基于自映像的光子晶体波导分束器的设计

通过研究2维正方晶格光子晶体波导多模干涉的自映像效应,优化设计了一种新型1×2光子晶体波导分束器,采用时域有限差分法对其传输特性进行模拟分析。设计过程中,根据多模干涉耦合区中周期出现的双重像的位置确定两个单模输出波导的位置,通过在分束器输入和输出耦合区的连结处设置介质柱,改变输入和输出耦合区中的模场分布,实现模式匹配,从而明显减小分束器的反射损耗。计算结果表明:设置的介质柱归一化半径分别为0.08和0.177时,对于波长为1.55μm的入射光,该分束器的透射率可高达93%。     

Analysis of single-mode condition and high-order modes discrimination in photonic crystal VCSEL

A new technique for analyzing the single-mode condition and mode discrimination of high-order modes in photonic crystal VCSEL is proposed and reported in this paper. The technique proposed is a semi-empirical approach that uses analytical analysis based on experimental data. For that purpose, single- and multimode photonic crystal VCSEL devices are fabricated and characterized for comparison of device performance. The mode width for first- and second-order mode of PhC VCSEL is computed for analyzing the mode behavior. In order to verify the findings, the fabricated single- and multimode devices are analyzed using finite difference frequency domain technique for the purpose of showing the loss as a function of mode order for the PhC VCSEL. The results show close agreement between the computed and experiment findings, justifying the use of the proposed semi-empirical technique for analyzing single-mode condition and of high-order modes discrimination in photonic crystal VCSEL.     

Multimodal and birefringence effects in magnetic photonic crystals

Photon trapping in magnetic photonic crystals leads to an enhanced Faraday rotation by increasing the optical path-length of the transmitted beam. The integration of these structures into on-chip photonic circuits, while advantageous from the point of view of component connectivity in multifunctional systems, faces several challenges. Photonic waveguide structures in magnetic films may support more than one mode depending on the waveguide thickness and refractive index. Differences in effective refractive indices between TE and TM modes engender phase disparities, thus hindering the Faraday response of the material. The effects of birefringence and multimodality on the performance of waveguide magnetic photonic crystals in magnetic garnets are addressed in paper. In particular, Faraday rotation enhancement in magnetophotonic crystals in the presence of waveguide birefringence and modal multiplicity on the photonic bandgap spectral response are discussed. Multiple stopbands and significant polarization rotation are observed in multimode Bi-substituted iron garnet film waveguides with single-defect photonic crystal structures. The photonic crystals for this study are patterned on ridge waveguide films by focused-ion-beam (FIB) milling.     

Optical surface modes of photonic crystals for dual-polarization waveguide

In this study, the design of a polarization-independent (dual-polarization) waveguide is presented by utilizing surface modes of photonic crystals. The waveguide structure operates in a frequency interval that is commonly shared by both transverse-electric (TE) and transverse-magnetic (TM) polarizations. The numerical calculations based on plane wave expansion and finite-difference time-domain methods are carried out to design and demonstrate a surface mode waveguide that provides confinement and guiding for both TE and TM modes. Once the relevant modes are properly excited, the high transmission efficiency of the photonic crystal surface waveguide is ensured. The demand to have polarization-insensitive devices makes our proposed design an important component for the photonic integrated circuit applications. Finally, we also propose a broadband surface mode photonic crystal waveguide with a bandwidth value of 28% for only TE polarization.     

超微光子晶体多模干涉型滤波器的设计与优化

根据对称入射时的自映像原理,设计了一种基于光子晶体多模波导的超微滤波器,对于进入该种器件的1.31 μm和1.55 μm两种通信波长的光信号,能够在通过一种波长的同时,对另一种波长进行阻隔,整个器件的长度不超过15 μm.讨论了该滤波器的输出效率,并通过调整位于单模波导与多模波导联结处的介电柱位置对结构进行优化.优化后的结构具有高的输出效率和较宽的带宽.时域有限差分法的模拟结果表明该器件是可行的.     

