Key technologies for high-speed digital coherent optical communications

Digital coherent optical communication technologies have shown superior performance in supporting high-bit-rate optical communications, including high fiber chromatic dispersion (CD) and polarization mode dispersion (PMD) tolerance, and high spectral efficiency. We review the typical system design and principles of dual-polarization digital coherent optical communications, and further show a new dual-stage cascaded frequency offset estimator in achieving very high performance.