Abstract
The emergence of digital coherent optical transmission technologies is being eagerly awaited by the world. This enables us to develop spectrally-efficient transmission systems by means of polarization-division multiplexing and multilevelmodulation formats such as quadrature-phase-shift keying (QPSK) and higher-order quadrature-amplitude modulation (QAM). Thanks to recent rapid advances in the research and development of electronics, demodulation of such signals can be realized effectively by utilizing sophisticated digital signal processors (DSPs). Such digital coherent technologies have successfully been implemented in commercial systems. However, the transmission performance of photonic networks is limited by system impairments that include crosstalk and spectrum narrowing caused at reconfigurable optical add/drop multiplexers (ROADMs) and the nonlinearity of optical fibers. Current digital coherent technologies do not resolve these problems comprehensively necessitating further research. In this paper, we investigate the impacts of the system impairments through intensive computer simulations and show the maximum transmission distances of multilevel-modulation signals. Various transmission schemes for gridless networks including Nyquist wavelengthdivision- multiplexing (WDM) networks, which need digital coherent technologies, are evaluated. We also discuss DSP algorithms that suit photonic networks and permit digital coherent technologies to become more effective in realizing future networks.
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