A star-shaped coupler for distributing the light signal emitted by any one of input-output devices among the other devices is disposed among these devices. The coupler comprises a plurality of single-core optical fibers for branching purposes. Each one end of those branching fibers whose number is less than the number of the input-output devices by one is bound together and is opposed to the end surface of the output single-core fiber for one of the input-output devices. The other end surfaces of the branching fibers are opposed to their respective end surfaces of the input single-core fibers for the other input-output devices. The end surfaces of the output fibers for the other input-output devices are connected with the end surfaces of the input fibers through the branching fibers. The other ends of the branching fibers which are opposed to their respective end surfaces of the input fibers are bound together to form bound portions corresponding to the input fibers.

FIELD OF THE INVENTION 
The present invention relates to a star-shaped coupler which is disposed 
among a plurality of input-output devices acting to receive and deliver 
optical signals for distributing the optical signal delivered from any one 
of the devices to the others. 
BACKGROUND OF THE INVENTION 
Examples of conventional star-shaped couplers of this kind are shown in 
FIGS. 1 and 2. The star-shaped coupler 1 shown in FIG. 1 comprises a 
plurality of optical fibers 2 which are bound and fused together by 
heating substantially at their centers to form a constricted bound portion 
3. The coupler branches into numerous fibers on both sides of the bound 
portion 3. The optical fibers 4a, 4b, 4c, 4d on the left side as viewed in 
the figure are used as input fibers, whereas the optical fibers 5a, 5b, 
5c, 5d on the right side are employed as output fibers. In use of the 
coupler 1, the input fiber 4a is connected to the output of one 
input-output device, and the output fiber 5a is connected to the input of 
the device. Similarly, the input fibers 4b, 4c, 4d and the output fibers 
5b, 5c, 5d are connected to their respective input-output devices. As an 
example, if the input-output device connected with the input fiber 4a 
emits light, the light travels through the input fiber 4a, and then it is 
substantially uniformly dispersed throughout the whole cross section of 
the waist 3a of the bound portion 3. Thereafter, the light is 
substantially equally distributed to the output fibers 5a, 5b, 5c, 5d for 
application to their respective input-output devices. 
In the star-shaped coupler 11 shown in FIG. 2 in exploded view, each one 
end of input optical fibers 12a, 12b, 12c, 12d is jointed together to form 
a bound portion 14. Likewise, each one end of output optical fibers 13a, 
13b, 13c, 13d is jointed together to form a bound portion 15. The opposed 
end surfaces of the bound portions 14 and 15 are bonded together via a 
mixing rod 16, though they are shown in spaced apart relation to each 
other. In use of this coupler, the input fibers 12a-12d and the output 
fibers 13a-13d are connected to their respective input-output devices in 
the same way as the aforementioned star-shaped coupler 1. For example, the 
light signal propagated through the input fiber 12a is dispersed 
substantially uniformly throughout the cross section of the mixing rod 16. 
Then, the light is substantially equally distributed to the output fibers 
13a, 13b, 13c, 13d before being applied to their respective input-output 
devices. 
In these conventional star-shaped couplers 1 and 2, the signal propagated 
through the input fiber 4a or 12a, for example, is distributed to all the 
output fibers 5a-5d 13a-13d via the constricted bound portion 3 or mixing 
rod 16. However, since the input fiber 4a or 12a and the output fiber 5a 
or 13a are connected to the same input-output devices, the delivered light 
signal returns to the same device. This reduces the quantity of light fed 
to the other input-output devices, thereby increasing the loss in the 
branching. In particular, it is desired that the light signal traveled 
through the input optical fiber 4a or 12a, for example, be distributed to 
three output fibers 5b, 5c, 5d, or 13b, 13c, 13d, but it is distributed to 
four fibers. It is now assumed that the quantity of light received by the 
input fiber 4a or 12a is A.sub.0 and that the propagation loss is 
negligible. Then, if the light is distributed to three fibers, the 
quantity of light supplied to each output fiber is A.sub.0 /3. In the 
conventional devices, however, the light is distributed to four fibers 
and, accordingly, the quantity of light distributed to each fiber is 
A.sub.0 /4. In this way, the loss in branching is greater than the ideal 
configuration. 
SUMMARY OF THE INVENTION 
In view of the foregoing situation, it is the main object of the present 
invention to provide a star-shaped coupler which supplies the light signal 
delivered from any one of input-output devices to the others in an 
efficient manner without allowing the signal to reenter the same device, 
thereby producing a less loss in branching. 
This object is achieved in accordance with the teachings of the present 
invention by a star-shaped coupler which comprises a plurality of 
single-core optical fibers for branching, and which is interposed among 
each end surface of output single-core optical fibers and each end surface 
of input single-core optical fibers, the input and output fibers forming 
pairs and being connected to their respective input-output devices that 
receive and deliver light signals, the coupler acting to distribute the 
light signal emitted from the output fiber for any one of the input output 
devices among the input optical fibers for the other devices. Each one end 
of those branching fibers whose number is less than the number of said 
input-output devices by one is bound together and is opposed to the end 
surface of the output fiber for one of the input-output devices. The 
branching fibers are separated from each other and each other end surface 
is opposed to respective one of the end surfaces of the input fibers for 
the other input-output devices. The end surfaces of the output fibers for 
the other input-output devices are connected via the branching fibers with 
the end surfaces of the input fibers in the same manner as the foregoing. 
