Diffractive optical element and endoscope

1. A diffractive optical element that is formed by laminating two optical material layers formed of different energy-cured resins;

in which a relief pattern is formed at an interface between the two optical material layers; and

that satisfies the following conditional expressions:

0.01 <d 2 /d 1 <0.2  (1)

0.05 <d 1 /φE< 1.0  (2)

0.0005 <d 2 /φE< 0.1  (3)

wherein

d 2 is the center plate thickness (mm) of one optical material layer that is cured first,

d 2 is the center plate thickness (mm) of the other optical material layer that is cured later, and

φE is the effective diameter (mm).

2. A diffractive optical element according to claim 1 , which satisfies the following conditional expressions:

0.03 <d 2 /d 1 <0.15  (1′)

0.07 <d 1 /φE< 0.6  (2′)

0.0007 <d 2 /φE< 0.06  (3′).

3. A diffractive optical element according to claim 1 , which satisfies the following conditional expressions:

0.05 <d 2 /d 1 <0.12  (1″)

0.1 <d 1 /φE< 0.4  (2″)

0.001 <d 2 /φE< 0.04  (3″).

4. A diffractive optical element according to claim 1 , which satisfies the following conditional expressions:

1.5 <n 1 <1.8  (4)

1.5 <n 2 <1.8  (5)

wherein

n 1 is the refractive index of the one optical material layer at the d-line, and

n 2 is the refractive index of the other optical material layer at the d-line.

5. A diffractive optical element according to claim 4 , which satisfies the following conditional expressions:

1.55 <n 1 <1.75  (4′)

1.55 <n 2 <1.75  (5′).

6. A diffractive optical element according to claim 4 , which satisfies the following conditional expressions:

1.6 <n 1 <1.71  (4″)

1.6 <n 2 <1.71  (5″).

7. A diffractive optical element according to claim 1 , which satisfies the following conditional expressions:

10<ν 1 <50  (6)

10<ν 2 <50  (7)

wherein

ν 1 is the Abbe number for the one optical material layer, and

ν 2 is the Abbe number for the other optical material layer.

8. A diffractive optical element according to claim 1 , which satisfies the following conditional expressions:

13<ν 1 <45  (6′)

13<ν 2 <45  (7′).

9. A diffractive optical element according to claim 1 , which satisfies the following conditional expressions:

15<ν 1 <40  (6″)

15<ν 2 <40  (7″).

10. A diffractive optical element according to claim 1 , wherein, for the relief pattern, the pitch thereof gradually decreases toward peripheries thereof from the center of the optical material layers; the groove depth is uniform over the entirety thereof; and the following conditional expression is satisfied:

0.8 <M 1 ×|n 1 −n 2 |/λd< 1.2  (8)

wherein

M 1 is the groove depth of the relief pattern,

n 1 is the refractive index of the one optical material layer at the d-line,

n 2 is the refractive index of the other optical material layer at the d-line, and

λd is the wavelength at the d-line.

11. A diffractive optical element according to claim 1 , which satisfies the following conditional expression:

0.0001 <|n 1 −n 2 |<0.4  (9)

wherein

n 1 is the refractive index of the one optical material layer at the d-line, and

n 2 is the refractive index of the other optical material layer at the d-line.

12. A diffractive optical element according to claim 11 , which satisfies the following conditional expression:

0.0005 <|n 1 −n 2 |<0.2  (9′).

13. A diffractive optical element according to claim 11 , which satisfies the following conditional expression:

0.001 <|n 1 −n 2 |<0.1  (9″).

14. A diffractive optical element according to claim 1 , which satisfies the following conditional expression:

0<|ν 1 −ν 2 |<30  (10)

wherein

ν 1 is the Abbe number for the one optical material layer, and

ν 2 is the Abbe number for the other optical material layer.

15. A diffractive optical element according to claim 14 , which satisfies the following conditional expression:

0.5<|ν 1 −ν 2 |<20  (10′).

16. A diffractive optical element according to claim 14 , which satisfies the following conditional expression:

1<|ν 1 −ν 2 |<15  (10″).

17. A diffractive optical element according to claim 1 , wherein the diffraction efficiency for first-order diffracted beams is 75% or greater in a wavelength range between 400 nm and 700 nm.

18. A diffractive optical element according to claim 1 , wherein the optical material layers are UV-cured resins, heat-cured resins, or thermoplastic resins.

19. An endoscope provided with a diffractive optical element according to claim 1 .

20. An endoscope according to claim 19 that is provided with an image acquisition element for capturing light transmitted through the diffractive optical element, and that satisfies the following conditional expression:

0.1 <M 1 /ccd _pitch<200  (11)

wherein

M 1 is the groove depth of the relief pattern, and

ccd_pitch is the pixel pitch of the image acquisition element.