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Fast tool servo (FTS) in ultra-precision machining (UPM) is an enabling and efficient technology for fabricating optical freeform surfaces or microstructures with submicrometric form accuracy and nanometric surface finish. There are many kinds of FTS in the different driving principle to present their various performances currently. Their kernel technologies influence the machining ability and accuracy of freeform surfaces, consequently receiving much research attention and interest. These technologies are generally summarized as the development of FTS structure, the advanced control algorithms, tool path planning, machining condition monitoring, and surface measurement and error compensation. This paper aims to survey the current state of the art in machining freeform optics by FTS. An analysis of the principle, performance, and application of FTS machining with regard to freeform optics is presented. And the key machining technologies for optical freeform surfaces by FTS are then introduced in detail. The challenges and opportunities for further studies are concluded according to the FTS machining difficult of optical freeform surfaces finally.

Phase compensation is a critical step for the optical measuring system using spatial light modulator (SLM). The wavefront distortion from SLM is mainly caused by the phase modulation non-linearity and non-uniformity of SLM’s physical structure and environmental conditions. A phase modulation characteristic calibration and compensation method for liquid crystal on silicon spatial light modulator (LCoS-SLM) with a Twyman-Green interferometer is illustrated in this study. A method using two sequences of phase maps is proposed to calibrate the non-uniformity character over the whole aperture of LCoS-SLM at pixel level. A phase compensation matrix is calculated to correct the actual phase modulation of the LCoS-SLM and ensure that the designed wavefront could be achieved. Compared with previously known compensation methods, the proposed method could obtain the phase modulation characteristic curve of each pixel on the LCoS-SLM, rather than a mono look-up table (LUT) curve or multi-LUT curves corresponding to an array of blocks over the whole aperture of the LCoS-SLM. The experiment results show that the phase compensation precision could reach a peak-valley value of 0.061λ in wavefront and this method can be applied in generating freeform wave front for precise optical performance.

Small overall dimensions, intricate geometries, and discontinuous local surface normals characterize complex small workpieces. These features impose stringent requirements on the alignment accuracy of the workpieces when using a profilometer for three-dimensional surface measurement. This paper presents a pre-measurement method based on a reverse projection algorithm. By capturing shadow contours from multiple viewing angles, the three-dimensional pointcloud of the workpiece can be reconstructed. The reconstructed pointcloud is then used to analyze the workpiece pose and guide the path planning of a point-scanning profilometer. Experimental results show that, for a standard sphere with a radius of 12,703 mm, the measured results of the proposed measurement device achieve a fitted radius deviation of 1.8 μm when measuring 70% of the area of the spherical surface. This accuracy meets the precision requirement for guiding the path planning of the profilometer. Furthermore, the measured results from the device are employed to correct the scanning path of a five-axis profilometer for complex workpieces, such as cross-cylinder workpieces, without the need for manual pose adjustment or high-precision fixtures.

Germanium is widely used for infrared components and high-speed transistors. Ultra-precision single-point diamond turning (SPDT) can be employed to achieve its nanometric surface, while subsurface damage has a significant influence on surface integrity due to its brittle feature. This study investigated its subsurface deformation after SPDT. A characterization model of micro-Raman spectroscopy was established with the aim to characterize the subsurface deformation, including phase transformation and residual stress, where the transformation from single crystal structure to amorphous structure is dominant. A spectral fitting method was utilized to analyze the variation of subsurface deformation in various machining parameters. The degenerated single crystal Raman peak was widened due to anisotropic stress. It was discovered that the turning operation at a moderately low spindle speed and tool feed rate reduces phase transformation and residual stress based on the value of Raman ratio and Raman shift. These findings provided significant basis for the manufacturing process of optical components with good surface integrity.

The two peaks characteristic of yellow and blue light in the spectrum of dual-wavelength white light emitting diodes (LEDs) introduce distinctive features to the interference signal of white light scanning interferometry (WLSI). The distinctive features are defined as discontinuities, so that the fringe contrast function cannot be modeled as a single Gaussian function, and causes the interferogram to have uneven distribution of fringes of different orders in the scanning interferometer. This phenomenon leads to the low accuracy of the zero-order fringe position in the envelope calculation, which affects the repeatability and accuracy of the interferometry. This paper proposes a new surface recovery algorithm based on the Hilbert phase envelope and adjacent reference points calculation, which can effectively overcome the influence of the discontinuous signal of dual-wavelength LED white light interference on the three-dimensional reconstruction of WLSI measurements. The reliability of the algorithm is verified by experiments, and the measurement accuracy of LED WLSI system is evaluated.

