Design of low-loss tapered waveguide by applying photonic-crystal-based microlenses in telescopic structure

In this paper, the design of low-loss tapered waveguides by applying photonic-crystal-based microlenses is presented. The microlenses are realized by using two-dimensional photonic crystal (PhC) structures with a hexagonal periodic lattice of air-holes. In order to make the PhC structures behave as homogeneous and isotropic microlenses, the PhC structures are designed to operate in the low-frequency domain with light wavelength sufficiently larger than the PhC lattice constant. The effective indices of the PhC are calculated and the structures are properly arranged in microlenslike shapes. By monolithically integrating the effective microlenses in the tapered area between the two optical waveguides with different core dimensions, the wavefront of the eigenmode can be coupled from the larger core to the smaller core effectively and also the radiation loss in the tapered area between the two connected optical waveguides can be effectively suppressed. The results show that the PhC-based microlens in the telescopic structure can serve as an optical element in the tapered waveguide with the advantages of low loss, high efficiency and compactness.     

折叠矩形槽波导行波管输入输出过渡波导

为了将槽波导端与标准矩形波导相连接,设计了一种适用于折叠槽波导结构行波管的输入输出过渡波导, 可将其视为槽宽渐变的双槽加载矩形波导。利用电磁仿真软件CST微波工作室对该结构进行仿真计算,讨论了各个结构参数对其性能的影响。对比了直线渐变、抛物线渐变和指数渐变3种槽宽渐变规律在W波段对其传输特性及损耗特性的影响。研究结果表明:指数渐变结构的驻波系数小于1.15的带宽比其他两种结构都要宽,且在90~99 GHz、驻波系数小于1.25时,该结构的整体长度也远小于另外两种结构,能够实现良好的过渡效果; 而直线渐变结构的损耗在90~97 GHz为最低。     

Design of hybrid optical waveguide with a 90° bend structure for high density photonics integrated circuits

A hybrid optical waveguide having a 90° sharp bend structure, composed of a dielectric straight waveguide, tapered dielectric strip waveguide, and microscale metal gap waveguide, is proposed and simulated to improve the efficiency of light coupling between dielectric and plasmonic waveguides. Our simulation result is a critical step for the hybrid integration of plasmonic components with conventional dielectric components.     

毫米波折叠波导行波管输入输出过渡波导设计

折叠波导以其宽频带、加工方便而成为一类重要的毫米波与太赫兹波段行波管慢波线。针对折叠波导慢波系统与标准波导的匹配过渡连接,通过等效电路分析与电磁计算软件模拟,设计出了直渐变波导、双曲圆弧渐变波导、切比雪夫阶梯渐变波导3种输入输出过渡波导结构,分析了各种过渡波导的优缺点及结构尺寸对性能的影响。计算表明,经过合理优化的设计,3种过渡结构均可以使反射系数在28~40 GHz波段小于0.05。     

Light Penetration through a Tapered Waveguide

An approximate pattern of calculation of the field behavior in waveguides of variable radius is proposed. It is based on the assumption of a slow dependence of the radius on the longitudinal coordinate and is reduced to the solution of an ordinary differential equation. The dependences of the waveguide transmission on the frequency of radiation, entrance and exit radii, as well as the rate of radius variation along the axis are studied. The analysis is carried out within the range of parameters typical for optical fibers used in optical near‐field microscopy. It is shown that the increase in the rate of radius variation with the longitudinal coordinate will cause the transmission coefficient of the tapered waveguide to increase.     

阵列波导光栅平坦化的研究

阵列波导光栅的平坦化在实际应用中有很重要的意义.本文系统地研究了阵列波导光栅的平坦化.在输入波导、输出波导、阵列波导输入端与输出端上分别引入了指数型锥形波导.通过改变锥形波导的形状和尺寸来实现平坦化的优化.本文首先从理论上论述了引入指数型锥形波导的输出光谱特性,给出了结构参量的关系表达式,阐明了输入波导处的锥形波导是影响输出光谱平坦化的主要因素,阵列波导和输出波导处的锥形波导对输出光谱的平坦化有一定的影响.其次采用数值模拟的方法模拟了输出光谱,优化了结构参量,总结出了指数型锥形波导对平坦化影响的趋势和规律.模拟结果显示,输出光谱1 dB带宽大于通道间隔的50%,插入损耗从5.2 dB减小到了4.0 dB,串扰小于-30 dB.最后,本文给出了实验结果,插入损耗减小了0.87 dB,串扰减小了3.67 dB,1 dB带宽增加0.1 nm,增加了54.7%.实验结果表明引入指数型锥形波导提高了阵列波导光栅器件的光谱性能.     

