RESUMEN

A spatially adaptive Mueller matrix imaging polarimeter is described, simulated, and demonstrated with preliminary experiments. The system uses a spatial light modulator (SLM) in the polarization state generator (PSG) to create spatial carriers that controlled by the pattern written to the SLM. The polarization state analyzer (PSA) is a commercial division of focal plane imaging polarimeter. The PSG/PSA pair form a 9-channeled partial Mueller matrix polarimeter that measures a 3 × 3 sub-matrix of the Mueller matrix. We demonstrate that adapting the PSG modulation to the spatial frequency structure of the scene can reduce channel crosstalk and improve reconstruction accuracy. Initial experiments are performed that demonstrate the SLM's ability to produce sufficient modulation diversity to create the desired channel structure. Though there are several experimental challenges to obtain accurate Mueller matrix imagery, we demonstrate a system that adapts to the particular scene spatial frequency structure.

RESUMEN

Analysis of data generated by Mueller matrix polarimeters using two photoelastic modulators has been evolving with the improvements in data acquisition and digital signal processing (DSP). Historical processing of the temporal data generated by these devices has involved isolating the frequencies via hardware signal processing (e.g., lock-in amplifiers) or the numerical computation of Fourier integrals of recorded temporal data. Both avenues have their advantages, but the DSP aspects of the latter provide greater flexibility in choice of harmonics for processing. While conventional processing uses one harmonic for each desired Mueller matrix element, recent work has demonstrated that theoretical improvements are possible by coherently combining the information in multiple harmonic channels for each element. We demonstrate some recent progress in DSP that enables these polarimeters' data to be more fully exploited by addressing two key issues in the Fourier domain: spectral leakage and phase recovery. Adequately addressing these issues enables numerical analysis of the temporal data in the complex Fourier domain and delivers Mueller matrix results in which spectral phase information is used to recover the matrix elements and determine their signs automatically. We explore the application of this complex analysis and how the precision and accuracy of the results are affected by common experimental and DSP limitations compared to the usual magnitude-only analysis in the Fourier domain. The multi-harmonic method can provide a theoretical factor of 1.3-1.7 improvement in instrumental precision, and our experimental results approach that theoretical range.

RESUMEN

Channeled spectropolarimetry (CSP) employing low-pass channel extraction filters suffers from cross talk and spectral resolution loss. These are aggravated by empirically defining the shape and scope of the filters for different measured. Here, we propose a convolutional deep-neural-network-based channel filtering framework for spectrally-temporally modulated CSP. The network is trained to adaptively predict spectral magnitude filters (SMFs) that possess wide bandwidths and anti-cross-talk features that adapt to scene data in the two-dimensional Fourier domain. Mixed filters that combine the advantages of low-pass filters and SMFs demonstrate superior performance in reconstruction accuracy.

RESUMEN

A channeled Stokes polarimeter that recovers polarimetric signatures across the scene from the modulation induced channels is preferrable for many polarimetric sensing applications. Conventional channeled systems that isolate the intended channels with low-pass filters are sensitive to channel crosstalk effects, and the filters have to be optimized based on the bandwidth profile of scene of interest before applying to each particular scenes to be measured. Here, we introduce a machine learning based channel filtering framework for channeled polarimeters. The machines are trained to predict anti-aliasing filters according to the distribution of the measured data adaptively. A conventional snapshot Stokes polarimeter is simulated to present our machine learning based channel filtering framework. Finally, we demonstrate the advantage of our filtering framework through the comparison of reconstructed polarimetric images with the conventional image reconstruction procedure.

RESUMEN

Channeled spectropolarimeters (CSPs) are capable of estimating spectrally resolved Stokes parameters from a single modulated spectrum. However, channel crosstalk and subsequent spectral resolution loss reduce the reconstruction accuracy and limit the systems' scope of application. In this paper, we propose a spectral-temporal modulation strategy with the aim of extending channel bandwidth and improving reconstruction accuracy by leveraging the hybrid carriers and allocating channels in the two-dimensional Fourier domain that yield optimal performance. The scheme enables spectral bandwidth and temporal bandwidth to be traded off, and provides flexibility in selecting demodulation strategies based on the features of the input. We present an in-depth comparison of different systems' performances in various input features under the presence of noise. Simulation results show that the hybrid-modulation strategy offers the best comprehensive performance as compared to the conventional CSP and dual-scan techniques.

RESUMEN

Snapshot channeled polarimeters forgo temporal modulation in favor of modulating polarization information in either space or wavenumber. We have recently introduced methodologies for describing both channeled and partial polarimeters. In this paper, we focus on the nine-reconstructables design, which limits the resolution loss by reducing the number of carriers. The architecture offers a number of favorable trade-offs: a factor of 5.44 increase in spatial bandwidth or a factor of 3.67 increase in spectral bandwidth, for a smaller amount of temporal bandwidth loss as dictated by the number of snapshots taken. The multi-snapshot structured decomposition given here allows one to analytically shape the measured space with optimal noise characteristics and minimum system complexity. A two-snapshot system can measure a premeditated set of 14 reconstructables; we provide the null space for the subset of optimal systems that also achieve better SNR than the baseline single-snapshot system. A three-snapshot system can measure all 16 Mueller elements while offering an overall 26.3% or 50.4% better bandwidth-SNR figure of merit for the spectral and spatial systems, respectively. Finally, four-snapshot systems provide diminishing returns, but may be more implementable.

