Production of optical glass fibers

In a method for producing optical glass fibers which comprises co-spinning glass melts of different kinds through coaxially disposed discharge nozzles of a multimember crucible composed of two or more crucible members having a discharge nozzle at their bottom, the improvement wherein the outermost nozzle has a length of at least 30 mm and is heated so that at least a part of it is maintained at a temperature equal to, or higher than, the temperature of the crucible thereby to increase the speed of spinning; and a high-speed spinning furnace used therefor.

This invention relates to the production of optical glass fibers, and more 
specifically, to a method and an apparatus for producing by a high-speed 
spinning process low-loss optical glass fibers suitable for use as 
transmission channels in optical communication. 
The term "optical glass fiber", as used in the present application, denotes 
a glass fiber consisting of a cylindrical elongated core and one or more 
cladding layers surrounding the core. A light beam entering one end of the 
optical glass fiber is transmitted within the core along the longitudinal 
direction of the fiber, and leaves the other end. 
According to the prior art, optical fibers are produced by a method which 
comprises using a multi-member crucible composed of two or more crucible 
members having a nozzle for discharging a glass melt at their bottom with 
the discharge nozzles being disposed coaxially, and co-spinning glass 
melts of different kinds from the discharge nozzles. For example, when an 
optical glass fiber is to be produced by using a two-member crucible, two 
large and small crucibles each of which has a nozzle for discharging a 
glass melt at its bottom are arranged so that the large crucible surrounds 
the small one and the two discharge nozzles are disposed coaxially. A 
core-forming glass is fed into the inner small crucible and a cladding 
glass, into the outer large crucible. The large crucible is externally 
heated to melt the glasses fed, and the core-forming and cladding glass 
melts are allowed to flow down respectively through the coaxially disposed 
nozzles, and drawn therefrom to form a glass fiber consisting of a core 
and a cladding layer which are coaxially arranged, and which, according to 
the take up speed, have an outside diameter of about 100 microns to about 
200 microns and a core diameter of about 20 microns to about 100 microns. 
By selecting a glass with a high refractive index as the core and a glass 
with a low refractive index as the cladding layer at this time, an optical 
glass fiber can be obtained. The resulting optical glass fiber has the 
cross-sectional shape shown schematically in FIG. 5, (a) of the 
accompanying drawings and the stepwise refractive index distribution as 
shown in FIG. 5, (b). In other words, the inside of the core 501 has a 
uniform high refractive index and the clad layer 502 has a uniform low 
refractive index, and a light beam entering one end of the glass fiber is 
transmitted through the core while totally reflecting on the interface 
between the core and the clad. In the present application, optical glass 
fibers having such a stepwise pattern of refractive index distribution are 
termed "step-type optical fibers". 
On the other hand, by selecting glass containing an ion (dopant ion) having 
a high degree of contribution to refractive index, such as a thallium ion, 
as the core glass, and glass containing an ion having a low degree of 
contribution to refractive index, such as an alkali metal ion, as the 
cladding glass, and by exchanging the thallium ion with the alkali metal 
ion through the boundary of the two glasses during melt co-spinning, there 
can be obtained an optical fiber which has the cross-sectional shape shown 
in FIG. 5, (a), and the gradient refractive index distribution shown in 
FIG. 5, (c), whereby in the inside of the core 501, the refractive index 
decreases progressively in the radial direction from the center toward the 
peripheral boundary, and a light beam entering one end of the optical 
fiber is transmitted through the core by refraction. Preferably, but not 
absolutely, the refractive index is continuous as in FIG. 5, (c) at the 
boundary between the core 501 and the clad 502. In the present 
application, optical glass fibers which show such a refractive index 
distribution pattern that the refractive index decreases progressively in 
the radial direction are termed "forcusing-type optical fibers". 
Conventional two-member crucibles are those in which the inside diameter of 
outer nozzle fixed to an outer crucible member is less than several 
millimeters, and the outer nozzle projecting from the lower end of the 
outer crucible member has a length of at most about 20 millimeters. 
Optical fibers requiring a low loss are produced by spinning at a speed of 
not more than several kilometers per hour while maintaining the 
temperature of the crucible at not more than about 1,000.degree. C. In 
recent years, there has been a tremendous advance in optical fibers and 
other optical communication devices, and the last problem which the 
manufacturers desire to solve in putting such optical communication 
systems into commercial practice is the reduction of the cost of producing 
the individual devices. 
The multi-member crucible method is a continuous process ranging from a 
step of melting glass to a step of producing spun fibers, and can 
continuously give optical glass fibers. Hence, it is a superior 
manufacturing process suitable for mass production. The speed of spinning, 
however, is less than one-tenth the speed of drawing of wires for 
conventional communication channels produced from copper as a raw 
material, and the cost of production is higher than in the case of 
producing copper wires. To obtain a drastic reduction in the cost of 
production, it is especially necessary to increase the speed of spinning. 
Generally, two methods are used to increase the speed of spinning in 
accordance with conventional techniques. One involves increasing the 
diameters of nozzles provided at the bottoms of multiple crucible members 
and thereby increasing the amount of molten glass that flows through the 
nozzles. According to this method, however, a deviation or turbulence 
occurs in the flow of the glass melt at the nozzle portion because of the 
influence of the convection of the glass melt within the crucible. This 
consequently tends to cause a variation in the outside diameter of the 
resulting fiber or the diameter of the core, and also in the eccentricity 
of the core. For this reason, the efficiency of connecting fibers at their 
ends is poor, and it is difficult to construct long-distance transmission 
channels from such fibers. 
The other method consists in increasing the temperature of the crucible to 
reduce the viscosity of glass and to increase the amount of molten glass 
flow. According to this method, however, when the temperature of the 
crucible is raised, impurity ions in the material that constitutes the 
crucible diffuse in the glass melt, and the loss of light absorption in 
the fiber increases. It is difficult therefore to build transmission lines 
of low loss. Accordingly, in practice, the conventional techniques cannot 
produce optical fibers of low loss by increasing the speed of spinning. 
The above description is directed mainly to the production of optical 
fibers by the two-member crucible method, but is applicable also to the 
production of optical fibers by using a crucible consisting of three or 
more members. The three-member crucible method is the one in which the 
outermost crucible member is provided outside the two-member crucible, and 
glass having a higher refractive index or light absorbability is fed into 
the outermost crucible member. Thus, an optical fiber of a three-layer 
structure is spun through three nozzles coaxially disposed at the bottom 
of the crucible. 
