Scattering from a double-strip grating: rigorous equivalent networkformulation

An exact solution for the problem of transverse electric (TE) or transverse magnetic (TM) plane-wave scattering from a periodic, planar double-strip grating at a dielectric interface is described. The metal-strip grating is assumed to be perfectly conductive and infinite in length, with two different strips within a unit-cell. The formulation is based on a multimode equivalent network representation, and uses a rigorous solution for the relevant integral equation that extends the novel solution developed previously for the single-strip grating. Expressions for the elements of the multimode coupling matrices are given, together with a comparison of results for power transmitted through the grating, obtained by using the networks developed with the present method and a simple point-matching solution. Results are presented to illustrate the differences between single and double-strip gratings     

基于光盘光栅的表面等离子体共振传感器

采用CD光盘光栅作为表面等离子体共振(SPR)传感器的耦合元件,通过检测共振角的变化对液体浓度进行传感测量。结果表明CD光栅耦合型SPR传感器对葡萄糖溶液的灵敏度可达3%(质量分数),理论分辨率为1%(质量分数)。通过模拟不同光栅周期、光栅槽形和待测介质折射率对共振曲线的影响,发现缩短光栅周期、使用正弦槽形的光栅作为耦合基底,都可以进一步提高这种传感器的灵敏度。     

Diffraction of a plane wave by a thick strip grating

The diffraction of a plane electromagnetic wave by a thick strip grating is solved rigorously by the Wiener-Hopf technique. The solution contains an infinite number of unknowns, which are shown to satisfy a certain infinite set of equations. By applying the modified residue calculus technique, this set of equations is solved and the approximate solution is derived. Representative numerical examples are given and the transmission characteristics of the grating are discussed     

Dielectric grating induced by cavity standing wave as a newexplanation of origin of nonlinear gain in semiconductor diode lasers

The nonlinear gain due to induced index and gain grating caused by the cavity standing wave is analysed using coupled-mode equations. The grating coupling coefficient is calculated using the rate equation with an ambipolar diffusion term. The self-consistent solution of the coupling coefficient and the optical field indicate the existence of a nonlinear gain term with a surprisingly large magnitude. This gain nonlinearity is shown to contribute significantly to the modulation damping factor of semiconductor diode lasers     

Plane wave scattering and absorption by flat gratings of impedance strips

The problem of the plane wave scattering by a flat impedance-strip grating located in free space is considered. The formulation involves a set of generalized boundary conditions relating limiting values of the fields to effective electric and magnetic currents. We develop an accurate numerical solution to this problem using the dual series equations and the method of analytical regularization (MAR) based on the main part inversion. This guarantees a fast convergence and controlled accuracy of computations. Reflected, transmitted, and absorbed power fractions as a function of the frequency and the grating parameters are analyzed. Sharp resonances are revealed in the H-polarized scattering near to the grazing of the higher-order modes; these resonances are absent for the perfect electrically conducting strip grating. Low-frequency asymptotics for the reflectance and transmittance in the single-mode regime are presented and compared with numerical solutions.     

Analysis of diffraction of a plane wave and of the field of a point source by a multirow grating

A multirow grating of impedance or dielectric elements is considered. The 2D problem of scattering by such a grating is solved with the help of the modified null field method and the pattern equation method. A system of integral equations is derived and the reflection and transmission coefficients are determined as functions of various parameters of the problem. The excitation of the grating by a current filament is investigated. An asymptotic solution is obtained for the case when the periods of the grating rows and interrow distances are rather large. In the low-frequency approximation, a solution to the problem for a grating of circular cylinders is presented.     

Nearly back-dissolution-free LPE growth from Sn solutions over gratings for DFB lasers

1.55 mu m GaInAsP-InP DFB laser wafers based on p-type substrates are grown in a two-step LPE process. The grating is effectively preserved from thermal deformation and back-dissolution by using an InP-Sn solution at only 464 degrees C for overgrowth. Data for the solubility and critical supersaturation for the Sn-In-P system are given.<>     

Nanoimprinted Grating‐Embedded Perovskite Solar Cells with Improved Light Management

Organic–inorganic halide perovskite solar cells are one of the most efficient photovoltaic devices, and their power conversion efficiency (PCE) continues to grow rapidly due to the optimization of device architecture and functional materials. The involved optical design has an important impact on performance by affecting the light harvesting capability. Here, demonstrated is a simple and effective strategy to boost device performance from the perspective of light management. A diffraction grating pattern into the TiO₂ scaffold and antireflection layer is simultaneously introduced to construct a double grating design for perovskite solar cells. The grating pattern obtained using a commercial compact disk as the hard mold can modulate light transport by enhancing the light transmittance and, thus, light absorption in perovskite solar cells. The grating pattern‐induced light trapping can significantly increase the short‐circuit current density and PCE by 4.8% and 7.9%, respectively, on average. The present work develops a feasible method by tailoring the morphology‐related optical property to further improve the performance of perovskite solar cells. The double grating design can also generate structural color and can provide an alternative solution for fabricating colorful solar cells.     

