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HELPING  PLASTIC  OPTICAL  FIBER 
CHALLENGE THE  PERFORMANCE OF  GLASS AND THE   SIMPLICITY OF USE OF  WIRE

By George J. Shevchuk - Bell Labs, Lucent Technologies


Introduction

Copper wire and optical fiber are among the most widely used physical media for communications. They share many common traits, particularly as regards their interconnection. Wire, however, has many forms of interconnection due to the simplicity of making electrical connections. Interconnecting optical fibers, on the other hand, requires the guiding of the light out of the end of one fiber into the end of another, without allowing too much of it to escape at the interface. Recent advances have produced plastic optical fiber (POF) capable of significant performance, and thus rekindled the interest in simplifying its interconnection. When comparing plastic optical fiber to glass fiber (GOF), there is appreciable simplification in its cutting and in eliminating the need for careful stripping of a coating. However, additional effort is still needed to ensure that the POF endface is smooth enough to maintain adequate guidance of the light. Additionally, connectors are needed to provide accurate alignment of the fiber ends to each other. Any improvement and/or simplification in these aspects could greatly improve the usability of POF.

High-Performance POF

Recent developments that make plastic optical fiber very attractive for high bandwidth applications involve materials and processes for the fabrication of low loss, graded­index fibers (GI­POF). Such a fiber, using a perfluorinated polymer (CYTOP) developed by Asahi Glass Company, has demonstrated data transmission at rates greater than 11 gigabits per second over lengths of 100 meters. Its cross­sectional geometry consists of a 120 µm core, a 220 µm cladding, which are in turn surrounded by a reinforcement layer of acrylic with a 0.5 mm outer diameter. These fibers have demonstrated losses as low as 20 dB per km.

Fiber End Preparation

Cutting POF with a razor or other sharp blade results in fiber ends which, when mated, introduce about 3 dB or more loss in the light passing through. After closer study of the factors contributing to this loss, the presence of multiple small subsurface cracks was found to be a significant contributor. Further investigation lead to the development of a method of blade­cutting the fiber that results in significant reduction or elimination of the cracking. This is accomplished by maintaining the fiber under some axial compression during the cut. Tests were conducted on the fiber described above using these compression cuts. With the cut fiber ends mated to a polished end, the average loss was .86 dB. For comparison, fibers cut without compression yielded losses of 2-3 dB when mated to the polished fiber.

Eliminating the Plug

While many applications of high­performance POF will require connectors, there remains the potential for truly maximizing the benefits of simplicity and low cost offered by such fibers for many other applications. One approach to such simplification in connections to lasers or photodetectors may be modeled on the wire terminals typically used on audio equipment, particularly speakers. A simple molded plastic receptacle provides for guiding the wire into place and for retaining it with a spring clip. In a similar fashion, a receptacle with a spring clip for terminating POF was prototyped. The receptacle provides the coarse alignment of the fiber end to a precision alignment bore in the device's housing, and contains the clamping spring for retaining the fiber in place. Measurements were made using these prototypes with packaged vertical cavity surface­emitting lasers (VCSELs) as sources, and a fiber compression­cut at both ends. The amount of light captured by the fiber, connected to such a receptacle was about 2 dB below what was output at the device's housing, without receptacle. The variation between multiple matings, mostly dependent on angular orientation, was 0.7 dB. While there are no connectorized equivalents to compare to yet, these results are very promising.

Summary

The new breed of graded-index, perfluorinated plastic optical fiber offers an opportunity for bringing the benefits of optics and the simplicity of wire to high- volume, short distance applications. Simple fiber end preparation using new techniques and tools, and very simple receptacles for connecting bare fiber to equipment will help in exploiting these opportunities.

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