Experimental Transmission with POLQAM and PS-QPSK Modulation Format Using a 28-Gbaud 4-D Transmitter

A transmitter with 4 digital-analog converters enabled the generation of 4-D optimized constellation with 6 polarization states (POLQAM) at 28-Gbd. A new feedback signal ensured adaptation of the CMA polarization demultiplexer in the off-line receiver and allowed comparison with polarization-switched QPSK (PS-QPSK) and DP-QPSK. Introduction In recent years signal constellation optimization in the 4 dimensions (4-D) of the optical carrier field have been discussed for coherent detection 1-3 systems . In general, the required generation of differently modulated signals for orthogonal X and Y polarization with a well defined dependency requires two synchronized transmitters. Therefore, experimental investigations on 4-D formats were so far limited to the comparatively simple polarizationswitched QPSK (PS-QPSK) format, which was generated (1) by signal polarization transmitter and time-multiplexing of X and Y signal, joint with fiber-delayed cross-polarization interleaving (PDM emulator), or (2) by an additional 5,6 polarization modulator , or (3) by a dual polarization modulator (DP-IQ) with fast (symbol rate) and simple logical gates for generation of 7,8 two modulator drives . In the following - thanks to a new 4-D transmitter working with a fast 4 channel digital-to-analog converter (DAC) driving a DP-IQ modulator - we demonstrate the generation and detection not only of PS-QPSK at 28-Gbd, but also of the more complex POLQAM format with QPSK in 6 1,9 distinct polarization states (6PolSK-QPSK ).