摘要
基于有效全向反射原理的布拉格光纤可以通过调谐周期性的包层结构参数来实现特定波长的高功率传输.为了克服传统空心布拉格光纤易于形变的缺点,制备了一种基于补偿叠层挤压技术的硫系玻璃纤芯全固态布拉格光纤,解决了传统叠层挤压引起的光纤层厚不均匀问题.该光纤含有三对厚度相同的包层对,在整根6m长光纤中包层对的厚度与光纤直径的比例保持恒定的3∶100.该光纤在4～10μm的波长范围内具有四个明显的低损耗窗口,证明了优化玻璃厚度的补偿叠层技术可以有效改善光纤结构及光传输性能.
Abstract
Objective Bragg fibers have multiple unique optical properties such as photonic bandgap light guides,single-mode transmission over a wide frequency range,dispersion management,and low transmission loss,which make them attractive for broad applications.The transmission ability of a traditional hollow Bragg fiber is restricted by air-core collapse and structured-cladding deformation during optical fiber preparation.Even under tiny fiber cladding deformations,the bandgap can be violently degraded.All solid-state structures have been proven to solve the core collapse and cladding deformation problems of hollow Bragg fibers.Therefore,an urgent requirement exists to develop novel fiber structures and effective fiber fabrication methods to improve fiber transmission capability.In this study,an all-solid Bragg fiber with a chalcogenide glass core is fabricated via a compensated-stacking extrusion technique to address the challenge of hollow-core deformation in traditional Bragg fibers.The fiber consists of three pairs of uniform periodic cladding and low-loss windows in the range of 4-10 μm.This experimental data can assist further study regarding mid-infrared bandgap-controlled fibers and unlock new directions for the development of high-quality laser transmissions or optical sensors in the mid-infrared region.Methods In this study,we first establish a theoretical model for all-solid-state Bragg fibers.Mid-infrared chalcogenide glasses Ge20As20Se15Te45and As2S3are chosen as high-and low-refractive-index cladding materials.The large difference in the refractive index between the alternating-layer materials helps to form the widest photonic bandgap.Two groups of fibers based on equal-or compensated-thickness glass are prepared for comparison.The cross sections,transmission loss values,and near-field energy distributions of these optical fiber types are calculated and analyzed.Results and Discussions According to the simulation results,the optimal structural parameters of all solid-state chalcogenide Bragg fibers are obtained.The experimental results show that optimized stacking extrusion based on compensated-thickness glass is the simplest and most effective method for improving fiber structural uniformity.The cross-sections of the all-solid Bragg fiber based on equal-thickness glass[Figs.7(a)-(c)]show that the core and innermost cladding are irregularly elliptical,with a large difference in the thickness of the three pairs of periodic claddings.The thickness of the layers ranges from 10 μm to 600 μm,which significantly differs the simulation results[Fig.8(a)].The fiber cross-sections based on thickness-compensated glass[Figs.7(d)-(f)]show that the fiber structure is highly circular,without deformation,and no obvious defects such as bubbles or holes are observed at the interfaces of adjacent layers.Three pairs of periodic claddings have similar thicknesses in a 6-meter-long fiber,and the average ratio of each layer thickness to the fiber diameter is approximately 3∶100 for an entire fiber length with 6 m length[Fig.8(b)].It is proven experimentally that it is feasible to solve the problem of uneven claddings and deformational cores using thickness-compensated glass.The average loss of fibers based on equal-thickness glass is 4 dB/m-6 dB/m,however,the uneven fiber structure results in light propagation in the cladding[Fig.9(a)].The fiber based on thickness compensated glass has four low loss windows[Fig.9(b)].For good light transmission effect,the light is confined in the core and almost no energy leaks into the cladding.Conclusions Bragg fibers based on the principle of effective omnidirectional reflection achieve high-power transmission at specific wavelengths by tuning the structural parameters of the claddings;however,some problems remain.In this study,an all-solid-state Bragg fiber with a chalcogenide glass core is fabricated using a compensated stacking extrusion technique to solve the problem of hollow core deformation in traditional Bragg fibers.Ge20As20Se15Te45 and As2S3 glasses are doped as high-and low-refractive-index cladding materials,respectively,and an all-solid-state chalcogenide glass Bragg fiber with three pairs of periodic cladding layers is successfully fabricated via compensated stacking extrusion.The superior structural uniformity of the prepared chalcogenide Bragg fibers is verified by comparing the cross-sections of the front,middle,and end of the Bragg fibers.Three pairs of periodic claddings have similar thicknesses in a 6-meter-long fiber,and the average ratio of each layer thickness to the fiber diameter is approximately 3∶100 for an entire fiber with length of 6 m.The light spot pattern proves that the optical fiber has good light transmission ability.It is proven experimentally that it is feasible to prepare chalcogenide Bragg fibers using the extrusion method.In future,our research will further improve the extrusion mold and conditions aiming to develop higher performance photonic crystal fibers based on chalcogenide glass.
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