Advanced Optical Modulation Formats and Their Comparison in Fiber-Optic Systems

This project started a few years ago and the initial scope of the research was mainly focused on the investigation of sub-carrier multiplexed (SCM) WDM optical systems. As directed by Sprint, during the course of the research, we have expanded our focus to other digital modulation formats for high-speed optical transmission and the comparison of the system performance in various fiber types. In an early work sponsored by Sprint, we have proved the feasibility of digital WDM/SCM optical transmission and demonstrated a four-channel digital SCM system with the aggregated capacity of 10Gb/s using optical single-sideband modulation. As the continuation, in this new effort we proposed and demonstrated a novel method to eliminate the effect of polarization mode dispersion (PMD) in highspeed SCM optical systems. We evaluated the effect of nonlinear crosstalks in SCM/WDM optical systems and the impact due to fiber characteristics. We also demonstrated a multi-channel ultra dense WDM optical transmitter using one laser diode and one electro-optic modulator. These new developments helped a better understanding of digital SCM optical transmission and made SCM/WDM optical systems more practical. In recent years there has been a great interest in other digital optical modulation formats for ultra-long distance optical transmission. The impact of optical modulation formats on transmission fiber types and data-rates in WDM systems is critically important for Sprint to make strategic decisions on the fiber plant investments and fiber-optic equipment purchasing. Our comparative study demonstrated that (1) for 10Gb/s DWDM optical transmission, standard single mode fiber (SMF-28) has the best performance for all optical modulation formats because of it low nonlinear crosstalks and (2) In 40Gb/s optical systems, self-phase modulation (SPM) in the transmission fiber becomes the major source of performance degradation, which makes SMF-28 vulnerable because of its high dispersion. In order to make SMF-28 more competitive for 40Gb/s optical transmission, optical phase modulation may be used, such as DPSK. Another way to reduce the effect of SCM is to use optimum prechirping and pre-distortion of optical signals in the transmitter. We proposed a possible method to achieve the optimum pre-chirping and pre-distortion, however, effectiveness may need to be further investigated.