Loss measurement of plasmonic modes in planar metal-insulator-metal waveguides by an attenuated total reflection method

We report experimental excitation and characterization of surface plasmon modes in planar metal-insulator-metal (MIM) waveguides. Our approach is based on determining the width of the reflection angular spectrum in the attenuated total reflection (ATR) configuration. Owing to its transverse character, the ATR configuration provides a more straightforward and simpler way to determine the loss of plasmonic modes in MIM structures, compared to using tapered end couplers with multiple waveguide samples or scanning near-field optical microscopy. In this Letter, two waveguide structures with Au claddings and 50/200 nm SiO(2) cores are investigated. The propagation lengths measured at λ = 1.55 μm are 5.7 and 18 μm, respectively, in agreement with the theoretical predictions.     

Resonant cavity based antireflection structures for surface plasmon waveguides

The concept of antireflection coating in the theory of multilayer films is introduced to the two-dimensional metal–insulator–metal (MIM) structures to realize total transmission of optical energy at the waveguide discontinuities. The antireflection structure consists of a resonant cavity which is constructed by changing the insulator width of the waveguide. A numerical method is used to achieve the optimal design directly. A T-splitter with zero reflection is proposed, utilizing a cavity structure in the input waveguide. A transformer with enhanced transmission between different waveguides is presented for further validating the efficiency and generality of these cavity based antireflection structures. The simulation results show that such a structure can realize a perfect antireflection function.     

Optical path design and evaluation in Tm doped glass channel waveguide for S-band amplification

To achieve high-gain S-band waveguide amplifiers and promote the practicality of integrated signal amplification devices, bent waveguide structures based on Tm³⁺ doped germanate glass substrate have been designed. Using simulated-bend method, the optimal radius for the curved structure is offered to be 1.90 cm with a loss coefficient of 0.04 dB/cm, as the substrate size is minimally schemed. For the folded-spiral waveguide, the internal gain at 1482 nm is derived to be 13.01 dB, which is higher than the values of 8.21 and 4.22 dB in the U- and S-bend waveguides, respectively, and nearly three times higher than that of the straight one. Simulation results indicate that the optical path design is attractive in realizing the high gain of Tm³⁺ doped germanate glass channel waveguides for practical S-band amplification.     

High-index and low loss polyethylene terephatate optical waveguides

In this paper, the fabrication and characterization of a high-index low loss polyethylene terephatate (PET) optical waveguide is presented. Dip-coating technique has been used to fabricate the optical waveguide using PET. Various waveguide properties, such as mode effective refractive index, optical loss, dispersion and waveguide thickness as a function of temperature of waveguides and substrates have been reported. Optical losses of as low as 0.7 dB cm^–1 waveguides can be obtained if the formation temperature can be controlled between 30 and 35°C. In addition, we suggested that it is a suitable material for optoelectronic waveguide devices.     

Modal attenuation in metal-clad graded-index slab optical waveguides

The paper presents the influence of refractive profile shape on the distribution of modal attenuation in planar metal-clad optical waveguide structures. A calculation method has been presented in which we applied the Snyder–Love expression on modal attenuation coefficients and matrix method 4 × 4 for the analysis of planar waveguides. The obtained results are almost identical with the exact results for refractive profiles: the parabolic, exponential, Gaussian and linear ones. It has been demonstrated that the applied calculation method can be used in waveguides of any profiles of the refractive index.     

Protection of phosphate glass by BK7 glass film in ion-exchange processing

Surface corrosion of phosphate glass during ion-exchange processing for optical waveguide fabrication is studied, and a protection method by using BK7 glass film is proposed and demonstrated in this paper. The protection coating processing is described and optical microscope and atomic force microscope observations are presented. Characteristics of the planar waveguide were measured also to show the protection coating would not introduce influence to waveguide formation in the phosphate glass. Experimental results show that the method can serve an effective protection, and will improve the performance of rare-earth-doped active phosphate glass optical waveguides.     

Complex angle of incidence for planar optical waveguide analysis

We propose the concept of an incident angle in the form of complex number and analyse planar optical waveguides by applying the concept. The complex angle of incidence is concerned with the modelling of prism–gap–waveguide structures. It is shown that, when optical waveguides are analysed by means of the transfer matrix method, the proposed concept enables us to find solutions faster and more accurately than by using the method utilizing the leaky structure. This revised version was published online in November 2006 with corrections to the Cover Date.     

