The inter-symbol-transition (IST) patterns of multi-level non-return-to-zero phase-shift-keying (NRZ-PSK) signals generated by two kinds of optical modulator implementations are analyzed to reveal the underlying relationship between IST patterns and optical filtering performance. In the tandem modulator implementations, some IST patterns are curved, and in the parallel implementations, all IST patterns are inherently straight lines. It′s proved that these IST patterns characteristics play a significant role in determining optical filtering performance. In the tandem implementations, optical filtering can lead to obvious IST pattern dependent signal distortion. As a comparison, the parallel implementations have less filtering-induced eye-opening penalty and consequently better tolerance against narrow filtering. Finally, the receiver sensitivity as a function of optical filter bandwidth for differential optical detection is studied together with the optimization of the demodulator parameter.