Chapter 13 - Ultralong-distance undersea transmission systems

Abstract

Ultralong-distance undersea transmission systems have gone through revolutionary changes since the introduction of coherent transponder technology. In the dry plant, transponders using higher order modulation formats and digital coherent technology are being routinely deployed with 100–400Gb/s per line card. In the wet plant, many undersea systems optimized for coherent transmission have been deployed. Today’s undersea cable operators require “open cable systems” with flexible capacity and improved cost per bit. In this chapter, we review the most recent technology evolutions and industry trends. We first introduce the Gaussian noise model, discuss generalized optical signal-to-noise ratio, and present the “open cable” concept. We then focus on technologies to combat fiber nonlinearities and achieve high capacity in single-mode fiber, including symbol rate optimization, various nonlinearity compensation technique, variable spectral efficiency, and nonlinear system optimization by proper system design. We summarize several different optical amplification technologies to increase optical bandwidth and enhance nonlinear tolerance including C+L band amplification, Raman-assisted C-band amplification, and compare the performance using different amplification schemes. Afterward, we review space division multiplexing as a means to achieve better power efficiency and higher overall system capacity. We discuss the trade-off among limited electrical power, spectral efficiency, number of fiber pair, and power efficient modulation formats. At the end, we discuss system value improvements using wet wavelength selective switch–based reconfigurable optical add-drop multiplexer and new cable types.