Optical fibre manufacture

Optical fibre (4) is drawn from a preform (1) by a capstan (5), and a control circuit (22) drives the capstan and the preform at respective predetermined rates during ramp-up. The capstan drive rate is modified (controller 31) in response to deviations from the nominal fibre diameter as measured by a fibre diameter monitor (9) over short-term periods, and the preform feed rate (20) is modified (algorithm 21) to maintain an average fibre pulling rate within .+-.5% of the present pulling rate, to thus control the glass melting rate.

BACKGROUND OF THE INVENTION 
This invention relates to the manufacture of optical fibre, particularly 
high quality optical fibre for telecommunication cables and systems. 
Our British patent application No. 8323692 describes and claims an optical 
fibre pulling tower constructed of a synthetic epoxy granite. We have 
found that this provides a resonant-free structure which is the key to 
manufacturing high quality fault-free fibre at high speed. This patent 
specification also shows how the fibre 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. 
However the preform is normally not of constant diameter, however hard one 
has tried to make it so. There are small variations in diameter and as the 
preform is fed into the furnace these variations in diameter can affect 
the resultant diameter of the pulled fibre. 
We have found the conditions inside the furnace are critical to pulling a 
high quality low-loss fibre; in particular the 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 arrangement 
for maintaining precise control of the optical fibre being drawn. 
SUMMARY OF THE INVENTION 
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 representative of a deviation of the measured 
diameter from the nominal diameter, and modifying the speed of the capstan 
in response to the deviation signal from the diameter monitor, whereby to 
tend to maintain the optical fibre as close as possible to the nominal 
preset diameter, and controlling the glass melting rate by varying the 
preform feed drive rate to maintain an average fibre pulling rate close to 
the second predetermined rate. 
According to another aspect of the present invention there is provided 
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 sensing the fibre 
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 
response to deviation from the fibre nominal diameter as measured by the 
fibre diameter monitor, and means for automatically varying the first rate 
to maintain an average fibre pulling rate close to the second 
predetermined rate and thereby control the glass melting rate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawing an optical fibre preform 1 held in a chunk 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 monitor 9. 
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 furnace 3 by the preform feed drive 20 at a 
predetermined rate controlled by the control algorithm 21. Control 
algorithm 21 receives a programme line speed signal from the programme 
line speed control 22 and also a signal from the tachometer 23 providing a 
signal representative of the capstan speed. The diameter of the fibre is 
preset and any deviation from the preset figure is sensed by the monitor 9 
and control algorithm 24 compares the deviation signal with the programme 
line speed signal 22 to adjust the capstan drive speed accordingly at the 
capstan drive control circuit 25. 
During ramp-up, after an initial portion of fibre has been threaded through 
the coating applicator and the curing stage and around the capstan, at an 
initial pulling speed of 4 meters per minute, then the preform 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 meters 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 by the 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 routine whereby the preform is inserted into the furnace 
during ramp-up, to initiate the withdrawal of the end portion of the 
preform when the usable body of the preform has been drawn, to initiate a 
ramp-down procedure. Autoinsert/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 constant fibre diameter 
using the signal from the diameter monitor 9 at the set speed. The preform 
1 will not have a constant diameter throughout its length. If an increased 
diameter portion enters the furnace it will tend to increase the diameter 
of the fibre. As soon as the commencement of any change is sensed by the 
monitor 9 the control controller 31 responds by making a short-term 
adjustment to the capstan drive 25 to increase the speed of the capstan 5 
to thus tend to reduce the diameter and maintain it as its nominal preset 
value. A typical value would be 125 microns. Thus the controller 31 
overrides the preset line speed for short-terms to prevent variations in 
preform diameter from affecting the diameter of the fibre drawn from the 
preform. 
The long-term capstan drive speed is preset by the programme line speed 22. 
Suppose a stable situation exists with the capstan running at a speed 
slightly greater than the preset line speed and the fibre diameter monitor 
showing no deviation from the nominal diameter. The preform will be 
feeding glass at a particular rate i.e. so many hilograms per hour. The 
capstan speed however is greater than the nominal or preset speed which 
has been calculated beforehand based on data decrived from an earlier 
measurement on the preform e.g. average diameter, length and weight, and 
we have found that it is important not to deviate by more than .+-.5% of 
the preset values. Thus the control algorithm 21 functions to maintain 
long term control of the preform feed drive and will thus, in the 
situation described, attempt to increase slowly the preform feed rate to 
match the measured capstan speed. The capstan speed will still be subject 
to short-term adjustment by the diameter monitor 9 should that sense any 
deviation from the preset diameter. 
In this way a predetermined glass through flow rate is maintained within 
.+-.5%. 
The log plus alarm circuit 29 and 30 function 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 indicates this control loop. This is only applied during setting 
up the furnace and without applying a coating via the coating applicator 
6. Thus the temperature e.g. 2000.degree. C. representative of a desired 
tension e.g. 20 grams, is found by trial and this temperature used for 
future production pulls.