The other ends of the branching fibers which are opposed to their 
respective end surfaces of the input fibers are bound together to form 
bound portions. 
Other and further objects of the invention will become obvious upon an 
understanding of the illustrative embodiment about to be described or will 
be indicated in the appended claims, and various advantages not referred 
to herein will occur to one skilled in the art upon employment of the 
invention in practice.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 3-5, there is shown an arrangement utilizing a 
star-shaped coupler embodying the concept of the present invention. This 
arrangement has output single-core optical fibers 22a, 22b, 22c, 22d, and 
input single-core optical fibers 23a, 23b, 23c, 23d. The fibers 22a and 
23a form a pair and are connected to an input-output device (not shown). 
Similarly, the fibers 22b and 23b, 22c and 23c, and 22d and 23d form pairs 
and are connected to their respective input-output devices (not shown). 
More specifically, the output fiber 22a is connected to the output of the 
input-output device which receives and delivers a light signal, the input 
fiber 23a being connected to the input of the device. The other pairs are 
connected in the same fashion. The star-shaped coupler 26 which acts to 
distribute the light signal from any one of the input-output devices to 
the others is interposed among the end surfaces 24a-24d of the output 
fibers 22a-22d and the end surfaces 25a-25d of the input fibers 23a-23d. 
The coupler 26 consists of (n -1) single-core optical fibers 27 for 
branching purposes, where n is the number of the input-output devices. In 
this specific example, n is four. The output fibers 22a-22d for one 
input-output device are connected to the input fibers 23a-23d for the 
other input-output devices through these branching fibers 27. 
More specifically, each one end of each three of these branching fibers 27 
is bound together as shown, and the bound ends are fused together by 
heating using a mold to make cylindrical bound portions 28 which are 
identical in diameter to the output fibers 22a-22d. As shown in FIG. 4, 
each one end surface 27a of three branching fibers 27 is shaped into a 
sector of a certain area such that the three end surfaces 27a form the 
circular end surfaces 29 of each bound portion 28. The end surfaces 29 of 
these bound portions 28 are bonded to the end surfaces 24a-24d, 
respectively, of the output fibers 22a-22d with adhesive, although they 
are shown in spaced apart relation for the sake of illustration. In 
discussing three branching optical fibers 27 joined to the output fiber 
22a, the bound portion 28 branches into the three fibers 27 the other end 
surfaces 27b of which are joined to the end surfaces 25b, 25c, 25d, 
respectively, of all the input fibers 23b, 23c, 23d except for the input 
fiber 23a that forms a pair with the output fiber 22a. The branching 
fibers 27 which are joined to the other outputs fibers 22b, 22c, 22d are 
also connected in the same way. 
Also, the other ends of the three branching fibers 27 are joined to the 
input fibers 23a, 23b, 23c, 23d by heating and binding together these ends 
using a mold to form cylindircal bound portions 30 having substantially 
the same diameter as the input fibers 23a-23d. As shown in FIG. 5, in the 
end surface 31 of each bound portion 30, the other end surfaces 27b of the 
three branching fibers 27 are each shaped into a sector of a certain area 
so that they may form a circle. The other end surfaces 27b are joined to 
the input fibers 23a-23d, respectively. 
The operation of the star-shaped coupler 21 constructed as described above 
is now described. As an example, a light signal which is passed through 
the output optical fiber 22a after it is emitted by an input-output device 
goes out of the end surface 24a, and then it enters the corresponding end 
surface 29 of the bound portion of the star-shaped coupler 26. Because 
this end surface 29 is tightly coupled to the end surface 24a of the 
output fiber 22a and has substantially the same diameter as the end 
surface 24a, only a small fraction of the light quantity is lost when the 
light enters the end surface 29. Then, the light signal is divided into 
three, which then travel through the branching fibers 27. Subsequently, 
the divided light signals exit from the other end surfaces 27b of the 
branching fibers 27 which lie in three different bound portions 30. Then, 
the light signals enter the end surfaces 25b, 25c, 25d, respectively, of 
the input fibers 23b, 23c, 23d, and they pass through these single-core 
fibers 23b, 23c, 23d. Finally, the signals are furnished to three 
input-output devices which are not connected with the output fiber 22a. 
The light signals from the other output fibers 22b, 22c, 22d are 
propagated in the same manner. Thus, employment of the novel star-shaped 
coupler 26 eliminates the possibility that the light signal emitted by any 
one input-output device returns to it. It is now assumed that the quantity 
of light emitted from each of the output fibers 22a-22d is equal to 
A.sub.0 and that all the losses other than the branching loss such as 
propagation loss can be neglected. Then, the quantity of light entering 
each of the input fibers 23a-23d is A.sub.0 /3. It can be seen that the 
loss due to branching is reduced as compared with the similar conventional 
devices by comparing this with the conventional cases where the quantity 
of the incident light is A.sub.0 /4. 
Although the star-shaped coupler 26 described in the above embodiment is 
designed to correspond to four input-output devices, it is obvious that 
the inventive coupler can be designed to correspond to any number of 
input-output devices. 
As thus far described, the coupler according to the invention is capable of 
distributing the light signal emitted from any one of input-output devices 
among the others without allowing the signal to reenter the device, thus 
reducing the loss due to the branching of light. Hence, the propagation 
loss of light signal can be reduced.