Patent ID: 7411884

Claim:
An optical system for an optical disc drive, comprising: a light source that emits first and second light beams, said first and second light beams utilized for recording and/or reproducing data to/from first and second optical discs, respectively, the second optical disc having a thicker protective layer and lower recording density than the first optical disc; an objective lens provided with a diffraction structure, said diffraction structure being designed to focus said first light beam on a recording layer of the first optical disc and said second light beam on a recording layer of the second optical disc; and a collimator lens disposed between said light source and said objective lens to adjust diverging/converging angle of said first and second light beams entering said objective lens, wherein change in spherical aberration of said first light beam caused by wavelength deviation from a design wavelength due to individual specificity of said light source is corrected by adjusting the diverging/converging angle of said first light beam emerging from said collimator lens, wherein said collimator lens is located between first and second optimum positions, the spherical aberration of said first light beam converged onto the recording layer of the first optical disc being minimized when said collimator lens is located at said first optimum position, the spherical aberration of said second light beam converged onto the recording layer of the second optical disc being minimized when said collimator lens is located at said second optimum position, wherein the collimator lens is fixed at a predetermined location during assembly of the optical systems, wherein said diffraction structure is designed so that change in spherical aberration caused by said objective lens in accordance with wavelength variation of said first light beam is generated mainly by third-order spherical aberration, and wherein said diffraction structure is designed so that change in fifth or higher order component of the spherical aberration caused by said objective lens in accordance with wavelength variation of said first light beam is less than 0.0005 λrms/nm.