Post-formation feature optimization

Details Post-formation feature optimization Post-formation feature optimization

2002-03-14

2003-09-16

Nguyen, Khiem (Department: 2839)

Optical waveguides

With disengagable mechanical connector

Optical fiber/optical fiber cable termination structure

Reexamination Certificate

active

06619855

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to components and processes for fiber optic related component fabrication. More particularly, the invention relates to the fabrication of optical coupling and waveguiding elements.
BACKGROUND OF THE INVENTION
Optical Fibers in commercial systems have been traditionally held by using a combination of pieces.
A connector assembly
100
, such as shown in
FIG. 1
as an exploded view is used to attach various fiber pieces (or fiber pieces and modules) together. A ferrule
102
is the part of the connector
100
into which the fibers
104
themselves are inserted before the ferrule
102
is inserted into the overall connector itself. The ferrule
102
is a ‘high-precision’ piece of the assembly
100
. It holds the fiber(s)
104
in a precise position and ensures that when two connector pieces are attached, that the fibers in the two pieces are held in accurate alignment. The remainder of the connector
106
is ‘low precision’ relative to the ferrule
102
.
In the multi-fiber connectors available today, most of the connections are for fiber arrays of 2 or more fibers, such as shown in U.S. Pat. No. 5,214,730, up to arrays of 1×12 (although some commercial 2×12 configurations have been tried). The connectors employed are referred to by various names depending upon who makes them. In 1×2 arrays, connectors are referred to as ST, LC, MT-RJ connectors while for 1×12 arrays the connectors are referred to as MTP®, MPO, MPX and SMC connectors, among others. In the 1×12 or 2×12 area, all of the various connectors use a common type of ferrule commercially available from, among others, US Conec Ltd. and Alcoa Fujikura Ltd. In addition, commercial connectors for small arrays (less than 12) fibers have also been proposed, for example, in U.S. Pat. No. 5,743,785.
Fiber holding pieces, such as ferrules
102
, can be made by molding plastic or epoxy pieces containing holes
108
into which optical fibers
104
can be inserted. Fibers must be able to be centered in each hole precisely and repeatably.
When an array of holes is made in a material for holding optical fibers, there are two aspects which need to be controlled. The spacing between holes (the “pitch” of the holes) and the diameter of each hole. Both have some margin of error due to the inherent inaccuracies of the fabrication techniques. If inaccuracies introduce errors in either (or both) pitch or size that are too large, then the fibers can be inserted at an angle or will not be positioned correctly in the ferrule. In either case, this negatively affects the ability to couple light efficiently, if at all, from one bundle to another or from an optical or opto-electronic component to a fiber bundle. If the hole pitch is inaccurate, then fibers from one bundle will not line up well with fibers of another bundle. However, even if the center-to-center pitch of the holes is very accurate, because the hole diameter is larger than the fiber (and each hole likely varies across an array) each fiber need not be in the exact same place in the hole as the other fibers in their holes, then that can cause misalignment, leading to inefficiencies or unacceptable losses. For example, if each of the holes in a ferrule piece was accurate to within 4 microns, then adjacent fibers could be off in pitch by up to 4 microns, since one fiber could be pushed to one side by 2 microns and the adjacent fiber could be pushed in the other direction by 2 microns. While this may be acceptable for multimode fibers, for single mode fibers this would be a huge offset that could make connections unacceptable or impossible.
In addition, fibers should generally not be placed in a hole at an angle or, if inserted at an angle, the particular angle should be specifically controlled.
FIG. 2
shows an example connector hole
200
and fiber
202
. The inner circle, represents an actual fiber
202
while the outer circle, represents the hole
200
in the ferrule. As shown, the difference in sizes is not to scale but is exaggerated for purposes of illustration. Nevertheless, in actuality, the ferrule hole
200
must be larger than the fiber
202
by enough of a margin to allow for easy insertion—ultra-tight tolerances can not be effectively used. While the fiber
202
should ideally be centered with respect to the hole
200
, as can be seen in
FIG. 3
, any individual fiber
202
could also be pushed in any hole
200
to somewhere else in the hole, for example, either the left or right edge (or any other edge) where it would not be centered within the hole
200
. Thus, even if the ferrule has an accurate pitch “P” between hole centers
204
, adjacent fibers
200
in an array may have an incorrect pitch “P+2&Dgr;P” due to the offset &Dgr;P between the center
206
of each hole
200
and where the fiber
200
lies within the hole
200
, in this case, causing an incorrect pitch of P plus 2 times the individual offset &Dgr;P in each hole.
The 1×12 and 2×12 ferrule technology currently in commercial use is based upon a glass filled epoxy resin (a high-performance plastic) which is fabricated using a common plastic molding technique called transfer molding. Today, ferrules molded out of epoxies or plastics can be made to the necessary tolerances for multimode fibers, but special care must be taken during fabrication. Plastic molding technology is very process sensitive and molds having the requisite precision are extremely difficult to make. Even so, yields tend to be poor due to the inherent manufacturing process errors that occur in plastics molding. Since the tolerances on these pieces must be very accurate (on the order of about 1 to 2 micrometers), high yield manufacture is difficult. As a result, the cost of terminating fiber bundles into these connectors can be quite expensive, running hundreds of dollars per side. In addition, the process is not scalable to larger numbers of fibers (particularly 30 or more) because of inaccuracies and yield issues associated with molding technology and reliable production of ferrules for similar numbers of single mode fibers is even more difficult.
There has been an increasing need among users in the fiberoptic field for larger groups of fibers, so demand for connectors to handle these groups has been increasing as well. As a result, creation of connectors for larger arrays, such as 5×12, have been attempted. One manufacturer is known to have made a 5×12 connector array, but achieved such poor yields that they deemed an array of that size unmanufacturable. Moreover, the cost of producing the pieces resulted in their being sold for $500 each, due to poor yield, and the mold for producing the pieces was destroyed during the process.
The problem is that in plastic molding pieces for holding higher fiber counts in small spaces results in less structural integrity for the molded piece. As such, the prior art has been forced to do without commercial connectors for such large arrays, because 5×12 arrays can not be reliably created and commercial connectors for larger format arrays (e.g. even a 6×12) are considered prohibitively difficult to even attempt.
The ferrule area is very small, since ferrules for the above MTP, MPO, MPX or SMC connectors are about 0.07″ high, 0.3″ wide and 0.4″ deep, so molding or machining of features in the ferrules of the sizes required to hold multiple optical fibers (which typically have about a 125 micron diameter for a multimode fiber and a 9 micron diameter core for a single mode fiber) is very difficult. Since single mode fibers have an even smaller diameter than multimode fibers, molding or machining ferrules to accommodate large arrays of single mode fibers is currently, for all practical purposes, impossible—particularly on a cost effective commercially viable scale.
Additionally, making ferrules for arrays is made more difficult due to process variations during production because, as the holes approach the edge of the ferrule, the structural integrity of the walls decrease causi

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