TORCH FOR FABRICATING OPTICAL FIBRE PREFORM

Abstract: A torch for fabricating an optical fiber preform which has a plurality of raw gas injection passages (2) at the center of the torch, a plurality of small-diameter combustion support­ ing gas injection passages (5) independent to each other and disposed to surround the raw gas injection passages on the outer periphery of the raw gas injection passages disposed at the centre of the torch, and an annular combustible gas injection passage (3) provided on the periphery of each of the small-diameter combustion supporting gas injection pass­ ages. Thus, the torch can stably manufacture the porous glass preform.

Description: This invention relates to a method of fabricating an optical fiber preform, in particular a porous glass preform using a VAD method or an OVD method. A VAD method or an OVD method, which mixes less impurities and OH group, is employed for fabricating a porous glass preform for an optical fiber, an image fiber, a light guide or a rod lens. In each of the above-mentioned methods, raw gas, combustible gas and combustion supporting gas, or these gases and a sealing gas, are supplied to a torch for fabricating a porous glass preform, producing a soot-state porous glass preform by flame hydrolysis and/or thermal oxidation, the preform being accumulated in a desired shape such as a rod or tube shape. A torch used in these methods has a multiwall tube structure, comprising three or more tubular walls. When the torch is formed, for example, of a quadruple tubular wall structure, the passages from the center to the outermost periphery of the torch comprise a raw gas injection passage (first passage: at the center), a sealing gas injection passage (second passage), a combustible gas injection passage (third passage) and a combustion supporting gas injection passage (fourth passage: the outermost periphery). The raw gas contains SiCl₄ as the main raw gas, and GeCl₄, POCl₃, BCl₃ as doping materials. The combustible gas comprises hydrogen (H₂), methane, propane, butane or a mixture of any two or more of these gases. The combustion supporting gas comprises oxygen (O₂), and the sealing gas comprises Ar and/or another inert gas. The principles of accumulating the optical fiber preform in the VAD and OVD methods are fundamentally the same, but in the VAD method the optical fiber preform is accumulated on the lower end of a vertical target drawn while rotating, and in the OVD method the optical fiber preform is accumulated on the outer periphery of a mandrel rotating in a horizontal position. The porous glass preform thus accumulated and formed by the above methods is dehydrated and transparently vitrified by a following heat treatment to become a transparent preform which contains no air bubble. In the above-mentioned VAD method, the porous glass preform is grown axially. In this case, as the preform grows, it is subjected to its own large weight. Thus, when a long and large porous glass preform is produced, the preform tends to be damaged by its own weight. Therefore, it is necessary to improve the strength of the preform to enhance the accumulating density of the optical fiber preform when fabricating a large-size porous glass preform by the VAD method. In the OVD method, where an optical fiber preform is accumulated on the outer periphery of a mandrel in a horizontal position, no damage occurs to the porous glass preform of the kind observed in the VAD method, but as the optical fiber preform grows, the diameter of the preform increases so that its surface area also gradually increases. Thus, the quantity of heat per unit area/unit time of the flame applied from the torch to the surface of the preform alters, and the quantity of heat at the end of accumulating the optical fiber preform becomes considerably smaller than that initially. The shrink-fitting degree of the porous glass preform becomes insufficient toward the end of the accumulation due to this phenomenon so that there is a difference in the density of the optical fiber preform over the radial direction of the preform between the central portion and the peripheral portion. The density of the porous glass preform is preferably 0,4 to 1.0 g/cm³. If the density of the porous glass preform decreases below this value due to insufficient shrink-fitting degree, a crack occurs in the preform along the longitudinal direction of the preform either whilst growing or subsequently whilst cooling. To eliminate this drawback, the rotating speed of the preform is reduced as the preform grows or the quantity of combustion gas is increased. However, in the former case (reducing the rotating speed of the preform), an uneven surface is produced on the porous glass preform or the preform develops an improper outer diameter. In the latter case (increasing the quantity of combusiton gas), it is an uncertain process to set experimentally the amount by which the gas must be increased and it is very difficult to gradually increase the combustible gas to avoid an uneven density in the optical fiber preform so as to prevent the preform from cracking, considering that the flame generated from the torch is of a converging shape. As described above, the method of fabricating a porous glass preform using a conventional torch of multiwall tube structure can hardly provide a large-size preform having uniform density without crack or improper outer