1×7锥形混合棒塑料光纤耦合器

利用光线追迹法进行数值仿真得到了锥形混合棒塑料光纤耦合器的分光比与其长度的关系.数值仿真表明:这种耦合器的分光比随锥形光波导长度增加,在0.143(即均分处)上下呈振荡型变化.选择一均分且分光比变化平缓的区域作为制作光波导的长度,通过拉伸粗的光波导棒成锥形且达到所需的长度,并采用精密的机械连接,制作出了1×7塑料光纤耦合器.测试数据表明:这种塑料光纤耦合器具有很高的均匀性.     

Perturbation analysis of finitely clad optical waveguides and couplers

In calculating the modal characteristics of a finitely clad optical waveguide, if the wavelength is much shorter than the core of the waveguide, then the cladding of the waveguide can be assumed to be infinite. On the other hand, if the wavelength is much longer than the core, the core can be ignored. These two approximations, although simple and accurate in the two respective extreme cases, do not provide adequate information in the transition region, where many tapered devices are operated. To overcome this a novel perturbation method is devised, in which these two approximations are combined in such a way that a result that is accurate in all cases can be obtained. Analytical expressions based on this method for describing the dispersion characteristics of finitely clad slab waveguides and circular step-index fibres are derived and compared with other conventional approximations. The combined use of this method and the coupled-mode theory for the analysis of optical couplers is also described.     

一种分析三维楔脊形光波导与光纤耦合的方法

和直波导的不同在于，三维楔脊形波导中的光场都是不稳定的.分别对波导-光纤耦合、光纤-波导耦合两种情况，将三维楔脊形波导等效为多段短的直波导，利用束传播法同时对三维楔脊形波导内不稳定的入射、反射场，及它们在传播方向上的偏导进行处理，再用自由空间辐射模法计算透射率和反射率.以SOI楔脊形波导和光纤的耦合为例，验证了该方法的可行性. 关键词： 自由空间辐射模法 束传播法 耦合 透射 楔脊形光波导 绝缘体上硅(SOI)     

Eigenmode calculation of dielectric waveguides near cutoff using the Rayleigh-Ritz method with rational basis functions

A scalar Rayleigh-Ritz method using rational functions as an orthonormal basis is presented. This formalism is especially useful for the calculation of the fundamental eigenmode of optical waveguides near cutoff. An application for the optimum design of the large-spot output waveguide in tapered spot-size transformers is described.     

Mode-order converter in a multimode waveguide

A novel device that converts the order of modes in an integrated-optic multimode waveguide was proposed and fabricated. The device consists of two mode splitters and tapered waveguides. Its operation at a wavelength of 1.55 microm was confirmed by observation of the output mode profiles. The cross talk measured by an indirect method was >10.2 dB. The excess loss with reference to a straight multimode waveguide was 1.5 dB, compared with the propagation loss of a straight multimode waveguide.     

InGaAsP/InP evanescent mode waveguide optical isolators and their application to InGaAsP/InP/Si hybrid evanescent optical isolators

We theoretically investigated InGaAsP/InP evanescent mode waveguide optical isolators and proposed their application to InGaAsP/InP/Si hybrid evanescent optical isolators. InGaAsP/InP evanescent optical isolators are composed of semiconductor optical amplifier (SOA) waveguides having InGaAsP multiple quantum well (MQW) active layer and upper InGaAsP waveguide layer with ferromagnetic layer. Optical isolation is obtained for evanescent optical mode in the InGaAsP waveguide layer. InGaAsP/InP/Si hybrid evanescent optical isolators are theoretically proposed based on the idea of InGaAsP/InP evanescent optical isolators. InGaAsP/InP/Si hybrid evanescent optical isolators are composed of ferromagnetic metal loaded silicon evanescent waveguides with wafer-bonded InGaAsP/InP optical gain material. The optical isolation and propagation loss are discussed with the structure of silicon evanescent waveguides, and optical isolation of 8.0 dB/mm was estimated. The concept of semiconductor evanescent mode optical isolators is feasible with InP based photonic integrated circuits and advanced silicon photonics.     

锥形脊结构半导体光放大器的有限元分析

为提高半导体光放大器与单模光纤耦合效率，建立了半导体放大器的锥形脊结构模型。在该模型下利用有限元数值模拟方法分析，计算了波导区折射率、锥尖宽度、条形波导尺寸、渐变折射率波导层对锥形脊结构模式扩展的影响。通过完善锥形脊结构参量的设计，获得了锥形脊结构半导体光放大器与单模光纤95％的耦合效率。