It is an object of this invention to provide a method for producing by 
high-speed spinning low-loss optical fibers with a greatly reduced 
variation in their outside and core diameters and a greatly reduced 
deviation in the arrangement of their core. 
Another object of this invention is to provide a method for producing by 
high-speed spinning optical glass fibers of low loss while maintaining the 
temperature of the crucible at a relatively low temperature, and therefore 
inhibiting the diffusion of impurities from the material constituting the 
crucible into glass melts. 
Still another object of this invention is to provide an apparatus for 
producing by high-speed spinning low-loss optical glass fibers with a 
greatly reduced variation in their outside and core diameters and a 
greatly reduced deviation in the arrangement of their core. 
Still other objects and advantages of the invention will become apparent 
from the following description. 
The present invention provides an improved method for producing optical 
glass fibers which comprises co-spinning glass melts of different kinds 
through coaxially disposed discharge nozzles of a multi-member crucible 
composed of two or more crucible members having a discharge nozzle at 
their bottom; wherein the outermost nozzle has a length of at least 30 mm 
and is heated so that at least a part of it is maintained at a temperature 
equal to, or higher than, the temperature of the crucible thereby to 
increase the speed of spinning. 
According to this invention, the above method can be performed by using a 
high-speed spinning furnace for optical glass fibers which comprises a 
multi-member crucible composed of two or more crucible members having a 
glass melt discharging nozzle at their bottom, the nozzles being disposed 
coaxially, and means for externally heating the multi-member crucible and 
the discharge nozzles, the outermost nozzle having a length of at least 30 
mm.

The invention is first described with reference to the method of spinning 
using a two-member crucible shown in FIGS. 1 to 3. It should be understood 
that the scope of the invention is not limited by the following 
description, and various modifications and changes obvious to those 
skilled in the art are possible within the scope of the invention. 
FIG. 1 is a cross-sectional view of a high-speed spinning furnace showing 
the principle of performing the present invention by using a method for 
externally heating the crucible indirectly by using an electric furnace. 
In FIG. 1, the crucible consists of an outer crucible member 102 and an 
inner crucible member 107, and is set at a predetermined position within 
the electric furnace. The outer crucible member 102 has as its bottom a 
cylindrical elongated metallic outer nozzle 101 which is heated to a high 
temperature by an electric furnace 103 for nozzle heating. On the other 
hand, the outer crucible member 102 is heated to a high temperature by an 
electric furnace 104 for crucible heating. The electric furnaces 103 and 
104 have refractory materials 106 and 106' and heaters 105 and 105' 
surrounded by the refractories. The heaters 105 and 105' generate heat by 
being electrically controlled so that the temperatures of the outer nozzle 
101 and the outer crucible member 102 are maintained at predetermined 
points. In the two-member crucible, the outer nozzle 10 is an outermost 
nozzle. 
The inner crucible member 107 is disposed within the outer crucible so that 
an inner nozzle 112 provided at the bottom of the inner crucible member is 
located coaxially with the outer nozzle 101. 
When an optical glass fiber is to be spun by the method of this invention 
using such a two-member crucible, a glass rod 108 for a cladding layer of 
the fiber is fed into the outer crucible member 102, and heated in it to 
become a melt 109 which flows down through the outer nozzle 101 and drawn 
from an exit 113 of the nozzle. Likewise, a glass rod 110 for a core of 
the fiber is fed into the inner crucible member 107 and heated in it to 
become a melt 111 which flows through the inner nozzle 112 and out into 
the glass melt 109 within the outer nozzle 102. It forms a laminar flow, 
and moves down through the center of the outer nozzle and drawn from the 
nozzle exit 113. Instead of the glass rods 108 and 110, glass already in 
the molten state may be supplied to the crucible members. 
In the production of step-type optical glass fibers, a glass composition 
having a relatively low refractive index is used as a material for a 
cladding layer, and a glass composition having a higher refractive index 
is selected as a core-forming glass. Glass compositions usually empoloyed 
in the art can be used as materials for the cladding and core in this 
invention. Examples of the cladding glass are silicate glass, borosilicate 
glass, and soda-lime-silicate glass which have a refractive index of 1.49 
to 1.54. Suitable materials for the core glass are silicate glass, 
borosilicate glass, and soda-lime-silicate glass having a refractive index 
of 1.50 to 1.59. 
For the production of focusing-type optical glass fibers, silicate glass 
containing an alkali metal ion having a low degree of contribution to 
refractive index, such as at least one of Li, Na, K, Rb and Cs, for 
example, is used as a material for a cladding layer. As a core-forming 
glass, glass having a metallic ion with a high degree of contribution to 
refractive index, such as a thallium ion, for example, is used. During the 
spinning, the Tl ion in the core glass diffuses into the cladding glass, 
and the alkali metal ion in the cladding glass diffuses into the core 
glass, both at the boundary between the core glass and the cladding glass. 
In the inside of the core of the resulting optical fiber, the Tl ion 
concentration decreases progressively, and the alkali metal ion 
concentration progressively increases, from the center toward the radial 
direction, and because of this concentration distribution, the inside of 
the core has such a refractive index distribution that the refractive 
index decreases progressively from the center toward the radial direction. 
Of the alkali metals, a Cs ion has a greater degree of contribution to 
refractive index than other alkali metal ions. For this reason, a 
focusing-type optical glass fiber can also be produced by using a Cs 
ion-containing glass as a core glass and a glass containing at least one 
of Li, Na, K and Rb as a cladding glass. 
Generally, platinum of high purity is used as a material for the crucible 
and nozzles. Other highly heat-resistant materials, such as 
platinum-iridium alloy, quartz glass, alumina, tungsten and molybdenum can 
also be used. 
When platinum is heated to a temperature of at least about 1200.degree. C., 
traces of impurities in it, especially an iron ion and a copper ion, 
diffuse into glass melts, and as a result, the glass has an increased loss 
of light absorption. Since the glass melts reside for a relatively long 
period of time in the crucible, the inside of the inner crucible member 
107 which accommodates the core glass melt 111 requiring a low loss of 
light absorption, and its vicinity are desirably maintained at a 
temperature sufficiently lower than about 1200.degree. C., preferably at a 
temperature above the melting temperature of the glass fed but below 
1000.degree. C., and more preferably about 850.degree. to about 
950.degree. C. 
