Electrostatic-Force-Modulated Microaspherical Lens for Optical Pickup Head

Abstract

This paper proposes a novel method for the modulation and fabrication of an aspherical microlens from a photocurable polymer for use in optical pickup heads. This novel modulation method can be employed not only to dynamically control the focal length and morphology of the microlens but also to fabricate aspherical lenses after UV curing of the photosensitive polymer. Forces in two dimensions provided by electrowetting and gradient electrostatic forces are applied to shape the polymer liquid from a hemispherical shape into a parabolic or a near- conical shape. Aspherical microlenses are designed based on constant optical-path-length theory and self-aligned by surface- tension forces. For morphology modulation, 2-D voltages are applied at the lower electrodes and between the upper and lower substrates, respectively, on a SU-8 spherical liquid-polymer droplet. The voltage applied at the lower electrodes provides an electrowetting effect on the polymer droplets, thereby reducing the contact angle of the spherical droplet, resulting in a desirable droplet height/baseline ratio for a desirable focal length. The voltage applied between the upper and lower substrates pulls up the droplet mostly at its center portion to form an aspherical shape that is close to a parabolic shape or even a conical shape. As a result, the Strehl ratio of the lens can be varied from 0.0076, as with a spherical shape, to 0.8362, as with a near-parabolic shape, and the focal-spot size can be reduced from 2.302 to 0.778 mum. The proposed method has been successfully implemented and has proven suitable for optical applications in which millimeter-diameter-size lenses with numerical apertures higher than 0.66 and focus-spot resolutions better than 0.8 mum are preferred.