56-Gbaud PDM-QPSK: Coherent Detection and 2,500-km Transmission. 56-Gbaud PDM-QPSK: Coherent Detection and 2,500-km Transmission (1) (1) (1) (2) (2) P. J. Winzer , A. H. Gnauck , G. Raybon , M. Schnecker , and P. J. Pupalaikis (1) Alcatel-Lucent, Bell Labs, 791 Holmdel-Keyport Road, Holmdel, NJ, USA, winzer@alcatel-lucent.com (2) LeCroy Corporation, Chestnut Ridge, NY, USA, michael.schnecker@lecroy.com Abstract A 56-Gbaud (224-Gb/s line rate) polarization-division multiplexed quadrature phase shift keyed (PDM- QPSK) signal is transmitted over 2,500 km (32 x 80 km) of fiber and is coherently de……