Optical fiber fusion splicing method

A method for fusion-splicing aligned optical fibers using heat generated by gas discharge produced between electrodes which are energized by an AC voltage. At first, a high trigger voltage is applied across the electrodes to generate the discharge, after which an AC voltage lower than the trigger voltage is applied across the electrodes to produce steady-state discharge. The AC voltage is selected to satisfy the relation, 10t.sub.O .gtoreq.(t.sub.f +t.sub.d +t.sub.r), where t.sub.O is the time necessary for ions created by the discharge to flow from one of the electrodes to the other and t.sub.f, t.sub.d and t.sub.r are the fall time, the quiescent time and the rise time of the AC voltage. Alternatively, a sine-wave AC voltage is used having a frequency higher than 500 Hz.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for fusion splicing optical 
fibers using heat generated by a gas discharge between electrodes. 
The optical fiber fusion splicing method employing gas discharge is 
disclosed, for example, in U.S. Pat. No. 3,960,531 "Method and apparatus 
for splicing optical fibers" issued June 1, 1976 and has been put to 
practical use. In the conventional fusion splicing method employing a gas 
discharge, optical fibers which are to be spliced are aligned end-to-end, 
and a pair of spaced electrodes are energized to generate an electrical 
field at the junction between the fibers which is of sufficient magnitude 
to create a gas discharge or electric arc that melts the ends of the 
fibers and fuses or splices them together. The power applied across the 
electrodes is a DC current or an AC current of the commercial power source 
frequency. For producing a gas discharge, a high voltage, for example, of 
3 to 6 KV must be applied across electrodes which are spaced, for example, 
1.5 mm apart and, when the discharge starts to effect a flow of current, 
the voltage between the electrodes drops to a discharge sustain voltage 
Vg. With the prior art method, a high AC voltage Vsi necessary for 
starting the discharge is always supplied to the electrodes, and during 
the discharge the difference voltage between the voltage Vsi and the 
sustain voltage Vg is produced across a stabilizing resistor connected in 
series with the discharge electrodes. Accordingly, the conventional method 
requires a high-voltage transformer for generating the high voltage Vsi at 
all times and this transformer is bulky and heavy. The power determined by 
the product of the abovesaid difference voltage, Vsi-Vg, and the discharge 
current is dissipated by the abovementioned stabilizing resistor and this 
power is not used for the fusion splice and hence it is wasteful. Since 
this wasteful power is large, the high-voltage transformer must have a 
large capacity and consequently the transformer is inevitably bulky and 
heavy. Further, such useless power dissipation is undesirable especially 
when the fusion splicing machine operates from a battery. For splicing 
optical fibers in places which exhibit difficult work conditions, such as 
a narrow manhole or hand hole and a telephone pole, a splicing machine 
operating from a small, lightweight battery is desired. With the prior art 
fusion splicing method, however, it is difficult to realize such a 
machine. 
Heretofore it has been proposed to make the transformer small and 
lightweight by raising the frequency of the AC voltage, for example, up to 
between 16 and 60 KHz. In this method, however, the AC voltage of the high 
frequency is used as the high voltage Vsi for starting the discharge and 
the AC voltage is supplied to the discharge electrodes at all times, so 
that much power is wasted by the stabilizing resistor as described above. 
Accordingly, this method does not provide improvement in power saving. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an optical fiber fusion 
splicing method which permits reduction of the size, weight and power 
dissipation of the fusion splicing machine. 
