Multicolor structured illumination microscopy and quantitative control of polychromatic light with a digital micromirror device.

Structured illumination microscopy (SIM) is a fast and gentle super-resolution fluorescence imaging technique, featuring live-cell compatible excitation light levels and high imaging speeds. To achieve SIM, spatial modulation of the fluorescence excitation light is employed. This is typically achieved by interfering coherent laser beams in the sample plane, which are often created by spatial light modulators (SLMs). Digital micromirror devices (DMDs) are a form of SLMs with certain advantages, such as high speed, low cost and wide availability, which present certain hurdles in their implementation, mainly the blazed grating effect caused by the jagged surface structure of the tilted mirrors. Recent works have studied this effect through modelling, simulations and experiments, and laid out possible implementations of multi-color SIM imaging based on DMDs. Here, we present an implementation of a dual-color DMD based SIM microscope using temperature-controlled wavelength matching. By carefully controlling the output wavelength of a diode laser by temperature, we can tune two laser wavelengths in such a way that no opto-mechanical realignment of the SIM setup is necessary when switching between both wavelengths. This reduces system complexity and increases imaging speed. With measurements on nano-bead reference samples, as well as the actin skeleton and membrane of fixed U2OS cells, we demonstrate the capabilities of the setup.

Structured illumination microscopy (SIM) is an attractive choice for fast super-resolution imaging. The generation of structured illumination patterns made by interference of laser beams is broadly employed to obtain high modulation depth of patterns, while the polarizations of the laser beams must be elaborately controlled to guarantee the high contrast of interference intensity, which brings a more complex configuration for the polarization control. The emerging pattern projection strategy is much more compact, but the modulation depth of patterns is deteriorated by the optical transfer function (OTF) of the optical system, especially in high spatial frequency near diffraction limit. Therefore, the traditional super-resolution reconstruction algorithm for interference-based SIM will suffer from much artifact in the case of projection-based SIM that possesses a low modulation depth. Here, we propose an alternative reconstruction algorithm based on image recombination transform (IRT), which provides an alternative solution to address this problem even in a weak modulation depth. We demonstrated the effectiveness of this algorithm in the multicolor super-resolution imaging of BPAE cells in our developed projection-based SIM system, which applies a computer controlled digital micro-mirror device (DMD) for fast fringe generation and multicolor LEDs for illumination. The merit of the system incorporated with the proposed algorithm allows for a low excitation intensity fluorescence imaging even less than 1W/cm2, which is beneficial for the long-term, in vivo super-resolved imaging of live cells and tissues.

In the structured illumination microscopy (SIM) family, interferometric implementations using traditional physical gratings or advanced liquid crystal on silicon spatial light modulators are popular but have the disadvantages of expensive and limited speed. A promising alternative of a digital micromirror device (DMD) has improved the situation for its lower cost and higher speed, furthermore, it has contributed to the proposal of a simple and compact projection DMD-SIM. However, the projection DMD-SIM method is accused of erroneously measuring the resolution because of the scattering characteristic of gold beads, despite the inferior experimental resolution that is because of the low fringe contrast and the resulting low signal-to-noise ratio (SNR). As an improvement, we use a high-brightness laser for illumination to achieve higher SNR, and the accompanying problems of speckle and inhomogeneity originating from the laser’s high coherence are solved by a high-frequency phase-randomization deformable mirror in conjunction with a square-core multimode fiber, which does not limit the image acquisition speed and with fewer energy losses compared to the existing methods. Also, in terms of the problem of low SNR, we apply a parallel-iteration Richardson–Lucy deconvolution algorithm for SIM reconstruction to further improve the imaging performance based on ensuring the resolution. Based on these advances, we image fluorescent nanoparticles and other non-biological and biological fluorescent samples, and the experimental results demonstrate the improved imaging performance of our modified system.

In 3‐D display system based on opto‐electronic reconstruction of digitally recorded hologram, the field of view of such a system is limited by the spatial resolution of the liquid crystal on silicon (LCOS) spatial light modulator (SLM) used to perform the opto‐electronic reconstruction. In this article, the spatial resolution limitation of LCOS SLM associated with fringe field effect and interpixel coupling is determined by liquid crystal director simulation and Finite Difference Time Domain (FDTD) simulation. The diffraction efficiency loss associates with the imperfection in phase profile is studied with an example of opto‐electronic reconstruction of an amplitude object. A high spatial resolution LCoS SLM with wide reconstruction angle is proposed.

Surface-stabilized antiferroelectric liquid crystals (AFLCs) and V-shape smectics share the spontaneous generation of intermediate transmission states, i.e. analogue grayscale. On the other hand, liquid crystal on silicon (LCOS) appears to be a promising alternative to thin-film transistors (TFTs) in the preparation of reflective microdisplays and spatial light modulators (SLMs). The feasibility of preparing LCOS SLMs using AFLC and V-shaped LCs has been analyzed. A comparative study of a number of AFLC and V-shape materials as candidates for LCOS SLMs has been carried out. Test cells below 1 µm have been prepared using an ITO-coated transparent glass plate and an aluminized silicon back plate mimicking the actual LCOS device. AFLC waveforms compatible to low-voltage operation of the silicon backplane have been demonstrated. V-shape waveforms have been designed and demonstrated as well. The results show that both LC families may be employed in this specific application. Dynamic behavior, with frame rates over 200 Hz, is especially remarkable.