Within the outer nozzle 101, the glass melts 109 and 111 flow down as a 
laminar stream. However, it is only the cladding glass melt 109 which 
directly makes contact with the wall surface of the heated platinum outer 
nozzle. Moreover, since the residence times of these glass melts in the 
outer nozzle 101 are relatively short, even when the impurity ions diffuse 
into the cladding glass melt 109, the diffused layer stops at the cladding 
glass melt 109, and does not reach the core-forming glass melt 111. Hence, 
the temperature of the outer nozzle 101 can be made higher than the 
temperature of the crucible, and even if the temperature of the outer 
nozzle exceeds about 1200.degree. C. for example, the low loss of the 
resulting optical glass fiber is not impaired. Thus, the outer nozzle 101 
can be heated and maintained at a temperature which is equal to or higher 
than the temperature of the crucible. Desirably, it is heated and 
maintained at a temperature of at least 25.degree. C., preferably at least 
50.degree. C., especially 75.degree.-150.degree. C., higher than the 
temperature of the crucible. Advantageously, heating is effected such that 
the temperature of the outer surface of the outer nozzle 101 is at least 
about 950.degree. C., preferably at least about 1000.degree. C., more 
preferably about 1050.degree. to about 1150.degree. C. 
It is desirable at this time that substantially the entire outer nozzle in 
its longitudinal direction be heated at the above-specified temperature. 
This is however not essential, and that portion of the outer nozzle which 
is near the outer crucible member and/or the exit or its vicinity of the 
outer nozzle may be at a temperature lower than the specified temperature. 
In short, in the present invention, at least a part of the outer nozzle, 
preferably at least the central portion of the outer nozzle in its 
longitudinal direction, is heated and maintained at a temperature which is 
equal to or higher than the temperature of the crucible. If the portion of 
the outer nozzle which is maintained at the above temperature is too 
small, it is likely that high-speed spinning will not be achieved. 
Desirably, therefore, at least one half, preferably at least two-thirds, 
of the length of the outer nozzle should be maintained at the 
above-specified temperature. 
The term "temperature of the crucible", as used in the present application, 
denotes the surface temperature of the outer wall of the outermost 
crucible member connected to the outermost nozzle, that is the outer 
crucible member designated at 102, 203, 303, 405 and 603 in FIGS. 1 to 4 
and 6, unless otherwise specified. 
The flow rate of glass which flows down through the outer nozzle 101 is 
determined according to the water heat value, the viscosity of the glass, 
and the shape of the nozzle, i.e. the inside diameter and length of the 
nozzle. It is empirically known that the flow rate is inversely 
proportional to the viscosity of the glass and the length of the nozzle, 
and is proportional to the fourth power of the inside diameter of the 
nozzle. 
Generally, at high temperatures at which spinning is performed, the 
viscosity of glass decreases to an order of one-tenth if its temperature 
rises by about 100.degree. C. This means that the flow rate of the glass 
increases to an order of 10 times, and in other words, the spinning speed 
for optical fiber formation increases to an order of 10 times. The heated 
long outer nozzle 101 shown in FIG. 1 serves to decrease the viscosity of 
the melt flowing therethrough and greatly increasing the flow rate of the 
glass melt while maintaining the melt in the form of a laminar flow. The 
flow rate of the glass melt flowing through the nozzle decreases 
proportionally to the length of the nozzle, but this decrease can be made 
up for by slightly increasing the inside diameter of the nozzle. 
If the length of the outer nozzle 101 is too short, it is difficult to heat 
the outer nozzle alone at the desired high temperature without increasing 
the temperature of the crucible to the undesired high temperature. Hence, 
it is recommendable to adjust the length of the outermost nozzle to at 
least 30 mm. There is no strict upper limit to the length of the outermost 
nozzle. However, if its length is too large, the flow rate of a glass melt 
flowing therethrough decreases markedly, and the object of high-speed 
spinning in accordance with the present invention is difficult to achieve. 
Generally, it is desirable for the length of the outermost nozzle not to 
exceed 2,000 mm. Advantageously, the length of the outermost nozzle is 
within the range of 50 to 1500 mm, especially preferably 100 to 1000 mm. 
In the present application, the term "length of a nozzle" denotes the 
distance of the nozzle from its one end fixed to the bottom of a crucible 
member to its forward end along the central axis of the nozzle. 
The inside diameter of the outermost nozzle (outer nozzle 101 in FIG. 1) is 
not critical, and can be varied over a wide range according to the type of 
the material glass, the heating temperature, the desired diameter of the 
glass fiber, etc. It is at least necessary that the glass melts flow 
therethrough in the form of a laminar flow. It is advantageous that the 
inside diameter of the outermost nozzle is generally 3 to 50 mm, 
preferably 4 to 30 mm, especially 5 to 20 mm. For the same reason as 
above, the ratio of the length to diameter of the outermost nozzle is 
chosen from the range of 3 to 100, preferably the range of 5 to 60, and 
more preferably the range of 10 to 50. 
The inner nozzle may be provided such that its exit 114 is opened into the 
inside of the outer nozzle 101 (for example, as shown in FIG. 3) so long 
as it is disposed coaxially with the outer nozzle. Or it may be provided 
such that its exit 114 is opened into the inside of the outer crucible 102 
before reaching the inlet of the outer nozzle 101 (for example, as shown 
in FIG. 1 or 2). In the case of the former, the exit of the inner nozzle 
may be located at the same level as the exit of the outer nozzle. However, 
if the length of the inner nozzle is made too long, various disadvantages 
are caused. For example, when the outer nozzle is heated to a high 
temperature, the inner nozzle is exposed correspondingly to high 
temperatures, and this increases the opportunity of impurities diffusing 
from the material of the inner nozzle into the core-forming glass melt. In 
the production of optical glass fibers having a gradient refractive index 
distribution as shown in FIG. 5, (c), a sufficient time for contact 
between the core-forming glass melt and the cladding glass melt for ion 
exchange will not be obtained. It is desirable generally that the exit of 
the inner nozzle is opened at a position one half the length of the outer 
nozzle or at a position nearer the crucible, preferably at a position 
one-third the length of the outer nozzle or at a possition nearer the 
crucible. It is especially desirable that the exit of the inner nozzle be 
opened at a position near the inlet of the outer nozzle. When the exit of 
the inner nozzle is opened into the inside of the outer crucible, the 
distance between the exit of the inner nozzle and the inlet of the outer 
nozzle is not critical. Advantageously, this distance is adjusted 
generally to not more than 20 mm, preferably not more than 10 mm, and more 
preferably not more than 5 mm. 
