Optical communication systems using star couplers

Optical communication system having repeater amplifiers cascaded with plurality of optical star couplers wherein each star coupler is coupled to an associated set of terminals. Each star coupler has two mixing sections serially disposed between its input and output terminal ports. Star couplers receive optical signals from their associated terminals, mix the received signals and retransmit the mixed signals to each terminal in its associated set and to each of the repeater amplifiers. Star couplers also receive amplified signals from the repeater amplifiers, mix the received amplified signal and retransmit the mixed optical signals only to their associated terminals. Star couplers, thus, enable optical signals transmitted by their associated terminals to be transmitted to each terminal in the system without permitting the same signal to be returned to the same star coupler, thereby avoiding the formation of continuous optical loops and hence avoiding the problem of optical lockup.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to optically linked data processing systems and, more 
particularly, to optical star couplers for coupling optical transmission 
lines and a plurality of stations in such systems. 
2. Background Art 
Fiber-optic transmission of data offers many advantages over the 
conventional form of data transmission in data processing systems. Optical 
signals are generally immune to errors caused by electromagnetic 
interference and radio frequency interference, and these systems do not 
spark or short circuit. Fiber-optic transmission also eliminates ground 
loop problems by providing electrical isolation between optically linked 
equipment. 
A fiber-optic system for data transmission may be employed in a distributor 
processing system, in a local area network, in a data bus system and the 
like. These systems frequently require the use of a plurality of 
processing stations or terminals to communicate with each other as well as 
with peripheral equipment. Fiber-optic transmission systems typically 
employ a ring or a star type architecture. 
A ring type architecture generally uses Tee type couplers. A ring type 
architecture using Tee couplers is disclosed in the U.S. Pat. No. 
4,072,399. Typically, in a ring type system, the light or the optical 
signal from one terminal is routed to the next terminal in the ring, where 
it is tapped off (received) and retransmitted. When an optical signal is 
tapped off, it reduces the signal level of the remaining signal being 
transmitted to the next terminal in the ring. In such an arrangement, 
there can be a substantial difference in the signal strength between a 
near terminal and a far terminal. This disparity in the signal strength 
can be avoided by each successive terminal receiving the signal, combining 
it with its own output and then retransmitting the new signal. One 
drawback of such a system is that the system fails when any terminal loses 
power, or when any cable breaks, or if any terminal is disconnected from 
the ring architecture. Because of these and other reasons, most 
fiber-optic data transmission systems use a star coupler architecture. 
There are two types of star couplers; reflective type and transmissive 
type. A star coupler has a plurality of input ports for receiving optical 
signals and a plurality of output ports for transmitting optical signals. 
The reflective star coupler combines the optical signals in a mixing 
section. The mixing section is terminated with a mirror which reflects the 
optical signals back into cables that first brought the signal to the 
mixing section. The transmissive optical couplers receive the signals at 
one set of ports, combine these signals and then distribute a portion of 
the mixed signals to each of its output ports. Both types of star couplers 
work equally well backwards, i.e. the light entering the output ports will 
be distributed to the input ports. But for clarity, the ports are 
generally designated as input ports or output ports. The present invention 
applies to the transmissive type star couplers. 
Although the figures in this application show the star coupler with inputs 
on one side and outputs on the other, such an arrangement is shown only 
for clarity and simplicity. As noted earlier, input ports and output ports 
are interchangeable. This arrangement may not be the best way to package 
the device. It is easier to control bends inside the coupler package than 
outside it. Therefore, the primary consideration should be to provide the 
best situation for the external optical fiber connection. In most 
applications, two fibers from each terminal are connected to the star 
coupler. These fibers should be kept together to the maximum extent 
possible instead of splitting them to go to different sides of the star 
coupler. Therefore, it may be desirable to have all fibers go to the 
coupler through one connector, which has fiber-optic pins or sockets. It 
is generally desirable to make the pin assignments (geometric 
configuration) in the star coupler such that the repeater circuits are 
separated from the terminals. In other words, the fibers going to the 
repeaters and the fibers going to the terminals go off in different 
directions. Additionally, a minimum separation of pairs of fibers from 
each other should be provided, and each pair should have a standardized 
color code -- one color for high level (transmitter output) and the other 
color for the low level (star coupler to the repeater). 