According to the present invention, at the start of discharge a trigger 
voltage high enough to produce a discharge is applied across the 
electrodes and, after the discharge is started at least once, a 
steady-state discharge is produced. In such a state of steady-state 
discharge, an AC voltage lower than the high trigger voltage is provided 
across the electrodes. This AC voltage has a waveform that satisfies the 
relationship, 10t.sub.0 .gtoreq.(t.sub.f +t.sub.d +t.sub.r), where t.sub.0 
is the time necessary for ions created by the discharge to flow from one 
of the electrodes to the other and t.sub.f, t.sub.d and t.sub.r are the 
fall, the quiescent and the rise time of the applied AC voltage. In other 
words, the parameters are such that discharge may be produced by the next 
half cycle of the applied AC voltage before the ions existing between the 
electrodes are extinguished. As a result of this, during the steady-state 
discharge, even if the applied AC voltage is considerably lower than the 
trigger voltage, the discharge is effected in each half cycle of the AC 
voltage. Accordingly, the power consumption by the stabilizing resistor is 
reduced and the boosting transformer may be small in boosting ratio and 
power capacity, and hence it can be made small and lightweight. It is 
preferred that after applying the high trigger voltage, the steady state 
discharge be effected by a lower AC voltage, and that the frequency of the 
AC voltage be raised. This will further reduce the size and weight of the 
transformer. In the present invention, the aforesaid sum, (t.sub.f 
+t.sub.d +t.sub.r) is selected to be smaller than 10t.sub.0, preferably, 
smaller than 2t.sub.0, and even more preferably, smaller than t.sub.0. 
Moreover, the frequency of the AC voltage is selected to be higher than 1 
KHz; in order to eliminate uncomfortable sound, it is preferred that the 
frequency of the AC voltage be higher than 15 KHz. In the case where the 
waveform of the AC voltage is sinusoidal, its frequency is selected to be 
higher than 500 Hz, preferably, higher than 5 KHz, regardless of the 
aforesaid 10t.sub.0 .gtoreq.(t.sub.f +t.sub.d +t.sub.r).

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To facilitate a better understanding of the present invention, a 
description will be given first of a conventional optical fiber fusion 
splicing method employing AC gas discharge. As shown in FIG. 1, for 
example, 50 Hz-100 AC voltage from a commercial power source 11 is boosted 
by a transformer 12 up to about 4 KV and the AC high voltage is applied 
via a stabilizing resistor 13 across a pair of electrodes 14 and 15 to 
produce therebetween a discharge. Optical fibers 16 and 17 disposed 
between the electrodes 14 and 15 are fused together by heat resulting from 
the discharge. 
FIG. 2 schematically shows the voltage characteristic between the 
electrodes 14 and 15 in this case. In FIG. 2, the abscissa represents time 
t and the ordinate represents an inter-electrode voltage V, the sign of 
the voltage being disregarded. In this gas discharge, a high initial spark 
voltage Vsi is required at the start t.sub.1 of a first discharge but, 
once the discharge is started, the subsequent discharge 10 takes place 
with a spark voltage Vs lower than the voltage Vsi and thereafter a 
steady-state discharge is sustained while changing the sign of the 
inter-electrode voltage in accordance with the applied AC voltage. Before 
the discharge is started in each cycle of the AC voltage, substantially no 
current flows and the applied AC voltage appears across the electrodes 14 
and 15 but, upon initiation of the discharge, a current flows and, at the 
same time, the voltage across the electrodes 14 and 15 drops to a constant 
value, i.e. a discharge sustain voltage Vg, at which a glow discharge 
occurs. The spark voltage Vs and the sustain voltage Vg of the 
steady-state discharge and the initial spark voltage Vsi bear the 
relationship Vsi&gt;Vs&gt;Vg. 
With the prior art optical fiber fusion splicing method shown in FIG. 1, 
the high voltage Vsi necessary for the start of discharge is also supplied 
to the electrodes 14 and 15 during the steady-state discharge, and the 
voltage across the electrodes 14 and 15 is lowered by the stabilizing 
resistor 13, having a large resistance value, down to the value Vs at the 
beginning of each discharge and, further to Vg during the discharge. 
Letting the resistance value of the stabilizing resistor 13 and the 
discharge current be represented by R and I, respectively, there holds an 
expression, Vsi=Vg+RI. The power W of the discharge is approximately given 
as follows: 
EQU W=Vg.multidot.I=Vg.multidot.(Vsi-Vg)/R. (1) 
The initial spark voltage Vsi and the sustain voltage Vg depend on the 
spacing d and the configurations of the electrodes 14 and 15. In order to 
obtain power for splicing quartz optical fibers of 125 to 150 .mu.m in 
outer diameter, the spacing d of the electrode is 1.5 mm and the 
resistance value R of the stabilizing resistor 13 is between 100 and 150 
K.OMEGA.. 
