Estimation of the background noise level with a horizontal array against spatially uncorrelated and structural interferences

A method of estimating the isotropic sea noise level with a horizontal array in the presence of uncorrelated interference and interference with a complex spatial structure is proposed and experimentally tested. The algorithm is based on the approximation of the Capon spatial spectrum of the received signal using a model Capon spectrum for the sum of isotropic noise and uncorrelated interference. A numerical simulation is carried out to study the dependence of the accuracy of the proposed method on the interference intensity, the distance from the array to the sources of structural interference, and the number of structural interference sources. It is shown that the use of the Capon spectrum provides a strong suppression of an intense structural interference source positioned near the array. The efficiency of the proposed method is confirmed experimentally.     

Numerical correction of coherence gate in full-field swept-source interference microscopy

A big problem in low-coherence interference microscopy is the degradation of the coherence signal caused by shift of the angular and temporal spectrum gates. It limits the depth of field in confocal optical coherence microscopy and degrades images of sample inner structure in most interference microscopy techniques. To overcome this problem we propose numerical correction of the coherence gate in application to full-field swept-source interference microscopy. The proposed technique allows three-dimensional sample imaging without mechanical movement of the microscope components and is also capable of determining separately the geometrical thickness and the refractive index of the sample layers, when the sample contains a transversal pattern. The applicability of the proposed technique is verified with numerical simulation.     

Switchable multi-wavelength fiber laser based on modal interference

A comb fiber filter based on modal interference is proposed and demonstrated in this paper. Here two cascaded uptapers are used to excite the cladding mode, and a core-offset jointing point is used to act as an interference component.Experimental results show that this kind of structure possesses a comb filter property in a range of the C-band. The measured extinction ratio is better than 12 dB with an insertion loss of about 11 dB. A switchable multi-wavelength erbium-doped fiber laser based on this novel comb filter is demonstrated. By adjusting the polarization controller, the output laser can be switched among single-, dual-, and three-wavelengths with a side mode suppression ratio of better than 45 dB.     

All-Optical Multi-Mode Interference Switch Using Non-Linear Directional Coupler as a Passive Phase Shifter

Abstract

This article presents an all-optical multi-mode interference switch in which the non-linear directional coupler is utilized to realize a passive phase shifter. The proposed structure can be used either as a 1 × 2 or a 2 × 2 switch, with two inputs applied simultaneously in the latter case. The operation of the device is mainly based on the phase difference of the inputs of the multi-mode interference section. The beam propagation method is used for the design and simulation of the structure. The simulation results approve the low sensitivity on wavelength and fabrication tolerances. The crosstalk of the structure is equal to ?31.6 dB.     

一种浅海声矢量场干涉结构提取方法

针对浅海Pekeris环境下声矢量场的干涉特征,基于简正波理论分析了在频率-距离域上声强谱、质点振速自谱、复声强谱和波阻抗谱干涉结构的形成机理,探讨了对干涉结构的波导不变量表征,并进行了宽带辐射声场干涉结构的海上测量试验。仿真研究和海试数据分析均表明,在频率-距离域上,上述4类物理量都会呈现稳定的、可用波导不变量β表征的干涉结构,而复声强独有的纯干涉分量更能体现波导的干涉效应。最后引入多尺度线性滤波器对海试实测LOFAR图进行处理,处理结果表明该滤波器可有效地增强干涉特征,更利于检测和提取干涉图案中的条纹信息。      

Steering light into logic patterns with two-dimensional cascaded multimode waveguide

Steering light into logic patterns with two-dimensional cascaded multimode waveguide is demonstrated. By employing the imaging properties of 2D multimode interference (MMI) and partial phase modulation method, the design ideas and the implementing methods of the 2^(2×2) bits type spatial logic steering are discussed; therefore the structure of logical pattern is proposed. Numerical simulation is carried out to verify the design in detail by using the beam propagation method. It is expected to realize logic coders by using the integrated optical methods and exploit their potential applications in the field of optical logic.     

Center positioning of circular interference fringe patterns for fine measurement

A kind of center positioning method of the circular interference fringe patterns was reported in this paper. The whole process was programmed by Matlab for processing the gray-scale images without denoising preprocessing and binary format transformation. The gradient vector field (GVF) based on a complex-valued cost function was used to extract the skeletons of circular interference fringe patterns, and the coordinate of the center was calculated by statistical method in which partial skeletons were selected to calculate the center according to the space topological relationship. Finally, subpixel accuracy and perfect anti-noise ability of the proposed method are demonstrated by the simulation and experimental fringe patterns, respectively. Thus the technique can be used in precision measurement and fine interferometry.     

Tunable comb-filter using thermally expanded core fiber and ytterbium doped fiber and its application to multi-wavelength fiber laser

We constructed a novel tunable comb-filter by splicing a commercial Yb³⁺ doped fiber (YDF) between 2 thermally expanded core (TEC) fibers at their core-expanded ends. This structure forms an all-fiber Mach-Zehnder interferometer due to the interference between the core mode and the cladding modes of the YDF. The interference fringes shifts to the longer wavelength region as the YDF is pumped by a 980 nm laser diode (LD). We then demonstrated application of this newly proposed tunable comb-filter to a SOA-based ring laser and obtained multi-wavelength lasing. The lasing wavelengths can be also tuned by pumping the YDF in the proposed comb-filter.     

