PLL-Free Quadrature-Amplitude Modulation in Coherent Optical Communication

In similarity to its electronic counterpart, conventional coherent optical communication requires the difficult task of phase synchronization of a local laser with a transmit laser indiscriminately without regard of individual phase noise contributors. By studying carefully the characteristic of each phase noise contributor, we developed a novel coherent detection algorithm empowered by advanced digital signal processing techniques for quadrature-amplitude modulation (QAM) to enable practical realization of coherent communication without the need of phase synchronization. This algorithm enables 9 photons/bit sensitivity at a bit error rate 10³ at 10 GBaud/s for QAM4 with an aggregate 100-MHz linewidth, providing us a cheaper alternative to utilize regular lasers of linewidth up to 12.5 MHz for 2.5-GBaud/s transmission.