Optical couplers with thermoformed fibers

An optical coupler using round optical fibers whose ends have been formed into a predefined shape to allow greater physical packing densities in order to achieve greater uniform illumination efficiency. The interstitial space between optical fibers is greatly reduced by thermoforming the ends of round optical fibers in a mold that makes a gradual transition from a circular shape to the predefined shape to avoid optical loss. Since the change in shape is gradual, the total cross-sectional area of the fibers remains constant; and optical loss is minimal becuase there is no reduction in mode volume. The predefined shape may be substantially square or rectangular. The core and cladding materials of the optical fiber are carefully chosen with respect to the glass transition state temperatures of both materials because the temperature at which the materials are formed by a mold are slightly above the glass transition state of the core or cladding, whichever is higher.

TECHNICAL FIELD 
the present invention relates to the transmission of light in an optical 
system and, in particular, to an optical coupler used in such a system. 
BACKGROUND OF THE INVENTION 
In many optical local area networks (LANs) and optical backplanes that have 
a bus-type architecture, an optical repeater receives and combines optical 
signals form optical transmitters in system nodes, as well as amplifies 
and individually retransmits signals to optical receivers in the system 
nodes. FIG. 1 illustrates such a prior art optical transmission system. 
Nodes 106 through 110 transmit and receive optical signals to and from 
optical repeater 100 by way of optical fibers 130 through 139. 
Specifically, optical repeater 100 receives and combines optical signals 
via optical fibers 135 through 139 and redistributes these optical signals 
via optical fibers 130 through 134. Optical combiner 115 receives and 
combines the optical signals; whereupon components 116, 117, and 118 
electrically process the combined signal. Optical unit 123 then converts 
the combined electrical signals to an optical signal and transfers it to 
optical splitter 124 via optical link 140 which communicates the optical 
signal on links 130 through 134. Nodes 106 through 110 are identical with 
each having a controller, receiver unit, transfer unit, and power control 
as illustrated for node 106. Optical combiners and splitters, such as 
units 115 and 116, are generically refereed to herein as optical couplers. 
FIG. 2 illustrates an optical coupler in accordance with U.S. Pat. No. 
4,913,508 which performs the functions of couplers 115 and 124. The 
optical signals from optical fiber bundle 201 are coupled via optical 
coupler 203 to optical fiber 204. Similarly, an optical signal from 
optical fiber 204 may be coupled to optical fiber bundle 201. The cavity 
of optical coupler 203 forms the optical core of the optical coupler and 
is filled with a material that creates a waveguide with substantially the 
same numerical aperture as optical fiber 204 and optical fiber bundle 201. 
By matching the numerical apertures, the efficient transfer of optical 
energy is achieved between the optical fibers in spite of possible 
refractive index mismatch between the optical core and optical fiber 
bundle 201 and optical fiber 204. 
One of the problems of making optical couplers of the type illustrated in 
FIG. 2 is the packing density achieved using round fibers in optical fiber 
bundle 201. The theoretical efficiency for perfectly uniform illumination 
of optical fiber bundle 201 by optical fiber 204 is given by the total 
core area of optical fiber bundle 201 divided by the total cross-sectional 
area of the cavity of optical coupler 203. In general, the theoretical 
efficiency for perfectly uniform illumination is limited to the range of 
50-60% for various numbers of fibers using the optical coupler illustrated 
in FIG. 2. 
The disadvantage of utilizing round optical fibers in a round cavity is 
illustrated in FIG. 3 and 4. FIG. 3 illustrates the case where the 
diameter of the cavity of the optical coupler is equal to four times the 
diameter of optical fiber 204. FIG. 4 illustrates the case where the 
diameter of the cavity of the optical coupler i equal to eight times the 
diameter of optical fiber 204. As illustrated, the optical coupler of FIG. 
3 has a theoretical efficiency of 49%; and the optical coupler of FIG. 4 
has a theoretical efficiency of 59%. Further, for optical fibers having 
polymeric cladding and cores, the cladding cross-sectional area is 
extremely small compared to the interstitial space between bundled optical 
fibers. Thus the removal of the cladding is costly and difficult with only 
a tiny increase in efficiency that might be gained. In the coupler 
illustrate din FIG. 2, the numerical apertures of the fibers and the 
polymeric mixing region are closely matched and the reflection is 
extremely small. Hence, the inefficiency caused by the interstitial space 
between round fibers is the only significant source of inefficiencies in 
this type of coupler. 
It is known in the art to mil optical glass preforms into D-shaped 
cross-sections and to draw D-shaped fibers from these preforms. These 
D-shaped optical fibers are then put together in circular cross-sections 
to make up 2.times.2 optical couplers. However, this method has the 
disadvantage of extremely high milling cots and material waste, since the 
entire optical fiber has to be in the D-shape. Similarly, other fiber 
shapes can be made by first milling an optical preform to form optical 
fibers but would suffer from the same disadvantages as the use of this 
method to form D-shaped optical fibers. 
SUMMARY OF THE INVENTION 
The aforementioned problem is solved and a technical advance is achieved in 
the art by reducing the interstitial space by using round optical fibers 
whose ends have been formed into a predefined shape to allow greater 
physical packing densities in order to achieve greater uniform 
illumination efficiency. Specifically, the interstitial space is greatly 
reduced in preferred embodiments by thermoforming the ends of round 
optical fibers in a mold that makes a gradual transition form a circular 
shape to the predefined shape to avoid optical loss. Since the change in 
shape is gradual, the total cross-sectional area of the fibers remains 
constant; and optical loss is minimal because there is no reduction in 
mode volume. The predefined shape may advantageously be substantially 
square or rectangular. The core and cladding materials of the optical 
fiber are carefully chosen with respect to the glass transition 
temperatures of both materials because the temperature at which the 
materials are formed by the mold are slightly above the glass transition 
temperature of the core or cladding, whichever is higher. If the core 
material of an optical fiber has a semicrystalline structure, the optical 
fiber is formed at a temperature near the melting temperature of the core 
material. Similarly, if the cladding is of sufficient thickness and is of 
a material having a semicrystalline structure with the core having a 
non-semicrystalline structure, the optical fiber is formed at a 
temperature near the melting temperature of the cladding material. 
The mold is capable of being used in field applications, allowing the 
optical fibers to be terminated in the field without difficulty. In one 
embodiment in accordance with the invention, the cladding material and the 
core material of the thermoformed optical fibers are a fluorinated acrylic 
polymer and poly(methyl methacrylate), respectively. 
Other and further aspects of the present invention will be become apparent 
during the course of the following description and by reference to the 
accompanying drawing.

