Optical head device and objective lens

1. An optical head device that converges first and second laser beams having different wavelengths through a single light converging optical system, the optical head device comprising:

an objective lens composing a part of the light converging optical system, the objective lens including a refracting surface divided into a central refracting surface region with an optical axis thereof being as a center and an outer circumferential refracting surface region that surrounds an outer circumference of the central refracting surface region, and a diffraction grating formed generally entirely over the central refracting surface region, and

a second diffraction grating formed over an entire area of the outer circumferential refracting surface region to diffract the first laser beam so as not to converge on a beam spot forming position of the beam spot, wherein the objective lens forms a beam spot of the second laser beam with the 1 st order diffracted beam obtained by the central refracting surface region and a 0 th order diffracted beam obtained by the outer circumferential refracting surface region,

wherein the objective lens forms a beam spot of the first laser beam at the first focal distance with a 0 th order diffracted beam or a 1 st order diffracted beam obtained by the diffraction grating of the central refracting surface region, and the objective lens forms a beam spot of the second laser beam at the second is focal distance with a beam portion that passes the outer circumferential refracting surface region and a 1 st order diffracted beam obtained by the diffraction grating in the central refracting surface region, and

wherein the beam spot of the first laser beam is formed at the first focal distance with the 0 th order diffracted beam obtained by the diffraction grating of the central refracting surface region.

2. An optical head device according to claim 1 , wherein the central refracting surface region has a refractive power that generally forms a beam spot of the first laser beam at the first focal distance, and the outer circumferential refracting surface region has a refractive power that generally forms a beam spot of the second laser beam at the second focal distance.

3. An optical head device according to claim 1 , wherein the beam spot of the first laser beam is formed at the first focal distance with the 1 st diffracted beam of the first laser beam obtained by the diffraction grating in the central refracting surface region.

4. An optical head device according to claim 3 , wherein the outer circumferential refracting surface region has a refractive power that generally forms a beam spot of the second laser beam at the second focal distance.

5. An optical head device that converges first and second laser beams having different wavelengths through a single light converging optical system, the optical head device comprising:

an objective lens composing a part of the light converging optical system, the objective lens including a refracting surface divided into a central refracting surface region with an optical axis thereof being as a center and an outer circumferential refracting surface region that surrounds an outer circumference of the central refracting surface region, and a diffraction grating formed generally entirely over the central refracting surface region, and

an outer circumferential diffraction grating formed over an entire area of the outer circumferential refracting surface region, wherein the objective lens forms a beam spot of the second laser beam at the second focal distance with the 1 st order diffracted beam obtained by the central refracting surface region and a 0 th order diffracted beam obtained by the outer circumferential refracting surface region,

wherein the beam spot of the first laser beam is formed at the first focal distance with the 1 st diffracted beam of the first laser beam obtained by the diffraction grating in the central refracting surface region, and

wherein the outer circumferential refracting surface region has a refractive power that generally forms a beam spot of the second laser beam at the second focal distance, and

wherein the objective lens forms a beam spot of the first laser beam at the first focal distance with a 0 th order diffracted beam or a 1 st order diffracted beam obtained by the diffraction grating of the central refracting surface region, and the objective lens forms a beam spot of the second laser beam at the second is focal distance with a beam portion that passes the outer circumferential refracting surface region and a 1 st order diffracted beam obtained by the diffraction grating in the central refracting surface region.

6. An optical head device that converges first and second laser beams having different wavelengths through a single light converging optical system, the optical head device comprising:

an objective lens composing a part of the light converging optical system, the objective lens including a refracting surface divided into a central refracting surface region with an optical axis thereof being as a center and an outer circumferential refracting surface region that surrounds an outer circumference of the central refracting surface region, and a diffraction grating formed generally entirely over the central refracting surface region, and

an outer circumferential diffraction grating formed over an entire area of the outer circumferential refracting surface region, wherein the objective lens forms a beam spot of the second laser beam at the second focal distance with the 1 st order diffracted beam obtained by the central refracting surface region and a 1 st order diffracted beam obtained by the outer circumferential refracting surface region,

