Optical pickup device with three beam tracking servo control using oval beams

An optical pickup device using a three-beam tracking servo system for writing or reading data from an optical information recording medium having a recording layer which changes from an original state to another state in response to a density of intensity of an irradiated light beam and a track being used for the tracking servo system, comprising:

• a light source for emitting a single light beam;
• an irradiating optical system including:
• an irregular interval diffraction grating for dividing said light beam into one writing or recording light beam and two controlling light beams for said tracking servo system, and
• an objective lens for converging said writing or recording light beam onto said recording layer to form a writing or reading spot having a first luminous flux density and, at the same time, said two controlling light beams to form two controlling spots which are radially displaced from said writing or reading spot on the recording layer by fractional track pitches,
• said irregular interval diffraction grating being formed of a large number of extremely narrow different parallel slits separated by opaque spaces of different widths so that astigmatism occurs resulting in elliptic spots appearing as said two controlling spots on the recording layer, characterized in that said irregular interval diffraction grating has a slit pattern symmetrical with respect to a central slit or space so that said two controlling spots are displaced symmetrically with said writing or reading spot, each of said controlling spots expanding in a track extending direction having a second luminous flux density lower than said first luminous flux density within a level which does not affect said original state of said recording layer;
• said objective lens collecting reflected lights from the recording medium;
• and
• an optical detecting means for receiving the reflected lights from said writing or reading spot and said controlling spots on the recording medium .