Liquid Crystal Variable Retarders Research Articles

Background suppression structured illumination microscopy based on polarization modulation

We report a structured illumination microscopy (SIM) reconstruction method based on polarization modulation (PM), which can suppress background noise by utilizing the polarization-sensitivity properties of fluorescent dipoles. The proposed method employs a ferroelectric liquid crystal (FLC) spatial light modulator (SLM) to generate multidirectional interference patterns, and uses a combination of two liquid crystal variable phase retarders (LCVRs) and a quarter-wave plate (QWP) for flexible and rapid polarization modulation. The results of simulations and experiments demonstrate that the combination of SIM and PM effectively suppresses background noise in SIM with a slight improvement in resolution. This technique also solves the problem of structural distortion that occasionally exists in SIM and improves the accuracy of the reconstruction.

Optimization analysis of a Stokes polarimeter forbroadband liquid crystal variable retarders underthe optimal objective function.

A liquid crystal variable retarder (LCVR) is the core device to realize fast and high-precision broadband polarization imaging, and its ability to suppress the noise will have an impact on the polarization measurement results. In order to obtain better imaging quality and measurement accuracy, it is crucial to solve the optimization problem of the LCVR. In this paper, the optimal objective function for solving the optimization problem of the LCVR is analytically derived and verified based on the genetic algorithm in the band range of 350-700nm. Meanwhile, considering that the minimum number of four measurements at this time cannot achieve the optimal state, the relationship between the number of measurements and the overall performance relative to the error propagation (optimized conditions number) is discussed. The results show that a better optimal set of angles can be obtained by using the optimal objective function. In this paper, a set of the most favorable angles is obtained, and the optimized average of the CN is 2.0000, which is reduced by 0.32% compared with previous optimization results and is closer to the ideal value of the CN. In addition, in this paper, the noise immunity of the set of most favorable angles is simulated and analyzed, and the optimized system can effectively improve the measured performance of the wide-band liquid crystal variable retarder polarimeter.

Polarized hyperspectral microscopic imaging system for enhancing the visualization of collagen fibers and head and neck squamous cell carcinoma.

Polarized hyperspectral microscopes with the capability of full Stokes vector imaging have potential for many biological and medical applications. The aim of this study is to investigate polarized hyperspectral imaging (PHSI) for improving the visualization of collagen fibers, which is an important biomarker related to tumor development, and improving the differentiation of normal and tumor cells on pathologic slides. We customized a polarized hyperspectral microscopic imaging system comprising an upright microscope with a motorized stage, two linear polarizers, two liquid crystal variable retarders (LCVRs), and a compact SnapScan hyperspectral camera. The polarizers and LCVRs worked in tandem with the hyperspectral camera to acquire polarized hyperspectral images, which were further used to calculate four Stokes vectors: , , , and . Synthetic RGB images of the Stokes vectors were generated for the visualization of cellular components in PHSI images. Regions of interest of collagen, normal cells, and tumor cells in the synthetic RGB images were selected, and spectral signatures of the selected components were extracted for comparison. Specifically, we qualitatively and quantitatively investigated the enhanced visualization and spectral characteristics of dense fibers and sparse fibers in normal stroma tissue, fibers accumulated within tumors, and fibers accumulated around tumors. By employing our customized polarized hyperspectral microscope, we extract the spectral signatures of Stokes vector parameters of collagen as well as of tumor and normal cells. The measurement of Stokes vector parameters increased the image contrast of collagen fibers and cells in the slides. With the spatial and spectral information from the Stokes vector data cubes (, , , and ), our PHSI microscope system enhances the visualization of tumor cells and tumor microenvironment components, thus being beneficial for pathology and oncology.

Measuring small longitudinal phase shifts via weak measurement amplification

Weak measurement amplification, which is considered as a very promising scheme in precision measurement, has been applied to various small physical quantities estimations. Since many physical quantities can be converted into phase signals, it is interesting and important to consider measuring small longitudinal phase shifts by using weak measurement. Here, we propose and experimentally demonstrate a novel weak measurement amplification-based small longitudinal phase estimation, which is suitable for polarization interferometry. We realize one order of magnitude amplification measurement of a small phase signal directly introduced by a liquid crystal variable retarder and show that it is robust to the imperfection of interference. Besides, we analyze the effect of magnification error which is never considered in the previous works, and find the constraint on the magnification. Our results may find important applications in high-precision measurements, e.g., gravitational wave detection.

