An ultra-short double-wavelength optical power splitter for two waveguides operation based on photonic crystal multimode interference

In this paper, we design an ultra-short 1 × 2 1310/1550 nm double-waveguide optical power splitter based on photonic crystal multimode interference. The device can be used to divide the input beam equally for both 1310 nm and 1550 nm at the same time. The total multimode waveguide length of this device is only about 13 μm, which is one 210th of the conventional dielectric counterparts reported. On the basis of the guided-mode propagation analysis method, the self-imaging effect is discussed for the case of symmetric incidence. The finite-difference time-domain method is used to simulate the propagation of the beam in the multimode interference. The results show that the repetitive appearances of single image and twofold image of the input field occur alternatively in this device.     

电光有机聚合物多模干涉分束器

分束器是光纤通信系统中光无源网络和光子集成回路的重要器件.1×2 MMI(multimode interferences)电光有机聚合物分束器是利用自成像效应设计的.分析了1×2 MMI 的工作原理.利用有效折射率方法(effective index method,EIM)计算出1×2 MMI 三维多模脊形波导的有效折射率分布.利用自成像效应的成像规律计算出成像位置并用导模传输分析法(guided-mode propagation analysis,MPA)对输出波导的光强进行模拟.分析表明:在波长为 1.55μm 的情况下,1×2 MMI 的输出波导的光强在未加电极时可以实现预定分束比输出;加上电极之后,由于电光效应可调控有机聚合物波导折射率的改变,利用有限差分光束传播法模拟输出波导的分束比可在7.6 dB 范围内可调.     

Splitting-on-demand optical power splitters using multimode interference (MMI) waveguide with programmed modulations

Reconfigurable multi-channel optical power splitter is proposed and its optical properties are calculated. The device can dynamically reconfigure the number of splitting channels by providing programmed refractive index modulations on a multimode interference (MMI) waveguide. A reconfigurable 3-channel optical power splitter is designed to work as 1 × 1, 1 × 2 or 1 × 3 optical power splitter depending on the state of the heat electrodes using thermo-optic modulation, and the input light can be distributed to three output channels with sequential orders. The device can work in the whole C-band (1530-1565 nm) with extinction ratio better than −29.0 dB, excess loss better than −0.45 dB, imbalance better than 0.08 dB and polarization dependent loss (PDL) better than 0.14 dB. The design conception is scalable to a multi-channel splitting-on-demand optical power splitter which can divide input light to 1, 2, …, N output channels equally by using the 3-channel reconfigurable optical power splitter as a building block.     

Tunable dual-wavelength terahertz wave power splitter

We demonstrate, for the first time, a tunable dual-wavelength terahertz wave power splitter based on the multimode interference effect and self-imaging principle in 1 × 2 × 2 photonic crystal waveguides. Both plane wave expansion method and finite-difference time-domain method are used to calculate and analyze the characteristics of the proposed device. The simulation results demonstrate that the power splitter not only split the input power into output1 and output2 branches with equal power at frequency of 1.09 THz, but also split the power into output3 and output4 branches symmetrically at frequency of 1.20 THz. Furthermore, for the frequency of 1.09 THz, the input terahertz wave power can be split into output1 and output2 branches with an arbitrary ratio by tuning the refractive index of the tuning rods.     

Design and optimization of novel ultra-compact SOI multimode interference optical switch

In this paper, an ultra-compact 3 × 3 SOI multimode interference optical switch is presented. There is only one hyperbolic multimode waveguide in the switch. By changing the refractive index of one or two parts within the hyperbolic multimode waveguide, the switch can realize on and off functions between any pair of input and output ports. The various states of the switch were simulated and analyzed using the finite difference beam propagation method (FD-BPM), the structure parameters of the switch were also designed and optimized. The optimized switch has excellent properties and the whole size of the switch is less than 4 mm.     

Analysis of the plasmonic power splitter and MUX/DEMUX suitable for photonic integrated circuits

In this paper, the metal-insulator-metal (MIM) plasmonic directional coupler (PDC) with 45° waveguide bends based on surface plasmon polaritons (SPPs) excitation has been analyzed by the finite-difference time-domain (FDTD) numerical method. Effects of the variations of the coupler length and the metal gap thickness on the output powers and the propagation loss at 1550 nm wavelength have been studied. By choosing proper coupler lengths, power splitters with various output power ratios at 1550 nm wavelength and multi/demultiplexers, as some applications of the directional couplers have been proposed and their performances have been simulated.     

