Micropolarizer Array Research Articles

Visible light full Stokes polarization imaging based on twisted liquid crystal micro-polarizer arrays

The micro-polarizer array is the key component of the division of focal-plane (DoFP) polarization imaging method. Currently, it is possible to achieve full Stokes information detection at a single wavelength using a designed array with the highest signal-to-noise ratio. To broaden the working wavelength to the visible range and achieve high-accuracy full Stokes polarization information, this paper proposes a micro-polarizer array structure based on twisted liquid crystal. The particle swarm optimization algorithm is utilized to optimize the device parameters, with the condition number as the evaluation function. A series of elliptically polarized lights are under measurement using the designed device. The results show that the errors of the degree of linear polarization (DoLP), degree of circular polarization (DoCP), and angle of polarization, are less than 2.00%, 1.50%, and 1.45%, respectively. All the results indicate that the twisted liquid crystal micro-polarizer array enables the high-accuracy measurement of full Stokes information within the visible range.

Reverse Design of Pixel-Type Micro-Polarizer Arrays to Improve Polarization Image Contrast.

Micro-polarizer array (MPA) is the core optical component of the Division of Focal-Plane (DoFP) imaging system, and its design is very important to the system's performance. Traditional design methods rely on theoretical analysis and simulation, which is complicated and requires designers to have profound theoretical foundations. In order to simplify the design process and improve efficiency, this paper proposes a 2 × 2 MPA reverse-design strategy based on particle swarm optimization (PSO). This strategy uses intelligent algorithms to automatically explore the design space in order to discover MPA structures with optimal optical properties. In addition, the all-pass filter is introduced to the MPA superpixel unit in the design, which effectively reduces the crosstalk and frequency aliasing between pixels. In this study, two MPA models were designed: a traditional MPA and an MPA with an all-pass filter. The Degree of Linear Polarization (DOLP) image contrast is used as the evaluation standard and compared with the traditional MPA; the results show that the contrast of the newly designed traditional MPA image is increased by 21%, and the MPA image with the all-pass filter is significantly increased by 82%. Therefore, the reverse-design method proposed in this paper not only simplifies the design process but also can design an MPA with enhanced optical performance, which has obvious advantages over the traditional method.

Method of Directly Writing MPA on Photosensitive Surface of Detector Based on FIB.

The division of focal plane (DoFP) polarization detector has great potential for the development of aerospace polarimeters, but the existing commercial DoFP polarization detector cannot satisfy all the missions due to the diversity of satellite payloads. Here, we propose a method of directly writing a micro-polarizer array (MPA) on the detector surface based on focused ion beams (FIB) and fabricating a push-broom scanning DoFP polarization detector. The feasibility and low crosstalk of the solution were proved through testing, and the reasons for the low extinction ratio caused by oxidation were explained through characterization and numerical calculations. This scheme is not only applicable to DoFP polarization detectors but also provides ideas for the integration of other metasurface structures and detectors.

Polarization image demosaicking based on homogeneity space

The advent of the micro-polarizer array allows focal plane sensors to capture polarized light information, resulting in the acquisition of subsampled polarization intensity images. The process of reconstructing a comprehensive representation of polarization information is called polarization demosaicking. Current polarization demosaicking methods are susceptible to encountering polarization artifacts along complex edges due to their failure to account for the intrinsic correlation between polarization channels. We define the homogeneity space by employing the metric neighborhood model to capture the underlying correlation among individual polarization channels. Using the homogeneity space as the foundation, we propose a DI-Fusion method that effectively adaptively diminishes polarization artifacts in the initial demosaicking performance. Additionally, our homogeneity space can be extended in other advanced polarization demosaicking techniques, effectively enhancing the overall demosaicking performance. The experimental results demonstrate the high efficacy of our method in reducing polarization artifacts and enhancing the initial polarization demosaicking performance across two widely recognized benchmark datasets. Our method significantly improves objective metrics and visual performance compared to the initial demosaicking performance.

Particle Swarm Optimization of Multilayer Multi-Sized Metamaterial Absorber for Long-Wave Infrared Polarimetric Imaging.