The inside diameter of the inner nozzle is not restricted strictly so long 
as it is smaller than the inside diameter of the outer nozzle. The ratio 
of the inside diameter of the outer nozzle to that of the inner nozzle is 
desirably within the range of 10:9 to 10:1, preferably 10:8 to 10:2, and 
10:7 to 10:3. The outside diameter of the inner nozzle needs to be smaller 
than the inside diameter of the outer nozzle especially when the inside 
nozzle is located inside the outer nozzle. Although depending upon the 
thickness of the clad required of the optical glass fiber, the outside 
diameter of the inner nozzle is at least 1 mm, preferably at least 2 mm, 
smaller than the inside diameter of the outer nozzle. 
The glass melt flowing down through the nozzle is taken up at the exit 113 
of the outer nozzle 101. By this take-up, the glass melt is rapidly 
attenuated in the vicinity of the exit 113. Thus, as it well known, the 
diameter of the resulting fiber is determined according to the take-up 
speed. If the take-up speed is low, a glass fiber having a relatively 
large diameter can be obtained. If the take-up speed is higher, an optical 
glass fiber with a smaller diameter can be obtained. 
The optical glass fiber so produced has the cross-sectional shape shown in 
FIG. 5, (a), and the refractive index distribution shown in FIG. 5, (b) or 
(c). 
With reference to FIG. 1, the performance of the present invention has been 
described in regard to a method of heating the crucible and nozzles by an 
electric furnace. The method of heating is not limited to the use of an 
electric furnace, and high frequency induction heating and electrical 
resistance heating are also possible. In FIGS. 2 and 3, these other 
heating methods are described specifically. It should be noted that where 
no description is made, the same description as in FIG. 1 will apply. 
FIG. 2 is a sectional view of a high-speed spinning furnace showing the 
principle of performing the present invention using a method of directly 
heating the crucible and nozzles with high frequency. A cylindrical, 
elongated outer nozzle 201 made of a metal such as platinum generates heat 
by induction heating induced by flowing a high frequency electric current 
through a coil 202 which surrounds the nozzle 201. This induction heating 
method has a high electric efficiency, and makes it relatively easy to 
maintain the temperature of the nozzle at as high as more than about 
1200.degree. C. A platinum outer crucible member 203 is similarly heated 
with a coil 204 by high frequency induction. 
FIG. 3 is a sectional view of a high-speed spinning furnace showing the 
principle of performing the present invention by a method of directly 
heating the crucible members and nozzles using a low-voltage and a large 
current. A cylindrical elongated outer nozzle 301 made of a metal such as 
platinum generates heat when a large current of low voltage is caused to 
flow through it via two electrodes 302 which are located approximately at 
the upper and lower ends of the nozzle 301. This direct resistance heating 
method has a high heat efficiency, easily permits a precise control of 
temperature, and can simply attain high temperatures of at least about 
1200.degree. C. A platinum outer crucible member 303 is similarly heated 
electrically through two electrodes 304. 
The above description has been made with regard to the performance of the 
present invention using a two-member crucible. The present invention can 
be equally applied to the production of optical glass fibers using a 
crucible consisting of two or more members. The invention is further 
described below with reference to the use of a three-member crucible. 
FIG. 4 shows an example of producing a low-loss optical fibers by an 
improved three-member crucible method in accordance with the present 
invention. The three-member crucible consists of an inner crucible 401 
into which a low-loss glass 402 for a core is fed, an intermediate 
crucible 403 into which a cladding low-loss glass 404 is fed and which 
surrounds the inner crucible, and an outer crucible 405 surrounding the 
intermediate crucible and into which a glass 406 for a protective layer is 
fed continuously. The three crucible members are fixed so that the inner 
nozzle 407, the intermediate nozzle 408 and the outer nozzle 409 provided 
at the bottoms of these members are located coaxially. The three-member 
crucible and nozzles are maintained at certain high temperatures by an 
electric furnace 410, and by the melting of the material glasses 402, 404 
and 406 fed from above, the glass melts within the crucible are maintained 
at certain water head values. These glass melts are associated at the 
nozzle portion, and drawn into an optical glass fiber having a three layer 
structure. 
The outer crucible member 405 is supported by a support member 411, and the 
support member 411 and the electric furnace 410 may optionally be fixed to 
a vertically movable stand 412 equipped with a mechanism (not shown) of 
micro-adjustment of position. The inner crucible 401 and the intermediate 
crucible 403 are suspended by a support member (not shown) provided above 
the crucible. Similarly, the material glasses are suspended by feed 
devices (not shown) provided above the crucible. 
The core-forming glass and the cladding glass may be the same as those 
described hereinabove. Examples of the glass which forms the protective 
layer are silicate glass, borosilicate glass and soda-lime-silicate glass. 
Al.sub.2 O.sub.3 and ZnO may be incorporated as glass ingredients for 
improving weatherability. 
The outer nozzle 409 can be heated in the same way as described hereinabove 
with regard to the two-member crucible method. The length and inside 
diameter of the outer nozzle and the ratio of its length to diameter are 
also the same as those described hereinabove with regard to the two-member 
crucible method. A detailed description of these is omitted here. 
The positions of the exits 414 and 415 respectively of the intermediate 
nozzle 408 and the inner nozzle 407 are not critical, and can be chosen 
freely. In an extreme case, the exits 414 and 415 may be at the same level 
as the exit 413 of the outer nozzle 409. But for the same reason as given 
hereinabove, for example to prevent a deviation or turbulence of the flow 
of the glass melt, the exit 414 of the intermediate nozzle 408 is 
desirable located inwardly of the exit 413 of the outer nozzle 409. 