With such a method heretofore employed, since the high voltage Vsi is 
always supplied to the electrodes 14 and 15, a transformer for high 
voltage generation use is required and, in addition, the resistor 13 must 
have a large resistance value and hence it consumes much power. Because of 
the large power consumption, the transformer 12 is required to be large in 
power capacity. In the conventional optical fiber fusion splice machine, 
the weight of the power source unit including the high-voltage transformer 
12 is about one-half of the overall weight of the machine, and the weight 
of the transformer 12 exceeds one-half of the weight of the power source 
unit. Therefore, the transformer 12 is an obstacle to any reduction of the 
size and weight of the machine. Further, the power that is dissipated by 
the stabilizing resistor 13 is not used for the fusion splicing, and hence 
it is entirely wasteful. This power consumption is so large that the 
conventional machine is poor in efficiency. Especially in the case of 
operating the fusion splicing machine from a battery, a large-capacity 
battery is needed; accordingly, the prior art machine is not suitable for 
use as a handy or portable splicing machine. 
According to the present invention, the high voltage Vsi (hereinafter 
referred to as the high trigger voltage Vsi) is applied only at the start 
of discharge and the steady-state discharge is sustained by an AC voltage 
Vs lower than the high trigger voltage Vsi, thereby to perform fusion 
splicing of optical fibers. If the AC voltage Vs is close to the high 
trigger voltage Vsi, the present invention is not very advantageous over 
the prior art method. Accordingly, it is preferred to minimize the value 
of Vs/Vsi. To this end, in the present invention the waveform of the AC 
voltage which is applied during the steady-state discharge is selected as 
follows: Defining the sum of the falling time t.sub.f, the quiescent time 
t.sub.d and the rise time t.sub.r of the AC voltage as a transient time t, 
and letting the time for ions created by discharge to flow from one of the 
electrodes to the other be represented by t.sub.0, the waveform of the AC 
voltage is selected to satisfy the following condition: 
EQU 10t.sub.0 .gtoreq.t. (2) 
FIG. 3 shows the abovesaid times t.sub.f, t.sub.d and t.sub.r in connection 
with a symmetrical waveform of the AC voltage. In FIG. 3, the abscissa 
represents time t and the ordinate represents voltage V. The time t.sub.f 
is a time in which the AC voltage waveform 20 falls from 90% to 10% 
thereof; the time t.sub.r is a time in which the AC voltage waveform 20 
rises from 10% to 90% thereof; t.sub.d is a time in which the AC voltage 
changes from 10% of the peak voltage V.sub.1 to 10% of the voltage 
-V.sub.1. In the case of an asymmetrical waveform, a longer one of the 
periods (t.sub.f +t.sub.d +t.sub.r) of changes from positive to negative 
and from negative to positive is used as the transient time t. 
Next, a description will be given of the fact that the value of Vs/Vsi is 
small when the relationship 10t.sub.0 .gtoreq.t is satisfied. 
The time t.sub.0 necessary for ions created by a discharge to flow from one 
of the electrodes to the other is given by the following expression: 
EQU t.sub.0 =d/v (3) 
where d is the electrode spacing and v is the migration velocity of ions. 
Letting the mobility of ions be represented by .mu., the ion migration 
velocity v is given by the following expression: 
EQU v=.mu.E (4) 
where E is an electric field set up by the applied voltage. When 
calculating the time t.sub.0, the sustain voltage Vg shown in FIG. 2 is 
used as a voltage to be applied. That is, it is determined that E=Vg/d 
and, accordingly, t.sub.0 =d.sup.2 /(.mu..multidot.Vg). In an AC 
discharge, the direction of the applied voltage is reversed every half 
cycle (T/2) and the direction of migration of ions is also reversed. When 
the time t in which the voltage is reversed is reduced to correspond to 
the time t.sub.0 defined by the expressions (3) and (4), the ions created 
by the discharge do not follow the change in the electric field set up by 
the applied voltage, resulting in a space charge layer being formed. This 
space charge layer acts to decrease the value of Vs/Vsi. The effect of the 
space charge layer produced by the accumulation of such ions between the 
electrodes is large, so that the value of Vs/Vsi can substantially be 
reduced. The reason for which t is defined to be smaller than 10t.sub.0, 
not t.sub.0 , is that, in the period (t.sub.r +t.sub.d +t.sub.f), a 
voltage lower than Vg is applied across the electrodes to cause a decrease 
in the ion migration velocity v, resulting in an increase in the ion 
migration time. This will hereinbelow be described based on experimental 
results. 
