Diffractive optical element having high diffraction efficiency at plural wavelengths and image-pickup optical system using the same
A diffractive optical element in which the maximum optical path length of light is an integral multiple of each of a plurality of wavelengths includes a diffractive grating configured by combining a first member and a second member having a refractive index lower than a refractive index of the first member and having dispersion higher than a dispersion of the first member. At least one of the first member and the second member is made of a glass material, and the diffractive grating is formed by the glass material which is heated to be softened using a thermal press forming, and has a concave grating shape that has a grating height increasing from a central part toward a peripheral part.
1. A diffractive optical element comprising:
a first member and a second member in contact with each other,
wherein the second member has a refractive index lower than a refractive index of the first member and has a dispersion higher than that of the first member,
wherein the first member is made of a resin material and the second member is made of a glass material,
wherein the second member includes a plurality of diffractive grating portions each having a grating wall surface and a grating surface that merge with each other,
wherein each of the diffractive grating portions is configured so that a grating height thereof increases as a distance increases away from an optical axis,
wherein the diffractive optical element has a positive power, and
wherein the following expressions are met:
ngd<nrd;
νgd≦ 125−50 ngd;
ngd≧ 1.8;
νrd≧ 125−50 nrd ; and
nrd ≧1.5,
where ngd and vgd are a refractive index and an Abbe number of the glass material at a d-line wavelength respectively, and nrd and νrd are a refractive index and an Abbe number of the resin material at the d-line wavelength.
2. The diffractive optical element according to claim 1 , wherein:
the diffractive grating is softened with heat using a thermal press, and
a linear expansion coefficient of a material of a mold used in the thermal press forming is smaller than a linear expansion coefficient of the glass material.
3. The diffractive optical element according to claim 1 , wherein the resin material is a material in which one of an organic nanoparticle material or an inorganic nanoparticle material, having a refractive index higher than a refractive index of a base material of a resin, is dispersed into the base material.
4. The diffractive optical element according to claim 1 , wherein the glass material is a low-melting-point glass having a glass transition temperature of at most 550 degrees C.
5. The diffractive optical element according to claim 1 , wherein the diffractive grating has a grating height of at least 3 μm.
6. An optical system comprising:
a stop; and
a diffractive optical element comprising:
a first member and a second member in contact with each other,
wherein the second member has a refractive index lower than a refractive index of the first member and has a dispersion higher than that of the first member,
wherein the first member is made of a resin material and the second member is made of a glass material,
wherein the second member includes a plurality of diffractive grating portions each having a grating wall surface and a grating surface that merge with each other,
wherein each of the diffractive grating portions is configured so that a grating height thereof increases as a distance increases away from an optical axis,
wherein the diffractive optical element has a positive power, and
wherein the following expressions are met:
ngd<nrd;
ν gd≦ 125−50 ngd;
ngd ≦1.8;
ν≧ rd 125−50 nrd ; and
nrd≧ 1.5,
where ngd and νgd are a refractive index and an Abbe number of the glass material at a d-line wavelength respectively, and nrd and vrd are a refractive index and an Abbe number of the resin material at the d-line wavelength.