Kagome-structured Large-pitch Hollow-core Photonic Crystal Fiber

Concurrent to the development of the low-loss hollow-core photonic crystal fibers(HC-PCFS), large-pitch Kagome-structured HC-PCF have excited interests because of its unique optical properties. Compared with hollow-core bandgap-guiding fibers, the kagome fibers show that air-guiding can be achieved in the absence of the PBG with large transmission bandwidth and relatively low loss. In 2008 another HC-PCF with large-pitch square-lattice was reported by F.couny et al. The fiber has similar guidance properties to the kagome-structure HC-PCF and can therefore be used in applications requiring broadband transmission.For such favorable properties of the kagome fiber, research reports are increasing in the world but little in China. As a matter of fact, for the guidance mechanism in these fibers there are various suggestions such as low cladding density of states, low overlap between core and cladding mode fields and high-order bandgaps. But the nature of guidance in these fibers is not fully understood. We have already known lots about the effects of the fiber structure on the bandgap HC-PCF. However, the impacts between the kagome fibers and fiber structure such as different core design and fiber size have not been fully discovered.In this paper, firstly we make a comparative analysis on the guidance mechanism, attenuation mechanism and bandgap characteristics of the bandgap-guiding and large-pitch kagome-lattice HC-PCF. Secondly we investigate the general features of the loss spectrum of silica-based large-pitch Kagome lattice HC-PCF by studying the effects of the core parameters and the size of the fiber structure on the guidance properties of such fibers. The numerical simulations show that the operational bandwidth of the fibers is drastically affected by improper core design parameters which cause the interaction among the fundamental core modes, surface modes and cladding modes. The interaction causes an'anti-crossing'between the fundamental core modes and surface modes which leads to the narrow operational bandwidth and the high confinement loss of the fiber. The transmission performance of large-pitch is also sensitive to the thickness of silica struts and pitches.