Phase-only Liquid-crystal Spatial Light Modulator Research Articles

Calibration of phase-only liquid-crystal spatial light modulators by diffractogram analysis

Phase-only spatial light modulator (SLM) is a key device for light manipulation. It is critical to calibrate the grayscale-phase response to the driving voltage of SLM, namely the look-up table (LUT), before use. The conventional calibration methods, which rely on interference, diffraction, or polarization to extract the phase, require extra optical devices and are sensitive to mechanical vibration, air turbulence, and systematic aberrations. To overcome these shortcomings, here we develop a novel method, which is based on the diffractogram analysis. By analyzing the intensity distribution of the diffractogram as a function of the specific SLM hologram, we identify both the global and the localized LUTs of SLM at high accuracy. Compared with the other ones, our method allows in-situ SLM calibration and is more tolerant to the exogenous factors.

Efficient wavefront sensorless adaptive optics based on large dynamic crosstalk-free holographic modal wavefront sensing.

The correction of wavefront sensorless adaptive optics (WFSless AO) can be significantly accelerated by using a holographic modal wavefront sensor (HMWFS). The HMWFS is realized by a computer-generated hologram (CGH) into which all aberration modes to be detected are encoded and only a single-shot image is required for simultaneous measurement of multiple modes. The conventional HMWFS suffers from a quite limited dynamic range and severe inter-modal crosstalk which deteriorates the sensing accuracy. We proposed a novel HMWFS with a large dynamic range and no crosstalk and validated its performance by simulation and experiment. In the improved HMWFS scheme, the aberration is represented by Lukosz modes whose gradients are orthogonal and the modal coefficients can be estimated independently. Instead of using a binary CGH in conventional HMWFS, a kinoform CGH with high diffraction efficiency is adopted in the improved HMWFS. The kinoform CGH is produced by a phase-only liquid-crystal spatial light modulator (LCSLM) which also serves as a wavefront corrector in our WFSless AO system.

Reduction of spatio-temporal phase fluctuation in a spatial light modulator using linear phase superimposition

Spatio-temporal phase modulation with a phase-only liquid-crystal spatial light modulator (SLM) plays an important role in the optics and photonics community. SLMs are generally affected by either or both spatial and temporal phase fluctuations, depending on driver electronics, thereby reducing the quality of a generated beam. In this study, to reduce phase fluctuations, we present an optical-based linear phase superimposition method with spatial bandpass filtering. We experimentally investigate the method’s effectiveness, particularly for holographic data storage applications. Experimental results show that the presented method is useful in robustly generating phase distributions against fluctuations, regardless of the SLM driving scheme.

Multiple optical vortices generated by azimuthal fractal spiral zone plates based on liquid crystal spatial light modulator

Azimuthal fractal spiral zone plates (AFSZPs) with particular fractal characteristics are proposed which have the ability to increase the number of foci comparing with conventional spiral zone plates. The generated beams possess sequence of optical vortices along the propagation direction with preset resolution. Numerical simulations are carried out under plane wave illumination and the good performance of this fractal diffractive optical elements are verified experimentally based on a programmable phase-only liquid-crystal spatial light modulator (SLM). In addition, the effects on the axial irradiance and resolution of foci caused by variations of parameters are also presented.

Passive ranging and a three-dimensional imaging system through wavefront coding**Project supported by the National Natural Science Foundation of China (Grant No. 61205158) and the Natural Science Foundation of Zhejiang Province, China (Grant No. LY15F050013).

A single-image passive ranging and three-dimensional (3D) imaging system with chiral phase encoding was proposed in 2011 [Opt. Lett. 36, 115 (2011)]. A new theoretical analysis of the system in space domain is presented in this paper. We deduce the analytic relationships between the object distance and the point spread function, and between the object distance and the encoded image, respectively. Both the point spread function and the processed spectrum of the encoded image have two spots, which will rotate with the variation of the object distance. Then the depth map is extracted from the encoded image and it can be used to set up 3D images. The theoretical analysis is verified by a wavefront coding system with a chiral phase which is generated by a phase-only liquid-crystal spatial light modulator. The phase generated by the liquid-crystal spatial light modulator is more flexible than the fixed phase mask and can be adjusted in real time. It is especially suitable for observing the object with a large depth of field.

Generating perfect polarization vortices through encoding liquid-crystal display devices.