Optimized configuration for liquid crystal Stokes polarimeters in the presence of fast-axis orientation errors.

We present an optimal configuration for Stokes polarimeters based on liquid crystal variable retarders, with the minimum number of measurements. Due to the inherent variations of the director orientation of the liquid crystal molecules, we propose a configuration that minimizes the sensibility of the polarimeter to fast-axis variations. For the optimization we consider a scheme that maximizes the volume of a tetrahedron inscribed in the Poincare sphere, to address additive and Poisson noise, with one of the vertices invariant to changes in the axis positions. We provide numerical simulations, considering misalignment errors, to analyze the robustness of the configuration. The results show that the proposed configuration helps to maintain the volume enclosed by the tetrahedron with high tolerance to fast-axis orientation errors. The condition number will remain below 3.07 for common misalignment errors and below 1.88 for more controlled liquid crystals. This optimization will improve the performance of liquid crystals polarimeters, with a more robust configuration that also considers misalignment errors, beyond additive and Poisson noise.

Research on one-to-two-point FSO system based on liquid crystal variable retarder cascade polarization grating.

We experimentally demonstrated a one-to-two-point free-space optical communication (FSO) system based on non-mechanical beam servo device in the laboratory. After the initial pointing, two sets of liquid crystal variable retarder cascaded polarization gratings perform non-mechanical beam servo and realized switching or working simultaneously of two communication links. The non-mechanical beam steerer had four diffraction fields; each can achieve beam steering with a 3.72° field and 30.77 µrad resolution, and the system emission efficiency was higher than 77%. The corresponding switching times of links at 2, 4, and 10 Hz were 46.7, 43.8, and 42.1 ms, respectively. In the quasistatic condition, the sensitivities of the two links under the data rate of 10.3125 Gbps were -23.18 and -23.01 dBm, respectively, indicating the service transmission capability of the multi-node beam control system.

Three-dimensional instrument polarization analysis and optimization of liquid-crystal-based Stokes polarimeter.

The Stokes polarimeter based on liquid crystal variable retarders (LCVRs) is a space polarization measurement technology widely used. However, due to the tilt of the optic axis of the LCVR with the driving voltage in the direction of light propagation and the interference in LCVR, the LCVRs-based Stokes polarimeter produces a large instrument polarization, which affects the accurate polarization measurement. In this paper, we combine polarization ray tracing with multi-beam interference, and establish a general three-dimensional polarization analysis model of the LCVRs-based Stokes polarimeter. The simulation results of adjusting the LCVR voltage to reduce the instrument polarization are analyzed, and the variation of polarization measurement accuracy with the field of view before and after optimization of the LCVRs-based Stokes polarimeter is simulated and analyzed. A LCVR structure with additional films for matching the refractive index is proposed. According to the simulation results, this structure can significantly reduce the interference effects and reduce the impact of variations in liquid crystal layer thickness on the interference effects.

Compressive hyperspectral microscopic imaging using spectral-coded illumination

A hyperspectral microscopic imaging system based on compressive sensing theory using spectral-coded illumination is presented in this paper. The spectral modulator, which consists of a liquid crystal variable retarder and two polarizers, is employed to encode the spectral transmittance of the incident light by adjusting the voltage of the liquid crystal variable retarder. The paper also provides the imaging model and reconstruction method, and the feasibility is confirmed through computer simulations and laboratory experiments. The imaging system combines traditional microscopy with spectrally encoded illumination and offers an alternative approach for designing a low-cost and compact hyperspectral microscope.

A simple polarimetric measurement based on a computational algorithm.

A simple and compact polarimeter comprising two electrically controlled liquid-crystal variable retarders (LCVRs) and a linear polarizer is demonstrated, which is enabled by analyzing the intensity variation of the modulated output light based on a computational algorithm. A proof-of-concept prototype is presented, which is mounted onto a power meter or a CMOS camera for the intensity data collection. The polarimetric measurement for the spatial variant polarization states of light is also verified, indicating the possibility of achieving a resolution-lossless polarimeter. Thus, our proposed method shows a cost-effective way to realize a compact polarimeter in polarization optics.

