Patent Application
20050069638
The invention relates to a method in the production of a doped glass material, particularly a glass material to be used in light amplification optical waveguides. The invention also relates to a system in the production of a doped glass material, particularly a glass material to be used in light amplification optical waveguides.
An important use of doped glass materials is light amplification waveguides, for example active optical fibres, whose light amplifying properties are based on utilizing stimulated emission. In order to make stimulated emission possible, the glass material in the core of the active optical fiber, and possibly also in the cladding layer surrounding the core, is doped with dopants, which are rare earth metals, for example erbium. In addition to optical fibers, the doped glass materials can also be used in different kinds of optical planar waveguides.
The active optical fibers are produced by drawing glass into optical fiber from a fiber preform, which fiber preform can be made in several different ways. A commonly used method for producing a fiber preform is to deposit glass material around a mandrel, or a corresponding substrate arranged to rotate, by flame hydrolysis deposition, FHD. When the above-mentioned deposition is performed from the outer periphery of the fiber preform, the so-called OVD method (outer vapour deposition) is the term often used in this context. The FHD method is also applied in forming glass layers required in optical planar waveguides on a planar substrate.
In the FHD method, a hydrogen-oxygen flame is typically used as a thermal reactor, and the glass forming basic materials used in the production of the glass material, for example silicon or germanium tetrachloride, are typically delivered in a vaporous form to the burner and the flame. The dopants of the glass material, such as, for example, erbium, are carried to the burner and the flame along with a carrier gas, typically in the form of a vapour or aerosol droplets formed of a liquid containing the dopants by vaporizing or by spraying, respectively.
In the flame used as a thermal reactor in the FHD or liquid flame spraying method, the basic materials and dopants further form aerosol particles, which aerosol particles are guided onto the substrate to be coated, thus forming-a doped porous glass material coating. In prior art references in the English language, these aerosol particles are often referred to as “glass soot”. When a suitable coating layer of porous glass material has been deposited on the mandrel or other substrate, the above-mentioned coating layer is sintered to form dense glass by a heat treatment of the substrate at a suitable high temperature. A so-called solution doping method is also known, in which method a fiber preform deposited of mere basic materials is dipped into a solution containing dopants first after depositing the fiber preform, before sintering.
Rare earth metals dissolve poorly into quartz glass and require that, for example, the structure of SiO2 based glass has been modified by admixing a suitable oxide with the glass. Oxides suitable for the purpose include, for example, Al2O3, La2O3, Yb2O3 or P2O5. Preferably, this oxide is aluminium oxide Al2O3, which at the same time increases the refractive index of the glass.
When the core of an optical fiber (or another waveguide) is doped with a rare earth metal, the aluminium oxide simultaneously provides an increase in the refractive index of the core in relation to the cladding layer, which is necessary for satisfying the operating principle of the optical fiber.
The capacity of liquids to discharge vapour into ambient air is represented by the vapour pressure of the substance, wherein the unit used is atm, kPa or mmHg. A liquid with a high vapour pressure is easily evaporated, and the vapour pressure of the substance will increase the higher, the warmer it is. Consequently, in a closed vessel and in a balanced state, saturated vapour will be formed above the liquid level, and the vapour concentration can be found out, for example, by calculating the concentration in a balanced situation. This concentration is dependent on the vapour pressure of the substance, wherein the concentration increases as the temperature rises. The concentration of the substance in vapour form in the air is normally given in parts per million (unit ppm). The composition of the carrier gas is thus changed in a way determined by the vapour pressure, which is known as such.
If the vapour is led from a heated vessel into a space or a pipe system which is cooler than the temperature of the vessel, the vapour will start to condense to a liquid, because the temperature is lower than the temperature of the saturated vapour. The condensation of the vapour into a liquid is not desirable in view of the management of the processes, because it has a direct effect on the vapour concentration and thereby on the mass flow of the substance conveyed with the carrier gas, and these parameters, in turn, are essential in view of the operation of the reactor. Problems occur particularly in cases in which separate flows of reactants or dopants are mixed, wherein the pipework in use is complex and simultaneously the temperature control becomes complicated.
One device of prior art, disclosed in U.S. Pat. No. 4,826,288, comprises several sources of a vapourous dopant. The means, in which the vapour is generated, are supplied with a carrier gas, and the outlets from all the sources are coupled together and led into a reactor. The temperature of each source as well as the temperature of the connecting pipes are controlled by a heating system according to prior art. The evaporating means, i.e. containers, are separate, each having a separate carrier gas inlet and outlet. For this reason, the control of the carrier gas and the dopants further requires a material feeding system, which is difficult to control, and a large number of valves, which must also be sufficiently heat resistant.
It is the main aim of the present invention to present a completely new system for mixing basic materials and reactants, whereby the above-described problems present in the processes of prior art are avoided.
The basic principle of the invention is particularly to collect dopants into the same carrier gas flow, wherein the evaporating means are coupled in series. One basic principle is also that the dopants are collected in the order of increasing temperature requirement. The temperature requirement, in turn, will be determined by the vapour pressure and the desired content of the dopant.
Because the vapour generating containers are arranged in such a way that the vapour is always transferred into a space or into a container warmer than the starting point, it is possible to avoid the condensation of the vapour into liquid onto the inner surfaces of the containers or conduits. At the same time, one avoids a change in the composition of the carrier gas, caused by a decrease in the temperature and the vapour pressure. The pipes connecting the containers are also heated, but their temperatures are selected to be suitable so that the temperature is higher than that in the preceding container and lower than (or equal to) the temperature in the next container. Thanks to the invention, the material feeding system becomes considerably simpler, the condensation is avoided, and the control of temperatures becomes easier. The composition of the carrier gas is now controlled primarily by controlling the temperature and not by using valves.
