In this paper, we present a method for measuring the focal length of large aperture lenses using reflective computergenerated hologram (CGH). The CGH is designed to emulate the properties of a large aperture retrosphere through diffraction when performing null test of a lens with an interferometer. To validate this test approach, we designed and fabricated a 450 mm × 450 mm reflective CGH specifically for testing the 440 mm × 440 mm spatial filter lenses with a focal length of 31984.222 mm (@ 632.8 nm). Experiments and error analysis were carried out. The results show that the CGH test approach features high accuracy and good repeatability.
The transmitted wavefront mid-spatial-frequency(MSF) errors of spherical lens has a great influence on the quality of the transmitted beam. Aiming at the problems of poor convergence precision and low convergence efficiency in mid-spatial frequency errors polishing of large square spherical lenses, this paper proposes a mid-spatial-frequency errors correction technology by using full-aperture rigid polishing combined with numerically controlled sub-aperture smooth polishing. In the full-aperture polishing stage, the surface shape distribution that is conducive to subsequent sub-aperture polishing is obtained through radius compensation technology to reduce the sudden change of surface shape in the corner area of the component. The more rigid polishing pad is used to smooth the whole surface then, so that the component has better MSF errors condition before the sub-aperture high-precision surface correction. In the stage of small tool CNC polishing, the transmission structure and mass distribution of the polishing disc are optimized, and the ideal transmission characteristics and size parameters of the polishing disc are obtained through mechanical simulation analysis to reduce the overturning moment of the polishing disc when the direction changes suddenly. This optimization also improves the pressure distribution of the polishing interface. Stability, a flexible polishing disc combined with a high dispersion polishing slurry is used to correct the surface errors. The concentration of the polishing slurry is optimized and the supply method is well changed. So when the surface shape errors convergence process is more efficient and controllable, it will not cause the deterioration of the MSF errors. The smooth tool is applied in the last stage with reducing the temperature change of the polishing interface. All these measures are aimed to increase the stability of smooth polishing and to achieve high-efficiency, high-precision and stable convergence of MSF errors. The experimental verification was carried out on four square spherical lenses with a size of 440mm×440mm. The final PSD1:RMS values have all reached within 1.8nm. Additionally the overall processing time has been greatly shortened.
Spectral beam combining technique is one of major approaches in power scaling of fiber lasers. In order to maximize beam-combining performance, maintain a high beam quality in the combining process, the absorption-induced wavefront distortion of the resulting combined beam must be considered. By reducing the film absorption, the dichroic filters will be promising competitors to dielectric gratings as crucial spectral beam combining elements. In this paper, two different structures: less-cavities (LC) structure and more-cavities (MC) structure of dichroic filter layers were carefully designed and fabricated by ion-beam sputtering deposition with high-precision layer thickness monitoring method. A photothermal scanning system based on laser-induced surface thermal lensing (STL) effect was used for analyzing the 1064nm wavelength absorption of dichroic filter layers in passband and stopband. Dichroic filter with reduced in-bandpass absorption was found in MC structure by relaxing the electric field strength in thin films. This approach is highly expected to have great potential for fabricating promising spectral beam combining dichroic filters with less thermal effect and higher laser-induced damage threshold (LIDT).
In this paper, we present a method of using computer-generated hologram (CGH) to measure the mid-spatial frequency error of large aperture lenses. To validate this test approach, we designed and fabricated a 450 mm × 450 mm reflective CGH for testing the 440 mm × 440 mm spatial filter lens with a focal length of 32500 mm. In our experiment, both the 0th and 1st order diffraction wavefront of CGH were measured, and the 0th order diffraction wavefront was used to calibrate the substrate error. The mid-spatial frequency error caused by the CGH fabrication errors were evaluated using the binary linear grating model and power spectral density theory (PSD). Experimental results and error analysis indicate that the measurement accuracy of PSD1 is ~0.9 nm RMS, which means the CGH test approach can be used to measure the mid-spatial frequency error of large aperture lenses.
