Spatial Light Modulator Pixels Research Articles

Modelling and simulation of a massively parallelised multi‐photon polymerization 3D microfabrication process

While additive manufacturing based on multi photon polymerization is currently considered to be a very promising technique for the fabrication of 3D micro and nano structures, long fabrication times are a major limitation of this approach. Parallelization of the fabrication process is an important technique to overcome this issue. The fabrication process is parallelized by imaging a 1920x1080 pixel Spatial Light Modulator (SLM) into an ultra‐sensitive Triplet‐Triplet Annihilation (TTA) resist. However, proximity effects between close pixels generate uncontrolled polymerization and make the controlled fabrication of 3D structures difficult. This work models light propagation and chemical interactions in our system to predict fabricated structures with a view to precompensating plot data and improving 3D resolution by performing optical and chemical proximity correction. Our simple model gives reasonable predictions of fabricated structures helping us fabricate fully 3D structures in parallel.This article is protected by copyright. All rights reserved.

Orbital Angular Momentum Encoded Beam Tracking and Wavefront Sensing

The working principle of conventional Shack-Hartmann-type wavefront sensors is based on a wavefront division by two-dimensional microlens arrays and a Poynting vector mapping by comparing the focal displacements to reference measurements. At complex phase distortions or steep phase gradients, however, wavefront reconstruction can suffer from ambiguities by crossing or overlapping neighboring sub-beams, or even by a complete loss of beam trajectories. Moreover, the recognition of sub-beams can additionally be affected by parasitic reflections at optical surfaces, or scattering in a medium. A possible solution of the problem is to work with arrays of well distinguishable sub-beams. Recently, high-resolution pixelated liquid crystal spatial light modulators became available which enable to flexibly generate spatially structured light and thus to realize advanced wavefront sensors with individually encoded beams. In our experimental study we investigated the specific advantages of orbital angular momentum beams for a robust beam tracking and wavefront reconstruction. As a key point for a wavefront reconstruction from vortex arrays, the reliable detection of spiral-like image features is addressed and related strategies are proposed.

Zero-order-free complex beam shaping

The unwanted zero-order diffraction is still an issue in beam shaping using pixelated spatial light modulators. In this paper, we report a new approach for zero-order free beam shaping by designing an asymmetric triangle reflector and introducing a digital blazed grating and a digital lens to the phase hologram addressed onto the spatial light modulator. By adding the digital lens phase to the previously reported complex-amplitude coding algorithms, we realized the generation of complex beams without the burden of zero-order diffraction. We comparatively investigated the produced complex light fields using a modified complex-amplitude coding algorithm to validate the proposed method.

Targeted generation of complex temporal pulse profiles

A targeted shaping of complex femtosecond pulse waveforms and their characterization is essential for many spectroscopic applications. A 4f pulse shaper combined with an advanced pulse characterization technique should, in the idealized case, serve this purpose for an arbitrary pulse shape. This is, however, violated in the real experiment by many imperfections and limitations. Although the complex waveform generation has been studied in-depth, the comparison of the effects of various experimental factors on the actual pulse shape has stayed out of focus so far. In this paper, we present an experimental study on the targeted generation and retrieval of complex pulses by using two commonly-used techniques: spatial-light-modulator (SLM)-based 4f pulse shaper and second-harmonic generation frequency-resolved optical gating (FROG) and cross-correlation FROG (XFROG). By combining FROG and XFROG traces, we analyze the pulses with SLM-adjusted complex random phases ranging from simple to very complex waveforms. We demonstrate that the combination of FROG and XFROG ensures highly consistent pulse retrieval, irrespective of the used retrieval algorithm. This enabled us to evaluate the role of various experimental factors on the agreement between the simulated and actual pulse shape. The factors included the SLM pixelation, SLM pixel crosstalk, finite laser focal spot in the pulse shaper, or interference fringes induced by the SLM. In particular, we observe that including the SLM pixelation and crosstalk effect significantly improved the pulse shaping simulation. We demonstrate that the complete simulation can faithfully reproduce the pulse shape. Nevertheless, even in this case, the intensity of individual peaks differs between the retrieved and simulated pulses, typically by 10–20% of the peak value, with the mean standard deviation of 5–9% of the maximum pulse intensity. We discuss the potential sources of remaining discrepancies between the theoretically expected and experimentally retrieved pulse.

