Polarization-insensitive nonlinear optical loop mirror demultiplexer with twisted fiber

• Published in: Optics letters 1999-06, Vol.24 (11), p.726-728
• Main Authors: Liang, Y, Lou, J W, Andersen, J K, Stocker, J C, Boyraz, O, Islam, M N, Nolan, D A
• Format: Article
• Language: English
• ISSN: 0146-9592; 1539-4794
• DOI: 10.1364/OL.24.000726

We experimentally demonstrate reduction of the polarization sensitivity of a nonlinear optical loop mirror (NOLM) from 5 to 0.5 dB by use of 550 m of twisted dispersion-shifted fiber with a twist rate of 8 turns/m (24 turns/beat length). The twisting of the fiber induces circular birefringence and equates the parallel-and the orthogonal-polarization nonlinear phase-shift terms. Experimental results show that the polarization sensitivity monotonically decreases from 5 dB for nontwisted fiber to 0.5 dB for fiber that is twisted at a rate of 8 turns/m, and the twist rate should be more than 4 turns/m (>10 turns/beat length) for emulation of circularly polarized fiber. The minimum polarization sensitivity occurs when the control-pulse polarization is aligned with one of the eigenmodes of the twisted fiber. With the fiber twisted at a rate of 8 turns/m in the NOLM, the nonlinear transmission is 23% at a switching energy of 4 pJ/pulse. Simulations confirm the observed behavior and show that the remaining polarization sensitivity results from energy transfer between orthogonal modes of the signal pulse.