Most Shack-Hartmann (S-H) sensors used to measure wavefront distortion depend on refractive microlens arrays, and are able to achieve sampling densities in the range from 1 to 10 lens/mm when the minimum diameter of the lenslets is 100 μm. Unfortunately, these microlenses are difficult to fabricate with lenslet diameters <100 μm and degrade in performance when trying to focus high-refractive-index beams, such as in aqueous environments.
To increase the sampling density to 50 lens/mm and measure wavefront distortion in high-refractive-index environments, scientists at the Institute of Electronic Materials Technology (Warsaw, Poland) and Heriot-Watt University (Edinburgh, Scotland) have replaced the microlens with a graded-index (GRIN) lens array in which each 1-mm-diameter hexagonal lenslet rod is formed in a stack-and-draw process from 7651 hexagonal glass rods with 0.5 mm diameter composed of two different optical materials (low-index silicate glass NC21 and high-index lead-silicate glass F2). Next, 469 of these hexagonal lenslets are stacked and further drawn down into a flat lens array with 318.5 μm diameter and 50-100 μm thickness consisting of the hexagonal core with 20 μm diameter lenslets and external circular cladding. Using the GRIN lens array in a S-H configuration, wavefront distortion was measured by analyzing the minute shifts in the beams refracted from each of the 469 GRIN lenslets through a regularized reconstruction method. Sampling resolution is much higher than in currently available configurations of Shack-Hartmann sensors. Reference: R. Kasztelanic et al., Opt. Express, 25, 3, 1680-1691 (2017).
