Objective Aspherical surfaces are widely used in modern optical systems. With the rapid development of various optical instruments, especially laser ignition devices, laser weapons, and satellite products, which have high standards for cleanliness, smoothness, and other quality aspects, the trend in customized development is quickly shifting towards modularization and batch production. As key components of the optical system, the automation of manufacturing, inspection, and assembly of aspherical surfaces directly determines the quality and efficiency of mass production. The automated optical inspection of aspherical surfaces has also raised increasingly higher requirements. Interferometry, especially compensation methods, is a rapid, precise, and non-contact technique with the potential for automated test of aspheric surfaces. In order to meet the stringent requirements for surface cleanliness of ultra-smooth aspherical surfaces used in high-energy lasers and other fields, and to eliminate the impact of cleanliness and thermal disturbances introduced by inspectors, researchers have developed an automated interferometric detection method for ultra-smooth aspherical surfaces with high cleanliness.
Methods The mathematical relationship between the amount of misalignment and wavefront aberration is determined by the compensation method based on the theory of vector wavefront aberration. Spherical and defocus aberrations are only affected by the position of the compensator or aspheric surface under test along the optical axis direction. Tilt, coma, astigmatism, field curvature, and distortion aberrations are influenced by the tilt and eccentricity of the optical elements. The order of the effects of tilt and eccentricity on aberrations varies. Based on the analysis, the automated interferometry test method is proposed. By establishing the sensitivity matrix between the misalignment amount of the aspheric surface and the wavefront aberration of the compensated interferometric method, it is possible to utilize the wavefront aberration to calculate the measured misalignment amount of the aspheric surface. The design utilizes the Stewart platform integrated test backplane to achieve the adjustment of the measured aspherical surface in six degrees of freedom. Taking the out-of-focus, coma, and dispersion of the ideal interference system as the optimization objectives, feedback control is implemented to achieve the automated adjustment of the measured aspherical surface. Subsequently, the automated interferometric detection of the high-cleanliness ultra-smooth aspherical surface is achieved.
Results and Discussions Simulation and experiment utilize the same aspheric surface under test, which is an ellipsoidal surface. The compensation consists of two lenses with plane wave incidence. In the simulation, random misalignment of the aspheric surface is introduced, which includes positional errors along the optical axis, tilt errors, and eccentricity errors. The wavefront aberrations, i.e., Zernike coefficients, can be obtained through simulation. The misalignment is addressed through simulation and continuously adjusted until the misalignment and aberration reach an acceptable level. Simulation initially verifies the feasibility of the automated interferometry test method. In the experiment, the aspheric surface is mounted on the back plane of the Stewart platform using a snap-in interface. The interferogram can be obtained through simple coarse adjustment as the initial state of the experiment. The experimental results show that within the measurable range of the interferogram, the convergence of the aspherical surface misalignment can be achieved through a few iterative steps using the sensitivity matrix. Combined with Stewart's six-degree-of-freedom adjusting stage, the system enables translation error adjustment with a precision of 2 μm and automated optical axis alignment with a precision of 2". Finally, the precision adjustment of the measured aspherical surface is completed, achieving automated interferometric detection of the high-cleanliness ultra-smooth aspherical surface.
Conclusions The automated interference detection method for aspherical surfaces utilizes a sensitivity matrix and adjustments in six degrees of freedom to enable a rapid solution and automatic correction of measured misalignments of aspherical surfaces. This method eliminates the influence of cleanliness and thermal perturbations introduced by inspectors, enhances the detection speed of aspherical surfaces, and achieves automated interference detection of ultra-smooth aspherical surfaces with high precision.
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