Optical fiber interconnection apparatus and methods of making interconnections

Apparatus for routing optical fiber comprises an elongated manipulator (20, FIG. 2) having a vertical axis which can be controlled to move in an X-Y plane and in the .theta. direction around its vertical axis. A rotatable wheel (21) is mounted on a free end of the manipulator, and a reel (19) containing optical fiber (17) is mounted on one side of the manipulator. The fiber is threaded over a peripheral portion of the wheel and the wheel presses the fiber against an adhesive-coated surface of a substrate (18) to cause it to adhere to the coated surface. The manipulator is then moved in a direction parallel to the flat surface at an appropriate speed and direction to cause the wheel to rotate and to exert tension on the optical fiber. The tension causes additional optical fiber to unwind from the reel and to be fed to the wheel for adherence to the coated surface, thereby to form a continuous optical fiber portion extending along, and adhered to, the coated surface.

TECHNICAL FIELD 
This invention relates to optical fiber interconnections and, more 
particularly, to optical backplanes. 
BACKGROUND OF THE INVENTION 
Electronics systems are typically organized by mounting various system 
components on printed wiring boards and interconnecting the printed wiring 
boards with a circuit transmission component known as a backplane. As the 
circuit density of printed wiring boards increases, it becomes 
increasingly difficult to provide the needed backplane interconnections 
because, as interconnection transmission lines become thinner, their 
impedances increase. Moreover, the distance over which information must be 
transmitted by backplane conductors is normally fairly long compared to 
the distances transmitted on the printed wiring boards. These factors may 
reduce the speed at which circuits can be operated, which may defeat a 
principal advantage of higher circuit densities. 
The copending applications of Holland et al., Ser. No. 07/785,112, field 
Oct. 30, 1991, now U.S. Pat. No. 5,155,785, granted Oct. 13, 1992, and 
Bonanni et al., Ser. No. 07/757,870, filed Sep. 11, 1991, now U.S. Pat. 
No. 5,204,925, granted Apr. 20, 1993, hereby incorporated herein by 
reference, describe the use of "optical" backplanes comprising optical 
fibers mounted on a substrate for interconnecting printed wiring boards. 
The electrical energy of each printed wiring board is translated to 
optical energy which is transmitted by an optical fiber on the optical 
backplane to another printed wiring board where it is translated back 
again to electrical energy for transmission on the other printed wiring 
board. Because optical fibers can transmit much greater quantities of 
information than electrical conductors, and with significantly less loss, 
such optical backplanes have a promising future. They could, of course, 
also be used to interconnect optical wiring boards, that is, circuit 
components on which signals are transmitted optically, and other 
electrical circuit modules such as multi-chip modules and hybrid 
integrated circuits. 
Even though the use of optical backplanes tends to simplify the 
interconnection problem, the optical fiber interconnections on a backplane 
may still be very complex and relatively difficult to fabricate. There has 
therefore been a long-felt need in the industry for methods for 
fabricating optical fiber backplanes that are amenable to mass production, 
that reduce the operator skill required for fabrication, and in which the 
optical fiber lengths are adequately precise. In many modern digital 
systems, deviations in optical fiber length may result in timing and 
synchronization errors. Various machines are available for automatically 
routing and bonding electrical wire to a substrate, but, in general, these 
machines cannot be adopted for optical fiber use because of the relative 
fragility of optical fiber and its relative inability to withstand heat 
and pressure, abrupt turns, etc. 
SUMMARY OF THE INVENTION 
An illustrative embodiment of the invention uses an elongated manipulator 
having a vertical axis which can be controlled to move in an X-Y plane and 
in the .theta. direction around its vertical axis. A rotatable wheel is 
mounted on a free end of the manipulator, and a reel containing optical 
fiber is mounted on one side of the manipulator. The fiber is threaded 
over a peripheral portion of the wheel and the wheel presses the fiber 
against an adhesive-coated surface of a substrate to cause it to adhere to 
the coated surface. The manipulator is then moved in a direction parallel 
to the flat surface at an appropriate speed and direction to cause the 
wheel to rotate and to exert tension on the optical fiber. The tension 
causes additional optical fiber to unwind from the reel and to be fed to 
the wheel for adherence to the coated surface, thereby to form a 
continuous optical fiber portion extending along, and adhered to, the 
coated surface. 