DETAILED DESCRIPTION 
FIG. 5 illustrates an optical coupler for communicating optical signals 
between optical fiber bundle 502 and optical fiber 504. In accordance with 
the invention, the end of each fiber of optical fiber bundle 502 has been 
formed--illustratively thermoformed--to have advantageously a 
substantially square end and to concomitantly minimize the amount of 
interstitial space between each optical fiber 1 through 16 of optical 
fiber bundle 502 when inserted into optical coupler 503. Fabrication of 
optical coupler 503 with optical fiber bundle 502 and optical fiber 504 is 
performed by first inserting optical bundle 502 and optical fiber 504 into 
optical coupler 503. After the insertion of optical fiber bundle 502 and 
optical fiber 504, the cavity of optical coupler 503 is filled via tube 
505 with an optical medium which creates a waveguide having substantially 
the same numerical aperture of optical fiber 504 and optical fiber bundle 
502. Details on the type of optical medium, the preparation, and insertion 
of this optical medium via tube 504 are given in U.S. Pat. No. 4,913,508, 
which is hereby incorporated by reference. 
Consider how each optical fiber of optical fiber bundle 502 is thermoformed 
to have substantially a square shaped end. Advantageously, the glass 
transition temperatures for the core and cladding materials of the optical 
fiber are similar because the temperature at which the materials are 
formed is slightly above the glass transition temperatures of the core or 
cladding, whichever is higher. Advantageously, one material system 
providing the desired characteristic is a poly(methyl methacrylate) core 
material and a fluorinated acrylic polymer cladding material. This 
material system provides a temperature range for molding purposes of 
approximately 100.degree. C. to 150.degree. C. One skilled in the art 
could readily utilize other materials for the cladding and core which 
would have the desired glass transition temperatures. 
If the core material of the optical fiber has a semicrystalline structure, 
the optical fiber is formed at a temperature near the melting temperature 
of the core material. In addition, if the cladding is of sufficient 
thickness and is of a material having a semicrystalline structure and the 
core has a non-semicrystalline structure, the optical fiber is formed at a 
temperature near the melting temperature of the cladding material. One 
example of such a cladding material is poly(4-methyl pentene-1) which is 
commercial used with a polycarbonate core material. The melting 
temperature of poly(4-methyl pentene-1) is 245.degree. C. 
FIG. 6 illustrates a front view of mold 600 for forming the end of optical 
fiber 603 into a substantially square as illustrated in FIG. 7. Optical 
fiber 603 represents one of the optical fibers of optical fiber bundle 
502. Optical fiber 603 is place din contact with subassembly 602, and 
subassembly 601 is positioned as illustrated in FIG. 6. Heating elements 
604 and 605 heat subassemblies 601 and 602, respectively, to approximately 
125.degree. C. for a sufficient amount of time to bring both the cladding 
and core of optical fiber 603 to a temperature above their glass 
transition temperatures. After this temperature is achieved throughout 
optical fiber 603, subassembly 601 is moved to the position illustrated in 
FIG. 7 resulting in the end of optical fiber 603 becoming substantially 
square. 
FIG. 8 illustrates a side view of subassembly 602, and FIG. 9 illustrates a 
top view of subassembly 602. Angle 801 defines the transition form a round 
fiber to a substantially square fiber. Angle 801 is defined by the 
following formula: 
##EQU1## 
where L is length 802 of FIG. 8 which is the length of the transitional 
portion of subassemblies 601 and 602. To minimize modal loss, the length 
802 is chosen to be at least 20 fiber diameters. R is the radius of 
optical fiber 603. Subassembly 601 is similar in shape to subassembly 602. 
Advantageously, it may be desirable to be able to produce substantially 
rectangular shaped ends of optical fibers in addition to substantially 
square shaped ends. FIG. 10 illustrates mold 1000 that produces 
substantially rectangular ends. First, optical fiber 1003 is placed in 
subassembly 1002, and subassembly 1001 is then brought into contact with 
optical fiber 1003. Subassemblies 1001 and 1002 are then heated to 
approximately 125.degree. C. Once optical fiber 1003 has reached a 
temperature above the glass transition temperatures of the cladding and 
core, subassembly 1001 is moved to the position illustrated in FIG. 11. 
It is to be understood that the above-described embodiments are merely 
illustrative of principles of the invention and that other arrangements 
may be devised by those skilled in the art without departing from the 
spirit or scope of the invention. In particular, other molds may be 
readily devised by those skilled in the art; and in particular different 
shapes may be utilized for the ends of the optical fibers. In addition, 
other material systems for the cladding and core materials may be readily 
devised by those skilled in the art.