean refractive-index difference ΔN with respect to undoped quartz glass. The normalization to ΔN takes into account the circumstance that the maximum refractive-index fluctuation δΔN is normally increasing with the mean refractive index ΔN.

[0080] Measurement of the Bubble Content

[0081] The TBCS value (English: Total Bubble Cross Section) designates the total cross-sectional area (in mm.sup.2) of all bubbles within a sample based on a unit volume of 100 cm.sup.3. The value is determined by visual detection of the bubbles and addition of the bubble cross-sections, where bubbles with diameters of less than 0.08 mm are not included.

[0082] Production of a Rod-Shaped Semifinished Product of Doped Quartz Glass

[0083] A slip of discrete, synthetically produced SiO.sub.2 particles with a mean particle size of about 10 μm is prepared in ultrapure water. An amount of 285.7 g of the slip with a residual moisture of 37.4% is diluted with 1000 ml ultrapure water. A pH of 14 is set by adding a concentrated ammonia solution in an amount of 75 ml. The alkaline suspension is homogenized. For the production of a quartz glass doped with Yb.sub.2O.sub.3 and Al.sub.2O.sub.3, an aqueous dopant solution of AlCl.sub.3 and YbCl.sub.3 (mole ratio 4:1) is prepared in parallel in 400 ml ultrapure water. Instead of the chlorides, other start substances can also be used, for instance organic compounds, nitrides or fluorides.

[0084] The suspension, which is moved by stirring, is fed with the dopant solution in the form of an atomized spray for a period of 65 minutes. For the generation of the atomized spray, the dopant solution is atomized by means of a spray nozzle, with a work pressure of 2 bar and a flow rate of 0.8 1/h being set. The atomized spray produced in this way contains drops with a mean diameter between 10 μm and 40 μm. The high pH value of the suspension leads directly to a mixed precipitation of hydroxides of the two dopants in the form of AI(OH).sub.3 and Yb(OH).sub.3. The solid particles formed thereby adsorb on the existing surfaces of the SiO.sub.2 particles and are thereby immobilized, thereby preventing a coagulation of the solid particles or a sedimentation. A dopant concentration of 2 mole % A1 and 0.5 mole % Yb (based on the Si content of the suspension) is thereby set. Subsequently, the slip mixed with the dopants is homogenized by stirring for another 2 hours. This procedure ensures that an optimally homogenously doped SiO.sub.2 slip is obtained.

[0085] The doped SiO.sub.2 slip is frozen and further processed by so-called freeze granulation into a granulate. The granulate sludge obtained after thawing is repeatedly washed with ultrapure water, and the excessive water is respectively decanted.

[0086] Subsequently, the granulate sludge which is freed of ammonia and purified is dried at a temperature of around 400° C. for 6 hours. The dried granulate is welded into a plastic mold and isostatically pressed at 400 bar.

[0087] The granulate compact obtained in this way is heated while being washed with helium and is then treated in a chlorine-containing atmosphere at about 900° C. for about 8 hours. Impurities are thereby removed from the compact and the hydroxyl group content is reduced to about 3 wt. ppm. The chlorine content can be lowered by an aftertreatment in oxygen-containing atmosphere at a high temperature. Low concentrations of hydroxyl groups and chlorine facilitate the bubble-free sintering.

[0088] The purified granulate compact has a cylinder shape with a diameter of 30 mm and a length of 100 mm. Its mean density is about 45% of the density of the doped quartz glass. It is an intermediate product and is molten in a gas-pressure sintering process into a component of the doped, transparent quartz glass.

[0089] The gas-pressure sintering process is carried out in a gas-pressure sintering furnace with an evacuated sinter mold of graphite. The interior of the sinter mold is made cylinder-like and defined by a bottom and a sidewall of annular cross-section.

[0090] The partly densified sintered bodies are vitrified in a graphite mold at a temperature of 1700° C. by gas pressure sintering. The mold is first heated to the sintering temperature of 1700° C. while maintaining a negative pressure. After the sintering temperature has been reached, an overpressure of 15 bar is set in the furnace and the mold is kept at this temperature for about 30 min. During subsequent cooling to room temperature the overpressure is further maintained up to a temperature of 400° C. After cooling down to room temperature the quartz glass block is removed, and rods of a length of 20 cm and a diameter of 15 mm are drilled out.