diameter. EP-A-0146659 discloses a method of fabricating an optical fiber preform, comprising injecting a raw gas through a raw gas injection passage at the center of a torch, injecting a combustible gas through an annular combustible gas injection passage of the torch disposed around the raw gas injection passage, and injecting a combustion supporting gas through a plurality of small-diameter combustion supporting gas injection passages of the torch, which are independent of each other, disposed within the annular combustible gas injection passage and spaced apart around the raw gas injection passage. However, also in EP-A-0 146 659 combustion supporting gas (O₂) is injected through an annular injection passage provided around the central raw gas injection passage. In accordance with this invention as seen from one aspect, the annular combustible gas injection passage (3) is disposed directly adjacent the raw gas injection passage (2) or is separated therefrom by an annular sealing gas injection passage (10) through which a sealing gas is injected. With this method, there is no reaction of oxygen with hydrogen immediately adjacent the injected raw gas, so that the risk of the raw gas injection passage becoming clogged with H₂O is minimised. By injecting the oxygen through small-diameter passages spaced apart within the annular H₂ injection passage, the high temperature generated by the reaction of the oxygen and hydrogen is distributed in the hydrogen flame and the density of the optical fiber preform is found to be comparatively even across its radius. In accordance with this invention as seen from another aspect, a combustion supporting gas is injected through an annular combustion supporting gas injection passage (4) of the torch, which is provided around the annular combustible gas injection passage (3). With this method, the heavier oxygen injected through the outer annular passage (4) encloses the hydrogen and helps to stabilise the flame. The above and other related objects and features of the invention will be apparent from a reading of the following description of the disclosure found in the accompanying drawings and the novelty thereof pointed out in the appended claims. Figure 1 is a plan view showing a first embodiment of torch used in the method according to the present invention; Figure 2 is a longitudinal sectional view of the torch of Figure 1; Figure 3 is a longitudinal sectional view of an essential portion of a second embodiment of torch used in the method according to the present invention; Figure 4 is a plan view showing a third embodiment of torch used in the method according to the invention; Figure 5 is a schematic view showing use of a VAD method in accordance with the invention; and Figure 6 is a schematic view showing use of an OVD method in accordance with the invention. Embodiments of a torch for use in a method of fabricating an optical fiber preform according to the present invention will be described in detail with reference to the accompanying drawings. Figures 1 and 2 show a first embodiment of a torch for use in a method according to the present invention. A torch 1 of a multiwall tube structure shown in Figs. 1 and 2 has a raw gas injection passage 2 provided at the center of the torch 1, an annular combustible gas injection passage 3 provided on the outer periphery of the raw gas injection passage 2, an annular combustion supporting gas injection passage 4 provided on the outer periphery of the combustible gas injection passage 3, and a plurality of small-diameter combustion supporting gas injection passages 5 independent of each other and provided at an (equal) interval circumferentially in the combustible gas injection passage 3. In case of such a torch 1, as apparent from Fig. 1, the small-diameter combustion supporting gas injection passages 5 disposed in the combustion gas injection passage 3 surround the raw gas injection passage 2 disposed at the center of the torch 1. Further, the small-diameter combustion supporting gas injection passages 5 are, as apparent from Fig. 2, directed toward a point P on the center line of the raw gas injection passage 2, of so-called a focus converging type, A distance L from the end of the torch 1 to the point P is ordinarily set to a range of approx. 30 to 350 mm, and more particularly to L = approx. 200 mm. As shown in Fig. 2, gas inlets 6, 7, 8 and 9 are provided correspondingly to the gas injection passages 2, 3, 4 and 5 at the lower portion of the torch 1. A torch 1 of multiwall tube structure of a second embodiment of the invention shown in Fig. 3 is constructed fundamentally the same as that in Figs. 1 and 2 except a raw gas injection passage 2, an annular combustible gas injection passage 3, an annular combustion supporting gas injection passage 4 and a small-diameter combustion supporting gas injection passage 5 are disposed in parallel with each other and that the injection passages 3, 5, 2 and 4 are sequentially protruded at the ends in the relative relationship of the passages. Further, in the torch 1 of Fig. 3, the ends of the raw gas injection passage 2 and the small-diameter combustion supporting gas injection passage 5 are formed in spherical shape. It is noted that the constructions of the difference (uneven state) of the projecting states of the ends of the injection passages or the spherical shape of the ends of the injection passages