Quantitative measurement of birefringence-induced astigmatism of optical pickup objective lenses

Birefringence in the plastic lenses of optical pickups (OPs) arises from pr eferential molecular orientation and internal stresses. This birefringence introduces astigmatic aberration to the transmitted wavefront when a polari zed light is incident on the lens. It is necessary to quantitatively measur e this birefringence-induced astigmatism (BIA) since it affects the aberrat ion of the focused spot on the disk which, in turn, influences the readout signals of the drive system. The measurement of this astigmatism is, howeve r, complicated by the fact that it couples with the astigmatism resulting f rom the deviation of the lens's shape, owing to fabrication errors of the m olding inserts, from its originally designed formula. This paper presents a method for analyzing these two components of astigmatism separately so tha t the BIA of OP lenses can be evaluated.