Photoinduced non-linear optical diagnostic of InI ferroelastic nanocrystals

Photoinduced second harmonic generation was established in the centrosymmetric layered crystals of InI. The effect was grounded on occurrence of photoinduced optical second harmonic generation in the ferroelastic phase of the InI crystals. As a subject of investigations layered ferroelastic crystal of InI was chosen. It was found that the ferroelastic phase disappears during decrease of the nanolayers from 15 nm up to 6 nm. Such effects are explained by occurrence of anharmonic phonon modes confined by the walls of the layered crystals. We have found that the effect may be of importance for the further investigations of the layered nanocrystals. At the same time the giant value of the photoinduced SHG indicates on a possible using the nanolayered crystals like promising non-linear optical crystals.     

离轴平顶高斯涡旋光束形成的合成光涡旋及其在自由空间中的传输

对两束拓扑电荷m_1,2=±1的平行、离轴平顶高斯涡旋光束在束腰面叠加形成的合成光涡旋及其在自由空间的传输做了研究.详细的数值计算和分析表明,合光涡旋的位置、数目和净拓扑电荷与光束的控制参数,包括相对位相,振幅比,束腰宽度比,相对离轴参数,光束阶数,以及传输距离有关,但拓扑电荷不总是守恒. 关键词： 奇点光学 合成光涡旋 平顶高斯涡旋光束 拓扑电荷     

Evaluation of a phase shifting method for vortex localization in optical vortex interferometry

The optical vortex interferometer uses a regular lattice of the optical vortices. In the previous papers we showed that changes in vortex lattice geometry can be related to physical quantities of the object being measured. The accuracy of such a measurement depends strongly on the precision of vortex point localization (vortex points are points where wavefront phase is undetermined). In this paper we compared the measurements of object wave tilt calculated from different localization methods applied to real interferograms and evaluate various localization methods.     

A new alternative approach of all optical frequency encoded clocked S-R flip-flop exploiting the non-linear character of semiconductor optical amplifiers

In all optical networking and computing system, the role of all-optical flip-flops is very much essential. For signal synchronization with a reference clock and for storage of digital bits the flip-flop has no alternative. In this communication the authors propose a method of developing an all optical frequency encoded clocked R-S flip-flop using the non-linear character of semiconductor optical amplifiers. Frequency is the basic character of light and several encoding/decoding problems in computations and communications can be solved using the frequency encoding principle of optical data. The proposed system is all-optical and therefore it can extend a super fast speed of operation (far above THz limit).     

Polarization vortex spatial optical solitons in Bessel optical lattices

We investigate the formation of polarization vortex spatial optical solitons in optical lattice induced by a non-diffracting Bessel beam. The properties of these solitons in zeroth-order and first-order Bessel lattices with focusing and defocusing Kerr nonlinearity are discussed. It is found that these solitons have some analogies with phase vortex solitons carrying single positive or negative topological charge in these lattices. Besides, these polarization vortex solitons have complicated dynamical characteristic and can be stabilized in some parameter region.     

Stability of Bragg grating solitons in a cubic-quintic nonlinear medium with dispersive reflectivity

We investigate the existence and stability of Bragg grating solitons in a cubic-quintic medium with dispersive reflectivity. It is found that the model supports two disjoint families of solitons. One family can be viewed as the generalization of the Bragg grating solitons in Kerr nonlinearity with dispersive reflectivity. On the other hand, the quintic nonlinearity is dominant in the other family. Stability regions are identified by means of systematic numerical stability analysis. In the case of the first family, the size of the stability region increases up to moderate values of dispersive reflectivity. However for the second family (i.e. region where quintic nonlinearity dominates), the size of the stability region increases even for strong dispersive reflectivity. For all values of m, there exists a subset of the unstable solitons belonging to the first family for which the instability development leads to deformation and subsequent splitting of the soliton into two moving solitons with different amplitudes and velocities.     