in the second embodiment of the invention may also be employed in the first embodiment in Figs. 1 and 2 and in the following third embodiment of the invention as will be described later within the spirit and scope of the invention. Further, it is also noted that in the torch 1 of the above-mentioned first and second embodiments, the annular combustion supporting gas injection passage 4 may be omitted within the spirit and scope of the invention. A torch 1 of multiwall tube structure of a third embodiment of the invention shown in Fig. 4 is constructed such that a raw gas injection passage 2, a first annular sealing gas injection passage 10, an annular combustible gas injection passage 3, a second annular sealing gas injection passage 11 and an annular combustion supporting gas injection passage 4 are sequentially provided from the center of the torch 1 toward the outer periphery and a plurality of small-diameter combustion supporting gas injection passages 5 independent of each other are provided at an interval in the circumferential direction within the annular combustion gas injection passage 3. As further embodiments, a plurality of raw gas injection passages and/or a plurality of combustible gas injection passages may be provided. For example, in these embodiments, when two (or more) adjacent raw gas injection passages are provided at the center of the torch 1, other gas injection passages provided in a multiwall tube structure is formed in an elliptical shape on the outer periphery of the raw gas injection passage. When the small-diameter combustion supporting gas injection passages are formed in a focus converging type in this case, two focuses of the ellipse are defined to correspond to the small-diameter combustion supporting gas injection passages. When two annular combustible gas injection passages are provided, one may be disposed at the position in the previous embodiments, and the other may be disposed, for example, on the outermost periphery of the torch 1. The tubular or cylindrical material of the torch 1 is formed of a quartz glass or ceramics having high heat resistance, and may also be formed at the end sides of the gas injection passages of a quartz glass or ceramics and at the remainder of a metal having excellent corrosion resistance and medicine resistance. Figs. 5 and 6 schematically show a VAD method and an OVD method executed by the torch 1 of the invention. In the VAD method in Fig. 5, a system has a reaction vessel 21 which contains an exhaust conduit 22, an electric furnace 23 which contains a heater (an electric heater) 24 provided at the top of the vessel 21, a target 25 of quartz and a rotary drawing unit 26 of the target 25. According to the VAD method in this known system, an optical fiber preform produced through the torch 1 is sequentially acccumulated on the lower end of the target 25 to form a porous glass preform 27, the porous glass preform 27 is transparently vitrified through the electric furnace 23 to provide a rod-shaped transparent glass preform 28. In the OVD method in Fig. 6, this system has a rotating and reciprocating type drive unit 31, and a mandrel 32 of quartz pipe supported by the drive unit 31. According to the OVD method in this known system, an optical fiber preform produced through the torch 1 is sequentially accumulated on the outer periphery of the mandrel 32 to provide a tubular porous glass preform 33. The raw gas, the combustible gas, the combustion supporting gas and the sealing gas used in the above-mentioned VAD and OVD methods employ those known per se. The torch 1 of the present invention is used in the above VAD and OVD methods as described above. In this case, the raw gas injected from the raw gas injection gas 2 is subjected to a flame hydrolysis and/or thermal oxidation with the combustible gas from the annular combustible gas injection passage 3 and the combustion supporting gas from the small-diameter combustion supporting gas injection passage 5 to become a soot-state optical fiber preform. In this case, since the small-diameter combustion supporting injection passage 5 is formed in a nozzle shape, the combustion supporting gas injected from the small-diameter combustion supporting gas injection passage 5 flows at a very high speed. As a result, the combustion reaction of the combustible gas injected from the annular combustible gas injection passage 3 disposed around the small-diameter combustion supporting gas injection passage 5 with the combustion supporting gas is accelerated to increase the combustion amount, thereby providing an extremely higher temperature than a conventional multiwall tube bumer. Therefore, the accumulation of the optical fiber preform is accelerated. Since the surface temperature of the porous glass preform formed by the accumulation of the optical fiber preform is very high and can be maintained at a uniform value, the optical fiber preform of the porous glass preform is uniformly and sufficiently shrink-fitted so that the porous glass preform of substantially uniform density of the optical fiber preform can be provided. More particularly, when the small-diameter combustion supporting gas injection passages 5 are of focus converging type, the above-mentioned advantage is remarkable. Since various gases are separately injected, for example, such that the combustion supporting gas is injected from the small-diameter combustion