Liquid Crystal Spatial Light Modulator Research Articles

Active compensation of extrinsic polarization errors using adaptive optics.

We present a scheme for active compensation of complex extrinsic polarization perturbations introduced into an optical system. Imaging polarimeter is used to measure the polarization state across a beam profile and a liquid crystal spatial light modulator controls the polarization of the input beam. A sequence of measurements permits determination of the birefringence properties of a perturbing specimen. The necessary correction is calculated and fed back to the polarization modulator to compensate for the polarization perturbation. The system capabilities are demonstrated on a range of birefringent specimens.

Dual-beam cross-correlation spectrometer for radial velocity measurements.

Measuring the radial velocity of an object can be achieved by quantifying the Doppler shift of Fraunhofer lines. Measurements are typically made using high-resolution conventional spectroscopy, in which the Doppler shift is calculated numerically on a computer. An alternative technique includes cross-correlation spectroscopy, which performs an optical correlation of the incident spectrum against a reference spectrum embedded in the instrument. Many existing correlation spectrometers leverage a chrome mask and obtain a single beam measurement, making the sensors more sensitive to atmospheric turbulence without moving parts. In this paper, we present a static dual-beam polarization-based technique for acquiring cross-correlation spectra that is insensitive to atmospheric turbulence and contains no moving parts. The instrument is based on acquiring light both inside and outside of the solar Fraunhofer lines using a twisted nematic liquid-crystal spatial light modulator. Correlation spectra can be calculated as a ratio of these two components. A model of the dual-beam cross-correlation spectrometer is presented and subsequently validated with experimental observations of Venus. Radial velocity accuracies, as calculated against reference ephemerides, yielded an absolute error less than 0.24%.

Self-referenced multiple-beam interferometric method for robust phase calibration of spatial light modulator.

Phase-only liquid crystal spatial light modulator has wide ranging applications that require accurate phase retardance. The phase calibration of the spatial light modulator is therefore of vital importance. Available self-referenced calibration methods face the challenges of high time consumption, low efficiency, and low stability against the conditions. A self-referenced multiple-beam interferometric method is proposed to derive the global grayscale-phase response. As is presented theoretically and experimentally, the proposed method reduces the measuring time and improves the calibration efficiency by encoding multiple fringes in a single hologram. Results also show that the method is equally accurate when compared with traditional two-beam interferometric method, whereas providing a greater robustness against measuring errors since the standard deviation is only 56% of that of the traditional method.

Partially coherent Bessel vortex superposition with linear charge increase and aligned maxima

In this paper, the generation of a partially coherent optical vortex composed of the incoherent superposition of N coaxial Bessel vortex beams with linearly increasing topological charges and aligned intensity maxima is proposed. To generate this beam, concentric ring slits with increasing topological charges and random phase shifts are optically Fourier transformed. The radii of the ring slits are chosen in such a way that the maxima of their corresponding Bessel functions are coincident. It is demonstrated that the resulting beam radius is independent of the topological charges involved in the superposition and the beam is propagation invariant. The Bessel vortex addition is experimentally generated by means of a phase-only liquid crystal spatial light modulator.

Holographic scanning confocal microscopy for both reflected light and fluorescence light imaging

In this paper, we propose a holographic scanning based confocal microscope that works in both the reflection and fluorescence modes. Here, the illumination beam is scanned using a reconfigurable binary hologram instead of a usual galvanometer mirror scanner. The reconfigurable hologram is implemented using a liquid crystal spatial light modulator. Two different lasers are used for fluorescence and reflection mode imaging with the phase profile of the illumination beam, derived from each laser, described by the holograms. We demonstrate the working of our microscope in both the modes by imaging a section of tooth specimen stained with a fluorescent dye.

Aberration-free multi-plane imaging of neural activity from the mammalian brain using a fast-switching liquid crystal spatial light modulator.