According to Engel Steenbeck, "Gasentladungen", Springer, 1932, the 
mobility .mu. of ions in the atmosphere is .mu..perspectiveto.2[cm.sup.2 
/SV] at 273.degree. K. under 1 atm. Calculating the time t.sub.0 in the 
case of d=1.5 mm based on this value, it is about 20 .mu.sec. The spark 
voltage Vs for the steady-state discharge was measured with varying 
transient time t using a rectangular wave of a frequency f=1 KHz for the 
AC voltage. The experimental results are shown in FIG. 4, in which the 
abscissa represents the normalized migration time obtained by dividing the 
transient time t by the ion migration time t.sub.0, and the ordinate 
represents the value obtained by dividing the spark voltage Vs by the high 
trigger voltage Vsi. Curves 21, 22, 23 and 24 (indicated by , 
.quadrature., .circle. and .increment., respectively) show the cases where 
the electrode spacing d was 0.5, 1.0, 1.5 and 2.0 mm, respectively. It 
appears from these curves that the reduction of the transient time t will 
decrease the spark voltage Vs for the steady-state discharge and that if 
10t.sub.0 =t, the spark voltage Vs will be less than one-half of the high 
trigger voltage Vsi even when d=2.0 mm. In the case of single-fiber 
splicing, the electrode spacing d is usually selected to be less than 2.0 
mm for the purpose of obtaining a stable discharge; from the viewpoint of 
easy operation of the splicing machine, it is preferred that the electrode 
spacing is about 1.5 mm. In the case of multi-fiber splicing, the 
electrode spacing is in the range of 3 to 5 mm. When 2t.sub.0 =t, the 
value of Vs/Vsi became approximately 0.2 and the spark voltage Vs could be 
appreciably reduced. With t.sub.0 =t, Vs/Vsi was nearly equal to 0.1. Even 
if t is further reduced, the value of Vs/Vsi cannot be decreased very 
much. The experimental results shown in FIG. 4 suggest that the transient 
time t should be less than 10t.sub.0, preferably less than 2t.sub.0, and 
more preferably less than t.sub.0. 
FIG. 5 shows results of similar experiments conducted where the waveform of 
the AC voltage was sinusoidal. In FIG. 5, the lower abscissa represents 
the frequency f, the upper abscissa represents the normalized period 
T/t.sub.0, and the ordinate represents Vs/Vsi. The electrode spacing d was 
1.5 mm. It appears from FIG. 5 that the value of Vs/Vsi becomes smaller 
than 0.4 at 500 Hz (100t.sub.0 in terms of the period T) and becomes very 
small at higher frequencies. In the case of the sine wave, the transient 
time t defined in FIG. 3 is T/3. Accordingly, considering that the 
transient time t is selected to be less than 10t.sub.0 also in the case of 
using the rectangular wave, it is sufficient that the period T be less 
than 30t.sub.0. However, the experimental results shown in FIG. 5 indicate 
that the period T may be less than 100t.sub.0. Accordingly, at the same 
frequency, it is preferred to employ a sine wave rather than a rectangular 
wave and this seems to result from the fact that the latter has a 
considerably larger t.sub.d than that of the former. Further, FIG. 5 shows 
that the value of Vs/Vsi approaches 1/10 at frequencies higher than 5 KHz 
and is saturated to about 0.1 at frequencies higher than 10 KHz. 
