Digital coherent receivers for 100GbE optical transmission

The optical fibre core network underpins the internet and the digital economy, with the present capacity of today's core networks being limited to ~ 1Tbit/s per fibre. While in current networks, the limited broadband data rates afforded by the copper based access network prevents the optical core network from being stretched to capacity, as optical fibre permeates the access network, the bottleneck will move from the access network to the core network. To overcome these limitations and to maximise the opportunities afforded by a fibre optic access network will require the capacity of the installed core network to be increased, either by increasing the number of wavelengths used or by increasing the data rate per wavelength. The proposed research aims to combine both techniques simultaneously - transmitting 100 gigabit Ethernet (GbE) on each wavelength, while employing wavelength division multiplexing (WDM) to increase the capacity of the core network to beyond 10Tbit/s.Using conventional intensity modulation schemes, much of the installed fibre base is unable to support data rates faster than 10Gbit/s due to imperfections in the installed fibre which causes pulse spreading. Current research at UCL, led by the principal investigator (PI), has recently experimentally demonstrated the potential of digital signal processing (DSP) combined with coherent detection of spectrally efficient modulation formats to overcome these limitations for 40Gbit/s transmission systems, with the same principles being equally applicable to 100GbE systems. In a digital coherent receiver the four components of the optical field, the in-phase and quadrature components of the two polarisations, are mapped into the electrical domain. This allows digital compensation of transmission impairments and the use of spectrally efficient four-dimensional modulation formats. Given the huge investment which has been made into installing the fibre base infrastructure, the ultimate aim of the research is to determine how this four-dimensional modulation space can be used in conjunction with DSP to maximise the capacity of the installed fibre.The proposed research combines fundamental theoretical research with a determinedly experimental research program into the nonlinear transmission of four-dimensional modulation formats at 100Gbit/s+ and beyond. The initial workpackage will investigate both experimentally and theoretically quadrature amplitude modulation, in combination with polarisation division multiplexing as a four dimensional modulation scheme for 100GbE transmission systems. Within this first workpackage, the system under investigation will be receiver centric, such that all of the DSP, both linear and nonlinear, is based at the receiver. In the second workpackage this assumption will be relaxed and combined transmitter and receiver DSP will be investigated, both experimentally and through simulation. The third and final workpackage which is a theoretical study, will draw on the conclusions of the previous workpackages, and will aim to answer the question Given the optical fibre is dispersive and nonlinear, what is the optimal modulation scheme which enables the capacity of the core network to be maximised assuming we are able to employ appropriate digital signal processing?