Preferably, the exit 414 is positioned at least one half of the length of 
the outer nozzle 409, more preferably at least one-third thereof, near the 
crucible. The length of the intermediate nozzle is desirably as short as 
possible. Generally, it is 1 to 200 mm, preferably 2 to 100 mm, especially 
3 to 50 mm. On the other hand, the inner nozzle 407 may be projected such 
that its exit 415 is opened into the inside of the outer nozzle 409, or as 
shown in FIG. 4, into the inside of the intermediate nozzle 408. Or in the 
same way as described hereinabove in regard to the two-member crucible, 
the inner nozzle 407 may be provided such that its exit is opened into the 
intermediate crucible 403. When the exit 414 is to be opened into the 
inside of the intermediate nozzle 408, it is advantageous to have the exit 
415 situated at least one half, preferably at least one-third, of the 
length of the intermediate nozzle 408 near the crucible. 
The inside and outside diameters of the intermediate nozzle 408 and the 
inner nozzle 407 are not critical, and can be determined optionally 
according to the characteristics required of the optical glass fibers to 
be produced. Generally, the intermediate nozzle 408 can have an inside 
diameter of 2 to 35 mm, especially 5 to 10 mm, and an outside diameter of 
4 to 40 mm, preferably 7 to 15 mm. Desirably, the inner nozzle 407 has an 
inside diameter of 2 to 20 mm, preferably 3 to 8 mm, and an outside 
diameter of 4 to 25 mm, especially 5 to 10 mm. 
The optical glass fiber so obtained has the cross-sectional shape shown in 
FIG. 5, (d), and the refractive index distribution shown in FIG. 5, (e) or 
(f) (in the drawing, n represents the refractive index, and r is the 
radius of the fiber). 
In conventional three-member crucibles, the lengths of all nozzles are less 
than several millimeters. For this reason, in the production of low-loss 
optical glass fibers, glass melts flow deviatingly or turbulently in the 
vicinity of the nozzle portion. This tends to cause a variation in the 
outside diameter of the optical fibers, a variation in the diameter of the 
core and the diameter of the cladding layer, and an eccentrical 
arrangement of the core and the cladding layer. Consequently, the 
efficiency of connecting the fibers to one another at their ends becomes 
poor, and it is difficult to construct long-distance transmission 
channels. Furthermore, when multi-mode transmission is performed using 
these optical fibers having an insufficient dimensional precision, mode 
conversion tends to occur, and in spite of the three-layer structure, the 
loss increases or the transmission band changes. By selecting the length 
of the outer nozzle in accordance with this invention from the range of 30 
to 2000 mm as shown in FIG. 4, and heating the outer nozzle at a high 
temperature within the above-specified range, it is possible to associate 
the glass melts flowing from the nozzles into a complete laminar flow, and 
the associated flow can be drawn from the exit of the nozzle for the outer 
crucible member to form an optical glass fiber. Hence, irregular sizes of 
fibers by the deviation or turbulent flowing of the melts can be 
prevented, and an optical fiber having a high dimensional precision over 
its large length can be obtained at high speeds. By increasing the length 
of the outer nozzle 409, the inside diameter of the outer nozzle can be 
increased over the conventional method. A nozzle having a large inside 
diameter has good processing precision and its position in coaxial 
arrangement is easy to control. As a result, an optical fiber having a 
small amount of deviation and good cross-sectional circularity can be 
obtained at a high speed. 
According to another aspect of this invention, an optical glass fibers 
having a three-layer structure shown in FIG. 5, (d) can be produced at a 
high speed by using a modified form of spinning furnace which consists of 
the aforesaid two-member crucible and an auxiliary nozzle. The principle 
of this process is shown in FIG. 6. 
In FIG. 6, the crucible is a two-member crucible consisting of an inner 
crucible 601 and a surrounding outer crucible 603 which are so disposed 
that the nozzles provided at the bottoms of these crucible members are 
coaxial. 
A low-loss glass 602 for a core and a low-loss glass 604 for a cladding 
layer are continuously fed into an inner crucible 601 and an outer 
crucible 603, respectively. The inner and outer crucible members are 
suspended by support members (not shown) provided above the crucibles, and 
fixed so that the nozzles for these crucible members are positioned 
coaxially. Likewise, the core-forming glass and the cladding glass are 
suspended from feed devices (not shown) provided above the crucible 
members. The two-member crucible is maintained at a certain high 
temperature by, for example, an electric furnace 605. The two material 
glasses fed change to glass melts 610 and 611 having certain water head 
values. The melt 610 flows down through an inner nozzle 612 provided at 
the bottom of the inner crucible. At the bottom of the outer crucible 
member 603, a long cylindrical outer nozzle 613 is provided coaxially with 
the inner nozzle 612, and heated by an electric furnace 605. 
In the embodiment shown in FIG. 6, the outer surface of the outer nozzle is 
covered with the auxiliary nozzle, but the auxiliary nozzle is just for an 
auxiliary purpose, and the outer nozzle 613 becomes the outermost nozzle. 
The conditions for heating the outermost nozzle and its length and inside 
diameter are chosen from the ranges described hereinabove. 
The melts 610 and 611 then gather coaxially in the form of a laminar flow, 
and go down through the outer nozzle 613. To the outer nozzle 613 having a 
length of more than 30 mm is fixed a cylindrical auxiliary nozzle 606 by 
means of a support stand 608. The auxiliary nozzle 606 is fixed coaxially 
with the outer nozzle 613 by means of a vertically movable stand 609 
equipped with a mechanism (not shown) for micron-adjustment of position. A 
material glass 607 for a protective layer is heated by the electric 
furnace 605, and changes to a melt 614 which is then fed continuously into 
the auxiliary nozzle 606 from an opening provided at the upper part of the 
auxiliary nozzle so that it is maintained at a certain water head value 
within the auxiliary nozzle. The melt 614 for the protective layer is 
associated in the form of a laminar flow with the glass melts for the core 
and the cladding layer, and drawn from an exit at the lower end of the 
auxiliary nozzle 606 to form an optical fiber having a three-layered 
structure at a high spinning speed. 
The present invention makes it possible to produce low-loss optical fibers 
of a multi-component glass at high spinning speeds reaching several tens 
of kilometer per hour by using a multi-member crucible having a long 
cylindrical nozzle of a specified length at the bottom of the outermost 
crucible member and heating the long nozzle to increase the flow rate of 
glass. This method can be performed continuously over long periods of time 
by continuously supplying material glasses or melts of the material 
glasses from the top of the crucible. By using one spinning apparatus, an 
optical fiber can be produced which measures several hundred thousand 
kilometers per year. The optical fibers produced by the present invention 
can have an exact reproduction of the low loss of the material glass, 
because the core and the neighboring glass can avoid contamination by 
impurity ions from the material that makes up the nozzles. Since the glass 
melt which flows down through a long outer most nozzle forms a laminar 
stream, the outside diameter of the resulting optical fiber, the diameter 
of the core, and the concentricity of the core and the outside diameter of 
the fiber have a high dimensional precision. If the present invention is 
applied to the production of focusing-type multi-component optical fibers 
for example, having refractive indices shown in FIG. 5, (c) and (f)!, a 
dopant ion in the core glass can be exchanged with the ion in the cladding 
glass over a sufficient period of time by utilizing a long nozzle. 
