1.6-b/s/Hz Spectrally Efficient Transmission Over 1700 km of SSMF Using 40 $\times$ 85.6-Gb/s POLMUX-RZ-DQPSK

We discuss high spectral efficient long-haul transmission using multilevel modulation formats. In particular, we investigate long-haul transmission with a 1.6-b/s/Hz spectral efficiency using polarization-multiplexed return-to-zero differential quadrature phase-shift keying. In an experimental demonstration, we show 40 times 85.6-Gb/s transmission with a 50-GHz channel spacing over 1700 km (18 times 94.5 km) of standard-single-mode fiber