Paraxial propagation of cosh-Airy vortex beams in chiral medium

Propagation dynamics of the cosh-Airy vortex(CAiV) beams in a chiral medium is investigated analytically with Huygens–Fresnel diffraction integral formula. The results show that the CAiV beams are split into the left circularly polarized vortex(LCPV) beams and the right circularly polarized vortex(RCPV) beams with different propagation trajectories in the chiral medium. We mainly investigate the effect of the cosh parameter on the propagation process of the CAiV beams.The propagation characteristics, including intensity distribution, propagation trajectory, peak intensity, main lobe's intensity, Poynting vector, and angular momentum are discussed in detail. We find that the cosh parameter affects the intensity distribution of the CAiV beams but not its propagation trajectory. As the cosh parameter increases, the distribution areas of the LCPV and RCPV beams become wider, and the side lobe's intensity and peak intensity become larger. Besides, the main lobe's intensity of the LCPV and RCPV beams increase with the increase of the cosh parameter at a farther propagation distance, which is confirmed by the variation trend of the Poynting vector. It is significant that we can vary the cosh parameter to control the intensity distribution, main lobe's intensity, and peak intensity of the CAiV beams without changing the propagation trajectory. Our results may provide some support for applications of the CAiV beams in optical micromanipulation.     

Effects of misalignments in the optical vortex transformation performed by holograms with embedded phase singularity

Spatial characteristics of diffracted beams produced by a “fork” hologram from an incident circular Laguerre-Gaussian beam whose axis differs from the hologram optical axis are studied theoretically. General analytical representations for the complex amplitude distribution of a diffracted beam are derived in terms of superposition of Kummer beams or hypergeometric-Gaussian beams. The diffracted beam structure is determined by combination of the “proper” topological charge m of the incident vortex beam and the topological charge l of the singularity “imparted” by the hologram. Evolution of the diffracted beam structure is studied in detail for several combinations of m and l and for various incident beam displacements with respect to the optical axis of the hologram. Variations of the intensity and phase distribution due to the incident beam misalignment are investigated and possible applications for the purposeful optical vortex beam generation and optical measurements are discussed.     

Diffraction characteristics of optical vortex passing through an aperture-iris diaphragm

The diffraction of an optical vortex through an iris diaphragm which is a close approximation to a circular aperture has been investigated. The results are compared with those obtained from the diffraction of a Gaussian beam through the same aperture. In our findings the diffraction of an optical vortex and a Gaussian beam produce ball bearing sort of structure of darkness and brightness. The singularity of the vortex beam is found to be consistent even after the diffraction through the aperture. The presence of singularity at the centre of diffraction pattern of an optical vortex has been confirmed by interferometry. There is a good agreement between the experimental and numerical results. We propose that these results may find various applications in optical trapping experiments.     

A method of optical implementation of frequency encoded different logic operations using second harmonic and difference frequency generation techniques in non-linear material

Optics has already been proved its successful roles for conduction of parallel logic, arithmetic and algebraic operations. Since last few decades many types of optical data processors were proposed. To implement these processors different data encoding/decoding techniques have been reported. In this context polarization encoding technique, tristate, quartenary logic, multivalued logic, symbolic substitution techniques etc. may be mentioned. Very recently, frequency encoding/decoding technique have also been well established. The potential advantage of frequency dependent encoding/decoding is that, as the frequency being the fundamental character of a signal; it will remain unaltered in reflection, refraction, absorption etc. during transmission of the signal. In this communication the authors propose a scheme for implementing different logic operations adopting frequency based encoding technique. For this purpose the second harmonic generation and difference frequency generation techniques are used by exploiting the non-linear response character of some materials.     

Linear and non-linear behavior of cavity polaritons

We report on the linear and non-linear emission of cavity-polaritons under resonant excitation. At low excitation density, in addition to polariton photoluminescence, strong Rayleigh scattering is observed. At higher excitation densities, a sudden transition to a highly emissive state is observed, accompanied by spatial patterning. We attribute such phenomena to a combination of nonlinear cavity-polariton relaxation mechanism and nonlinear response of the cavity, leading to transverse pattern formation. A careful analysis of near- and far-field emission patterns with spatial filtering as well as reflectivity shows that an inhomogeneous situation develops, the center of the excited region undergoing a strong to weak coupling transition while the periphery is still in strong coupling. Despite the complex non-linear behavior we observe no signatures of Bose–Einstein condensation or Boser action.     

The new method of topological charge determination of optical vortices in the interference field of the optical vortex interferometer

In this paper, the new method of determination of the topological charge of vortex points in the interference field obtained by three plane waves interference is presented. Such optical fields are used in the optical vortex interferometer (OVI) and the determination of vortex points’ topological charge allows of unique determination of the relative phase between interfering waves (phase unwrapping problem). The new method uses additional plane wave, which produce a characteristic fork-like fringe structure in the neighbourhood of vortex points. By analysing the orientation of these fork-like patterns one can read the sign of the topological charge of the given vortex point. The method is simple and can be used for OVI calibration performed before the measurements.     

