Contents 1. Introduction
To keep up with the growing demand for more data transmission capacity in wavelength-division-multiplexed (WDM) optical transmission systems, technologies to achieve per-channel bit rates of 400 Gbps and higher are strongly desired. Although some early commercial 400-Gbps systems employ multicarrier (or superchannel) approaches, which use multiple optical subcarriers in a single WDM channel, single-carrier transmission with high baud rates are also being studied actively [1], [2]. The merits of single-carrier transmission over multicarrier transmission include a simpler optical configuration and lower peak-to-average-power ratio (PAPR), which leads to higher tolerance to nonlinear effects in fiber. The key enablers of recent high-baud-rate single-carrier transmission experiments are high-speed analog electronics, such as time-domain multiplexers (MUXs) and digital-to-analog converters (DACs), and optical filters to shape the output optical spectrum right after the optical modulators. Optical time-division multiplexing (OTDM) is another promising approach for achieving high bit rates with a single optical carrier [3], [4]. However, in most of the demonstrations, OTDM transmitters are just emulated by using fiber delay lines. An integrated 80-Gbps OTDM modulator consisting of optical switch and two Mach-Zehnder modulators (MZMs) was reported [5], but implementation of an integrated OTDM transmitter for bit rates of 400 Gbps and higher remains a challenge.