In a system configuration using star couplers, each star coupler receives 
the transmitted signal and divides the signal evenly to all the receiving 
terminals, thereby minimizing the differences in the signal levels between 
near and far terminals. However, this division of optical signal 
proportionally reduces the level of signal received by each terminal. As 
an example, if a star coupler receives a signal of level "Y" and transmits 
it to 24 terminals, then each terminal will receive a signal of level Y 
divided by 24. Thus, for a large system there is not much signal left for 
any terminal. The diminution of an optical signal because of its division 
in the star coupler is generally referred to as a "fan-out" or a 
"furcation" loss. Thus, the use of fiber-optics to interconnect a local 
area network or a data base transmission system is generally limited by 
these furcation losses. The present limitation is the use of approximately 
64 terminals for each star coupler. Additionally, for large systems the 
terminals must be designed for a low signal level which is highly 
undesirable. 
When a large number of outputs are required or inputs are clustered in 
several locations, it would be desirable to use a plurality of couplers 
cascaded with repeater amplifiers. However, when conventional star 
couplers are cascaded in this way, as shown in FIG. 1, the optical signal 
forms a continuous loop. This continuous loop phenomenon is called a 
"lockup". The lockup occurs because light output from the first star 
coupler is amplified by the repeater amplifier and sent to the second star 
coupler, where it is divided and a part of the signal is amplified again 
by the repeater amplifier and returned to the first star coupler, thereby 
constituting a loop. This problem has been avoided in the past by using 
only one star coupler in a system with enough ports for all terminals. 
The present invention addresses the problem by providing a star coupler 
which when cascaded with repeater amplifiers does not lockup, while at the 
same time preserving the advantage of equal signal strength and the 
ability to monitor the system's integrity by monitoring its own signal 
returning from the star coupler. 
SUMMARY OF THE INVENTION 
The present invention discloses optical communication systems employing 
optical star couplers that do not lockup. The star coupler disclosed in 
the present invention comprises a plurality of input and output ports, at 
least one repeater input port, at least one repeater amplifier output port 
and two mixing sections connected in series between the input and output 
ports. The input ports are coupled via optical fibers to the first mixing 
sections which in turn is coupled to the second mixing section and the 
repeater amplifier output ports. The output ports are coupled via optical 
fibers to the second mixing section which in turn is coupled to the first 
mixing section and the repeater amplifier input ports. The optical signals 
received at the input ports are transmitted to the output ports and the 
repeater amplifier output port. The optical signals received at the 
repeater amplifier input terminals are transmitted only to the output 
ports. These star couplers when cascaded with the repeater amplifiers in 
an optical communication system do not permit the formation of a closed 
optical loop and thus avoiding the lockup problem. 
The optical communication systems disclosed in the present invention 
comprise at least one repeater amplifier cascaded with a plurality of 
optical star couplers of the present invention and a plurality of 
terminals. Each star coupler is coupled to a corresponding set of 
terminals. Each star coupler receives optical signals from the terminals 
coupled to it, mixes these optical signals and retransmits the mixed 
optical signals to its associated terminals and also transmits a portion 
of the mixed signal to each of the repeater amplifiers. The repeater 
amplifiers receive the signals from the optical star couplers, amplify and 
retransmit the amplified signals to the star couplers in the system. Each 
star coupler receives optical signals from the repeater amplifiers, mixes 
them in one of its mixing sections and retransmits the mixed signal only 
to its associated terminals. 
Examples of the more important features of this invention have thus been 
summarized rather broadly in order that the detailed description thereof 
that follows may be better understood, and in order that the contribution 
to the art may be better appreciated. There are, of course, additional 
features of the invention that are described hereinafter and which also 
form the subject of the claims appended hereto. 
These and other features and advantages of the present invention will 
become apparent with reference to the following detailed description of a 
preferred embodiment thereof in connection with the accompanying drawings 
wherein like reference numerals have been applied to like elements, in 
which:

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to the drawings, several embodiments of the present invention 
will be explained. 
In order to more fully understand communication system configurations using 
star couplers which do not lockup, it is considered helpful to first 
describe a system configuration using conventional star couplers that does 
lock-up. 