高功率弯波导TE_(01)—TM_(11)模式变换临界角分析

在模式耦合理论的基础上 ,采用传统的波导轴线圆弧弯曲的方法 ,对TE0 1—TM11模式变换器的临界角进行了全面的优化分析。得出在临界角情况下 ,若考虑多模因素 ,则不能使TE0 1—TM11的能量发生全转换 ,而真正的最优化能量全转换角在临界角的附近 ,且转换的效率与弯波导曲率相关。     

Multimode fiber devices with single-mode performance

A taper transition can couple light between a multimode fiber and several single-mode fibers. If the number of single-mode fibers matches the number of spatial modes in the multimode fiber, the transition can have low loss in both directions. This enables the high performance of single-mode fiber devices to be attained in multimode fibers. We report an experimental proof of concept by using photonic crystal fiber techniques to make the transitions, demonstrating a multimode fiber filter with the transmission spectrum of a single-mode fiber grating.     

紧凑型圆极化模式转换器

 提出了一种结构紧凑的、能将圆波导TM01模或同轴波导TEM模转换为圆极化TE₁₁模的高功率微波模式转换器。该转换器由前后2个十字转门波导结对接组成,前者首先把圆波导TM₀₁模转变为4个矩形波导中的TE₁₀模,4个矩形波导的长度不等;后者再把4个经过不同相位延迟的矩形波导TE₁₀模转变为圆波导中的圆极化TE₁₁模。对所设计的1.75 GHz模式转换器进行了仿真研究,在中心频率上,该模式转换器转换效率为99%,轴比为0.03 dB;在1.575~1.900 GHz的频率范围内,转换效率大于90%,轴比小于2.5 dB,对应带宽为18.6%。     

Analysis of multiple reflections in hybrid photonic crystal multimode interference coupler

In this paper the analysis of multiple reflections in photonic crystal (PhC) multimode interference (MMI) couplers using eigen-mode expansion method is presented. The analysis is conducted on a hybrid PhC structure which consisted of 1-D PhC multimode waveguide sandwiched between 2-D PhC input/output waveguides. In PhC multimode waveguide, where the mechanism of wave confinement is not due to total internal reflection but due to photonic bandgap properties, the reflectivity at 2-D PhC facet wall would be very large for all the guided modes in the waveguide when ever the image formed due to MMI effect does not coincides with the output access waveguide.     

Demonstration of systematic photonic crystal device design and optimization by low-rank adjustments: an extremely compact mode separator

We present a powerful design and optimization method for devices in a photonic crystal. The method is based on a Wannier basis field expansion and efficient matrix analysis techniques for searching through a vast number of designs. The method permits the design of many compact optical devices with complex and novel functions. We present a design example of a very compact mode separator that is 8.2 microm x 13.3 microm in size that demultiplexes the three modes of an input photonic crystal multimode waveguide into three single-mode output waveguides. We verify the method with finite-difference time-domain calculations.     

Dispersionless slow light by photonic crystal slab waveguide with innermost elliptical air holes

We report a low-loss photonic crystal slab waveguide formed by deforming the innermost circle air holes in the conventional photonic crystal slab waveguide into elliptical ones. We obtain the photonic bands and group index of guided modes in this photonic crystal waveguide by guided-mode expansion method and investigate the dependence of photonic bands and group index of guided modes on the parameters of the innermost elliptical air holes. The group velocity and group velocity dispersion of this waveguide strongly depend on the innermost elliptical air holes. Photonic crystal slab waveguide with the optimum innermost elliptical air holes possesses a wider single mode region below the light line, in which light can easily propagate without intrinsic loss. At the same time, the guided mode supported by this waveguide has nearly constant group velocity and vanishing group velocity dispersion in a 3-5 nm bandwidth.     

Superbending effect in two-dimensional graded photonic crystals

We study the superbending effect in a two-dimensional graded photonic crystal (GPC) for both TE and TM modes. We show that the lateral beam shift is extremely sensitive to the wavelength for the TM mode but not TE mode, indicating the potential in designing some special superbending devices, for example, TE/TM splitter and polarization-indifference waveguide at prescribed wavelengths.     

Theoretical study on the multimodeness of a commercial endlessly single-mode PCF

We report a theoretical study on the characteristics of a commercial photonic crystal fiber claimed to be endlessly single mode, i.e. the LMA-15 fiber. The study was carried out using a finite element leaky mode solver. By using a loss discrimination between the fundamental and higher order leaky modes as a multi/single-mode criterion, we obtained that the fiber is obviously multimode in short wavelengths, even for long length beyond the practically usable length of the fiber.     