Digital micromirror devices (DMDs) are spatial light modulators that employ the electro-mechanical movement of miniaturized mirrors to steer and thus modulate the light reflected off a mirror array. Their wide availability, low cost and high speed make them a popular choice both in consumer electronics such as video projectors, and scientific applications such as microscopy. High-end fluorescence microscopy systems typically employ laser light sources, which by their nature provide coherent excitation light. In super-resolution microscopy applications that use light modulation, most notably structured illumination microscopy (SIM), the coherent nature of the excitation light becomes a requirement to achieve optimal interference pattern contrast. The universal combination of DMDs and coherent light sources, especially when working with multiple different wavelengths, is unfortunately not straight forward. The substructure of the tilted micromirror array gives rise to a blazed grating, which has to be understood and which must be taken into account when designing a DMD-based illumination system. Here, we present a set of simulation frameworks that explore the use of DMDs in conjunction with coherent light sources, motivated by their application in SIM, but which are generalizable to other light patterning applications. This framework provides all the tools to explore and compute DMD-based diffraction effects and to simulate possible system alignment configurations computationally, which simplifies the system design process and provides guidance for setting up DMD-based microscopes.This article is part of the Theo Murphy meeting ‘Super-resolution structured illumination microscopy (part 1)’.

Liquid crystal on silicon (LCoS) spatial light modulators (SLMs) are versatile scientific tools relevant to an increasingly wide variety of research and technological applications including digital holography, wavefront correction, optical tweezing, and non-mechanical beam steering to name a few. Since SLMs are used in a multitude of different ways, some aspects of device performance (e.g., response time) are crucial to certain applications while being irrelevant to others. In this work we couple our standard SLMs with a thermo-electric cooler, allowing for tunability of the device operating temperature from 0° to 75 °C. We show that there is an inherent tradeoff between the liquid crystal response time and the phase stability of an SLM, and that the operating temperature offers a means of controlling this tradeoff. Furthermore, this paper aims to provide the reader with a brief but thorough explanation of SLM operating principles and device structure, defines the performance metrics of the SLM, and provides a methodology for measuring the specifications. By allowing control over the SLM operating temperature and detailing how temperature affects device functionality, SLM users are afforded greater experimental flexibility and will be better able to tailor the performance of their device for the given project or application at hand.

Digital projectors based on liquid crystal or micro-mirror technologies provide higher spatial resolution than the CRTs typically used in flight-simulator applications. However, their temporal response is limited by long hold-times. Hold-time is the interval that pixels are activated during each frame, and it is related to both the design of the driver circuitry and the display itself. Long hold-times result in the blurring of moving imagery, which can affect the detection and identification of simulated targets as well as the perceived realism of simulated scenes. Described here are the results of an evaluation of several digital projectors that implement various techniques for reducing pixel hold-time. The evaluation included estimates of both spatial and temporal resolution, as well as perceptual tests used to assess the validity of those estimates. The techniques described here can be used to assess the potential utility of candidate projectors for a given simulator application. The following projectors were tested: ferro-electric liquid crystal on silicon (FLCoS), liquid crystal on silicon (LCoS), liquid crystal (LCD), digital light projector (DLP, a digital micro-mirror device, or DMD), and cathode ray tube (CRT). The hold-time of the FLCoS and LCD projectors was varied using mechanical shutters, and the hold-time of the DLP projector was varied using an electronic shutter. The CRT and LCD projectors were used mainly for comparison and so their hold-times were not varied.

.Significance: Confocal laser scanning enables optical sectioning in clinical fiber bundle endomicroscopes, but lower-cost, simplified endomicroscopes use widefield incoherent illumination instead. Optical sectioning can be introduced in these simple systems using structured illumination microscopy (SIM), a multiframe digital subtraction process. However, SIM results in artifacts when the probe is in motion, making the technique difficult to use in vivo and preventing the use of mosaicking to synthesize a larger effective field of view (FOV).Aim: We report and validate an automatic motion compensation technique to overcome motion artifacts and allow generation of mosaics in SIM endomicroscopy.Approach: Motion compensation is achieved using image registration and real-time pattern orientation correction via a digital micromirror device. We quantify the similarity of moving probe reconstructions to those acquired with a stationary probe using the relative mean of the absolute differences (MAD). We further demonstrate mosaicking with a moving probe in mechanical and freehand operation.Results: Reconstructed SIM images show an improvement in the MAD from 0.85 to 0.13 for lens paper and from 0.27 to 0.12 for bovine tissue. Mosaics also show vastly reduced artifacts.Conclusion: The reduction in motion artifacts in individual SIM reconstructions leads to mosaics that more faithfully represent the morphology of tissue, giving clinicians a larger effective FOV than the probe itself can provide.