Infrared polarization imaging holds significant promise for enhancing target recognition in both civil and defense applications. The Division of Focal Plane (DoFP) scheme has emerged as a leading technology in the field of infrared polarization imaging due to its compact design and absence of moving parts. However, traditional DoFP solutions primarily rely on micro-polarizer arrays, necessitating precise alignment with the focal plane array and leading to challenges in alignment and the introduction of optical crosstalk. Recent research has sought to augment the performance of infrared detectors and enable polarization and spectral selection by integrating metamaterial absorbers with the pixels of the detector. Nevertheless, the results reported so far exhibit shortcomings, including low polarization absorption rates and inadequate polarization extinction ratios. Furthermore, there is a need for a comprehensive figure of merit to systematically assess the performance of polarization-selective thermal detectors. In this study, we employ the particle swarm optimization algorithm to present a multilayer, multi-sized metamaterial absorber capable of achieving a remarkable polarization-selective absorption rate of up to 87.2% across the 8-14 μm spectral range. Moreover, we attain a polarization extinction ratio of 38.51. To elucidate and predict the resonant wavelengths of the structure, we propose a modified equivalent circuit model. Our analysis employs optical impedance matching to unveil the underlying mechanisms responsible for the high absorption. We also introduce a comprehensive figure of merit to assess the efficacy of infrared polarization detection through the integration of metamaterials with microbolometers. Finally, drawing on the proposed figure of merit, we suggest future directions for improving integrated metamaterial absorber designs, with the potential to advance practical mid-infrared polarization imaging technologies.

Analysis of multispectral polarization imaging image information based on micro-polarizer array.

As a new detection technology, polarization imaging is of great significance in the field of target detection. At present, polarization imaging technology usually adopts visible light polarization imaging. The technique is difficult to image the target in complex background due to its narrow working spectrum and short detection distance. Therefore, based on the principle of full Stokes micro-polarizer array, this paper proposes a multi-spectral polarization imaging scheme and designs a multi-spectral polarization imaging detection system penetrating haze. Conducting indoor and outdoor polarized imaging experiments. Finally, image quality was assessed using metrics such as information entropy (EN), average gradient (AG), and standard deviation (STD). The results show that compared with traditional strength detection, the imaging system has significantly improved detection distance and imaging quality in smoky environments. The imaging system can effectively enhance the contours and details of the target object and improve detection and recognition capabilities.

Analysis of phase calculation error introduced by the extinction ratio in instantaneous phase shifting interference.

Instantaneous phase shifting interferometry technology, the core component of which is the pixel micropolarizer camera, has been widely used in commercial interferometers. This technology has the superiority of single-frame acquisition, vibration insensitivity, and no need for phase shifting devices. However, due to manufacturing defects and accuracy limitations, the extinction ratios (ER) of the micropolarizer array are different and fairly small, directly affecting the phase calculation accuracy. This paper initially derives a theoretical expression for the phase calculation error introduced by the extinction ratio (ER) and proposes the error correction model to reduce phase calculation errors caused by the extinction ratio. The theoretical analysis can serve as an important basis for accurately assessing the polarization characteristics of a pixel micropolarizer camera. Quantifying the impact of the extinction ratios provides significant support for the selection of polarization equipment. In addition, the paper proposes a calibration model to improve measurement accuracy, which can serve as an effective means to reduce the impact of the extinction ratio (ER). The innovative research content revealed the influence of extinction ratio (ER), serving as a valuable complement to the existing analysis and research on extinction ratio (ER).

Optical Biosensing of Polarized Light

Interactions between liquid crystal molecules and target analytes open up various biosensing applications for quick screening and point-of-care applications. In this review, we categorized biosensors by type, depending on the liquid crystal mesophase, and considered several applications for the detection of biomolecules, point-of-care diagnostics and environmental monitoring. We also discuss interactions between polarized light and target pathogens dispersed in biological fluids, which result in the change of the polarization state. An array of the Stokes parameters can be compared with the pattern, and a proper pathogen can be manifested. We suggest that a combination of a micropolarizer array and a complementary metal oxide semiconductor sensor is an optimal setup for the detection of pathogens. Herein, we discuss the working principles of liquid crystal biosensors and their fabrication principles. In addition, relevant theoretical and practical issues related to liquid crystal biosensors are outlined. In general, this review gives an in-depth survey of the research on liquid crystal-based sensors, making it easier for researchers to locate their niche and make contributions to this subject from multiple viewpoints.

