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www.sea-technology.com July 2014 / st 35 O cean observatories require both an effcient means of data transmission and the reliable delivery of electrical power. Over the past decade, a signifcant number of cabled ocean observatory projects have been installed and are now opera- tional. Fiber-optic connections to shore, or to other offshore facilities, provide low-latency, high-band- width, bidirectional communication and control of the observatory. As observatory networks ex- tend farther from shore and grow in complexity and size, undersea amplifcation of transmitted data is often required. The undersea telecom in- dustry has provided customized transmission and powering solutions for such cabled observatories. Products and technology have been developed to support these projects, and they can be custom- ized for systems with a variety of data transmission and powering architectures. Basic Architectures Undersea telecom transmission systems are ei- ther repeatered or nonrepeatered. Nonrepeatered systems are characterized by point-to-point trans- mission over a submarine cable between terminal stations. The fber-optic transmission distance is limited by transceiver (transmitter-receiver) performance, fber nonlin- ear propagation effects and optical attenuation over the f- ber path. There are no powered amplifers between terminal stations, though the undersea cable often has an electrical conductor for electroding and fault localization only. De- pending upon the capacity required (a few high-data-rate channels or many), nonrepeatered systems are limited to a digital line section (DLS) length in the range of 300 to 500 kilometers. If the geography is suitable, distributed network topologies (rings, festoons, collapsed ring) can also be sup- ported. In the case of small ocean observatories, the elec- troding conductor could be used for observatory power. Repeatered systems rely on a number of powered un- dersea optical amplifers spaced regularly along the cable (50 to 150 kilometers). In addition to the factors that affect nonrepeatered systems, the repeater amplifer noise also im- pacts transmission performance. Repeatered technology can provide nonregenerated DLS lengths in excess of 11,000 ki- lometers supporting trans-Pacifc dense wavelength division multiplexing (DWDM) applications. The amplifer pump lasers are driven by a constant current carried by the cable conductor from shore power sources. The necessary volt- age level is a function of cable length, quantity of repeaters, number of amplifed fber pairs, component voltage drops and earth potential. Depending upon the length of a repeatered segment, power feed equipment (PFE) may need to supply between 1,000 to 15,000 volts DC at line currents in the range of 0.6 to 1.5 amperes. Repeatered systems can be protected against the effects of a single shunt fault by placing power supplies at each end of the system with enough voltage ca- pability to power the system from only one end (single-end feed). Subsea Data Transmission And Electrical Powering Flexibility for Cabled Ocean Observatories By Robert Thomas • Adnan Akhtar • Marsha Spalding (SL DCC Photo Credit: TE SubCom) Single- and dual-conductor powering of a cabled ocean observatory.