Abstract

Multilayer-coated focussing systems for XUV imaging, by their very nature, generally require laterally graded multilayer period or thickness across the clear aperture of mirrors to optimize performance. This requirement stems from the variation of incidence angles given by the optical design, and the inherint finite bandwidth of x-ray multilayers [1,2]. The ideal period (d) variation at each mirror is prescribed for small-field imaging systems by the variation in incidence angle θ across the clear aperture through the Bragg condition for constructive multilayer interference, λ= 2dcosθ. For large field systems the field size can make an additional contribution to angular variations which can lead to vignetting under extreme cases [3]. The actual d variation should conform to the prescribed ideal d variation to a tolerance set by the multilayer bandwidth. Otherwise, degraded performance may result from unwanted phase and intensity variations across the clear aperture. The phase errors may contribute to limiting imaging performance if wavefront errors resulting from mirror figure and system alignment errors are reduced to much less than λ. The intensity errors result in nonuniform intensity across the wavefront, and will limit the effective aperture if d variation problems are severe. These effects can and should be investigated with appropriate calculations for specific optical systems of interest [4,5].

© 1993 Optical Society of America