IMPROVED SINTERING OF OPTICAL FIBER PREFORMS

Abstract: A light beam is directed through the deposited layer in an optical fiber preform during sintering of the deposited material. The degree of transmission of the light indicates the degree of sintering of the deposit that has occurred due to the moving hot zone. The speed of traversal of the hot zone can thereby be maximized. This technique is especially desirable for use with the tandem hot zone process utilizing a plasma fireball for high-speed preform manufacture.

Description: IMPROVED SINTERING OF OPTICAL FIBER PREFORMS Background of the Invention 1. Field of the Invention The present invention relates to a method of improving the sintering of preform material in the manufacture of optical fibers. 2. Description of the Prior Art Optical fibers are typically made by depositing glass particulate material in or on a support. The porous deposited material is then consolidated by heating to an elevated temperature, which process is also referred to as "sintering1,. The resulting structure is referred to as a "preform" and can be in the form of either a rod or a tube. An optical fiber is drawn from the preform. One commercially successful method of making an optical fiber preform is by the modified chemical vapor deposition (MCVD) technique, described in U. S. Patent No. 4,217,027. In the MCVD technique, particulate material is formed inside a tube, and the deposited particulates are then sintered by means of a moving hot zone produced by a heat source external to the tube. The external heat source also provides the reaction of the precursor material within the tube that results in the deposits on the inside of the tube. The longitudinal velocity of the heat source relative to the tube is usually determined by considerations that relate to the reaction rate of the precursor material, and also the need to produce a relatively constant deposit layer along the length of the tube; that is, to avoid or minimize a taper of deposited materials along the tube. Another constraint is that the heat source should produce adequate sintering of the deposited material. In practice, this latter requirement is typically provided for by monitoring the temperature in or near the hot zone by means of an optical pyrometer which is an infrared radiation sensing device. A control signal is then provided that adjusts the flow rate of the gases within the torch used as the external heat source to provide an adequate temperature for sintering the deposited material. It is also possible to make a preform by reacting precursor material in a flame and depositing the material on a support. In one technique, the support is a rod, with the flame moving axially along its length to deposit material thereon; see, for example, U. S. Patent Nos. 3,826,560 and 3,823,195. In another technique, particulate material is directed perpendicularly onto the surface of a rotating substrate, with the deposit forming axially on the substrate. These techniques also require sintering of the deposited particulate material, which can be accomplished by heating the unconsolidated preform in a furnace. A high-rate technique for making an optical fiber preform using a plasma is disclosed in U. S. Patent No. 4,331,462. As frequently practiced, that technique is an MCVD technique, wherein the hot zone is in the form of two zones, at least one of which is generated primarily by a plasma fireball. As typically practiced, the speed at which the plasma technique can produce a preform is limited primarily by the rate at which the deposited material can be sintered to form a smooth, glassy layer. The rate of traverse of the sintering torch has in the past been manually adjusted. Automation of this operation and an increase in the sintering rate are very advantageous. Summary of the Invention The present invention is a technique for making an optical fiber preform wherein a light beam is directed through at least partially sintered preform material. The intensity of the transmitted light beam controls the rate at which the material is sintered to produce a nonporous glassy layer. The technique is advantageously used when the preform material to be sintered is produced or deposited with the assistance of a plasma in the interior portion of a preform tube. It can also be used to aid the sintering of porous glassy material formed by other techniques. Brief Description of the Drawing FIG. 1 shows a plasma deposition technique suitable for practicing the present invention; FIG. 2 illustrates an optical arrangement suitable for practicing the present invention; and FIG. 3 shows an electrical comparison circuit suitable for use in practicing the present invention. Detailed Description The following detailed description relates to an improved method of making a preform from which an optical fiber is drawn. In the present technique, a light beam, typically produced by a laser, is directed through an at least partially sintered portion of a preform while the preform is moving relative to a heat zone that produces the sintering. A control signal produced by the intensity of the light beam transmitted through the preform is used to control the rate at which the heat source moves relative to the preform. The present technique is advantageously used in conjunction with a plasma-assisted deposition technique, whereby a high rate of preform manufacture is accomplished. Referring to FIG. 1, a reactant flow 101 of precursor materials is directed through a rotating preform tube 102. The preform tube, also referred to as a substrate tube, starter tube, or support tube, is typically fused silica. Typical precursors include, for example, SiC14, GeC14, POC13, a fluoride, etc. Oxygen is also flowed through the tube, and a reaction is initiated