具有分光功能的电吸收调制器分析

利用有限差分光束传播法和导模传输分析法,对GaAs/AlGaAs多量子阱波导型电吸收调制器和多模干涉型分束器的光传播特性进行了理论分析,提出了制作具有分光功能的电吸收调制器的设想,并进行了模拟和实验验证.     

Second-order temporal interference of two independent light beams at an asymmetrical beam splitter

The second-order temporal interference of classical and nonclassical light at an asymmetrical beam splitter is discussed based on two-photon interference in Feynman's path integral theory. The visibility of the second-order interference pattern is determined by the properties of the superposed light beams, the ratio between the intensities of these two light beams, and the reflectivity of the asymmetrical beam splitter. Some requirements about the asymmetrical beam splitter have to be satisfied in order to ensure that the visibility of the second-order interference pattern of nonclassical light beams exceeds the classical limit. The visibility of the second-order interference pattern of photons emitted by two independent single-photon sources is independent of the ratio between the intensities. These conclusions are important for the researches and applications in quantum optics and quantum information when an asymmetrical beam splitter is employed.     

Slow light transmission in a photonic crystal power splitter with parallel outputs

In this paper, we investigate coupling of light to slow modes in a photonic crystal power splitter composed of a Y-junction and two 60° bends. First, a combination of two cascaded bends which is commonly used in integrated photonic crystal circuits is studied in slow light frequency regime. We propose a structure that its transmission spectrum covers the high group-index frequencies near the band edge. Also, by structural modifications, high transmission near to 95% is achieved in slow light bandwidth. Next, we study the complete structure of a photonic crystal power splitter with parallel outputs based on a Y-junction integrated with two 60° bends. Using modified bends and reducing sharpness of Y-junction, the efficiency of splitting increases in both high and low group-index frequency bands. The optimized structure has an average efficiency of 82% in slow mode regime. This structure can be used in photonic crystal based slow light devices, such as Mach-Zehnder interferometers.     

Micromachining by CO2 laser ablation: Building blocks for a multiport integrated device

We report on the design and fabrication of complex microcomponents based on multimode optical interference, using a CO₂ laser ablation technique. Mode confinement, power division and losses are assessed. Power splitters show a good balance in the intensity division but beam combiners exhibit a variation in output power. The devices are compact and show a low sensitivity to imperfections in the fabrication process. The results demonstrate the technique's potential to develop multiport integrated circuits.     

Microdisk resonator filters made of dielectric-loaded plasmonic waveguides

Microdisk resonator filters, an alternative to microring resonator filters, are studied by means of vectorial three dimensional finite element method simulations. Their performance characteristics are highlighted for different microdisk radii, and compared with those of the respective, footprint-wise, microring filters. We show that microdisk filters are advantageous, as the resonator involved exhibits smaller radiation losses. Extinction ratios as high as 30 dB are possible by properly tuning the gap separating the waveguide from the microdisk in each case. Transmission dips due to higher-radial-order modes that drastically change the transmission picture appear only for very large microdisk radii.     

Comparison of subwavelength focusing properties of diffraction and Fresnel zone plate plasmonic planar lenses

The subwavelength focusing properties of diffractive plasmonic planar lenses (DPLs) and Fresnel zone plate plasmonic planar lenses (FZPs) have been compared in this paper. To that end, we use the same lens material, incident wavelength, thickness and focal length for comparison. Both DPLs and FZPs consist of central circular slits surrounded by transparent and opaque zones and can get clear focusing performances. By using the rigorous electromagnetic numerical method, the fields in the focal region are analyzed in detail, and our results demonstrate that FZP can generate higher transmission efficiency, and higher peak field intensity at the focal plane. Focusing polarization properties of the lens illuminated by linearly polarized state, are calculated and analyzed also. The numerical results show that both the DPL and the FZP produce asymmetrical focal spot distributions with a low f-number. To the FZP, the full-width at half-maximum (FWHM) varies from 256 nm (along y-axis) to 516 nm(along x-axis) and to the DPL, the FWHM varies from 256 nm(along y-axis) to 580 nm(along x-axis), respectively. But for a high f-number, the asymmetrical performance of the focal spot will be reduced due to complicated electromagnetical field interferences and micro waveguide effect. Otherwise, the DPL can get a higher depolarization effect than the FZP does.     

