We report in this paper an instrument for measuring the Stokes parameters of a light beam. This Stokes polarimeter employs two low birefringence photoelastic modulators (PEMs) operating at different resonant frequencies. The two PEMs in the instrument are oriented 45° apart. An analyzer, following the PEM-pair, bisects the PEMs' optical axes. Electronic signals from a detector are analyzed using lock-in amplifiers at the PEMs' first and second harmonics. A computer program calculates and displays the intensity parameter and the normalized Stokes parameters of the light beam measured. Common laboratory lasers are measured as examples.
Author(s): Gerald E. Jellison, Jr.; C. Owen Griffiths; David E. Holcomb; Christopher M. Rouleau
The two-modulator generalized ellipsometer (2-MGE) is a spectroscopic polarization-sensitive optical instrument that is sensitive to both standard ellipsometric parameters from isotropic samples as well as cross polarization terms arising from anisotropic samples. In reflection mode, teh 2-MGE has been used to measure the complex dielectric functions of several uniaxial crystals, including TiO2, ZnO, and BiI3. The 2-MGE can also be used in the transmission mode, in which the complete Mueller matrix of a sample can be determined (using 4 zone measurements). If the sample is a linear diattenuator and retarder, then only a single zone is required to determine the sample retardation, diattenuation, the principal axis direction, and the depolarization. These measurements have been performed in two different modes: 1) Spectroscopic, where the current wavelength limits are 260 to 850 nm, and 2) Spatially resolved (Current resolution ~30-50 microns) at a single wavelength. The latter mode results in retardation, linear diattenuation, and principal axis direction "maps" of the sample.
Two examples are examined in this paper. First, a simple Polaroid film polarizer is measured, where it is seen that the device behaves nearly ideally in its design wavelength range (visible), but acts more as a retarder in the infrared. Second, congruently grown LiNbO3 is examined under bias. These results show that there are significant variations in the electric field-Pockels coefficient product within the material. Spectroscopic measurements are used to determine the dispersion of the r22 Pockels coefficient.
Birefringence in quartz and sapphire plates was measured at 632.8 nm. The observed birefringence is presumed to be caused by a tilt in the optic axis with respect to the plate geometry. Two instrumental methods were used to make the measurements. A Mueller matrix laser polarimeter was used at the Air Force Research Laboratory, and the Exicor system was used at Hinds. The measurement techniques are described and results are presented.
The angular distribution of the photon pairs of a Zeeman laser scanning confocal microscope (ZLSCM) is measured in turbid media. By scanning the pinhole at different locations on a focal plane, the angular distribution of the snake photon pairs that is contributed by the object plane in the scattering medium is measured. The narrower width of the angular distribution of the snake photon pairs implies the better performance of the depth resolution of ZLSCM in turbid media. In this study, the dependence between depth resolutions of ZLSCM with respect to different vol. % concentrations of the scattering medium is observed. In addition, the correlation between angular distribution and depth resolution in different concentrations is also demonstrated and discussed.
We have studied the polarization characteristics of light scattered from randomly distributed spherical particles using the 4x4 Mueller matrix. The experimental system consists of a Helium-Neon laser, polarizers (vertical, horizontal, 45-degree linear, left-hand circular) and six analyzers (vertical, horizontal, 45-degree linear, 135-degree linear, right-hand circular, left-hand circular). If the six polarized states of the scattered light for a given incident polarization are measured with analyzers, we can calculate the Stokes vector. By repeating this measurement for four independent incident polarizations, we can obtain the complete Mueller matrix. Random media consist of spherical particles of different concentrations suspended in water. The numerical study is based on the complete solution of the radiative transfer equation. Using the discrete ordinate method and matrix solver, we obtain the Stokes vector for a given incident polarization. By calculating Stokes vector for four independent polarizations, we can obtain a full Mueller matrix. The experimental results are compared with the numerical analysis.
New Thin Crystal Film (TCF) polarizers and retarders are produced by deposition of water based lyotropic liquid crystals formed by supramolecules of dichroic dyes. The supramolecules have an aspect ratio of more than 100 with a "spaghetti-like" structure. The supramolecules are formed by disk-shaped aromatic molecules that self-assemble into columnar stack (or strand-like) structures. In the course of deposition, shear force is applied and the supramolecules are aligned along the shear force direction in a wet layer about 10 microns thick. While drying, the layer of lyotropic liquid crystal phase-transitions into a sub-micron TCF functional layer with the dichroic dye molecules aligned in the same fashion over the entire coated surface. Layers of TCF can be deposited on a wide variety of materials including plastic and glass. The coatings exhibit properties of an E-type polarizer in wavebands where molecules exhibit absorption and birefringence up to 0.8 in areas of spectra where absorption is minimal. Several types of TCF polarizers and retarders have been produced and tested in order to improve the performance of LCDs. These films enhance the contrast, viewing angle performance and color rendering of many types of LCDs.
