Fusion splicing method for optical fibers

A fusion splicing method for optical fibers in which optical fibers are fusion spliced in an inert gas atmosphere after water adsorbed on their surfaces is removed by decomposition in a plasma of an inert gas containing a halogen.

BACKGROUND OF THE INVENTION 
The present invention relates to a fusion splicing method for optical 
fibers. 
The transmission loss of a quartz glass fiber has now been reduced as low 
as 0.2 dB/km (substantially equal to its theoretical value) at a 1.55 
.mu.m wavelength; this has made possible transmission without any repeater 
over a distance of 100 km. Furthermore, it is predicted, theoretically, 
that the transmission loss of a fluoride glass fiber of ZrF.sub.4, 
HfF.sub.4 series will be 0.01 dB/km or less in a 2 to 4 .mu.m wavelength 
band, and it is now drawing attention as an optical fiber of the next 
generation. 
With such a low transmission loss of the filter as mentioned above, an 
optical signal could be transmitted over several thousand kilometers 
without regenerative or amplifying repeating of the signal. 
At present, however, the length of a one-piece fiber which can be produced 
even by the most advanced quartz fiber manufacturing technique is in the 
order of 100 km at best, and techniques capable of fabricating a fiber 
1000 km or more in length have not yet been established. In addition, a 
fiber which has sufficient required strength in its entire length for 
incorporation into a cable is 10 to 20 km at the longest, so that in even 
a system of a 50 km or so transmission distance is forced to have at least 
several joints of optical fibers. Accordingly, as the transmission loss of 
the optical fiber is reduced, an optical fiber splicing technique for low 
loss and high strength splicing acquires a greater importance. 
The most common method that is now employed for splicing optical fibers of 
quartz system is a fusion splicing method, in which two fibers to be 
spliced are butted tightly at one end and the butted end portions are 
fused together. With this method, the splice loss is low and very small in 
aging with time; hence this method has been one of factors in the 
development of optical communications so far. However, this method 
possesses a defect such that the strength of the joint decreases to 1/3 to 
1/5 the strength of fiber strands. This is ascribed to the fact that water 
adsorbed on the fiber surface causes the formation of crystallites during 
the fusion splicing process. 
Moreover, it is considered that fusion splicing of fluoride glass fibers is 
difficult because when heated in the air, fluoride glass crystallizes 
before it softens. 
Factors contributing to the crystallization of the fluoride glass fall into 
the basic one in which the glass crystallizes by its own instability and 
the external one in which oxygen or water vapor contained in the 
atmosphere, or water adsorbed on the glass surface reacts with the glass 
to form crystal nuclei which grow into crystals. 
With respect to the former, our studies on the viscosity temperature 
characteristics of glass of various compositions and the crystallization 
temperatures thereof have revealed that short-time heating in an inert gas 
atmosphere would not cause the crystallization as long as the composition 
of the glass used falls within the range in which fibers can be produced; 
this means that the fusion splicing of the fluoride glass fibers has no 
inherent disadvantage. 
Therefore, it is necessary only to completely remove impurities such as 
water adsorbed on the glass surface, which forms the external cause of the 
crystallization, and to perform the fusion splicing in an inert gas 
atmosphere. 
Also in the case of the quartz glass fiber, the removal of water adsorbed 
on the glass surface would suppress the formation of crystallites, 
permitting high-strength fusion splicing. 
At present, heat treatment of the glass in a dry atmosphere or in a vacuum 
is the most effective for the removal of the impurities such as water 
adsorbed on the glass surface. However, this method cannot be applied to a 
material which reacts with water or like impurities and crystallizes even 
at low temperatures. Further, even the quartz glass fiber which is stable 
thermally is impossible of sufficient dehydration because heating of its 
butted portion to a temperature for dehydration will degrade its resin 
coating. Thus no effective means is available, at present, for completely 
removing the adsorbed water and like impurities from the glass fiber of 
its butted portion to obtain a clean glass surface. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a high-strength, low-loss 
fusion splicing method for quarts and fluoride glass fibers. The fusion 
splicing method of the present invention is characterized in that 
impurities such as water adsorbed on the fiber surfaces near the butted 
portion of the fibers to be connected are removed through use of a plasma 
of an inert gas containing a halogen, after which their fusion splicing is 
performed by an arc discharge, or through use of infrared light such as 
CO.sub.2 laser light, in an atomsphere of an inert gas such as argon, 
helium or the like. 
By treating the fiber surfaces in a plasma containing a halogen, as 
mentioned above, the halogen in its nascent state reacts, at a range of 
low temperature, with impurities on the fiber surfaces and is decomposed 
into materials which have a small energy of adsorption on glass, such as 
hydrogen halide or oxygen; accordingly, adsorbed water and other 
impurities on the fiber surfaces can easily be removed at a range of low 
temperature, permitting the fusion splicing of the fluoride glass fibers 
which has been difficult in conventional art. Moreover, high-strength 
fusion splicing of the quartz fibers also becomes possible. 
The principles of the present invention will be described first with 
reference to the reaction of the fluoride glass with water on the glass 
surface. The reaction of the fluoride glass with water basically proceeds 
in accordance with the following reaction formula (1l ): 
EQU (--M--F)+H.sub.2 O.fwdarw.(--M--OH)+HF (1) 
where M indicates a metallic element forming the glass. Further,--in the 
above formula means a bond; for example, when M is zirconium (Zr), the 
formula (1) becomes as follows: 
EQU ZrF.sub.4 +H.sub.2 O.fwdarw.ZrF.sub.3 OH+HF (2) 
This reaction does not occur at room temperature but proceeds as 
temperature rises. 
Therefore, as the water on the fiber surface is heated, the metal M forming 
the fluoride glass becomes a hydroxide, which forms a nucleus for 
crystallization. 
On the other hand, when a halogen in the nascent state exists, the water 
reacts with highly reactive nascent halogen atoms even at a range of low 
temperature and is decomposed into hydrogen halide and oxygen, as 
indicated by the following formula (3): 
EQU H.sub.2 O+2X.fwdarw.2HX+(1/2)O.sub.2 ( 3) 
The hydrogen halide and oxygen thus formed by decomposition are far smaller 
than the water in the energy of adsorption on the glass, and hence leave 
the glass surface even at low temperatures. That is, the treatment of the 
glass in the presence of nascent halogen atoms enables the water adsorbed 
on the glass surface to be removed with ease even at a range of low 
temperature. 
The nascent halogen atoms can easily be created, even at a range of low 
temperature, by ionizing an inert gas containing halogen gas or 
halogenated gas, into a plasma through a radio-frequency glow discharge or 
the like. 
Accordingly, fluoride glass fibers can be fusion spliced by the following 
steps: At first, two fibers to be joined are butted together tightly with 
no fiber axis deviation therebetween, and the pressure in a splicing 
vessel is evacuated. Then, water adsorbed on the glass surfaces is removed 
by generating a plasma of an inert gas containing halogen gas or 
halogenated gas, and an inert gas dried for avoiding re-adsorption of 
water is introduced into the vessel to the atmospheric pressure, in which 
the butted portion of the fibers is heated. 
Also in the case of the quartz glass fiber, water adsorbed on the glass 
surface can be removed by the reaction indicated by the formula (3), by 
which the formation of crystallites can be suppressed, making it possible 
to achieve high-strength fusion splicing.

