GB2179339A

GB2179339A - Optical fibre manufacture

Info

Publication number: GB2179339A
Application number: GB08520945A
Authority: GB
Inventors: Ian Douglas Harding
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1986-08-14
Filing date: 1985-08-21
Publication date: 1987-03-04
Also published as: GB2179339B; GB8520945D0

Description

1 GB 2 179 339 A 1

SPECIFICATION

Optical fibre manufacture h A 55 This invention relates to them an ufacture of optica I fibre, particularly high quality optical fibre fortelecommunication cables and systems.

Our British Patent Application No. 8323692describes and claims an optical fibre pulling towercon- structed of a synthetic epoxy granite. We havefound thatthis provides a resonant-free structure which is the keyto manufacturing high qualityfault-free fibre at high speed. This patent specification also shows howthefibre diameter is measured by a diameter measuring device and the capstan draw-off rate is dependent upon this and the preform feed rate to maintain constant fibre diameter.

Howeverthe preform is normally not of constant diameter, however hard one has tried to make itso.

There are small variations in diameter and asthe perform is fed into the furnacethese variations in diameter can affectthe resultant diameter of the pulledfibre.

We have found the conditions inside thefurnace are critical to pulling a high quality low-loss fibre; in particularthe flow rate of glass through the furnace at a predetermined furnace temperature and a predetermined fibre diameter must remain within close limits.

It is an object of the present invention to provide an improved arrangementfor maintaining precise control of the optical fibre being drawn.

According to the present invention there is provided a method of manufacturing optical fibre comprising feeding an optical fibre preform into a furnace at a first predetermined rate, pulling a fibre from the preform around the capstan at a second predetermined rate, said first and second predetermined rates being calculated to produce a fibre of a predetermined nominal diameter, sensing the diameter of the drawn optical fibre and providing a signal representa- 105 tive of a deviation of the measured diameterfrom the nominal diameter, and modifying the speed of the capstan in responseto the deviation signal from the diameter monitor, wherebyto tend to maintain the optical fibre as close as possibleto the nominal preset 110 diameter, and controlling the glass melting rate by varying the preform feed drive rateto maintain an averagefibre pulling rate closeto the second predetermined rate.

According to anotheraspect of the present invention there is provided apparatusfor manufacturing optical fibre comprising means for holding an optical fibre preform, a furnace for melting the tip of the preform, means forfeeding the preform into the furnace, means forsensing the fibre diameter, a capstan 120 for drawing the fibre from the preform, and a control system arranged to drive the preform into thefurnace at a first predetermined rate and to drive the capstan at a second predetermined rate, and wherein the cap- stan drive rate is modified over short-terms in responseto deviation from the fibre nominal diameter as measured bythe fibre diameter monitor, and means for automatically varying the first rateto maintain an average fibre pulling rate close to the second predetermined rate and thereby control the glass melting rate.

In orderthatthe invention can be clearly understood referencewill now be madeto the accompanying drawing which shows a block schematic diagram of fibre manufacturing apparatus according to an embodiment of the present invention.

Referring to the drawing an optical fibre preform 1 held in a chuck 2 is advanced slowly into a furnace 3 which melts the tip of the preform 1. An optical fibre 4 is drawn from the molten tip by a capstan 5. A primary coating is applied to the optical fibre 4 by a coating applicator 6, and a coating curing station 7 such as a U-V device, cures the coating.

The diameter of the fibre is measured by a diameter monitor9.

The preform feed drive 20 controls the vertical positioning of the chuck 2 in which the preform is held and has a servo-motor (not shown). The preform 1 is fed into the fu rnace 3 by the preform feed drive 20 at a predeterm ined rate controlled by the control aigorithm 21. Control algorithm 21 receives a programme line speed signal from the programme line speed control 22 and also a signal from thetachometer23 providing a signal representative of the capstan speed. The diameter of the fibre is preset and any deviation from the presetfigure is sensed bythe monitor 9 and control algorithm 24 comparesthe deviation signal with the programme line speed signai 22to adjustthe capstan drive speed accordinglyat the capstan drive control circuit25.

During ramp-up, afteran initial portion of fibre has been threaded through the coating applicatorand the curing stage and around the capstan, at an initial pulling speed of 4 metres per minute,then the pre- form feed speed and capstan take-up speed are both rapidly increased under control of the computer through the program line speed 22 and control algorithms 21 and 24, respectively. When the preset line speed e. g. 120 metres per minute is achieved, the existing control loop from the diameter monitor is switched to an electronic controller 31 which takes over control of the capstan speed in response to changes in diameter represented bythe deviation signal from the monitor 9.

