Theoretical model of a planar waveguide refractive index sensor assisted by a corrugated long period metal grating

A theoretical model of a novel planar integrated refractive index sensor based on surface plasmon-polariton (SPP) excitation with a corrugated metal long period grating (LPG) is presented and comprehensively investigated. The main principle of the operation this device is based on co-directional energy transfer by means of a corrugated metal LPG between a p-polarized guided mode propagating in a waveguide layer and the SPP propagating a metal layer separated from the waveguide layer by a buffer. The corrugated LPG is engraved in the metal layer in contact with the sensed medium. The power transmitted through the LPG in the guided mode serves as an input signal for an interrogation unit. This device is free from any moving parts and can be simply integrated into any planar waveguide system. Our sensor simulations are based on the local-normal-mode transfer matrix method and performed in telecom wavelength range.     

折射率平面波导光栅耦合特性分析

利用波导系统等效网络分析法,分析了折射率平面波导光栅的耦合特性.计算了影响光栅输入输出耦合的重要参数——损耗系数α,并通过数值计算分析了平面波导光栅各参数对损耗系数的影响.     

Influence of parameters on light propagation dynamics in optically induced planar waveguide arrays

The diffraction and refraction of light beam in optical periodic structures can be determined by the photonic band-gap structures of spatial frequency. In this paper, by employing the equation governing the nonlinear light propagations in photorefractive crystals, we study the photonic band-gap structures, Bloch modes, and light transmission properties of optically induced planar waveguide arrays. The relationship between the photonic band-gap structures and the light diffraction characteristics is discussed in detail. Then the influence of the parameters of planar waveguide arrays on the band-gaps structures, Bloch modes, and linear light transmissions is analyzed. It is revealed that the linear light transmission properties of waveguide arrays are tightly related to the diffraction relationships determined by band-gap structures. And the Bloch modes corresponding to different transmission bands can be excited by different excitation schemes. Both the increases of the intensity and the period of the array writing beam will lead to the broadening of the forbidden gaps and the concentration of the energy of the Bloch modes to the high-index regions. Furthermore, the broadening of the forbidden gaps will lead to separation and transition between the Bloch modes of neighboring bands around the Bragg angle. Additionally, with the increase of the intensity of the array writing beams, the influences from light intensity will tend to be steady due to the saturation of the photorefractive effect. Supported by the Youth for Northwestern Polytechnical University (NPU) Teachers Scientific and Technological Innovation Foundation, the NPU Foundation for Fundamental Research, and the Doctorate Foundation of NPU (Grant No. CX200514)     

Large refractive index modulation tilted holographic planar structured grating based on dichromated gelatin

A new type grating, tilted holographic planar structured dichromated gelatin (DCG) grating with large refractive index modulation, is presented in this paper. The grating is inscribed by superposing two coherent laser beams from the front and the thin side surfaces, respectively. The relationship between the tilted angle and the three types of coupling modes is analyzed when coupling with fiber. Besides, the couple mode theory and experimental investigation are applied on major characteristics analysis of the radiation mode DCG grating of tilted angle 45°. Due to the tilted planar structured grating has large refractive index modulation, it not only offers convenience for connecting (or integrating) with other fiber or waveguide-based devices, but also provide some special characteristics such as low wavelength selectivity (which is required for some applications such as multiple spectrum biomedical imaging). Therefore, this unique grating device has a wide application in optical fields such as for the waveguide-based, parallel, multiple spectrum imaging system.     

Collimation and focusing properties of a planar waveguide with hyperbolic secant refractive index profile: light deflector and collimator design

Light transmission through an inhomogeneous waveguide whose refractive index profile can be represented by one-dimensional hyperbolic secant function is analysed from a geometrical optics point of view. Focusing and collimation conditions are obtained and the design of a light deflector is proposed. The possibility of obtaining almost perfect collimation for an off-axis point source by polishing the exit face of the waveguide is considered.     

