Feature ArticleSubsea Data Transmission And Electrical Powering
By Robert Thomas • Adnan Akhtar • Marsha Spalding
Undersea telecom transmission systems are either repeatered or nonrepeatered. Nonrepeatered systems are characterized by point-to-point transmission over a submarine cable between terminal stations. The fiber-optic transmission distance is limited by transceiver (transmitter-receiver) performance, fiber nonlinear propagation effects and optical attenuation over the fiber path. There are no powered amplifiers between terminal stations, though the undersea cable often has an electrical conductor for electroding and fault localization only. Depending 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 supported. In the case of small ocean observatories, the electroding conductor could be used for observatory power.
Repeatered systems rely on a number of powered undersea optical amplifiers spaced regularly along the cable (50 to 150 kilometers). In addition to the factors that affect nonrepeatered systems, the repeater amplifier noise also impacts transmission performance. Repeatered technology can provide nonregenerated DLS lengths in excess of 11,000 kilometers supporting trans-Pacific dense wavelength division multiplexing (DWDM) applications. The amplifier pump lasers are driven by a constant current carried by the cable conductor from shore power sources. The necessary voltage level is a function of cable length, quantity of repeaters, number of amplified fiber 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 capability to power the system from only one end (single-end feed).
Despite the significant difference between available telecom capacity and the more modest needs of scientific observatories, the same fundamental amplifier product can be used in both applications. Standard undersea telecom systems provide tremendous amounts of capacity. DWDM systems presently in production can transmit hundreds of channels (wavelengths) of 100 gigabits per second data rates. For example, TE SubCom (Eatontown, New Jersey) recently demonstrated transmission of 49.3 terabytes per second (81 x 162-gigabits-per-second channels and 201 x 180-gigabits-per-second channels) over 9,100 kilometers on a single-fiber core incorporating state-of-the-art bandwidth utilization techniques. Such extremely high capacities are not needed for the operation of cabled ocean observatories, even those in their earliest planning stages, but the repeaters used for commercial telecom systems can, and have been, adapted for use with the lower capacities and data rates of cabled ocean observatories.
The basic repeater architecture consists of pairs of amplifiers (amp-pairs) that boost the signal on transmission paths (fibers) in opposite directions along the cable. These erbium-doped fiber amplifiers (EDFAs) are driven by shared 980-nanometer pump lasers and are implemented with undersea-qualified optical components. The optical design of the repeater can be tailored to accommodate a wide range of system applications through erbium-doped fiber design, selection of the pump drive power level and appropriate specification of other passive optical components. Each amp-pair can be independently customized to optimize the performance for its particular fiber paths. The amp-pairs are clear channel, meaning that they boost the signal independent of transmission data rate or format. The amplifier design offers broad bandwidth, a low-noise figure, gain flatness over the band and bit-rate independence. Designs are available with both single and dual stages of amplifier gain.
A key TE SubCom repeater feature is a modular design that can accommodate up to 16 amplifiers, making it possible to facilitate customized system design needs on a qualified undersea-proven hardware platform. Repeater designs can include multiple amplifier designs within the same repeater housing that support different capacity requirements for varying fiber-pair functions. For example, a system architecture may need one fiber pair for high-capacity telecom use, while another fiber pair could support a relatively low data rate for scientific needs. To continue this article please click here.
Bob Thomas is with TE SubCom in Eatontown, New, Jersey, where he has system engineering responsibility for telecom projects serving scientific and offshore markets. His undersea cable background includes work in the development and production of undersea vehicles and in cableship construction.
Adnan Akhtar is with TE SubCom where he works on undersea optical transmission design, including modeling, simulation, and optimization of transmission paths for scientific and offshore platform networks. He graduated with a Ph.D. in electrical engineering from the University of Toronto in 2008.
Marsha Spalding is the director of cable and offshore planning and engineering at TE SubCom. She has been responsible for the design and qualification of numerous generations of undersea optical fibers and SL cables. Her undersea system experience also includes technical sales, application engineering, and project and product management.