Polarization Artifacts Research Articles

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.

Polarization-insensitive optical coherence tomography based on partly depolarized light.

Polarization-related image artifacts are frequently observed in optical coherence tomography (OCT) data. As most modern OCT layouts rely on polarized light sources, only the co-polarized component of the light scattered from within a sample can be detected after interference with the reference beam. Cross-polarized sample light does not interfere with the reference beam and thus produces artifacts ranging from a reduction to the full absence of OCT signals. Here we present a simple yet effective technique to prevent polarization artifacts. By partly depolarizing the light source at the interferometer entrance, we achieve OCT signals regardless of the sample polarization state. We demonstrate the performance of our approach in a defined retarder as well as in birefringent dura mater tissue. This simple and cost-effective technique can be applied to obviate cross-polarization artifacts in virtually any OCT layout.

Mueller matrix imaging of pathological slides with plastic coverslips.

Mueller matrix microscopy is capable of polarization characterization of pathological samples and polarization imaging based digital pathology. In recent years, hospitals are replacing glass coverslips with plastic coverslips for automatic preparations of dry and clean pathological slides with less slide-sticking and air bubbles. However, plastic coverslips are usually birefringent and introduce polarization artifacts in Mueller matrix imaging. In this study, a spatial frequency based calibration method (SFCM) is used to remove such polarization artifacts. The polarization information of the plastic coverslips and the pathological tissues are separated by the spatial frequency analysis, then the Mueller matrix images of pathological tissues are restored by matrix inversions. By cutting two adjacent lung cancer tissue slides, we prepare paired samples of very similar pathological structures but one with a glass coverslip and the other with a plastic coverslip. Comparisons between Mueller matrix images of the paired samples show that SFCM can effectively remove the artifacts due to plastic coverslip.

Study on relaxor polymer interface matrix for piezoelectric nanocomposite generators

Piezoelectric nanocomposites consisting of a polymer matrix and ceramic fillers are candidate components of flexible, wearable, and self-powered electronic devices. The physical discrepancy between ferroelectric polymers and ceramics in a piezoelectric interface makes it difficult to assemble efficient hybrid piezoelectric nanocomposites without polarization and extraneous artifacts. Here, we describe the effect of a relaxor ferroelectric terpolymer matrix on the piezoelectric output, which can enhance the energy harvesting or sensor performance of piezoelectric composite device of nanocomposite generators with filler nanoparticles of lead zirconium titanate. The dielectric property and reduced ferroelectric hindrance of the proposed terpolymer matrix provides more poling to align the polarization of piezoelectric ceramic fillers compared with a normal ferroelectric copolymer matrix. Therefore, relaxor ferroelectric polymers can be better than normal ferroelectric polymers as the matrix of hybrid polymer-ceramic piezoelectric nanocomposite. This research provides important physical information about the interface between a polymer matrix and ceramic fillers in flexible piezoelectric nanocomposite applications.

Accurate Determination of Enantiomeric Excess Using Raman Optical Activity

The optical purity of a chiral sample is of particular importance to the analytical chemistry and pharmaceutical industries. In recent years, the vibrational optical activity (VOA) has become established as a sensitive and nondestructive technique for the analysis of chiral molecules in solution. However, the relatively limited accuracy in the range of about 1–2% reported in published papers and the relatively small spread of experimental facilities to date have meant that vibrational spectroscopy has not been considered a common method for determining enantiomeric excess. In this paper, we attempt to describe, in detail, a methodology for the determination of enantiomeric excess using Raman optical activity (ROA). This method achieved an accuracy of 0.05% for neat α-pinene and 0.22% for alanine aqueous solution, after less than 6 h of signal accumulation for each enantiomeric mixture, which we believe is the best result achieved to date using vibrational optical activity techniques. An algorithm for the elimination of systematic errors (polarization artifacts) is proposed, and the importance of normalizing ROA spectra to correct for fluctuations in excitation power is established. Results comparable to those obtained with routinely used chemometric analysis by the partial least squares (PLS) method were obtained. These findings show the great potential of ROA spectroscopy for the quantitative analysis of enantiomeric mixtures.

Computational refocusing of Jones matrix polarization-sensitive optical coherence tomography and investigation of defocus-induced polarization artifacts.

