Optical pickup apparatus

An optical pickup apparatus comprising:

• a three-wavelength laser diode including
• a first laser element configured to emit a first laser beam having a first wavelength for reading a signal recorded on a signal recording layer of a first optical disc;
• a second laser element configured to emit a second laser beam having a second wavelength, longer than the first wavelength, for reading a signal recorded on a signal recording layer of a second optical disc of a standard different from a standard of the first optical disc;
• and
• a third laser element configured to emit a third laser beam having a third wavelength, longer than the second wavelength, for reading a signal recorded on a signal recording layer of a third optical disc of a standard different from the standards of the first optical disc and the second optical disc,
• the first, second, and third laser elements housed in the same case;
• an objective lens on which the first to third laser beams are incident through the same optical path, the objective lens configured to focus the first to third laser beams onto the signal recording layers of the first to third optical discs, respectively;
• a first photodetector configured to be irradiated with first return light reflected from the signal recording layer of the first optical disc,
• a second photodetector configured to be irradiated with second return light and third return light respectively reflected from the signal recording layers of the second optical disc and the third optical disc;
• a polarizing beam splitter configured to reflect the first to third laser beams in a direction of the objective lens so that the first to third laser beams are incident on the objective lens through the same optical path, and to allow the first return light to the third return light having passed through the objective lens to pass therethrough;
• a semitransparent mirror on which the first return light to the third return light having passed through the polarizing beam splitter are incident, the semitransparent mirror configured to guide the first return light in a direction of the first photodetector, and to guide the second return light and the third return light in a direction of the second photodetector, and
• an astigmatism plate provided between the semitransparent mirror and the second photodetector, the astigmatism plate configured such that a spacing between the second return light and the third return light after passing through the semitransparent mirror and the astigmatism plate is the same as a spacing between the second return light and the third return light before passing through the semitransparent mirror.