Abstract
Computer-generated holography enables efficient light pattern generation through phase-only wavefront modulation. While perfect patterning usually requires control over both phase and amplitude, iterative Fourier transform algorithms (IFTA) can achieve phase-only approximations which maximize light efficiency at the cost of uniformity. The phase being unconstrained in the output plane, it can vary abruptly in some regions leading to destructive interferences. Among such structures phase vortices are the most common. Here we demonstrate theoretically, numerically and experimentally, a novel approach for eliminating phase vortices by spatially filtering the phase input to the IFTA, combining it with phase-based complex amplitude control at the spatial light modulator (SLM) plane to generate smooth shapes. The experimental implementation is achieved performing complex amplitude modulation with a phase-only SLM. This proposed experimental scheme offers a continuous and centered field of excitation. Lastly, we characterize achievable trade-offs between pattern uniformity, diffraction efficiency, and axial confinement.
Highlights
Computer generated holography (CGH) enables projection of patterns in the focal plane of a lens
Numerically and experimentally, a novel approach for eliminating phase vortices by spatially filtering the phase input to the iterative Fourier transform algorithms (IFTA), combining it with phase-based complex amplitude control at the spatial light modulator (SLM) plane to generate smooth shapes
We proposed solutions to perform computer generated holography with reduced speckle contrast
Summary
Computer generated holography (CGH) enables projection of patterns in the focal plane of a lens. Phase-only Spatial Light Modulators (SLMs) enable arbitrary intensity pattern generation in the objective lens image plane. The field at any given point in the back focal plane of the objective is the average of the field coming from two (or more) adjacent SLM pixels This allows for an extra (or more) degree of freedom to modulate simultaneously amplitude and phase in the pupil plane. Modulating the amplitude of a one-dimensional grid (or a checkerboard) at the SLM allows intensity modulation at the back focal plane of the objective, by stopping ±1 diffraction orders by the beam block. Considering that p must correspond to an even number of SLM pixel widths, it is maximal when p = 2a In this case, the size of the clear aperture in the beam block, is only reduced by a factor two along the one-dimension grid axis compared to regular CGH. The axial field of excitation is expected to be redu√ced by a factor of 2 (if amplitude modulation is performed with a 2 by 2 pixel checkerboard) or 2 ( with a one-dimensional 2-pixel periodic grid)
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