[0091] Thermo-Mechanical Homogenization by Way of Oxidizingly Acting Plasma

[0092] The rod-shaped semifinished product is then homogenized by thermo-mechanical homogenization (twisting) and formation of a cylinder of rare earth metal-doped quartz glass. This treatment operation is schematically shown in FIG. 1. To this end, two holding rods 3 of undoped quartz glass are welded to the front ends of the rod-shaped semifinished product 1 by way of plasma burners. The holding rods 3 are clamped in the spindles 6, 7 of a glass lathe. The glass lathe is equipped with a plasma burner 2 which is fed with pure oxygen as plasma gas. The plasma burners 2 generate a plasma flame 5 which ignites in atmospheric pressure and has an oxidizing effect with respect to silicon and the rare earth metals. The plasma is excited by microwave excitation with a frequency of 2.45 GHz at a power of 6000 watts.

[0093] The plasma flame 5 is guided along the semifinished product 1 clamped in the glass lathe, and the product is thereby locally heated to more than 2000° C. Disparate rotation speeds (ω1, ω2) of the two glass lathe spindles 9, 10 create a twisting region 9 which is positioned in the heating region of the plasma flame 5. Thorough mixing takes place in this twisting region 9 and thus a homogenization of the glass. The plasma burner 2 is reversingly moved along the semifinished product 1 at a low speed (as outlined by the directional arrows 8) and the rod-shaped semifinished product 1 is twisted zone by zone about its longitudinal axis 10 and the softened glass mass is thereby intensively mixed over the whole length of the semifinished product. A glass cylinder with a diameter of about 15 mm and a length of about 100 mm is thereby obtained.

[0094] The oxidizingly acting microwave oxygen atmospheric pressure plasma 5 reduces the amount of chlorine and reduced species in the rare earth metal-doped quartz glass. This manifests itself by discoloration of the glass.

[0095] The rods of homogenized quartz glass were used as core rods for producing a preform for a laser fiber. To this end a fluorine-doped quartz glass as the cladding glass was built up by way of a plasma coating process on the core rods previously purified by etching in HF solution, thereby producing a laser fiber preform. This preform was subsequently further processed in the fiber drawing tower into a laser fiber. The laser fiber obtained thereby showed laser activity.

[0096] To determine the refractive index fluctuation, disc-shaped measurement samples are cut with a thickness of 10 mm out of the homogenized glass cylinder.

[0097] FIG. 2 shows the typical refractive-index profile of a Yb—Al—F-doped quartz glass sample before (A1) and after homogenization (A2). On the y-axis, the refractive index difference ΔN (×10.sup.−3) is plotted (as difference value with respect to undoped quartz glass) against the radius r (in mm; normalized to the same radius). Before homogenization (refractive index profile A1), distinct refractive-index fluctuations manifest themselves. It is visible that after homogenization (refractive index profile A2), the refractive index fluctuations are considerably reduced. The evaluation of the refractive index profile (4n) over the measurement length of ⅓×r to ½×r (measured from point 0 in the sample center to the outside) yielded a maximum refractive-index difference of 0.43×10.sup.−3, which corresponds to the refractive index fluctuation Δn at the same time. The ratio Δn/Δn which is normalized to ΔN is here 7%. By comparison, the ratio δΔn/Δn before homogenization is 20%. This also manifests itself visually by way of improved transparence and reduced scattering of the homogenized sample.

[0098] The roundings of the refractive index profiles which are respectively observed in the edge region of the samples are due to artifacts of the algorithm with which the profile is calculated. These roundings are not real. The increasing refractive-index fluctuation in the central region is also due to an artifact. In regions where these artifacts are less dominant, even a fluctuation of the reactive index, δΔn, of 0.3×10.sup.−3 (standard deviation 0.1×10.sup.−3) is reached within the short range (here measured from radial position −3 mm to −2 mm) on the basis of the profile measurement in homogenized rods.

[0099] The photo of the measurement sample of FIG. 3 shows that the edge region of the sample is already more strongly homogenized due to the higher shear on the outside than the sample center which is comparatively inhomogeneous and shows an increased yellow coloration. The plasma homogenization was interrupted in this case before the oxidizing effect of the plasma, which is also determined by time-dependent diffusion processes, had also reached the central region of the torsion region.