Also in the case of the rectangular wave depicted in FIG. 3, when the 
frequency of the AC voltage rises, the transient time t automatically 
decreases to reduce the value of Vs/Vsi. In other words, to reduce the 
value of Vs/Vsi, it is preferred, regardless of the waveform of the AC 
voltage, to raise the frequency. If the frequency is too high, however, it 
is difficult, because of stray capacitance and other factors, to construct 
the machine in a manner to obtain intended characteristics, or the machine 
cannot be formed using inexpensive parts. Therefore, it is practical at 
present that the frequency of the AC voltage be lower than 100 KHz. 
The high trigger voltage Vsi and the discharge sustain voltage Vg do not 
depend on the frequency of the AC voltage. For example, as depicted in 
FIG. 6 which shows the measured value of the frequency characteristic of 
the voltage Vg with respect to a sine-wave voltage in the case of the 
electrode spacing d being 1.5 mm, the voltage Vg is constant with respect 
to the frequency f. The voltage Vg depends on the electrode spacing d. For 
example, as shown by measured values in FIG. 7, an increase in the 
electrode spacing d raises the voltage Vg. In FIG. 7, the measurement was 
conducted at 1 KHz. 
FIG. 8 shows the experimental results of the characteristic of the value 
(Vs/Vg) of the spark voltage Vs normalized by the voltage Vg with respect 
to the frequency f of the sine-wave AC voltage. The electrode spacing d is 
1.5 mm and the value of Vsi/Vg is about 6. The value of Vs/Vg gradually 
decreases with an increase in the frequency f and becomes nearly equal to 
2 at 5 KHz. In the range above 10 KHz the voltage Vs is appreciably close 
to Vg and, therefore, wasteful power consumption by the stabilizing 
resistor 13 is markedly diminished. 
As described above, wasteful power consumption can be reduced by producing 
the steady-state discharge using the AC spark voltage Vs lower than the 
high trigger voltage Vsi after starting discharge by the voltage Vsi and 
by selecting 10t.sub.0 .gtoreq.t or raising the frequency. Next, a 
description will be given of reduction of the size and weight of the 
transformer by decreasing power consumption and raising the frequency. 
Letting the overall cross-sectional area of the primary and secondary 
windings of the transformer, the overall area of a window of a core, the 
effective cross-sectional area of each core, the magnetic flux density of 
each core, the permissive current density of the winding, the effective 
length of the winding and the ratio of occupation by the winding be 
represented by Ac (m.sup.2), Aw (m.sup.2), Ae (m.sup.2), B (Wb/m.sup.2), 
.delta. (A/m.sup.2), le and .beta.=Ae/Aw, respectively, the power capacity 
P and the capacity Q of the transformer bear the following relationships: 
EQU P=2.beta..multidot.B.multidot.Ae.multidot.Aw.multidot..delta..multidot.f 
(5) . 
and 
##EQU1## 
Assuming that Aw is proportional to Ae, the capacity Q is in proportion to 
.sqroot.Ae.sup.3. The power P is proportional to Ae.sup.2 f, so that if 
the capacity Q and the weight W are proportional to each other, when the 
power capacity P is constant, the weight W is in proportion of f.sup.-3/4. 
This relation is shown in FIG. 9, where a proportional constant is 
determined using measured values of a 50 Hz transformer. In FIG. 9 the 
abscissa represents the frequency f and the ordinate represents the weight 
W of the transformer. Curves 25, 26 and 27 show the cases where power 
capacity is 100, 50 and 15 W, respectively. It will be seen from FIG. 9 
that an increase in the frequency f will decrease the weight W of the 
transformer to make it smaller in size, and that the smaller the power 
capacity is, the more the transformer is reduced in weight and size. 
As described in the foregoing, according to the present invention, the 
discharge is started by applying the higher trigger voltage Vsi across the 
electrodes and the subsequent steady-state discharge is sustained by 
applying the voltage Vs lower than that Vsi. Such a voltage application 
may be effected, for example, as follows: For example, as depicted in FIG. 