Calibration method for thermal infrared division-of-focal-plane polarimeters considering polarizer reflection characteristics

Owing to manufacturing defects of micropolarizer arrays and differences in the pixel response of detectors, division-of-focal-plane (DoFP) polarimeters have severe nonuniformity, which affects the measurement accuracy of the polarimeters and the calculation of the polarization information. This study proposes a calibration method for thermal infrared DoFP polarimeters considering polarizer reflection characteristics. The temperature-controlled adjustable infrared polarized radiation source is calibrated by a division-of-time polarimeter and is, in turn, used to calibrate a thermal infrared DoFP polarimeter. Through laboratory blackbody and external scenes, the performance of the proposed method is compared to that of state-of-the-art techniques. The experimental results indicate that the proposed method effectively avoids overcalibration and improves the accuracy of polarization information.

Sky Brightness Evaluation and First Coronal Signal Detection from Concordia Base (Antarctica) with a Calibrated Micropolarizer Array Camera

Sky Brightness Evaluation and First Coronal Signal Detection from Concordia Base (Antarctica) with a Calibrated Micropolarizer Array Camera

Defocus compensation scheme for femtosecond laser direct writing of large-area non-periodic micro-structures

The optical path error, the position-independent geometric error, and the form and position tolerance of the substrate will lead to position deviations of the laser focus resulting in the inconsistency of the large area micro-structure and limiting the processing range of the femtosecond laser fabrication platform. To realize continuous defocus compensation along the processing trajectory, an autofocus scheme consisting of an optimized depth from focus (DFF) method based on grid division and the bilinear interpolation algorithm is proposed in this paper. The optimized DFF method includes the Laplace-DWT evaluation function combining the advantages of spatial domain and frequency domain sharpness evaluation function, an adaptive focusing window selection method, and an adaptive step length three-step hill-climbing search method. The optimized DFF method was used to obtain the best focus position of each grid point. Compared with the traditional DFF method, the optimized algorithm has higher focusing precision and avoids the disadvantage of time-consuming focusing. Then, to compensate for interpolating points of the processing trajectory, the bilinear interpolation algorithm was introduced to achieve online focusing during two-photon lithography processing. Finally, the feasibility and effectiveness of the auto-focusing scheme were validated by experiments. Through defocus compensation, the sharpness evaluation value of the focus image on the processing trajectory improved by 90%, which indicated the successful focus of the laser beam on the workpiece surface. Moreover, a high-precision and large-area encoded micro-polarizer array with consistent structure were fabricated successfully. This paper provides a simple way to identify and compensate the defocus errors online with high accuracy and efficiency. The proposed method can improve the machining accuracy and quality of the femtosecond laser direct writing.

Multi-Parameter Model-Based Polarimetric Calibration for Dual-Coded Spectral Polarization Imaging System

A polarization analysis method based on a multi-parameter model is proposed to address the polarization effect analysis and calibration requirements of a dual-coded snapshot spectral polarization imaging system. A full-link polarization effect model for a spectral polarization imaging system is established that includes a digital micromirror array (DMD), prism grating prism (PGP), micro-polarizer array detector (MPA), and multi-film. The influence of parameters such as the refractive index, incident angle, grating refractive index, constant, prism refractive index, vertex angle, multi-layer film complex refractive index, and film thickness on the optical transmittance of the system are analyzed. Using a dynamic data exchange mechanism to perform full-link, full-FOV, and full-pupil ray tracing on the optical system, the polarization effect distribution of the system under different degrees of polarization (DOP) and wavelengths is obtained. A calibration experiment for the controllable incident wavelength and DOP using narrowband filters and glass stacks is established. The experimental results show that in the 420 nm, 532 nm, and 635 nm wavelength bands, the MSEs of the calibrated values are 1.3924 × 10−4, 1.6852 × 10−4, and 1.6735 × 10−4, respectively. It is proven that the calibration method based on a multi-parameter model is feasible. Finally, the spectral polarization image at 532 nm is calibrated. The contrast ratio of metallic aluminum is calibrated from 7.13 to 15.33. This study provides a theoretical basis for the analysis and calibration of polarization effects in a dual-coded snapshot spectral polarization imaging system.

