Complementary Filtering Approach to Enhance the Optical Reconstruction of Holograms from a Spatial Light Modulator

“Electrically-addressed spatial light modulators (SLMs) are programmable diffractive optical elements (DOE), which are capable to modulate the amplitude and/or the phase of the incident light through a matrix structure. They are organized in a two-dimensional array of discrete pixels, which is geometrically specified by pixel pitch and pixel gap. Pixel pitch is the distance between the centres of neighboring pixels and pixel gap is the non-active area between them. The ratio of the SLM active area to its total area is referred to as its fill factor. SLMs can be used to optically reconstruct the complex amplitude of a wavefield. However, the reconstructed wavefield in the far field is corrupted by effects arising from the discrete nature, the finite pixel size and the limited fill factor of nowadays light modulators. One of the principal drawbacks is the appearance of unwanted laterally shifted replicas which are called higher diffraction orders. Another is the modulation of the wavefield in the reconstruction domain by an envelope function that affects the quality of the recovered signal. In this report, a complementary two-step filtering process is described to enhance the quality of the signal in the reconstruction plane. Firstly, the high diffraction orders are filtered by means of a limiting aperture in the Fourier domain of a classical 4f-arrangement. The second filter is digitally applied by changing the signal to be encoded into the SLM in order to compensate the effect of the envelop function.”