High-resolution Spatial Light Modulators Research Articles

Periodic and aperiodic liquid crystal-polymer composite structures realized via spatial light modulator direct holography

In this work we present the first realization and characterization of two-dimensional periodic and aperiodic POLICRYPS (Polymer Liquid Crystal Polymer Slices) structures, obtained by means of a single-beam holographic technique exploiting a high resolution spatial light modulator (SLM). A first investigation shows that the gratings, operating in the Raman Nath regime, exhibit a morphology and a electro-optical behavior that are typical of the POLICRYPS gratings realized by two-beam interference holography.

Lensless digital holography with diffuse illumination through a pseudo-random phase mask

Microscopic imaging with a setup consisting of a pseudo-random phase mask, and an open CMOS camera, without an imaging objective, is demonstrated. The pseudo random phase mask acts as a diffuser for an incoming laser beam, scattering a speckle pattern to a CMOS chip, which is recorded once as a reference. A sample which is afterwards inserted somewhere in the optical beam path changes the speckle pattern. A single (non-iterative) image processing step, comparing the modified speckle pattern with the previously recorded one, generates a sharp image of the sample. After a first calibration the method works in real-time and allows quantitative imaging of complex (amplitude and phase) samples in an extended three-dimensional volume. Since no lenses are used, the method is free from lens abberations. Compared to standard inline holography the diffuse sample illumination improves the axial sectioning capability by increasing the effective numerical aperture in the illumination path, and it suppresses the undesired so-called twin images. For demonstration, a high resolution spatial light modulator (SLM) is programmed to act as the pseudo-random phase mask. We show experimental results, imaging microscopic biological samples, e.g. insects, within an extended volume at a distance of 15 cm with a transverse and longitudinal resolution of about 60 μm and 400 μm, respectively.

Invariant correlation filter with linear phase coefficient holographic realization in 4-F correlator

Realization of distortion invariant correlation filters in optical image correlators open possibilities for object identification with remarkably high computational capabilities. Application of the linear phase coefficient composite filter (LPCCF) is attractive for recognition of binary edged images. We use methods of digital holographic synthesis to realize LPCCF in a coherent 4-F correlator as a computer-generated amplitude holographic filter. A high resolution spatial light modulator (SLM) has to be implemented for such a filter representation. Transparency limitations of high frame rate and high resolution SLM's and its effect on recognition performance of holographic filter in the 4-F correlator are discussed in the given paper.

What spatial light modulators can do for optical microscopy

AbstractWith the availability of high‐resolution miniature spatial light modulators (SLMs) new methods in optical microscopy have become feasible. The SLMs discussed in this review consist of miniature liquid crystal displays with micron‐sized pixels that can modulate the phase and/or amplitude of an optical wavefront. In microscopy they can be used to control and shape the sample illumination, or they can act as spatial Fourier filters in the imaging path. Some of these applications are reviewed in this article. One of them, called spiral phase contrast, generates isotropic edge enhancement of thin phase samples or spiral‐shaped interference fringes for thicker phase samples, which can be used to reconstruct the phase topography from a single on‐axis interferogram. If SLMs are used for both illumination control and spatial Fourier filtering, this combination for instance allows for the generalization of the Zernike phase contrast principle. The new SLM‐based approach improves the effective resolution and avoids some shortcomings and artifacts of the traditional method. The main advantage of SLMs in microscopy is their flexibility, as one can realize various operation modes in the same setup, without the need for changing any hardware components, simply by electronically switching the phase pattern displayed on the SLMs.

Depth of field multiplexing in microscopy

We demonstrate "depth of field multiplexing" by a high resolution spatial light modulator (SLM) in a Fourier plane in the imaging path of a standard microscope. This approach provides simultaneous imaging of different focal planes in a sample with only a single camera exposure. The phase mask on the SLM corresponds to a set of superposed multi-focal off-axis Fresnel lenses, which sharply image different focal planes of the object to non-overlapping adjacent sections of the camera chip. Depth of field multiplexing allows to record motion in a three dimensional sample volume in real-time, which is exemplarily demonstrated for cytoplasmic streaming in plant cells and rapidly swimming protozoa.

Optically addressed high-resolution liquid crystal spatial light modulator with a diffraction efficiency above 50%

An optically addressed liquid crystal spatial light modulator (SLM) with a photosensitive amorphous hydrogenated silicon carbide (a-SiC:H) layer has been studied using holographic techniques. The maximum diffraction efficiency (DE) of the device amounted to ηmax = 50.5%. A decrease in the DE to a level of 0.5ηmax was observed at a spatial frequency of 75 mm−1. The high resolution and DE of the proposed SLM are due to features in the formation of an asymmetric grating line profile in the given structure.

