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Various types of pick-up lenses and optical systems for pick-ups are discussed. We evaluated optical performances of objective lenses. Aspherical lenses for CD player pick-ups have wider field angle compared to the conventional triplet objective lens. We also evaluated reliability of both plastic and glass molded lenses which are designed in the same way. Plastic lens caused spherical aberration due to temperature change. Optical pick-ups for CD players are remarkably simplified by means of aspherical lens or holographic element. On the other hand, pick-ups for write once or erasable optical disk system consist of many optical components. For these pick-ups, we developed a single bi-aspheric molded glass lens which has high NA (0.55) and remarkable light weight (85mg).

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We have developed a bi-aspheric lens, intended for a compact disc (CD) player pick-up, which is manufactured by a high precision glass molding process and has been mass-produced. The lens realizes a single-lens system for a CD player pickup, that has function of both an objective lens and a collimator lens, while conventional system use 4 or 5 spherical lenses. The lens has a small size (5.0 mm diameter X 4.2 mm thickness) and has weight of only 300 mg. With the small lens we have also developed a very small size CD player pick-up, having a height of 13.3 mm, much shorter than 18.5 mm height of our former one. The off-axis wave-front aberration at an image height of 150 microns has been designed to be 0.035 wave rms. Such performance should be considered excellent, where a relative local shift between a laser diode and a lens is avoidable due to a tracking, a focusing and a set up error. The observed on-axis transmitted wave-front aberrations of the molded lenses are less than 0.03 wave rms.