Focus grid generation by in-line holography

We describe a simple way to generate a wide-area high-resolution focus grid by in-line holography and study the factors that impacts its quality. In our holographic recording setup, the reference beam was the direct transmission of the incoming collimated laser beam through a mask coating with thin metal film, and the sample beam was the transmission of the laser through small apertures fabricated on the mask. The interference of the two beams was then recorded by a holographic plate positioned behind the mask. Compared with other recording schemes, the in-line holography scheme has many distinct advantages and is more suitable for generating a wide-area focus grid. We explored the dependence of diffraction quality, including reconstructed focus spot intensity and spot size, on different parameters for recording, such as optical density of the metal film, size of the apertures, and focal lengths. A wide-area focus grid (170 x 138 spots with area 5.1 mm x 4.1 mm) was recorded using the in-line holography scheme for a demonstration.