5.94-Tb/s 1.49-b/s/Hz (40/spl times/2/spl times/2/spl times/40 Gb/s) RZ-DQPSK polarization-division multiplex C-band transmission over 324 km

The combination of return-to-zero differential quadrature phase-shift keying with polarization-division multiplex, a 16-ary modulation scheme, allows for ultimate spectral efficiency. We raise C-band fiber capacity with phase-shift keying transmission beyond previously reported figures, achieving forward-error correction limit performance over four fiber spans.