wherein the beam spot of the first laser beam is formed at the first focal distance with the 1 st diffracted beam of the first laser beam obtained by the diffraction grating in the central refracting surface region, and

wherein the objective lens forms a beam spot of the first laser beam at the first focal distance with a 0 th order diffracted beam or a 1 st order diffracted beam obtained by the diffraction grating of the central refracting surface region, and the objective lens forms a beam spot of the second laser beam at the second is focal distance with a beam portion that passes the outer circumferential refracting surface region and a 1 st order diffracted beam obtained by the diffraction grating in the central refracting surface region.

7. An optical head device that converges first and second laser beams having different wavelengths through a single light converging optical system, the optical head device comprising:

an objective lens refracting surface composing at least a part of the light converging optical system, the objective lens refracting surface including a plurality of ring-band shape refracting surfaces concentrically arranged about an optical axis thereof, and step differences formed in a direction of the optical axis at boundaries of the plurality of ring and shape refracting surfaces,

wherein each of the plurality of ring-band shape refracting surfaces has a refractive power that generally forms a beam spot of the second laser beam at the second focal distance,

the step differences of the ring-band shape refracting surfaces formed in a central region of the objective lens refracting surface corresponding to an aperture number set for the first focal distance are set such that a beam spot of the first laser beam is formed at the first focal distance, and

a diffraction grating is formed on at least one of the ring-band shape refracting surfaces formed in an outer circumference region that surrounds the central region of the objective lens refracting surface,

wherein the diffraction grating has a diffraction property that does not allow beam components of the first laser beam that pass the outer circumference region to converge at the first focal distance.

8. An optical head device according to claim 7 , wherein each of the step difference and a wavelength .mu.2 of the second laser beam have a relation generally defined by Aλ2/(n−1), where A is a positive integer, and n is an index of refraction of the ring-band shape refracting surface.

9. An optical head device according to claim 8 , wherein the step difference is defined by λ2/(n−1).

10. An objective lens that converges first beam and second laser beams having different wavelengths, the objective lens comprising:

a refracting surface divided into a central refracting surface region with an optical axis thereof being as a center, and an outer circumferential refracting surface region that surrounds an outer circumference of the central refracting surface region; and

a diffraction grating formed generally entirely over the central refracting surface region,

wherein the objective lens forms a beam spot of the first laser beam at the first focal distance with a 0 th order diffracted beam or a 1 st order diffracted beam obtained by the diffraction grating of the central refracting surface region, and the objective lens forms a beam spot of the second laser beam at the second focal distance with a beam portion that passes the outer circumferential refracting surface region and a 1 st order diffracted beam obtained by the diffraction grating in the central refracting surface region, wherein the objective lens forms a beam spot of the second laser beam with the 1 st order diffracted beam obtained by the central refracting surface region and a 0 th order diffracted beam obtained by the outer circumferential refracting surface region, and

wherein the objective lens forms a beam spot of the second laser beam at the second focal distance with the 1 st order diffracted beam obtained by the central refracting surface region and a 0 th order diffracted beam obtained by the outer circumferential refracting surface region.

11. An objective lens that converges first beam and second laser beams having different wavelengths, the objective lens comprising:

a plurality of ring-band shape refracting surfaces concentrically arranged about an optical axis thereof, and step differences formed in a direction of the optical axis at boundaries of the plurality of ring-band shape refracting surfaces,

wherein each of the plurality of ring-band shape refracting surfaces has a refractive power that generally forms a beam spot of the second laser beam at the second focal distance,

the step differences of the ring-band shape refracting surfaces formed in a central region of the objective lens refracting surface corresponding to an aperture number set for the first focal distance are set such that a beam spot of the first laser beam is formed at the first focal distance, and

a diffraction grating is formed on at least one of the ring-band shape refracting surfaces formed in an outer circumference region that surrounds the central region of the objective lens refracting surface,

wherein the diffraction grating has a diffraction property that does not allow beam components of the first laser beam that pass the outer circumference region to converge at the first focal distance.