Referring to FIG. 1, this figure illustrates a system configuration using 
conventional star couplers that will lock-up. The conventional star 
couplers shown in FIG. 1 simply have their input and output ports 
connected via optical fibers which form a mixing section in between the 
input and the output ports. Starting at terminal or station 36, an optical 
signal is transmitted through its transmitter port 34 into the fiber optic 
cable 22a. The signal then enters the star coupler 20 where the signal is 
mixed in the mixing section 30 with signals being received from other 
terminals via input ports 26. Normally, system protocol would prevent more 
than one terminal from transmitting a signal at any particular time. 
Therefore, the star coupler normally should receive only one signal at any 
time. One output port of the star coupler 20 is connected to the receiver 
input 32 of the terminal 36 with the fiber-optic cable 24a. Therefore, 
each terminal would receive signals from itself and all other inputs to 
the star coupler 20. By receiving its own signal back, the terminal can 
determine that it is communicating to and from the star coupler. 
Generally, one star coupler is sufficient for a small system 
configuration. Current state-of-the-art permits the connecting up to 
approximately 64 terminals to one star coupler. However, a plurality of 
star couplers and at least one repeater may be necessary for large 
systems. 
Still referring to FIG. 1, this figure further illustrates why lockup 
occurs when an attempt is made to expand the system described thus far by 
connecting a repeater amplifier to more than one conventional star 
coupler. In such a system configuration, an optical signal from any 
terminal, such as terminal 36, is transmitted through cable 22(a) to the 
star coupler 20. This signal is mixed in the mixing section 30 and 
transmitted to the repeater amplifier 44 via cable 50, where it is 
electrically or optically amplified and retransmitted through cable 56 to 
an input port of another star coupler 21. In star coupler 21 the signal is 
combined with signals from terminals (not shown) connected to this star 
coupler and any signal received by star coupler from any other repeater 
amplifier. A portion of the signal received by star coupler 21 is routed 
through an output port to cable 54. Cable 54 is connected to a receiver 
port of the repeater amplifier 44, where this optical signal is amplified 
and retransmitted via cable 52 to an input port of the first star coupler 
20. In the star coupler 20, the signal is again combined with the other 
inputs to this coupler and a portion of the mixed signal is transmitted 
through cable 50 to the repeater amplifier 44. This signal again follows 
the path described by the cables 56, 54, 52, and 50 thereby constituting a 
continuous loop and thus causing a lockup (continuous loop) of the optical 
signal. 
FIG. 2 discloses a novel star coupler in accordance with the present 
invention. The star coupler 70, as illustrated in FIG. 2, comprises a 
housing 71, a plurality of input ports 72 and 73, a plurality of output 
ports 74 and 75, optical fibers 76, 78, 84, 86, and 88. 
Still referring to FIG. 2, a majority of the input ports 72-1 through 72-N 
are connected to the output ports 74-1 through 74-M with optical fibers 
via two mixing sections 80 and 82. One end of the first mixing section 80 
is directly coupled to the input ports 72 with optical fibers 76a-76d, 
while the other end is coupled to both the second mixing section 82 and at 
least one of the output ports 75 via optical fibers 86. Similarly, one end 
of the second mixing section 82 is directly coupled to the output ports 
74-1 through 74-M with optical fibers 78a-78d, while the other end is 
coupled to the first mixing section 80 via optical fiber 83 and at least 
one input port 73 via optical fibers 84. In this manner, optical fibers 
from input ports 73 of the coupler are routed only through the mixing 
section 82, which is directly coupled to the output ports 74; and optical 
fibers from output ports 75 are routed only through the mixing section 80, 
which is directly coupled to the input ports. 
In large communication system configurations, star couplers are generally 
connected to several terminals or stations and at least one repeater 
amplifier, as illustrated in FIG. 6. Each terminal having its transmitter 
and receiver port coupled to a star coupler while repeater amplifier is 
cascaded between the star couplers. Now referring back to FIG. 2, the 
light signals or the optical signals transmitted by the terminals are 
inputted to input ports 72 of the star coupler. The optical signals from 
the terminals are combined in the mixing section 80. The amplified signals 
from the repeater amplifier are received only in the input ports 73. These 
signals bypass the first mixing section 80 and mix with other signals only 
in the second mixing section 82 before being outputted to the output 
terminals 78. The repeater amplifier receives optical signals transmitted 
by the terminals from ports 75 only. Since ports 75 receive optical 
signals from mixing section 80, they are not mixed with any of the signals 
transmitted by the repeater amplifier to the star coupler via ports 72. 
