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High-Bandwidth, Expendable Cable Enables New-Generation Unmanned Underwater Vehicles to Communicate Effectively in Real Time
Advancements in electronic miniaturization, energy-storage components, higher efficiency propulsion systems, sensor suites, and composite housing materials have significantly increased the capability of UUV designs to meet expanded mission areas that were previously considered impractical with traditional UUVs. However, as the operational capabilities of these new UUVs increase, so too must UUV communication capability expand to reliability retrieve, transfer, and process increased amounts of data in a timely fashion. Few operating environments place more limitations on the ability to transfer high-bandwidth data than the underwater medium. Radio frequency propagation is limited by the conductive medium; low visibility and particulate scattering restrict optical transmission; and noise, high-frequency attenuation, and multipath interference dominate acoustic transmission capabilities. Unless a hard connection via a cable is used, data transfer rates are usually neither real-time nor highbandwidth. Improved underwater communication is realized by incorporating an umbilical cable but this can result in major impacts on the UUV design that limits the operational capabilities of the UUV, and may require a specialized platform from which to deploy and operate the UUV. An expendable fiber-optic communication link based upon a small diameter microcable (less than 0.035 inches in diameter) offers the best trade-off for many potential UUV applications.
Microcable History
Early microcables incorporated a protective jacketing that was composed of heat-polymerized resin impregnated with reinforcing fibers. Use of these heat-polymerized resinous materials in the production of microcable had several manufacturing disadvantages that limited the longterm production capabilities of the microcable. The high temperatures required to cure the heat-polymerized resins damaged the ultraviolet-lightcured buffer layers present on state-ofthe-art optical fibers resulting in unacceptable degradation in optical performance. Reducing the curing temperature to a more moderate level could be achieved with a corresponding increase in the exposure time of the resin. However, at these reduced production rates (approximately 4 inches/second), jamming of the microcable forming die with resin and reinforcing fibers limited the production lengths of microcables to between five and 10 kilometers. These production problems combined to increase the costs of microcable to levels that limited the applications for an expendable telemetry link. To meet the communication requirements of this new generation of UUVs, it became evident that there...