Here we demonstrate a long-depth-of-focus imaging method using polarization sensitive optical coherence tomography (PS-OCT). This method involves a combination of Fresnel-diffraction-model-based phase sensitive computational refocusing and Jones-matrix based PS-OCT (JM-OCT). JM-OCT measures four complex OCT images corresponding to four polarization channels. These OCT images are computationally refocused as preserving the mutual phase consistency. This method is validated using a static phantom, postmortem zebrafish, and ex vivo porcine muscle samples. All the samples demonstrated successful computationally-refocused birefringence and degree-of-polarization-uniformity (DOPU) images. We found that defocusing induces polarization artifacts, i.e., incorrectly high birefringence values and low DOPU values, which are substantially mitigated by computational refocusing.

Polarization Independent Optical Coherence Tomography

Optical coherence tomography (OCT) is a well established imaging modality for high-resolution three-dimensional imaging in clinical settings. While imaging, care must be taken to minimize the imaging artifacts related to the polarization differences between the sample and the reference signals. Current OCT systems adopt complicated mechanisms, such as the use of multiple detectors, polarization-maintaining fibers, polarization controllers to achieve polarization artifacts free sample images. Often the polarization controllers need readjustment which is not suitable for clinical settings. In this work, we demonstrate a simple approach that can minimize the polarization-related artifacts in the OCT systems. Polarization artifact-free images are acquired using two orthogonally polarized reference signals where the orthogonal polarization is achieved using a Faraday mirror. In the current approach, only a single detector is required which makes the current approach compatible with swept-source or camera-based OCT systems. Furthermore, no polarization controllers are used in the system which increases the system stability while minimizing the artifacts related to the sample birefringence, polarization change due to the sample scattering, and polarization change due to the optical fiber movements present in the system.

Measuring the spectral response of a division-of-focal-plane polarization imager using a grating monochromator.

Spectral characterizations are performed on imagers to obtain a relative spectral response (RSR) curve. This process often utilizes a grating monochromator with an output that changes polarization as a function of wavelength (our monochromator's degree of linear polarization was found to vary from less than 10% to more than 70%). When characterizing a polarization-sensitive imager, this introduces polarization artifacts into the RSR curve. We present a simple method to avoid these polarization artifacts for division-of-focal-plane polarization imagers by directly illuminating the camera with the monochromator output and calculating the S0 Stokes parameter at each super pixel, then we show consistent results from this method for two division-of-focal-plane polarization imagers. We also show that ignoring the monochromator polarization results in order-of-magnitude RSR errors. The recommended method uses an iris to limit the spatial extent of the monochromator output, which was found experimentally to increase the minimum signal-to-noise ratio by more than a factor of 2.

Measurement of Raman Optical Activity with High-Frequency Polarization Modulation.

Many chiroptical spectroscopic techniques have been developed to detect chirality in molecular species and probe its role in biological processes. Raman optical activity (ROA) should be one of the most powerful methods, as ROA yields vibrational and chirality information simultaneously and can measure analytes in aqueous and biologically relevant solvents. However, despite its promise, the use of ROA has been limited, largely due to challenges in instrumentation. Here, we report a new approach to ROA that exploits high-frequency polarization modulation. High-frequency polarization modulation, usually implemented with a photoelastic modulator (PEM), has long been the standard technique in other chiroptical spectroscopies. Unfortunately, the need for simultaneous spectral and polarization resolution has precluded the use of PEMs in ROA instruments. We combine a specialized camera system (the Zurich imaging polarimeter, or ZIMPOL) with PEM modulation to perform ROA measurements. We demonstrate performance similar to the current standard in ROA instrumentation while reducing complexity and polarization artifacts. This development should aid researchers in exploiting the full potential of ROA for chemical and biological analysis.

Passive shortwave-infrared polarimetric sensing of cloud thermodynamic phase

Cloud thermodynamic phase (CTP) classification is a vital piece of information for determining how clouds affect climate and electromagnetic wave propagation. We describe a passive shortwave infrared (SWIR) three-channel polarimeter for CTP remote sensing. This instrument adds polarimetric sensitivity to the traditional radiance ratio method that capitalizes on the difference in spectral absorption between liquid water and ice. The ground-based polarimeter was used to measure the first three Stokes parameters in three SWIR spectral bands alongside a dual-polarization lidar system for validation. Additionally, we operated the polarimeter in division-of-time and division-of-aperture modes to explore the severity of polarization artifacts that arose from fast-moving clouds. Although temporal smearing of polarization signatures was identified, the impacts were found to be minimal. Using a surface fitting technique, the radiance ratio method was directly compared with a method that combined radiance ratios with the S1 Stokes parameter. We found that the addition of polarimetry improved cloud phase classification ability from ∼73 % to 95%.