Image processing algorithms for measuring two-dimensional distributions of linear birefringence using a pair of variable retarders are proposed. Several algorithms using from 2 to 5 recorded frames allow to make fast or real time measurements, increase sensitivity and suppress measurement errors. Moreover, the described algorithms can be applied for proposed birefringence imaging systems with fixed retarders and/or Faraday rotators, including systems with images acquired in time sequences or as sets of parallel images.
Recent years have seen the development of several new results that predict and explain the optimum configurations for passive Stokes polarimeters. Stokes polarimeters work by analyzing an unknown input Stokes vector by taking its projection on several known polarization states. The polarimeter is optimized when these states are maximally spaced on the surface of the 4-dimensional conical space of allowed polarization states. Active Mueller matrix polarimetry can be thought of as the combination of two passive polarimeters - one to generate the polarization state and one to analyze the scattered state. It is demonstrated here that an active polarimeter has maximal SNR when the polarization state generator and analyzer are each individually optimized.
Polarization in optical waveguides is always an aspect of optical loss and consequently impacts device performance. Wavelet image processing allows a means to detect optical signals buried under noise. Orthogonality is an essential element in wavelet bases. There are three primary types of multiresolution bases: orthogonal wavelet bases, semiorthogonal wavelet bases, and biorthogonal wavelet bases. Wavelet image processing will be applied to laser beam propagation in lithium niobate and nonlinear polymer waveguides to achieve detection of signals below noise and a better understanding of polarization as an aspect of device performance.
Imaging through random media is an important problem with many applications including optical remote sensing and bio-optics. As the optical depth gets larger, the imaging resolution and contrast deteriorates because of the effect of scattering. In this paper, we present the solution to the vector radiative transfer equation (VRTE) and its application to the optical imaging problem. Since the incoherent component created by the scattering in random media is responsible for the deterioration of the quality of images, several techniques are proposed to improve the imaging by reducing the incoherent component. The Off-Axis Intensity Subtraction (OAIS) and Cross-Polarization Intensity Subtraction (CPIS) imaging techniques are based on the fact that off-axis and cross polarization contains most of the incoherent component. Photon Density Waves (PDW) is a frequency-domain method which exhibits less effect of multiple scattering from the random media. We investigate the techniques mentioned above using numerical solution of VRTE and show the effectiveness, the limitations and the conditions of these techniques. Because we consider the polarized pulse wave case, we also discuss the time-domain behavior and the application of time-gating to the imaging problem. The time-gating method is investigated in both position and duration. Since in practice an array of detectors are often used, we also include the effect of Field Of View of a detector (pixel FOV) in our calculations. We quantitatively measure the performance of imaging techniques by contrast. Also, we apply these techniques to numerical simulations of cross images and show the improvement of the quality of the images.
Photoelastic modulators (PEMs) are polarization modulation devices used in a wide range of experiments to probe the interaction between polarized light and matter. Experimental setups using PEMs rely on common detector types (photodiodes, photomultipliers, etc.) but care must be taken with their use. Specifically,
1. Care must be exercised to ensure proper impedance matching between the detector and the signal analyzing electronics, for example by using a trans-impedance preamplifier.
2. For silicon detectors, the responsivity at PEM frequencies (e.g. 100 kHz) decreases markedly for light wavelengths above about 900 nm
3. For circular dichroism experiments in the UV and visible, care must be taken in selecting a photomultiplier tube to minimize signal "artifacts" due to birefringence in optical components such as the sample cell and the PEM optical element.
Polarimetric imagery that is collected from time-sequential and multiple image format sensors all have potential for image misregistration. Since polarization is usually measured as small differences between radiometric measurements, it is highly sensitive to misregistration, especially at regions of high contrast. The general consensus in the polarization community is that image misregistration on the order of 1/10th of a pixel can introduce artifacts in polarization images. If the registration is not achieved and maintained to this resolution, the data must be registered in software. Typically, rotation and translation (horizontal and vertical) are the main transformations that need to be corrected. It is desirable to have a registration algorithm that determines rotations and translations to 1/10th of a pixel, does not require user intervention, takes minimal computation time, and is based on analytical (non-iterative), automated calculations. This paper details an analytical, automated registration algorithm that corrects for rotation and translations by using a Fourier transform technique. Examples of images registered with this algorithm, and estimates of residual misregistrations are presented. Typical processing times are also given.