in a heat zone 107, produced in part by flame 108 from torch 103. The reacted precursor material forms particulate matter that deposits on the inside of tube 102. The deposition is assisted by means of plasma fireball 105, which enhances the thermophoretic forces that drive the particulates to the wall of the tube. The plasma is produced by coils 104 surrounding the tube and is energized typically by an rf source. Water is fed through nozzles 106 located in baffle 109 to cool the tube. This prevents the high temperature of the fireball from causing the tube to sag, and also further enhances the thermophoretic deposition forces. The plasma technique typically allows a high rate of reaction of the precursor material, such that a deposition rate of particulate material of several grams per minute is readily achieved. The consolidation of the particulate material into a smooth, glassy layer is accomplished primarily by means of torch 103 as it traverses to the right, as viewed in FIG. 1. However, other arrangements are possible. For example, the plasma fireball 105 can precede the torch 103; that is, be located to the left of torch 103, as viewed in FIG. 1. In many cases, torch 103 and plasma fireball 105 travel at the same, or approximately the same, rates. In that case, sintering is continuously accomplished during each deposition pass, as the tandem heat zone moves from left to right, as viewed in FIG. 1. However, other deposition and sintering schemes are possible. For example, deposition can be accomplished using only plasma fireball 105 or else plasma fireball 105 assisted by torch 103 moving at a sufficiently rapid rate so that sintering does not occur on each pass. Also, torch 103 can be held stationary near the entrance of tube 102 to assist in the reaction of the particulate material, with fireball 105 moving axially along the tube to produce the majority of the deposition on the inside of the tube. Following a multiplicity of such passes, torch 103 can then be moved along tube 102 to produce the sintering of a multiplicity of layers sa deposited. In the present technique, the velocity of the heat source used for'sintering (torch 103 in FIG. 1) is adjusted in response to the transmission of light through the sintered or partially sintered zone of the preform tube. Referring to FIG. 2, a light source 201 directs a beam of light 202 through the preform tube 102. Source 201 is typically a helium-neon laser, having an output of about 2 milliwatts. The beam is passed through the tube in close proximity to the sintering torch 103. On emerging from the substrate tube, the beam first passes through a lens 208 and then through a narrow band filter 209, which allows the beam to pass through but blocks out most of the radiation from the hot tube and rf plasma. A filter having transmission wavelengths of 6328 Angstroms is suitable for use with a helium-neon laser. The beam then passes onto a detector 210, typically a silicon photodiode. The lens 208 primarily ensures that the beam, which may be dispersed on its passage through the tube by bubbles or other imperfections in the tube wall, is directed onto the detector. The torch 103 is mounted on fire carriage 207, which moves to the right, as viewed in FIG. 2. This produces sintering of the deposited material, producing a sintered region 204 from the unsintered material in region 206. The signal from the detector is then used to adjust the velocity of fire carriage 207 as it traverses tube 102. The operation of the controller will be more fully illustrated by means of the following Example: EXAMPLE This Example illustrates the improved sintering technique for use with a plasma-assisted deposition scheme using a tandem heat zone, as further described in abovenoted U. S. Patent No. 4,331,462. In this Example, a fused silica tube, approximately 1 meter long having an inside diameter of 46 millimeters and an outside diameter of 50 millimeters, is used as the substrate for deposition of particulate material. Through this tube was directed a reactant flow of precursor materials, resulting from passing oxygen through bubblers at a temperature of 46 0C. Oxygen gas flowed through a silicon tetrachloride bubbler at a rate of 800 cm3 per minute, and oxygen also flowed through a POC13 bubbler at a rate of 200 cm3 per minute. The resulting gases, and CC12F2 gas at a rate of 80 cm3 per minute, were flowed through substrate tube 102. An oxyhydrogen torch 103 produced a flame 108, producing the first portion of a tandem heat zone 107. The precursor chlorides were substantially oxidized in the first portion of the heat zone, producing particulate matter. The particulates were directed onto the inner wall of the tube by means of plasma fireball 105. The plasma was generated by 11 kilowatts of rf energy at a frequency of 3 megahertz applied to coil 104, being 4 turns of 0.635 cm. (1/4 inch) diameter copper having an inside diameter of 56 millimeters. The plasma portion of the tandem heat zone was cooled with deionized water flowing through nozzles 106 at a rate of 0.7 liters per minute. During the deposition process, the plasma fireball 105 moved axially along the length of the preform tube at a varying velocity in order to produce a substantially constant thickness of particulate material inside the tube according to principles known in the art. The minimum velocity of plasma fireball 105 was approximately 2.5 millimeters per second near the beginning of the tube (to the left, as viewed in FIG. 1), with a maximum velocity of approximately 6 millimeters per second near the end of the tube. At the above flow rates, approximately 4.5 grams per minute of glass particulate material was deposited on the inside of the tube. A control circuit suitable for use with the present invention