We present a method of using computer-generated hologram (CGH) to measure the radius of curvature of large
aperture long-focal-length lens. In this method, a large aperture transmission CGH is used as a transmission sphere to
generate the test and reference wavefronts by means of diffraction. To verify the feasiblity of this method, a 450 mm ×
450 mm transmission CGH is designed and fabricated for measuring the radius of 440 mm × 440 mm spatial filter lens.
Experimental results and error analysis show that the CGH test method features high accuracy and good repeatability.
Automatic measurement of single points schema by coordinate measuring machine(CMM) is used to measure the Ultra-Long curvature radius of spherical optical element. The removal quantity of each measuring point can be calculated through contrasting the measure value and the theoretical value. A removal model of spherical optical element polishing is established based on Preston equation, and the required machining parameters are predicted by removal simulation in MATLAB. A processing test on a fused silicon with an aperture of 440mm×440mm was performed and the result shows that the model is effective in Ultra-Long curvature radius control of spherical optical element during full aperture polishing.
In large-scale high-power laser devices, the mid-spatial-frequency(MSF) error of the transmitted wavefront of the large-aperture spherical lens has a direct impact on the energy scattering of the high power laser. This paper proposes a technology about correction of MSF error of large-aperture spherical lens based on computer numerical control polishing. A smooth polishing theoretical model is established for spherical lens and the removal function morphology is optimized. To make a better MSF error convergence, the rigid conformed tool is designed and assembled. The polishing tool and the main axle are connected by a high-precision universal joint, so that the polishing pad can be flexibly attached to the surface of the workpiece when the tool is running. This makes it able to polish the square spherical workpiece with large curvature radius by the processing method of planar workpiece. In the earlier stage, two kinds of path are applied to converge the low- frequency error by crossing each other. Then the random path is applied for MSF error convergence. By the experimental varification of Four fused silica spherical lens with aperture of 440 mm × 440 mm, the RMS value of the PSD1 frequency band of the lens’ transmitted wavefront error is finally converg-ence to 2.2 nm through once MSF error correction.
For large-scale high-power laser devices, the transmitted wavefront gradient root mean square (GRMS) is one of the most important indexes for evaluating the low-frequency distortion of spherical lenses. In this paper, for the low-frequency error index requirement of large-diameter spatial filter lens, a conformed polishing method based on composite tool is proposed in the spherical CCOS polishing stage. Based on the detection results of the optical components, the GRMS distribution is separated by low-pass filtering, and the high value of GRMS is partitioned by K-means algorithm to determine the polishing path and dwelling time. A composite conformed polishing tool is designed and the simulation analysis of the polishing tool structure and polishing stress distribution are carried out to optimize the parameters, in order to obtain an ideal GRMS convergence removal function. After experiments and production verifications of several 440×440 sized spatial filter lenses, the results show that the GRMS can be quickly converged below 7nm/cm, and the convergence rate is improved by about 50% compared with the traditional CNC polishing.
The ghost reflections can cause spurious fringes in the interferograms and lead to error in the measurement. So it is necessary to evaluate the influence of the ghost reflection spot for a correct interpretation of interferograms. In this paper, the ghost reflections of testing long focal length lens with computer-generated hologram (CGH) are investigated and geometrical model is established to obtain an expression for the size of the ghost reflection spot. Moreover, simulations and experiments are carried out by studying the ghost reflections of the long focal length lens in Shenguang III system.
A method for measuring the wavefront of wedged focus lens by using the Fizeau interferometer with large aperture and a reflective computer generated hologram (CGH) is proposed. The CGH has 6 zones: one main zone for the null testing of wedged focus lens, one additional zone for alignment the CGH substrate with the interferometer, and four others for the pre-alignment of wedged focus lens by projecting four marks. CGH design process was performed, including the ray trace model for multizone CGH and the optical testing configuration. Simulation results show that the desired precision can be reached with use of CGH and confirm the feasibility of this measurement method.
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