Dual-frequency division de-multiplexer based on cascaded photonic crystal waveguides

A dual-frequency division de-multiplexing mechanism is demonstrated using cascaded photonic crystal waveguides with unequal waveguide widths. The de-multiplexing mechanism is based on the frequency shift of the waveguide bands for the unequal widths of the photonic crystal waveguides. The modulation in the waveguide bands is used for providing frequency selectivity to the system. The slow light regime of the waveguide bands is utilized for extracting the desired frequency bands from a wider photonic crystal waveguide that has a relatively larger group velocity than the main waveguide for the de-multiplexed frequencies. In other words, the wider spatial distribution of the electric fields in the transverse direction of the waveguide for slow light modes is utilized in order to achieve the dropping of the modes to the output channels. The spectral and spatial de-multiplexing features are numerically verified. It can be stated that the presented mechanism can be used to de-multiplex more than two frequency intervals by cascading new photonic crystal waveguides with properly selected widths.     

Magnetically Tunable Terahertz Switch and Band-Pass Filter

A novel design of a tunable terahertz switch and band-pass filter using liquid-crystal-filled photonic crystal waveguide is demonstrated. The effects of magnetic field on the photonic bandgaps and transmitting properties of a THz wave are investigated by using the plane wave expansion method and finite difference time domain method. The efficient photonic bandgap tuning is predicted such that the two-dimensional liquid-crystal-filled photonie crystal waveguide can serve as a switch and continuously tunable band-pass filter with controlling of the magnetic field.     

二维三角晶格光子晶体波导的出射光集束传播

为了解决光子晶体波导出射端光场控制, 同时解决二维三角晶格光子晶体波导出射光辐射困难的问题。利用二维三角晶格光子晶体设计了一种新型光子晶体波导出射口结构。在二维三角晶格光子晶体波导出射端引入两个微腔, 通过光波与微腔发生共振, 形成类似于三个点光源干涉的出射光, 同时进一步提出波导出射端喇叭口干涉出射光定向辐射的设计。通过这种微腔喇叭口设计, 利用时域有限差分法分析结果表明光波实现很好的定向辐射, 并且辐射距离显著提高。     

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.     

Air waveguide in a hybrid 1D and 2D photonic crystal hetero-structure

An air waveguide in hybrid one-dimensional photonic crystal and two-dimensional photonic crystal slab hetero-structure is designed. Light propagating in air waveguide can be confined by two-dimensional photonic crystal slab in x-y plane and one-dimensional photonic crystal films in z direction. Theoretical calculations show that air waveguide in the hetero-structure can achieve some functions as 3D PhCs but could be made more easily than 3D PhCs.     

2D FDTD simulation of low loss small angle bend and Y branch configurations in a photonic crystal waveguide layout with a Mach–Zehnder device design configuration

A technique to produce low loss small angle bends in photonic crystal waveguides is presented. The technique consists of bridging parallel input and output waveguide segments with an inclined waveguide region of the same basic design that has a lateral dielectric shift. Results are presented for waveguides produced by enlarging the silicon gap along the central line, separating air holes in a square array photonic crystal for the TE polarization and an operating wavelength of λ_o = 1.55 μm. This low loss waveguide bending technique is applied to the design of Y branch and Mach–Zehnder photonic crystal structures. Simulation of the performance of the dielectric structures is achieved using 2-D FDTD, similar results are anticipated when applied to 3-D waveguide configurations and for other photonic crystal layouts.     

Roughness losses and volume-current methods in photonic-crystal waveguides

We present predicted relative scattering losses from sidewall roughness in a strip waveguide compared to an identical waveguide surrounded by a photonic crystal with a complete or incomplete gap in both 2d and 3d. To do so, we develop a new semi-analytical extension of the classic “volume-current method” (Green’s functions with a Born approximation), correcting a longstanding limitation of such methods to low-index contrast systems (the classic method may be off by an order of magnitude in high-contrast systems). The resulting loss predictions show that even incomplete gap structures such as photonic-crystal slabs should, with proper design, be able to reduce losses by a factor of two compared to an identical strip waveguide; however, incautious design can lead to increased losses in the photonic-crystal system, a phenomena that we explain in terms of the band structure of the unperturbed crystal.     

非线性一维光子晶体波导光双稳

利用非线性折射率系数较大且非线性时间响应较快的CdS_xSe_1-x玻璃为材料,设计并制备了非线性一维光子晶体波导光双稳器件,该器件的折射率空间分布呈正弦形式。实验测得双稳开关的阈值功率密度为1.60×10⁵W/cm²,开关时间为63ps。采用时域有限差分方法讨论了光子晶体带隙随入射光强变化而移动的情况,随着入射光功率密度的增加,光子晶体的带隙中心向短波方向移动。同时计算了该器件的双稳特性,理论计算得到双稳开关的阈值功率密度为1.40×10⁵W/cm²,开关时间约为50ps。获得了理论与实验基本一致的结果。     

Photonic crystal waveguide sampled gratings

Photonic crystal waveguide sampled gratings (PCWSG) are proposed to realize multi-channel optical filter in a photonic crystal (PC) waveguide. The reflection characteristics of the filter are analyzed by using the coupled-mode theory (CMT) together with transfer-matrix method (TMM). It is shown that the reflection spectrum of the filter exhibits a group of reflection peaks, which are distributed with equal frequency spacing within the photonic crystal bandgap (PBG). The theoretically calculated results are numerically verified by using two-dimensional (2D) finite-difference time-domain (FDTD) method.     

