Optics: measuring and testing – By light interference
Reexamination Certificate
1999-01-07
2001-04-10
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
C356S073100, C356S511000
Reexamination Certificate
active
06215555
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to fiberoptic connectors, and more particularly to methods and apparatus for reliably testing multi-fiber fiberoptic connectors to ensure that all fiber ends are properly aligned with, and in physical end-to-end contact with, corresponding fiber ends in a mating fiberoptic connector.
The MT (Mechanically Transferable) connector shown in FIG.
1
and MPO (Multi-path Push On) connector shown in
FIGS. 2A and 2B
originally developed by NTT in Japan, have been deployed primarily in Japan for several years. The advantageous technical features and price/performance ratios of MT and MPO connectors have meant that this non-traditional style of fiberoptic connector is gradually becoming more widely accepted, and is becoming quite widely used in both the U.S. and other worldwide markets. The main advantages of MT and MPO connectors are high optical fiber density (typically 2-12 fibers), small physical size, and low cost. A variant of the MT connector is the MT-RJ connector, which has a smaller design that fits within the footprint of a standard 8 pin modular telephone jack, and is being considered as one of the main contenders by the standards organizations for fiberoptic premise network wiring.
This type of fiberoptic connector has extremely critical dimensional tolerances that must be maintained to ensure acceptable performance and “intermatability” of connectors. As these connectors (the MT, MPO, and MT-RJ) become more widely used in single-mode applications, their geometric tolerances are expected to become even tighter. As fiberoptic cable bandwidth requirements increase, the fiberoptic connectors can become one of the most critical components affecting the overall system performance of a fiberoptic transmission system.
An optical fiber
1
shown in
FIG. 4
typically is constructed in three distinct concentric layers, including a 250 micron diameter acrylic jacket
2
which coats the outside of the glass optical fiber. The jacket's main function is to provide basic environmental protection to the glass optical fiber. Without this jacket, just brushing the fiber over another surface could score the glass, leading to a crack which eventually would propagate through the glass, fracturing the fiber and rendering it inoperable. Since the acrylic coating can be colored, it also provides a useful method of fiber identification. The second layer is a 125 micron diameter cladding
3
. This has become the standard outer diameter for all but the most unusual and application specific fiber designs. The purpose of the cladding is to contain the light within the 8 micron fiber core
4
, using the principal known as “total internal reflection”. The secondary purpose of the cladding
3
is to increase the fiber diameter to a level that provides it sufficient mechanical strength, can be fairly easily seen and can be manipulated by human hands. The core is the part of the fiber that carries the light. The core
4
and the cladding
3
constitute one contiguous piece of glass; however, they have different refractive indexes to keep the light within the core. Multi-mode fibers have a typical core diameter of 62.5 microns, as opposed to the 8 micron core diameter typically used in single-mode fibers.
Although the core of a single-mode fiber is much smaller than that of a multi-mode fiber, allowing only a single “mode” to propagate from the input to the output of the fiber dramatically increases the amount of data or “bandwidth” offered by single-mode fibers, when compared to multi-mode fibers. With the rapidly increasing demand for voice, video, and Internet communications, bandwidth can be a scarce and valuable resource. As such, most new long distance fiber deployment is single-mode. Even when using sophisticated multiplexing techniques, the maximum bandwidth capacity of a single fiber may be used up, and there is no other option than to add additional fibers to increase communication capacity. As the number of single fibers being added to a bundle increases, so does the diameter of the cable necessary to contain and protect them. Not only is this expensive, but it can also create problems in already crowded ducts and passages used to route cables. Therefore, manufacturers are looking for ways to achieve smaller physical size, higher performance, more manageable, and less expensive systems, and have begun to manufacture “ribbon fibers”. A ribbon fiber as shown in
FIG. 5
includes a number of optical fibers (typically 2-12) laid side by side and sleeved with an additional outer coating. This technique provides very high fiber densities, while having the added advantage that installation and maintenance workers are able to handle up to 12 or more fibers at one time.
Fiber preparation can be a very labor intensive and expensive part of terminating or joining fibers together. Having the ability to work on multiple fibers at one time using specialized tools has led to dramatic time and cost savings in optical fiber installation and maintenance for ribbon fiberoptic cable users.
Optical fiber multiplexing and transmitter and receiver technology have made such great technological advances that data transfer rates of the order of Terabytes per second over a “perfect” optical fiber link have been demonstrated, using a combination of various multiplexing and data compression technologies. As a result, engineers and scientists now face the difficult task of simplifying system implementation (without losing performance) to a level such that workers with little experience, crawling down through manhole covers in harsh environments, can be reasonably expected to install and maintain such ribbon fiber links with a high degree of success and reliability.
One of the most important and frequently overlooked factors involved when installing a fiberoptic transmission system is the proper installation and use of fiberoptic connectors. When it is necessary to join or patch two ribbon cables together, there are two primary choices: fusion of optical fibers and use of optical fiber connectors. Fusion involves accurately cleaving all of the fibers to the same length across the ribbon on the two cables to be joined, and then using a specialized machine known as a ribbon fusion splicer, which brings all of the fiber pairs together very accurately along the X,Y and Z axes. An electrical arc applied with a small compressive force pressing the cleaved surfaces together then is used to physically fuse the individual fiber pairs together as one contiguous fiber. This process, when performed properly, and after the application of additional splice protection, can join two multi-fiber ribbon cables together almost as effectively as if they were manufactured as a contiguous piece of fiber. For permanent joints, fusion splicing provides the most economical and robust solution to joining optical fibers.
On the other hand, there are many situations where a permanent joint is not desired, not required, or not feasible, in which case connectors become the only viable alternative technique for joining the fibers. Examples of such applications would include (1) patch panels where reconfiguring of fiber routes may be necessary, and (2) attachment to system or test equipment and applications such as high speed optical back-planes which require automatic connection and disconnection of the optical path as circuit boards are inserted and removed. At the user level “consumers” expect multi-fiber fiberoptic connectors to work in the same way as electrical connectors, that is, the fiberoptic connectors are simply “plugged in” for a pair of MPO or a pair of MT connectors, and everything works. In reality, a great deal of sophisticated technology and precision engineering has to occur to make this happen.
It is important to recognize that the performance of optical connectors can have a dramatic impact on the overall performance, integrity and reliability of an entire optical link. The main “enemies” of an optical signal at a connectorization point are “loss” and “back-reflection”. Since the
Cahill Sutton & Thomas P.L.C.
Direct Optical Research Company
Font Frank G.
Lee Andrew H.
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