Advanced Differential Modulation Formats for Optical Access Networks

The use of incoherent multilevel modulation formats with high spectral efficiency (more than two bits per symbol) has been proposed in order to enable the next generation of very high-speed Time-Division Multiplexing Passive Optical Networks (TDM-PONs). Incoherent multilevel modulation is attractive for access applications since multilevel formats allow the scaling of the bit rate with electronic and photonic components operating at a fraction of the bit rate. On the other hand, incoherent detection reduces the requirement for complicated Digital Signal Processing (DSP) and crucially an additional local oscillator, compared to coherent receivers. The modulation formats examined are Differential 8 Phase-Shift Keying (D8PSK) and three versions of incoherent 16 Quadrature Amplitude Modulation (QAM), specifically Star 16QAM, coded square 16QAM and 16QAM with pre-integration. Generation and detection of these formats is discussed, as well as the implementation challenges associated with such advanced modulation formats. The performance of these modulation formats was evaluated through extensive simulation and experimental work. Results indicate that incoherent modulation can fulfill important requirements of networks operators, namely increased bit rate and increased splitting ratio, and provide a cost-effective solution for Next-Generation Optical Access Networks.