A novel method enabling to produce a flat-top band-rejection filter is firstly proposed and experimentally demonstrated, which is achieved by using a phase-modulated helical long-period fiber grating (HLPG). Unlike most of the long-period fiber grating (LPG)-based and the HLPG-based band-rejection filters reported so far, the proposed band-rejection filter is inherently polarization-independent. Moreover, the proposed HLPG does not need a complex apodization in the grating’s profile, which considerably facilitates the fabrication processes and makes the designed HLPG particularly suitable for fabrication with using the CO2 laser-writing platform. As a typical example, a polarization-independent band-rejection filter with a bandwidth of ~10 nm@-20dB and a rejection depth of ~28 dB has been successfully achieved.

中文翻译：

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基于相位调制HLPG的偏振无关平顶带阻滤波器

首次提出并通过实验证明了一种能够生产平顶带阻滤波器的新方法，该方法是通过使用相位调制螺旋长周期光纤光栅 (HLPG) 实现的。与迄今为止报道的大多数基于长周期光纤光栅 (LPG) 和基于 HLPG 的带阻滤波器不同，所提出的带阻滤波器本质上与偏振无关。此外，所提出的 HLPG 不需要在光栅轮廓中进行复杂的切趾，这大大促进了制造过程，并使设计的 HLPG 特别适合使用 CO2 激光写入平台进行制造。作为一个典型的例子，已经成功实现了带宽为~10 nm@-20dB 和抑制深度为~28 dB 的偏振无关带阻滤波器。