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.     

Double-distance propagation of Gaussian beams passing through a tilted cat-eye optical lens in a turbulent atmosphere

By using the extended Huygens-Fresnel diffraction integral and the method of expanding the aperture function into a finite sum of complex Gaussian functions, an approximate analytical formula of the double-distance propagation for Gaussian beam passing through a tilted cat-eye optical lens and going back along the entrance way in a turbulent atmosphere has been derived. Through numerical calculation, the effects of incidence angle, propagation distance, and structure constant on the propagation properties of a Gaussian beam in a turbulent atmosphere are studied. It is found that the incidence angle creates an unsymmetrical average intensity distribution pattern, while the propagation distance and the structure constant can each create a smooth and symmetrical average intensity distribution pattern. The average intensity peak gradually deviates from the centre, and the central average intensity value decreases quickly with the increase in incidence angle, while a larger structure constant can bring the average intensity peak back to the centre.     

Negative Refraction in One-Dimensional Photonic Crystals with Tilted Interface

We investigate the wave propagation through the tilted interface of one-dimensional photonic crystals. Negative refraction can be realized by excitation of the Bloch states in the extended Brillouin zone with suppressed reflection. Equi-frequency surface analysis shows that the positive refraction, negative refraction or birefringence in this configuration can be achieved under a proper incident angle, which is confirmed by finite-difference timedomain simulations. The results may be useful in applications in the new devices based on one-dimensional photonic or optical waveguide arrays.     

Propagation of a pair of vortices through a tilted lens

The propagation of a pair of vortices nested in a Gaussian beam through a tilted lens is studied. It is shown that after passing through the tilted lens, the relation between the transverse position of vortices and the tilt coefficient is linear in the propagation for isopolar vortex pair and vortex dipole, and in the three-dimensional (3D) case the vortex trajectories are sometimes like U or X shapes. With increasing the orientation angle of the vortex pair, the trajectories of vortices are circles for the both cases. The overlap of vortices may take place for the isopolar vortex pair, while the annihilation and revival of vortices occur for the vortex dipole in the propagation.     

A 795 nm gain coupled distributed feedback semiconductor laser based on tilted waveguides

The 795 nm distributed feedback lasers have great application in pumping the Rb D1 transition. In this paper, in order to realize specific 795 nm lasing, we designed tilted ridge distributed feedback lasers based on purely gain coupled effect induced by periodic current injection windows through changing the angle of the tilted ridge. The fabricated devices were cleaved into 2 mm-cavity-length, including 5 tilted angles. The peak output powers of all devices were above 30 m W.Single longitudinal mode lasing was realized in all tilted Fabry–Perot cavities using periodic current injection windows,with side mode suppression ratio over 30 d B. The total wavelength range covered 8.656 nm at 20℃. It was disclosed theoretically and experimentally that the output powers, threshold currents, and central wavelengths of the tilted ridge purely gain coupled DFB lasers were relevant to the tilted angles. The results will be instructive for future design of DFB laser arrays with different central wavelengths.     

Analysis of integrated corner mirrors by using a wide-angle beam propagation method

An integrated corner mirror is analyzed by using a two-dimensional (2D) wide-angle beam propagation method (WA-BPM) combining with an effective index method (EIM). Both the reflection efficiency and the phase variation due to a total internal reflection (TIR) for the TE and TM modes are calculated. A non-perfect fabrication (such as a tilted or displaced mirror) introduces an additional reflection loss and some errors of the phase variation. The additional reflection loss due to a tilted mirror increases rapidly as the tilted angle increases and becomes more sensitive to the tilted angle when the confinement of the waveguide becomes weaker. It is shown that the displacement of a mirror influences the phase variation greatly and the difference of the phase variation between the TE and TM modes is not sensitive to the displacement of the mirror.     

倾斜Bragg光纤光栅反射谱特性理论和实验研究

光纤Bragg光栅在通信和传感领域有着广泛的应用。根据倾斜光纤Bragg光栅的结构特点，利用耦合模原理，运用传输矩阵法，模拟分析了倾斜角度对光纤Bragg光栅反射特性的影响，并用实验对模拟结果进行了验证。实验和模拟结果表明：随着倾斜角的增大，中心反射波长向长波方向移动，反射带宽变窄，反射率减小。这个结论对倾斜Bragg光纤光栅的生产和应用有一定的指导作用。     