Theoretical study of micro-optical structure fabrication based on sample rotation and two-laser-beam interference

A method for fabricating a micro-optical structure based on sample rotation and two-laser-beam interference is proposed. The rotation process is analyzed using the coordinate transformation in matrix presentation and the theoretical expressions of the optical field distributions corresponding to different sample rotations. By rotating the samples and changing the laser wavelength, various special micro-optical structures can be obtained, such as equally spaced concentric rings and irregular trapezoidal lattices; these structures are demonstrated by simulating the corresponding optical field distributions. The proposed approach may be developed into a low-cost laser interference lithography technology for the fabrication of various micro-optical structures.     

Comparative analysis of long-haul system based on SSB modulation utilising dual parallel Mach–Zehnder modulators

In this paper, we have proposed a long-haul optical transmission system, based on a single sideband (SSB) modulation scheme. Analytical and simulation models have been developed, optimised and demonstrated for the proposed SSB system configurations. The SSB modulation scheme was proposed to overcome dispersion in the fibre. We have shown that the related link losses can be minimized by increasing the quality of the optical signal at the modulation. We have optimised the radio over fibre configuration scheme based on dual parallel dual drive Mach–Zehnder Modulator, thereby increasing transmission length of the fibre. With the proposed SSB, by suppressing some of the harmonics and cancelling one of the sidebands, we have halved the RF power fading and interference. The developed analytical (theoretical/mathematical) model agrees very well with the simulation results using two (both) different commercial simulation tools. The optical signal is boosted while minimizing the number of repeaters. We report a SSB configuration, compensation and amplification with individual spans of 150 km, by extending the length of the link up to 3250 km. The proposed system configuration exhibits high performance with less complexity and lower cost.     

Interference structure of low-frequency reverberation signals in shallow water

Using numerical simulation, an analysis was conducted of the interference structure of a bottomscattered sound field generated by a wideband point source in shallow water under winter and summer conditions. The scattered signals were received from the place where the source was located and were subjected to Fourier transform with a sliding window. The paper demonstrates the possibility of estimating the waveguide invariant for backscattered signals when processing the sound intensity distributions in wide frequency and distance ranges up to the scattering area. A technique is proposed for reconstructing the twodimensional field of internal waves using variations of the interference pattern of reverberation signals. The influence of wind surface waves on the degree of interference band contrast is illustrated.     

Broadly tunable one-way terahertz plasmonic waveguide based on nonreciprocal surface magneto plasmons

One-way-propagating broadly tunable terahertz plasmonic waveguide at a subwavelength scale is proposed based on a metal-dielectric-semiconductor structure. Unlike other one-way plasmonic devices that are based on interference effects of surface plasmons, the proposed one-way device is based on nonreciprocal surface magneto plasmons under an external magnetic field. Theoretical and simulation results demonstrate that the one-way-propagating frequency band can be broadly tuned by the external magnetic fields. The proposed concept can be used to realize various high performance tunable plasmonic devices such as isolators, switches and splitters for ultracompact integrated plasmonic circuits.     

运动目标辐射声场干涉结构映射域特征研究

浅海低频声场呈稳定而显著的干涉特征,并蕴含声源状态和波导特性等信息.本文研究运动目标辐射声场干涉结构的简化映射以及映射域能量分布对目标运动状态的指示特征.理论分析了声压场和矢量声场空(时)频干涉谱图的二维傅里叶变换映射特征,推导了匀速运动目标时频干涉谱图映射域能量脊斜率与波导不变量以及距变率、航向角的关系式,证明了映射域脊斜率符号、脊斜率绝对值变化等对目标来袭或远离以及目标威胁程度的指示,并进行了数值仿真和海上试验研究.实测结果与理论、仿真分析有较好的一致性.研究结果表明:二维傅里叶变换可将声压场和矢量场时(空)频谱图干涉结构简化,匀速运动目标辐射声场干涉结构映射域的能谱脊斜率、距变率、航向角与波导不变量有解析关系式,声强谱、动能密度谱、声强流谱等声场干涉结构经映射后更为一致,映射域脊能简明的指示目标运动状态和威胁程度.     

Independently tunable dual resonant dip refractive index sensor based on metal—insulator—metal waveguide with Q-shaped resonant cavity

A plasmonic resonator system consisting of a metal—insulator—metal waveguide and a Q-shaped resonant cavity is proposed in this paper. The transmission properties of surface plasmon polaritons in this structure are investigated by using the finite difference in time domain (FDTD) method, and the simulation results contain two resonant dips. The physical mechanism is studied by the multimode interference coupled mode theory (MICMT), and the theoretical results are in highly consistent with the simulation results. Furthermore, the parameters of the Q-shaped cavity can be controlled to adjust the two dips, respectively. The refractive index sensor proposed in this paper, with a sensitivity of 1578 nm/RIU and figure of merit (FOM) of 175, performs better than most of the similar structures. Therefore, the results of the study are instructive for the design and application of high sensitivity nanoscale refractive index sensors.     