supporting gas injection passages 5 and the combustible gas such as hydrogen is injected from the annular combustible gas injection passage 3 in the torch 1 of the invention, different from the case that the hydrogen and the oxygen are injected together in the preliminarily mixed state, a flame is formed at a position separate at a predetermined distance from the end of the torch 1. More specifically, since the flowing speed of the combustion supporting gas is accelerated as described above in this invention, a flame can be formed at the position separate from the end of the torch 1. As a consequence, such a problem that the soot formed by the combustion of the combustible gas and the combustion supporting gas and the optical fiber preform formed by the combustible gas, the combustion supporting gas and the raw gas are bonded to the end of the torch to cause the torch to clog is hardly taken place. When the ends of the raw back injection passage 2 and the small-diameter combustion supporting gas injection passage 5 are formed of spherical shape, it can preferably prevent the optical fiber preform from being bonded to the end of the torch 1. The first annular sealing gas injection passage 10 has an effect of preventing the raw gas injection passage 2 from being blocked by the optical fiber preform and the second annular sealing gas injection passage 11 protects against the thermal deformation of the end of the tubular or cylindrical member for partitioning the annular combustible gas injection passage 3 and the annular combustion supporting gas injection passage 4. Further, the combustion supporting gas injected from the annular combustion supporting gas injection passage 4 contributes to the stability of a flame without direct relation to the synthesis of the optical fiber preform. In other words, the flame is uniformly formed in the temperature distribution over the center and the periphery. As a result, it can prevent the surface of the porous glass preform from becoming uneven, and additionally equalize the density of the optical fiber preform. The reason is because the combustion supporting gas injected from the annular combustion supporting gas injection passage 4 interrupts the contact of the atmosphere with the combustible gas injected from the inside annular combustible gas injection passage 3 to allow the remaining unburnt combustible gas to be completely burnt by the combustion supporting gas. In other words, it prevents the remaining unburnt combustible gas injected from the inside annular combustion gas injection passage 3 from being subjected to an unstable combustion with oxygen in the air so that a disorder in the periphery of the flame is induced. As an experimental example, the OVD method of Fig. 6 was executed under the following conditions with the torch 1 in Figs. 1 and 2. I. Raw gas injection passage 2: SiCl₄ (at 50°C) = 3 ℓ /min. (with Ar of carrier gas) Combustible gas injection passage 3: H₂ = 35 ℓ /min. to 45 ℓ /min. Combustion supporting gas injection passage 4: O₂ = 5 ℓ /min. to 8 ℓ /min. Combustion supporting gas injection passage 5: O₂ = 16 ℓ /min. II. Outer diameter of mandrel 32: 15 mm in diameter Traverse speed: 100 mm/min. Traverse zone: 350 mm Rotating speed of mandrel 32: 60 r.p.m. Fabricating time: Approx. 6.5 hrs. The porous glass preform fabricated under the above conditions had outer diameter X length = 120 mm × 350 mm., and the optical fiber preform density over radial direction was approx. 0.4 g/ cm³ being substantially constant and no crack observed. According to the first embodiment the present invention as described above, the torch for fabricating the optical fiber preform in accordance with the invention disposes the small-diameter combustion supporting gas injection passages independent of each other and disposed to surround the raw gas injection passage on the outer periphery of the raw gas injection passage disposed at the center of the torch, and provides the annular combustible gas injection passage around the small-diameter combustion supporting gas injection passages, Accordingly, the porous glass preform can be stably fabricated by the specific arrangement of the small-diameter combustion supporting gas injection passages. According to the second embodiment of the present invention as described above, the torch for fabricating the optical fiber preform further disposes in the first embodiment the annular combustion supporting gas injection passage on the outer periphery of the annular combustible gas injection passage. Therefore, the torch of this embodiment can not only provide the above-mentioned advantages, but stabilize the flame through the annular combustion supporting gas injection passage over the entire radial direction. According to the third embodiment of the invention as described above, the torch for fabricating the optical fiber preform further disposes in the above first and second embodiments the first and the second annular sealing gas injection passages betueen the predetermined gas injection passages. Consequently, the torch of this embodiment can not only provide the above-mentioned advantages, but prevent the raw gas injection passage from clogging by the optical fiber preform to protect the end of the tubular or cylindrical member for partitioning the annular combustible gas injection passage and the annular combustion supporting gas injection passage.

