System performance improvement in optical wavelength division multiplexing systems and networks using optical gratings

The low-loss transmission window of silica fibers in the 1.55-μm wavelength region is about 25 THz. The usable bandwidth of the erbium-doped fiber amplifier is about 3.5 THz. As a result, full utilization of the entire usable bandwidth is a very important research issue in optical fiber communication system. One of the most promising techniques to accomplish this is wavelength-division multiplexing (WDM). WDM corresponds to the scheme in which optical carriers at different wavelengths are modulated using independent electrical bit streams and then transmitted over the same fiber. Though WDM techniques can dramatically increase transmission capacity, there are several phenomena which limit the transmission performance, especially for long-haul links. These limiting factors include: non-uniform EDFA gain profiles, fast power transients of the cascaded EDFA chains, accumulated amplified spontaneous emission (ASE) noise, fiber dispersion, and nonlinearities.; Many techniques have been developed to solve these problems. Most of these technologies are passive and have proved successfully in long distance point-to-point transmission. However, in a highly dynamic reconfigurable network, each individual WDM channel might be dropped or added frequently. For such a non-static network, the system performance will definitely be degraded if there is no associated technology to carefully take care of the problems induced by the dynamic conditions. In addition, there is also a slow performance variation due to the degradation of the deployed optical components in the system. As a result, active/dynamic solutions of these problems are necessary.; To guarantee the robustness of communication systems, we have successfully demonstrated techniques to solve these problems dynamically. These techniques include: (1) using acousto-optical (AO) modulators and amplitude-tunable long period gratings to equalize channel power nonuniformity due to non-uniform EDFA gain, (2) using AO modulators to cancel fast power transients in WDM systems, (3) developing a novel tunable dispersion compensator based on a nonlinearly chirped fiber Bragg grating (FBG), which dynamically compensates the accumulated chromatic dispersion in a 10 Gb/s system, and (4) designing a new sampled nonlinearly chirped fiber Bragg grating to simultaneously compensate accumulated chromatic dispersion for three WDM channels at 10 Gb/s.