Ultralow-loss silica glass and optical fibers using the same

This ultralow-loss glass is characterized in that high purity silica glass contains 1 to 500 wt.ppm of at least one network modifying oxide. It is assumed that the network modifying oxide appropriately loosens the tetrahedral network structure of silica and hence Rayleigh scattering is decreased. Examples of the network modifying oxide include Na.sub.2 O, K.sub.2 O, Li.sub.2 O, MgO, CaO, and PbO. Since Rayleigh scattering losses are minimal in comparison with those of high purity silica glass, this impurity-added silica glass is excellent as a base material of a glass fiber for a long-distance transmission.

TECHNICAL FIELD 
This invention relates to impurity-added silica glass from which 
ultralow-loss optical fibers, etc. can be produced, and optical fibers 
produced by using this impurity-added silica glass. 
BACKGROUND ART 
In general, followings can be listed as factors of light transmission loss 
of optical fibers. 
(1) Loss inherent in materials constituting optical fibers, such as 
Rayleigh scattering and infrared absorption loss. 
(2) Scaterring loss due to fiber structure imperfection and glass flaws, 
such as scattering by irregularities of the interface of a core and a 
cladding, strias and bubbles. 
(3) Absorption loss by impurities remained in fibers, such as absorption by 
iron and other transition metals, absorption by intermolecular vibration 
of hydroxyl group. 
Silica optical fibers, which are currently in wide practical use, are free 
from losses caused by external factors mentioned in (2) and (3). When high 
purity silica glass is used as a core, it becomes possible to produce 
fibers with losses close to the theoretical limit of 0.15 dB/km. However, 
since the age of internet and other multimedia is coming, it is required 
to develop fibers with losses less than those of silica fibers, in order 
to reduce communication costs by increasing the repeater span and to 
enlarge communications networks, and studies save been constinued both 
inside and outside Japan. 
In the stage of developing materials, it has been especially demanded to 
decrease Rayleigh scattering, which is the main cause of loss, and efforts 
have been made to find multi-component glass with minimal Rayleigh 
scattering. For example, Japanese Unexamined Patent Publication (KOKAI) 
No.105483/1993 discloses a large number of multi-component glasses which 
can lessen density fluctuations, which are the main cause of Rayleigh 
scattering. It has been expected that multi-component glasses can realize 
superior fibers than silica fibers, but, at present, none of the 
multi-component glasses are practically used as fibers for long distance 
communication. 
The multi-component glasses have following disadvantages: 
(1) Density fluctuations are increased and hence, light scattering losses 
are increased, in comparison with one-component glass. 
(2) Degree of crystallinity is high and microcrystallization in spinning 
fibers is hard to be controlled, and therefore light transmittance is 
degraded in comparison with one-component glass. 
(3) It is difficult to control impurities which absorb light with 
wavelengths used currently for communications, such as hydroxyl group and 
transition metals, and transmission losses are increased. 
It is an object of the present invention to obviate the disadvantages of 
multi-component glasses and produce glass materials which can be produced 
into optical fibers with minimal losses, which cannot be realized by 
optical fibers formed of the conventional silica glasses, especially 
optical fibers which have superior Rayleigh scattering loss 
characteristics. 
DISCLOSURE OF THE INVENTION 
The present inventors have found that addition of a very small amount of 
Na.sub.2 O to silica glass decreases Rayleigh scattering losses and have 
completed the present invention. 
The ultralow-loss glass of the present invention is characterized in that 
silica glass contains at least one network modifying oxide, such as 
Na.sub.2 O, in an amount of 1 to 500 wt.ppm. It is assumed that the 
network modifying oxide appropriately loosens the tetrahedral network 
structure of silica and hence that Rayleigh scattering losses are 
decreased. 
The ultralow-loss glass of the present invention comprises silica glass and 
at least one network modifying oxide which is uniformly dispersed in the 
silica glass on the atom order. High purity silica glass is used as the 
silica glass. 
The network modifying oxide added to silica glass is in an amount of 1 to 
500 wt.ppm. The modifying oxide thus added in a very small amount should 
be regarded as a very small amount of impurities in one-component silica 
glass rather than as a constitutional component of multi-component glass. 
The ultralow-loss glass of the present invention can obviate the 
disadvantages of the conventional multi-component glass. 
Examples of the above network modifying oxide include Na.sub.2 O, K.sub.2 
O, Li.sub.2 O, MgO, CaO, and PbO, and at least one of them is selected. 
This glass has minimal Rayleigh scattering in comparison with high purity 
silica glass, and when used as optical fiber materials, this glass can 
offer optical fibers with losses less than those of the conventional 
silica optical fibers. 
As the cause of a decrease in Rayleigh scattering, following two are 
assumed. 
(1) The glass transition temperature is lowered, and as a result, scattered 
light intensity, which is considered to be proportional to the glass 
transition temperature, is also decreased. 
(2) Owing to diffusion of the modifying oxide, freezed density 
fluctuations, which are the cause of light scattering, are promoted to be 
lessened, and consequently, scattered light intensity is decreased. 
When the content of the network modifying oxide is less than 1 wt. ppm, the 
decrease in Rayleigh scattering is hardly observed. When the content 
exceeds 500 wt.ppm, the disadvantages of the multi-component glass cannot 
be obviated and hence low-loss optical fibers cannot be realized. 
The ultra-low loss glass of the present invention can be used as a core, or 
as a core and a cladding of an optical fiber. 
In the ultra-low loss glass of the present invention, the tetrahedral 
structure of silica glass is loosened by the network modifying oxide and 
Rayleigh scattering is reduced. Therefore, the ultralow-loss glass of the 
present invention can stably transmit light for a longer distance, and 
when the ultralow-loww glass of the present invention is used as optical 
fibers, the repeater span can be increased.

