On-machine measurements can improve the form accuracy of optical surfaces in single-point diamond turning applications; however, commercially available linear variable differential transformer sensors are inaccurate and can potentially scratch the surface. We present an on-machine measurement system based on capacitive displacement sensors for high-precision optical surfaces. In the proposed system, a position-trigger method of measurement was developed to ensure strict correspondence between the measurement points and the measurement data with no intervening time-delay. In addition, a double-sensor measurement was proposed to reduce the electric signal noise during spindle rotation. Using the proposed system, the repeatability of 80-nm peak-to-valley (PV) and 8-nm root-mean-square (RMS) was achieved through analyzing four successive measurement results. The accuracy of 109-nm PV and 14-nm RMS was obtained by comparing with the interferometer measurement result. An aluminum spherical mirror with a diameter of 300 mm was fabricated, and the resulting measured form error after one compensation cut was decreased to 254 nm in PV and 52 nm in RMS. These results confirm that the measurements of the surface form errors were successfully used to modify the cutting tool path during the compensation cut, thereby ensuring that the diamond turning process was more deterministic. In addition, the results show that the noise level was significantly reduced with the reference sensor even under a high rotational speed.
Diamond turning of high-precision molds is a vital process for the roll-to-roll-based ultraviolet resin imprinting process in fabricating subwavelength gratings. The effects of the grating shape and grating period on diffraction efficiencies and diffraction angles were simulated. Experiments were then conducted to examine the effects of shape design, grating period, and cutting speed on machinability of the mold. According to the optical measurement results, the performance of the subwavelength gratings matched the design well at various incident angles. The results confirm that diamond turning of high-precision molds is a feasible approach for ensuring the continual mass production of subwavelength gratings.
Glass molding process has emerged as a promising way to produce complex optical elements with high precision. Glass material shows explicit viscoelasticity at molding temperature, therefore studying the viscoelastic properties of glass at elevated temperature is important for the molding process. In this paper, Young's modulus and viscosity of glass were tested by compressing cylindrical glass gobs above the transition temperature, and obtained by curve fitting using the Burgers model of viscoelastic deformation. Based on the viscoelastic parameters obtained from experiments, a numerical model was developed to simulate the glass molding process with the help of a commercial finite element method software package. The simulation results provide an easy way to analyze and understand the molding process in detail, such as temperature distribution, stress, strain and strain rate, which are difficult or impossible to measure in experiments. A good agreement between the calculated pressing loads and the experimental results verified the validity of the numerical model, which can be applied to predict the pressing load during complex-shape glass lens molding process.
This note summaries some fundamental aspects on single-point diamond turning technology of brittle crystalline materials for infrared optical applications. The ultra-precision machining mechanism is discussed concerning material property, tool geometry and machining parameters. Some latest results on the fabrication of aspherical optics on brittle materials are also presented.
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