Optical head device with diffraction grating for separating a light beam incident on an optical recording medium from a light beam reflected therefrom

An optical head device comprising:

• a light source;
• condensing means for condensing a light beam emitted from said light source on an optical recording medium;
• a diffraction grating arranged in the optical path of an incident light beam entering the recording medium from said light source to transmit the incident light beam through said diffraction grating and to diffract light reflected from the recording medium for separating the reflected light from the incident light beam;
• and
• a photodetector for receiving the light diffracted by said diffraction grating, said photodetector including a light receiving surface which is divided into a plurality of areas by at least a dividing line extending substantially in a direction in which a spot of the light diffracted by said diffraction grating moves on said light receiving surface due to the wavelength fluctuation of said light source.