A position transducer 26 associated with the preform feed drive 20 is an incremental length transducer and provides signals, following an autoinsertion routing whereby the preform is inserted into thefurnace during ramp-up, to initiatethe withdrawal of the end portion of the preform when the usable body of the preform has been drawn, to initiate a ramp-down procedure. Autionsert/withdrawal circuit 28 controls these functions and count circuit 27 measures the preform insertion as it proceeds.

The capstan speed is also controlled to maintain constantfibre diameter using the signal from the diameter monitor9 at the set speed. The preform 1 will not have a constant diameter throughout its length. If an increased diameter portion enters the furnace itwill tend to increase the diameter of the fibre. As soon as the commencement of any change is sensed bythe monitor 9 the control controller 31 responds by making a short-term adjustmentto the capstan drive 25 to increasethe speed of the capstan 5 to thus tend to reduce the diameter and maintain it 2 GB 2 179 339 A 2 as its nominal preset value. Atypical value would be 125microns. Thus the controller 31 overrides the preset line speed for short-terms to prevent variations in preform diameterfrom affecting the diameter of the fibre drawn from the preform.

The long-term capstan drivespeed is preset bythe programme line speed 22. Suppose a stable situation existswith the capstan running ata speed slightly greaterthan the preset line speed and thefibre dia- meter monitorshowing no deviation from the nominal diameter. The preform will befeeding glass ata particular rate i.e. so many hilograms per hour. The capstan speed however is greaterthan the nominal or presetspeed which has been calculated beforehand based on data decrived from an earlier measurement on the preform e.g. average diameter, length and weight, andwe havefoundthat itis important notto deviate by morethan -5% of the presetvalues. Thus the control algorithm 21 functionsto maintain long term control of the preform feed driveand will thus, in the situation described, attemptto increase slowly the preform feed rateto match the measured capstan speed. The capstan speed will still besubjectto shortterm adjustment bythe diameter monitor9 should thatsense any deviation fromthe presetdiameter.

Inthiswaya predetermined glassthrough flow rate is maintainedwith -t5%.

The log plus alarm circuit29 and 30function to provide chart records and an alarm if the measured parameter goes outside preset boundaries.

The tensiometer 8 controls the temperature of the furnace 3 and a broken line 8A indicatesthis control loop. This is only applied during setting up thefurnace and without applying a coating via the coating applicator6. Thus thetemperature e.g. 2000'C representative of a desired tension e.g. 20 grams, is found bytriai and this temperature used forfuture production pulls.

Claims

1. A method of manufacturing optical fibre comprising feeding an optical fibre preform into furnace at a first predetermined rate, pulling a fibre from the preform around the capstan at a second predetermined rate, said first and second predetermined rate, said first and second predetermined rates being calculated to produce a fibre of a predetermined nominal diameter, sensing the diameter of the drawn optical fibre and providing a signal representative of a deviation of the measured diameterfrom the nominal diameter, and modifying the speed of the capstan in responseto the deviation signal from the diameter monitor, wherebyto tend to maintain the optical fibre as close as possibleto the nominal preset diameter, and controlling the glass melting rate byvarying the preform feed drive rate to maintain an averagefibre pulling rate closed to the second predetermined rate.

2. A method as claimed in claim 1, wherein the average fibre pulling rate remains within plus or minus of the second predetermined rate.

3. A method as claimed in claim 1 orclaim 2, wherein the rotational speed of the capstan is measured and a signal derived representative thereof, and this signal is compared with a signal representa- tive of the preset second rate to derive a preform feed control signal for adjusting said f irst rate.

4. A method of manufacturing optical fibre substa ntia 1 ly as herein before described with reference to and as illustrated in the accompanying drawings.

5. Apparatus for manufacturing optical fibre comprising means for holding an optical fibre preform, a furnace for melting the tip of the preform, means for feeding the preform into the furnace, means for sens- ing thefibre diameter, a capstan for drawing the fibre from the preform, and a control system arranged to drive the preform into the furnace at a first predetermined rate and to drive the capstan at a second predetermined rate, and wherein the capstan drive rate is modified over short-terms in responseto deviation from the fibre nominal diameter as measured bythe fibre diameter monitor, and meansfor automatically varying the first rate to maintain an averagefibre pulling rate close to the second predetermined rate and thereby control the glass melting rate.

6. Apparatus as claimed in claim 5, comprising means for measuring the speed of the capstan a control algorithm circuit for comparing the measured speed with the preset speed, and arranged to provide a control signal for adjusting the f irst rate at which the preform is fed into the furnace.

7. Apparatus substantially as hereinbefore described with reference to and as illustrated in the accompnaying drawing.

Printed for Her Majesty's Stationery Office by Croydon Printing Company (UK) Ltd, 1187, D8817356. Published byThePatentOffice,25 Southampton Buildings, LondonWC2A lAY, from which copies maybe obtained.

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