Method for controlling fiber diameter during optical fiber drawing process

There is provided a drawing process for producing an optical fiber which comprises drawing the optical fiber from a preform therefor under tension to form the optical fiber while heating and melting the preform, wherein an outer diameter of the optical fiber on which no coating has been provided is measured at a position at which shrinkage of the outer diameter of the optical fiber, while stretched, is not larger than 0.5% and drawing conditions are controlled based on the deviation of the measured diameter from a preselected outer diameter.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for optical fiber drawing. 
2. Description of the Related Art 
In a conventional drawing process for producing an optical fiber, the 
optical fiber is produced by heating and melting a preform for the optical 
fiber in a drawing furnace and drawing the fiber from the preform at a 
certain rate by a winding up device. The optical fiber which has just left 
the furnace and is remaining intact, that is, a so-called "bare fiber", 
tends to be considerably damaged and influenced with moisture. Therefore, 
the bare fiber is usually coated with an ultraviolet curable resin or a 
thermosetting resin in a resin coating device comprising, for example, a 
die, the resin is consequently cured in a resin curing device, and then 
the fiber is wound as a coated optical fiber. A diameter of the bare fiber 
is measured by a measuring device before the coating steps, whereby 
conditions during the drawing are controlled so that the outer diameter of 
the fiber is to be a preselected one. 
A position at which the diameter measuring device is disposed has not been 
thought to be critical, and the device is usually located immediately 
below the drawing furnace as shown in Japanese Patent Kokai Publication 
No. 295260/1986. 
If there is anything to limit the position of the measuring device, it has 
been that the measuring device should not be directly subjected to a 
strong radiation light from a lower portion of the furnace to avoid being 
heated to a remarkably high temperature. 
In addition, it is preferred to locate the measuring device near the 
furnace in order to shorten the time lag and to increase a control gain 
when fluctuation in the diameter of the optical fiber has to be suppressed 
by controlling a drawing rate depending on an output signal from the 
measuring device. 
Thus, in the conventional production of the optical fiber, usually a 
distance between the outer diameter measuring device and the coating die 
is longer than that between the drawing furnace and the measuring device, 
or a forced cooling device is disposed between the measuring device and 
the coating die in order to achieve a better resin coating. 
In the conventional drawing process for producing the optical fiber, the 
drawing rate of the optical fiber was in the order of 100 m/min. Recently, 
the drawing rate is remarkably increased and it is reported that, in an 
experimental scale, a rate of 1000 m/min. has been realized. However, when 
such a high drawing rate is employed in the conventional process in which 
the measuring device is located immediately below the furnace, it has been 
found that the outer diameter of the finished optical fiber is extremely 
smaller than the diameter which is measured with the measuring device. As 
the requirements for accuracy in optical fiber diameter increase and the 
minimization of fluctuation in that diameter become critical as the result 
of the connection between fibers becoming better, development of a process 
which improves the accuracy of the outer diameter of the optical fiber is 
highly desired. 
For example, the accuracy of the diameter of a quartz base optical fiber is 
usually required to be in 125.mu.m.+-.1.mu.m. Taking account into an 
accuracy of the measuring device itself and the fluctuation in the 
diameter of the optical fiber during the production, a deviation of the 
measured diameter with the measuring device from a true diameter of the 
finished fiber should be not larger than 0.5% of the outer diameter of the 
finished fiber. Thus, it is desirable to develop a process which can 
achieve the deviation of 0.5% or less. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a drawing process for 
producing an optical fiber in which an improved accuracy of an absolute 
value of an optical fiber diameter is ensured and, especially, a deviation 
of a measured outer diameter with a diameter measuring device from a true 
diameter of a finished optical fiber can be smaller than that as obtained 
in the conventional process. 
It is found that when the optical fiber is drawn with controlling 
conditions on the basis of an output signal from the measuring device for 
the outer diameter of the optical fiber, a position of the measuring 
device considerably affects the diameter of the finished optical fiber, 
and suitable control of the position minimizes the deviation though such 
positioning has not been noted in high speed drawing. 
According to the present invention, there is provided a drawing process for 
producing an optical fiber which comprises drawing the optical fiber from 
a preform therefor under tension to form the optical fiber while heating 
and melting the preform, wherein an outer diameter of the optical fiber on 
which no coating has been provided is measured at a position at which 
shrinkage of the outer diameter of the optical fiber, while stretched, is 
not larger than 0.5% preferably 0.5 to 0.3% and drawing conditions are 
controlled with a deviation of a measured diameter from a preselected 
outer diameter. 