Holographic generation of complex fields with spatial light modulators: application to quantum key distribution

There has been considerable interest recently in the generation of azimuthal phase functions associated with photon orbital angular momentum (OAM) for high-dimensional quantum key distribution. The generation of secure quantum keys requires not only this pure phase basis but also additional bases comprised of orthonormal superposition states formed from the pure states. These bases are also known as mutually unbiased bases (MUBs) and include quantum states whose wave functions are modulated in both phase and amplitude. Although modulo 2pi optical path control with high-resolution spatial light modulators (SLMs) is well suited to creating the azimuthal phases associated with the pure states, it does not introduce the amplitude modulation associated with the MUB superposition states. Using computer-generated holography (CGH) with the Leith-Upatnieks approach to hologram recording, however, both phase and amplitude modulation can be achieved. We present a description of the OAM states of a three-dimensional MUB system and analyze the construction of these states via CGH with a phase-modulating SLM. The effects of phase holography artifacts on quantum-state generation are quantified and a prescription for avoiding these artifacts by preconditioning the hologram function is presented. Practical effects associated with spatially isolating the first-order diffracted field are also quantified, and a demonstration utilizing a liquid-crystal SLM is presented.

Implementation of Spatial Light Modulator(SLM) using a Commercial LCD Beam Projector

In this paper, a new high resolution XGA-SLM is implemented through modification of a commercial TFT-LCD beam projector and its optical modulation characteristics as a spatial light modulator(SLM) is also analyzed.BR First, the optics module, projection lamp and fans are removed from a commercial beam projector and instead, some electric circuits to compensate their removal are manufactured and then, by inserting them into the beam projector, a new XGA-SLM is finally implemented.BR Second, from some optical experimental results, this TFT-SLM is found to have a good optical linearity in amplitude and phase modulation characteristics as a function of the input gray levels. Especially, through implementation of a binary phase-type correlator such as BPEJTC by using the suggested TFT-LCD panel, the implemented SLM is proposed as a new relatively low-cost and high resolution SLM for optical information processing.

Spiral phase contrast imaging in microscopy

We demonstrate an optical method for edge contrast enhancement in light microscopy. The method is based on holographic Fourier plane filtering of the microscopic image with a spiral phase element (also called vortex phase or helical phase filter) displayed as an off-axis hologram at a computer controlled high resolution spatial light modulator (SLM) in the optical imaging pathway. The phase hologram imprints a helical phase term of the form exp(i phi) on the diffracted light field in its Fourier plane. In the image plane, this results in a strong and isotropic edge contrast enhancement for both amplitude and phase objects.

Analysis of a high-resolution optical wave-front control system*1

We consider the formulation and analysis of a problem of automatic control: correcting for the distortion induced in an optical wave front due to propagation through a turbulent atmosphere. It has recently been demonstrated that high-resolution optical wave-front distortion suppression can be achieved using feedback systems based on high-resolution spatial light modulators and phase-contrast techniques. We examine the modeling and analysis of such adaptive optic systems, and show that under certain conditions, the nonlinear dynamical system models obtained are gradient systems (with energy functions that also serve as Lyapunov functions). These gradient systems (employing fixed phase-contrast sensors) serve as a starting point for understanding the design of practical high-resolution wave-front correction systems, in which the phase-contrast sensor itself is subject to control.

Holographic Memories Using 2-Dimensional Phase-Code Multiplexing Method

When implementing a phase-code multiplexing method in real holographic storage systems, there are several factors of cross-talk noise which degrade signal-to-noise ratio (SNR). To reduce such problems, we developed a new and practical phase-code multiplexing method using a 2-dimensional high-resolution phase-only spatial light modulator (P-SLM) which encodes a reference beam with orthogonal phase codes. Theoretically and experimentally, we found that removing a particular all 0 phase code which is always generated by Walsh-Hadamard matrices is quite effective in reducing the cross-talk noise. Moreover, we found that periodically tiled phase-code patterns reduce the cross-talk noise due to inhomogeneous illumination of the P-SLM in real systems. By combining these methods, we achieved bit error rates (BER) for multiplexed holograms on the order of 10-4–10-3 without error correction.

Diffractive optical tweezers in the Fresnel regime

We demonstrate a flexible setup for holographic steering of laser tweezers in microscopy using a high resolution spatial light modulator (SLM). In contrast to other methods, hologram read-out is done in the off-axis Fresnel regime rather than in the typically used on-axis Fourier regime. The diffractive structure is calculated as a Fresnel hologram, such that after reflection at the SLM only the desired first diffraction order is guided to the input of an optical microscope, where it generates a tailored optical tweezers field. We demonstrate some advantageous features of this setup, i.e. undesired diffraction orders are suppressed, the optical traps can be easily steered in real-time by just "mouse-dragging" a hologram window at the SLM display, and a number of independently steerable optical traps can be generated simultaneously in a three-dimensional arrangement by displaying a corresponding number of adjacent hologram windows at the SLM screen.