BEST MODE FOR CARRYING OUT THE INVENTION 
Hereinafter, the present invention will be concretely discussed by way of 
examples, but this invention should not be limited to these examples. 
EXAMPLE 1 
Na ions were implanted into a high purity silica glass specimen containing 
0.01 ppm or less metal impurities (Al, Ca, Cu, Fe, Na, K, Li, Mg, Mn, and 
Ti) by an ion implantation apparatus, to prepare a specimen of this 
example. The specimen dimensions were 20.times.10.times.1 mm.sup.3 and the 
amount of Na ions implanted was 4.2.times.10.sup.17 cm.sup.-2. As a 
result, the concentration of Na.sub.2 O in the silica glass became 50 ppm. 
After that, the Na ion-implanted specimen was subjected to a diffusion 
treatment of heating at 600.degree. C. for 24 hours. Thus, ultralow-loss 
glass of this example was obtained. 
Scattered light intensity of the ultralow-loss glass of this example at a 
scattering angle of 90.degree. was measured by employing an argon laser of 
488 nm. The scattered light intensity at room temperature is shown in FIG. 
1 and Table 1. 
In FIG. 1, the axis of ordinate shows scattered light intensity and the 
axis of abscissa shows the amount of Na.sub.2 O added. 
COMATIVE EXAMPLE 1 
High purity silica glass containing 0.01 ppm or less metal impurities (Al, 
Ca, Cu, Fe, Na, K, Li, Mg, Mn, and Ti) was used, as it was, as a specimen 
of Comparative Example. Scattered light intensity of the specimen of 
Comparative Example 1 was measured in the same way as in Example 1. The 
scattered light intensity at room temperature is also shown in FIG. 1 and 
Table 1. 
EXAMPLES 2 TO 8 
Na ions were implanted into high purity silica glass specimens containing 
0.01 ppm or less metal impurities (Al, Ca, Cu, Fe, Na, K, Li, Mg, Mn, Ti) 
in the same way as in Example 1, and the respective specimens with 
Na.sub.2 O concentrations of 40, 30, 20, 15, 10, 5, 1 ppm were thus 
prepared. These specimens were subjected to a diffusion treatment of 
heating at 600.degree. C. for 24 hours in the same way as in Example 1, 
and ultralow-loss glasses of Examples 2 to 8 were thus obtained. 
Scattered light intensity of the ultralow-loss glasses of Examples 2 to 8 
at a scattering angle of 90.degree. were measured by using the argon laser 
of 488 nm. The scattered light intensity of these examples at room 
temperature are also shown in FIG. 1 and Table 1. 
TABLE 1 
______________________________________ 
Na.sub.2 O SCATTERED LIGHT 
CONCENTRATION 
INTENSITY 
(wt. ppm) (arbitrary intensity) 
______________________________________ 
Ex. 1 50 25,500 
Ex. 2 40 25,800 
Ex. 3 30 26,000 
Ex. 4 20 26,400 
Ex. 5 15 26,500 
Ex. 6 10 27,300 
Ex. 7 5 27,900 
Ex. 8 1 30,000 
Com. Ex. 1 &lt;0.01 30,500 
______________________________________ 
As apparent from FIG. 1 and Table 1, by adding 1 wt.ppm Na.sub.2 O, the 
scattered light intensity was decreased from 30,500(A.U.) to 30,000(A.U.), 
and by adding Na.sub.2 O in amounts of 5 wt.ppm, 10 wt.ppm, 15 wt.ppm, 20 
wt.ppm, 30 wt.ppm, 40 wt.ppm, 50 wt.ppm, respectively, the scattered light 
intensity was further decreased to 27,900(A.U.), 27,300(A.U.), 
26,500(A.U.), 26,400(A.U.), 26,000(A.U.), 25,800(A.U. ), and 25,500(A.U.). 
Possibility of Industrial Utilization 
As discussed above, since light scattering, which remarkably increases 
transmission losses, is minimal, the ultralow-loss silica glass of the 
present invention is superior as a base material of a glass fiber for 
long-distance transmission. In addition, since the present inventive glass 
can be produced only by adding a very small amount of at least one 
modifying oxide to silica glass, the present production line in the 
vapor-phase axial deposition method (the soot method) of producing silica 
glass preforms can be utilized only by being changed slightly. This is a 
big advantage of the present invention.