Method of and Apparatus for Coating Optical Fiber With Plastics Material

The present invention relates to the provision of a plastics material coating on an optical fiber in general, and more particularly to a method of and apparatus for providing such coating at a high advancement speed on a glass optical fiber.

In the manufacture of glass optical fibers, it is desirable to provide the freshly drawn fiber with a plastics material protective coating as soon as conveniently possible after drawing in order to limit the reduction of strength consequent upon damage to the pristine surface of the optical fiber occasioned by atmospheric attack. This coating is conventionally applied by passing the fiber downwardly through a bath containing a suitable liquid, typically a resin solution or prepolymer. The fiber leaves the bath with a liquid coating which is then cured to form a plastics protective sheath for the fiber. If the fiber is passed too quickly through the coating bath, there arises an increasing tendency for incomplete wetting of the fiber and formation of bubbles in the coating, or more usually for lengths of fiber to emerge entirely unwetted and completely devoid of coating. One approach to ameliorating this problem, which is described in United Kingdom Patent Specification No. 1,441,086, involves placing ballotini in the coating bath to strip bubbles from the fiber. A disadvantage of this approach is the risk of damage to the fiber by the bubble-stripping action of the ballotini.

Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.

More particularly, it is an object of the present invention to develop a method of coating optical fibers with plastics materials, which method does not possess the disadvantages of the conventional optical fiber coating methods.

Still another object of the present invention is to provide an optical fiber coating method which assures the formation of a continuous and bubble-free plastics material coating on the optical fiber.

A concomitant object of the present invention is to devise an apparatus which is especially suited for performing the method of the present invention.

It is yet another object of the present invention to so construct the apparatus of the type here under consideration as to be simple in construction, inexpensive to manufacture, easy to use, and reliable in operation nevertheless.

In pursuance of these objects and others which will become apparent hereafter, one feature of the present invention resides in a method of applying a plastics coating to a glass optical fiber wherein the uncoated fiber is drawn downwardly through an aperture in the base of a vessel containing a plastics precursor in liquid form, wherein the freshly prepared fiber enters, without previous contact with any other surface, the plastics precursor liquid in a region of reduced pressure produced by the action of an air induction device creating a partial vacuum in an open ended tube that surrounds the fiber and dips into the liquid in the vessel, and wherein the liquid coating on the fiber issuing from the vessel aperture is subsequently cured to form the plastics coating.

According to another concept of the invention, there is provided an apparatus for applying a plastics coating to a glass optical fiber, which apparatus includes a coating station located above a curing station and means for drawing the optical fiber through the two stations, wherein the coating station includes a coating vessel for containing a plastics precursor in a liquid form, which vessel is provided with an aperture in its base through which the fiber can freely pass, wherein above the aperture and located in alignment therewith there is an open ended tube whose lower end protrudes into the vessel and whose upper end is connected with an air induction device adapted to create a partial vacuum in the tube, and wherein the curing station is provided with means for curing the liquid coating on fiber issuing from the vessel aperture to transform it into a plastics coating.

Above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a diagrammatic view depicting the general layout of components of an apparatus according to the present invention used in processing a silica optical fiber preform rod into plastics primary coated fiber, and

FIG. 2 is a somewhat simplified sectional view of a coating unit of FIG. 1.

Referring now to the drawing in detail, and first to FIG. 1 thereof, it may be seen that the reference numeral 1 has been used to identify a silica optical fiber preform. The preform 1 is progressively lowered, lower end first, at a controlled rate into a furnace 2 where it is softened by the heat so that a fiber 3 can be drawn from its tip. This fiber 3 passes through a monitoring station 4 at which its diameter is monitored to provide a feedback signal to operate a control loop regulating the rate at which the preform 1 is lowered into the furnace 2 so as to maintain a constant fiber diameter. The fiber 3 next passes through a coating station 5 where it is coated with a liquid resin to protect it from degradation by atmospheric attack. Beneath the coating station 5 is a curing station 6 where the liquid resin is cured, typically by heat or by ultra-violet light depending upon resin type. The fiber 3 complete with its plastics coating is then reeled up on a drum 7, or alternatively directed elsewhere, such as for further processing, either by a pulley or some form of traction device such as a capstan or caterpillar drive.

One of the factors governing the rate at which the fiber 3 can be coated is the distance between the furnace and the coating station. It is believed that it is the temperature of the fiber 3 as it enters the resin that is responsible. Thus when silica fiber approximately 125 microns in diameter is drawn through a coating station containing a polysiloxane resin, marketed under the designation Sylgard 182, in an open vessel to a depth of between 2 and 10 cm, it is found that the fiber can be satisfactorily coated at a rate of up to about 60 meters per minute in a drawing rig providing approximately 1.5 meters between the bottom of the furnace and the surface of the resin in the curing station; whereas in a smaller rig where the corresponding separation is approximately 0.5 meters, the safe coating top speed limit is in the region of 30 meters per minute.

Turning now to FIG. 2, it may be seen that the coating station 5 has a vessel 20 containing a body 21 of a coating resin constituting a plastics precursor. At the bottom of this vessel 20, which may be water cooled to inhibit premature curing of the resin, there is provided an orifice 22 through which the fiber 3 passes. Although not shown in FIG. 2, it is convenient to provide this orifice on the line of abutment of two shutters which can be moved apart to facilitate threading of the fiber 3 through the vessel 20 before it is filled.

Dipping into the resin body 21 is a suction tube 23 approximately 25 cm long and 5 cm in bore diameter. At its upper end, this suction tube 23 is connected to an air induction device 24, marketed by F. Brauer Limited, Harpenden, Herts. under the designation `Airmover`. The device 24 uses a flow of compressed air into a pipe 25 to create a partial vacuum in the tube 23. Optionally, in order to keep the air flow from the air induction device 24 away from the fiber 3, the fiber 3 is threaded through a funnel 26, the outside of whose conical region serves as a deflector while its stem extends just through the device 24 and preferably into the upper end of the tube 23.

By admitting sufficient compressed air into the air induction device 24 to raise the height of the resin within the suction tube 23 about 10 cm above its height outside the tube 23, it has been found possible to increase the coating speed in the larger rig referred to previously substantially above the region of 60 meters per minute.

If the compressed air supply is shut off, the level of resin in the suction tube 23 begins to fall, but, long before it has fallen to any appreciable extent, the resin ceases properly to wet the moving fiber 3. This shows that the increased coating speed is not attributable merely to the increased length of column of resin in contact with the fiber 3.

A particular advantage of the invention in comparison with the method described in United Kingdom Patent Application 2 048 726A is that the reduced pressure at the point of entry of the fiber 3 into the plastics precursor is maintained without recourse to vacuum seals or adjustable end diaphragms. In this way the risk of damage to the fiber 3 by contact with such seals or diaphragms is avoided.

While we have discussed above the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the accompanying claims.