We report intensive experimental and theoretical data corresponding to helium extreme ultraviolet (EUV) emission in the 30-60 nm wavelengths range for e- + He, H+ + He, and Hn+ + He (n=2-3) collisions with impact velocities from 1.4 to 8.5 a.u. Specifically, the degree of linear polarization associated with HeI (1snp) 1Po levels, n=2-5, (λ=51.7 to 58.4 nm for n=2-5 to n=1 transitions) and HeII (2p) 2Po states (λ=30.4 nm) have been measured at intermediate- to high-energy range using a compact lightweight molybdenum/silicon multilayer mirror polarimeter (MLM). The combination of these polarization measurements with our previous total EUV cross sections has enabled us to determine experimental magnetic substate scattering angle-integrated cross sections σ0 and σ1for ML = 0, ±1, and 1 following excitation of He (1s2) 1S to HeI (1s2p) 1Po levels and ionization-excitation to He+ (2p) 2Po for electron and proton impact at a wide range velocities. The degree of linear polarization and obtained magnetic sublevel cross sections are compared to earlier theoretical calculations together with our recent 23-state R-Matrix, 1st and 2nd Born calculations. Such a detailed comparison may shed more light on the collision processes involved in positively and negatively single charge particle impact in helium. In particular it shows that electron-electron interactions play an important role in the ionization-excitation mechanism. In addition, this comprehensive database possibly will provide a deeper understanding of electron and proton induced jets in solar flare astrophysical investigations.
This paper provides a summary of research undertaken to quantify the degree of polarization that occurs in thermal infrared radiance in the natural Earth environment. Water surfaces provide one of the only significantly polarized natural sources of thermal infrared radiation, while emission from the atmosphere and ground is almost always unpolarized to any practical degree. However, the actual degree of polarization from water depends upon a balance of orthogonally polarized emission and background reflection terms, weighted by atmospheric transmittance. This summary describes computer simulations of polarized radiance from water viewed in a realistic atmosphere and a comparison of the model with measurements of the spectral degree of thermal infrared polarization for water.
Author(s): Thomas Ruhtz; Eyk Boesche; Marc Schroeder; Juergen Fischer
This paper describes the first results of measurements with the new sensor module FUBISS-Polar. The system measures hyper spectral polarimetric sky radiances in the spectral range from 250 to 1080 nm at four different polarization angles (0°, 45°, 90°, 135°) and enables the calculation of the degree of the linear polarization (DOLP) and its azimuth. This paper will give an overview of the instrument and the data calibration procedures and will present the first results of measurements taken at the laboratory and at three different field campaigns. The field campaign measurements were done in the principle plane of the sun during clear sky conditions in arctic environments (Ny Alesund, Svalbard), on top of the mountain Zugspitze (Alps, Germany) and in urban environments (Berlin, Germany).
Using passive IR polarization enhances the possibility to detect man made objects. Usually, man made objects consist of flat surfaces which polarize the emitted and reflected radiation, thereby increasing the contrast relative to an almost completely depolarized background. This makes reconnaissance and surveillance sensors and warners more efficient. In order to protect an object from being detected by a polarization sensitive sensor the surface of the object should be less sensitive to polarization. This is achieved by increasing the surface roughness. In this paper describes the use of cenosphere particles to increase the surface roughness and hence reducing the polarization. At the same time it is important to decide the value of the emissivity due to the need of contrast reduction. Evaporating gold with different coverage on the surface does this.
The equipment used here consist of two IR Thermovision cameras, one for the wavelength region 3-5 μm and the other for the region 8-12μm. By applying linear polarizing filters in front of the cameras, it is possible to measure the emissivity curves.
The emissivity of the used cenosphere samples varies between about 0.90 and 0.23. For all samples the radiation is close to Lambertian and they all depolarize the radiation completely.
