Abstract
Computer generated holograms (CGHs) are powerful optical elements used in many fields, such as wavefront shaping, quality testing of complex optics, and anti-counterfeiting devices. The Lee algorithm is the most used to generate binary amplitude Fourier holograms. Grayscale CGHs are known to give a higher reconstruction quality than binary holograms, but they usually require a cumbersome production process. A very simple and straightforward method of manufacturing rewritable grayscale CGHs is proposed here by taking advantage of two key components: a digital micro-mirror device (DMDs) and a photochromic plate. An innovative algorithm, named Island algorithm, able to generate grayscale amplitude Fourier CGHs, is reported and compared with the standard Lee approach, based on 9 levels. A crucial advantage lies on the fact that the increase or decrease of the quantification does not affect the spatial resolution. In other words, the new coding leads to a higher spatial resolution (for a given CGH size) and a reconstructed image with an order of magnitude higher contrast with respect to the classical Lee-coded hologram. In order to show the huge potential of our approach, a 201 level Island hologram is designed, produced and reconstructed, pushing the contrast to values higher than104. These results reveal the potential of our process as well as our algorithm for generating programmable grayscale CGHs.
Highlights
Computer Generated Holograms (CGHs) are attractive optical elements that are finding applications for beam shaping, particles manipulation, interferometric optical testing and anticounterfeiting [1,2,3,4]
Binary amplitude CGHs are obtained when a single mask is projected until the material becomes transparent, but, with a smart control of the Digital Micro-mirror Device (DMD), we demonstrated grayscale CGHs obtained in a one-exposure process of the same duration, without any developing step [10]
Leecoded holograms will be taken as reference for judging our approach in order to compare the performances of the algorithm we developed to generate grayscale amplitude Fourier CGHs
Summary
Computer Generated Holograms (CGHs) are attractive optical elements that are finding applications for beam shaping, particles manipulation, interferometric optical testing and anticounterfeiting [1,2,3,4]. Binary amplitude CGHs are obtained when a single mask is projected until the material becomes transparent, but, with a smart control of the DMD, we demonstrated grayscale CGHs obtained in a one-exposure process of the same duration, without any developing step [10] This ready to use hologram can be erased and rewritten, thanks to the reversibility of the transformation in P-type photochromic materials. Other interesting works focus on the real-time holography based on imidazole dimers [19,20] and on photorefractive materials [21] In the latter case, it has been demonstrated the possibility to reach high efficiency in phase holograms, but an electric field is required to enhance the refractive index modulation and this makes the system more complex than the pure photochromic one. Very large improvements in terms of Signal to Noise ratio and fidelity are obtained by keeping the same efficiency
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