Comparison of four-wave mixing techniques for studying orientational relaxation

Three four-wave mixing techniques for measuring orientational relaxation of molecules in solution using time-separated pulses are compared both theoretically and experimentally. This comparison is shown to be greatly facilitated by modifying the conventional third-order response functions to incorporate a portion due to the slowly varying reorientation of the transition dipoles. Ordinary Bragg diffraction of a probe field, in which a transient grating formed by excitation pulses of the same polarization is probed with parallel and perpendicular polarizations, has the advantage of yielding both the time dependence and the magnitude of the induced anisotropy. However, the accompanying acoustic grating interferes with the polarization-sensitive excited state concentration grating unless the probe wavelength falls within a very strong absorption band. The acoustic grating can be eliminated by using perpendicularly polarized pumping pulses (the "crossed grating" configuration). Polarization spectroscopy in which the generated field is collinear with the probe gives the same anisotropy decay kinetics as the crossed grating, but is more sensitive to interference from background birefringence. These three methods, together with time-resolved fluorescence polarization, have been used to examine rotational dynamics in the ground and excited states of rhodamine 6G, 9-aminoacridine, and benzophenone.     

Metallic photonic band-gap materials (MPBG) as angular selective reflector or radome: Application to antenna grating lobe reduction

This paper presents numerical and experimental results on metallic photonic band-gap materials (mpbg), composed of metallic wires and used as antenna reflector or radome. Attention is focused on mpbg angular response. In fact, with specified angular characteristics, MPBG materials can be interesting in order to reduce antenna grating lobes. One possible grating lobe reduction solution is based on the use of mpbg as angular filter. The radiation characteristics obtained with this solution are then improved by using a mpbg reflector to enhance the gain. Regarded to a classical metallic reflector, 20 dB and 12 dB gain improvement in the theoretical and in the experimental cases are respectively obtained, when the reflector is composed of more than two layers. The scanning performances are also presented. We show that angular filtering characteristics can be improved either by changing the array interface or by changing the mpbg lattice. In order to select the structure with the best angular behavior, an experimental and numerical characterization method is also presented.     

Plane wave diffraction by dielectric gratings finite-differenceformulation

Plane-wave diffraction by dielectric gratings of arbitrary profile (groove shape and size) is analyzed using an analytical-numerical technique. The solution method is simple, general, and numerically efficient. It involves expansion in a Fourier series of the periodic permittivity function in the inhomogeneous grating region and application of the finite differences to solve numerically the inhomogeneous vector wave equation in the region. The obliquely incident radiation is of linear polarization with either of its fields parallel to the rulings. Numerical results are presented for several gratings to demonstrate the convergence, accuracy, and reliability of the method. These also show the effect of the grating profile on its diffraction characteristics     

大间距相控阵天线栅瓣抑制技术研究

适当增加阵元间距有利于降低相控阵天线的成本,但是会带来不期望的栅瓣。文中针对5×5大间距相控阵天线栅瓣抑制技术进行了研究。改进了传统遗传算法,增加了种群的多样性。首先采用改进的遗传算法优化得到局部解,然后运用模式搜索算法优化得到最优解,并对阵列口径及阵元最小间距进行约束。其次,结合实际的工程应用,对阵元位置进行约束,优化出便于生产应用的最佳阵列排布。仿真结果表明,优化后的阵列,最大栅瓣电平得以有效抑制,满足工程应用的要求。     

An Analytical Study of the Echelette Grating with Application to Open Resonators

Reflection coefficients of various order diffracted waves are calculated for an echelette grating which finds important use as a component of suboptical resonators. The classical optics approach has been found to be inadequate for deriving reliable results for this configuration. Accurate solution of the boundary value problem associated with the echelette grating is derived by a new method. The results are useful for the design of suboptical resonators which use the echelette grating as a coupling mirror to achieve a single wavelength operation.     