Since the optical signals from the repeater amplifier bypass the mixing 
section 80, the repeater amplifier simply never receives any signal which 
it had transmitted into the system previously. This system configuration 
thus cuts off the return path of the amplified optical signals back to the 
repeater amplifier, thereby preventing the formation of continuous optical 
loops and hence does not lock-up. 
FIG. 3 illustrates the star coupler of FIG. 1 wherein the mixing sections 
80 and 82 are fabricated by fusing the optical fibers into mixing sections 
or zones. The fused zones may be fabricated by etching the cladding from a 
section of each fiber so that the light signal can exit from the core of 
each optical fiber, and then twisting these cores with each other or 
bonding together these fibers along a side to enable the optical signals 
to communicate from fiber-to-fiber. The star coupler of FIG. 2 having two 
mixing sections, however, may be fabricated by any of several techniques. 
One such technique is where the input and output fibers are fused to a 
"mixing rod". A mixing rod is basically a large diameter fiber-optic bar. 
Generally, optical couplers will be fabricated with at least two optical 
fibers branching off from the first fused section 80 to be connected to 
redundant repeaters via ports 75; similarly, fibers from the redundant 
repeaters are connected to the star coupler via ports 73. 
The principles and advantages of the coupler of the invention can be 
further demonstrated by referring first to a system wherein two 
conventional star couplers are connected in series as illustrated in FIG. 
4. This particular system, like the system of the invention, will not lock 
up; however, it has other disadvantages which will become clear. In FIG. 
4, all but one of the outputs of the first coupler 100 are connected to 
the input ports of the second coupler 101. The remaining output 108 from 
the first coupler 100 is inputted to a repeater amplifier. The return 
signal from the repeater amplifier is connected to the remaining input 116 
of the second coupler 101 after routing through a third optical star 
coupler 103 via optical cables 126 and 128. This coupler system will not 
lock-up with any amount of delay in the associated amplifier/star coupler 
system. 
In FIG. 4, if the star couplers are conventional star couplers, the light 
signal received by the light detector 122 will be a composite of light 
from the original source 120 and the light returning from the associated 
coupler 103 and the repeater amplifier 124. Such a composite light signal 
would result in a very distorted wave form as shown in FIG. 5, which would 
probably be unusable. 
FIG. 6 illustrates an optical communication system utilizing star couplers 
of the present invention. The system of FIG. 6 will not lock-up, and in 
addition such a system will not distort the optical signal as will the 
system shown in FIG. 5. In the system of FIG. 6, two star couplers are 
coupled to a common repeater amplifier 156 and separate sets of stations 
(terminals). Stations 152 (S1-SN) are connected to a first star coupler 
150 wherein the transmitter port "T" of each station in its set or cluster 
is connected to a corresponding input port 180 of the first star coupler, 
and the output port "R" of each station is connected to a corresponding 
output port 181 of the first star coupler 150. A second set or cluster of 
stations 154 (S1-SM) is similarly connected to a second star coupler 151. 
A repeater amplifier 156 having at least two sets of transmitter and 
receiver ports is connected to the star couplers 150 and 151. The output 
terminal 182a of the first star coupler 150 is connected to the receiver 
port 190 of the repeater amplifier 156. The transmitter port 191, which is 
associated with the receiver port 190, is connected to the input terminal 
183a of the second star coupler 151. In a similar fashion, the second set 
of receiver and transmitter ports of the repeater amplifier 156 are 
respectively connected to the output terminal 183b of the second star 
coupler 151 and the input terminal 182b of the first star coupler 150. 