Perfect polarization vortices (PPVs) are a new kind of vector beams with identical diameters. In this paper, we propose an approach that uses encoded liquid-crystal display devices to generate PPVs and is based on the Fourier transformation of high-order Bessel-Gaussian (BG) beams. The liquid-crystal display device used here is a phase-only liquid-crystal spatial light modulator (SLM). In the experiment, a hologram consists of a holographic axicon, and a spiral phase plate is encoded on a SLM to generate high-order BG beams. Then, another SLM and a convex lens can realize the transformation from BG beams into PPVs. We analyze the diameter of the PPV obtained by our approach, and find it is impacted by the focal length of the lens as well as the period of the holographic axicon, but is little influenced by polarization order.

基于纯相位液晶空间光调制器的全息光学捕获与微操纵

an improved and rapid three dimensional gerchberg-saxton (gs) algorithm based on the classic gs algorithm for computer generated holograms is proposed and applied to holographic optical tweezers. theoretical simulations and experimental results have demonstrated the rapidity and efficiency of the proposed algorithm. a robust holographic optical tweezers setup based on phase-only liquid-crystal spatial light modulator(slm) is built, and stable trapping and dynamic manipulation of yeast cells and silica beads with large array traps in three dimensions are demonstrated. two special traps, i.e., line trap with intensity gradient distribution and optical vortex trap, are generated to transport and rotate micro-particles respectively. the system is verified to be robust on particles manipulations, which provides a new and powerful tool for researches on biology, colloid physics and so on. © 2016, chinese laser press. all right reserved.

Experimentally generating any desired partially coherent Schell-model source using phase-only control

A technique is presented to produce any desired partially coherent Schell-model source using a single phase-only liquid-crystal spatial light modulator (SLM). Existing methods use SLMs in combination with amplitude filters to manipulate the phase and amplitude of an initially coherent source. The technique presented here controls both the phase and amplitude using a single SLM, thereby making the amplitude filters unnecessary. This simplifies the optical setup and significantly increases the utility and flexibility of the resulting system. The analytical development of the technique is presented and discussed. To validate the proposed approach, experimental results of three partially coherent Schell-model sources are presented and analyzed. A brief discussion of possible applications is provided in closing.

Demonstration of polarization-insensitive spatial light modulation using a single polarization-sensitive spatial light modulator.

We present a simple configuration incorporating a single polarization-sensitive phase-only liquid crystal spatial light modulator (LC-SLM) to facilitate polarization-insensitive spatial light modulation. The polarization-insensitive configuration is formed by a polarization beam splitter (PBS), a polarization-sensitive phase-only LC-SLM, a half-wave plate (HWP), and a mirror in a loop structure. We experimentally demonstrate polarization-insensitive spatial light modulations for incident linearly polarized beams with different polarization states and polarization-multiplexed beams. Polarization-insensitive spatial light modulations generating orbital angular momentum (OAM) beams are demonstrated in the experiment. The designed polarization-insensitive configuration may find promising applications in spatial light modulations accommodating diverse incident polarizations.

Dynamic holographic three-dimensional projection based on liquid crystal spatial light modulator and cylindrical fog screen

A dynamic holographic three-dimensional (3D) projection based on phase-only liquid crystal spatial light modulator (LC-SLM) and cylindrical fog 3D screen is introduced. Sequential kinoforms of a 3D real existing object are calculated from sixty viewing angles using the slice-based fresnel diffraction algorithm. To suppress speckle noise of reconstructed images, sub-kinoforms for each viewing angle are calculated by adding dynamic pseudorandom initial phase factor into each object plane. The sequential kinoforms are reconstructed by a holographic reconstruction system based on phase-only LC-SLM. A specially designed cylindrical fog 3D screen is used as the scattered carrying medium to project the dynamic 3D images. Through our holographic 3D projection system, a vivid dynamic holographic reconstructed projection image can be observed by some observers at the same time.

Generation and three-dimensional characterization of complex nondiffracting optical beams