Accordingly, broad-band optical fibers having a large core diameter can be 
obtained. Focusing-type optical fibers having a large core diameter easily 
permit incoming of light beams or can be easily connected to one another, 
and have superior practical applicability. 
In the present invention, the nozzles and the crucibles may be heated by 
the same or different methods. Furthermore, it is not altogether necessary 
that the inside diameter of the nozzle be the same along the entire length 
of the nozzle. The nozzle may be of a type whose inside diameter decreases 
progressively. On the other hand, the present invention can be applied not 
only to the production of optical fibers, but also to the production of 
pre-forms for drawing optical fibers. 
The present invention can also give optical fibers of a three-layered 
structure which have stable multi-mode transmission characteristics and a 
high dimensional precision. When the outermost nozzle has a length of more 
than 30 mm, a variation in the outside diameter of the optical fibers can 
be limited to 1 micron or less. Furthermore, since the outer nozzle is 
long, its outside and inside diameters can be increased, and processing 
precision and concentricity can be increased. The use of a pure platinum 
crucible with the outer nozzle having an outside diameter of at least 10 
mm can give optical fibers having a circularity of more than 99% and a 
core deviation of less than 1 micron. Accordingly, the optical fibers so 
produced can be formed into a long, low-loss light transmission channel by 
a simple method. The optical fibers of a three-layered structure produced 
by the present invention can permit a drastic decrease in the amount of a 
cladding material glass which requires low loss. For example, an optical 
fiber of a multi-component glass having a core diameter of 60 .mu.m and an 
outside diameter of 150 .mu.m, if it is of a two-layer structure, requires 
about 37 kg of the cladding glass per 1000 km. But if the structure is 
changed to a three-layer structure and the outside diameter of the 
cladding layer is decreased to 100 .mu.m, the amount of the cladding glass 
required decreases to 13 kg per 1000 km. The cost of producing optical 
fibers of a multi-component glass is dominated by the cost of the material 
glasses if the amount of the optical fibers produced increases. Hence, the 
reduction of the amount of the low-loss glass has a great effect on the 
reduction of the cost of producing optical fibers. The use of a material 
having good water resistance, alkali resistance and acid resistance for a 
protective glass layer which constitutes the outermost layer of the 
three-layer structure is preferred since it makes possible the production 
of optical fibers having good weatherability. Furthermore, by making the 
coefficient of expansion of the glass in the protective layer lower than 
that of the cladding glass, a compression strain is formed in the 
protective layer, and the mechanical strength of the optical fibers can be 
increased. 
If the method of the present invention is applied to the production of 
focusing-type three-layered optical fibers utilizing the diffusion of a 
dopant ion by ion exchange, the nozzle for the intermediate crucible 
member (in the case of a three-member crucible) or the nozzle for the 
outer crucible member (in the case of a two-member crucible) is made 
sufficiently long so as to secure a diffusion time sufficient for forming 
the desired distribution of refractive index when the melts of the core 
glass and the cladding glass gather coaxially and are flowing down. As a 
result, optical glass fibers having favorable distributions of refractive 
index can be provided. 
The optical glass fibers provided by the present invention are used, for 
example, for communication light transmitting channels, and light 
transmission lines for processing or controlling information or signals. 
The following Examples illustrate the present invention in greater detail. 
EXAMPLE 1 
Using a spinning furnace adapted to be heated indirectly as shown in FIG. 
1, a step-type optical fiber was spun from a soda-borosilicate glass 
having a viscosity of about 1000 pieses at 970.degree. C. Specifically, 
glass consisting of 55% by weight of SiO.sub.2, 20% by weight of B.sub.2 
O.sub.3, 19% by weight of Na.sub.2 O and 6% by weight of CaO and having a 
refractive index of 1.533 was used as a material for the core, and glass 
consisting of 67% by weight of SiO.sub.2, 11% by weight of B.sub.2 O.sub.3 
and 22% by weight of Na.sub.2 O and having a refractive index of 1.513 was 
used as a material for the cladding layer. The spinning furnace included 
an outer nozzle 101 having a length of 50 mm and an inside diameter of 8 
mm, and an inner nozzle having an outside diameter of 7 mm, an inside 
diameter of 6 mm and a length of 25 mm with the tip of the inner nozzle 
located 3 mm above the inlet opening of the outer nozzle. The temperature 
of the crucible was maintained at 970.degree. C., the temperature of the 
central portion of the outer nozzle at 1050.degree. C., and the 
temperature of its lower end at 1000.degree. C. An optical fiber having an 
outside diameter of 150 microns and a core diameter of 100 microns could 
be obtained at a spinning speed of 15 km per hour under these conditions. 
The above procedure was repeated except that an inner nozzle having an 
outside diameter of 9 mm and an inside diameter of 8 mm with the other 
specifications being the same as described above and an outer nozzle 
having a length of 100 mm and an inside diameter of 10 mm were used, and 
the temperature of the crucible portion, the temperature of the center of 
the outer nozzle, and the temperature of the forward end of the outer 
nozzle were maintained respectively at 970.degree. C., 1100.degree. C., 
and 1060.degree. C., an optical glass fiber having an outside diameter of 
150 microns and a core diameter of 100 microns could be obtained at a 
spinning speed of 35 km per hour. Low loss values were obtained even when 
the inner nozzle 112 was replaced by one having an inside diameter of 3 to 
7 mm and the core diameter was changed within the range of 20 microns to 
120 microns. The fluctuations of the outside diameter and the core 
diameter were less than 1%, and the amount of eccentricity between the 
outside diameter and the core was less than 1 micron. 
EXAMPLE 2 
Using a spinning furnace adapted to be heated indirectly as shown in FIG. 
