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Three-dimensionally modulated graded-index diffractive elements by thermal ion exchange in glass.

Thermal ion exchange was introduced recently [Appl. Phys. Lett. 66, 917 (1995)] as a method to generate graded refractive-index distributions that permit continuous one-dimensional phase modulation in diffractive optics. We apply enhanced diffusion-modeling techniques and a novel three-step optimization procedure to design, for the first time to our knowledge, efficient three-dimensional graded-index profiles with two-dimensional phase-modulation capability. A discrete, nonseparable inversion-symmetric signal pattern is demonstrated experimentally in which the uniformity error in the intensities of the seven signal spots is less than 5%.