Optical pickup device for detecting a tracking error of a main spot on an optical disk

An optical pickup device for detecting a tracking error, comprising:

• a pair of laser diodes for emitting laser beams having different wavelengths;
• a beam converting system for converting the light beams emitted by the laser diodes into a main beam and four side beams so that a main spot and first through fourth side spots are provided on an optical disk by the main beam and the side beams, wherein said first and said second side spots are at preceding positions to the main spot and said third and said fourth side spots are at following positions to the main spot, and wherein said first and said fourth side spots are symmetrical about the main spot and said second and said third side spots are symmetrical about the main spot;
• a photodetector having photodetecting elements for receiving reflection beams of said first through said fourth side spots and for outputting a first intensity difference signal related to the reflection beams of the first and fourth side spots and a second intensity difference signal related to the reflection beams of the second and third side spots;
• and
• a tracking error detector for outputting a tracking error signal based on a difference between the first intensity-difference signal related to the reflection beams of the first and fourth side spots and the second intensity-difference signal related to the reflection beams of the second and third side spots,
• the beam converting system including:
• a dichroic mirror for reflecting a laser beam from one of the pair of laser diodes and for transmitting a laser beam from the other of the pair of laser diodes to pass through the dichroic mirror;
• a grating for converting the laser beam from the dichroic mirror into five diffracted light beams;
• a relay lens for converting the diffracted light beams from the grating by a magnification factor adjusted in response to a wavelength of the emitted laser beam, said relay lens being moved along an optical axis of the relay lens in response to the wavelength of the emitted laser beam, thereby adjusting the magnification factor;
• a polarization beam splitter for converting the beams from the relay lens into linearly polarized light beams passing through the beam splitter, wherein said polarization beam splitter converts reflection beams, reflected from the disk to the polarization beam splitter, into lateral reflection beams;
• a quarter wave plate for converting the linearly polarized light beams from the polarization beam splitter into circularly polarized light beams, wherein said quarter wave plate converts the circularly polarized light beams, reflected from the disk to the quarter wave plate, into linearly polarized light beams;
• an objective lens for directing the circularly polarized light beams from the quarter wave plate to a track of the optical disk;
• and
• a focusing lens for directing the lateral reflection beams from the polarization beam splitter to the photodetector.