Objective lens having diffractive structure for optical pick-up

An objective lens for an optical pick-up that converges light beams of at least two different wavelengths onto recording layers of optical discs of at least two different recording densities, respectively, said objective lens comprising:

• a refractive lens having a positive power;
• and
• a diffractive lens structure having a plurality of concentric ring areas with minute steps at the boundaries therebetween, said diffractive lens structure being formed on at least one surface of said refractive lens to correct chromatic aberration of said refractive lens,
• wherein said diffractive lens structure is designed such that the diffraction order where the diffraction efficiency is maximized at the working wavelength of said optical disc having high recording density is higher than the diffraction order where the diffraction efficiency is maximized at the working wavelength of said optical disc having low recording density,
• wherein said diffractive lens structure maximizes the diffraction efficiency of the third order diffracted light at the working wavelength of said optical disc having high recording density and said diffractive lens structure maximizes the diffraction efficiency of the second order diffracted light at the working wavelength of said optical disc having low recording density.

An objective lens for an optical pick-up that converges light beams of at least three different wavelengths onto recording layers of optical discs of at least three different recording densities, respectively, said objective lens comprising:

• a refractive lens having a positive power;
• and
• a diffractive lens structure having a plurality of concentric ring areas with minute steps at the boundaries therebetween, said diffractive lens structure being formed on at least one surface of said refractive lens,
• wherein said diffractive lens structure maximizes the diffraction efficiency of the third order diffracted light at the working wavelength of said optical disc having the highest recording density and said diffractive lens structure maximizes the diffraction efficiency of the second order diffracted light at the working wavelengths of said optical discs having lower recording densities.