When coherent light such as laser beam is projected on a rough target, the scattering field will exhibit random bright spots, known as speckle phenomenon. If the target is in motion, the speckle field also changes over time, and the dynamic characteristics of the speckle field contain the motion information of the target. We refer to the area around the rotation axis of the target as the polar region. In this paper, computer simulation technology is used to study the power spectrum of the dynamic speckle field in the backward and nearby areas when a laser is incident around the polar region direction. The research results indicate that there exists a direction of minimum characteristic frequency of the speckle field near the polar region. When the laser direction is deviated from the rotating axis, the minimum frequency direction is symmetric with respect to the rotation axis of the incident direction, and the minimum characteristic frequency is proportional to the magnitude of the pointing error. The method proposed in this article can be used for remote orientation of the target rotation axis.
During the preparation of optical films, each interface of the film system will deviate from the ideal shape of the smooth surface, and micro-roughness optical surface formed by random fluctuation of film thickness. With the rapid development of thin-film/semiconductor chip micro-nano periodic structures, periodic units and defect sizes are gradually approaching the nano-level, the influence of the roughness of the optical surface on the overall performance of the system cannot be ignored. Based on the three dimensional FDTD/MRTD algorithm, the system studies the light field distribution characteristics of the surface of complex optical structures at the micro-nano scale. Numerically analyze the optical field characteristics of the film surface with a multilayer periodic structure, and obtain the key parameters that affect the optical field of the film surface by analyzing the changes of the structure parameters. In view of the special structure of the multilayer periodic structure film, the concept of roughness is introduced to analyze the surface field changes on the surface of the multilayer film when the roughness exists, discuss the influence of the change of roughness parameters on the light field distribution, and give the influence law a detailed physical explanation.
The surface plasmon has the advantages of breaking through optical diffraction limitation and local field enhancement. With the development of micro nanometer cycle structure of film/semiconductor rapidly, the cycle unit size tends to be nanometer level, therefore the requirement for purity and clarity is higher and higher. The existence of defect influence the quality of surface plasmon obviously , so it should not be ignored on the overall performance of optical system. In order to detect effectively the deficiency of surface plasmon, a surface plasmon structure model based on the plane type surface of periodic square hole which contains defective particles is proposed by Multi- Resolution Time-Domain (MRTD) method. The changes of the light scattering field when different defects appear in the structure of periodic square hole vitro exciter are focused on micro-and nanoscale, and we can analyze the influence of different structural parameters and defect parameters on the total scattering field to achieve effective control of the defects and modulation of the light field.
The beam polarization characteristics is developed to assess the photocarrier properties of semiconductors. Based on three carrier effects (bandfilling, bandgap shrinkage, and free-carrier absorption) and Fresnel reflection theories, calculations and analyses are performed to investigate the effects of the excess carrier concentration, the photo energy and the incident angle of the probe beam on the polarization azimuth angle. The results indicate that the azimuth angle of the reflected light is approximately proportional to the excess carrier concentration at high injection level (>1018 cm-3 ), and the proportionality constant depends on the incident photon energy and the incident angle. These studies provide an alternative for determining the photocarrier properties of semiconductors.
The Rytov method is used to derive the distribution of the scintillation index on the receiving aperture plane of a laser beam reflected from a retro-reflector in the ground-air-ground double-path propagation. According to the mutual independence of the turbulence effect and the speckle effect caused by the random vibration of the retro-reflector array, the intensity covariance function and the scintillation index on the aperture plane are calculated. The variations of the aperture smoothing effect and scintillation index of the beam on the finite aperture receiving plane with the aperture size are revealed. The study results could also provide theoretical and technical guidance for the application of laser target detection and recognition.
On the basis of the extended Huygens-Fresnel principle and the cross-spectral density function (CSDF), the intensity and average orbital angular momentum (OAM) of the partially coherent flat-topped vortex beams in the slant atmospheric turbulence are presented. The effects of the order, topological charge, waist radius, and propagation distance of the beam on the intensity and average OAM are discussed. Results obtained show that the intensity of the partially coherent flat-topped vortex beam is changed due to the variations of the propagation distance, waist radius, topological charge and beam order, the average OAM is constant during the beam propagation in the atmospheric turbulence and related only to the waist radius and beam order. Results obtained by this paper may serve as theory bases for future applications in the atmospheric optical communication.
As targets in space are usually very far from the ground, some targets containing smooth reflecting components can be seen to be composed of a single or multiple glints when they are detected by a Lidar (laser radar) system located on the ground. The received intensity of the detector fluctuates, which caused significant noise on the system, for two reasons. One is the randomness of positions of the glints and the other is the perturbations of the atmospheric turbulence. The formulation of the scintillation index of the reflected intensity is derived by using incoherent superposition of the reflected field. The results show that the scintillation index can be divided into two parts, corresponding to the two sources that cause the intensity fluctuations. The results show that the target composed by multiple glints has two different effects on the fluctuation of the reflected intensity, one is the amplification effect of the incoherent superposition, and the other is some similar aperture averaging effect.
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