Performance analysis of coherent ultrashort light pulse CDMA communication systems with nonlinear optical thresholder

We theoretically analyze the performance of coherent ultrashort light pulse code-division multiple-access (CDMA) communication systems with nonlinear optical thresholders. The coherent ultrashort light pulse CDMA is one of the promising systems for optical local area networks due to the advantages of asynchronous transmission, high information security, multiple access capability, and optical processing. The nonlinear optical thresholder is based on frequency chirping induced by self-phase modulation (SPM) in optical fibers, and discriminates the ultrashort pulse from multiple access interference (MAI) having picosecond duration. Our results show that the nonlinear optical thresholder can discriminate 400-fs ultrashort pulse from ≈100-ps MAI even if the response time of the photodetector is limited to 100-ps, and that the system performance does not always become better as the total number of users becomes smaller because of thermal noise, which is a unique characteristic of this system.