Transmission of 9$\,\times\,$138 Gb/s Prefiltered PM-8QAM Signals Over 4000 km of Pure Silica-Core Fiber

We experimentally demonstrated the application in ultrahigh spectral density long-haul optical systems of tight prefiltering to generate “quasi-Nyquist wavelength division multiplexing” signals together with the use of pure-silica core fiber and hybrid Raman/erbium-doped fiber amplification. The transmission of polarization-multiplexed quadrature-amplitude modulation with 8 symbols (PM-8QAM) at 138 Gb/s and channel spacing equal to 1.22 $\,\times\,$(symbol rate) was demonstrated, reaching over 4000 km at net spectral efficiency (SE) of 4.1 b/s/Hz. In our experiment, we achieved to date highest per-carrier symbol rate (23 Gbaud), SE (4.1 b/s/Hz), and (SE)$\,\times\,$ (distance) product (16655 b/s/Hz$\cdot$km) for non-orthogonal frequency-division multiplexing transmission using PM-8QAM modulation.