We report a novel two-photon fluorescence microscope based on a fast-switching liquid crystal spatial light modulator and a pair of galvo-resonant scanners for large-scale recording of neural activity from the mammalian brain. The spatial light modulator is used to achieve fast switching between different imaging planes in multi-plane imaging and correct for intrinsic optical aberrations associated with this imaging scheme. The utilized imaging technique is capable of monitoring the neural activity from large populations of neurons with known coordinates spread across different layers of the neocortex in awake and behaving mice, regardless of the fluorescent labeling strategy. During each imaging session, all visual stimulus driven somatic activity could be recorded in the same behavior state. We observed heterogeneous response to different types of visual stimuli from 3,300 excitatory neurons reaching from layer II/III to V of the striate cortex.

Color LED DMD holographic display with high resolution across large depth.

Holographic displays employing digital micromirror devices (DMDs) reconstruct 3D images at high diffraction orders. For LED displays, this geometry introduces large dispersion at the DMD surface, reducing image resolution and depth. This work proposes a color DMD LED holographic display with dispersion compensation utilizing an additional diffraction grating in an illumination module. The solution allows to obtain image depth up to 100mm, which is comparable to the one achieved by a liquid crystal spatial light modulator, where the limiting factor is spatial coherence of the source. Experimental comparison of the results obtained with the laser and LED source gives evidence of effective speckle noise reduction, even from a single frame.

Model-based compensation of pixel crosstalk in liquid crystal spatial light modulators.

Spatial light modulators (SLMs) based on liquid crystals are widely used for wavefront shaping. Their large number of pixels allows one to create complex wavefronts. The crosstalk between neighboring pixels, also known as fringing field effect, however, can lead to strong deviations. The realized wavefront may deviate significantly from the prediction based on the idealized assumption that the response across a pixel is uniform and independent of its neighbors. Detailed numerical simulations of the SLM response based on a full 3D physical model accurately match the measured response and properly model the pixel crosstalk. The full model is then used to validate a simplified model that enables much faster crosstalk evaluation and pattern optimization beyond standard performance. General conclusions on how to minimize crosstalk in liquid crystal on silicon (LCoS) SLM systems are derived, as well as a readily accessible estimation of the amount of fringing in a given SLM.

Holographic display technology based on liquid crystal device

AbstractHolographic display technology can reconstruct the same images of the original objects exactly, which has played a more and more important role and become the goal of the three‐dimensional display. As liquid crystal devices can modulate the polarization state of light, they have been widely used in the holographic display to modulate the phase and amplitude of information. Among them, liquid crystal spatial light modulator and liquid crystal lens are two important devices in the holographic diffraction. In this paper, several liquid crystal devices have been introduced and the holographic display technologies based on these liquid crystal devices have been discussed. The merits and demerits of these technologies based on the liquid crystal devices have been analyzed, and the outlook of the holographic display is given in the end.

Correction of Fabry-Pérot interference effects in phase and amplitude pulse shapers based on liquid crystal spatial light modulators

Broadband femtosecond laser pulses manipulated by pulse shapers based on a liquid crystal spatial light modulator (LC-SLM) inevitably experience periodic spectral distortions due to Fabry-Perot interference effects within the LC-SLM. We present a method, applicable to phase and amplitude pulse shapers based on dual LC-SLMs, that enables the calibration and suppression of the undesired spectral intensity modulations in a non-iterative fashion. We demonstrate that the method considerably improves the amplitude shaping fidelity of phase and amplitude pulse shapers without compromising the phase shaping properties.

A conversion method of two-dimensional image into three-dimensional holographic display

A method to convert two-dimensional(2D) image into three-dimensional(3D) holography display is proposed. The conjectured phase distribution is directly obtained by a polynomial equation based on the 2D image, and is superimposed over the object plane. The parameters of polynomial equation are automatically calculated by the gray value of 2D image. A special kinoform is designed, which contains the phase information generated by Fresnel diffraction algorithm. It can effectively suppress the speckle noise. The sequential kinoforms are reconstructed by the 3D dynamic holographic display system with phase-only liquid crystal spatial light modulator (LC-SLM) and the cylindrical fog screen. The results of simulation and optic experiment exhibit that the proposed method can reconstruct 3D scenes with a better vision effect. The display results show more advantages, such as high contrast, clear detail and low noise. The method presented in this paper can be applied to high performance 3D dynamic holographic display.

