Laser measurement technology is widely used in antenna main reflector and subreflector deformation or pose measurement. Based on the laser measurement technology, atmospheric turbulence caused by the atmospheric characteristics or the convergence effect of the antenna itself will affect the refractive index disturbance during laser propagation, which affects the accuracy of the laser measurement of the antenna deformation or pose. In order to solve the problem of laser atmospheric propagation deflection caused by atmospheric turbulence on the near-ground of a large-aperture antenna, firstly, this paper decomposes the laser atmospheric propagation path into multiple isotropic air layers, calculates the atmospheric refractive index of each air layer, obtains the atmospheric refractive index of the whole laser propagation path through curve fitting methods, and then assesses the laser deflection. Secondly, the turbulence intensity under sunny daytime and cloudy nighttime are evaluated, the laser spot position deviation is compared and analyzed, and the matching relationship among turbulence intensity, theoretical deviation of the laser spots, and actual spot deviation is obtained. Finally, the deflection of laser atmospheric propagation is fitted and calculated by measuring environmental data matched to the actual experimental data of the Nanshan 26m radio telescope antenna, which verifies the effectiveness and feasibility of the proposed method.
In the measurement of equi-inclination and Equi-attitude parameters, the four-step phase-shifting method using white light illumination is used to obtain the principal stress direction angles within the range of [-π/2, π/2], which are consistent with the attributes. This replaces the "inconsistent" isoclinic angles in the six-step phase-shifting method, avoiding the "sawtooth-like" errors in the isoclinic line-wrapped phase map and the "distortions" in the isochromatic line-wrapped phase map. The wrapped phase maps of the isochromatic lines are processed using general unwrapping methods, such as the Goldstein branch-cut method and the quality-guided method, to obtain full-field fractional fringe order. Finally, the separation of stresses is conducted to obtain σx, σy, and τxy for the entire field. A comparison is made between the experimental processing results of a radially compressed circular disk and the theoretical values. The error in the static equilibrium check on the 7.5mm diameter section of the disk, located at a distance of 30mm from the horizontal diameter, is 2.27%, meeting the engineering accuracy requirements.
The light intensity, polarization degree and orbital angular momentum density of partially coherent radially polarized beams carrying double vortexes after being focused by a high numerical aperture lens are obtained based on Richards– Wolf vectorial integral theory. The tight focusing properties of the beam including polarization degree and orbital angular momentum density are discussed. The influence of the source coherence length of the incident beam and the numerical aperture of the lens on the focusing characteristics is studied. Results show that the distribution of polarization degree and orbital angular momentum density presents a petal-like pattern, and the number of petals is related to the topological charge. The changes in the source coherence length and the numerical aperture affect the focusing properties of the beam. More useful orbital angular momentum is obtained after vortex beams are focused through a high numerical aperture lens. These results may provide a theoretical reference for the tight focus of vortex beams and the application of polarization and orbital angular momentum.
Based on angular spectrum diffraction theory, the scattering field of vector vortex beams (VVBs) passing through a rotating random rough surface is derived. The speckle produced by VVBs passing through the rotating ground glass is collected experimentally. The effects of ground glass roughness and rotational speed on the speckle and orbital angular momentum (OAM) of different order VVBs are discussed in detail. Furthermore, the speckle characteristics of the VVB are compared with the scalar vortex beam. The change in the OAM states of VVBs before and after passing through the rotating ground glass was detected by double-slit interference. The results show that the speckle of VVBs is generated after passing through the static ground glass, and the speckle contrast increases with the increment of roughness. Compared with the scalar vortex beam, the speckle contrast of the VVB changes faster with the roughness of ground glass. When the ground glass rotates, the speckle reduction effect appears and the contrast is reduced. When the rotational speed is >15 r / s, the speckle reduction effect is the strongest. The intensity distributions and OAM states of VVBs are not affected by the rotating ground glass. The research results provide a theoretical basis for the application of VVBs in optical communication, optical detection, and other fields.
The rotating speed of a target is usually measured using superposition vortex beams with opposite topological charges. Based on angular spectrum representation and Goodman’s speckle theory, the speckle characteristics of superposition vortex beams after passing through a rotating ground glass disk (RGGD) were studied. The statistical characteristics of the speckle field were analyzed using the theory of the orbital angular momentum (OAM) spectrum. The analysis determined the relationship between the spatial coherence length and the topological charge of a superimposition vortex beam and its speckle field. Moreover, the study explored the relationship between the speed of the ground glass disk (GGD) and the speckle field of the superposition vortex beam. Results show that the rotating speed of the GGD increases and the intensity distributions of generated partially coherent superposition vortex beams become increasingly uniform after passing through the RGGD. The speckles gradually disappear but the OAM spectrum becomes increasingly dispersive with increased rotating speed. These results might be useful for practical applications of the rotational Doppler effect in remote sensing and metrology.
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