Circular-birefringence fiber for nonlinear optical signal processing

The circular-birefringence fiber (CBF) or the so-called twisted fiber provides' novel attractive potential for all-optical signal-processing applications. Unlike a standard non-polarization-maintaining (non-PM) fiber, where the polarization dependence of X(1) nonlinearity is averaged out by the residual birefringence, the CBF behaves as a perfectly isotropic fiber in terms of nonlinear wave propagation. The CBF, therefore, allows the utilization of the intrinsic absolute polarization dependence of chi((3)) nonlinearity in realizing novel functions that can never be obtained with a non-PM fiber. Several types of CBF are successfully fabricated by twisting commercial non-PM fibers at 15 turns/m, and their unique potential is experimentally demonstrated. First, a novel type of all-optical intensity discriminator is realized using the effect of ellipse rotation in CBF, which is absent in a non-PNI fiber. The demonstrated device is directly applicable to all-optical data reshaping, pulse compression, pedestral suppression, and enhanced mode-locking of fiber lasers. Next, a 160-Gb/s polarization-insensitive wavelength conversion is demonstrated by using cross-phase modulation in CBF with a circularly polarized probe wave. In addition to its simplicity and robustness, the scheme is readily upgradable to polarization-insensitive all-optical data regeneration.