[0100] The diagram of FIG. 4 shows the spectral absorption curve in the wavelength range of 250 to 3500 nm, measured on a Yb—Al-doped quartz glass sample which is only homogenized in part. The absorption A (in normalized unit) is plotted on the y-axis. The absorption curve B1 is assigned to the center of the measurement sample that is not homogenized yet, and the absorption curve B2 to the homogenized edge region. The basic damping (scatter fraction) in the associated absorption spectrum B2 of the homogenized sample region decreases due to the significant reduction of the scatter centers and the improvement of the material homogeneity in the edge region, wherein the ytterbium concentration does not significantly change between the individual sample regions if the scatter fraction in the spectra is deducted. Due to the plasma homogenization the Yb.sup.3+/Yb.sup.2− balance is thus shifted in favor of Yb.sup.3+. The absorption due to the divalent Yb.sup.2+ ions, which do not contribute to the laser effect, is eliminated; this is visible in that the yellow coloration of the sample disappears in the homogenized edge region, and that the absorption curve B2 of the homogenized sample region decreases in the blue spectral range, whereby transmission is improved.

[0101] Due to the plasma homogenization the Yb.sup.3+/Yb.sup.2+ balance can be shifted in favor of Yb.sup.3+, and it thus acts as an aftertreatment of the glass by heating in oxygen atmosphere, but much more efficiently in that constantly newly generated surface is exposed to the oxidizingly acting atmosphere during twisting. The plasma homogenization according to the invention fulfills a homogenization of the rare earth metal-doped quartz glass at different levels, namely on the one hand a mechanical thorough mixing that leads to an elimination of differences in the composition and a standardization of the refractive index of the glass and on the other hand a chemical treatment that effects a change and standardization of the electrical properties of the glass and also of the proportion of the refractive index that can be influenced by the electron configuration.

[0102] The aforementioned yellow coloration of the glass prior to plasma homogenization is due to the fact that, by comparison with Yb3+, Yb.sup.2+ has additional absorption bands in the blue spectral range. The shift of the Yb.sup.3+/Yb.sup.2+ balance after plasma homogenization can be observed in a purely visual manner by the yellow coloration decreasing in the glass.

[0103] The measurement samples only showed a few recognizable bubbles. The result of a visual evaluation of several typical measurement samples in the form of core rods and preforms is summarized in Table 1:

TABLE-US-00001 TABLE 1 Dimensions [mm] Mean L = Length Bubble bubble diameter Φ = Diameter number [μm] TBCS value Preform L = 845 8 100 8 Φ = 1.1 Core rod L = 1500 25 300 6 Φ = 15 Core rod L = 1500 20 200 3 Φ = 15

[0104] In all of these measurement samples the bubble content is smaller than the one represented by a TBCS value of 10.

[0105] The hydroxyl group content which is preset by the purification of the granulate compact does not increase due to the subsequent treatments. As is evident from FIGS. 5 to 8, the homogenization is however significant with respect to the chemical composition. These diagrams respectively show the distribution profile of specific components of the rare earth metal-doped quartz glass before and after plasma homogenization. The distribution profiles are based on electron probe micro analysis (EPMA), based on wavelength-dispersive X-ray spectroscopy (WDX)). On the y-axis, the respective dopant concentration is plotted (in mole % or in wt. ppm, respectively) over the measurement position (in mm). The measurement length is 2 mm.

[0106] The diagram of FIG. 5 shows the radial concentration distribution profiles of the dopant Al.sub.2O.sub.3 before (curve C1) and after (curve C2) plasma homogenization, and the diagram of FIG. 6 shows the radial distribution profiles of the dopant Yb.sub.2O.sub.3 before (curve D1) and after (curve D2) plasma homogenization. In both cases the profile smoothing after plasma homogenization (curves C2, D2) is significant.

[0107] The diagram of FIG. 7 also shows a distinct smoothing of the radial concentration distribution profile for the main component of the material —SiO.sub.2— after plasma homogenization (curve E2) by comparison with the profile before plasma homogenization (curve E1).

[0108] It is apparent from the diagram of FIG. 8, which shows the radial distribution profiles of chlorine before (curve F1) and after (curve F2) plasma homogenization, that the chlorine concentration is considerably reduced on the one hand by plasma homogenization to about ¼ of the initial value, and that a more homogeneous distribution of the chlorine concentration is obtained on the other hand.

[0109] This effect of the plasma homogenization is above all desirable for the reason that the mean value of the chlorine concentration is set to a value of about 1500 wt. ppm, which turns out to be a suitable compromise in relation to refractive index and UV radiation resistance of the glass and that unnecessary concentration maxima for chlorine are avoided on the other hand, as shown by curve F1 in the sample center. The chlorine concentration values indicated in the figure refer to pure quartz glass; dopants are not considered.

[0110] It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.