Diffractive optical element

摘要

A diffractive optical element (DOE) comprising at least two layers of materials in close proximity or in contact with at least one optical interface therebetween; an optical path through the DOE leading through the first and second layers; the first layer being configured to provide a first optically diffracting pattern (405, 602) of phase retardation; the second layer being configured to provide a second optically diffracting pattern (404, 604) of phase retardation; the first and second diffracting patterns being selected to optimise the DOE's diffraction efficiency at two predetermined design wavelengths. Preferably the first and second diffracting pattern satisfy the condition that the sum of the phase retardations of the two patterns at a first design wavelength is substantially equal to the sum of their phase retardations at a second design wavelength.