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

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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.