Mode multiplexing in optical frequency mixers

Asymmetric Y junctions allow the development of a new class of optical frequency mixers that utilize higher-order waveguide modes for signal processing. We measure high-contrast (>30 dB) mode sorting in asymmetric Y junctions by use of a novel technique: efficient TM00, TM10, and TM20 mode mixing in a periodically poled lithium niobate waveguide. We also demonstrate an odd-to-even mode wavelength converter capable of spectral inversion without offset or bidirectional wavelength conversion.     

光子晶体缺陷结构的特性

基于平面波展开法,设计了二维正方晶格和三角晶格光子晶体缺陷结构,数值模拟了TE模、TM模式的光子晶体禁带特性,计算了二维三角晶格光子晶体缺陷结构的模式分布。结果表明,光子晶体缺陷结构能够形成很好的禁带特性和模式结构,研究结论为光子晶体波导器件的开发提供参考。     

Research of band gap properties based on two-dimensional photonic crystal with mixed shapes of rods

Two new structures of photonic crystals were designed. The band gap properties of photonic crystals with square and circular dielectric rods mixed arrangement are researched. The band gap properties of mixed shapes rods photonic crystal are calculated and compared with the crystals with square rods or round rods by using plane wave expansion method. Simulation results show that for the square lattice, mixed shapes of rods make the higher-order bands of TM modes moving toward the low frequency range. The gap bands’ widths and locations are between the parameters of square and round rods photonic crystal. In triangle lattice, a significant band gap is presented in photonic crystal with mixed shapes of rods in TE mode, while it is almost not presented in square and round rods crystals. The phenomenon of bands moving toward the low frequency range is also found in the triangle lattice mixed shapes rods photonic crystal. The reasons of the results in the vision were analyzed.     

二维正方晶格光子晶体禁带特性

基于平面波展开法,以碳化硅构成二维正方晶格光子晶体,数值模拟了TE模、TM模二维光子晶体的禁带特性,结果表明,TE模更容易形成光子禁带。同时设计了以碳化硅构成二维正方晶格光子晶体波导,数值模拟了TE模、TM模波导的传输特性和禁带特性,结果表明,TE模构成的波导电磁波能够较好的传播,它们的光子禁带都没有出现。研究结论为光子晶体波导器件的开发提供参考。     

Analysis of directional emission in square resonator lasers with an output waveguide

Square microcavity laser with an output waveguide is proposed and analyzed by the finite-difference time-domain (FDTD) technique. For a square resonator with refractive index of 3.2, side length of 4 μm, and output waveguide of 0.4-μm width, we have got the quality factors (Q factors) of 6.7 × 102 and 7.3 × 103 for the fundamental and first-order transverse magnetic (TM) mode near the wavelength of 1.5 μm, respectively. The simulated intensity distribution for the first-order TM mode shows that the coupling efficiency in the waveguide reaches 53%. The numerical simulation shows that the first-order transverse modes have fairly high Q factor and high coupling efficiency to the output waveguide. Therefore the square resonator with an output waveguide is a promising candidate to realize single-mode directional emission microcavity lasers.     

All-fiber controller of radial polarization using a periodic stress

Our aim is to transpose the polarization control by mechanical stress, usually applied to single-mode fibers, to the (TM(01), TE(01), HE(21)(ev), HE(21)(od)) annular mode family. Nevertheless, the quasi-degeneracy of these four modes makes the situation more complex than with the fundamental mode HE(11). We propose a simple device based on periodic perturbation and mode coupling to produce the radially polarized TM(01) mode or at least one of the four modes at the extremity of an arbitrarily long fiber, the conversion to TM(01) mode being achievable by classical crystalline plates.     

Design of a novel wideband single-mode waveguide in a photonic crystal slab structure

We propose a novel photonic crystal slab waveguide that provides a single-mode band with large bandwidth. The proposed waveguide is obtained by introducing a line defect in a triangular lattice of air holes in a dielectric slab. This line defect consists of holes which are not located in the original lattice points. The plane wave expansion (PWE) method is used to extract the band diagram of guiding modes and based on the results, photonic crystal holes in the defect row and its adjacent rows are modified to maximize the waveguide bandwidth. We show that using the proposed structure, a single-mode bandwidth of 17% can be achieved.