As used herein, the term "shrinkage" is intended to mean a ratio of 
difference in diameters between the optical fiber at the measuring 
position and the optical fiber once it has finished shrinking. 
In one preferred embodiment of the present invention, a temperature of the 
optical fiber at the measuring position of the outer diameter is lower 
than the glass softening point of the material of the optical fiber. 
In another preferred embodiment of the present invention, a drawing rate 
(or linear velocity) from the preform is varied depending on the deviation 
in order to control the outer diameter of the optical fiber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows one embodiment of the present invention, in which the 
numerical number 1 indicates the preform for the optical fiber, 2 does a 
drawing furnace, 3 does the outer diameter measuring device, 4 does a die 
for resin coating, 5 does a curing device for the resin and 6 does a 
winding up device for the optical fiber. The preform 1 which is heated and 
melted in the furnace 2 is stretched under tension to form the optical 
fiber 11, which is taken up by a spool (not shown) installed in the 
winding up device. In FIG. 1, Z is a distance from an outlet of the 
drawing furnace to the measuring device 3. Generally, an additional set of 
the coating die and the curing device is disposed between the curing 
device 5 and the winding up device 6. The present invention is 
characterized in that the outer diameter measuring device 3 is located at 
a position at which the shrinkage of the outer diameter of the optical 
fiber, while stretched, is not larger than 0.5%, preferably 0.5 to 0.3%. 
As a result, the position is shifted downward from the conventional 
position of the measuring device. 
Generally, the outer diameter of the preform is gradually reduced in the 
furnace corresponding to an axial change of the preform temperature 
(therefore, a viscosity change of the preform material). Further, a size 
of the shrinking portion of the preform is dependent on the drawing rate, 
and the outer diameter of the preform and the preform temperature at the 
outlet of the furnace increase, as the rate increases. Of course, the 
outer diameter of the optical fiber depends on a preform diameter, 
structural factors of the drawing furnace such as a heating length, a size 
of the furnace outlet, and a flow rate and a kind of an inert gas. Thus, 
the present invention resides in not only limiting the distance between 
the outlet or a center of the drawing furnace and the measuring device for 
the outer diameter but also, as a whole, limiting such factors described 
above. 
It is known that a temperature T (.degree. C) of the optical fiber at a 
position which is Z (m) away from the outlet of the drawing furnace is 
estimated according to the following equation (I): 
EQU T=T.sub.O +(T.sub.S -T.sub.O)exp(-a.multidot.Z/V.sub.F) (I) 
wherein T.sub.O is a room temperature (.degree. C.), T.sub.S is a 
temperature (.degree. C.) of an optical fiber immediately after leaving a 
furnace, Z (m) is a distance from an outlet of the furnace to a position 
at which an outer diameter of the optical fiber is measured, V.sub.F is a 
drawing rate (or linear velocity) (m/min.) and "a" is a constant 
determined with the diameter, a specific heat of the optical fiber and a 
thermal conductivity between the optical fiber and an atmosphere. 
As seen from the above equation (I), the higher the linear velocity, that 
is, the larger V.sub.F is, the higher the temperature of the optical fiber 
when Z is fixed to a certain value. 
With an apparatus comprising the devices as shown in FIG. 1 in which an 
stable operation up to 300 m/min. of the drawing rate can be carried out, 
the optical fiber was repeatedly produced with varying Z which is the 
distance from the shrinking part of the preform 1 to the outer diameter 
measuring device. During the production, the diameter of the optical fiber 
was measured by the measuring device and the diameter of the obtained 
optical fiber of which coating was stripped (that is, a true diameter of 
the optical fiber) was actually measured by a precise micrometer. Thus, it 
is found that, in the case of the drawing rate of 300 m/min., the 
difference between the measured diameter of the optical fiber with the 
measuring device and the true diameter of the optical fiber is less than 
0.5%, when the optical fiber is cooled to below a temperature at which the 
shrinkage of the optical fiber diameter under tension, at the point where 
the outer diameter is measured with the measuring device, is 0.5% or less. 