光纤布拉格光栅的折射率传感研究

采用光纤布拉格光栅制备折射率传感器,研究光纤光栅的折射率传感灵敏度与其包层直径之间的关系。理论分析可得,光栅包层直径越小,Bragg波长的偏移量随环境折射率变化的影响越大,这样就能使实验中光栅所反射的LD光功率变化(传感灵敏度)越明显。利用氢氟酸溶液腐蚀光栅包层的方法,得到不同包层直径的光纤Bragg光栅折射率传感器。实验指出,包层直径减小时,光栅可传感的折射率范围会缩小,而其折射率的传感灵敏度却会增大,如包层直径为8.9 μm时,折射率的检测范围为1.3872~1.4730,其最大灵敏度值达到了224.0320 dBm/RIU。     

Multimode network description of a planar periodic metal-stripgrating at a dielectric interface. III. Rigorous solution

For pt.II see (ibid., vol.37, no.3, p.542-52, 1989). In pt.I the solution to the scattering problem posed by a plane-wave incident at an angle on a plane, periodic, metal-strip grating at a dielectric interface was formulated in terms of novel multimode equivalent network representations. The analytical phrasing followed in pt.I led to two Fredholm integral equations of the first kind with singular kernels. Here, the authors derive a rigorous analytical solution for one of the two integral equations derived in pt.I (see ibid., vol.37, no.3, p.534-41, 1989). Closed-form, rigorous, analytical expressions for the elements involved in the equivalent network representations derived are obtained. Numerical results obtained by using these rigorous equivalent network representations are presented for TE-mode and TM-mode incidence for the case in which the same dielectric is present on both sides of the grating as well as for the case in which the two dielectric media are different     

基于M-Z滤波和LPFG的液体折射率传感特性研究

利用飞秒激光逐线刻写方式在HI1060光纤中制作谐振波长为1548.5 nm,谐振强度为7 dB,栅区长度为3.2 mm的长周期光纤光栅。通过纤芯错位熔接方法在HI1060光纤中制作马赫曾德-干涉仪(MZI),对LPFG的透射光谱进行滤波优化。设计使用不同折射率的酒精溶液、氯化钠溶液和蔗糖溶液分别对基于MZI滤波的LPFG的折射率特性进行了测试和研究。实验中随着三种溶液折射率增加,LPFG的谐振波长发生红移,该LPFG在酒精溶液、氯化钠溶液和蔗糖溶液中的折射率灵敏度分别为301.78 nm/RIU、138.80 nm/RIU和132.67 nm/RIU。     

Two analytic frequency dependent current density approximations forTE scattering from a conducting strip grating

Two analytic frequency dependent current density approximations are presented for TE mode incidence on a conducting strip grating. The current densities are assumed to have both a spatial and a frequency dependence. The first is based upon using the classic edge mode approximation. The second is derived from applying conformal mapping methods and invoking Babinet's theorem. The solutions are compared to the moment method solution. The resultant reflection coefficients are found to be good approximations at all frequencies for narrow strips and at moderately low frequencies for arbitrary strip widths. Although the solutions are not as accurate as a moment method solution at all frequencies, they are computationally faster and provide analytic expressions that demonstrate the relationship between grating dimensions and performance     

Equivalent Network of a Multimode Planar Grating

In this paper a plane-parallel, perfectly conducting, zero-thickness grating is analyzed by an integral equation procedure. This procedure generalizes the well-known single-mode method by taking multimode propagation into account. The grating is located in free space; viewed transversely to the plane of the structure, it is a zero-thickness shunt discontinuity in the free-space waveguide. The solution of the integral equation is obtained for E (or TM) mode excitation, when the spacing between conducting strips is small compared to a period, and it is interpreted in network terms. Finally, an explicit equivalent network is presented for the grating under multimode conditions.     

飞秒直写长周期光纤光栅及其液体折射率特性研究

利用800 nm飞秒激光在未经载氢处理的HI1060光纤中制作了长周期光纤光栅(LPFG).采用逐线刻写方式实现了谐振波长为1 548.4 nm的LPFG,谐振强度为12 dB,栅区长度小于4 mm.使用不同折射率的氯化钠溶液、蔗糖溶液和酒精溶液分别对LPFG的折射率特性进行了测试和研究.实验中随着三种溶液折射率增加,LPFG的谐振波长发生红移,该LPFG在氯化钠溶液、蔗糖溶液和酒精溶液中的折射率灵敏度分别为175.34、175.31和331.89 nm/RIU.实验结果表明,这种基于飞秒激光制作的LPFG对液体折射率变化有较高的灵敏度,可用于液体折射率传感测试.     

A novel frequency-scanned reflector antenna

Theoretical and experimental results are presented for a frequency-scanned antenna composed of a line source and a frequency-scanned reflection grating that is shaped to a cylindrical reflector. The principle considering is to select the grating geometry such that the first higher-order diffracted wave propagates and serves as the frequency-scanned beam. An analytical solution for the cylindrical reflector geometry that gives a line focus is given. The grating structure considered consists of an array of dipoles placed over a ground plane. The design of the dipole grating for optimum blazing (i.e., for optimal power conversion from the incident wave to the diffracted wave) is discussed. For the theoretical analysis Floquet's theorem and the method of moments are used