Patent ID: 7403454

Claim:
An optical head for carrying out at least one of recording and reproduction of an information signal with respect to an optical recording medium including a light transmission layer on a recording portion having at least two recording layers where the information signal is recorded, the optical head comprising: a light source for emitting a beam; converging means for converging the beam onto the recording layer of the optical recording medium; light detecting means for detecting a reflected beam converged onto the recording layer of the optical recording medium by the converging means and reflected by the recording layer; and aberration correcting means provided on an optical path extending from the light source to the converging means for controlling, by an arbitrary pattern, spherical aberration and defocus of the beam converged onto the recording layer of the optical recording medium, wherein when the radius of beam spot of the beam converted onto the recording layer is assumed to be r, and variables different from each other are assumed to be A and B so that B/A=K is provided, the aberration correcting means allows the beam to generate phase distribution indicated by the following phase distribution formula: A ⁢ ( - r 4 ) - B ⁡ ( - r 2 ) = ⁢ A ⁢ { ( - r 4 ) - B / A ⁡ ( - r 2 ) } = ⁢ A ⁢ { ( - r 4 ) - K ⁢ ( - r 2 ) } where either a value of K is caused to be a value which cancels change of the optimum value of focus bias produced by switching of selection of recording layer of two recording layers or more in the optical recording medium, or plural values are set in advance in correspondence with two recording layers or more in the optical recording medium as value of K, and value of K corresponding to a selected recording layer is selected and is used.