Thus, the position at which the measuring device is disposed is determined 
on the basis of the estimation of the fiber temperature according to the 
equation (I) and the several experiments as follows: 
Firstly, the difference between the measured outer diameter and the true 
one is obtained with varying the position of the measuring device. Then, a 
relation between the difference and the measuring position is established. 
Finally, the position is determined at which the difference is less than 
0.5%. Thus, the measuring device can be located at that position and an 
optical fiber having a better accuracy is produced. 
A rough position near which the measuring device should be disposed can be 
determined with the tension during the production and physical properties 
on an elasticity or a viscosity of the fiber at a fiber temperature. 
EXAMPLES 
With an apparatus as shown in FIG. 1 in which an stable production at the 
velocity up to 300 m/min. can be carried out, an optical fiber was drawn 
with varying Z from 0.4 to 0.8 m and measured the true outer diameter of 
the produced fiber after stripping the coating. As the outer diameter 
measuring device at the measuring position, Laser Diameter Monitor 551 A 
commercially available from Anritsu Corporation was used. Other conditions 
were as follows: 
______________________________________ 
Outer diameter of preform 
25 mm 
Drawing rate 300 m/min. 
Room temperature (T.sub.0) 
25.degree. C. 
Fiber temperature immediately 
1600.degree. C. 
after leaving furnace (T.sub.S) 
______________________________________ 
When Z was 0.4 m, the measured outer diameter with the measuring device was 
125.0 .mu.m and the true outer diameter was 123.7 .mu.m. 
When Z was 0.8 m, the measured outer diameter with the measuring device was 
125.0 .mu.m and the true outer diameter was 124.9 .mu.m. The fiber 
temperature at the measuring position was estimated to be about 900 
.degree. C. according to the equation (I). It is seen that the optical 
fiber is under shrinking at the position of Z=0.4 m as employed in the 
conventional manner. 
In the embodiment as shown in FIG. 1, it is contemplated to produce an 
optical fiber with a measured diameter of 126.3 .mu.m at the outer 
diameter measuring position so as to produce the optical fiber with a 
diameter of 125 .mu.m. But such conditions are not essential. 
The results for other Z values are shown in Table: 
TABLE 
______________________________________ 
Measured outer diameter (.mu.m) 
Drawing rate 
100 m/min. 200 m/min. 
300 m/min. 
______________________________________ 
Z = 0.4 m 125.1 125.2 126.1 
0.5 m 125.0 125.1 125.5 
0.6 m 125.0 125.1 125.2 
True diameter 
125.0 125.0 125.0 
______________________________________ 
It can be seen that the present invention is particularly effective in the 
drawing of the optical fiber at a drawing rate higher than 300 m/min. 
Series of experiments as described above were repeated, and it is found 
that the optical fiber should be cooled to a temperature at which the 
shrinkage of the optical fiber under tension is not larger than 0.5% at a 
position where the outer diameter measuring device 3 is located when the 
drawing is carried out at a rate higher than 300 m/min. 
Another embodiment of the present invention is shown in FIG. 2, in which 
the drawing rate is controlled with results from arithmetic operation (by, 
for example, a PID controller) on the deviation of the output signal of 
the measured outer diameter with the measuring device from the preset 
outer diameter. 
A further embodiment of the present invention is shown in FIG. 3. In the 
embodiment as shown in FIG. 1, it takes time to detect the outer diameter 
of the fiber which is under increase in its diameter in the case of small 
drawing rate, whereby a time lag arises in the control. In the embodiment 
as shown in FIG. 3, when the drawing rate is small, detection of the outer 
diameter is carried out with the measuring device 31 and when the rate is 
increased, the detection is carried out with the measuring device 32. 
Alternatively, only one measuring device is used which can move along the 
optical fiber depending on the drawing rate. 
Further, a forced cooling device for the optical fiber is provided between 
the furnace 2 and the outer diameter measuring device 3, whereby the 
distance between them can be shortened. In this embodiment, the diameter 
of the optical fiber is also measured at a position at which the shrinkage 
of the outer diameter is not larger than 0.5%. When the drawing rate is 
largely exceeds 300 m/min., such a construction is especially preferred 
since large scaling of the apparatus can be avoided and a prompt response 
can be obtained. 
As described above, according to the present invention, the absolute value 
of the outer diameter of the optical fiber which has been shrunk is 
measured correctly, whereby the optical fiber with better accuracy in its 
size is produced.