Liang Y, Lou J W, Andersen J K, Stocker J C, Boyraz O, Islam M N, Nolan D A
Department of Electrical Engineering and Computer Science, University of Michigan, 1301 Beal Avenue, Ann Arbor, Michigan 48109-2122, USA.
Opt Lett. 1999 Jun 1;24(11):726-8. 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.
我们通过使用550米的扭转色散位移光纤（扭转率为8圈/米（24圈/拍长）），实验证明了非线性光学环形镜（NOLM）的偏振灵敏度从5分贝降低到了0.5分贝。光纤的扭转会引起圆双折射，并使平行和正交偏振的非线性相移项相等。实验结果表明，偏振灵敏度从非扭转光纤的5分贝单调降低到扭转率为8圈/米的光纤的0.5分贝，并且扭转率应大于4圈/米（>10圈/拍长），以模拟圆偏振光纤。当控制脉冲偏振与扭转光纤的本征模之一对齐时，偏振灵敏度最小。在NOLM中，光纤以8圈/米的速率扭转时，在4皮焦/脉冲的开关能量下，非线性传输为23%。模拟结果证实了观察到的行为，并表明剩余的偏振灵敏度是由信号脉冲的正交模之间的能量转移引起的。
