US Patent 7898919 Optical pickup apparatus

An optical pickup apparatus reading signal recorded on signal recording layer of first and second optical discs different in distance from incident surface of laser light to the layer, comprising: an objective lens to be changed in numerical aperture for reading signal from the discs; a focus coil to displace the lens in direction toward surfaces of the discs; a tracking coil to displace the lens in radial direction of the discs; a tilt coil to correct optical-axis angle of the lens to the surfaces; a numerical aperture changing element to change the numerical aperture so that the light is focused on the layers of the discs; an incident angle changing element to change incident angle of the light to the lens so that the light is focused on the layers; and a tilt control unit to control driving of the tilt coil to correct coma aberration.