The continuous optical fiber is routed in a complex pattern by changing the 
direction of motion of the manipulator. Various loops are formed along the 
edge of the continuous optical fiber pattern. After the formation of the 
single continuous optical fiber pattern, the fiber is severed from the 
manipulator. The substrate is then cut along a line which severs the loops 
so as to form a large plurality of optical fiber interconnections crossing 
the backplane surface in the desired manner. An advantage of laying the 
fiber along one continuous path in this manner is that, prior to severing 
the loops, all the fiber can be tested in a single operation by directing 
light through the single continuous fiber. Any break or other serious 
anomaly in the fiber would of course be detected during the testing. After 
testing, the substrate is severed, as described before, and it can be 
assumed that the various optical fiber links thus formed are capable of 
properly transmitting light. 
These and other objects, features and advantages of the invention will be 
better understood from a consideration of the following detailed 
description taken in conjunction with the accompanying drawing.

DETAILED DESCRIPTION 
The various components as shown in the drawings are not necessarily drawn 
to scale, and in some cases dimensions have been purposely distorted to 
aid in clarity of exposition. Referring of FIG. 1, there is shown 
schematically an electronic system comprising a plurality of printed 
wiring boards 11 which are interconnected by optical fiber ribbons 12 and 
a pair of optical backplanes 13. The printed wiring boards 11 constitute 
part of an electronic switching system and, as such, comprise electronic 
circuits and other electrical components. At the edges of the printed 
wiring boards, outgoing electrical energy is converted to optical energy 
for transmission by the optical backplane or the optical fiber ribbons, 
and incoming optical energy is converted to electrical energy for 
processing within the printed wiring board. The above-mentioned 
application of Holland et al., for example, shows how optical fibers can 
be coupled to photodetectors for conversion of optical energy to 
electrical energy, and how lasers can be coupled to the optical fibers for 
converting electrical energy to optical energy for transmission by optical 
fibers. 
As described in the aforementioned Bonanni et al. application, optical 
backplanes 13 comprise optical fibers 15 which interconnect opposite ports 
or tabs 16 of the optical backplanes. The optical fibers are supported on 
a flexible substrate which is then coated with adhesive to which the 
optical fibers adhere. The substrate is preferably a polymer of the type 
commercially identified by the trademark "Kapton." Because the optical 
backplanes are made of a flexible material, they can be bent for mounting 
in an appropriate structure to reduce the volume required by the system 
and to aid in connection to other electronic systems of an electronic 
switching machine. 
Although some of the tabs 16 of the optical backplane are shown as not 
being used, it is nevertheless desirable that all of the optical fibers 
shown be mounted to provide access to additional components that may be 
added to the system. Further, it is desirable that three optical fibers be 
included for each optical fiber 15 which is shown. This allows for a 
plurality of input-output ports for each printed wiring board, with one 
optical fiber transmitting input energy, one optical fiber transmitting 
output energy, and one optical fiber transmitting a clock signal for 
synchronization purposes. For clarity, only one of the three optical 
fibers extending along each path have been shown. Some of the optical 
fiber paths have been shown with heavy lines or dotted lines as a further 
aid in understanding the layout. It is intended that each port or tab 16 
of the optical backplane contains six groups of three fibers each, or a 
total of eighteen optical fibers. The optical fibers are two hundred fifty 
microns in diameter and, as is discussed in the aforementioned Holland et 
al. application, each fiber must have an appropriately large curvature for 
each change of direction to avoid damage to the fiber. Thus, the acute 
changes of direction of the optical fiber routing shown should, in actual 
practice, be more gradual changes of direction. 
In implementing the FIG. 1 embodiment, each of the optical fibers 15 have 
been manually placed on the adhesive-coated substrate. Care has to be 
taken so that each optical fiber in each group of three fibers is of the 
same length to avoid synchronization errors in the data transmitted and 
received. Although all of the backplanes 13 may be identical in structure, 
mass-production apparatus of the prior art has been found to be 
unsatisfactory for making such backplanes automatically, due primarily to 
the fragility of optical fiber. 
