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An electromagnetic objective lens actuator for optical disc drives has been developed in order to reduce higherorder resonance amplitude in frequency response characteristics of the actuator. The actuator has a configuration in which each set of three adjacent magnets is located to oppose each of the two sides of the lens holder and each polarization of the center magnet is set in reverse to those of the two outside magnets. The driving force in the positive focusing direction is generated at the focusing coil portions opposing the two outside magnets and the driving force in the negative focusing direction is generated at the focusing coil portions opposing the center magnet. This driving force distribution cancels the bending deformation of the vibration mode at the higher-order resonance. The resulting amplitude of the higher-order resonance in the focusing direction of the developed actuator with this three-polar magnetic structure for 9.5 mm-height Blu-ray Disc drives was successfully reduced to 2 dB or under.

Our previously proposed method for arranging adhesive joints for an optical assembly was adopted to resist long-term exposure to severe environments. This method is expected to passively compensate for permanent optical misalignment due to long-term exposure to hot and humid conditions by arranging the bonding direction and position of each optical component in order to tend to displace or tilt a desired amount in a desired direction over time. We calculated the beam spot deviation on a detector, which is an indicator of misalignment of the optical assembly due to heat, humidity, and time effects on adhesive joints. We arranged to adhesively bond the laser diode to the holder from the top so that the laser diode could be lifted up due to the moisture expansion of the adhesive joint over time. The developed adhesive joints arrangement had a great advantage in that upward displacement of the laser diode compensates for beam spot deviation caused by tilt of the prism and displacement of the detector along the warp deformation of the housing. The calculated and measured results showed that the beam spot deviation was within about ±5 % after the optical pickup was left at 60 °C and 90 %RH for 336 hours. It is concluded that our method for arranging adhesive joints of each component effectively compensates for passively permanent optical misalignment that occurs after long-term exposure to hot and humid conditions.

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