Multilevel coding based on IQ modulation such as optical quadrature phase-shift keying, quadrature amplitude modulation, and amplitude-phase-shift keying has attracted increased attention for high spectral efficiency transmission systems beyond 1 bit/s/Hz.
Phase diversity coherent receiver based on digital signal processing without the need for locking the phase of the local oscillator enables us to investigate the IQ vector space and conveniently compensate transmission impairments such as nonlinear effects and group-velocity dispersion.
At the same time, dense wavelength-division multiplexing (DWDM) has become a suitable technique for taking full advantage of the increased spectral efficiency of multilevel coding. Since optical coherent receiver relying on electrical filters has sharp cutoff characteristics for channel demultiplexing, DWDM channels can be demultiplexed without optical filtering impairments. Based on this method, we elucidate the channel spacing limit for 20-Gbit/s QPSK signals in a 1000-km long-haul transmission system.
Sang-Yeup Kim received Ph.D. degree in Electronic Engineering at Kwangwoon University in 2004. He is now a postdoctoral researcher in Research Center for Advanced Science and Technology (RCAST) at Tokyo University and his research interests include optical multilevel coding based on digital signal processing, DWDM transmission systems and analog circuit design.