Advanced transmission technologies for high-capacity optical networks

© 2016 Dr. Qian HuIn order to keep up with the exponential growth of Internet traffic, there emerges an urgent demand for high-capacity optical networks. The optical networks can be classified into two categories, the long-haul and the short-reach optical networks, in consideration of their different requirements in transmission distance, data rate, and construction cost. Great challenges exist in both long-haul and short-reach optical networks when upgrading the capacities. Various multiplexing techniques have been proposed to explore the available modulation freedoms in optical fibers, leading to a constant capacity increase in long-haul optical networks over the past few decades. A significant capacity increase has been witnessed during the last decade with the revival of the coherent optical technique, which makes full utilization of the available degrees of modulation freedoms in single-mode fiber (SMF). With the development of the coherent technique, the capacity of the long-haul optical networks rapidly approaches the Shannon limit, which imposes an upper limit for the channel capacity in SMF. To support a sustainable capacity increase, the long-haul optical networks are faced with the challenge to achieve a capacity beyond the Shannon limit in SMF. On the contrary, the traffic is relatively low in the short-reach optical networks, where the service is provided for fewer users. The system cost shared by each user is dramatically increased, making the cost a primary consideration. A cost-efficient solution can be readily obtained using direct modulation and direct detection (DM/DD) system. However, the data rate and transmission distance of such system are limited due to the signal distortion resulting from the nonlinear channel. To meet with the increasing traffic demand between the data centers, the short-reach optical networks are confronted with the challenge to improve the capacity while maintaining the system cost at an acceptable level. In this thesis, we explore the promising techniques for high-capacity optical networks with various transmission distances, from short-reach to long-haul. For the long-haul optical networks, a channel capacity beyond the Shannon limit of SMF can be achieved with the help of few-mode fiber (FMF), where the signal is multiplexed in different spatial modes. The multiple-input multiple-output (MIMO) digital signal processing (DSP) is necessary to recover the signal after the random mode coupling. The channel matrix plays a critical role in the DSP, describing the linear impact from the FMF channel. By investigating into the properties of the FMF channel matrix, we obtain the essential knowledge to design a FMF transmission system with optimal performance and efficient DSP, providing a feasible way towards the ultra-high-speed optical transmission. For short-reach optical networks, various advance modulation and detection schemes have been proposed to improve the channel capacity with low-cost transceivers. In this thesis, we will report the recent progress on high-performance short-reach optical networks, where a linear channel is obtained to reach a capacity beyond that of simple DM/DD system. We will introduce serveral advanced modulation formats, which can linearize the channel in direct detection (DD) systems using self-coherent approach. The emphasis will be placed on the Stoke Vector Direct Detection (SV-DD) scheme where superior electrical spectral efficiency can be achieved. After that, we will investigate into two advanced detection schemes for high-performance direct modulation (DM) system. By utilizing the phase modulation from the directly modulated laser (DML) due to the frequency chirp effect, the receiver sensitivity can be significantly improved, leading to a greater potential of high-capacity transmission