We present a 3D electromagnetic simulation of a digital micromirror device (DMD) from 0.4 µm to 5 µm, which accurately models DMD reflectance and contrast ratio, including the effects of diffraction. A DMD is a spatial light modulator with a wide range of applications, including projection displays, 3D printing, and imaging spectroscopy. The physical structure of the DMD induces strong wavelength-dependent diffraction effects that impact the stray light, optical throughput, and pupil illumination distribution of a system. To quantify this, we perform a 3-dimensional electromagnetic finite-difference time-domain simulation, illuminating the DMD with a focused, incoherent beam, explicitly calculating the near-field electric fields, and calculating the far-field distribution of light. The far-field data determines diffraction efficiency and the distribution of light across the pupil. With these models, we are able to study the DMD’s optical efficiency in three key regimes: the specular regime, where the DMD behaves like a segmented mirror with a diffractive component (λ < 1 µm); the diffraction-dominated regime, which is also described by analytic diffraction grating theory (3 µm < λ < 5 µm); and, uniquely, the transition region, where the specular reflection and diffraction contributions are comparable (1 µm < λ < 3 µm). Our results inform system performance parameters, provide optical design constraints, and create a framework of use cases for DMDs.

The cavity supports the orthogonal reference beam families as its eigenmodes while enhancing the reference beam power. Such orthogonal eigenmodes are used as additional degree of freedom to multiplex data pages, consequently increase storage densities for volume Holographic Data Storage Systems (HDSS) when the maximum number of multiplexed data page is limited by geometrical factor. Image bearing holograms are multiplexed by orthogonal phase code multiplexing via Hermite-Gaussian eigenmodes in a Fe:LiNbO3 medium with a 532 nm laser at multiple Bragg angles by using Liquid Crystal on Silicon (LCOS) spatial light modulators (SLMs) in reference arms. Total of nine holograms are recorded with three angular and three eigenmode.

We have developed a high-resolution, multi-color retinal imaging system using liquid crystal adaptive optics. A liquid crystal on silicon (LCOS) spatial light modulator (SLM) is used to correct ocular aberrations. In order to compensate for the dependency of an LCOS SLM on optical wavelength and acquire aberration-corrected images at different color, we apply an open-loop technique. In the open-loop technique, the imaging light is separated from the sensing light and the optimal phase modulation is applied only to the imaging light while the sensing light is not phase-modulated. With the system, in vivo imaging of the human retina is achieved by using illumination light at wavelength of 655nm and 593nm and sensing light at 780nm. Photoreceptors are clearly revealed at each illumination wavelength with the liquid crystal adaptive optics.

2-D optical phased array (OPA) antenna based on a liquid crystal on silicon (LCOS) spatial light modulator is introduced. Diffraction efficiency of the 1-D OPA for fine angle beam steering and 2-D OPA for aberration correction is characterized experimentally and by computer simulation. Excellent agreement between the simulation, measurement and empirical model is observed. Large aperture transmitter for long distance free space communication is analyzed by wave optics simulation. The system performance for beam steering and correction for wavefront aberration in large aperture transmitter is discussed.

We present a method for reducing the phase flicker originated by the pulsed modulation of a Liquid Crystal on Silicon (LCoS) Spatial Light Modulator (SLM). It consists in reducing the temperature of the LCoS in a controlled way, in order to increase the viscosity of the liquid crystal. By doing this, we increase the time response of the liquid crystal, and thus reduce the amplitude of phase fluctuations. We evaluate the efficacy of this method quantifying the temporal evolution of phase shift using an experiment that is insensitive to optical polarization fluctuations. Additionally, we determine the effect of the temperature reduction on the effective phase modulation capability of the LCoS. We demonstrate that a reduction of up to 80% of the flicker initial value can be achieved when the LCoS is brought to -8 °C.

Our company has developed a new technology of phase modulation of light wave by means of tunable diffraction phase gratings. On this basis, a new class of electro‐optical Light Modulators Relief (LMR) with the following characteristics has been developed. Pixel switching frequency: up to 0.5 MH, efficiency: 98.6% (key competitors: Digital Micromirror Device (DMD) has 40%, Liquid Crystal on Silicon (LCoS) has 25%). High resistance to the laser radiation ‐ up to 100 W. Light polarization is not required. Diffraction optics with light flux up to 32 klm. Response time is 2‐7 microsecond. The theoretical resolution limit is 2000 lin/mm. The research program is over. The serial manufacture cost of LMR can be compared with the one of LCoS. The most promising applications of LMR are 1) pico‐projectors built into phone or laptop, 2) solid state scanning device without moving parts for LiDAR (Light Detection and Ranging) and other applications, 3) optical transponder for autonomous robots and vehicles, 4) holographic projectors, 5) terabyte memory, 6) next generation laser light source for car headlights, for office and household lights with controllable spectrum of light at any time. The technology is ready for industrial production. Modern industrial needs require not only minimising the price and physical volume of electro‐optical products, but also increasing the efficiency and processing speed, improving the information visualization technique with better functional and operational characteristics. These properties are fully possessed by the ultra‐fast electro‐optical LMR, which has no analogues in the world, developed by NanoRelief Display Ltd.