1, a focusing-type optical fiber was spun from a soda-borosilicate base 
glass having a viscosity of about 1000 poises at 950.degree. C. A glass 
composition for a core had a refractive index of 1.533 and consisted of 
55% by weight of SiO.sub.2, 20% by weight of B.sub.2 O.sub.3, 18% by 
weight of Na.sub.2 O and 7% by weight of Ti.sub.2 O, and a glass 
composition for a cladding layer had a refractive index of 1.513 and 
consisted of 65% by weight of SiO.sub.2, 12% by weight of B.sub.2 O.sub.3 
and 23% by weight of Na.sub.2 O. An outer nozzle having a length of 500 mm 
and an inside diameter of 10 mm, and an inner nozzle having an outside 
diameter of 8 mm and an inner diameter of 6 mm were used. The forward end 
of the inner nozzle was located 3 mm above the inlet of the outer nozzle. 
The temperature of the crucible was 950.degree. C. The temperature of the 
central portion of the outer nozzle was maintained at 1080.degree. C., and 
its lower end, at 1030.degree. C. An optical fiber having an outside 
diameter of 150 microns could be obtained at a spinning speed of 20 km per 
hour under these conditions. The core diameter of the fiber was 50 
microns. The distribution of the refractive index of the inside of the 
core was parabolic, and a band of 1000 megahertz per kilometer was 
obtained. 
An optical fiber having a core diameter of 75 microns which was obtained by 
increasing the head of the core-forming glass melt in the crucible for the 
core thereby to increase the flow rate of the core-forming glass had a 
band of 400 megahertz, and exhibited a low loss and a good dimensional 
precision. 
EXAMPLE 3 
Using a direct heating method using high frequency as shown in FIG. 2, a 
step-type optical fiber was spun from a soda-lime-silicate glass having a 
viscosity of about 1000 poises at 1050.degree. C. A glass composition for 
a core had a refractive index of 1.546 and consisted of 66% by weight of 
SiO.sub.2, 15% by weight of Na.sub.2 O and 19% by weight of CaO, and a 
glass composition for a cladding layer had a refractive index of 1.527 and 
consisted of 68% by weight of SiO.sub.2, 17% by weight of Na.sub.2 O and 
15% by weight of CaO. An outer nozzle having a length of 200 mm and an 
inside diameter of 10 mm was used. The inner nozzle used had the same size 
and location as that used in Example 2. The crucible was maintained at a 
temperature of 1050.degree. C., and a high frequency of 10 kilocycles at 5 
kilowatts was applied to the outer nozzle to maintain the central portion 
of the outer nozzle at 1230.degree. C., and its lower end at 1120.degree. 
C. An optical fiber having a core diameter of 60 microns, an outside 
diameter of 150 microns, a good dimensional precision and a low loss could 
be obtained under these conditions at a spinning speed of 45 km per hour 
over long periods of time. 
EXAMPLE 4 
Using a direct resistance heating method as shown in FIG. 3, a step-type 
optical fiber was spun from a soda-lime-silicate glass having a viscosity 
of about 1000 poises at 1080.degree. C. A glass composition for a core had 
a refractive index of 1.545 and consisted of 68% by weight of SiO.sub.2, 
14% by weight of Na.sub.2 O and 18% by weight of Ca. A glass composition 
for a cladding layer had a refractive index of 1.525 and consisted of 70% 
by weight of SiO.sub.2, 16% by weight of Na.sub.2 O and 14% by weight of 
CaO. 
An alternate current of 2 volts and 300 amperes was passed through an outer 
nozzle having a length of 1000 mm and an inside diameter of 20 mm, and the 
temperature of the central portion of the nozzle was maintained at 
1250.degree. C., and the temperature of its lower end at 1130.degree. C. 
The crucible was maintained at 1080.degree. C. The inner nozzle used had 
an outside diameter of 16 mm and an inside diameter of 14 mm with its 
forward end being at the same height as the inlet of the outer nozzle. An 
optical fiber having an outside diameter of 150 microns and a core 
diameter of 60 microns could be obtained at a spinning speed of 60 km per 
hour under these conditions. When the temperature of the central portion 
of the nozzle was lowered to 1080.degree. C., and the temperature of its 
lower end to 1000.degree. C., and the spinning speed was set at 1 meter 
per hour, performs having an outside diameter of 10 mm and a core diameter 
of 4 mm could be produced. 
EXAMPLE 5 
Using a three-member crucible equipped at its bottom with an outer nozzle 
having an inside diameter of 20 mm and a length of 200 mm, a step-type 
optical fiber consisting of a core 103, a cladding layer 104 and an 
outermost layer 105 as shown in FIG. 5, (d) was spun from a 
soda-lime-silicate glass having a viscosity of about 1000 poises at a 
temperature of 1020.degree. C. by the method based on the principle shown 
in FIG. 4. A glass composition for a core had a refractive index of 1.549 
and consisted of 64% by weight of SiO.sub.2, 16% by weight of Na.sub.2 O 
and 20% by weight of CaO. A glass composition for a cladding layer had a 
refractive index of 1.530 and consisted of 66% by weight of SiO.sub.2, 18% 
by weight of Na.sub.2 O and 16% by weight of CaO. A glass composition for 
an outermost layer had a refractive index of 1.535, and consisted of 66% 
by weight of SiO.sub.2, 20% by weight of Na.sub.2 O, 10% by weight of CaO 
and 4% by weight of BaO. 
The crucible also included an inner nozzle 407 having an outside diameter 
of 10 mm, an inside diameter of 6 mm and a length of 50 mm with its 
forward end being positioned 20 mm below the fixing part of the outer 
nozzle, and an intermediate nozzle 408 having an outside diameter of 16 
mm, an inside diameter of 14 mm and a length of 100 mm with its forward 
end being positioned 80 mm below the fixing part of the outer nozzle. 
A fiber spun at a speed of about 30 km per hour while maintaining the 
temperature of the crucible portion at 1020.degree. C. the temperature of 
the control portion of the nozzle at 1050.degree. C., and the temperature 
of its lower end at 1030.degree. C. could retain a high dimensional 
precision, represented by a fiber outside diameter of 150 .mu.m .+-. 1 
.mu.m, a clad outside diameter of 120 .mu.m .+-. 1 .mu.m, a core outside 
diameter of 80 .mu.m .+-. 1 .mu.m, and a core eccentricity of less than 1 
.mu.m, over its large length. The distribution of its refractive index had 
the configuration shown in FIG. 5, (e). The light which was transmitted 
through the fiber over a distance of 5 km showed a steady distribution of 
mode, and had a number of apertures of 0.20. When light beams fall upon 
the optical fiber at this aperture number, the loss of light is less than 
3 dB/km at a wavelength of 0.80 .mu.m to 0.85 .mu.m, and a band of 50 
megahertz.km was obtained. When the optical fibers were coated with 
plastics and bundled to make an optical cable, variations in loss and band 
in the cable were negligibly small. 