The Analysis and Experiment of Pixel-Matching Method for Space-Dimensional Dual-Coded Spectropolarimeter

In order to meet the high accuracy pixel-matching requirements of space-dimensional dual-coded spectropolarimeter, a dual-coded image pixel-matching method based on dispersion modulation is proposed. The mathematics of the dispersion power and the pixel matching is modeled. The relationship between different pixel-matching coefficients and the peak signal-to-noise ratio (PSNR) and structure similarity index measure (SSIM) of reconstructed images is analyzed. An imaging system experiment consisting of a digital micromirror device (DMD), a micro-polarizer array detector (MPA), and a prism–grating–prism (PGP) is built to reconstruct a spectral linear polarization data cube with 50 spectral channels and linear polarization parameters. The contrast ratio of the reconstructed spectropolarimeter image was raised 68 times against the ground truth. It can be seen from the reconstruction evaluation analysis that the spectral data and polarization data can be matched effectively by optimizing the dispersion coefficient of the PGP. The system can effectively reconstruct when the noise SNR is greater than 15 dB. The PSNR and SSIM of the reconstruction images can be improved by increasing the pixel-matching spacing. The optimal choice of the dual-coded pixel-matching spacing is one super-polarized pixel. The spectral resolution and quality of the spectropolarimeter are improved using the pixel-matching method.

Synchronous Phase-Shifting Interference for High Precision Phase Imaging of Objects Using Common Optics.

Quantitative phase imaging and measurement of surface topography and fluid dynamics for objects, especially for moving objects, is critical in various fields. Although effective, existing synchronous phase-shifting methods may introduce additional phase changes in the light field due to differences in optical paths or need specific optics to implement synchronous phase-shifting, such as the beamsplitter with additional anti-reflective coating and a micro-polarizer array. Therefore, we propose a synchronous phase-shifting method based on the Mach-Zehnder interferometer to tackle these issues in existing methods. The proposed method uses common optics to simultaneously acquire four phase-shifted digital holograms with equal optical paths for object and reference waves. Therefore, it can be used to reconstruct the phase distribution of static and dynamic objects with high precision and high resolution. In the experiment, the theoretical resolution of the proposed system was 1.064 µm while the actual resolution could achieve 1.381 µm, which was confirmed by measuring a phase-only resolution chart. Besides, the dynamic phase imaging of a moving standard object was completed to verify the proposed system's effectiveness. The experimental results show that our proposed method is suitable and promising in dynamic phase imaging and measurement of moving objects using phase-shifting digital holography.

Design of an Au–Pt–Ti wire-grid polarizer based on midinfrared pixelated micropolarizer arrays

Polarization information is widely used in complex applications such as industrial detection, biomedicine, earth remote sensing, modern military, and aerospace. However, traditional polarization devices cannot adequately satisfy the requirements of increasingly complex environments. A subwavelength grating consisting of three types of materials, namely, aurum, platinum, and titanium, was designed to improve the performance of the polarimetric components of a midwavelength infrared polarization imaging system. The morphological structure parameters of the designed grating were optimized by analyzing the effects on the polarization performance through the rigorous coupled wave theory. The proposed Au–Pt–Ti composite grating could address the problem of grating peeling off from the substrate because of high temperature during preparation. The composite grating has some advantages such as high mechanical stability, lower oxidation, and great polarization properties. The diffraction can be reduced and the extinction ratio (ER) can be effectively enhanced in a midwave infrared (MWIR) focal plane array through appropriate reduction in the termination edge spacing and the addition of a gold frame between polarizer pixels. The grating with a period of 0.8 μm was designed for application in the MWIR band. The fill factor was 0.5 and the depths of Au–Pt–Ti in the grating region were 0.44 μm. Each pixel was separated by a 3.2-μm wide gold frame. Modeling results revealed that the ER of grating was higher than 28.68 and 32.77 above the incident wavelengths of 3.89 and 4.57 μm, respectively. Additionally, excellent polarization capability was observed over a wide MWIR spectrum. Furthermore, the current manufacturing process level may generate polarizers with the aforementioned structural specifications.

Efficient fabrication method for non-periodic microstructures using one-step two-photon lithography and a metal lift-off process.

