Integrated tunable fiber gratings for dispersion management in high-bit rate systems

Dispersion management is becoming paramount in high-speed wavelength-divisi on-multiplexed lightwave systems, that operate at per-channel rates of 40 G b/s and higher. The dispersion tolerances, in these systems, are small enou gh that sources of dispersion variation, that are negligible in slower syst ems, become critically important to network performance, At these high-bit rates, active dispersion compensation modules may be required to respond dy namically to changes occurring in the network, such as variations in the pe r-channel power, reconfigurations of the channel's path that are caused by add-drop operations, and environmental changes, such as changes in ambient temperature. We present a comprehensive discussion of an emerging tunable d ispersion compensating device, based on thermally actuated fiber gratings. These per-channel devices rely on a distributed on-fiber thin film heater, deposited onto the outer surface of a fiber Bragg grating. Current flowing through the thin film generates resistive heating at rates that are governe d by the thickness profile of the metal film. A chirp in the grating is obt ained by using a thin-film, whose thickness varies with position along the length of the grating in a prescribed manner; the chirp rate is adjusted by varying the applied current, The paper reviews some of the basic character istics of these devices and their implementation, in a range of different a pplications, including the mitigation of power penalties associated with op tical power variations. We present detailed analysis of the impact of group -delay ripple and polarization-mode dispersion on systems performance, and present results from systems experiments, that demonstrate the performance of these devices at bit rates of 10, 20, 40 and 160 Gb/s. We also discuss a dvantages and disadvantages of this technology, and compare to other device s.