Binary Amplitude Holograms Research Articles

Binary amplitude holograms for shaping complex light fields with digital micromirror devices

Abstract Digital micromirror devices are a popular type of spatial light modulators for wavefront shaping applications. While they offer several advantages when compared to liquid crystal modulators, such as polarization insensitivity and rapid-switching, they only provide a binary amplitude modulation. Despite this restriction, it is possible to use binary holograms to modulate both the amplitude and phase of the incoming light, thus allowing the creation of complex light fields. Here, a didactic exploration of various types of binary holograms is presented. A particular emphasis is placed on the fact that the finite number of pixels coupled with the binary modulation limits the number of complex values that can be encoded into the holograms. This entails an inevitable trade-off between the number of complex values that can be modulated with the hologram and the number of independent degrees of freedom available to shape light, both of which impact the quality of the shaped field. Nonetheless, it is shown that by appropriately choosing the type of hologram and its parameters, it is possible to find a suitable compromise that allows shaping a wide range of complex fields with high accuracy. In particular, it is shown that choosing the appropriate alignment between the hologram and the micromirror array allows for maximizing the number of complex values. Likewise, the implications of the type of hologram and its parameters on the diffraction efficiency are also considered.

3D scattering-enabled vectorial holographic projection with a binary amplitude hologram.

Empowered by wavefront shaping (WFS) techniques, scattering materials (SMs) hold significant potential in high-capacity, high-fidelity, and crosstalk-free 3D holographic projections. Here, we present an optimal accumulation algorithm (OAA) to generate binary amplitude holograms that enable simultaneous control of 3D intensity and polarization distributions through SMs. In particular, OAA is efficient for creating binary holograms since only addition and comparison operations are required. Using such a binary hologram, we demonstrate complete polarization control on four planes simultaneously, and an average degree of polarization over 95% is achieved. Moreover, a 3D holographic projection of polarization-multiplexed images on multiple planes is also presented with an average Pearson correlation coefficient over 0.80. By exploiting the rapid switching ability of a digital micromirror device, we further demonstrate dynamic 3D vectorial holographic projections with reconfigurable binary amplitude holograms. Our proposed approach offers a competitive way to generate holograms for 3D scattering-enabled vectorial holographic projections.

Non-iterative generation of binary amplitude holograms applied to holographic projection with digital micromirror devices

In this work, we implement a fast non-iterative method for the generation of binary amplitude Fresnel holograms and demonstrate their application in a holographic projection scheme based on a digital micromirror device. To achieve this, we perform the binarization of phase-only holograms generated using an optimized Fresnel random phase. We analyze the quality of the resulting binary amplitude holograms and compare them with holograms obtained with the conventional iterative Fresnel algorithm as a function of the number of iterations and the propagation distance. Additionally, we evaluate the diffraction efficiency in both methods. We demonstrate that the holograms produced with our method present significantly advantages in computation speed without a significant reduction in the reconstruction quality. Both experimental and numerical results confirm the effectiveness of our proposal.

Generation of rotationally symmetric power-exponent-phase vortex beams based on digital micromirror devices.

In this paper, what we believe to be a new method for the generation of rotationally symmetric power-exponent-phase vortex beams (RSPEPVBs) based on digital micromirror devices (DMD) was proposed and demonstrated. Based on the theory of binary amplitude holography and Lee method, the two-dimensional amplitude holograms for the generation of RSPEPVBs were obtained. Then, the experimental setup was established for the generation of RSPEPVBs based on DMD and to verify the phase structure of RSPEPVBs by the Mach-Zehnder interferometer. The experimental results showed that the RSPEPVBs can be generated based on DMD with high beam quality and stability, and the ±1st-order diffracted beams were respectively corresponding to the RSPEPVBs with contrary TCs, which was the first time to report the RSPEPVBs with negative TC. Besides, the overall and ±1st-order diffraction efficiencies of RSPEPVBs generated by DMD were 7.18% and 1.73%, respectively. The method can be applied for the generation of RSPEPVBs with different parameters and quickly achieve mode switching by loading different binary amplitude holograms, which provides a new choice for the generation of new structure beams based on DMD.

Dynamic modulation of spatial intensity profile of a laser beam using a binary hologram

In this paper we first describe two existing schemes to construct binary amplitude or phase holograms for arbitrary complex amplitude modulation in the +1 order diffracted beam. We then propose another binarization scheme for complex amplitude modulation that uses a combination of fixed as well as statistically dependent threshold values for binarization. We perform a numerical investigation to compare the accuracy of the three binarization schemes to incorporate spatial intensity modulation in a laser beam and observe that the proposed scheme offers significant relative advantage in terms of aliasing. We present experimental results that supports the above observations.