Nondiffracting optical beams play an important role in contemporary optics due to their special propagation characteristics, i.e., nondiffracting in a diffraction-free zone, shape recovering behind obstacles or self-healing property. Liquid crystal spatial light modulators (LC-SLM) are widely used for generating nondiffracting optical beams in virtue of programmable and dynamic features. In this paper, we propose a complex amplitude modulation technique that can encode any scalar complex fields for generating the complex nondiffracting beams. Before experiment, the phase modulation curve of the phase-only LC-SLM is optimized into being linear in a range of 0-2πby gamma correction in the way of variable binary phase gratings. Then, we experimentally generate the nonaccelerating beams, e.g., two zero-order Bessel beams with variable intensity distributions, and the nondiffracting petal-like beams generated by interfering with two coaxial Bessel beams. By scanning a reflection mirror near the focal region along the optical axis, a stack of two-dimensional images is acquired, and then a three-dimensional intensity profile of the beam is reconstructed with a software. We also experimentally demonstrate a new kind of multi-main-lobe accelerating beam with parabolic accelerating trajectory by modifying the spatial spectrum of classical Airy beam. Compared with the so-called vectorial accelerating beam with multiple main lobes in spheroidal coordinates, our generated two-main-lobe accelerating beam has a very high energy efficiency. The self-healing property of the two-main-lobe accelerating beam is also demonstrated. The presented technique can generate a variety of complex nondiffracting optical beams rapidly and obtain their three-dimensional intensity distributions accurately, which has potential applications in the fields of optical microscope, optical date storage, optical trapping, optical micromachining, etc.

Compensation of phase nonlinearity of liquid crystal spatial light modulator for high-resolution wavefront correction

The ability of phase modulation enables liquid crystal spatial light modulator (LCSLM) to control wavefront. However, the disadvantage of its inherent nonlinear phase response will decrease the wavefront control accuracy. In this paper, a compensation for the nonlinear phase response is proposed based on Inverse Interpolation method. Characteristic curve of phase retardation versus gray levels for a 256×256 pixels phase-only LCSLM has been measured and calibrated by Inverse Interpolation. A mapping relationship between input gray levels and driving gray levels has been built and recorded by a linear look-up table ANTI2.LUT. The nonlinear error of the phase drops from 15.9% to 2.42% by using ANTI2.LUT. Further more, the mapping curve of ANTI2.LUT is almost consistent with 290.LUT from the manufacturer, which proved the efficiency of the compensation of phase nonlinearity. Finally, the distorted wavefront caused by a liquid crystal flake is corrected using LCSLM based on ANTI2.LUT. Experimental results show that the peak-valley value of the distorted wavefront decreases from 1.56λ to 0.26λ (λ =0.6328 μm), the root-mean-square value decreases from 0.25λ to 0.02λ and the Strehl ratio of diffractive spots increases from 0.08 to 0.97. So LCSLM can be applied to realize high-precision and high-resolution wavefront correction with linear phase response.

Variable shape or variable diameter flattop beam tailored by using an adaptive weight FFT-based iterative algorithm and a phase-only liquid crystal spatial light modulator

The variable shape or variable diameter flattop beam shaping technique by using an adaptive weight FFT-based iterative algorithm (AWFFT-IA) and a phase-only liquid crystal spatial light modulator (PO-LCSLM) is demonstrated. The algorithm is used to design the phase distribution for tailoring central symmetric laser beam into the desired shape or diameter flattop beam, the PO-LCSLM serves as dynamically programmable phase filters, which depend on the designed phase distributions, to retard the incident wavefront as we would expect. The experimental results show that the technique can realize variable shape or variable diameter flattop beam shaping.

Hollow Gaussian beam generated by beam shaping with phase-only liquid crystal spatial light modulator

Based on the refractive beam shaping system, the transformation of a quasi-Gaussian beam into a dark hollow Gaussian beam by a phase-only liquid crystal spatial light modulator (LC-SLM) is proposed. According to the energy conservation and constant optical path principle, the phase distribution of the aspheric lens and the phase-only LC-SLM can modulate the wave-front properly to generate the hollow beam. The numerical simulation results indicate that, the dark hollow intensity distribution of the output shaped beam can be maintained well for a certain propagation distance during which the dark region will not decrease whereas the ideal hollow Gaussian beam will do. By designing the phase modulation profile, which loaded into the LC-SLM carefully, the experimental results indicate that the dark hollow intensity distribution of the output shaped beam can be maintained well even at a distance much more than 550 mm from the LC-SLM, which agree with the numerical simulation results.

Generation of dark hollow femtosecond pulsed beam by phase-only liquid crystal spatial light modulator

Based on the refractive laser beam shaping system, the dark hollow femtosecond pulse beam shaping technique with a phase-only liquid crystal spatial light modulator (LC-SLM) is demonstrated. The phase distribution of the LC-SLM is derived by the energy conservation and constant optical path principle. The effects of the shaping system on the temporal properties, including spectral phase distribution and bandwidth of the femtosecond pulse, are analyzed in detail. Experimental results show that the hollow intensity distribution of the output pulsed beam can be maintained much at more than 1200 mm. The spectral phase of the pulse is changed, and the pulse width is expanded from 199 to 230 fs, which is caused by the spatial-temporal coupling effect. The coupling effect mainly depends on the phase-only LC-SLM itself, not on its loaded phase distribution. The experimental results indicate that the proposed shaping setup can generate a dark hollow femtosecond pulsed beam effectively, because the temporal Gaussian waveform is unchanged.

