Lavery, DJP; (2013) Digital Coherent Receivers for Passive Optical Networks. Doctoral thesis , UCL (University College London).

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Abstract

The work presented herein explores the use of digital coherent receivers in loss limited transmission with a view to implementation in a 100 km long-reach passive optical network (LR-PON) with a net data rate of 10 Gbit/s per optical network unit. Optical power receiver sensitivity limits are investigated for C-band coherent receivers. Coherent-enabled advanced (amplitude, phase, and polarisation) modulation schemes are characterised in terms of electronic and optical bandwidth requirements and power efficiency to determine the optimum modulation format for a high capacity LRPON. Including the net coding gain achievable with forward error correction, the high power efficiency of polarisation switched (PS) quadrature phase shift keying (QPSK) enables an experimental demonstration of 4 photons/bit receiver sensitivity, while polarisation division multiplexed (PDM) QPSK enables transmission with 5 photons/bit sensitivity; a 0.5 dB power penalty. Nevertheless, PDM-QPSK is identified as the optimum modulation format for coherent LR-PON, due to its 1.25 dB bandwidth efficiency advantage over PS-QPSK. A coherent access network architecture is developed using 10 Gbit/s PDM-QPSK channels in a wavelength division multiplexed configuration. Multiple access is achieved by using the frequency selectivity of the coherent receiver to provide gain to the channel of interest. Combined with high receiver sensitivity, this demonstrates the feasibility of colourless network operation supporting 1024 channels. In bidirectional transmission, crosstalk from backscattering of optical power is mitigated using the receiver frequency selectivity and by using pulse shaping to restrict the optical channel bandwidth. A reflection-to-signal power ratio of 18.5 dB is tolerated without penalty. Practical realisation is addressed by exploring low complexity, multiplier-free digital signal processing (DSP) algorithms for adaptive channel equalisation; algorithms are identified that can be used without penalty. Finally, to address issues of integration, tunable local oscillator lasers, suitable for monolithic integration, are investigated. The receiver DSP is modified to overcome the additional intensity noise from these lasers. In this scenario, the reduced receiver sensitivity would still enable an LR-PON with 128 channels.

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