GRADED-INDEX DIFFRACTIVE STRUCTURES FABRICATED BY THERMAL ION-EXCHANGE

High-efficiency diffractive elements with a continuous phase-modulatio n capability can be realized either by means of a surface profile or a refractive-index distribution. We consider the latter approach, expan ding on our recent proposal [Appl. Phys. Lett. 66, 917 (1995)] to fabr icate graded-index diffractive structures by thermal ion exchange in g lass. A rapid approximate method is developed to obtain a predesign of the ion-exchange mask, which is then refined by parametric optimizati on based on solutions of the full diffusion problem. The use of two co nsecutive ion-exchange steps and postbaking to enhance the designs is investigated, and some experimental results are provided. (C) 1997 Opt ical Society of America.