Polarization and Possible Differences in Backscatter Measurements

Dual photoelastic modulator polarimeter systems are widely used for the measurement of light beam polarization, most often described by Stokes vectors, that carry information about an interrogated sample. The sample polarization properties can be described more thoroughly through its Mueller matrix, which can be derived from judiciously chosen input polarization Stokes vectors and correspondingly measured output Stokes vectors. However, several sources of error complicate the construction of a Mueller matrix from the measured Stokes vectors. Here we present a general formalism to examine these sources of error and their effects on the derived Mueller matrix, and identify the optimal input polarization states to minimize their effects in a dual photoelastic modulator polarimeter configuration. The input Stokes vector states leading to the most robust Mueller matrix determination are shown to form Platonic solids in the Poincaré sphere space; we also identify the optimal 3D orientation of these solids for error minimization.

Recently, it was pointed out that polarization effects must be considered in hard target calibration of lidars. A vector theory of radiometry is developed, and it is demonstrated for a real nonideal target that the reflectance is a matrix quantity and not a scalar quantity, and all its components must be measured. These concepts can be extended to actual field measurements of the volume backscatter coefficients. The volume backscatter coefficient at range R is an averaged (4 x 4) matrix, which is averaged over the sampling depth dR = ctau/2. The transmitted beam is polarized in a definite sense, the received beam is still polarized, and both are represented as (4 x 1) Stokes vectors so that the interaction must be represented by a (4 x 4) matrix called the volume backscatter coefficient beta. Present experiments are in error for data are considered a scalar quantity with only one value not a matrix with sixteen components. Some of these components may be zero but many are not.

This article focuses on a polarization measurement method based on Mueller calculus for determining the polarization properties of metal substrates. As the Mueller matrix of a medium provides all its polarization-altering properties, the presented method enables to predict the effect of any optically absorbing media on the polarization state of light beam. For the measurement, a polarized beam of light is reflected off the substrate under study and the change in state of polarization is measured in terms of Stokes parameters. Simple formulae have been derived to deduce the values of matrix elements from the measured Stokes parameters. The presented method facilitates determination of the optical properties of any absorbing medium even though the optical constants are unidentified. It has been validated by the estimation of Mueller matrix elements of a few metal reflectors (Cu, Al and SS304) using Fresnel’s equations and comparing the theoretical values with the experimentally determined values. It has been verified further by measuring the complex refractive index of a sample (brass: Cu–Zn alloy) using the principle angle method and comparing the results with that obtained by newly developed method. For the materials under study, the average absolute error in the measurement of matrix elements is found to be <0.01 and the standard deviation is <0.02. The main advantage of the method is that the polarization properties of any optically absorbing medium can be determined using a simple optical setup at any desired wavelength.

The state of polarization (SOP) is the key characteristic of light; its evaluation is typically based on measurements of the Stokes parameters. However, conventional approaches often have bulky and complex optical systems with long acquisition times. In this Letter, an ultracompact polarimeter based on dielectric metasurfaces is proposed. The pixel of the proposed metasuface has four areas; each area acts not only as a polarization separator, but also a flat focusing lens. It can acquire a certain polarization component of the incident light and focus the component on the receiver. Therefore, the Stokes parameters of the incident light can be immediately determined through the data from the receiver, fully describing its SOP. The incident waves with different SOPs are simulated to demonstrate that the reconstructed Stokes parameters are in good agreement with the original ones.

For the measurement of Mueller matrix in an optical system with birefringence and small polarization-dependent loss or gain (PDL/G), we theoretically derive the statistical relationship between the Mueller matrix measurement error and three input states of polarization (SOP). Based on this theoretical relation and simulation results, it can be concluded that the three input SOPs, that are coplanar with an angle of 120 degrees between any two of them in Stokes space, can be considered as a substitute for the best input SOPs which can statistically lead to the minimum measurement error. This conclusion is valid when the PDL/G of the optical system under test is less than 0.35 dB.

