A novel evaluation and compensation method for ultra-precision machining of hybrid lens

In this study, a diffractive-refractive hybrid objective lens (DHOL) compatible with multi-type optical discs (Blu-ray Disc (BD), China Blue High-Definition Disc (CBHD), DVD and CD) was designed and fabricated. The surface of the DHOL was an aspherical (refractive) substrate superimposed on a diffractive sub-structure, and by using ultra-precision single point diamond turning, the form error was reduced to be less than λ/4. However, because of the finite dimensions of the tool radius (R), the pitch of the diffractive structure must be larger than R. This limits the performance of the DHOL and has a bigger effect on the diffractive performance than on the refractive performance. To counteract the limitation, a novel refractive compensation method is proposed. Through superimposing a small refractive curvature, a significant improvement in the diffractive performance was achieved. By reconstructing the wavefront aberration using Zernike expansion and taking the horizontal rotation angle, tool radius and usable cutting edge into account, an optimal design was determined. The optimal design was fabricated and found to perform satisfactorily.