Iterative synthesis of binary inline Fresnel holograms for high-quality reconstruction in divergent beams with DMD

Aim of this paper is formation of single focused diffraction order using binary amplitude modulation for digital micromirror device (DMD) applications. This can be achieved by using inline binary Fresnel holograms with divergent incident beam. However, extremely low reconstruction quality and diffraction efficiency of such holograms, synthesized by conventional means, prevent it. Here, we present new iterative method of synthesis of binary inline Fresnel holograms with much higher image reconstruction quality and diffraction efficiency compared to standard technique of hologram direct calculation. Synthesis procedure was carried out in two stages. The first stage of the new technique is based on Gerchberg-Saxton algorithm with two principal differences. First one is that synthesized diffractive optical element (DOE) is not a phase one, but an inline binary amplitude hologram. And second difference is that incident wavefront is spherical instead of a plane one. The second stage includes an application of the direct search with random trajectory method which greatly improves hologram characteristics by consequent processing of every pixel in it. The resulting binary inline Fresnel holograms in comparison to ones, synthesized using direct calculation method, have up to 11 times lower reconstruction error and up to 4 times higher diffraction efficiency. Results of optical reconstruction using DMD also demonstrate superiority of the new approach.

Deep-learning-based binary hologram.

Binary hologram generation based on deep learning is proposed. The proposed method can reduce the severe effect of quality degradation from binarizing gray-scaled holograms by optimizing the neural network to output binary amplitude holograms directly. In previous work on binary holograms, the calculation time for generating binary holograms was long. However, in the proposed method, once the neural network is trained enough, the neural network generates binary holograms much faster than previous work with comparable quality. The proposed method is more suitable for opportunities to generate several binary holograms under the same condition. The feasibility of the proposed method was confirmed experimentally.

Control of broadband optically generated ultrasound pulses using binary amplitude holograms.

In this work, the use of binary amplitude holography is investigated as a mechanism to focus broadband acoustic pulses generated by high peak-power pulsed lasers. Two algorithms are described for the calculation of the binary holograms; one using ray-tracing, and one using an optimization based on direct binary search. It is shown using numerical simulations that when a binary amplitude hologram is excited by a train of laser pulses at its design frequency, the acoustic field can be focused at a pre-determined distribution of points, including single and multiple focal points, and line and square foci. The numerical results are validated by acoustic field measurements from binary amplitude holograms, excited by a high peak-power laser.

Shaping diffraction-free Lommel beams with digital binary amplitude masks.

Here, we report on experimental observations of various Lommel modes that possess distinct diffraction-free behaviors. The binary amplitude masks are designed to accurately encode the complex field information with the superpixel method. Then, the generation of the desired beams is demonstrated with these binary patterns projected onto the digital micromirrors device (DMD). Remarkably, we find that the field distribution and orbital angular momentum can be continuously engineered by tuning the beam parameters. Furthermore, the shape-invariant feature of such beams is verified by their far-field ring-like structures. Our observations are in good accordance with the theoretical predictions, and our methods may find potential applications in optical guiding and imaging. Moreover, apart from the DMD, the binary amplitude hologram can also be presented with well-fabricated elements, and thus, our method will also enable new applications for surface plasmon polaritons as well as electron beams.

Binary amplitude holograms made from dyed photoresist

A method for fabricating binary amplitude holograms from a dyed photoresist is described. It is of particular interest for holograms that are used as null optics in the form metrology of aspheric surfaces and wavefronts. A pigment that strongly absorbs light near 633 nm was dissolved in a positive photoresist and the dyed resist was spun onto silica glass substrates. Stable resist layers were obtained that were essentially opaque at 633 nm with little effect on the transmittance of the resist in the UV. A Fresnel zone plate was fabricated from the dyed resist layer using contact lithography, and its performance was demonstrated at 633 nm.

Wavefront aberrations of rotationally symmetric CGHs fabricated by a polar coordinate laser plotter

Computer-generated holograms (CGHs) were fabricated by a polar coordinate laser plotter. The wavefront aberrations of these CGHs caused by fabrication inaccuracies were measured interferometrically. We tested Fresnel zone plates as typical examples of CGHs with focal ratios of ƒ/10 and ƒ/5 with a Fizeau interferometer. The tests comprised both binary phase holograms and binary amplitude holograms. The wavefront aberrations of the CGHs had an rms variation of about λ /50. Two types of aberrations were identified and could be referred to certain writing errors. Their size was deduced from the interferograms.

Millimetre-wave Bessel beams using computer holograms

A computer-generated binary amplitude hologram is used to transform an initial Gaussian electromagnetic field with spherical phase front at 310 GHz into a non-diffracting Bessel beam. The beam profile is measured with the help of a near field scanner. In contrast to the situation in the optical region, both amplitude and phase information is readily obtainable from the generated field.

Recording complex holograms on photoresist by using an ion-implantation method

Photoresists have traditionally been used as a holographic recording medium for phase holograms. We demonstrate the recording of a binary amplitude hologram on photoresist by using an ion-implantation method. This amplitude recording property is based on the fact that the optical transmittance of photoresist can be changed by a high-dose ion implantation. A novel complex-hologram recording method is proposed in which the phase is represented by photoresist thickness and the amplitude is encoded on its optical transmittance.