The detection technology of infrared polarization has gained significant attention due to its ability to provide better identification and obtain more information about the target. In this paper, based on the expression of the full polarization state in Stokes space, we designed micro-nano metasurface functional arrays to calculate the polarization state of the incident light by reading the Stokes parameters (a set of parameters that describe the polarization state). Metalens with linear and circular polarization-dependent functions are designed based on the propagation and geometric phases of the dielectric Si meta-atoms in the infrared band, respectively. The device exhibits a high polarization extinction ratio. The influence of incident angle on polarization-dependent metalens is discussed, and the analysis of incident angle is of great significance for the practical application. An infrared six-foci metalens is proposed, each corresponding to the Poincaré sphere's coordinate component (a graphical polarization state method). By matching the six polarization components of the incident light and the Stokes parameters, the polarization detection function can be realized by calculating the polarization state of the incident light. There is a slight error between the theoretical value and the calculated value of the unit coordinate component of the Stokes parameters. At the same time, the intensity distribution of different incident light polarization azimuth angles and ellipticity angles on the focal plane agrees with the theory. The advantage of the device is that the polarization state of the incident light can be directly calculated without passing through other components. The six-foci metalens have potential applications in polarization detection and imaging, space remote sensing, etc.

Polarimetry is a noninvasive method that uses polarised light to assess biophysical characteristics of tissues. A series of incident polarisation states illuminates a biological sample, and analysis of sample-altered polarisation states enables polarimetric tissue assessment. The resultant information can, for example, help quantitatively differentiate healthy from pathologic tissue. However, most bio-polarimetric assessments are performed using free-space optics with bulky optical components. Extension to flexible fibre-based systems is clinically desirable, but is challenging due to polarisation-altering properties of optical fibres. Here, we propose a flexible fibre-based polarimetric solution, and describe its design, fabrication, calibration, and initial feasibility demonstration in ex vivo tissue. The design is based on a flexible fibre bundle of six multimode optical fibres, each terminated with a distal polariser that ensures pre-determined output polarisation states. The resultant probe enables linear 3 × 3 Mueller matrix characterization of distal tissue. Potential in vivo Mueller matrix polarimetric tissue examinations in various directly-inaccessible body cavities are envisioned.

Mueller matrix polarimetry is recently attracting more and more attention for its diagnostic potentials. However, for prevalently used division of time Mueller matrix polarimeter based on dual-rotating retarder scheme, beam drift induced by rotating polarizers and waveplates introduces spatial misalignment and pseudo-edge artifacts in imaging results, hindering following accurate microstructural features characterization. In this paper, we quantitatively analyze the beam drift phenomenon in dual-rotating retarder Mueller matrix microscopy and its impact on linear retardance measurement, which is frequently used to reflect tissue fiber arrangement. It is demonstrated that polarizer rotation induces larger beam drift than waveplate rotation due to surface non-uniformity and stress deformation. Furthermore, for waveplates rotated constantly in dual-rotating retarder scheme, their tilt within polarization state analyzer can result in more drift and throughput loss than those within polarization state generator. Finally, phantom and tissue experiments confirm that beam drift, rather than inherent optical path changes, dominates the systematic overestimation of linear retardance in boundary image regions. The findings highlight beam drift as a dominant error source for quantifying linear retardance, necessitating careful optical design alignment and a reliable registration algorithm to obtain highly accurate polarization data for training machine learning models of pathological diagnosis using Mueller matrix microscopy.

The developments in polarized light have spawned a multitude of novel applications in optical fiber systems, but the design and fabrication of practical fiber wave plates with high degree of integration still remain a challenging issue. To address this problem, an all-fiber spun wave plate (SWP) for arbitrary state of polarization (SOP) conversion is proposed in this work, and its principle is analyzed with Mueller matrix. Simulations are conducted to exhibit the arbitrary SOP conversion capability of the proposed SWP, and two key parameters, including the maximum spinning rate (ξmax) and linear birefringence (δ), are investigated for efficient conversion of desired SOP. Different functions to increase the spinning rate ξ from 0 to ξmax, computational efficiency and accuracy related to N are discussed in detail. Furthermore, the depolarization effect caused by retardation of SWP is also considered. The results of this research suggest that the proposed SWP exhibits promising performance in arbitrary SOP conversion, and the meticulous analysis of the numerical computation, design, and implementation of SWP presented in this work can provide novel insights for devloping fiber wave plates.