Unveiling Excited-State Chirality of Binaphthols by Femtosecond Circular Dichroism and Quantum Chemical Calculations.

Time-resolved circular dichroism (TR-CD) is a powerful tool for probing conformational dynamics of biomolecules over large time scales that are crucial for establishing their structure-function relationship. However, such experiments, notably in the femtosecond regime, remain challenging due to their extremely weak signals, prone to polarization artifacts. By using binol and two bridged derivatives (PL1 and PL2) as chiral prototypes, we present here the first comprehensive study of this type in the middle UV, combining femtosecond TR-CD and quantum mechanical calculations (TD-DFT). We show that excitation of the three compounds induces large variations of their transient CD signals, in sharp contrast to those of their achiral transient absorption. We demonstrate that these variations arise from both the alteration of the electronic distribution and the dihedral angle in the excited state. These results highlight the great sensitivity of TR-CD detection to signals hardly accessible to achiral transient absorption.

NONCONFOCAL ULTRA-WIDEFIELD SCANNING LASER OPHTHALMOSCOPY: Polarization Artifacts and Diabetic Macular Edema.

Bowtie-shaped polarization artifacts are often present in nonconfocal ultra-widefield scanning laser ophthalmoscope (SLO) images. We studied these artifacts and evaluated their potential value as clinical biomarkers in screening for center-involving diabetic macular edema (DME). We performed a retrospective, observational, cohort study on 78 diabetic adult patients (143 eyes) who had spectral domain optical coherence tomography and nonmydriatic nonconfocal ultra-widefield SLO testing on the same day. Scanning laser ophthalmoscope green-only (532 nm), red-only (635 nm), and composite pseudocolor (532 plus 635 nm) images were examined for the presence of a foveal bowtie polarization artifact. Polarization artifacts were absent in all but one eye with center-involving DME (32 of 33 eyes). Polarization artifacts were also absent in many eyes without center-involving DME (49 of 110 eyes in pseudocolor images). As clinical biomarkers of center-involving DME, artifact absence has high specificity (99, 100, and 98% for green, red, and pseudocolor images, respectively) but poor sensitivity (49, 31, and 40% for green, red, and pseudocolor images, respectively). Foveal bowtie-shaped polarization artifacts occur routinely in nonconfocal ultra-widefield SLO images. Their presence indicates preserved foveal Henle fiber layer structure. Contemporary nonconfocal ultra-widefield SLO images lack the sensitivity for their bowtie artifacts to serve as reliable biomarkers in screening for center-involving DME.

New quaternary BNT–BT–PMN–PT ceramic: ferro-/piezo-/pyroelectric characterizations

A near morphotropic phase boundary composition of a new quaternary ceramic 0.47(Bi0.5Na0.5)TiO3–0.04BaTiO3–0.31Pb(Mg1/3Nb2/3)O3–0.18PbTiO3 [BNT–BT–PMN–PT] has been synthesized using the solid state reaction method. The ceramics were characterized for their ferroelectric properties along with dielectric and piezoelectric characteristics. High value of maximum dielectric permittivity (~ 4140) and transition temperature (Tc ~ 190 °C) with low dielectric loss (< 0.5) were displayed. Well-shaped saturated PE hysteresis loop with high value of remanent polarization (Pr = 13.09 μC/cm2) and coercive field (Ec = 16.56 kV/cm) with excellent fatigue resistance (over 107 cycles) were observed. Together with the conventional P–E hysteresis used for recording the Pr value, which contains false contributions from various polarization artifacts, true remanent hysteresis task was used to unveil the practically usable polarization which was found to be ~ 93% that of the Pr value obtained using conventional hysteresis, suggesting high ferroelectric quality of the quaternary ceramic system. ‘Time-dependent compensated’ ferroelectric loops were traced to analyze the resistive-leakage nature of BNT–BT–PMN–PT system which showed that the system does not show degradation of ferroelectric properties due to change in frequency of applied field. This validates the potential of the material in devices such as actuators where fields are switched at different rates. The ceramic displayed good pyroelectric character in temperature range 95–110 °C. Further, the butterfly loop revealed the piezoelectric coefficient d33* value to be 177 pm/V. Overall, high Tc, high ferroelectric quality, stability under cyclic electric field and high d33 show that BNT–BT–PMN–PT (with lesser toxicity) is an attractive ceramic for consumer electronic device applications.