Properties of a 1 × 4 optical power splitter made of photonic crystal waveguides

Optical power splitters made of photonic crystal waveguides are believed to become essential components for compact photonic integrated circuits used in fiber optic networks. We design a 1 × 4 optical power splitter made of linear-defect waveguides in photonic crystals, and analyze its properties using the finite-difference time-domain method. Our simulation results show that the transmission properties vary with wave frequency and branch geometry, and that an incident wave is divided equally into four output ports. To reduce the reflections at the three branching regions in the 1 × 4 splitter, we place the defects of extra rods in the branching region, and achieve high transmission in each output waveguide.     

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%。     

Selective modes excitation in multimode photonic crystal waveguides

We investigate modes excitation with the input field of different positions in two-dimensional multimode photonic crystal waveguides. Odd modes can be selectively excited by the input field of odd symmetry. The input field with different positions can excite different modes due to the field intensity distribution of modes. When the input field locates at the position of the zero field, intensity of waveguide modes is zero and the modes are not excited. The finite-difference time-domain method is used to obtain the excited field distributions.     

Optical guided dispersions and subwavelength transmissions in dispersive plasmonic circular holes

The light transmission through a dispersive plasmonic circular hole is numerically investigated with an emphasis on its subwavelength guidance. For a better understanding of the effect of the hole diameter on the guided dispersion characteristics, the guided modes, including both the surface plasmon polariton mode and the circular waveguide mode, are studied for several hole diameters, especially when the metal cladding has a plasmonic frequency dependency. A brief comparison is also made with the guided dispersion characteristics of a dispersive plasmonic gap [K.Y. Kim, et al., Opt. Express 14, 320–330 (2006)], which is a planar version of the present structure, and a circular waveguide with perfect electric conductor cladding. Finally, the modal behaviour of the first three TM-like principal modes with varied hole diameters is examined for the same operating mode.     

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1.55 μm的入射光,该分束器的透射率可高达93%。     

基于热膨胀效应的可调光功率分束器设计

可调光功率分束器是集成光子系统中的重要功能器件,其光功率输出具有动态可调控特性,诸多光子系统对其有着广泛需求.本文基于空隙槽型Y分支波导结构,利用热膨胀效应,提出了一种新型可调光功率分束器实现方法,通过调控温度来改变分支处空隙槽的宽度,以实现其分支波导的光功率输出的动态变化.采用有限元方法,对其热膨胀形变和光学性能进行了模拟分析.其模拟结果表明,该器件能实现大范围光分束调控,且对波长和偏振态依赖性低;同时,该器件具有结构简单、易于设计和制作、易于调控等优点,这种新型器件在各种光子系统中具有重要应用前景.     

Phase‐controlled directional switching of surface plasmon polaritons via beam interference

A novel method for switching the direction of surface plasmon polarition (SPP) launching is proposed. The principle behind the switching mechanism is based on the relative phase difference between the reference beam and the control beam. Selective guidance of the direction of SPP can be explained by the composition of the charge parity that is induced near the slit edge, which is caused by interference created by two incident beams. The switching of SPP launching direction could be implemented by simple phase modulation of the control beam when the slit parameters satisfy the specific conditions at the designed specific wavelength. An experimental demonstration, in good agreement with numerical results, shows that our configuration of SPP switching can be operated by a quite simple interaction.     

Thermo-optic multimode interference switches with air and silicon trenches

A novel thermo-optic multimode interference (MMI) switch with air and silicon trenches was proposed, and the performance of the switch was simulated. In the design, one heating electrode is used to alter the refractive index at a spot image which changes the phase of this image to realize the switching function. The simulation results clearly indicate that the MMI switch can satisfy −39 dB crosstalk at two states. The electric power consumption for the MMI switch with these trenches is less than half of that of a conventional MMI switch.