This paper presents a mask-less, flexible, efficient, and high-resolution fabrication method for non-periodic microstructures. Sub-wavelength micro-polarizer arrays, (MPAs) which are the most essential part of the focal plane polarimeters, are typical non-periodic structures. The grating ridges of each polarizer were oriented in four different directions offset by 45°, corresponding to different polarization directions. The finite element method was introduced to optimize the structural parameters of the MPA in the far-infrared region. The numerical results demonstrated that the designed MPA had a TM transmittance of more than 55% and an extinction ratio no less than 7dB. An aluminum MPA that operates in the 8-14µm infrared region was prepared by one-step two-photon lithography (TPL) and the metal lift-off process. The femtosecond laser exposed the photoresist with only a single scan, making TPL very efficient. The fabricated single-layer sub-wavelength MPAs with a period of 3µm, a duty cycle of 0.35-0.5, and a height of 150nm, were analyzed by an optical microscope and an atomic force microscope. The successful fabrication of the MPA indicated that one-step TPL could be a viable and efficient method for pattern preparation in the fabrication of non-periodic microstructures.

Pixelated Micropolarizer Array Based on Carbon Nanotube Films.

A micropolarizer array (MPA) that can be integrated into a scientific camera is proposed as a real-time polarimeter that is capable of extracting the polarization parameters. The MPA is based on highly aligned carbon nanotube (CNT) films inspired by their typical anisotropy and selectivity for light propagation over a wide spectral range. The MPA contains a dual-tier CNT pixel plane with 0° and 45° orientations. The thickness of the dual-tier structure of the CNT-based MPA is limited to less than 2 μm with a pixel size of 7.45 μm × 7.45 μm. The degree of polarization of the CNT-MPA reached 93% at a 632 nm wavelength. The specific designs in structure and semiconductor fabrication procedures are described. Compared with customary MPAs, CNT-based MPA holds great potential in decreasing the cross-talk risk associated with lower film thickness and can be extended to a wide spectral range.

Numerical study of a high-extinction-ratio micro-polarizer array with a metal grid substrate.

In this paper, a complete investigation and simulation of the extinction ratio (ER) in a setup with a micro-polarizer array (MPA) is performed. A metal grid substrate structure, which can effectively reduce optical crosstalk and improve the ER, is proposed. The numerical calculation results show that the ER of our proposed structure is more than 10-fold higher than that of the traditional structure. Moreover, the effect of its structural parameters to the ER has been researched, and the results showed that when the substrate is thicker, the incident angle of light is less than 15°, the metal width is larger than 900nm, and a complete grating with more than 20 periods on a single pixel can obtain a high ER.

Design and Experimental Analysis of Micropolarization Array Based on a Long-Wave Infrared Optical System

Aiming at the shortcomings of the traditional infrared polarization imaging device, such as large volume, complex structure, difficult registration, and inability to recognize moving targets, this paper proposes an infrared polarization imaging method based on a micropolarizer. The long refractive wave infrared polarization optical system is designed to achieve the imaging detection of a 0.25 m target at 1 km; for this, a design of a micropolarizer array suitable for the long-wave infrared band is proposed. In the simulation analysis, the effects of the grating substrate and line grating material, grating period, duty cycle, and grating slot depth on the polarization performance of grating are discussed, respectively. Through infrared polarization imaging experiments on typical targets, the unique advantages of infrared polarization technology in distinguishing metal and non-metal, natural and artificial objects, and high-temperature object recognition are verified, which provides practical support for physical evidence searches and camouflage target recognition.

Mixed noise removal based on Stokes residual noise removal for division of focal plane polarimetric images

The division of focal plane (DoFP) polarimeter has the advantages of simultaneous imaging and compact optical structure. It is inevitable to introducing the noise in the process of micro-polarizer array integration, polarization image acquisition and transmission. In this paper, we propose a novel mixed noise suppression method based on Stokes residual noise removal (SRNR) for mixed additive white Gaussian noise (AWGN) and impulse noise (IN) removal in DoFP polarimetric images. The Laplacian scale mixture (LSM) model is introduced to estimate the IN and the nonlocal low-rank regularization (NLR) is adopted to enhance the denoising performance. The residual noise of the Stokes parameters is taken into consideration. This crucial processing achieves the acquisition of high-quality polarization data. The experiments demonstrate the superior denoising performance for polarimetric imagery in terms of objective assessment and visual evaluation.