DETAILED DESCRIPTION 
In FIG. 1, reference numeral 1 indicates a splicing vessel which can be 
evacuated to a vacuum, 2 an inlet port for halogen gas or halide gas, 3 an 
inlet port for an inert gas, 4 a vacuum exhaust port, 5 a vacuum pump, 6 
an exhaust port, 7, 8, 9 and 10 stop valves, 11 and 11a fibers, 12 a joint 
of the fibers, 13 and 13a fiber coatings, 14 and 14a packings for 
maintaining a vacuum in the splicing vessel, 15 and 15a jigs for adjusting 
the fiber axes, 16 and 16a electrodes for generating a plasma, 17 and 17a 
reidle electrodes for producing an arc discharge, 18 and 18a insulators 
for insulating the electrodes 16 and 17 from each other, 19 a 
radio-frequency power source for creating the plasma, 20 a high-voltage 
power source for generating the arc discharge, and 21 a switch for 
switching between the radio-frequency power source and the high-voltage 
power source. 
In the splicing of the fibers 11 and 11a, the fibers stripped of coatings 
on their top end portions are fixed to the fiber centering jigs 15 and 
15a, by which the end faces of the fibers are contacted tightly without a 
gap therebetween and their optical axes are adjusted into alignment with 
each other. The vessel 1 is evacuated by the vacuum pump 5 to a vacuum of 
approximately 0.001 Torr, after which argon or like inert gas containing 
halogen gas or halide gas is introduced from the gas inlet port 2 into the 
splicing vessel 1 to raise its inside pressure to 0.1 to 0.01 Torr. After 
this, radio-frequency power is applied via the switch 21 to the electrodes 
16 to create a plasma, by which impurities on the surfaces of the fibers 
11 and 11a are removed. Then, the stop valves 7 and 9 are closed but 
instead the other stop valves 8 and 10 are opened to introduce pure argon 
or a similar inert gas from the inert gas inlet port 3 into the vessel 1 
to increase its inside pressure to one atmosphere pressure, at which an 
arc discharge is produced, thereby fusion splicing the fluoride glass 
fibers. 
It has been ascertained by experiments on this embodiment that the fusion 
splicing of fluoride glass fibers can be achieved which has been regarded 
as impossible with the prior art. 
As described above, according to the present invention, optical fibers are 
fusion spliced after chemically removing, at a low temperature, impurities 
such as water adsorbed to the glass surface in a plasma containing a 
halogen; so the invention permits the fusion aplicing of fluoride glass 
fibers which could not have been achieved in conventional art. Also in the 
fusion splicing of quartz glass optical fibers, the present invention is 
capable of preventing the joint strength from being diminished by adsorbed 
water on the glass surface, making it possible to fusion splice the 
optical fibers with substantially the same strength as that of the fiber 
strands.