Contents I. Introduction
With the rapid development of the next generation of optical fiber communication systems, high speed transmission systems have been the focus of research. Against such background as the continued exponential growth of the Internet services, transmission capacity is a tremendous challenge to networks. It is expected that the evolution of broadband services will further accelerate the demand for capacity in optical communication networks with higher channel bit rate. Recently, as a next generation bit rate beyond 40Gbit/s, 100Gbit/s transmission technologies have been intensely studied. The most straight-forward method to realize 100Gbit/s transmission is by using electrically time division multiplexing non-retum-to-zero on-off keying (NRZ-OOK). This modulation format is currently utilized in most commercial optical transmission systems with bit rates up to 10Gbitls [1]. The main advantage of NRZ-OOK is that it requires the least electrical and optical components for generation and detection. However, at the same time it puts the largest constraint on the bandwidth of these components. When the single-channel rate is 100Gbit/s, a variety of physical penalty tolerance puts forward higher requirements such as group velocity dispersion (GVD) and PMD tolerance. Some sophisticated modulation formats yielding a compact spectrum would promise better performance against GVD and PMD. Various kinds of advanced optical modulation formats have been proposed due to the traditional NRZ-OOK would not have met the demand for high speed transmission.
