40 Gb/s Optical Single Sideband DWDM Transmission System using Polar Return-to-Zero Signalling Format

Abstract

Optical single sideband (oSSB) 40 Gb/s transmission systems using binary polar coded return-to-zero (RZ) pulse shape are proposed to enable dense wavelength division multiplexing networks employing efficient electrical dispersion compensation (EDC). The detuning of ITU 50, 100, and 200 GHz grids optical filters is optimized by simulation. In presence of optical fibre and EDC, polar RZ oSSB clearly surpasses common nonreturn-to-zero oSSB for all the considered grids. After 160 km of standard single mode fibre, polar RZ retrieves Q-factors above 6 for the 50 GHz grid and above 7 for the remaining grids; whereas NRZ Q-factor is never superior to 3 after only 80 km. Keywords: signalling format, electrical dispersion compensation (EDC), optical single sideband (oSSB), optical communication systems, dense wavelength division multiplexing (DWDM). INTRODUCTIONThe increasing offer of broadband applications is leading to higher transmission bandwidth requirements. This demand can be satisfied by dense wavelength division multiplexing (DWDM) core networks; these offer high capacity and long reach transmission. Advanced modulation formats are essential to the cost-efficiency of DWDM networks. Among these, differential binary phase shift keying (DPSK) [1], differential quadrature phase shift keying (DQPSK)[2], duobinary [3], carrier-suppressed return-to-zero (CS-RZ) [4], and optical single sideband (oSSB)[5] have presented promising characteristics.Optical SSB allows DWDM channel density increase and improved tolerance to group velocity dispersion (GVD) due to the suppression of one of the signal sidebands. Moreover, efficient electrical dispersion compensation (EDC) can be combined with direct detected (DD) oSSB signals to further mitigate the effects of GVD. Simple EDC methods have been proposed with results at 10 Gb/s and 40 Gb/s, such as simple dispersive lines, adaptive filters, or Butterworth filters [5]- [8]. EDC can be used as a stand alone technique or to minimize residual dispersion arising from mismatched optical dispersion compensation.The combination of optical SSB modulation, coding formats and the return-to-zero (RZ) pulse shape has been proposed to enhance the system performance due to enhanced tolerance to intersymbol interference (ISI) of RZ shape. Promising results have been recently presented in [9], where polar coding and RZ pulse shape are combined with carrier-unsuppressed oSSB signals at 10 Gb/s. The proposed transmitter enables improved receiver sensitivity and transmission distances when compared to a common non-return-to-zero (NRZ) oSSB system. In this work, polar RZ signalling is explored for the first time at 40 Gb/s. The potential of the generated oSSB signal for inclusion in the 200 GHz, 100 GHz and 50 GHz ITU grids is assessed in back-to-back situation and after fibre transmission and EDC. The results are compared with those obtained by common oSSB-NRZ.