Very High Speed Coherent Passive Optical Networks

Abstract

This work investigates the viability of coherent technology in a passive optical network (PON) setting. Coherent PON offers an alternative to IM/DD that can scale to higher datarates. This thesis focuses on datarates of 200 Gb/s per wavelength for next generation PON. Improving the cost competitiveness of coherent PON is achieved by simplifying receivers and transmitters. The asymmetry of the PON network, with many optical network units (ONUs) and a single optical line terminal (OLT), offers up ways in which the complexity can be moved to the OLT, reducing the overall cost. These simplifications require different approaches in the downstream and upstream transmissions. The downstream solution uses Alamouti-coded transmission, which allows single polarisa- tion detection at the receiver. This enables greatly simplified receivers, such as the single polarisation heterodyne receiver, to be used at the ONU. Two such receivers are considered, one based on a balanced photodiode and one on a single ended photodiode. For further improvement of the power budget, linear and non-linear pre-distortion techniques are in- vestigated. The solution with the balanced photodiode exceeds the E1 class power budget requirement (>33 dB), whereas the single ended photodiode solution meets the N1 class requirement (29 dB). This is supported by a parallel implementation of the adaptive equaliser. The upstream solution uses dual polarisation PAM4 signals, which are generated using low cost electro-absorption modulators combined with DFB lasers. To further reduce the non-linear distortions, introduced by the low cost transmitter components, more complex DSP was investigated. The effects of a look-up table based pre-distortion combined with non-linear equalisers, such as Volterra or MLSE were studied. The solution was shown to exceed N1 class requirements. The above mentioned solutions were combined and tested bidirectionally on both lab and field installed fibre. Testing on 20 km and 40 km in the lab, as well as 19 km of field fibre showed no impairment due to bidirectional transmission. In addition, loss profile of the field fibre enabled higher launch powers improving the solution’s loss margin. To assess the operating costs of coherent PON, a power consumption analysis was performed. This analysis compared the most likely coherent PON candidates to previous standards in terms of transmission efficiency, i.e. J/bit. It was found that coherent PON at 200 Gb/s per wavelength improves the transmission efficiency, and simplified coherent PON can provide a further 5% improvement.