Research On High-Order Complete Photonic Bandgap Enhancement In Moderate Refractive Index Ratio And Broadband Single-Mode Slow-Light Fiber

The ability to achieve complete photonic bandgap(CPBG)in photonic crystals(PCs)has enabled the unprecedented ability to control and confine light.Optical devices based on the two-dimensional CPBG,such as high-Q cavity,slow-light bandgap fiber,polarization-independent waveguides,polarization beam splitters,etc,have a wide range of applications in optical communications,optical sensors,non-linear enhancement,etc.However,to obtain a CPBG usually requires a material system with high refractive index ratio(RIR),severely restricting our choice of material systems,such as chalcogenide glass and silicon nitride,and restricting applications in the wavelength range of visible light and near-infrared.Therefore,it is necessary to extend the CPBG to moderate RIR material system.At present,though numerous structural adjustment methods have been proposed to enhance 2D CPBG,only limited CPBG bandwidths could be achieved in few moderate RIR systems,and continually enhance CPBGs by constructing excellent PCs gets harder and harder.Thus,new stream of thought to further enhance CPBGs in moderate RIR systems is needed.In this paper,high-order CPBG enhancement in moderate RIR system is studied meticulously,and some meaningful results are obtained.Furthermore,based on the results,a broadband low-dispersion single-mode slow-light photonic bandgap fiber is designed.The specific research results are as follows:(1)By using the plane-wave expansion method,the variation of the maximum CPBG with RIR in air-hole triangular lattice,honeycomb lattice and honeycomb connecting-rod PCs are compared and analyzed.Through the investigations,we find that high-order CPBG is less effect by the RIR than low-order CPBG.Based on the phenomenon,a triangular lattice of annular-hole-connecting-rods PC is proposed by combing the structural advantage with RIR-insensitivity.After optimization of the structural parameters,the PC can not only obtain a normalized high-order CPBG of9.27%for RIR=2.4:1,but can also support a CPBG for RIR greater than 2.1:1.(2)To achieve CPBG enhancement in moderate RIR,a new method for the design of a PC that supporting high-order CPBG is proposed.In this method,a hole-shaped structure is set as a central-scatter object of the PCs firstly.Then,serval connecting-rods filled with high-refractive index material are set at the periphery of the hole-shaped structure as the marginal-scatter object.The closed area delineated by the central and marginal scatter objects is denoted as scatter region.When the scatter regions are delineated by a finite number of central and marginal scatter objects as many as possible,a PC we designed should theoretically supports higher-order scattering,then,a high-order CPBG should be obtained accordingly.As a result,CPBG enhancement in moderate RIR should be achieved due to the RIR-insensitivity of high-order CPBG.Based on the approach,a square lattice of annular-hole-multi-rods PC is proposed.After optimization of the structural parameters,a high-order CPBG of 3.348%could be obtained for RIR=2.1:1.CPBG could be existed for RIR greater than 1.9:1,which is lower than the lowest RIR of2.1:1 to support CPBG in 2D PCs we used to know.(3)A broadband slow-light photonic bandgap fiber with single-mode and low dispersion properties has been designed.The fiber is composed by a supercell as the background cladding and an air hole in the center of the supercell as fiber core.Slow light is obtained at the normalized propagation constantβ→0 by exploiting the strong confinement of the CPBG.Furthermore,an annular-hole fill with high-refractive index is introduced around the core.Through a subtle adjustment for the outer and inner radius of the annular-hole,the design of a broadband slow optical bandgap PC fiber with single-mode low dispersion properties is achieved.The calculation results according to the plane wave expansion method show that the fiber could support single-mode slow light with an average group refractive index of7 in the frequency range of 0.80827-0.81816.Its single-mode slow light bandwidth is about 20nm,the delay bandwidth product is 0.085,and the group velocity dispersion is controlled within an acceptable range of±150 ps~2/mm.