Fast generation of controllable equal-intensity four beams based on isosceles triangle multilevel phase grating realized by liquid crystal spatial light modulator

Liquid crystal spatial light modulator (LCSLM) realizing equal-intensity multiple beams often has some features, i.e., phase valley between two adjacent pixels, flyback region when phase decreases immediately from 2π to 0, and inevitable backplane curvature, which are different from those of most conventional diffractive optical elements (DOEs), such as static DOEs. For optimal intensity uniformity, equal-intensity multi-beam generation must be considered for these artifacts. We present a tunable-grating method in which the intensity uniformity can be improved by considering the LCSLM artifacts. For instance, tuning phase modulation depth of the grating, called isosceles triangle multilevel phase grating (ITMPG), can be used not only to improve the intensity uniformity, but also to fast steer four beams with narrow beamwidths, determined by the same effective aperture of ITMPG. Improved intensity uniformity and high relative diffraction efficiency are demonstrated through experiments with phase-only LCSLM.

Adaptive conversion of multimode beam to near-diffraction-limited flattop beam based on dual-phase-only liquid-crystal spatial light modulators

We propose and demonstrate the adaptive conversion of a multimode beam into a near-diffraction-limited flattop beam in the near field based on a combination of dual-phase-only liquid-crystal spatial light modulators (LC-SLMs) and the stochastic parallel gradient descent (SPGD) algorithm. One phase-only LC-SLM redistributes the intensity of the multimode beam, and the other compensates the wavefront of the output beam. The SPGD algorithm adaptively optimizes the phase distributions of dual-phase-only LC-SLMs to reduce the variance between the actual beam shape and the target beam shape. The experimental results on a fiber multimode beam show that the system is capable of adaptively creating square and rectangle flattop beams with desired parameters. Beam quality can be greatly improved by this system. The power in the main lobe of the far-field spot is about 4 times larger than that of the input multimode beam.

Experimental validation of closed-loop adaptive optics based on a self-referencing interferometer wavefront sensor and a liquid-crystal spatial light modulator

A closed-loop adaptive optics system based on a self-referencing interferometer wavefront sensor (SRI WFS) using phase-shifting point-diffraction technique and an electrically addressed phase-only liquid-crystal spatial light modulator (LC SLM) is built and validated experimentally. The wavefront aberration of incoming beam is directly measured by analyzing four frames phase-shifted interferograms captured by a single CCD camera in two camera shots, and then by loading the conjugate function onto the SLM the wavefront aberration is corrected. The proposed scheme does not rely on any complicated control algorithm or wavefront-reconstruction algorithm and can achieve high-resolution and high-accuracy correction. Closed-loop correction results of single order Zernike aberrations and a Kolmogorov turbulence phase screen show that considerable improvements in the Strehl ratio (of greater than 0.94) is achieved.

利用纯相位型液晶空间光调制器实现空心光束

To improve the power efficiency of the laser system, it is necessary to transform Gaussian intensity distribution to dark hollow intensity distribution. The phase-only liquid crystal spatial light modulator (LC-SLM) has been used to transform a quasi-Gaussian beam to a dark hollow one. The phase distribution of the shaping system was derived according to the energy conservation law and the constant optical length principle. The influences of input beam waist and shape on shaping effect were discussed based on numerical simulation. The generation of the dark hollow beam has been realized experimentally with the phase-only LC-SLM, and the conversion efficiency was larger than 99%．

The performance of phase-only liquid crystal spatial light modulator used for generating Zernike terms

The liquid crystal spatial light modulators (LC-SLM) are recently emerging as wavefront aberration correctors for use in low cost adaptive optics system (AO). As a novel aberration corrector，the wavefront generation performance of a phase-only LC-SLM is a governing factor that determines its ability for wave-front correction in an AO system. In this article，the performance of wave-front generation of LC-SLM is demonstrated by the production and quantification of the first 36 Zernike terms with different values. The factors，such as the fill factor，fringe field effect，pixel structure，the phase wrapping method and different phase strokes to impact wave-front generation are analyzed.