Tunable optical vortex arrays using spontaneous periodic pattern formation in nematic liquid crystal cells

Controllable liquid crystal (LC) defects can provide an effective approach to creating tunable optical vortices. We develop a method to create tunable matter vortex arrays in an LC cell, in which +1 and −1 defects are periodically arranged in a square grid lattice. Spontaneous formation of the periodic defect array is achieved using a spontaneous standing pressure wave without using any patterned electrode or patterned alignment layer. The +1 and −1 defects in the array can induce optical vortices with opposite handedness, and the matter vortex array produces a periodic optical vortex array with orbital angular momenta of −2ℏ and +2ℏ in the same grid lattice. Because the pitch of the grid can be controlled, the method can provide a useful pathway to producing tunable optical vortex arrays for various applications such as advanced optical communication and quantum computation.     

部分相干涡旋光束传输中的光斑分析

在近轴光束近似条件下,采用交叉谱密度传输公式推导了 部分相干涡旋光束传输一段距离后观测平面上交叉谱密度矩阵元的解析表达式, 在此基础上对观测平面上的光强分布进行了分析.研究表明, 和完全相干涡旋光束不同,部分相干涡旋光束传输后光斑中心点的光强会逐渐凸现出来, 随着传输距离的增加,观测平面上的光强会逐渐演变为类似高斯型分布的特性. 这种演变规律与源平面上光源的拓扑电荷数和相干长度有关, 在其他参数不变的情况下,拓扑电荷数越小,相干长度越短, 演变为高斯型光斑的速度越快.最后针对一阶部分相干高斯涡旋光束, 通过观测平面上光强极值研究,对光斑随传输距离演变的过程进行了详细的分析, 在理论上对这种演变规律给出了严格的证明.     

部分相干涡旋光束在湍流大气中的传输特性

根据广义的惠更斯-菲涅耳原理,推导了部分相干涡旋光束在湍流介质中传输时光强分布情况的理论公式,详细研究了部分相干涡旋光束在湍流介质中的传输变化规律。研究结果表明,湍流介质的强弱,光源的相干性以及光束所带拓扑电荷数大小均会影响光束传输特性。     

Numerical study of an optical regenerator exploiting self-phase modulation and spectral offset filtering at 40 Gbit/s

In this work, we numerically investigate the performances of optical regenerators based on self-phase modulation and spectral offset filtering at 40 Gbit/s. We outline the different effects affecting the device performances and explain the choice of the optimal working power. The impact of the regenerator on the output signal is also analysed through a statistical approach. Both single- and double-stage configurations are investigated.     

两束离轴高斯光束干涉场中的横向光涡旋

研究了两束离轴高斯光束干涉场中的波前结构以及横向光涡旋.研究表明干涉场中的相位鞍点以及涡旋点的位置与光束的离轴参数、束腰宽度、相位以及相对振幅有关.相位鞍点既可位于涡旋点的内侧，也可以位于涡旋点的外侧，且控制参数取一定值时，相位鞍点将与涡旋点重合.对于离轴光束，XZ平面两侧的相位鞍点与涡旋点相遇时所对应的控制参数并不相同，且XZ平面上相位鞍点相遇时所对应x值，一般情况下与涡旋点相遇时所对应的x值不相同.     

Alternating approach of implementing frequency encoded all-optical logic gates and flip-flop using semiconductor optical amplifier

In conduction of parallel logic, arithmetic and algebraic operations, optics has already proved its successful role. Since last few decades a number of established methods on optical data processing were proposed and to implement such processors different data encoding/decoding techniques have also been reported. Currently frequency encoding technique is found be a promising as well as a faithful mechanism for the conversion of all-optical processing as the frequency of light remains unaltered after refection, refraction, absorption, etc. during the transmission of light. There are already proposed some frequency encoded optical logic gates. In this communication the authors propose a new and different concept of frequency encoded optical logic gates and optical flip-flop using the non-linear function of semiconductor optical amplifier.     

Propagation-mode near-field method for measuring optical waveguides of graded index profile: some considerations on TM-modes

A new set of formulae is developed for profiling refractive index of optical waveguides using the propagation-mode near-field method. Effects of graded-index profile on wave equations for TM-modes are taken into account. Anisotropic waveguides are also studied.