Alternative use of reflection spatial light modulator in phase shifting digital Gabor holography

The aim of the present work is to present an alternative way of using SLM in the experimental phase shifting setup to obtain a hologram by using only an illuminating beam. This beam is modified using a Spatial Light Modulator (SLM), where a number of selected SLM pixels generate a plane-illuminating wave with determinate phase, and other pixels generate a plane wave for to be used as reference. The position of these pixels in the SLM is selected symmetrically, and the percentage of them regarding all the SLM pixels is used as a control of the intensity relation between the reference and object beams. An experimental setup of Phase-Shifting Digital Gabor Holography using a reflection SLM, model LR-C-1080 was created. An ad hoc software, that allows modifying the phase of illuminating beam and of course, the object wave phase, the hologram recording and the image reconstruction, was developed in this work. A human hair is used as testing sample, with diameter previously calibrated.

3D photolithography through light field projections.

A methodology of 3D photolithography through light field projections with a microlens array (MLA) is proposed and demonstrated. With the MLA, light from a spatial light modulator (SLM) can be delivered to arbitrary positions, i.e., voxels, in a 3D space with a focusing scheme we developed. A mapping function between the voxel locations and the SLM pixel locations can be one-to-one determined by ray tracing. Based on a correct mapping function, computer-designed 3D virtual objects can be reconstructed in a 3D space through a SLM and a MLA. The projected 3D virtual object can then be optically compressed and delivered to a photoresist layer for 3D photolithography. With appropriate near-UV light, 3D microstructures can be constructed at different depths inside the photoresist layer. This 3D photolithography method can be useful in high-speed 3D patterning at arbitrary positions. We expect high-precision 3D patterning can also be achieved when a femtosecond light source and the associated multi-photon curing process is adopted in the proposed light field 3D projection/photolithography scheme. Multi-photon polymerization can prevent the unwilling patterning of regions along the optical path before arriving to the designed focal voxels as observed in our single photon demonstrations.

Optical patterns controlled by computerized spatial light modulator

In this work, a contemporary optical system is designed using a technology of spatial light modulator (SLM). This system is depending on the ability of altering light beam into different optical patterns according to a signal feedback injected computationally to the SLM. The effects of control parameters on the SLM operation are studied such us brightness (B) which determines the constant DC voltage applied to SLM pixels, and contrast (C) determines the maximum AC voltage amount that can be applied to the SLM. The influences of the transmission linearity of image intensity are controlled by the gamma adjustment which can be used to align non linearity of image intensity. The grey scale is used to control and imagine which signals corresponding to grey levels and image location. The resolution and orientation of the obtained images are altered by the screen format to make optical experiments more satisfied, hence different ratios of amplitude and phase modulation can be understood.

The effects of source resolution on resolution enhancement through shifted superimposition projection

AbstractResolution in a projected display is traditionally defined by the number of pixels in the projector's spatial light modulator (SLM). In recent years, different techniques that increase the resolution on the screen above the number of SLM pixels have gained popularity. In one such technique, called pixel‐shifting or shifted‐superimposition, the display physically shifts every nth frame on the projected screen, and the overlapping pixel grids forms a finer subpixel grid with a higher pixel count.There is still an open question how much this method increases the resolution and how to quantify it. The resolution on the screen also depends upon the resolution of the input image fed to the projector.In this work, we experimentally investigate how the projector performs with resolution enhancement through pixel‐shifting and how this method relates to the source resolution. We also investigate some known methods of resolution measurement and evaluate how these methods perform for the shifted‐superimposition case.We find that the resolution enhancement through shifted‐superimposition enhances the resolution to about 40% over native resolution, and we also find two different measurement methods (grille contrast and least resolvable line pair method) that is relevant for effectively measuring resolution within such systems.

