Analysis of beam spot deviation considering vibration of components in optical pickup

Abstract

An optical disc drive uses an optical pickup to read and write data by focusing a laser beam on the data plane of a rotating disc. The pickup is required to keep the beam spot on the current track. The latest trend of replacing metal pickup housing with plastic easily excites elastic deformation modes of the pickup housing. Optical components mounted on the housing are vibrated at the mode frequencies of the housing as well. These vibrations tilt the incident angle of a laser beam entering the objective lens and results in the displacement of the beam spot on the data plane. Our purpose in this paper is to improve the accuracy of a vibration model for optical pickups by considering the effect of the elastic deformation modes of the housing in addition to the commonly used mechanical dynamics of an objective lens actuator in order to predict stability in a tracking servo system. The effect of the elastic deformation modes of the housing was formularized as the incident angle deviation and was calculated by adding and subtracting the products of a ray transfer matrix chain representing the transfer from each optical component to the objective lens with its small displacement and tilt vector. This new vibration model reproduced an increased gain around a frequency of 20 kHz in the measured open-loop transfer function of the tracking servo system with 4% error in frequency, which was entirely unexpected in the context of existing vibration models for optical pickups. The calculated results also identified the most influential vibration modes of the housing and estimated the effectiveness of stiffening the housing with 17% error.