Applications of linear and nonlinear fiber grating properties for optical communications

We discuss an application of nonlinear characteristics of optical fiber gratings. A cascaded long-period fiber grating pair shows a very sharp interference fringe patterns in wavelength domain. This can be applied to the modulation of optical beam intensity by the use of cross-phase modulation. The core mode can be switched to the cladding mode by the method, and the pulse intensity needed in the switching is reduced by an order of magnitude compared to the previous method. We also discuss bidirectional wavelength add/drop multiplexers (B-WADM's) using fiber Bragg gratings (FBG's) and multiport optical circulators. A further modification of this structure that uses two separate multiplexer (MUX) and demultiplexer (DEMUX) pairs and reflection-type FBG filters is also discussed. The proposed B-WADM alleviates the formerly stringent requirement on MUX/DEMUX crosstalk characteristics, while suppressing the power penalty induced by the Rayleigh backscattering or optical reflection. In experiment, the measured crosstalk induced by the backscattering was successfully suppressed to negligible level (-30 dB) while at the same time achieving high B-WADM span gain over 25 dB.