Referring to FIG. 2, there is shown schematically apparatus for routing 
optical fiber 17 on an adhesive-coated substrate 18 in accordance with an 
illustrative embodiment of the invention. A reel 19 of optical fiber is 
mounted on the side of a manipulator 20 which is capable of moving 
parallel to the X-Y plane of the surface of substrate 18 and of rotating 
about a central vertical axis. Mounted on an end of manipulator 20 is a 
wheel 21 over which the optical fiber 17 is threaded. Reel 19 and wheel 21 
are located in the same plane of rotation, and the plane of rotation can 
rotate with manipulator 20. 
The optical fiber 17 is initially threaded around wheel 21 and the wheel 21 
is pressed against the adhesive-coated upper surface of substrate 18. The 
manipulator 20 then moves parallel to the upper surface of substrate 18. 
As it moves, friction causes the wheel 21 to rotate which exerts tension 
on optical fiber 17 causing the reel 19 to rotate. In this manner, as the 
manipulator is moved, optical fiber is fed from reel 19 to wheel 21 for 
adherence to the upper surface of substrate 18. Manipulator 20 is 
rotatable in a .theta. direction around its central axis. When one desires 
to change the direction, one rotates the manipulator 20 to "steer" the 
wheel 21 in a direction to be moved. The change of direction should be 
sufficiently gradual to avoid undue stress on the optical fiber 18. When 
the manipulator 20 is rotated, reel 19 and wheel 21 are rotated with it so 
that their planes of rotation remain parallel with the direction of 
movement in the X-Y plane. 
We have found that, with reel 19 being freely rotatable, the tension 
exerted on optical fiber 17 is not sufficient to damage it. That is, the 
tension is accommodated by the rotation of reel 19 which unwinds optical 
fiber from the reel. A spring 22 in the manipulator permits vertical 
movement of wheel 21 to accommodate crossovers, such as that shown at 
location 24, without stressing unduly the optical fiber. 
An important advantage of the method of FIG. 2 is that it can be 
implemented with commercially available robotic apparatus. Specifically, 
apparatus shown in somewhat more detail in FIGS. 3 and 4 was mounted on 
one of the manipulators of an FWS-200 flexible robotic workstation, a 
product of AT&T; a similar product is commercially available from the 
Megamation Company of Princeton, N.J. This is a gantry style robot 
designed for precise positioning of moderate weight work pieces. The X-Y 
motion of the manipulator is produced by linear stepper motors that ride 
on an air bearing. Z motion is produced through a lead screw, and .theta. 
motion is produced by a dc servo motor with encoder feedback. Referring to 
FIG. 3, a mounting rod 26 was mounted at the end of the robot manipulator. 
Mounted on the side of the mounting rod 26 are three optical fiber reels 
27, 28 and 29, each of which feeds optical fiber 30 through optical fiber 
guides 31 to a wheel 32. As shown in the detail of FIG. 5, wheel 32 
comprises three peripheral grooves for containing each of the three 
optical fibers 31. By including three reels, it is possible to lay three 
optical fibers side by side on the substrate which is desirable for the 
reasons described above. The lengths of the three fiber sections can be 
made to be of equal length with a very high degree of accuracy in this 
manner. The three reels 27-29 are independently rotatable to permit slight 
differences of angular velocity, as is required when a curved path is 
being formed. 
Another advantage of the method of FIG. 2 is that it facilitates the 
formation and testing of a plurality of interconnections extending across 
a backplane. This is illustrated in FIG. 6 in which optical fiber 34 
represents a single continuously laid optical fiber path. The continuous 
path is formed by laying the fiber such that it forms loops 35 and 36 on 
opposite sides of the pattern. Notice that to form the pattern, the 
manipulator 20 of FIG. 2 must be rotated on its .theta. axis by more than 
three hundred sixty degrees. After the continuous optical fiber 34 has 
been laid, the substrate, along with the optical fiber 34 overlying it, is 
cut along lines 37 and 38. After the cut, it can be seen that four 
distinct optical fiber interconnections are provided between lines 37 and 
38 due to the severing of loop portions 35 and 36. 