Liquid Crystal Spatial Light Modulator with Optimized Phase Modulation Ranges to Display Multiorder Diffractive Elements

A liquid crystal on silicon spatial light modulator (LCoS SLM) with large phase modulation has been thoroughly characterized to operate optimally with several linear phase modulation ranges (π, 2π, 3π, 4π, 6π, and 8π) for an intermediate wavelength of the visible spectrum (λG = 530 nm). For each range, the device response was also measured for two additional wavelengths at the blue and red extremes of the visible spectrum (λB = 476 nm and λR = 647 nm). Multiorder diffractive optical elements, displayed on the LCoS SLM with the appropriate phase modulation range, allowed us to deal with some widely known encoding issues of conventional first-order diffractive lenses such as undersampling and longitudinal chromatic aberration. We designed an achromatic multiorder lens and implemented it experimentally on the SLM. As a result, the residual chromatic aberration reduces to one-third that of the chromatic aberration of a conventional first-order diffractive lens.

Calibration of a phase-only spatial light modulator for both phase and retardance modulation.

Liquid crystal spatial light modulators (SLMs) are usually configured and calibrated for phase modulation. However, as they are variable retarders, they also have application as polarization modulators. We show that conventional phase-only calibrations are insufficient for this purpose, and a separate retardance calibration is needed. To overcome this shortcoming we report a simple Twyman-Green interferometer-based setup to realize SLM phase and retardance calibration. For phase calibration, we identify the non-linear, spatially variant response to the drive voltage of the SLM using fringe analysis and both horizontally and vertically polarized incident light. For retardance calibration, we use incident light polarized at 45° and assess the intensity variation. The methods presented are compatible with in situ calibration of SLMs.

Hybrid cryptosystem based on diffraction transfer function and phase-truncated Fourier transform encryption

A hybrid cryptosystem based on phase-truncated Fourier transform (PTFT) encryption and diffraction transfer function is designed. The designed cryptosystem cannot be decrypted by phase-retrieval-based attacks to which conventional phase-truncated Fourier transform cryptosystems are proven vulnerable. Compared with conventional PTFT encryption, the encryption process of this proposed cryptosystem needs no extra devices. With the help of Rayleigh–Sommerfeld diffraction theory, the inverse diffraction optically is realized and the decryption process can be performed using a liquid crystal spatial light modulator. Unlike conventional PTFT cryptosystem, no useful information can be retrieved in case of lacking any of ciphertext, private keys and diffraction parameters. The optical setup and numerical simulation results are provided to verify the effectiveness, security and robustness of proposed hybrid cryptosystem.

Electronic holographic three-dimensional display with enlarged viewing angle using non-mechanical scanning technology

A non-mechanical scanning method based on a liquid-crystal spatial light modulator (LC-SLM) is proposed to increase the viewing angle of electronic holographic three-dimensional (3D) displays. A scanning off-axis Fresnel lens is simulated by the LC-SLM to deflect the reconstructed light beam. Using the time-division multiplexing method, the viewing zones are sequentially projected to different positions. When the eyes of the observer are located at the viewing zones, the reconstructed 3D images can be observed. The experimental results and analyses show the feasibility and advantages of the proposed method.

High precision beam steering using a liquid crystal spatial light modulator

Liquid crystal spatial light modulators (LCSLMs) are novel phase modulation devices. Due to the dielectric properties of the orientation layer, the orientation layer has a voltage division effect on the driving voltage. A more effective analysis of the relationship between the driving voltage and phase modulation properties of LCSLMs is needed, as well as improved beam steering accuracy. In this paper, we establish an electrical model of an LCSLM, and propose a method for quantitatively calculating the liquid crystal (LC) layer voltage. This differential iterative method based on nonlinear least squares is used to analyze the LC molecular directors. Considering the thickness of the orientation layer, we calculate the LC director distribution and the voltage division effect of the LC layer, obtaining a more accurate electro-optic characteristic curve of the LCSLM, and verify the calculations by experiment. Experimentally, the average beam pointing error of the LCSLM was 0.0098° at 532 nm, representing a significant improvement of the beam steering accuracy.