High-accuracy and high-efficiency calibration method for determining voltage-phase characteristics of LCVR based on a Wollaston prism

Liquid crystal variable retarder (LCVR), as a promising and precision device in applications of light field modulations and manipulations, has been widely used to change the phase and thus the polarization state of light based on electronically controlled rotations of liquid crystal molecules. However, the LCVR usage inevitably involves an important task, i.e., the calibration, since the phase retardation of LCVR changes with the temperature of the environment and the wavelength of the incident light. In this paper, based on using a Wollaston prism, we propose an effective calibration method to tailor the voltage-phase characteristic curves of LCVR. In the experiment, given a wavelength of light to be calibrated, similar calibration results are always obtained, regardless of the angle of polarization (AoP) of the incident light and the angle of LCVR placement. Experimental results show that the root-mean-square error (RMSE) of the voltage-phase calibration is no more than 0.132 radian. Moreover, excellent robustness and efficiency of this LCVR calibration method are also demonstrated and verified.

High Light Efficiency Spectral Polarization Imaging Method Based on Mach–Zehnder Structured Liquid Crystal Tunable Filters and Variable Retarders

Liquid crystal tunable filters (LCTFs) are extensively used in hyperspectral imaging systems to obtain spectral information of target scenes. However, a typical LCTF can only filter linearly polarized light, greatly reducing the transmittance of the system and limiting its application in spectral and polarization imaging. In this paper, a spectropolarimeter using Mach–Zehnder structured LCTFs (MZ-LCTFs) combined with liquid crystal variable retarders (LCVRs) is proposed. The polarized beam splitter (PBS) can make full use of the two polarization components of the incident light to improve the transmittance of the system. Specifically, the results show that the mean pixel intensity (MPI) of spectral images is improved by 93.48% compared to a typical LCTF. Subsequently, the average signal to noise ratio (SNR) of filtered and unfiltered images when simultaneously using polarization S and P channels is increased by 2.59 dB compared to a single channel. In addition, the average Standard Deviations (STDs) of DoLP and DoCP are 0.016 and 0.018, respectively. The proposed method has the potential to be applied to obtain polarization information with high optical efficiency and a full spectrum in a wide band.

Phase degradation of liquid crystal variable retarders and preconfiguring method for high-power continuous-wave laser

The effects of laser-induced heat on the performance degradation of liquid crystal (LC) devices are critically important in applications involving laser systems. In this study, we investigated the influences of 1064-nm continuous-wave (CW) laser loading on an LC variable retarder (LCVR). The results show that thermal deposition initially causes an overall downward shift in the electronically controlled phase retardation curve. This phase retardation reduction is restored under a certain voltage and laser power by reducing the driving voltage. For common nematic LC wave plates that do not require voltage application, the decrease in phase retardation can be pre-compensated by increasing the corresponding LC cell thickness. For electrically controlled LC devices, the phase retardation under a high-power CW laser can be preconfigured by plotting phase response curves versus voltages at different laser powers. Based on these calibration curves, the LC devices can still service high-power CW lasers as long as the resulting temperature does not reach the clear point of the LC material. These results clarify the fundamental mechanisms of phase modulation variation induced by thermal deposition and could be useful for the fabrication optimization and effective implementation of LC devices in high-power laser systems.

Flexible broadcast UWOC system using an LCVR-based tunable optical splitter.

For underwater wireless optical communication (UWOC) systems, using an omnidirectional light source to construct a broadcast system will require considerable energy due to high geometric loss and water attenuation. In addition, high-sensitivity photon detectors usually have a limited dynamic range, therefore limiting communication distance. In this Letter, a broadcast UWOC system, based on liquid crystal variable retarders (LCVRs) and polarization beam splitters (PBSs), is proposed to allocate user power in accordance with user-specific channel conditions. By adjusting the driving alternating current (AC) voltage of LCVRs to change the input light polarization, different proportions of light can be allocated to different PBS ports before broadcasting to different users. In a dual-user transmitter for the proof-of-concept, the output power dynamic range and the additional insertion loss for the first user are 19.17 dB and 0.91 dB, respectively. For the second user, the performance degrades to 17.33 dB and 1.26 dB, respectively. The step size of power adjustment is less than 0.063 dB. To verify the effectiveness of power adjustment in UWOC systems, a 7-m/243.2-Mbps single-user UWOC system is designed with a water attenuation coefficient ranging from 0.50 dB/m to 2.35 dB/m. All bit error rates (BERs) can decrease to below the forward error correction (FEC) limit by adjusting the LCVR driving voltage. The adjustable range of communication distance could be extended from 4.2 m to 13.19 m with a channel attenuation coefficient of 1.44 dB/m. Finally, a dual-user UWOC experiment is conducted and proves that the proposed system can still work in a multi-user system. The proposed system is proven to be effective for improving the anti-jamming capability and flexibility of UWOC networks.