Still referring to FIG. 6, the processing of an optical signal transmitted 
by any of the stations in the system is now described. As an example, an 
optical signal transmitted by the terminal S1 in the set 152 is first 
transmitted to the first optical coupler 150 through an input terminal 180 
via optical cable 158. This signal is then mixed in the mixing section 194 
with all of the other signals transmitted only by any of the stations in 
the station cluster 152 (S1-SN) connected to the optical coupler 150. A 
portion of the mixed signal from the mixing section 194 is transmitted to 
the repeater 156 via optical cables 198a and 160, while the rest of the 
mixed signal is further mixed in the mixing section 195 with signals 
received by the optical coupler 150 from the repeater amplifier 156 via 
cables 166 and 198(b) and then transmitted to the terminal set 152 (S1-SN) 
via optical cables 168, 170, 172 and the like. The signal received at the 
repeater amplifier via cable 160 is amplified and transmitted by the 
transmitter terminal 191 via cable 162 to the second optical coupler 151, 
where it is mixed in the mixing section 197 and transmitted only to each 
station in the terminal cluster 154 (S1-SM) connected to the second 
optical coupler 151. It will be noted that the optical signal received by 
the second coupler 151 from the repeater amplifier 156 via cable 162 has 
no path for it to go back to the repeater transmitter and hence to the 
first optical coupler 150. In other words, the optical fiber 164, which is 
connected between the second optical coupler 151 and the repeater 
amplifier 156, is isolated from the signal received by the second coupler 
151 from any of the terminals in the cluster 152 (S1-SN) or the repeater 
amplifier 156. In a similar fashion, a signal transmitted by any of the 
stations in the cluster 154 (S1-SM) will be first transmitted to the 
second optical coupler 151, and only a portion of such signal will then be 
transmitted to the repeater amplifier 156 via cable 164, which is 
amplified and retransmitted to the stations 152 (S1-SN) via cable 166 and 
198b, the mixing section 195 and the cables 168, 170 and 172. It will be 
seen that the optical signal transmitted by any of the stations in the 
cluster 152 (S1-SN) after amplification in the repeater amplifier 156 will 
never return back to the stations 152 (S1-SN), thereby avoiding the 
possibility of a lockup. Similarly, a signal transmitted by any of the 
terminals in the cluster 154 (S1-SM) after amplification in the repeater 
amplifier 156 will never return to the terminals 154 (S1-SM), thereby 
avoiding the possibility of a lockup. 
The reliability of the system as described above and illustrated in FIG. 6 
can be increased by using redundant repeater amplifiers as illustrated in 
FIG. 7. Such a system involves branching out the mixing section of each of 
the star couplers 150 and 150b into two or more input and output ports. As 
an example, star coupler 150a shows two ports 200a and 200b for inputs 
from repeaters 210 and 212, and two ports 202a and 202b for outputs to 
repeater amplifier 210 and 212. Similarly, the second star coupler 150b 
has two input terminals 206a and 206b, and two output terminals 204a and 
204b. Each star coupler 150a and 150b is thus coupled to redundant active 
repeaters 210 and 212 via separate receiver cables 201a, 201b and 202a, 
202b, etc. In this manner, if either path, such as, 200a or 200b fails, 
the signal strength received by the terminals in the cluster 152 will be 
reduced to half which can still be detected and also can be used to warn 
the operator of the failure. The system redundancy is obviously expandable 
to more parallel routes by the same technique. 
FIG. 8 illustrates a large system architecture using the star coupler of 
the present invention. Such a system configuration is useful when stations 
in several geographical areas 230-233 need to communicate to each other. 
As illustrated in FIG. 8, several stations S1-SN in first geographical 
area 230 are connected to one star coupler 260 which is then connected to 
a central repeater amplifier 250. The repeater 250 in such a configuration 
is designed such that each input is interlocked to the corresponding 
output such that the input from any star coupler is not returned to the 
same star coupler, but is distributed to all other star couplers. FIG. 8 
illustrates a system with nonredundant repeaters for clarity only. It will 
be noted that a system with redundant repeaters can be designed easily. 
The repeater amplifier may further comprise as many channels as desired, 
and may be designed as modules, for example, four to eight channels which 
can be interconnected via the common electrical signal bus to form any 
size system. 
The fiber-optic system architectures disclosed in the present invention are 
very flexible and expandable but have a few interconnection combinations 
which are not viable: (1) a star coupler can connect to only one repeater. 
This repeater, however, may be a redundant type; and (2) repeaters may 
transmit optically to other repeaters but alternate routes (not included 
in the redundancy discussed herein) are not acceptable.