Diffraction grating, aberration correction element and optical head device

1. A diffraction grating, comprising:

a convex portion; and

a concave portion,

wherein the convex portion and the concave portion function with lights having different wavelengths λ 1 , λ 2 , and λ 3 ,

wherein the convex portion and the concave portion are alternately disposed on at least one surface of a substrate with a predetermined pitch,

wherein an average refractive index of the convex portion is smaller than an average refractive index of the concave portion, and

when phase differences of the lights having the wavelengths λ 1 , λ 2 , and λ 3 , which are transmitted through the convex portion and the concave portion are, respectively, a 1 π, a 2 π, and a 3 π,

0≦| a 1 −a 2 |0.2,

0.2<| a 3 −a 1 |, and

0.2 <|a 3 −a 2 |,

are satisfied.

2. The diffraction grating of claim 1 , wherein the convex portion comprises SiO 2 as a main component; and

wherein the concave portion comprises TiO 2 , Nb 2 O 5 , or Ta 2 O 5 as a main component.

3. An optical head device, comprising:

a light source which emits lights having three different types of wavelengths;

a diffraction grating of claim 2 ;

an objective lens which focuses the lights emitted from the light source onto an optical recording medium; and

a detector which detects the emitted lights reflected from the optical recording medium,

wherein the diffraction grating is disposed in an optical path between the light source and the objective lens.

4. The diffraction grating of claim 2 , wherein the concave portion comprises TiO 2 as the main component.

5. The diffraction grating of claim 2 , wherein the concave portion comprises Nb 2 O 5 as the main component.

6. The diffraction grating of claim 2 , wherein the concave portion comprises Ta 2 O 5 as the main component.

7. The diffraction grating of claim 1 , wherein the convex portion is a single-layer film comprising SiO 2 as a main component; and

wherein the concave portion is a multi-layer film in which

a first layer comprising SiO 2 as a main component and

a second layer comprising TiO 2 , Nb 2 O 5 , or Ta 2 O 5 as a main component,

are alternately stacked.

8. The diffraction grating of claim 7 , wherein the multi-layer film is an antireflection film.

9. An optical head device, comprising:

a light source which emits lights having three different types of wavelengths;

a diffraction grating of claim 8 ;

an objective lens which focuses the lights emitted from the light source onto an optical recording medium; and

a detector which detects the emitted lights reflected from the optical recording medium,

wherein the diffraction grating is disposed in an optical path between the light source and the objective lens.

10. An optical head device, comprising:

a light source which emits lights having three different types of wavelengths;

a diffraction grating of claim 7 ;

an objective lens which focuses the lights emitted from the light source onto an optical recording medium; and

a detector which detects the emitted lights reflected from the optical recording medium,

wherein the diffraction grating is disposed in an optical path between the light source and the objective lens.

11. The diffraction grating of claim 7 , wherein the second layer comprises TiO 2 as the main component.

12. The diffraction grating of claim 11 , wherein the multi-layer film is an antireflection film.

13. The diffraction grating of claim 7 , wherein the second layer comprises Nb 2 O 5 as the main component.

14. The diffraction grating of claim 13 , wherein the multi-layer film is an antireflection film.

15. The diffraction grating of claim 7 , wherein the second layer comprises Ta 2 O 5 as the main component.

16. The diffraction grating of claim 15 , wherein the multi-layer film is an antireflection film.

17. The diffraction grating of claim 1 , wherein the convex portion and the concave portion are each a multi-layer film in which

a first layer comprising SiO 2 as a main component and

a second layer comprising TiO 2 , Nb 2 O 5 , or Ta 2 O 5 as a main component,

are alternately stacked.

18. An optical head device, comprising:

a light source which emits lights having three different types of wavelengths;

a diffraction grating of claim 17 ;

an objective lens which focuses the lights emitted from the light source onto an optical recording medium; and

a detector which detects the emitted lights reflected from the optical recording medium,

wherein the diffraction grating is disposed in an optical path between the light source and the objective lens.

19. The diffraction grating of claim 17 , wherein the second layer of at least one of the concave portion and the convex portion comprises TiO 2 as the main component.

20. The diffraction grating of claim 17 , wherein the second layer of at least one of the concave portion and the convex portion comprises Nb 2 O 5 as the main component.

21. The diffraction grating of claim 17 , wherein the second layer of at least one of the concave portion and the convex portion comprises Ta 2 O 5 as the main component.

22. An optical head device, comprising:

a light source which emits lights having three different types of wavelengths;

a diffraction grating of claim 1 ;

an objective lens which focuses the lights emitted from the light source onto an optical recording medium; and

a detector which detects the emitted lights reflected from the optical recording medium,

wherein the diffraction grating is disposed in an optical path between the light source and the objective lens.