Improvement of Fiber Optic System Performance by Synchronous Phase Modulation and Filtering at the Transmitter

In this chapter, the performance improvement obtained from phase modulation (PM) and filtering on return-to-zero (RZ) signal format at the transmitter is investigated. The primary purpose is to understand the mechanism of each type of PM that leads to performance improvement relative to the RZ signal format with no PM. Three types of phase modulation on an RZ pulse train are considered in this chapter: square-wave phase modulation (SWM), sinusoidal alternating-phase modulation (APM), and sinusoidal same-phase modulation (SaPM). Additionally, the continuous-wave square-wave (CWSW) signal format is also investigated. It was shown in the previous chapter (Chapter 4) that individual pulses in the CWSW signal format can undergoes PIE, the phenomenon in which the pulse peak can be greater than that at the fiber input under the presence of dispersion. However, only transmission of a single CWSW pulse was considered in the previous chapter. The extent of the performance improvement provided by PIE in practical systems still needs to be investigated. Since the investigation is at the system level where there are many system components involved, it is difficult to find a closed-form expression that fully describes the system performance. Consequently, numerical simulation is utilized instead. In this Chapter, the optical fiber used in the simulations is assumed to be linear and lossless in order to focus on how each modulation format minimizes the impairment caused by dispersion. The normalized system parameters involved in the simulations are the modulation index B of the phase modulation, normalized transmitter filter bandwidth