To counteract accuracy degradation in micrometer-scale precision marking—where the precision marking (PM) system denotes the precision marking platform and the Optical Microscope (OM) system denotes the camera-based visual guidance module—a genetic-algorithm-based framework for motion-system calibration and error control is introduced. A kinematic error model is established to capture multi-source coupled errors in the PM system, and the propagation mechanisms of axis misalignment, pose misregistration, and flatness-induced errors are analyzed. Building on this model, a GA-driven multi-objective calibration scheme and a coordinated optimization model jointly address axis-orthogonality correction, PM-OM extrinsic-pose calibration, and workpiece flatness compensation. Furthermore, a dynamic error-compensation framework leveraging real-time monitoring and adaptive adjustment sustains long-term high-precision marking. In post-calibration tests-after correcting axis orthogonality, aligning the PM-OM extrinsic pose, and compensating workpiece flatness, the PM system achieves dimensional accuracies of ±0.05, ±0.08, and ±0.10 μm for nominal 1, 2, and 3 μm marks, respectively, with positional accuracy better than ±0.2 μm. Marking consistency improves markedly, and the indentation force closely matches the target mark size, validating the approach. These techniques provide both theoretical and practical support for the engineering deployment of PM systems and are significant for improving the quality and productivity of micrometer-scale precision marking.

Silicon (Si) and silicon carbide (SiC) are second- and third-generation semiconductor materials with excellent properties that are particularly suitable for applications in scenarios such as high temperature, high voltage, and high frequency. Si/SiC wafers face warpage and bending problems during production, which can seriously affect subsequent processing. Fast, accurate, and comprehensive detection of thickness, thickness variation, and flatness (including bow and warpage) of SiC and Si wafers is an industry-recognized challenge. Frequency scanning interferometry (FSI) can synchronize the upper and lower surfaces and thickness information of transparent parallel thin wafers, but it is still affected by multiple interfacial harmonic reflections, reflectivity asymmetry, and phase modulation uncertainty when measuring SiC thin wafers, which leads to thickness calculation errors and face reconstruction deviations. To this end, this paper proposes a high-precision facet reconstruction method for SiC/Si structures, which combines harmonic spectral domain decomposition, refractive index gradient constraints, and partitioning optimization strategy, and introduces interferometric signal “oversampling” and weighted fusion of multiple sets of data to effectively suppress higher-order harmonic interferences, and to enhance the accuracy of phase resolution. The multi-layer iterative optimization model further enhances the measurement accuracy and robustness of the system. The flatness measurement system constructed based on this method can realize the simultaneous acquisition of three-dimensional top and bottom surfaces on 6-inch Si/SiC wafers, and accurately reconstruct the key parameters, such as flatness, warpage, and thickness variation (TTV). A comparison with the Corning Tropel FlatMaster commercial system shows that this method has high consistency and good applicability.

Off-axis reflective optical systems find wide applications in various industries, but the related manufacturing issues have not been well considered in the design process. This paper proposed a design method for cylindrical reflective systems considering manufacturing constraints to facilitate ultra-precision raster milling. An appropriate index to evaluate manufacturing constraints is established. The optimization solution is implemented for the objective function composed of primary aberration coefficients with weights and constraint conditions of the structural configuration by introducing the genetic algorithm. The four-mirror initial structure with a good imaging quality and a special structural configuration is then obtained. The method’s feasibility is validated by designing an off-axis four-mirror afocal system with an entrance pupil diameter of 170 mm, a field of view of 3° × 3° and a compression ratio of five times. All mirrors in the system are designed to be distributed along a cylinder.

The microlens array is a key microstructure component in imaging systems, such as in light field cameras. A resolution of the light field camera can facilitate the use of a small-area microlens array. However, fabricating a large-area microlens array with a highly accurate and consistent surface finish is still difficult and high-cost. A low-cost manufacturing method is proposed in this study, where the mould core of the microlens array is fabricated with high precision and high uniformity by cylindrical ultra-precision diamond turning. The proposed tool path strategy is employed to achieve superior surface quality and avoid the dynamic vibration of an unsmooth path. Light field camera prototypes are developed using a commercial DSLR camera with a fabricated microlens array. The successful performance of the prototypes confirms that the proposed manufacturing method satisfies the application demands of large-area microlens arrays.

Transparent films are significant industrial components that are widely used in modern optics, microelectronics, optical engineering, and other related fields. There is an urgent need for the fast and stable thickness measurement of industrial films at the micron-grade. This paper built a miniaturized and low-cost film thickness measurement system based on confocal spectral imaging and the principle of thin-film spectral interference. The reflection interference spectrum was analyzed to extract the phase term introduced by the film thickness from the full spectrum information, where local spectral noise can be better corrected. An efficient and robust film thickness calculation algorithm was realized without any calibrating sample. The micron-grade thickness measurement system had an industrial property with a measurement range of up to 75 μm with a measurement uncertainty of 0.1 μm, presenting a good performance in single-layer film thickness measurement with high efficiency.