High Spectral Efficiency 400 Gb/s Transmission by Different Modulation Formats and Advanced DSP

We experimentally demonstrate probabilistically shaped 64-ary quadrature-amplitude-modulation (64QAM) 50-GHz-grid wavelength-division-multiplexing (WDM) transmission with a bit rate of 400-Gbit/s per channel over 3,600-km Raman-amplified ultra-large effective area fiber (ULAF). Our results show that the probabilistically shaped 64QAM outperforms the hybrid-32/64QAM by 50% reach improvement. Thanks to pre-equalization and look-up-table pre-distortion, a 5.5-dB optical signal-to-noise ratio (OSNR) gain can be obtained. Furthermore, utilizing ULAF with Raman amplifier causes an apparent improvement on the transmission performance compared with using the SSMF with EDFA. We also compare the optical coupler with the wavelength selective switch utilized in the WDM system to explore a suitable way for WDM transmission. Advanced DSP including Butterworth low-pass digital filter is also used at the receiver end.