Patent ID: 8050168

Claim:
An optical head device, comprising: a plurality of light sources switchably usable; an objective lens for converging light emitted from one of the plurality of light sources to an information recording layer of an optical disc; a light detector for receiving the light reflected by the information recording layer and outputting an electric signal based on the amount of the received light; and an optical element located on an optical path on which the light reflected by the information recording layer proceeds until being incident on the light detector, the optical element giving astigmatism to the light transmitting therethrough; wherein: the plurality of light sources include a first light source for emitting light having a first wavelength and a second light source for emitting light having a second wavelength shorter than the first wavelength; a defocus detection range of a focusing error signal obtained from an electric signal based on the amount of received light having the first wavelength is wider than a defocus detection range of a focusing error signal obtained from an electric signal based on the amount of received light having the second wavelength; the light having the first wavelength reflected by the information recording layer and the light having the second wavelength reflected from the information recording layer are both incident on the light detector; the light detector performs photoelectric conversion on the incident light to generate an electric signal for obtaining a focusing error signal; and the light detector receives the light reflected by the information recording layer and given the astigmatism to generate the focusing error signal by an astigmatism method.