Investigation of 2-b/s/Hz 40-gb/s DWDM transmission over 4/spl times/100 km SMF-28 fiber using RZ-DQPSK and polarization multiplexing

We have investigated transmission of 14/spl times/40 Gb/s dense wavelength-division-multiplexing channels with 20-GHz spacing over 4/spl times/100 km of SMF-28 fiber. A spectral efficiency of 2 b/s/Hz was demonstrated by 4-b (8-state) per-symbol coding using return-to-zero differential quadrature phase-shift keying (RZ-DQPSK) and polarization multiplexing using polarization bit interleaving. All 12.5-Gsymbols/s transmitted channels included 25% overhead bandwidth reserved for forward-error correction (FEC) and showed uncorrected bit-error rate below the FEC threshold required for error-free operation. Demodulation of the RZ-DQPSK signal at the receiver was achieved using a fiber-based asymmetric Mach-Zehnder interferometer as well as a packaged integrated lithium niobate optical 90/spl deg/ hybrid.