10, the output from the transformer 12 is rectified by a DC booster 
circuit 28 composed of a capacitors and diodes and, at the same time, the 
rectified output is boosted. One end of the input side of the booster 
circuit 28 is connected to the one electrode 14 via a capacitor 29 and the 
stabilizing resistor 13, whereas the other end is connected to the other 
electrode 15. The output side of the DC booster circuit 28 is connected 
via a resistor 31 of a large resistance value to the connection point of 
the capacitor 29 and the resistor 13. The resistor 31 has a resistance 
value of, for example, 10 M.OMEGA. and the stabilizing resistor 13 has a 
resistance value of 2 K.OMEGA.. For instance, the transformer 12 provides 
at its output a 20 KHz-1 KV voltage, which is rectified by the booster 
circuit 28 and, at the same time, it is boosted up to six times in this 
example and charged across the capacitor 29. About 7 KV, which is the sum 
of the voltage of the capacitor 29 and the output voltage of the primary 
side of the transformer 12, is applied across the electrodes 14 and 15 and 
this voltage serves as the high trigger voltage Vsi to produce discharge 
between the electrodes 14 and 15. Upon occurrence of the discharge, the 
impedance through the resistor 13 and the discharge gap becomes 
sufficiently smaller than the impedance of the resistor 31, so that the 
output AC voltage from the transformer 12 flows through the route 
[capacitor 29-resistor 13-gap between electrodes 14 and 15] and the DC 
booster circuit 28 is effectively disconnected from the electrodes 14 and 
15. After the discharge between the electrodes 14 and 15 has started, 
therefore, the 20 KHz-1 KV AC voltage is applied across the electrodes 14 
and 15 from the transformer 12, thereby providing the steady-state AC 
discharge. 
An alternative arrangement is shown in FIG. 11. The output from the 
transformer 12 is applied via the stabilizing resistor 13 across the 
electrodes 14 and 15 and, at the same time, a piezoelectric high voltage 
generator 32 is connected across the electrodes 14 and 15 via the resistor 
31. The piezoelectric high voltage generator 32 operates instantaneously 
to create the high trigger voltage Vsi needed to produce discharge between 
the electrodes 14 and 15. Once the discharge is generated between the 
electrodes 14 and 15, the steady-state discharge is sustained between them 
by the 20 KHz-1 KV AC voltage from the transformer 12. 
As has been described in the foregoing, according to the optical fiber 
fusion splicing method of the present invention, discharge is started by 
the high trigger voltage Vsi and the subsequent steady-state discharge is 
maintained by the lower AC voltage Vs, whereby the power source unit can 
be reduced in size and weight and its power consumption can be diminished. 
In the case where the frequency f of the AC voltage is 20 KHz and the 
electrode spacing d is 1.5 mm, the value of Vs/Vsi is about 1/10 from the 
measured value shown in FIG. 5. By such lowering of the voltage Vs as 
compared with the voltage Vsi and the raised frequency of the AC voltage, 
the weight of the transformer 12 is decreased to 80 g from 1.6 Kg which is 
the weight of the transformer in the case where the frequency f of the AC 
voltage is 50 Hz and the voltage Vsi is applied during the steady-state 
discharge, too. That is, the weight of the transformer 12 is reduced to 
1/20 of that in the prior art, greatly reducing the weight of the power 
source unit. Furthermore, by making the voltage Vs for the steady-state 
discharge lower than the voltage Vsi, the resistance value of the 
stabilizing resistor 13 connected in series with the electrodes 14 and 15 
can be decreased. In this example, the resistance value could be decreased 
from 100 K.OMEGA. to 1 K.OMEGA.. Accordingly, the power consumption was 
cut down to 12 W from about 100 W in the prior art. In addition, when 
graded fibers 50 .mu.m in core diameter and 125 .mu.m in outer diameter 
were spliced by this machine, the same splice characteristics as those by 
the prior art were obtained. Moreover, splicing could be achieved in 
succession more than 90 times using a 12 V, 0.45 Ah Ni-Cd battery. 
With the method of the present invention, the power source unit can be made 
small in size and lightweight and its power consumption is cut down. In 
addition, the AC voltage Vs for the steady-state discharge can be made 
close to the sustain voltage Vg to facilitate maintenance of the glow 
discharge, permitting excellent splicing of optical fibers. 
It will be apparent that many modifications and variations may be effected 
without departing from the scope of the novel concepts of this invention.