Generation of composite vortex beams by independent Spatial Light Modulator pixel addressing

The composite optical beams being a result of superposition, are a promising way to study the orbital angular momentum and its effects. Their wide range of applications makes them attractive and easily available due to the growing interest in the Spatial Light Modulators (SLM). In this paper, we present a simple method for generating composite vortex patterns with high symmetry. Our method is simple, flexible and gives perfectly aligned beams, insensitive to mechanical vibrations. This method is based on the ability to split SLM cells between phase patterns that are to be superposed. This approach allows control of the intensity relation between those structures, enables their rotation and is capable to superpose more than two such structures. In this paper, we examine its ability to produce superposition of two optical vortices by presenting both theoretical and experimental results.

Impact of spatial light modulator pixel structure on wavefront reconstruction

Impact of spatial light modulator pixel structure on wavefront reconstruction

Broadband transparent optical phase change materials for high-performance nonvolatile photonics

Optical phase change materials (O-PCMs), a unique group of materials featuring exceptional optical property contrast upon a solid-state phase transition, have found widespread adoption in photonic applications such as switches, routers and reconfigurable meta-optics. Current O-PCMs, such as Ge–Sb–Te (GST), exhibit large contrast of both refractive index (Δn) and optical loss (Δk), simultaneously. The coupling of both optical properties fundamentally limits the performance of many applications. Here we introduce a new class of O-PCMs based on Ge–Sb–Se–Te (GSST) which breaks this traditional coupling. The optimized alloy, Ge2Sb2Se4Te1, combines broadband transparency (1–18.5 μm), large optical contrast (Δn = 2.0), and significantly improved glass forming ability, enabling an entirely new range of infrared and thermal photonic devices. We further demonstrate nonvolatile integrated optical switches with record low loss and large contrast ratio and an electrically-addressed spatial light modulator pixel, thereby validating its promise as a material for scalable nonvolatile photonics.

Programmable pulse shaping for time-gated amplifiers.

We experimentally demonstrate a novel use of a spatial light modulator (SLM) for shaping ultrashort pulses in time-gated amplification systems. We show that spectral aberrations because of the device's pixelated nature can be avoided by introducing a group delay offset to the pulse via the SLM, followed by a time-gated amplification. Because of phase wrapping, a large delay offset yields a nearly-periodic grating-like phase function (or a phase grating). We show that, in this regime, the phase grating periocidity defines the group delay spectrum applied to the pulse, while the grating's amplitude defines the fraction of light that is delayed. We therefore demonstrate that a one-dimensional (1D) SLM pixel array is sufficient to control both the spectral amplitude and the phase of the amplified pulses.

Generation of vector Bessel beams with diffractive phase elements based on the Jacobi-Anger expansion.

We report the design and optimization of a diffractive phase element whose phase modulation is derived from the Jacobi-Anger relation, which allows the simultaneous generation of multiple scalar Bessel beams of different integer orders. In addition, by the appropriate treatment of a couple of such Bessel beams, we generate a vector Bessel beam of arbitrary order m. This beam is constructed by the collinear superposition of the scalar Bessel beam modes of orders m-1 and m+1, with circular orthogonal polarizations. We demonstrate experimentally both the simultaneous generation of the multiple scalar Bessel beams and the generation of vector Bessel beams of orders m=0 and m=1. These tasks are performed in an optical setup based on a pixelated liquid-crystal spatial light modulator.

Four-element division algorithm for focusing light through scattering medium

Light transport in complex disordered medium, such as white paint, milk, is a fundamental physical phenomenon, and it plays an important role in numerous applications including imaging through turbid layers, and quantum information processes. However, all spatial coherence is lost due to the distorted incident wavefront caused by repeated scattering and interference. Incident coherent light diffuses through the medium and cannot form a geometric focus but a volume speckle field on the imaging plane. In this paper, we propose a four-element division algorithm and experimentally demonstrate that using this algorithm to modulate the incident light, the shaped wavefront can focus through disordered material. At the beginning, we start with four segments on spatial light modulator (SLM), changing the phase of each segment from 0-2πup to search for the optimal phase in terms of the maximal output intensity at a certain field. After the optimal phase of these four segments is found, each of all segments is divided further into four subsegments, so 16 subsegments are formed on the SLM. Just like the first step, the optimal phase is found by cycling the phases of these 16 subsegments. Sequentially, this procedure is repeated several times, so more and more subsegments are obtained. As a result, the modulated input light from SLM can be focused after it has passed through the turbid scattering medium. By employing this approach in the forward scattered experiment, the total pixels of spatial light modulator are divided into 4-4096 segments to shape the incident light. After separately searching for all the optimal phase distributions, we can see that a sharp focusing is gradually achieved. Likewise, in backscattered experiment, 4-1024 segments are used to focus the incident light after passing through the diffuse material. In comparison with stepwise sequential algorithm, the main advantage of our method is that the interference effect of all segments on SLM is taken into consideration, which means that the modulated and the modulating segments are connected with each other. In this way, the signal-to-noise ratio is higher and no iteration is needed. All this experiment shows that the four-element division algorithm can be employed to focus the incident light passing through a disorder material efficiently, which maybe provide a new idea and method in the field of biomedical imaging through scattering medium.