Optimization of polarization parameters for an LCVR polarization spectrometer under non-oversampling.

The spectral polarization measurement can obtain not only the spectral information of the target but also its polarization information, which can improve the detection and identification of the measured target. In the polarization spectrometer based on a liquid crystal variable retarder (LCVR) and acousto-optic tunable filter (AOTF), the LCVR is a core device for achieving fast and high-precision polarization detection. The AOTF is a new, to the best of our knowledge, filter device for spectral tuning. To reduce the sensitivity of an LCVR-based Stokes polarization spectrometer system to errors and Gaussian noise, and to maintain the advantage of fast electrical tuning of the system for spectral polarization detection, the phase retardation and azimuth angle of the polarization device LCVR is calculated and analyzed optimally under the minimum number of samples N=4 of the Stokes vector measurement method in this paper. The optimization algorithm considers the constraints, such as the number of types of LCVR phase retardation and the number of adjustments, and the azimuth and phase retardation to be optimized are searched for optimality step by step. The simulation results show that the number of adjustments of the phase retardation δ of LCVRs is only three times when four Stokes parameters are obtained. The LCVRs' number of species is four kinds (2×2). The condition number of the optimized measurement matrix is 1.742, which converges to the ideal condition number, the optimal azimuth angle (θ 1,θ 2) is (18.9°, 41.9°), and the optimal phase retardation δ is (179.9°, 156.6°, 0.4°, 46.3°). Its corresponding tetrahedral volume is closer to the ideal value. The optimized system is less sensitive to errors and Gaussian noise.

In-flight validation of the Metis visible-light polarimeter coronagraph on board Solar Orbiter

Context. The Metis coronagraph is one of the remote sensing instruments of the ESA-NASA Solar Orbiter mission. The goal for the instrument is to enable the study of the solar atmosphere and solar wind by simultaneously acquiring images of the solar corona at two different wavelengths: visible light (VL), within a band ranging from 580 nm to 640 nm, and ultraviolet light, in the HI Lyα 121.6 ± 10 nm. The visible-light channel of the coronagraph includes a polarimeter with electro-optically modulating liquid crystal variable retarders to measure the linearly polarized brightness of the K-corona and derive the electron density. Aims. In this paper, we present the first in-flight validation results of the Metis polarimetric channel together with a comparison to the on-ground calibrations. This paper seeks to validate the first use of an electro-optical device, the liquid crystal-based polarimeter, in deep space and within a hard radiation environment. Methods. We used the orientation of the K-corona’s linear polarization vector during the roll maneuvers of the Space Orbiter spacecraft for the in-flight calibration. Results. The Metis coronagraph on board the Solar Orbiter shows good agreement with the on-ground measurements. The in-flight validation confirms the expected performance of the visible-light channel polarimeter. Furthermore, a comparison between the first polarized brightness value obtained by Metis and the polarized brightness values obtained by the space-based coronagraph LASCO and the ground-based coronagraph K-Cor shows the consistency of the Metis calibrated results.

Comparative study of liquid crystal variable retarder with rubbed polyimide and SiO2 thin films by glancing angle deposition

A comparative study on the optical, morphological and laser-induced damage characteristics of rubbed polyimide, SiO2 thin films by glancing angle deposition, and the corresponding liquid crystal (LC) retarders, is presented in this paper. SiO2 inorganic thin films were prepared with a deposition angle of 45[Formula: see text], and traditional polyimide (PI) films were prepared by spin coating and rubbing. The experimental results indicate that the SiO2 thin films have a good transmittance and resistance to laser damage. The transmittance of the SiO2 thin films was above 92% in the wavelength range 400–1200 nm, and that of the corresponding LC retarder was approximately 90% in the wavelength range 800–1200 nm. The laser-induced damage threshold of the SiO2 thin films reached 32.6 J/cm2, and that of the corresponding LC retarder was 1.84 J/cm2. These results are better than those of the PI alignment films. SiO2 thin films have potential for use in the LC optical devices for laser applications.

Partial Mueller matrix polarimeter using liquid crystal modulators.