Claims: 1. A method of fabricating an optical fiber preform, comprising injecting a raw gas through a raw gas injection passage (2) at the center of a torch, injecting a combustible gas through an annular combustible gas injection passage (3) of the torch disposed around the raw gas injection passage (2), and injecting a combustion supporting gas through a plurality of small-diameter combustion supporting gas injection passages (5) of the torch which are independent of each other, disposed within the annular combustible gas injection passage (3) and spaced apart around the raw gas injection passage (2), characterised in that the annular combustible gas injection passage (3) is disposed directly adjacent the raw gas injection passage (2) or is separated therefrom by an annular sealing gas injection passage (10) through which a sealing gas is injected. 2. A method according to claim 1, characterised in that a combustion supporting gas is injected through an annular combustion supporting gas injection passage (4) provided around said annular combustible gas injection passage (3). 3. A method of fabricating an optical fiber preform, comprising injecting a raw gas through a raw gas injection passage (2) at the center of a torch, injecting a combustible gas through an annular combustible gas injection passage (3) of the torch disposed around the raw gas injection passage (2), and injecting a combustion supporting gas through a plurality of small-diameter combustion supporting gas injection passages (5) of the torch, which are independent of each other, disposed within the annular combustible gas injection passage (3) and spaced apart around the raw gas injection passage (2), characterised in that a combustion supporting gas is injected through an annular combustion supporting gas injection passage (4) of the torch, provided around said annular combustible gas injection passage (3). 4. A method according to claim 2 or 3 characterised in that a sealing gas is injected through an annular sealing gas injection passage (11) of the torch, provided between said annular combustion supporting gas injection passage (4) and said annular combustible gas injection passage (3). 5. A method according to any preceding claim, characterised in that the small-diameter combustion supporting gas injection passages (5) are directed towards a point (P) on the center line of the torch. 6. A method according to any one of claims 1 to 4, characterised in that the ends of the raw gas injection passage (2) and each of said small diameter combustion supporting gas injection passages (5) are formed in a spherical shape. 7. A method according to any preceding claim, characterised in that the gas injection passages are defined by regular concentric circles. 8. A method according to any one of claims 1 to 6, characterised in that a plurality of raw gas injection passages (2) are provided adjacent one another at the center of the torch and the annular combustible gas injection passage (3) is disposed around all of said raw gas injection passage (2). 9. A method according to claim 8, characterised in that said annular combustible gas injection passage (3), said annular combustion supporting gas passage (4) (where provided) and said sealing gas injection passage or passages (10, 11) (where provided) are formed to concentrically elliptical shapes. 10. A method according to any preceding claim, characterised in that a body of said torch which is formed with said passages is formed of a quartz glass. 11. A method according to any one of claims 1 to 9, characterised in that a body of said torch which is formed with said passages is formed of ceramics.

Inventors: MATSUBARA, KUNIHIRO C/O CHIBA CABLE WORKS, MIKAMI, MASATOSHI C/O CHIBA CABLE WORKS
Applicants: THE FURUKAWA ELECTRIC CO., LTD.
Application year: 1987
Main classification: C03B37
IPC codes: C03B37, F23D14, C03B8, G02B6