Forming the optical fiber interconnections in this manner has two distinct 
advantages. First, it is not necessary to sever the fiber and rethread the 
fiber after each interconnection has been defined; rather, they are all 
defined by a single continuous operation that begins at the location 
labeled "start" and finishes at the location labeled "end." Second, all 
four optical fiber interconnections can be tested in a single operation 
prior to cutting by directing light in one end such as the "start" end and 
detecting light from the opposite end, the "end" end. If light has been 
transmitted satisfactorily, the cuts along lines 37 and 38 can be made and 
it can be assumed that the four interconnections will operate 
satisfactorily without requiring individual testing. The optical fiber 34 
may, of course, consist of three optical fibers as was described before, 
all three fibers being laid in a single operation and being tested 
together. 
The principle illustrated in FIG. 6 is applied in FIG. 7 to generate an 
optical fiber pattern 40 that will perform the interconnection function of 
one of the optical backplanes 13 of FIG. 1. The pattern is formed on a 
substrate having ports or tabs 16' that correspond to the tabs 16 of FIG. 
1. When the pattern 40 is being routed, coated substrate portions are 
located to the left of the left-most tabs 16' and to the right of the 
right-most tabs 16'. Thus, as each optical fiber routing traverses one of 
the tabs 16', it makes a loop and doubles back onto another tab 16' in the 
manner illustrated in FIG. 6. The entire pattern 40 is made with a single 
optical fiber, or alternatively with three side-by-side fibers, that are 
laid continuously to form a single continuous pattern. After, the single 
continuous pattern has been formed, it is tested as described before by 
directing light through the three continuous optical fibers that make up 
the pattern. The substrate is then cut to sever the loops on the extreme 
portions of the pattern and thereby to define the ends of tabs 16' on 
opposite sides of the pattern. The various curves within the pattern 40 
are designed to provide appropriate lengths for the various optical fiber 
transmission lines constituting the pattern 40. 
FIG. 8 shows a cross-sectional view of three fibers 41, 42 and 43 which may 
make up the pattern 40 of FIG. 7. Each fiber is a conventional dual 
acrylic coated glass fiber having a total outside diameter of two hundred 
fifty microns. The substrate 45 is preferably made of a flexible material 
such as Kapton having a thickness of 0.002 inch. Other materials such as 
Mylar (a trademark) could be used, but Kapton was found to have both added 
dimensional stability and flame retardancy compared to Mylar. Coated on 
the substrate 45 was a pressure-sensitive adhesive 46 which may, for 
example, be number 711 adhesive, available from the Adchem Corporation of 
Westbury, N.Y. This is an acrylic adhesive supplied on coated release 
paper. This adhesive requires the application of some pressure for 
adhesion and is therefore easier to work with than conventional adhesive. 
After routing, the optical fibers were encapsulated by a layer 48 of 
thermoplastic material as shown in FIG. 9. Over the thermoplastic 
material, another thin layer 49 of Kapton, 0.001 inch in thickness, was 
placed. A roller 50 was heated sufficiently to cause the thermoplastic 
material to flow around the optical fibers as shown. The thermoplastic 
material 48 is preferably a polyurethane, commercially available as number 
3205 from Bemis Associates of Shirley, Mass. This material could be flowed 
at a temperature low enough to avoid damage to the optical fibers and 
provided good moisture protection to the optical fibers. 
The FWS-200 flexible workstation that was used for controlling the 
manipulator is controlled by a stored computer program using the Modular 
Manufacturing Language, a language similar to BASIC. Although the Modular 
Manufacturing Language contains commands for coordinated straight moves, 
it does not have the capability of making radius moves. For this purpose, 
software routines were devised and written to approximate the continuous 
arc by a series of short and equal linear movements. Trigonometric 
relations were used to linearly interpolate a forty-five degree arc 
segment in thrity equal angular steps (1.5 degree each). A numerical file 
containing the corresponding series of X and Y increments was tabulated. 
Because of the resolution limit in the X-Y motion, it was necessary to 
insure that all vectors and arcs join together to form traces of the 
proper dimensions. This was achieved by rounding all incremental movements 
to the nearest 0.001 inch and slightly adjusting values to sum to the 
overall dimensions of 0.707 by 0.293 inches. This data file was the basis 
for all possible orientations and directions of forty-five degree arc 
segments, obtained through a series of sixteen subroutines using proper 
phasing and sign of the motion. 
For placing fibers in array, a problem arose from the fact that three 
fibers have an overall width of approximately 0.0295 inch and the 
manipulator is limited to steps of 0.001 inch. Whrn fiber groups were 
placed using a spacing of 0.030 inch, small gaps occurred as all six 
groups were laid and the resulting eighteen-wide array was not acceptable. 
