I. Introduction
Due to the emergence of high-speed digital signal processors (DSP), coherent communication has revived, and dramatic increases in spectral efficiency (SE) and transmission capacity have been reported in the last decade [1], [2]. DSP-based schemes can now realize rectangular shape Nyquist filters for achieving higher SE, and can easily estimate the optical carrier phase even if the light source of the optical carrier has very low coherency, i.e., spectral linewidth exceeds 1 MHz and its frequency fluctuates and drifts within a range of 5 GHz (The drift range is specified in the OIF) [3], [4]. For realizing much higher SE by adoption of higher-order modulation formats, a highly coherent light source will be required in order to suppress the phase noise. We also note that the frequency drift restricts the optical bandwidth by 5 GHz per channel, which decreases the actual SE. The frequencies of light sources should be stabilized such that the wavelength-division multiplex (WDM) is dense enough to match the Nyquist filter.