光子晶体光纤色散的无量纲化计算方法

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Enhanced localization of light in slow wave slot photonic crystal waveguides

A flexible design of slot photonic crystal waveguide with a wide comb is investigated. Introduction of a carefully designed comb within the photonic crystal waveguide allows an accurate dispersion engineering in order to achieve slow light and increase the optical confinement within the comb. The strong light confinement results in an extremely small nonlinear effective area around 0.015 μm². We report experimental realization of a comb photonic crystal waveguide with measured group indices higher than 100 in a Mach-Zehnder interferometer configuration and extract losses limited to 3.7?dB for a 100?μm device at n_g=37.     

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。     

An Integrative Biosensor Based on Contra-Directional Coupling Two-dimensional Photonic Crystal Waveguides

We propose an integrative biochemical sensor utilizing the dip in the transmission spectrum of a normal singleline defect photonic crystal （PC） waveguide, which has a eontra-directional coupling with another PC waveguide. When the air holes in the PC slab are filled with a liquid analyte with different refractive indices, the dip has a wavelength shift. By detecting the output power variation at a certain fixed wavelength, a sensitivity of 1.2 × 10^-4 is feasible. This structure is easy for integration due to its plane waveguide structure and omissible pump source. In addition, high signal to noise ratio can be expected because signal transmits via a normal single-line defect PC waveguide instead of the PC hole area or analyte.     

Analysis of ultra-small photonic large scale integrated circuits

A detailed study of a platform of ultra-small photonic large-scale integrated circuits was conducted. Bandgap structure calculations of silicon-on-insulator (SOI) based photonic crystals have been investigated. The photonic crystal consists of dielectric cylinders in air. Using the band structure calculations we obtained design parameters for the proposed structures. The coupling between the photonic crystal and a waveguide fabricated from SOI system has been analysed. It is shown that the optical coupling is improved by interfacing different types of spot-size converters (SSCs) between the SOI waveguide and the photonic crystal. Also, the possibility and limitations of silicon doped germanium and SOI photonic crystals to analyse the light guiding in the third dimension is discussed.     

Parallel microgenetic algorithm design for photonic crystal and waveguide structures

We have developed a powerful parallel genetic algorithm design tool for photonic crystal and waveguide structures. The tool employs a small-population-size genetic algorithm (microgenetic algorithm) for global optimization and a two-dimensional finite-difference time-domain method to rigorously design and optimize the performance of photonic devices. We discuss the implementation and performance of this design tool. We demonstrate its application to two photonic devices, a defect taper coupler to connect conventional waveguides and photonic crystal waveguides, and a sharp 90 degrees waveguide bend for low index contrast waveguides.     

Low-dimensional waveguide microwave photonic crystals

Structures containing periodically alternating elements, which are a source of high-order waves, are investigated as low-dimensional waveguide microwave photonic crystals. The band character of the transmittance and the reflectance of a photonic crystal, which consists of sequentially alternating dielectric layers and thin metallic plates partially overlapping the waveguide section, is revealed by the numerical modeling and the experimental investigation of amplitude-frequency characteristics. It has been shown that the application of metallic plates with gaps in the structure of the photonic crystal made it possible to decrease its longitudinal size substantially compared with photonic crystals fabricated based on elements made of alternating dielectric layers with various permittivities.     

Low group velocity in a photonic crystal coupled-cavity waveguide

A two-dimensional photonic crystal coupled-cavity waveguide is designed and optimized, the transmission spectrum is calculated by using the finite-difference time-domain method, and the group velocity of c/1856 is obtained. To our knowledge, this value of group velocity is the lowest group velocity in a photonic crystal waveguide calculated from its transmission spectrum so far. The result is confirmed by the photonic band structure calculated by using the plane wave expansion method, and it is found that the photonic crystal waveguide modes in a photonic band structure are in accordance with those in the transmission spectrum by using the finite-difference time-domain method. The mechanism of slow light in the coupled-cavity waveguide of photonic crystal is analysed.     

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%。     

Compact interrogator for fiber optic Bragg sensors based on an acousto-optic filter formed by photonic crystal rows of air holes

Fiber optic sensors are typically used with expensive tunable lasers or optical spectrum analyzers for wavelength interrogation. We propose to replace the tunable laser by a broadband optical source incorporated with a novel thin linewidth acousto-optic tunable filter. It utilizes optical beam expanders constituted by photonic crystal rows of air holes in LiNbO(3) waveguide. A new design is numerically studied for a short structure (with 32 photonic crystal rows) by a two-dimensional finite-difference time-domain method. Extrapolation of these results to larger structure sizes (about 1 cm) demonstrates the possibility to develop compact interrogators with 0.4 pm wavelength resolution and 40 nm tunable range around 1550 nm.     

Numerical characterization of nanopillar photonic crystal waveguides and directional couplers

We numerically characterize a novel type of a photonic crystal waveguide, which consists of several rows of periodically arranged dielectric cylinders. In such a nanopillar photonic crystal waveguide, light confinement is due to the total internal reflection. A nanopillar waveguide is a multimode waveguide, where the number of modes is equal to the number of rows building the waveguide. The strong coupling between individual waveguides leads to the proposal of an ultrashort directional coupler based on nanopillar waveguides. We present a systematic analysis of the dispersion and transmission efficiency of nanopillar photonic crystal waveguides and directional couplers. Plane wave expansion and finite difference time domain methods were used to characterize numerically nanopillar photonic crystal structures both in two- and three-dimensional spaces.