1. Field of the Invention
The present invention relates to a process for producing an optical fiber preform by depositing fine glass particles around a starting member in the form of a transparent glass rod and, more particularly, to a process capable of producing an optical fiber preform of high quality by ensuring that the nonsteady outside diameter portion at either end of a soot preform is protected against local temperature elevation.
2. Description of the Related Art
One of the conventional processes for producing an optical fiber preform uses a starting member in the form of a transparent glass rod composed of either a core or a core cladding combination, which is installed either horizontally or vertically in a vessel and rotated about its own axis while gaseous glass forming materials are supplied into a burner to synthesize fine glass particles which are deposited on the starting member making reciprocal movement relative to the burner. This is generally referred to as an "Outside Vapor Phase Deposition method" and the resulting soot preform (optical fiber preform) is fused with heat until it vitrifies (turns into a transparent glass) thereby synthesizing a jacket (see, for example, Unexamined Japanese Patent Publication (kokai) No. Hei. 2-172838).
An exemplary apparatus for producing an optical fiber preform by the Outside Vapor Phase Deposition method (OVD method) is shown in FIG. 1. A vessel 1 has a starting member 2 fitted vertically as it is held on a rod 7 such that it is rotatable on its own axis and capable of recical movement in a vertical direction by means of an elevator 5. A glass synthesizing burner 3 is fitted on a lateral side of the vessel 1 at right angles to the rotating axis of the starting member 2 such that fine glass particles synthesized with the starting materials being supplied from a feed supply unit 8 are deposited on the starting member 2. The glass synthesizing burner 3 is adapted to be movable towards or away from the starting member 2 by means of a burner moving unit 4. An exhaust port 6 is provided on the lateral side of the vessel 1 which is remote from the glass synthesizing burner 3 and an exhaust containing any excess of the fine glass particles being synthesized is discharged through the port.
When an optical fiber preform is produced with the apparatus shown in FIG. 1, the middle portion of a soot preform 9 in FIG. 2 which is being formed around the starting member 2 is a steady outside diameter portion 10 having a constant outside diameter. However, a nonsteady outside diameter portion 11 having a varying outside diameter forms at both the upper and lower ends of the deposit. Since the burner makes a round trip in each of these end portions and both ends are smaller in outside diameter, hence, heat capacity, the temperature of the soot preform may rise locally if the heating power of the burner is constant. In addition, the supply of the starting materials for the synthesis of the fine glass particles is either reduced or stopped in the nonsteady outside diameter portion and, hence, given constant flow rates of hydrogen and oxygen, the surface temperature of the deposit will increase further. If a plurality of burners are used to synthesize the fine glass particles (soot), the supply of the starting materials is stopped in such an order that the supply from the outermost burner is stopped first and the supply from the second outermost burner is stopped thereafter in order to shorten the length of the nonsteady outside diameter portion at either end of the soot preform. However, this is another cause of the high likelihood for local temperature elevation. For example, when the target is moving at a constant speed; even if the burners are operated such that the temperature of the steady outside diameter portion is about 900.degree. C., the temperature at either end of the soot preform may sometimes rise up to about 1,100.degree. C. and, as a result, the glass deposit will have a bulk density gradient that introduces irregularities in hardness and the thermal expansion mismatch between the transparent glass rod and the soot preform will cause cracking in the soot. In addition, temperatures higher than 1,100.degree. C. will cause vitrification.
A method is also known for producing rare earth incorporating quartz such as a quartz rod having a rare earth element added thereto; according to the method, a soot of fine glass particles is deposited on a rod of starting member to prepare a soot boule by a technique such as a VAD or OVD method and the soot boule is impregnated with an alcoholic solution of a rare earth metal chloride by immersion and thereafter dried and heated to vitrify thereby forming a rare earth incorporating quartz rod. A problem with this method is that the soot boule formed by a conventional technique such as a VAD process is apt to have a great difference in bulk density between the central and surface areas. If such a soot boule is impregnated with an alcoholic solution of a rare earth metal chloride, the content of the rare earth element will vary between the central and surface areas, making it difficult to produce a quartz rod having a uniform radial profile of rare earth element. In order to solve this problem, it has been proposed that a soot boule having smaller radial changes in bulk density should be produced by depositing a soot on a rod of starting member by an OVD method in such a way that the temperature of the peripheral surface of the soot is held constant (see Unexamined Japanese Patent Publication (Kokai) No. 4-367536).
As the outside diameter of the soot boule increases, its surface area and volume increase accordingly and a decreasing proportion of the soot boule is contacted by the flame of the burner. At the same time, the heat capacity of the soot boule per unit volume and, hence, its surface temperature will decrease. The process described in the preceding paragraph is primarily intended to deal with this situation and as the surface temperature of the soot boule decreases, the supply of hydrogen and oxygen into the burner is increased so that it will produce a greater amount of flame to maintain the surface temperature of the soot boule constant. In other words, the process under consideration is intended to maintain a constant temperature on the surface of the soot boule but not for preventing the above-described problem of local temperature elevation of the nonsteady outside diameter portion at either end of the soot preform.
A similar process for the production of an optical fiber preform has been proposed and it comprises basically the steps of depositing a soot of glass forming materials on a starting member by an OVD method and heating it to vitrify, with the additional step being included for controlling the bulk density of the soot being deposited by adjusting the distance from the soot deposition area to the burner and the temperature of that deposition area on the basis of the results of measurement of these factors (see Examined Japanese Patent Publication (kokoku) No. 4-13299). In the process of soot growth, the temperature of the soot deposition area fluctuates on account of the burner coming close to the soot and because of changes in such factors as the heat capacity of the soot and the efficiency of its cooling and as a result of this temperature fluctuation, the bulk density of the soot fluctuates in a radial direction, eventually leading to such problems as cracking in the soot and the fluctuation in the amount of dopants during sintering. The process under consideration is primarily intended to prevent these phenomena but not for preventing the local temperature elevation of the nonsteady outside diameter portion at either end of the soot preform.
A technology is also available for controlling the temperature near both ends of the soot preform and it is an apparatus for controlling a cladding forming burner comprising a gas source for supplying cladding forming gases, the cladding forming burner which burns the cladding forming gases to produce a flame that is thrown onto a porous core on which fine glass particles have been deposited, a flow rate regulating valve provided between the gas source and the cladding forming burner, a distance meter for measuring the distance over which the distal end of said core moves and a flow rate adjusting means by which the opening of said flow rate regulating valve is controlled to increase at a specified rate such that an empirically predetermined distance of movement is reached on the basis of an output value from said distance meter, characterized in that the porous cladding is formed as the temperature of the flame being thrown from said cladding forming burner is made lower at the distal end of the core than any other sites such that the refractive index profile created upon forming the cladding is generally uniform at any site in an axial direction (see Japanese Patent No. 2592359). This technology is primarily intended to deal with the abnormal index profile that occurs on account of the temperature fluctuations at the distal end of the core; in this process, fine glass particles are formed on the porous core, so unlike in the aforementioned case of depositing fine glass particles on the transparent glass rod, there will occur no problems due to thermal expansion mismatch such as cracking in the soot and irregularities in hardness.