A simple partial Mueller polarimeter is described, consisting of two linear polarizers and two liquid crystal variable retarders. The measurement leads to an incomplete Mueller-Scierski matrix, in which the elements from the third row and the third column are simultaneously missing. The proposed procedure for determining information about the birefringent medium from such an incomplete matrix is based on numerical methods and carrying out measurements with the rotated azimuthal sample. Acquired from the obtained results, the missing elements of the Mueller-Scierski matrix are reconstructed. The correctness of the method was verified by numerical simulations and test measurements.

Automated Polarized Hyperspectral Imaging (PHSI) for ex-vivo and in-vivo Tissue Assessment.

Polarized light interactions with biological tissues can reveal information regarding tissue structure, while spectral characteristics are closely related to tissue composition. An integration of both modalities in a compact system could better assist tissue assessment. This study aims to develop a polarized hyperspectral imaging (PHSI) system that fulfills both linearly and circularly polarized hyperspectral imaging for in vivo and ex vivo applications. The system is comprised of a white LED, two linear polarizers, two liquid crystal variable retarders (LCVRs), and a hyperspectral snapshot camera. The system was calibrated to compute the full Stokes polarimetry. For tissue differentiation, fresh ex vivo mouse tissue specimens from kidney, liver, spleen, muscle, lung, and salivary gland of mice were imaged. The spectra of three features, named degree of polarization (DOP), degree of linear polarization (DOLP), and degree of circular polarization (DOCP), were generated. A k-nearest neighbor (k-NN) classifier was trained with multi-class spectra and 5-fold cross validation. It was found that DOP better differentiates tissue with an average accuracy of 0.87. Additionally, support vector machine (SVM) classifiers were trained to differentiate between each two of the organs, and it was determined that DOLP better identified kidney, liver, and spleen, whereas DOCP better identified muscle and lung tissues. Then, the setup was employed to image in vivo human fingers with and without a blood occlusion to qualitatively estimate oxygen saturation. Preliminary results demonstrate that both DOLP and DOCP reveal a distinction of oxygen saturation states. These results demonstrate the feasibility of the PHSI system for distinguishing between optical properties of tissues, which has the potential to reveal disease-related information for diverse medical applications.

Polarized hyperspectral microscopic imaging for collagen visualization on pathologic slides of head and neck squamous cell carcinoma.

We developed a polarized hyperspectral microscope to collect four types of Stokes vector data cubes (S0, S1, S2, and S3) of the pathologic slides with head and neck squamous cell carcinoma (HNSCC). Our system consists of an optical light microscope with a movable stage, two polarizers, two liquid crystal variable retarders (LCVRs), and a SnapScan hyperspectral camera. The polarizers and LCVRs work in tandem with the hyperspectral camera to acquire polarized hyperspectral images. Synthetic pseudo-RGB images are generated from the four Stokes vector data cubes based on a transformation function similar to the spectral response of human eye for the visualization of hyperspectral images. Collagen is the most abundant extracellular matrix (ECM) protein in the human body. A major focus of studying the ECM in tumor microenvironment is the role of collagen in both normal and abnormal function. Collagen tends to accumulate in and around tumors during cancer development and growth. In this study, we acquired images from normal regions containing normal cells and collagen fibers and from tumor regions containing cancerous squamous cells and collagen fibers on HNSCC pathologic slides. The preliminary results demonstrated that our customized polarized hyperspectral microscope is able to improve the visualization of collagen on HNSCC pathologic slides under different situations, including thick fibers of normal stroma, thin fibers of normal stroma, fibers of normal muscle cells, fibers accumulated in tumors, fibers accumulated around tumors. Our preliminary results also demonstrated that the customized polarized hyperspectral microscope is capable of extracting the spectral signatures of collagen based on Stokes vector parameters and can have various applications in pathology and oncology.

Liquid-crystal based drift-free polarization modulators: Part II. Ultra-stable Stokes and Mueller polarimeters.

We have previously reported a new design for drift-free liquid-crystal polarization modulators (LCMs) based on liquid-crystal variable retarders (LCVRs). Here, we study their performance on Stokes and Mueller polarimeters. LCMs have polarimetric responses similar to LCVRs and can be used as temperature-stable alternatives to many LCVR-based polarimeters. We have built an LCM-based polarization state analyzer (PSA) and compared its performance to an equivalent LCVR-based PSA. Our system parameters remained stable over a wide range of temperature, precisely from 25°C to 50°C. Accurate Stokes and Mueller measurements have been conducted, paving the way to calibration-free polarimeters for demanding applications.