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 nonpolarization-maintaining (non-PM) fiber, where the polarization dependence of chi⁽³⁾ 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-PM 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