The high accuracy required in traditional ellipsometric measurements necessitates the absolute calibration of both the source and the detector. We demonstrate that these requirements can be circumvented by using a non-classical source of light, namely, a twin-photon polarization-entangled source that produces type-II spontaneous parametric down-conversion, in conjunction with a novel polarization interferometer and coincidence-counting detection scheme. Our scheme exhibits two features that obviate the requirements of a calibrated source and detector. The first is the twin-photon nature of the source; we are guaranteed, on the detection of a photon in one of the arms of the setup, that its twin will be in the other, effectively serving as calibration of the source. The second is that the polarization entanglement of the source serves as an interferometer, thereby alleviating the need for calibrating the detector. The net result is that absolute ellipsometric data from a sample may be obtained. We present preliminary experimental results showing how the technique operates.
Surface plasmon resonance (SPR) imaging system is presented as a novel technique based on modified Mach-Zehnder phase-shifting interferometry (PSI) for biomolecular interaction analysis (BIA), which measures the spatial phase variation of a resonantly reflected light in biomolecular interaction. In this technique, the micro-array SPR biosensors with over a thousand probe NDA spots can be detected simultaneously. Owing to the feasible and swift measurements, the micro-array SPR biosensors can be extensively applied to the nonspecific adsorption of protein, the membrane/protein interactions, and DNA hybridization. The detection sensitivity of the SPR PSI imaging system is improved to about 1 pg/mm2 for each spot over the conventional SPR imaging systems. The SPR PSI imaging system and its SPR sensors have been successfully used to observe slightly index change in consequence of argon gas flow through the nitrogen in real time, with high sensitivity, and at high-throughout screening rates.
The biosensors based on surface plasmon resonance (SPR) are often used as tools for directly detecting the kinetic interaction of unlabelled biological molecules at surface in real time. With the measured SPR reflection spectrum, we can detect a shift in the location and quantity of the reflection spectrum minimum and the half width at half maximum due to the change in the thickness or the refractive index of a thin dielectric film layer. The interested parameters of analyte layer or monolayer, like the molecular size and concentration, can be determined either with analytical approaches or linear data analysis approaches. Depends on the number of parameters need to be resolved, we may need either multiple spectra (two color method) or only one sensing spectrum under the assumption that the other film parameter is given for multiple parameters case. Although it is possible to estimate multiple parameters from only one sensing spectrum by linear estimation techniques, it suffers from not only the shortcoming for larger variance in the estimates from those techniques than that of multiple spectra method but also the difficulty for choosing the appropriate initial value in the estimation process. In this paper, we propose a modified analytic approach to attain suitable initial parameters that close enough to the exact value. Furthermore, we incorporated multi-experiment method into linear estimation algorithms to determine the optimal estimated parameters with smaller variability of the estimated parameters. In that manner, it would be benefit to reject the colored noise accidentally results from experiment process. The experimental data with the multi-experiment linear data analysis demonstrates that it has ability to sense slightly index change in consequence of argon gas flow through the nitrogen.
We present the results of efforts to understand the polarimetric characterization properties of gold- and silver-coated mirrors, furthering our abilities for design and calibration of optical sensors. A FT spectropolarimeter was used to determine the experimental Mueller Matrix for both the gold- and silver-coated mirrors, thus fully characterizing the polarization altering properties of each sample. Presented is the total, linear and circular polarizance for incident angles ranging from 15° to 60° and between 0.7 and 1.1 μm.
The Jones matrix formalism allows to consider separately the behavior of polarization ellipse and amplitude of radiation (Azzam R.M.A. and N.M. Bashara, Ellipsometry and polarized light. North-Holland. Amsterdam. 1977). In scope of the Mueller matrix formalism this question received considerably less attention. On the one hand, it is probably explained by apparent evidence of this question. Indeed, it is well known that polarization ellipse is completely described by three lower components and total intensity by first component of the Stokes vector (Azzam R.M.A. and N.M. Bashara, ibidem). On the other hand, many properties of the polarization transfer function were established basing on the exploitation of the conformality of bilinear transformation of polarization. However, the polarization transformation in scope of the Mueller matrix formalism has no such property. At the same time, the existence of the generalized deterministic Mueller matrix (Mar'enko V.V. and S.N. Savenkov, Representation of arbitrary Mueller matrix in the basis of matrices of circular and linear anisotropy. Optics and Spectroscopy, 76(1), pp. 94-96, 1994) allows to analyze the properties of the polarization transfer function in scope of the Mueller matrix formalism. Basing on this analysis it is offered the optimal procedure to measure elements of the Mueller matrix, which are the coefficients of polarization transfer function.