is shown in FIG. 3. An operational amplifier 301 is used to convert the output of photodiode 210 to a voltage of 0 to 15 volts D.C., corresponding to 0 to full diode illumination. The gain of the circuit is set by potentiometer 302. The signal is supplied to a fire carriage controller (Litton 9665 microprocessor-controlled carriage). A supplied voltage of 0 to 5 volts sets an internal flag to 0, corresponding to the case of incomplete sintering, whereas a voltage of 5 to 15 volts sets the flag to 1, which indicates complete sintering. The gain adjustment 302 is used to set the output voltage to the level of 5 volts at the required level of sintering. During fire carriage control, the microprocessor examines the internal flag set by the sintering sensor circuit every 0.01 second. If the flag is set to 0 (incomplete sintering), the sintering torch speed is reduced. The torch is halted for a maximum of 1 second (100 flag examinations) before it is moved forward 1 millimeter, and the process repeats. If the internal flag is set to 1 (complete sintering), the torch is allowed to progress at its preset maximum speed. This speed is set so that it can never catch up and collide with the plasma fire carriage. In typical operation, this control provides a minimum speed of 1 millimeter per second and a maximum speed of 5 millimeters per second for the sintering torch. The minimum speed is adjusted to be sufficient to prevent tube collapse from excessive heating from torch 103. Under the above-noted conditions, it was found that when adjusting the velocity of torch 103 by manually observing the sintering process and thereby adjusting the rate of torch 103 to produce adequate sintering, typically a maximum torch velocity of approximately 4 millimeters per second was obtained. Thus, approximately 3 minutes are required to deposit and sinter a single layer over a usable preform length of approximately 70 centimeters. When the above-noted fusion monitor is used, it was found that the maximum velocity of torch 103 can increase typically to about 5 millimeters per second under the above conditions, allowing a substantial decrease in the sintering time. Since typically 10 to 40 layers of cladding material are deposited in a tube, substantial time-saving results from the above-noted technique. While an increase in sintering speed is an advantage of the present technique, an independent advantage is that it frees an operator from the necessity to closely monitor the sintering process. The more automated process facilitated by the present technique thus allows a saving in overall operator time over a number of lathes operating simultaneously. As noted above, other optical fiber production techniques depend upon the sintering of porous material. For example, with axial preforms produced by any of the above-noted flame deposition techniques, the unsintered preforms can be inserted into a furnace and sintered as they pass through the hot zone inside the furnace. In addition, it is known to make optical fiber preforms from colloidal gels, wherein a deposition step per se is not necessarily present; see, for example, "New Optical Fibre Fabrication Method," K. Susa et al, Electronics Letters, Vol. 18, page 499 (1982). A porous structure results from that process, which can be sintered according to the present technique. By directing a light beam through the preform in the sintering zone, the present technique can be used advantageously to maximize the rate of sintering, or reduce operator requirements, or both. All such uses to which the present technique has advanced that art are considered to be within the scope of the present invention.

Claims: Claims 1 An improved method of making an optical fiber by steps comprising forming a preform comprising porous glassy material sintering said porous material to form a nonporous glassy material and drawing an optical fiber from said preform, CHARACTERIZED IN THAT said sintering is accomplished by steps comprising directing a light beam through an at least partially sintered portion of said preform while causing relative movement of said preform and a heat source that produces said sintering, detecting the amount of light transmitted through said at least partially sintered portion; and controlling the velocity of said relative movement. 2. The method according to claim 1, CHARACTERIZED BY producing said light beam by a laser. 3. The method according to claim 2 CHARACTERIZED IN THAT said laser is a helium-neon laser. 4. The method according to claim 1 or 2 or 3, CHARACTERIZED IN THAT said detecting of said light beam results in an increase in said velocity when the degree of light transmission exceeds a certain level, and wherein said detecting of said light beam results in a decrease in said velocity when said intensity is below a certain level. 5. The method according to any one of preceding claims 1-4, CHARACTERIZED IN THAT said porous glassy material is made by depositing particulate material inside a tube. 6. The method according to claim 5, CHARACTERIZED BY accomplishing said depositing of particulate material at least in part by means of a plasma fireball that moves axially within said tube. 7. The method according to claim 5, CHARACTERIZED BY producing particulate material by a chemical reaction within a flame. 8. The method according to claim 5 or 6 or 7, CHARACTERIZED BY producing said sintering at least in part by means of a flame produced by an oxyhydrogen torch that moves axially within said tube. 9. An optical fiber drawn from a preform made according to the method according to any one of preceding claims 1-8.

Inventors: FLEMING, JAMES WILLIAM, JR.
Applicants: WESTERN ELECTRIC COMPANY, INCORPORATED
Application year: 1983
Main classification: C03B37
IPC codes: C03B37, G02B6