We present recent progress and challenges toward ultrahigh-speed optical time-division multiplexed (OTDM) transmission with a single-channel bit rate beyond 10 Tbit/s using optical Nyquist pulses. First, we review the technological progress made on OTDM/ETDM and digital coherent transmission over the last 20 years and highlight the challenges that must be met to achieve a higher bit rate and a higher spectral efficiency. We then present our proposed scheme that consists of an optical Nyquist pulse and its OTDM transmission and show that coherent Nyquist pulses are very attractive in terms of realizing an extremely high bit rate and high spectral efficiency simultaneously. We describe our recent demonstrations of a single-channel 10.2 Tbit/s (2.56 Tbaud) DQPSK transmission over 300 km using non-coherent Nyquist pulses, and a 15.3 Tbit/s (1.28 Tbaud) 64 QAM transmission over 160 km using coherent Nyquist pulses, in which the highest bit rate we achieved was 15.3 Tbit/s with a spectral efficiency as high as 8.7 bit/s/Hz. We also present a 12.8 Tbit/s (1.28 Tbit/s/ch × 10 ch) transmission of 320 Gbaud DQPSK Nyquist pulses over 1500 km. These results indicate that Nyquist pulses are advantageous not only for achieving an extremely high bit rate but also for realizing a WDM transmission with a 1 Tbit/s channel capacity over long distances.
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