Analysis of a high-resolution optical wave-front control system

We consider the formulation and analysis of a problem of automatic control: correcting for the distortion induced in an optical wave front due to propagation through a turbulent atmosphere. It has recently been demonstrated that high-resolution optical wave-front distortion suppression can be achieved using feedback systems based on high-resolution spatial light modulators and phase-contrast techniques. We examine the modeling and analysis of such adaptive optic systems, and show that under certain conditions, the nonlinear dynamical system models obtained are gradient systems (with energy functions that also serve as Lyapunov functions). These gradient systems (employing fixed phase-contrast sensors) serve as a starting point for understanding the design of practical high-resolution wave-front correction systems, in which the phase-contrast sensor itself is subject to control.

Räumliche und zeitliche Kontrolle über Ultraschallfelder mittels optoakustischer Holographie

The present paper presents a new method for generating ultrasound, based on the interaction of laser-induced ultrasound generation and ultrasonic holography. An ultrasonic field generated in a water tank via the optoacoustic effect is spatially and temporally controlled: In order to produce defined ultrasonic frequencies in the MHz range, the laser pulses incident on a light-absorbing layer are modulated in time using an electro-optic modulator (EOM). Additionally, a high-resolution liquid crystal spatial light modulator (SLM) is used to imprint a pre-calculated phase front to the laser beam. A computer-generated binary hologram is also displayed at the SLM. The expanded laser beam projects the corresponding pattern to the plane absorptive layer in the water tank. The projection of specific patterns for the generation of ultrasonic beams resembles the use of “diffractive optical elements” in optics. Optical ultrasound generation with holographic steering is a flexible tool with promising numerous new applications in medical and technical ultrasound diagnostics.

Optical design of a miniature Fourier transform lens system for a hybrid digital-optical correlator

The availability of high resolution spatial light modulators (SLMs) and modern digital signal processors (DSPs) allows a convenient hybridization of digital and optical image processing to produce a small and compact correlator system. The hybridization allows the optical Fourier transform lens system for a correlator to be compact, such that it will fit inside a conventional PC. We describe the design of the optical system. © 2002 Society of Photo-Optical Instrumentation Engineers.

Motionless multiresolution interferometry based on Talbot effect

We describe the principle of a motionless interferometer based on the Talbot effect. The test object is scanned by a convergent spherical beam with variable curvatures and lateral shifts. The wave- fronts are generated by a programmable off-axis Fresnel zone plate, implemented on a high resolution and high speed binary phase spatial light modulator. The aberrated field is then analyzed by passage through a fixed binary phase grating, detection by a sensor located in a fractional Talbot plane, and numerical phase retrieval from the Fourier analysis of the reconstructed intensity distribution. We describe the principle of the object multiresolution analysis. The system operation is then illustrated experimentally in a simple case exhibiting subwavelength resolution ca- pability. Finally, we give some orders of magnitude for the dynamic range that can be expected with such a system, as well as possible extension toward very high resolution measurements. © 1999 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(99)01212-X)

Practical issues for the use of liquid crystal spatial light modulators in adaptive optics

Low cost, accurate, high resolution spatial light modulators are of increasing interest for adaptive optics applications. Most adaptive optics systems currently use expensive segmented or deformable mirrors. Nematic liquid crystal spatial light modulators have been suggested as possible substitutes for these mirrors as the phase modulating arrays in adaptive optics systems. This paper discusses broad wavelength band, and polarisation insensitive operation of two different liquid crystal spatial light modulators currently available and under investigation as possible phase modulating arrays for adaptive optics systems. The results of our investigations give a good indication of how the devices will operate in practical adaptive optics systems.

High resolution adaptive optics using an interference phase loop

The Interference Phase Loop is an interferometer with feedback of the output intensity to a phase modulator in one of the inteferometer arms. When a spatial optical phase modulator is used, the modulator phase distribution approximates to the conjugate of the spatial phase distribution in the interferometer, and so the device has potential for application in adaptive optics. We have implemented an interference phase loop using a high-resolution liquid crystal spatial light modulator (PAL-SLM) and a common path radial shearing interferometer. We show how the system can be analysed in simple terms, derive some design and operating criteria, and demonstrate diffraction-limited real-time aberration correction of arbitrary input wavefronts using the system. Because of the nature of the interferometer used, our system has the potential to operate in white light.

Photovoltaic spatial light modulator

A novel concept for implementing a high-resolution spatial light modulator using a thin slab of photorefractive LiNbO3 crystal is described. This method uses the photovoltaic effect to impress phase information onto the crystal without the use of holography where coherent reference beams are required. Experimental demonstration as well as an analysis of the operation and performance of the device are given.

Real-time hologram construction and reconstruction using a high-resolution spatial light modulator

Real-time construction and reconstruction of Fresnel holograms is demonstrated using an optically-addressable ferroelectric liquid-crystal spatial light modulator with a resolution up to 175 line pairs/mm. A bright holographic image is read out due to the optical separation between the write and read sides by a dielectric mirror.