Direct VCSEL Launch into Large Core Multimode Fiber: Enhancement of the Bandwidth*Distance Product

Traditionally, large core (> 100 micron) step index multimode optical fiber has occupied a relatively small niche of applications in data communications. While the large diameter of this type of fiber makes it easy to align to optoelectronic devices, its Bandwidth*Distance (BW*D) product is low due to modal dispersion between the large number of modes supported by a fully filled fiber. Recently, interest has been renewed in using an underfilled launch to excite 62.5 micron core graded index multimode fiber as a way to improve its bandwidth performance. With the proper launch conditions, this same effect has been measured in large core fiber. A vertical cavity surface emitting laser (VCSEL) is used to provide a low numerical aperture launch into a large core fiber, which has a relatively larger numerical aperture. The laser thus underfills the modes of the fiber, and a bandwidth enhancement for the fiber is obtained. Results of experiments performed on step and graded index large core multimode fibers using a direct VCSEL launch are presented. In addition, these relaxed alignment tolerance fibers allow the utilization of very low cost cabling and connectorization procedures for parallel optical fiber cables. Data, including skew, bandwidth, and insertion loss, are presented on these cables.