A comparative study of pulse interactions in optical fiber transmission systems with different modulation formats

One of the principal problems in the design of high-data rate optical fiber communications systems is the choice of the modulation format. The conventional non-return-to-zero (NRZ) format was the first format employed in optical fiber communications, and it remains the predominant modulation scheme at this time. The NRZ format has evolved into the chirped return-to-zero (CRZ) format, where both phase and amplitude modulation are used to improve the system performance. At the same time, there has been a considerable amount of work done to investigate the possibility of using classical solitons in optical fiber communications. The classical soliton format eventually evolved into the dispersion-managed soliton (DMS) format. Classical and dispersion-managed solitons are similar in important respects. For example, the balance between chromatic dispersion and nonlinearity is the key condition for the existence of periodically stationary dispersion-managed solitons. By contrast, the CRZ pulse shape is not periodic and the pulse evolution is mostly determined by the dispersion. However, we find that with respect to nonlinear pulse interactions, the DMS and CRZ systems resemble each other and their behavior differs dramatically from that of classical soliton systems. We stress that in our work here, when we refer to a DMS system, we mean a periodically-stationary DMS system, rather than a quasilinear DMS system. The quasilinear DMS systems resemble CRZ systems more than they resemble the periodically-stationary DMS systems. We demonstrate that in the DMS and CRZ systems, the performance degrades as the number of frequency channels increases, as opposed to classical soliton systems, where adding channels does not affect the performance.