Design Examples Of Hybrid Refractive-Diffractive Lenses

Abstract

ABSTRACT Diffractive lenses have recently become realizable elements in optical systems. However, their application to wide band optical imaging systems are limited because they are highly dispersive and exhibit unacceptable amounts of color related aberrations. In this paper we will present a design study of the achromatization of hybrid refractive-diffractive lenses over wide spectral bands. Design examples of hybrid lenses which operate in the visible, mid-infrared (3-5 micron) and long­ wave infrared (8 - 12 micron) spectral bands will be presented and a comparison with conventional lenses designed to operate in these spectral bands will be discussed. 1. ACHROMATIZING DIFFRACTIVE LENSES Swanson and Veldkamp1 and others2 ^ have shown and demonstrated that diffractive structures can be etched into various substrate materials to achieve high diffraction efficiency and high quality optical imaging components. Stone andGeorge^ have shown that diffractive lenses can be achromatized if they are combined with refractive elements and achromatized using the traditional methods. However, when diffractive optical elements are combined with a refractive lens to form a hybrid achromatic doublet, color related aberrations such as spherochromatism and secondary color remain unconnected and limit the overall performance of the resulting hybrid lens. The use of low dispersion materials helps reduce the effects of the color aberrations but, does not totally eliminate them. The fact that chromatic aberrations dominate the performance of hybrid lenses is no surprise since diffraction itself is a highly dispersive process. In the design examples to follow, we will show how diffractive lenses can be used to achromatize a variety of refractive materials and through a proper choice of refractive materials both spherochromatism and secondary color in a hybrid diffractive-Mangin mirror element can be simultaneously corrected.For optical design purposes the diffractive lens can be thought of as a refractive material with unique optical properties. The optical power of a diffractive lens varies linearly with the illuminating wavelength and its dispersion characteristics can be described by an effective Abbe number, V, and an effective partial dispersion, P. Both the effective Abbe number and partial dispersion value of diffractive lenses are functions of wavelength only. Stone and George describe the effective Abbe number of diffractive lenses as :and the partial dispersion as:The classical method for achromatization can be applied to a hybrid refractive-diffractive lens combination by balancing the optical power between two elements such that two separated wavelengths focus at a common point. The equations which describe this condition relate the optical power distribution of the elements to the Abbe numbers of the