Objective lens design method, lens, and optical system, optical head, and optical disc apparatus using the same

An objective lens for plural types of optical storage medium having transparent substrates of different thickness, the objective lens receiving a light beam of a different wavelength for each of the plural types of optical storage medium and having a positive power to condense, by refraction, the light beam onto an information surface on each of the transparent substrates of the plural types of optical storage medium;

• wherein the objective lens substantially cancels out spherical aberration caused by a difference in thickness between the transparent substrates with chromatic aberration caused by a difference in wavelength λ of each of the light beams, by which the objective lens condenses the light beam onto the information surface with a Root Mean Square (RMS) wavefront aberration of no more than 0.035λ for each of the plural types of optical storage medium.

An objective lens for plural types of optical storage medium having transparent substrates of different thickness, the objective lens receiving a light beam of a different wavelength for each of the plural types of optical storage medium and having a positive power to condense, by refraction, the light beam onto an information surface on each of the transparent substrates of the plural types of optical storage medium;

• wherein the objective lens substantially cancels out spherical aberration caused by a difference in thickness between the transparent substrates with chromatic aberration caused by a difference in wavelength λ of each of the light beams, by which the objective lens condenses the light beam onto the information surface with a RMS wavefront aberration satisfying a following formula:
• {square root}{square root over ((ΣWi2)/i )}≦0.028 Formula 1:
• (λi(i=1, 2, . . . ) is a wavelength of the i-th light beam, ΣWi2 is sum of square of each RMS wavefront aberration for all wavelengths, and Wi·λi is a RMS wavefront aberration of light beam having wavelength λi), for each of the plural types of optical storage medium.

An objective lens for plural types of optical storage medium having transparent substrates of different thickness, the objective lens receiving a light beam of a different wavelength for each of the plural types of optical storage medium and having a positive power to condense, by refraction, the light beam onto an information surface on each of the transparent substrates of the plural types of optical storage medium;

• wherein the objective lens substantially cancels out spherical aberration caused by a thickness difference between the transparent substrates with chromatic aberration caused by a difference in wavelength of each of the light beams, by which the objective lens condenses the light beam onto the information surface with a RMS wavefront aberration ratio of
• 1≦Wmax/Wmin<1.8
• where Wmax is a maximum RMS wavefront aberration among RMS wavefront aberrations of each of the light beams and Wmin is a minimum RMS wavefront aberration among RMS wavefront aberrations of each of the light beams, for each of the plural types of optical storage medium.

An objective lens for plural types of optical storage medium having transparent substrates of different thickness, the objective lens receiving a light beam of a different wavelength for each of the plural types of optical storage medium and having a positive power to condense, by refraction, the light beam onto an information surface on each of the transparent substrates of the plural types of optical storage medium;

• wherein the lens substantially cancels out spherical aberration caused by a difference in thickness between the transparent substrates with chromatic aberration caused by a difference in wavelength λ of each of the light beams, and
• the lens has different focal point positions respectively for each wavelength and each storage medium.