Progress in Phase Calibration for Liquid Crystal Spatial Light Modulators

Phase-only Spatial Light Modulator (SLM) is one of the most widely used devices for phase modulation. It has been successfully applied in the field with requirements of precision phase modulation such as holographic display, optical tweezers, lithography, etc. However, due to the limitations in the manufacturing process, the grayscale-phase response could be different for every single SLM device, even varying on sections of an SLM panel. A diverse array of calibration methods have been proposed and could be sorted into two categories: the interferometric phase calibration methods and the diffractive phase calibration methods. The principles of phase-only SLM are introduced. The main phase calibration methods are discussed and reviewed. The advantages of these methods are analyzed and compared. The potential methods for different applications are suggested.

Phase modulation characteristics analysis of liquid crystal spatial light modulator under oblique incidence.

A correction method for obtaining the phase modulation curves of liquid crystal spatial light modulator (LCSLM) systems under oblique incidence is proposed. Considering the reflection and refraction of the interface, the influence of oblique incidence on the LCSLM phase modulation was simulated and analyzed by solving the extended Jones matrix. The polarization interference principle was applied to construct a phase modulation system, the phase modulation was measured at different incident angles, and the measured data were subjected to Fourier fitting. The phase modulation curve was corrected to a straight line using the inverse interpolation method, and the lookup table (LUT) for the gray level and phase was then generated through the line coordinates. The measured results revealed that a larger angle of oblique incidence led to a smaller range of phase modulation. For LCSLM to function over a range from 0 to 2π, the oblique incidence angle should be less than 45°. Experimental results using 25° as an example confirmed the validity of the LUT. The correlation coefficient R between the phase modulation curve and the ideal linear curve was 0.9893, and the root-mean-square error was 0.1888. At incident angles from 0° to 45°, the linearity of the corrected LUT was as good as that of the LUT for vertical incidence, thereby laying the foundation for high-precision beam pointing control.

Accurate aberration correction in confocal microscopy based on modal sensorless method.

Confocal microscopy has the advantages of high resolution and optical sectioning ability over conventional microscopy. However, aberration induced by the optical system can compromise these advantages and considerably reduce the energy reaching the pointlike detector. We propose an accurate aberration correction method with a liquid-crystal spatial light modulator (LCSLM) in the confocal system. Each coefficient of Zernike aberration modes is calculated by directly measuring the variance of the images with different bias aberration modes. Large-coefficient (>0.7 rad) aberration is compensated first by LCSLM, following which aberrations with small coefficients are measured precisely, minimizing the cross talk between different kinds of aberrations. With this predistortion strategy, the aberration correction is much more accurate, and maximum image intensity in the normal and nonconjugated systems is improved by 2.5 times and 4 times compared to the normal correction method, respectively, demonstrating the effectiveness of our method.

Computer-generated holograms for complex surface reliefs on azopolymer films

The light-driven superficial structuration observed on the surface of films of azobenzene-containing polymers follows the optical field distribution of the illuminating light pattern, i.e. the light polarization state and the intensity distribution. The ability to precisely manipulate the illuminating intensity pattern can hence provide a new level in the range of complex light-induced superficial textures accessible onto azopolymer film surfaces. In this respect, digital holography, based on the principles of the Computer-Generated Holograms (CGHs), and actually implemented by means of a versatile liquid crystal spatial light modulator, can represent a unique experimental tool in the field of the light-induced mass migration in azo-materials. In the present work, we demonstrate the possibility to precisely control the features and the quality of complex light patterns generated through CGHs in order to induce arbitrarily complex surface reliefs onto the surface of an azopolymer. The results shown here can potentially broaden the range of possible applications of photo-responsive azopolymer films in the fields of surface engineering, biology and photonics.