Sensor properties of planar waveguide structures with grating couplers

The paper presents the results of theoretical analysis as well as the results of experimental research involving planar sensor structures with input grating couplers of the period Λ = 800 nm. In the theoretical part of the paper we discussed the influence of the parameters of a sensor structure on it sensitivities. The experimental part of the work presents the results of experimental research involving the influence of refractive index of the cover on the coupling characteristics of sensor structures with grating couplers. The full widths at half maximum (FWHM) were from 0.023° to 0.029°. For the investigated structures we estimated detection thresholds for the changes of refractive index of the cover and the changes of sensitive film thickness. It has been demonstrated that by the application of the elaborated structures we can detect minimal changes of the refractive index (Δn_c)_min = 2.1×10⁻⁶ when the refractive index of the cover n_c = 1.333 and (Δn_c)_min = 1.0×10⁻⁶ when n_c = 1515. For sensitive films of the thickness w < 100 nm, by using the elaborated structures, we can detect mean changes of the thickness along the values lower than 10⁻³ nm.     

A novel plasmonic refractive index sensor based on gold/silicon complementary grating structure

A novel complementary grating structure is proposed for plasmonic refractive index sensing due to its strong resonance at near-infrared wavelength.The reflection spectra and the electric field distributions are obtained via the finite-difference time-domain method.Numerical simulation results show that multiple surface plasmon resonance modes can be excited in this novel structure.Subsequently,one of the resonance modes shows appreciable potential in refractive index sensing due to its wide range of action with the environment of the analyte.After optimizing the grating geometric variables of the structure,the designed structure shows the stable sensing performance with a high refractive index sensitivity of 1642 nm per refractive index unit(nm/RIU)and the figure of merit of 409 RIU^-1.The promising simulation results indicate that such a sensor has a broad application prospect in biochemistry.     

Liquid refractive index sensor based on slow light in slotted photonic crystal waveguide

A high sensitive and compact refractive index sensor based on slotted photonic crystal waveguide (S-PhCW) is demonstrated. This design is worked on a Mach–Zehnder interferometer (MZI) configuration with S-PhCW as the measuring arm, which can be used to detect any changes in refractive index that correspond to different concentration of the measuring liquid. Combining the slow light enhancement in photonic crystal waveguide (PhCW) with the advantage of excellent optical confinement in slot waveguide, the sensitivity of this simple scheme can reach to 2.3 × 10⁹ nm/RIU with the active region of only 1 mm long.     

变波长逆WKB方法确定单模渐变平面光波导的折射率轮廓

将渐变波导模式方程(WKB积分方程)化为分段积分,以波导某一模式在不同波长下的转折点为分段点,当波长相差很小时,相应的转折点相差也很小,可在各个分段积分中作折线近似,从而从理论上推出确定波导轮廓数据的递推式.以所得轮廓必须满足光滑条件为判据,最后定出波导的轮廓.该方法尤其适用于单模渐变波导,而且无需事先假设待定轮廓的函数形式.本文对双曲止割和抛物线轮廓的理想波导进行了计算机模拟,结果证明该方法的精度达到10~(-3)甚至于更高.而且理论上具有分割愈密,精度愈高的优点.     

Dynamic analysis of erbium-doped optically pumped waveguide lasers using a time-domain travelling wave model

A simple and straightforward dynamic model of Er3+-doped optically pumped waveguide lasers using a time-domain algorithm is developed. The theoretical model is based on (a) time-dependent rate equations of a quasi-two-level system for the population densities and (b) time-dependent travelling wave equations for the pump and signal power which are solved simultaneously in the time domain. In addition, the longitudinal distribution of spontaneous emission noise is also taken into consideration. Using the fast Fourier transform, the spectrum profile of the time-domain signal power is calculated. In this paper, the transient responses of population densities, pump and signal power are investigated for Er3+-doped Ti:LiNbO3 waveguide lasers. To demonstrate the sophistication of this time-domain model, the influence of the cross-coupled effect of the travelling waves due to the optical feedback of an etched grating is also studied. Dynamic response of the waveguide laser with and without a grating are compared. This revised version was published online in June 2006 with corrections to the Cover Date.     

Slow light in a dielectric slab waveguide with a negative refractive index photonic crystal substrate

Slow light effect in a dielectric slab waveguide with a negative refractive index photonic crystal substrate is numerically investigated. The guided modes are confined in the core layer by total internal reflection. Dispersion and wave propagating properties are explored in details. The result demonstrates that light speed can approach zero at two adjacent frequencies, which is also verified by finite-difference time-domain (FDTD) simulation. Calculated Q-factors both exceed several thousand. The structure has the potential to be used as high-Q open cavity for dual-wavelength lasers or traveling wave amplifiers.     