EXAMPLE 6 
Using a three-member crucible equipped at the bottom of the outermost 
crucible member with an outer nozzle having an inside diameter of 25 mm 
and a length of 1000 mm, a focusing-type optical fiber was spun from a 
soda-borosilicate glass having a viscosity of about 1000 poises at a 
temperature of 950.degree. C. 
Glass compositions for a core and a cladding layer were the same as those 
used in Example 2. A glass composition for an outermost layer had a 
refractive index of 1.518 and consisted of 65% by weight of SiO.sub.2, 
10% by weight of B.sub.2 O.sub.3, 20% by weight of Na.sub.2 O and 5% by 
weight of ZnO. 
The crucible also included an inner nozzle having an outside diameter of 12 
mm, an inside diameter of 10 mm and a length of 130 mm with its forward 
end being positioned 100 mm below the fixing part of the outer nozzle, and 
an intermediate nozzle having an outside diameter of 20 mm, an inside 
diameter of 16 mm and a length of 200 mm with its forward end being 
positioned 180 mm below the fixing part of the outer nozzle. 
The ion exchange distance over which the core glass and the cladding glass 
flowed down in contact with each other within the nozzle for the outermost 
crucible member was adjusted to 900 mm, and the temperature of the center 
of the outer nozzle was maintained at 950.degree. C., and the temperature 
of its lower end at 900.degree. C. The crucible temperature was 
950.degree. C. Under these conditions, a fiber was produced at a spinning 
speed of about 15 km per hour. The resulting fiber had a fiber outside 
diameter of 150 .mu.m, a cladding outside diameter of 100 .mu.m and a core 
outside diameter of 60 .mu.m, and a distribution of refractive index 
having the configuration shown in FIG. 5, (f). The dimensional precision 
of the fiber was high over its large length. An optical cable produced 
from the resulting optical fibers could give a low loss of less than 5 
dB/km and a broad band of at least 1 gigahertz km in multimode 
transmission when a semiconductor laser was used as a light source. 
EXAMPLE 7 
Using the apparatus shown in FIG. 6 which included an outer nozzle 613 
having an outside diameter of 10 mm, an inside diameter of 6 mm and a 
length of 100 mm, a steptype optical fiber having a three layer structure 
was spun from a soda-lime-silicate glass having a viscosity of about 1000 
poises at a temperature of 1020.degree. C. The glass compositions used 
were the same as in Example 5. The apparatus also included an inner nozzle 
having an outside diameter of 4 mm, an inside diameter of 3 mm and a 
length of 3 mm with its forward end being positioned 2 mm below the fixing 
part of the outer nozzle, and an auxiliary nozzle having an outside 
diameter of 20 mm, an inside diameter of 16 mm and a length of 40 mm with 
its forward end being on the same level as the forward end of the outer 
nozzle. 
While maintaining the temperature of the center of the outer crucible 
member at 1020.degree. C., the temperature of the auxiliary nozzle 606 and 
the central part of the outer nozzle at 1050.degree. C., and the 
temperature of the forward end of the outer nozzle at 1000.degree. C., an 
optical fiber was obtained at a speed of about 20 km per hour. The fiber 
could retain a high dimensional precision, represented by a fiber outside 
diameter of 150 .mu.m .+-. 1 .mu.m, a cladding outside diameter of 120 
.mu.m .+-. 1 .mu.m, a core outside diameter of 80 .mu.m .+-. 1 .mu.m, and 
a core eccentricity of less than 1 .mu.m, over its large length. The fiber 
had a distribution of refractive index shown in FIG. 5, (e). Light beams 
which had been transmitted through the fiber over a distance of 5 km 
showed a steady mode distribution, and had a number of apertures of 0.20. 
When multimode transmission was performed by passing light beams having 
this aperture number, the light loss was less than 3 dB per kilometer at a 
wavelength of 0.80 .mu.m to 0.85 .mu.m, and a band of 50 megahertz.km was 
obtained. When the fiber was coated with a plastic material, and coated 
optical fibers were formed into an optical cable, variations in loss and 
band in the cable were negligibly small. 
EXAMPLE 8 
A two-member crucible of the type shown in FIG. 6 was used which had an 
outer nozzle 613 having an outside diameter of 22 mm, an inside diameter 
of 18 mm and a length of 1000 mm at the bottom of outer crucible member 
603 and an inner nozzle having an outside diameter of 12 mm, an inside 
diameter of 10 mm and a length of 100 mm with its forward end being 
positioned 900 mm above the forward end of the outer nozzle. An auxiliary 
nozzle 606 having an outside diameter of 30 mm, an inside diameter of 26 
mm and a length of 100 mm with its forward end being of the same level as 
the forward end of the outer nozzle was provided at the exit end of the 
outer nozzle. A focusing-type optical fiber was spun from a 
soda-borosilicate glass having a viscosity of about 1000 poises at a 
temperature of 950.degree. C. in the spinning furnace shown in FIG. 6. 
The glass compositions were the same as in Example 6. 
The ion-exchange distance over which the coreforming glass and the cladding 
glass flowed down in contact with each other within the outer nozzle 613 
was adjusted to 900 mm, and the temperature of the center of the nozzle 
was maintained at 950.degree. C. and the temperature of its lower end at 
900.degree. C. The crucible was maintained at 950.degree. C. An optical 
fiber was produced under these conditions at a spinning speed of about 10 
km per hour. The fiber obtained had an outside diameter of 150 .mu.m, a 
cladding outside diameter of 100 .mu.m and a core outside diameter of 60 
.mu.m, and showed a refractive index distribution of the pattern shown in 
FIG. 5, (f). The dimensional precision of the fiber was high along its 
large length. With an optical cable produced from the resulting optical 
fibers, a low loss of less than 5 dB/km and a broad band of about 1 
gigahertz.km could be obtained in multimode transmission using a 
semiconductor laser as a light source.