Position alignment of laser beam spots in optical disc drives using cross-correlation

Optical pickup unit (OPU) is a data read/write head in optical disc drives, and precision orientation angle adjustment of a diffraction grating (GT) is an important process in OPU manufacturing for aligning laser beam spots on the same data track of an optical disc. This paper proposes a new method for detecting alignment of laser beam spots for utilization in automation of the GT adjustment process. While a conventional method uses complicated and expensive image processing techniques to an eye diagram of a data radio frequency (RF) signal, the proposed method measures the cross-correlation of two binary signals corresponding to two of the laser beam spots in consideration of the geometric relationship between the laser beam spots and data tracks. The proposed method is not only so simple as to be implemented using a digital logic circuit but also tolerant to poor signal condition due to radial off-Track run-out and disc rotational speed error. A FPGA-based implementation of the proposed method is presented, and experimental results are provided to show its validity¹.