Automatic spectral calibration for polarization-sensitive optical coherence tomography.

Accurate wavelength assignment is important for Fourier domain polarization-sensitive optical coherence tomography. Incorrect wavelength mapping between the orthogonal horizontal (H) and vertical (V) polarization channels leads to broadening the axial point spread function and generating polarization artifacts. To solve the problem, we propose an automatic spectral calibration method by seeking the optimal calibration coefficient between wavenumber kH and kV. The method first performs a rough calibration to get the relationship between the wavelength λ and the pixel number x of the CCD for each channel. And then a precise calibration is taken to bring both polarization interferograms in the same k range through the optimal calibration coefficient. The optimal coefficient is automatically obtained by evaluating the cross-correlation of A-line signals. Simulations and experiments are implemented to demonstrate the performance of the proposed method. The results show that, compared to the peaks method, the proposed method is suitable in both Gaussian and non-Gaussian spectrums with a higher calibration accuracy.

Noise-bias and polarization-artifact corrected optical coherence tomography by maximum a-posteriori intensity estimation.

We propose using maximum a-posteriori (MAP) estimation to improve the image signal-to-noise ratio (SNR) in polarization diversity (PD) optical coherence tomography. PD-detection removes polarization artifacts, which are common when imaging highly birefringent tissue or when using a flexible fiber catheter. However, dividing the probe power to two polarization detection channels inevitably reduces the SNR. Applying MAP estimation to PD-OCT allows for the removal of polarization artifacts while maintaining and improving image SNR. The effectiveness of the MAP-PD method is evaluated by comparing it with MAP-non-PD, intensity averaged PD, and intensity averaged non-PD methods. Evaluation was conducted in vivo with human eyes. The MAP-PD method is found to be optimal, demonstrating high SNR and artifact suppression, especially for highly birefringent tissue, such as the peripapillary sclera. The MAP-PD based attenuation coefficient image also shows better differentiation of attenuation levels than non-MAP attenuation images.

Correcting Systematic Polarization Effects in Keck LRISp Spectropolarimetry to < 0.05%

Spectropolarimetric measurements at moderate spectral resolutions are effective tracers of stellar magnetic fields and circumstellar environments when signal-to-noise ratios (S/Ns) above 2000 can be achieved. The LRISp spectropolarimeter is capable of achieving these S/Ns on faint targets with the 10 m aperture of the Keck telescope, provided several instrumental artifacts can be suppressed. We describe here several methods to overcome instrumental error sources that are required to achieve these high S/Ns on LRISp. We explore high S/N techniques such as defocusing and slit-stepping during integration with high spectral and spatial oversampling. We find that the instrument flexure and interference fringes introduced by the achromatic retarders create artificial signals at 0.5% levels in the red channel which mimic real stellar signals and limit the sensitivity and calibration stability of LRISp. Careful spectral extraction and data filtering algorithms can remove these error sources. For faint targets and long exposures, cosmic ray hits are frequent and present a major limitation to the upgraded deep depletion red-channel CCD. These must be corrected to the same high S/N levels, requiring careful spectral extraction using iterative filtering algorithms. We demonstrate here characterization of these sources of instrumental polarization artifacts and present several methods used to successfully overcome these limitations. We have measured the linear to circular cross-talk and find it to be roughly 5%, consistent with the known instrument limitations. We show spectropolarimetric signals on brown dwarfs are clearly detectable at 0.2% amplitudes with sensitivities better than 0.05% at full spectral sampling in atomic and molecular bands. Future LRISp users can perform high-sensitivity observations with high-quality calibration when following the described algorithms.

