Digital Signal Processing (DSP) Technology For High Speed Transmission System

During the past decade, the demand for network bandwidth has been significantly raised by high performance applications such as high-definition video and cloud computing, which urges the backbone network to achieve further development. To approach challenges mentioned above, there is a strong motivation to promote the transmission capacity of the optical system.With a view to DSP technologies and all optical clock recovery scheme for long-haul, high capacity transmission system, we have done several research works. As for DSP, the clock recovery algorithm for Nyquist system is analyzed, which mainly considers the performance of algorithm under negligible roll-off factor. In terms of the demultiplexed-feedback clock recovery scheme, the principle and experimental system are characterized and demonstrated.Main research works in this dissertation are as follows.1. Clock recovery algorithm for Nyquist system.Most timing recovery algorithms implemented in optical communication have been designed for non-return-to-zero (NRZ) or return-to-zero (RZ) pulse shapes. However, to Nyquist signal with small roll-off factor, these conventional algorithms would fail to work. We propose and realize the modified clock recovery algorithm based on traditional Gardner’s method for such condition. Through system simulation, the timing jitter performance of conventional and modified algorithms are evaluated and compared to verify the feasibility. We also introduce the all-optical Nyquist OTDM transmission system and its DSP implementation, with corresponding constellations to illustrate the key roles of algorithms in the receiver. Meanwhile, BER of the whole experimental system is calculated and given to prove the success of the experiment scheme.2. Demultiplexed-feedback clock recovery scheme utilizing OEO loop.The base-rate clock component in a4x40Gbaud serial data signal is characterized through numerical simulations to draw principle of the proposed demultiplexed-feedback clock recovery scheme. And the numerical simulation shows that the clock component would feature high strength when there is only40Gbaud signal. Such a scenario can be practically realized by feeding the clock recovery unit with the demultiplexed signal. Thus a feedback structure by combining the demultiplexing unit and the clock recovery unit can be set up for this task.Then, the principle is applied to a4x40Gbaud optical time-division multiplexing (OTDM) transmission experiment to prove the concept, resulting in a40GHz recovered clock with timing-jitter approaching RF source’s instrument index. Significant timing-jitter reduction, from58fs to31fs in back-to-back configuration and from59fs to34fs after400km transmission (in the100Hz to10MHz range), is observed when compared to the results of a conventional feedforward scheme. Error-free performance of the whole OTDM system is also enabled by using the proposed scheme.