Athermalized diffractive optical elements

摘要

A diffractive optical element, ("DOE"), and particularly a diffractive lens having annular zones about an optical axis and known as a Fresnel zone plate or kinoform lens, is athermalized so that its effective focal length ("EFL") does not shift substantially with temperature over a temperature range by, respectively, increasing and decreasing the zone widths with respect to widths which are optimized to focus at the EFL at room temperature. Half of the zones (alternate, adjacent zones) provide the correct EFL for the expanded condition of the lens at the hot end of the temperature range; the other half of the zones provide correct EFL for the contracted condition of the lens at the cold end of the temperature range. The exact spacings and zone widths are obtained, in the same manner as a diffractive lens is corrected for spherical and chromatic aberration, from the coefficients of the equation governing the phase shift through the lens as a function of radial distance from the optical axis, by averaging the aberration properties at the hot and cold ends of the range, the net effect of which is to dictate the spacing of alternate, successive zones which are slightly greater and less than the zone spacings optimized at room temperature, respectively.