High-frequency modulation of light in diffraction by the Bragg grating with a refractive index traveling wave

The diffraction by a Bragg grating with high-frequency modulation of the refractive index of the medium is analyzed theoretically. The effect of the grating parameters and parameters of an rf signal on the efficiency of modulation of optical signals at frequencies above 10 GHz is investigated.     

内开槽矩形波导栅慢波电路高频特性的数值分析

介绍了用德国CST公司提供的MWS 4.2软件模拟计算色散特性和耦合阻抗的理论方法,对内开槽矩形波导栅结构的高频特性进行了数值模拟。结果表明：在传统矩形波导栅结构的基础上,开槽后的矩形波导栅结构比开槽前的相速更低,用作行波管的慢波结构时,可降低工作电压,工艺上容易实现,而且耦合阻抗均在45 W以上,满足毫米波行波管的要求;在内开槽宽度一定的前提下,槽深、栅间距和栅周期的增大对降低系统相速有利。     

内开槽矩形波导栅慢波电路高频特性的数值分析

 介绍了用德国CST公司提供的MWS 4.2软件模拟计算色散特性和耦合阻抗的理论方法,对内开槽矩形波导栅结构的高频特性进行了数值模拟。结果表明：在传统矩形波导栅结构的基础上,开槽后的矩形波导栅结构比开槽前的相速更低,用作行波管的慢波结构时,可降低工作电压,工艺上容易实现,而且耦合阻抗均在45 W以上,满足毫米波行波管的要求;在内开槽宽度一定的前提下,槽深、栅间距和栅周期的增大对降低系统相速有利。     

Study of refractive index and thickness of TiO2／ormosil planar waveguide

Hybrid titania/ormosil waveguide films have been prepared by sol-gel method at low thermal treatment temperature of 150℃. The influence of processing parameters including the molar ratios of Ti(OBu)_{4}/glycidoxypropyltrimethoxysilane (GLYMO) and H_{2}O/Ti(OBu)_{4} (expressed as R), especially aging of sol, on the refractive index and thickness of film was investigated. The optical properties of films were measured with Scanning Electron Microscope and m-line spectroscopy. The results indicate that the film thickness increases with the aging time of sol, but the variation of refractive index as a function of aging time of sol depends on the relative ratio of GLYMO to Ti-alkoxide. The relation between film thickness and corresponding sol viscosity is linear as the volume of GLYMO is 80% within the range of measured data.     

等离子体覆盖立方散射体目标雷达散射截面的时域有限差分法分析

采用分段线性电流密度递归卷积时域有限差分（PLJERC-FDTD）算法计算了均匀非磁化等离子体覆盖三维立方体目标的散射特性.分析了等离子体厚度、密度和碰撞频率对雷达散射截面（RCS）的影响.计算结果表明：等离子体包层能有效地减小雷达目标的RCS，当等离子体频率比入射电磁波频率小得多时，主要靠增大等离子体的厚度使立方散射体目标的RCS值减小，增大等离子体碰撞频率对立方散射体目标的RCS值影响不大；当等离子体频率约为入射电磁波频率的一半时，增大等离子体厚度和碰撞频率都对立方散射体目标的RCS值减小有影响；当等 关键词： FDTD算法 电磁波 等离子体隐身 雷达散射截面     

Plasmonic gold nanoshell induced spectral effects and refractive index sensing properties of excessively tilted fiber grating

We demonstrate a fiber refractive index(RI) sensor based on an excessively tilted fiber grating(ExTFG)immobilized by large-size plasmonic gold nanoshells(GNSs). The GNSs are covalently linked on ExTFG surface.Experimental results demonstrate that both the intensity of the transverse magnetic(TM) and transverse electric(TE) modes of ExTFG are significantly modulated by the localized surface plasmon resonance(LSPR) of GNSs due to the wide-range absorption band. The wavelength RI sensitivities of the TM and TE modes in the low RI range of 1.333–1.379 are improved by ～25% and ～14% after GNSs immobilization, respectively, and the intensity RI sensitivities are ～599%/RIU and ～486%/RIU, respectively.     

由光栅常数测量液体及固体的折射率

介绍由透射光栅分出来的一级衍射光用于测量固体或液体折射率的方法,该法只要测出一级衍射光透过特测介质的后的位置读数,由已知的光栅常数就可计算待测介质的折射率。