The evolutions of polarization and orbital angular momentum (OAM) states of light in helically twisted birefringent photonic crystal fibers (TB-PCFs) are analyzed. It is shown that a circular polarization (CP) component (S3 of a Stokes parameter) is periodically excited when usual linearly polarized (LP) modes of PCF are launched. The excitation originates from a geometric phase in TB-PCFs. The S3 excitation is larger for larger linear birefringence for a fixed twisting rate. If the linear birefringence is large enough, a CP filtering behavior can be seen in addition to the S3 excitation. From the analytical consideration of the sign of the geometric phase, the TB-PCF with periodical inversion of twisting is proposed to generate arbitrary polarization state on the Poincare sphere. Next, an OAM state generation in multimode TB-PCFs is shown for higher-order LP mode input. By observing a far-field interference pattern from TB-PCF mixed with LP01 mode, a vortex associated with the OAM state can be seen. Similar to the single-mode case, by using periodical twisting inversion, efficient OAM generation is possible. These results indicate that by simply launching fiber’s LP mode into TB-PCF, arbitrary polarization and OAM states can be generated, leading to a novel mechanism for the manipulation of the spatial state of light.

中文翻译：

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螺旋扭曲双折射光子晶体光纤中几何相位诱导的任意偏振和轨道角动量的产生

分析了螺旋扭曲双折射光子晶体光纤 (TB-PCF) 中光的偏振和轨道角动量 (OAM) 状态的演变。结果表明，当发射 PCF 的通常线性偏振 (LP) 模式时，会周期性地激发圆偏振 (CP) 分量（斯托克斯参数的 S3）。激发源于 TB-PCF 中的几何相位。对于固定扭曲率的较大线性双折射，S3 激发较大。如果线性双折射足够大，除了 S3 激发外，还可以看到 CP 滤波行为。从几何相位符号的解析考虑，提出了具有周期性扭转反转的TB-PCF在庞加莱球上产生任意偏振态。下一个，多模 TB-PCF 中的 OAM 状态生成显示为高阶 LP 模式输入。通过观察 TB-PCF 与 LP01 模式混合的远场干涉图，可以看到与 OAM 状态相关的涡流。与单模情况类似，通过使用周期性扭曲反转，可以有效地生成 OAM。这些结果表明，通过简单地将光纤的 LP 模式发射到 TB-PCF 中，可以产生任意偏振和 OAM 状态，从而形成一种用于操纵光空间状态的新机制。