Patent ID: 7233562

Claim:
An optical pickup device which selectively records or reproduces a first optical recording medium, a second optical recording medium having a recording density lower than that of the first optical recording medium, and a third optical recording medium having a recording density lower than that of the second optical recording medium, the device comprising: a first laser light source which emits a first laser light for the first optical recording medium; a second laser light source which emits a second laser light having a wavelength longer than that of the first laser light for the second optical recording medium; a third laser light source which emits a third laser light having a wavelength longer than that of the second laser light for the third optical recording medium; and an objective lens whose numerical aperture (NA) is set to 0.75 or more for the first optical recording medium and in which at least one of first and second surfaces backing on to each other is formed as an aspheric surface and which converges the first to third laser lights onto signal surfaces of the first to third optical recording mediums, the objective lens satisfying the following expression: t<a·f+b wherein t: an axial thickness of the objective lens, f: a focal distance of the objective lens, a: a coefficient, and b: a constant, and the coefficient a and the constant b satisfies that a=2.02, b=−1.94; a=2.08, b=−1.84; or a=2.11, b=−1.77.