Birefringence in the plastic lenses of optical pickups (OPs) arises from preferential molecular orientation and internal stresses. This birefringence introduces astigmatic aberration to the transmitted wavefront when a polarized light is incident on the lens. It is necessary to quantitatively measure this birefringence-induced astigmatism (BIA) since it affects the aberration of the focused spot on the disk which, in turn, influences the readout signals of the drive system. The measurement of this astigmatism is, however, complicated by the fact that it couples with the astigmatism resulting from the deviation of the lens’s shape, owing to fabrication errors of the molding inserts, from its originally designed formula. This paper presents a method for analyzing these two components of astigmatism separately so that the BIA of OP lenses can be evaluated.
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