倾斜入射时偏振分束棱镜的角宽压缩效应

从理论上计算了由于光束倾斜入射产生的棱镜起偏系统角谱宽度的压缩效应,计算结果表明,入射光束相对于棱镜入射面的入射角越大对角谱宽度的压缩就越明显,不同材料的基底折射率的差别对角谱宽度的压缩影响不大。通过举例从膜系设计的角度进一步论证了这一结论。还指明在实际系统中可以保持入射光束的相对角度不变的情况下,通过改变棱镜的形状来达到倾斜入射的目的从而实现角谱宽度的压缩。对如何设计出高性能的宽角宽光谱偏振分束镜具有重大的指导意义。     

Transverse laser induced thermoelectric voltage effect in tilted La0.5Sr0.5CoO3 thin films

The transverse laser induced thermoelectric voltage effect has been investigated in tilted La_0.5Sr_0.5CoO₃ thin films grown on vicinal cut LaAlO₃ (1 0 0) substrates when films are irradiated by pulse laser at room temperature. The detected voltage signals are demonstrated to originate from the transverse Seebeck effect as the linear dependence of voltage on tilted angle in the range of small tilted angle. The Seebeck coefficient anisotropy ΔS of 0.03 μV/K at room temperature is calculated and its distorted cubic structure is thought to be responsible for this. Films grown on a series of substrates with different tilted angles show the optimum angle of 19.8° for the maximum voltage. Film thickness dependence of voltage has also been studied.     

圆锥透镜对倾斜球面波光束的衍射模式

由于球面波入射圆锥透镜所产生的出射光束具有焦深长和中心光斑直径较小的特点,适合激光通信以及长距离测量.在实际使用中,入射球面波很可能与圆锥透镜的光轴发生倾斜.研究了圆锥透镜对倾斜球面波光束的衍射特性,通过基尔霍夫衍射理论以及稳相法分析,分析了其径向衍射光场分布形式,并用计算机进行了仿真.使用了半径为15 mm、底角为1°的圆锥透镜进行实验,并对仿真结果进行了验证.仿真和实验证明,减小入射球面波曲率半径、出射光束传输距离和入射光束倾斜角都可以减弱入射光束倾斜对衍射图的影响.     

An improvement of tilted AlN for shear and longitudinal acoustic wave

This study investigates c-axis tilted aluminum nitride (AlN) piezoelectric films for the improvement of both shear and longitudinal acoustic wave resonances. Solidly-mounted resonator (SMR) structure is adopted for the applications of high frequency wireless communications and high sensitivity sensors. As to the piezoelectric layer, c-axis tilted AlN has the capability to excite the dual-mode resonances, namely, the longitudinal and shear mode resonances. In this study, SMR devices made with a seven-layer molybdenum/silicon dioxide (Mo/SiO₂) Bragg reflector and the c-axis tilted AlN are carried out. A conventional off-axis sputtering technique is applied to grow the tilted AlN. The outcome frequency responses show dual resonant characteristics. However, the longitudinal resonance fades away with the AlN c-axis tilted angle, and the quality factor of the longitudinal resonance decreases. Consequently, we make an improvement by tilting the off-center substrates toward the sputtering source and successfully enhance the longitudinal resonance while preserving the shear resonance at the same time. Not only the shear resonance for the liquid-based sensing application, but also an outstanding longitudinal resonance could be obtained. The practicability of the dual-mode resonator is extended.     

基于倾斜阵的距离与方位联合估计*

针对阵列倾斜引起的阵不变量方法定位精度下降的问题,该文结合倾斜阵声源定位的三维模型提出一种基于倾斜阵的距离与方位联合估计算法。该算法通过联合自校正方位距离联合估计方法对阵列倾角在声源-接收平面的投影量进行估计和补偿,改善目标距离估计误差的同时,利用匹配出的阵列倾角投影量反向估计目标方位,获得对声源二维位置的估计。通过仿真验证了该算法能够在阵列倾角较小时实现目标方位与距离的联合估计。     

Dispersive optical power splitter: assessment through time domain techniques

A comparative study has been performed between the finite difference time domain technique and the time domain beam propagation method (TD-BPM) while assessing the ultra short pulsed beam propagation inside dispersive power splitter. The dispersive GaAs material, used in the device, is modeled by Lorentzian dispersion relation that satisfies the refractive index of the material at a certain wavelength and shows a strong dispersion over a wide wavelength range. A smarter moving window technique is needed to be applied rather than at pulse’s group velocity for the TD-BPM, especially for bifurcated opto-electronic devices at the ultra short pulse.     

