Electrical encoding and band-limiting of advanced modulation formats for 40Gbit/s WDM transmission systems

Abstract

This thesis first proposes a novel method to generate advanced modulation formats that includes carrier suppressed return-to-zero (CSRZ) and CSRZ-differential phase shift keying (DPSK), in order to provide a cost-effective transmitter configuration for 400bit/s wavelength division multiplexed (WDM) transmission systems. The proposed method requires a broadband radio frequency (RF) mixer for electrical data encoding, and a single stage Mach-Zehnder (MZ) intensity modulator for optical signal generation. Thus, it saves one modulator in comparison with the conventional method, which uses two cascaded MZ intensity modulators. The proposed scheme is validated numerically and experimentally. We next evaluate the effects of electrical band-limitation on the transmission performance of CSRZ and CSRZ-DPSK signals. The proposed method makes it possible to band-limit the generated signals in the electrical domain rather than in the optical domain by employing a band-limiting low pass filter (LPF). The electrical band-limitation causes spectral compactness as optical pre-filtering, however, it is immune to the stability of optical filters. Numerical results for eye opening penalty (EOP) show that the tolerances against both the chromatic dispersion of the optical fiber and the coherent crosstalk from adjacent WDM channels are improved due to the narrowed optical spectrum bandwidth. On the other hand, signal distortion is increased due to electrical filtering in the transmitter side. In addition, fiber nonlinear effects are expected to have greater influence on the transmission performance of the electrically band-limited signals; and, the simulation shows that the additional penalty is negligibly small in the simulated 7-channel WDM transmission over 640 km single mode fiber link. Besides, taking into account all of the impacts of electrical band-limitation, we show that the optimal bandwidth for the electrical band-limitation can be selected at around 240Hz for 40Gbit/s...