640-Gb/s/Channel Single-Polarization DPSK Transmission Over 525 km With Ultrafast Time-Domain Optical Fourier Transformation

A single-channel 640-Gb/s differential phase-shift-keying transmission over 525 km was successfully demonstrated in a single polarization. A new ultrafast time-domain optical Fourier transformation (OFT) circuit was developed, in which the strong chirp required for the OFT of a subpicosecond optical pulse was realized with a phase modulator operated in a round-trip configuration. By applying this technique to a 640-Gb/s transmission, the tolerance to higher order polarization-mode dispersion was greatly improved and error-free transmission was achieved.