Xianlei Liu

Compensation strategy for machining optical freeform surfaces by the combined on- and off-machine measurement.

Freeform surface is promising to be the next generation optics, however it needs high form accuracy for excellent performance. The closed-loop of fabrication-measurement-compensation is necessary for the improvement of the form accuracy. It is difficult to do an off-machine measurement during the freeform machining because the remounting inaccuracy can result in significant form deviations. On the other side, on-machine measurement may hides the systematic errors of the machine because the measuring device is placed in situ on the machine. This study proposes a new compensation strategy based on the combination of on-machine and off-machine measurement. The freeform surface is measured in off-machine mode with nanometric accuracy, and the on-machine probe achieves accurate relative position between the workpiece and machine after remounting. The compensation cutting path is generated according to the calculated relative position and shape errors to avoid employing extra manual adjustment or highly accurate reference-feature fixture. Experimental results verified the effectiveness of the proposed method.

Highly efficient machining of non-circular freeform optics using fast tool servo assisted ultra-precision turning

Freeform optics has been regarded as the next generation of the optical components, especially those with non-circular apertures are playing an increasingly significant role in scanning field and specialized optical system. However, there still exist challenges to machine non-circular optical freeform surface. This paper is focused on highly efficiently generating freeform surfaces with optical surface quality by ultra-precision turning using a fast tool servo (FTS). A systematic strategy of machining smooth freeform surfaces with rectangular aperture is proposed in this paper. The contour of freeform optics is decomposed and assigned to the motions of slide and FTS back-and-forth. An optimized model is established for deriving the profile of the rotational component to cater for the capacity of FTS. Tool path reconstruction is carried out to generate a smooth tool trajectory and modified the contour to cater for the stroke of FTS. Simulation is adopted to analyze the machining property of a typical rectangular freeform surface. A rectangular freeform surface is efficiently machined via the proposed method, where a micron level profile error and nanometric finish in Ra are realized. Characteristics of reflection are analyzed via experiment and simulation. Prospects of such machining approach are discussed to provide guidance to future study.

High angular accuracy manufacture method of micro v-grooves based on tool alignment by on-machine measurement.

Micro v-groove has found wide applications in optical areas as one of the most important structures. However, its performance is significantly affected by its angular geometry accuracy. The diamond cutting has been commonly used as the fabrication method of micro v-groove, but it is still difficult to guarantee the cutting tool angle, which is limited by the measurement accuracy in the manufacture and mounting of the diamond tool. A cutting tool alignment method based on the on-machine measurement is proposed to improve the fabricated quality of the v-groove angle. An on-machine probe is employed to scan the v-groove geometrical deviation precisely. The system errors model, data processing algorithm and tool alignment methods are analyzed in details. Experimental results show that the measurement standard deviation within 0.01° can be achieved. Retro-reflection mirrors are fabricated and measured finally by the proposed method for verification.

Wavefront aberration metrology based on transmitted fringe deflectometry

A simple, precise, and innovative technique for wavefront aberration measurement is presented in this paper. This technique is based on transmitted fringe deflectometry (TFD) used to measure wavefront slope and wavefronts. The system measurement error when measuring wavefront aberration is analyzed, including the aberration of the reference wavefront caused by shape distribution and the tilt and decentration of the phase object. Based on this, the system error model is established, and the system error compensation method is also presented. In addition, the system parameters calibration method is discussed in detail. Simulation and experimental results show that the proposed TFD wavefront aberration measurement model can achieve high measurement accuracy.

Controllable fabrication of freeform optics

Freeform optics is the next–generation optics to be used widely in advanced optical systems. The approach of trial and error loops is always employed to achieve the required form accuracy. Controllable fabrication based on on–machining measurement is proposed in this study. The laser probe is mounted on an ultra–precision turning machine to measure the freeform in the spiral path. A unit decomposition algorithm (UDA) is proposed to supply the proper arrangement of measuring data for the filtering and composition. A modified robust weight function (MRWF) is also presented to improve the performance of the 3D robust Gaussian filtering (RGF). Simulation experiments and case studies are carried out to verify the validity of the controllable fabrication method.