Numerical Simulations of the Long-Haul RZ-DPSK Optical Fibre Transmission System

Return-to-zero differential phase shift keying (RZ-DPSK) is a promising modulation format for the long-haul large capacity optical fibre transmission system, and it demonstrated a superior performance compared to the conventional intensity-modulation direct-detection (IM-DD) system experimentally (Cai et al., 2006; Inoue et al., 2004; Rasmussen et al., 2004). For the IM-DD based long-haul system, dispersion map is a commonly used technology to improve the transmission performance (Bergano, 2005). The dispersion map was developed to compromise following two incompatible conditions: close to zero dispersion is preferred from the point of the waveform distortion due to the chromatic dispersion, while a finite dispersion is required to reduce the nonlinear degradation due to the optical fibre. The dispersion map realizes close to zero end-to-end dispersion while keeping the finite local dispersion by combining positive and negative dispersion fibre in the transmission line. For example, the positive dispersion fibre is a standard single mode fibre (SMF), and the negative dispersion fibre is a non-zero dispersion shifted fibre (NZDSF). For the long-haul IM-DD system, so-called “block type” dispersion map is the most popular style. For this block type dispersion map, both the positive and negative dispersion fibres are combined to compose one dispersion block of several hundred to one thousand kilometres, and an entire long-haul system is composed of several dispersion blocks (Bergano, 2005). Even though the block type dispersion map is effective to improve the performance of the conventional IM-DD based system, it was reported that the performance of the long-haul 10Gbit/s RZ-DPSK system with the block type dispersion map using the NZDSF and the SMF showed performance degradation near the system zero dispersion wavelength (Dupont et al., 2007; Moh et al., 2007; Vaa et al., 2004). It is a strange feature of the RZ-DPSK transmission system, because the conventional IM-DD system shows better performance near the system zero dispersion wavelength rather than the other wavelengths. Then, this chapter has been intended to clarify the reason why the long-haul RZ-DPSK system shows such behaviour using the numerical simulations. Section 2 describes the method of the numerical simulations. The simulator is using the standard calculation scheme of the long-haul optical fibre transmission. It solves the nonlinear Schrödinger equation using the split-step Fourier method (Agrawal, 2006). Using this simulator, the transmission performance of the long-haul RZ-DPSK system is evaluated, and the difference between the block type and block-less type maps is compared in section 3. The results obtained through the simulation agree to the experimental results