Calibrating and stabilizing spectropolarimeters with charge shuffling and daytime sky measurements

Well-calibrated spectropolarimetry studies at resolutions of $R>$10,000 with signal-to-noise ratios (SNRs) better than 0.01\% across individual line profiles, are becoming common with larger aperture telescopes. Spectropolarimetric studies require high SNR observations and are often limited by instrument systematic errors. As an example, fiber-fed spectropolarimeters combined with advanced line-combination algorithms can reach statistical error limits of 0.001\% in measurements of spectral line profiles referenced to the continuum. Calibration of such observations is often required both for cross-talk and for continuum polarization. This is not straightforward since telescope cross-talk errors are rarely less than $\sim$1\%. In solar instruments like the Daniel K. Inouye Solar Telescope (DKIST), much more stringent calibration is required and the telescope optical design contains substantial intrinsic polarization artifacts. This paper describes some generally useful techniques we have applied to the HiVIS spectropolarimeter at the 3.7m AEOS telescope on Haleakala. HiVIS now yields accurate polarized spectral line profiles that are shot-noise limited to 0.01\% SNR levels at our full spectral resolution of 10,000 at spectral sampling of $\sim$100,000. We show line profiles with absolute spectropolarimetric calibration for cross-talk and continuum polarization in a system with polarization cross-talk levels of essentially 100\%. In these data the continuum polarization can be recovered to one percent accuracy because of synchronized charge-shuffling model now working with our CCD detector. These techniques can be applied to other spectropolarimeters on other telescopes for both night and day-time applications such as DKIST, TMT and ELT which have folded non-axially symmetric foci.

Removing the polarization artifacts in Mueller matrix images recorded with a birefringent gradient-index lens

We obtained backscattering Mueller matrix images on a cancerous tissue sample through a gradient-index (GRIN) lens and observed strong distortion in all the Mueller matrix elements. By measuring the intrinsic polarization properties of the GRIN lens, which is dominated by birefringence following a radial profile, and applying a matrix inversion method to the distorted Mueller matrix, we are able to remove the artifacts because of the birefringent GRIN lens and recover the polarization features of the sample. The results demonstrate the feasibility to take Mueller matrix measurements using GRIN lenses or other optical components with strong birefringence.

HIGH-FIDELITY RADIO ASTRONOMICAL POLARIMETRY USING A MILLISECOND PULSAR AS A POLARIZED REFERENCE SOURCE

A new method of polarimetric calibration is presented in which the instrumental response is derived from regular observations of PSR J0437-4715 based on the assumption that the mean polarized emission from this millisecond pulsar remains constant over time. The technique is applicable to any experiment in which high-fidelity polarimetry is required over long time scales; it is demonstrated by calibrating 7.2 years of high-precision timing observations of PSR J1022+1001 made at the Parkes Observatory. Application of the new technique followed by arrival time estimation using matrix template matching yields post-fit residuals with an uncertainty-weighted standard deviation of 880 ns, two times smaller than that of arrival time residuals obtained via conventional methods of calibration and arrival time estimation. The precision achieved by this experiment yields the first significant measurements of the secular variation of the projected semi-major axis, the precession of periastron, and the Shapiro delay; it also places PSR J1022+1001 among the ten best pulsars regularly observed as part of the Parkes Pulsar Timing Array (PPTA) project. It is shown that the timing accuracy of a large fraction of the pulsars in the PPTA is currently limited by the systematic timing error due to instrumental polarization artifacts. More importantly, long-term variations of systematic error are correlated between different pulsars, which adversely affects the primary objectives of any pulsar timing array experiment. These limitations may be overcome by adopting the techniques presented in this work, which relax the demand for instrumental polarization purity and thereby have the potential to reduce the development cost of next-generation telescopes such as the Square Kilometre Array.

A Simple, Single-measurement Methodology to Account for Electrode Polarization in the Dielectric Spectra of Colloidal Dispersions

Abstract A single-measurement methodology to produce accurate dielectric spectra is developed and validated by measurements on a dilute aqueous suspension containing 1 µm polystyrene particles suspended in 0.1 mM KCl. Electrode polarization artifacts in the spectra are eliminated down to the kilohertz frequency regime. Corrected dielectric spectra agree with previously established extrapolation methods and the standard electrokinetic theory. The method is simple yet effective and allows for accurate time-dependent measurements without the need for disassembling and reassembling the dielectric cell.