This problem was overcome by taking advantage of the compliant nature of 
the tacky adhesive, allowing the fiber to "slide" a small amount 
laterally. Fiber groups were laid in a staggered fashion by alternating 
the pitch by 0.030 and 0.029 inches, resulting in close packing in the 
array. One may increase the resolution of the linear motors to 0.0001 inch 
which will overcome this problem. During the fiber routing, the speed of 
the wheel 32 was limited to the range of one to five inches per second. 
Described below is a summary of the process flow used for making the 
flexible optical circuits. 
1. The acrylic transfer adhesive (10.8 inch wide on paper release liner) 
was pressure laminated onto Type HN Kapton film (12 inches wide). 
2. An FR4 board (12 by 24 inches) was thoroughly cleaned with solvent, 
dried, and lightly sprayed with 3M.RTM. spray adhesive. 
3. The Kapton/Adhesive material was rolled under pressure onto the board 
with the Kapton side against the board. Edges were trimmed and taped to 
secure the sheet. 
4. The board was positioned onto the work area and clamped. 
5. The paper liner was removed to expose the pressure sensitive adhesive. 
6. The manipulator wheel was threaded with optical fiber. 
7. Under software control, the manipulator lowers to contact the wheel to 
the board, routes fiber groups, and raises at endpoint. 
8. Manually assisted operations include cutting, releasing, and 
re-threading the fiber. 
9. Routed board is removed from work area and covered with one sheet of 
0.005 inch polyurethane and one sheet of 0.001 inch Kapton. 
10. Board is laminated between heated rollers at a speed of about five 
inches per minute, temperature of one hundred fifty degrees Centigrade, 
and a pressure of ten to thirty pounds per square inch. 
11. Razor bade and metal template was used to cut border and tab edges. 
12. Completed circuit was separated from carrier board and cleaned. 
Referring to FIG. 10, there is shown another optical fiber routing method 
for making a "fan-out" arrangement of transmission lines in which the 
transmission lines of one input port are directed to each of several 
output ports. The optical fiber path 52 is a continuous path which is 
begun at the end labeled "start" and ends at the location labeled "end." 
After the continuous path 52 has been made, the optical fiber is tested 
as, for example, by directing light from a source 61 into one end of the 
fiber and detecting the light with a photodetector 62. Next, the substrate 
and the optical fiber on it are cut along line 53. After cutting, it can 
be seen that two input port regions 54 and 55 have been made, each of 
which comprise four optical fibers with optical fibers from each of the 
input ports being directed to all of the output ports 56-59. As before, 
the plurality of optical fiber lengths are made from a single continuous 
fiber by including a plurality of loop regions which are cut or severed by 
the cutting along line 53. The optical fiber path 52 may, as before, 
comprise one, two, three or more optical fibers. 
The various embodiments which have been described are intended to be merely 
illustrative of the inventive concepts involved. The substrate used for 
the optical backplane may be of any of various materials which may be 
flexible or inflexible. While it is convenient to use a pressure-sensitive 
adhesive on the substrate, conventional adhesives could alternatively be 
used. The materials and methods described for encapsulating the optical 
fibers have been found to be consistent with requirements for protection 
from the environment, particularly moisture, and have been found to adhere 
well under rigorous conditions, but various other encapsulation methods 
could alternatively be used. In theory, the method of FIG. 2 could be 
practiced manually which would be an improvement over manual fabrication 
of the FIG. 1 embodiment, but, as mentioned before, a principal advantage 
of the method is its amenability to robotic implementation. The reel 19 
illustrated in FIG. 2 has been found to be a convenient way of feeding out 
optical fiber in response to small increments of optical fiber tension so 
that undue stresses on the relatively fragile-fiber are avoided, but other 
containers or magazines for optical fiber could probably alternatively be 
developed. A principal advantage of the method of FIG. 2 is that patterns 
such as that shown in FIG. 6 can be applied as a single continuous optical 
fiber which may then be conveniently tested, but individual 
interconnection lines can be made by the method of FIG. 2 in which the 
optical fiber is severed after each interconnection is made. Various other 
embodiments and modifications may be made by those skilled in the art 
without departing from the spirit and scope of the invention.