Photonic crystal fibers with disordered claddings - a new path towards improved light guidance in defect cores

光子晶体光纤（PCFs）是一种特殊类型的光纤，具有复杂的内部微观结构，在生物光子学和非线性光学等各个领域都有应用。一种特殊的PCF几何结构包含折射率高于主材料的纵向介电束的规则阵列，并延伸到光纤的整个长度。这种阵列充当光子带隙（PBG）介质，通过省略阵列内的一个或多个空穴，即掺杂晶格，实现沿光纤的光引导。由于PBG效应，这些光纤在特定的光谱间隔内引导光，即所谓的传输带，其带宽由链和/或阵列特性预定义。另一种带隙光纤是空心芯（HC） PBG光纤，包括具有中心HC的空气孔阵列。这些纤维最近由于在粒子加速或非线性紫外线产生等方面有前景的应用而受到了极大的关注。两种PBG设计都包含规则的链阵列，结果显示出一些不希望看到的特性，例如有限的传输带宽，这表明规则的孔阵列并不适合所有可能的应用。在该项目中，我们计划研究和了解不同类型的无序对pcf光学特性的影响，并开发新型无序光纤包层，其总体思路是超越目前使用的pbg - pcf的最新技术。该项目的主要目标是评估pcf包层内的无序是否会导致相位和群速度色散、模型衰减、抗弯曲鲁棒性和传输窗口带宽方面的制导性能改善。这种失序是由材料变化或直径或形状变化引起的。第一个基准几何是具有无序包层的全固体纤维，允许以可控的方式研究与无序相关的所有相关包层特性。在第二步，该项目将专注于PBG纤维与液体填充的hc。这些纤维有一个完全固体的包层，由低折射率矩阵中的HI股的无序阵列组成，中心有一个液体芯，其折射率通常低于基体本身。该项目一方面包括深入的理论调查和设计研究（Weiss组），另一方面包括对这种新型纤维的实验实现和精确表征（Schmidt组）。该项目将定义一类具有工程特性的新型微结构光纤，并将其应用于生物光子学或非线性光学等高度相关的领域。

Photonic crystal fibers (PCFs) represent a special type of optical fiber including sophisticated internal microstructure and having led to applications in various areas such as biophotonics or nonlinear optics. One particular PCF geometry contains regular arrays of longitudinal dielectric strands with refractive indices higher than the host material and extending through the entire length of the fiber. Such an array acts as a photonic band gap (PBG) medium and light guidance along the fiber is achieved by omitting one or more holes within the array, i.e. doping the lattice. Due to the PBG effect, these fibers guide light within particular spectral intervals - so-called transmission bands - with the bandwidth predefined by the strand and/or array properties. Another type of band gap fiber are hollow-core (HC) PBG fibers, including air hole arrays with a central HC. These fibers have recently gained significant attention due to promising applications in e.g. particle acceleration or in nonlinear ultraviolet generation. Both PBG designs include regular arrays of strands and, as a result, reveal particular undesired properties such as limited transmission bandwidths, showing that regular hole arrays are not favorable for all possible applications.In the proposed project we plan to investigate and understand the influence of different types of disorder on the optical properties of PCFs and develop new types of disordered fiber claddings with the overall idea of going beyond the state-of-the-art of currently used PBG-PCFs. The main objective of the project is to evaluate if disorder within the PCF-cladding can lead to improved guidance properties in terms of phase and group velocity dispersion, model attenuation, robustness against bending and transmission window bandwidth. The disorder is introduced either by material variation or by diameter or shape modifications. The first benchmark geometry are all-solid fibers with disordered claddings allowing studying all relevant cladding properties related to disorder in a controlled manner. In a second step, the project will be focused on PBG fibers with liquid-filled HCs. These fibers have an entirely solid cladding consisting of disordered arrays of HI strands in a low index matrix with a central liquid core, which typically has a lower index than the matrix itself.The project comprises on the one hand in-depth theoretical investigation and design studies (Weiss group) and, on the other hand, the experimental implementation and precise characterization of such novel kind of fiber (Schmidt group). The project will define a new class of microstructured fibers with engineered properties and applications in highly-relevant areas such as biophotonics or nonlinear optics.