Interference grating structures in photonic crystal circuits

Photonic Crystal Circuits have been widely investigated for various optoelectronics device applications. And the gratings in photonics devices are indispensable tool, which is very common for light manipulation. In this study, an interference grating structure formed by Gaussian beam interferences in photonic crystal waveguide such that having Kerr type nonlinearity is proposed and its transmission characteristics are investigated. Finite-Difference Time-Domain method is used to analyze the device characteristics. According to simulation results, the interference grating has shown special transmission characteristics that can bring tunability for photonic crystal devices. This can be an effective method for controlling optical signals in the photonic crystal for all optical switching/routing applications as a part of add/drop multiplexing.     

深海近距离声场频率-距离干涉结构反演海底声学参数

地声反演一般存在多解性的问题,不易有效分离沉积层声速与厚度,本文提出了一种当声源与接收水听器分别位于海面与海底附近时匹配声场干涉周期反演深海沉积层声速与厚度的方法。该声场周期结构由直达波与海底沉积层反射回波干涉形成,其干涉周期受沉积层声速和厚度影响,通过匹配实测与理论计算的声场干涉周期可有效反演沉积层声速和厚度。开展了深海实验验证,利用水听器接收到的近距离船舶噪声的声场干涉条纹,反演得到的实验海域海底沉积层声速与该海域海底表层采样平均值差小于20 m/s。数值仿真与实验结果表明该方法可以有效反演沉积层的声速与厚度。      

同步辐射束流尺寸测量干涉仪的设计与仿真

基于同步光的干涉法,是一种非拦截高精度的束流截面测量手段。相比传统成像法,干涉法可以测量更小的束团尺寸、分辨率更好,较短测量波长下有望获得亚μm级的分辨率,因此在同步辐射光源中得到广泛应用。对合肥光源HLS II的原有同步光干涉装置,提出了将原有的干涉光路中第一面聚焦透镜换成RC结构聚焦反射镜,第二面单透镜换成双胶合透镜,以达到在不改变光路光轴情况下减小色散和几何像差,从而提高光路成像质量的目的。采用几何光路设计方法对成像质量进行评价,并进行物理光学仿真计算,得到测量系统的干涉条纹。仿真结果表明:光学系统成像的艾里斑半径减小约35%,点列图的均方根半径减小了约99%,波前差也减小了约75%,调制传递函数（MTF）的截止频率提高了约65%,采用聚焦反射镜代替原有的聚焦透镜可大幅提升光路成像质量。     

高灵敏度光纤应变传感器

提出一种基于锥形光纤和光纤F-P腔组合结构的光纤应变传感器。该传感器包含单模光纤拉锥形成的锥区和石英毛细管构建的F-P腔2个应变敏感区域。理论分析了光波在该传感器中的传播过程,获得了该传感器的光强传输函数。由于锥形光纤中激发出的包层高阶模参与干涉,导致传感器干涉光谱具有调制特性。实验获得了该传感器的干涉光谱,通过分析谐振波长偏移或消光比变化对应变实现独立测量,在0~500 με的测量范围内,该传感器的应变灵敏度为14.6 pm/με。利用锥形光纤引发的模式干涉和F-P腔的双光束干涉效应共同作用形成受调制的干涉谱型进行应变传感,应变灵敏度高,同时具备2种独立的应变检测手段(谐振波长和消光比检测)。     

采用双方波信号和B-样条小波解调弱反射光纤布拉格光栅

提出了一种采用双方波信号和B-样条小波解调弱反射光纤布拉格光栅(WFBG)的方法,并进行了实验验证.单个方波周期设置为相邻WFBG间光纤中激光往返传输的时间,对单个方波进行猝发操作形成双方波,则前WFBG反射的后方波与后WFBG反射的前方波重叠干涉.采用B-样条小波变换降低干涉信号的噪声,利用Hilbert变换对干涉信号进行π/2相移,对原干涉信号和相移后干涉信号比值进行反正切运算,得到干涉信号的相位信息.将间隔为50m的5-WFBG阵列置于木地板上,分别接收不同振幅和频率的正弦声波,采用上述方法解调的干涉相位信号能较好地反映声波信息.该解调方法解调光路简单,数据处理简单.     

Information hiding based on the optical interference principle

An information hiding method based on the optical interference principle is proposed. In this method, a secret image can be obtained by two light beams' interference. One of the beams is modulated by our assigned host image; and the other is modulated by a noise-like complex distribution, which is regard as the encrypted image. The transmission of the encrypted image can be implemented by hiding it in the host image to prevent the communication from being perceived by unauthorized person. In addition, this method can also realize simultaneous encryption and hiding for two images. A series of numerical simulation results are presented to verify the feasibility of our proposed method.