Experimental demonstration of direct-detection quaternary differential polarisation-phase-shift keying with electrical multilevel decision

Direct-detection quaternary differential polarisation-phase-shift keying (DPolPSK) has been experimentally demonstrated for optical communications. DPolPSK encodes information on both the polarisation and phase of lightwave. The polarisation-phase symbols are decoded using optical interferometric demodulation and electrical multilevel decision; no dynamic optical polarisation control is required at the receiver. In the experiment, 20 Gbit/s quaternary DPolPSK was transmitted through 3×80 km standard singlemode fibre with a measured BER of less than 10^-11.