Stokes parameters (S) on the Poincaré sphere are very useful values to describe the polarisation state of photons. However, the fundamental principle on the nature of polarisation is not completely understood, yet, because we have no concrete consensus on how to describe spin of photons, quantum-mechanically. Here, we have considered a monochromatic coherent ray of photons, described by a many-body coherent state, and established a fundamental basis to describe the spin state of photons, in connection with a classical description based on Stokes parameters. We show that a spinor description of the coherent state is equivalent to Jones vector for polarisation states, and obtain the spin operators (Ŝ) of all components based on rotators in an SU(2) group theory. Polarisation controllers such as phase-shifters and rotators are also obtained as quantum-mechanical field operators to change the phase of the wavefunction for polarisation states. We show that the Stokes parameters are quantum-mechanical average of the spin operators, S=⟨Ŝ⟩.

Polarization is capable of probing microstructures and has unique sensitivity to fibrous anisotropic structure. Polarimetric imaging has demonstrated promising potential in diverse applications ranging from biomedicine, material science, and atmospheric remote sensing. The polarization properties of samples can be comprehensively described by a Mueller matrix (MM). However, the relationship between individual MM elements and properties of the sample is often not clear. There have been consistent efforts to derive polarization parameters from MM based on certain assumptions for better description of the samples, e.g., MM polar decomposition (MMPD), MM transformation (MMT) and MM differential decomposition. Usually, the MM imaging requires sequential measurements with different polarization states of incident light and the imaging process is time consuming. In addition, for movable samples, we cannot guarantee the consistency during the imaging. This may cause precision issues since the images cannot be well-registered. In this work, we built a statistical translation model to generate polarization parameters from a single Stokes vector which can be obtained by one-shot imaging. This will improve the imaging efficiency, simplify the optical system and avoid introducing errors by the image registration. In the model design, we adopted the generative adversarial network (GAN) where the generator is based on a U-net architecture. We demonstrated the effectiveness of our approach on liver tissue, blood smear and porous anodic alumina (PAA) film, and quantitatively evaluated the results by similarity assessment methods. The model can generate a parameter image within 0.1 second on a desktop computer, which shows the potential to achieve real-time performance.

Mueller matrix-based optimization of reflective type twisted nematic liquid crystal SLM at oblique incidences

The study and measurement of the state of polarization (SOP) of the light propagating along a single-mode optical fibre, has been receiving particular attention in recent applications, such as coherent optical communications and fibre sensors [1, 2]. In both cases the polarization analysis requires a dynamical measurement of the SOP at the fiber output, which can be done on the basis of the Jones vector [3] (in terms of the amplitude ratio R and the relative phase Φ of the two components of the electric field) or of the Stokes vector [4] (in terms of the intensity parameters usually represented on the Poincare sphere to describe a fully polarized beam: s0, s1, s2, s3).

The state of polarization (SOP) is the inherent property of light and has important applications in the field of electromagnetic waves, which can be usually evaluated by measuring the Stokes parameters. The conventional methods of measuring the SOP are based on numerous bulky devices and complex optical systems that make measurement difficult and cumbersome. Here, we propose an ultra-compact polarizer based on the gallium nitride metasurface in visible range, which consists of three parts to steer the four SOPs by utilizing double-phase modulation. X-, Y-, 45° linear polarized light and left circular polarized light can be simultaneously focused on spatially symmetric positions. The Stokes parameters can be immediately recovered through simultaneously measuring the focusing energy of the four polarizations. Our device shows reliable performance on polarization analysis as well as paves the way toward promoting the development of material analysis, chiral detections, and so on.

The operation of a photoelastic modulator (PEM) type ellipsometer is described in terms of Mueller matrix elements. The phase shift (5 and the relative amplitude attenuation ratio tan * between orthogonal polarization components are contained in the Mueller matrix elements and can be obtained through combinations of measured matrix elements. A phase shift measurement accuracy of ±0.2° has been obtained without calibration. A computer-controlled implementation is described along with simple algorithms to extract the ellipsometric constants. The Mueller matrix of a magneto-optic (MO) film medium system has been used along with the Mueller matrix for a "leaky" beamsplitter to form the Mueller matrix for an ideal MO read-back system. One of the Stokes parameters is proportional to the differential detection signal commonly used in MO data detection schemes.