| Summary: | © 2016 Feng Yuan During the past decade, with the rapid development of cloud computing, smart phones and mobile Internet, there has been an increasing demand for high-capacity and high reliable metropolitan area networks and access networks. Unlike long-haul transmission systems, where bulky and expensive optical components are normally used, the short-to-medium reach optical link which is in the range of a few kilometres to hundreds of kilometres is naturally costsensitive. Therefore, in recent years, instead of using the traditional LiNbO3-based intensity and I/Q modulators, the compact and low-cost directly modulated laser (DML) has drawn plenty of attention and becomes an attractive candidate in such short-reach applications. DML-based optical transmission systems have been extensively explored and successfully demonstrated with various configurations, including using different modulation, detection and multiplexing methods. Among these demonstrations, DML has already shown its capability to realize optical access links for a data rate of 100 Gbps and beyond. Despite its promising performance that has been reported, not only the potential of DML has not been fully exploited, but also its limitations as an optical transmitter has not been thoroughly discussed. It is well-known that one of the major drawbacks of ordinary DML being used as a transmitter is its intrinsic frequency chirp, which, after combining with the fibre chromatic dispersion, becomes an increasingly detrimental factor to data rate enhancement. Efforts have been made to overcome such obstacles, such as employing inverse dispersion fibre, optical injection locking, optical filters and specially designed DMLs. The effectiveness of the above approaches on extending the transmission reach and increasing the data rate has been rather limited. Additionally, they always require extra devices, adding more complexity and cost to the system, and are thus not desirable for low-cost implementations. In this work, utilizing the digital signal processing (DSP) facilitated coherent detection (COHD), we propose a new modulation scheme on DML-based systems, which is called complexmodulated DML (or CM-DML). It has been shown that a significant optical signal-to-noise ratio (OSNR) sensitivity improvement can be achieved compared with the traditional intensityonly detection method. Besides, we also experimentally demonstrated that the CM-DML system exhibits a reasonable tolerance for reduced receiver bandwidth. In a nutshell, it is feasible to realize CM-DML systems using cost-effective receivers with narrow bandwidth.
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