Patent ID: 7463430

Claim:
A design method for designing a diffraction element including a first and second layer attached to one another and a diffraction pattern in which each of step-like protruding portions located at predetermined intervals between the first and second layers includes s steps wherein the first and second layers are respectively made from a first and second material through which a first, second and third wavelength λ 1 , λ 2 and λ 3 of an optical beam passes, the design method comprising a selection step of selecting the first material and the second material such that the first material's refraction indexes n 1 (λ 1 ), n 1 (λ 2 ) and n 1 (λ 3 ) for the first, second and third wavelengths λ 1 , λ 2 and λ 3 and the second material's refraction indexes n 2 (λ 1 ), n 2 (λ 2 ) and n 2 (λ 3 ) for the first, second and third wavelengths λ 1 , λ 2 and λ 3 satisfy one of the following conditions (1) or (2): δ1:δ2·ρ12 :s ·δ3·ρ13= p 1+ε1 :p 2+ε2 :q 3+ε3; and gcd ( s,q 3)=1 (1) wherein i=1,2,3 j=1,2,3 and i≠j; δi=n 2 (λi)−n 1 (λi); “ρij” represents a ratio of wavelengths or ρ ⁢ ⁢ ij = λ ⁢ ⁢ j λ ⁢ ⁢ i ; “p1”, “p2” and “q3” represents any whole numbers; “εi” represents a difference from whole numbers or ɛ ⁢ ⁢ i < 0.2 s - 1 ; and “gcd(a,b)” represents the greatest common divisor of whole numbers a and b or δ1 :s ·δ2·ρ12 :s ·δ3·ρ13 =p 1+ε1 : q 2+ε2 : q 3+ε3; gcd ( s,q 2)=1; and gcd ( s,q 3)=1 (2) wherein “q2” represents any whole numbers.