空间光调制器像素尺寸对非球面检测误差影响分析

空间光调制器像素尺寸对非球面检测误差影响分析

Multi-receivers and sparse-pixel pseudo-thermal light source for compressive ghost imaging against turbulence

For remote sensing with computational ghost imaging, the atmospheric turbulence will influence much on the performance since the transmitted patterns from the pseudo-thermal source will be distorted. As the distribution of refractive index on the propagation path varies with time, reducing the time consumption of data acquisition will be an effective way for mitigating the influence. We propose using fewer pixels of spatial light modulator to increase the modulation rate of the pseudo-thermal source, with the positions of pixels being randomly distributed. Multi-bucket detectors are employed for further reducing of the data acquisition time. The sparse recovery algorithm from compressed sensing is employed to enhance the quality of image. The performance of this system is demonstrated by comprehensive numerical analyses.

Holography using pixelated spatial light modulators—Part 2: applications

Dynamic holography can replace or enhance traditional basic optical functionalities within a lot of different optical systems. In particular, the possibility of considerably changing the function of a system is an important strength that is rarely possible in conventional optical systems. Today, different spatial light modulators can be employed in order to achieve dynamic holography, but most often liquid crystal on silicon phase modulators are used. We give an overview of applications for the usage of pixelated spatial light modulators in dynamic holography based on the functionalities that the dynamic holograms fulfill.

Holography using pixelated spatial light modulators—part 1: theory and basic considerations

Pixelated spatial light modulators are used today in large quantities for projecting information. Apart from this standard application, the elements have found widespread use as dynamic changeable holograms for a multitude of applications ranging from microscopy over optical measurement systems to lithography. In this first part of our contribution, we give an overview concerning the necessary theory for the successful application of pixelated modulators in holography. We focus on the limitations imposed by current and future technology.

Experimental system of coupled map lattices

We design an optical feedback loop system consisting of a liquid-crystal spatial light modulator (SLM), a lens, polarizers, a CCD camera, and a computer. The system images every SLM pixel onto one camera pixel. The light intensity on the camera pixel shows a nonlinear relationship with the phase shift applied by the SLM. Every pixel behaves as a nonlinear map, and we can control the interaction of pixels. Therefore, this feedback loop system can be regarded as a spatially extended system. This experimental coupled map has variable dimensions, which can be up to 512 by 512. The system can be used to study high-dimensional problems that computer simulations cannot handle.

How to use a phase-only spatial light modulator as a color display.

We demonstrate that a parallel aligned liquid crystal on silicon (PA-LCOS) spatial light modulator (SLM) without any attached color mask can be used as a full color display with white light illumination. The method is based on the wavelength dependence of the (voltage controlled) birefringence of the liquid crystal pixels. Modern SLMs offer a wide range over which the birefringence can be modulated, leading (in combination with a linear polarizer) to several intensity modulation periods of a reflected light wave as a function of the applied voltage. Because of dispersion, the oscillation period strongly depends on the wavelength. Thus each voltage applied to an SLM pixel corresponds to another reflected color spectrum. For SLMs with a sufficiently broad tuning range, one obtains a color palette (i.e., a "color lookup-table"), which allows one to display color images. An advantage over standard liquid crystal displays (LCDs), which use color masks in front of the individual pixels, is that the light efficiency and the display resolution are increased by a factor of three.