Tb/s superchannels for optical communications

The exponential growth of data traffic has motivated the researchers to find the transmission technologies which can offer high capacity transmission with high spectral efficiency (SE) at economical cost. The next-generation optical transmission systems are expected to operate at 400 Gb/s or 1 Tb/s per line rate, whereas today’s recent commercial system operates at 100 Gb/s. The traditional ways to increase the data rate, by increasing the symbol rate and encoding more bits per symbol, face several challenges that make their practical implementations difficult. Increasing the symbol rate to generate 400 Gb/s or 1 Tb/s requires a very high speed optical and electronic components, some of which are not yet commercially available. Superchannel is the emerging technology having the potential to enable Tb/s transmission. In superchannels, optical carriers can be spaced tighter than the standard ITU-T WDM grid, thus allowing higher spectral efficiency (SE). Superchannel approaches, such as all-optical orthogonal frequency division multiplexing (AO-OFDM) and Nyquist wavelength-division multiplexing (Nyquist-WDM) are based on a concept where signal overlapping is allowed either in time or frequency domain as long as the orthogonality between subcarriers is preserved. Nyquist-WDM superchannel is generated by spectrally shaping each subcarrier to occupy a bandwidth equal to the symbol rate. The channel capacity and optical reach of the Nyquist-WDM superchannel can be adjusted to the traffic demand by changing the symbol rate, bits per symbol, and the number of subcarriers. In Nyquist-WDM, spectral shaping has been previously performed in the electrical and digital domains, and it has been demonstrated that digital filters require ultra-high-speed digital-to-analogue converters (DACs) and high-speed digital-signal-processing (DSP) for the precise shaping of the optical signal which can be possibly implemented only with high complexity and cost. To overcome the challenges, the objective of this thesis is to investigate novel spectral shaping techniques which can reduce the complexity of current state-of-the-art DSP by utilising the now commercially available optical filters such as a wavelength selective switch (WSS). However, the currently available WSS has a limited resolution of 10 GHz. In this thesis, we propose for the first time; two novel filter input optimisation approaches for enhancing the output response of the WSS. This thesis also details the simulation implementation of five-channel Nyquist WDM superchannel system to test and validate the proposed spectral shaping technique with higher modulations formats at different data rates. We show that by implementing optical spectral shaping utilising a WSS and a simple adaptive algorithm, it is possible to reduce the complexity of state-of-the-art by 19.8 %