In the following, the invention and some of its advantageous embodiments will be described in more detail with reference to the appended drawings, in which
All the reactants required for the production of doped glass material according to the invention, as well as the basicmaterials (for example Si or Ge compounds) and dopants (for example Al compounds and compounds of rare earth metals) are first brought to a vaporous form, i.e. the gas phase, by suitably raising the temperature of said materials and by selecting a suitable chemical composition for each reactant. Technically, the heating of the containers of the reactants can be implemented by methods known as such. For the glass material, for example silicon tetrachloride SiCl4 is used as the basic material and aluminium and erbium as the dopants, the latter ones in the form of either nitrates or chlorides. The compounds used as the sources of aluminium and erbium are, for example, dissolved in suitable liquids and evaporated further to a gas phase by heating the solutions. Suitable carrier gases are utilized for carrying the reactants brought to the gas phase.
The basic materials and the dopants, which are in a gaseous and reduced form and are mixed with each other, are then guided as gas flows into a reactor, and the temperature is simultaneously kept at such a level that the basic materials and the dopants remain in their vapour form.
According to prior art, the basic materials and the dopants are kept separate from each other, and their mutual ratio can be adjusted, if necessary, by changing the mutual ratio between the gas flows by means of, for example, control valves, such as mass flow controllers, or in another suitable way. In the present invention, at least a part of the basic materials and the dopants are mixed into the same gas flow, but it is also possible to mix other dopants according to prior art, for example by means of control valves. The carrier gas can also be formed by combining separate gas flows. Gases with the function of reaction control are also supplied in separate flows via separate conduits into the reactor.
In the embodiment of
Each conduit and container comprises a separate controlled heating system which is controlled, for example, centrally by a control system. The operation of the system is normally also provided with temperature sensors to provide information about the temperature. Moreover, the control of the supply of the carrier gas can also be provided with a control valve and the necessary sensor means for receiving information about the carrier gas flow. In the system of the invention, it is possible to apply measuring and sensor systems known as such.
From the conduit 4, the gas mixture 12 is led into a container 3 of a dopant 11 which is, in this case, erbium chloride ErCl3. The composition of the carrier gas, i.e. the gas mixture 12, is changed again in a way determined by the vapour pressure of the dopant 11, resulting in a gas mixture 13. From the container 3, the gas mixture 13 is led into a conduit 5. The conduit 5, in turn, is heated to a temperature T5 which is higher than T3 to avoid the condensation of the vaporous gas mixture 13 onto the inner surface of the conduit 5. The temperature T3, in turn, is higher than T1, to avoid the condensation of the vaporous gas mixture 12 inside the container 3 and the change in the composition of the carrier gas 12 with respect to erbium chloride ErCl3.
Along the conduit 5, the gas mixture which forms the gas flow 13 of reactants to be supplied into the reactor 6 is, in turn, led into an oven-like reactor 6 in which is it processed in a way known as such. If necessary, along separate conduits, the reactor 6 is also supplied with oxygen O2, an inert gas, such as nitrogen N2, and hydrogen H2, whose use will depend on the thermal reactor and on the method, and whose purpose is to control the reactions. The reactor 6, in turn, is heated e.g. by means of an induction coil 7 to a temperature T6 which is higher than T5 and which is preferably also higher than the temperature required by the reactor process. The carrier gas, the dopants and the auxiliary gases react in the reactor 6 in a way known as such, to produce a fibre preform.
In the reactor, the hot and mixed gases/vapours in reduced form in the gas flow are oxidized and condensed to oxides to form the glass material. The method of oxidation will depend on the intended final result. In particular when homogeneity is aimed at, the oxidation/condensation is performed at such a temperature and under such gas conditions that all the reactants achieve a state of multiple supersaturation (oven temperature from 1000 to 2000° C.). As a result, the quick condensation of all the constituents produces droplets and immediately further glass particles with a homogeneous reciprocal and internal composition. The quick condensation is triggered, for example, by quick oxidation of the reactants and/or by quick adiabatic expansion of the gas flow of the reactants. The quick oxidation, in turn, is achieved by strong jets of the oxidizing gas (O2).
When producing doped glass materials, the basic materials used can also be chlorine-free reactants, such as TEOS (tetraethylortosilicate) or GEOS (tetraethoxygermanium) in a suitable form. In addition to the dopants mentioned above, it is also possible to use other rare earth metals and lantanides, such as, for example, neodymium, and further also phosphorus, borium and/or fluorine.
We shall now discuss in more detail one embodiment of the invention in which the reactor 6 is an OVD burner. The OVD burner is shown in a reduced cross-sectional view in
The burner 6 is supplied with oxygen O2 for the combustion and a fuel gas, for example nitrogen H2, via gas inlets 22 and 24, respectively. An inert gas, for example nitrogen N2, is supplied via a gas inlet 23 to prevent the mixing of the fuel gas and the oxygen O2 on the surface of the burner 6. The fuel gas and the oxygen O2 react outside the burner 6, and the mixture is ignited by, for example, and electric spark. The reactants supplied from the conduit 18 react in a flame and form glass particles which can be further collected, for example by thermophoresis, onto the surface of the first mandrel used for the manufacture of the fibre preform.
In one embodiment, the reactor 6 also comprises two quartz glass tubes forming the conduit 18 and the shield 20 shown in
The invention is not limited solely to the above-presented embodiments, but it can be varied within the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20031398 | Sep 2003 | FI | national |