Entropy-stabilized smectic C phase in a system of zigzag-shaped molecules

We report Monte Carlo simulations of a system of rigid zigzag-shaped molecules that demonstrate that simple excluded-volume interactions are sufficient to produce a fluid tilted lamellar [smectic C (SmC)] liquid crystal phase. The molecules are composed of three rigidly linked hard spherocylinders arranged in a zigzag fashion. By varying the zigzag angle we have mapped out the whole phase diagram as a function of pressure and zigzag angle Psi. For Psi between 35 degrees and 80 degrees our model simulation exhibits the SmC phase. This is the first conclusive evidence where steric interactions arising out of molecular shape alone induce the occurrence of the SmC phase for a wide range of zigzag angles. For smaller Psi, a transition from tilted crystal to crystal is observed.     

Avoiding temporal distortions in tilted pulses

Tilted femtosecond laser pulses, having an intensity front with an angle to the propagation direction, can be generated by a dispersive element and a lens or mirror for imaging. Here we show that conventional geometries, for example with a grating at Littrow's condition, produce significant temporal distortions over the beam profile. The aberrations are the result of a mismatch between the grating's surface and the object plane of the imaging system. This changes the chirp of the pulses over the beam profile and lengthens the pulses to picoseconds for millimeter-sized beams. The distortions can be avoided by choosing a geometry in which the propagation direction of the tilted pulses is perpendicular to the grating's surface.     

质子交换Y分支光波导弯曲损耗研究

对采用退火质子交换技术(APE)制备的具有正弦型弯曲和余弦型弯曲两种不同的S型弯曲的Y分支光波导,利用宽角有限差分光束传播法,就过渡区长度、波导表面折射率增量、掩模版开口宽度等对波导弯曲损耗的影响进行了研究。结果表明,两种S弯曲下的Y分支波导的弯曲损耗,随波导结构参数的变化基本上是相同的,但二者的表现并不完全一致。数值计算结果为相应波导器件的设计和制备提供了一定的参考。     

Magnetization reversal of single domain permalloy nanowires

Magnetization reversal process and magnetoresistance (MR) hysteresis of single domain permalloy nanowires are numerically investigated by using OOMMF. It is shown that the abrupt jumps in the magnetoresistance are due to the domain formation and domain wall propagation so that a magnetic domain suddenly switches from one state into another. A nonmonotonic angular dependence of the jump (switching) field is found. Coherent rotation mode is responsible for the smooth variation of MR curves. The nucleation pattern of newly born domains depends on the tilted angle of external field.     

基于同心球透镜的四镜头探测器阵列拼接成像系统

为了同时实现成像系统的大视场、长焦距和高分辨率,设计了基于同心球透镜的四镜头探测器阵列拼接成像系统.首先,阐述了四镜头探测器阵列拼接方案的原理;介绍了同心球透镜的结构特点,阐述了其成像优点.然后,完成了满足实际拼接应用的同心球广角、长焦成像系统(拼接子系统)的光学设计.最后,给出了拼接子系统的像质评价并对其进行公差分析.结果表明:拼接后的系统可实现100 mm焦距和120°视场成像.该系统解决了大视场和长焦距之间的矛盾,可实现超高像素成像,相对于传统光电成像系统具有巨大的优势.     

Discrete diffraction and waveguiding of optical beams in a cascade-induced lattice

Regularities of anisotropic discrete diffraction in a cascade-induced lattice formed by two crossed pump waves in a quadratically nonlinear medium have been studied. The transition from diffraction of a signal beam in free space to discrete diffraction with further capture into a waveguide in a defocusing lattice with an increase in the pump wave intensity has been traced. Diffraction-free propagation of a tilted beam is described.     

Intensity depending on object distance in a two-dimensional photonic crystal

The propagation of electromagnetic (EM) waves in two-dimensional triangular-lattice photonic crystals (PCs) is investigated through dispersion characteristics analysis and numerical simulation of field pattern. The designed PC structure can exhibit all angle negative refraction in the second band. A flat superlens formed from such a PC has been designed and its imaging properties have been investigated systematically. Good-quality images and focusing, with relative refractive index of ?1, have been observed in this system for TE mode waves. In contrast to the images in near-field region for the lowest valence band, non-near-field images, explicitly following the well-known wave-beam negative refraction law, have been demonstrated. The relation between the intensity of the image and the object distance has also been discussed in this paper. The extensive applications of such a phenomenon to optical devices are anticipated.