RESUMEN

Most channeled polarimeters modulate the intensity in a single independent domain such as space, time, or wavenumber. Recently we proposed and modeled a concept for a system modulated simultaneously in space and time [Opt. Lett.43, 2768 - 2771 (2018)] and demonstrated that superior performance could be obtained by trading off spatial and temporal bandwidth in the system. Here we present results from a prototype realization of such a system and demonstrate quantitatively that the spatial modulation transfer function of the imager can be improved by choosing the appropriate modulation strategy for a given scene spatial and temporal bandwidth. We demonstrate that a hybrid modulation system can achieve the high spatial frequency performance of a time modulated system for static scenes, or it can achieve the high temporal frequency performance of a spatially modulated system for rapidly varying scenes, and it can out perform both systems for scenes with intermediate bandwidth in both domains. Moreover, the physical system implementation is essentially the same for each system type, which in principle allows the reconstruction strategy to be selected in real-time by choosing the appropriate reconstruction filters.

RESUMEN

Photoelastic modulator-based polarimeters use multi-carrier modulation schemes that are more complicated than the single carriers of rotating optics. Current state-of-the-art reconstruction implementations favor mathematical simplicity by using significantly abridged subsets of channels. In this Letter, we extend our generalized channeled polarimetry principles to address the challenges associated with multi-carrier modulation schemes. We demonstrate the performance forfeited by existing systems through the use of an incomplete set of information channels, as well as propose a set of more optimal system parameters that achieve better reconstruction noise characteristics. The overall improvement corresponds to a better sensitivity of up to a factor of six.

RESUMEN

We present the analysis and design of spatio-temporal channeled Stokes polarimeters. We extend our recent work on optimal pixelated polarizer arrays by utilizing temporal carrier generation, resulting in polarimeters that achieve super-resolution via the tradeoff between spatial bandwidth and temporal bandwidth. Utilizing the channel space description, we present a linear-Stokes design and two full-Stokes imaging polarimeter designs that have the potential to operate at the full frame rate of the imaging sensor of the system by using hybrid spatio-temporal carriers. If the objects are not spatially bandlimited, the achievable temporal bandwidth is more difficult to analyze; however, a spatio-temporal tradeoff still exists.

RESUMEN

Conventional imaging devices are often compared using their optical transfer functions (OTFs) in space and their impulse responses in time. Modulated polarimeters cannot be directly compared this way, since they are frequency multiplexed. Here we define a spectral density response function that describes how the spectral density matrix of the Stokes parameters for an object transfers through a modulated polarimeter. This response function facilitates the objective comparison of polarimeters in a way that is analogous to the OTF for conventional imaging systems. The spectral density response is used to calculate a Wiener filter for a rotating analyzer polarimeter as an example of filter optimization for modulated polarimetry.

RESUMEN

Vectorial vortex analysis is used to determine the polarization states of an arbitrarily polarized terahertz (0.1-1.6 THz) beam using THz achromatic axially symmetric wave (TAS) plates, which have a phase retardance of Δ = 163° and are made of polytetrafluorethylene. Polarized THz beams are converted into THz vectorial vortex beams with no spatial or wavelength dispersion, and the unknown polarization states of the incident THz beams are reconstructed. The polarization determination is also demonstrated at frequencies of 0.16 and 0.36 THz. The results obtained by solving the inverse source problem agree with the values used in the experiments. This vectorial vortex analysis enables a determination of the polarization states of the incident THz beam from the THz image. The polarization states of the beams are estimated after they pass through the TAS plates. The results validate this new approach to polarization detection for intense THz sources. It could find application in such cutting edge areas of physics as nonlinear THz photonics and plasmon excitation, because TAS plates not only instantaneously elucidate the polarization of an enclosed THz beam but can also passively control THz vectorial vortex beams.

RESUMEN

Channeled polarimeters measure polarization by modulating the measured intensity in order to create polarization-dependent channels that can be demodulated to reveal the desired polarization information. A number of channeled systems have been described in the past, but their proposed designs often unintentionally sacrifice optimality for ease of algebraic reconstruction. To obtain more optimal systems, a generalized treatment of channeled polarimeters is required. This paper describes methods that enable handling of multi-domain modulations and reconstruction of polarization information using linear algebra. We make practical choices regarding use of either Fourier or direct channels to make these methods more immediately useful. Employing the introduced concepts to optimize existing systems often results in superficial system changes, like changing the order, orientation, thickness, or spacing of polarization elements. For the two examples we consider, we were able to reduce noise in the reconstruction to 34.1% and 57.9% of the original design values.

RESUMEN

Axially symmetric half-wave plates have been used to generate radially polarized beams that have constant phase in the plane transverse to propagation. However, since the retardance introduced by these waveplates depends on the wavelength, it is difficult to generate radially polarized beams achromatically. This paper describes a technique suitable for the generation of achromatic, radially polarized beams with uniform phase. The generation system contains, among other optical components, an achromatic, axially symmetric quarter-wave plate based on total internal reflection. For an incident beam with a constant phase distribution, the system generates a beam with an extra geometrical phase term. To generate a beam with the correct phase distribution, it is therefore necessary to have an incident optical vortex with an azimuthally varying phase distribution of the form exp( + iθ). We show theoretically that the phase component of radially polarized beam is canceled out by the phase component of the incident optical vortex, resulting in a radially polarized beam with uniform phase. Additionally, we present an experimental setup able to generate the achromatic, uniform-phase, radially polarized beam and experimental results that confirm that the generated beam has the correct phase distribution.

RESUMEN

A generalized van Cittert-Zernike theorem for the cross-spectral density matrix of quasi-homogeneous planar electromagnetic sources is introduced. We present theoretical examples of using this theorem to generate fields with interesting polarization and spatial coherence properties by choosing the appropriate spectral density distribution of the source. We found that under certain conditions, a quasi-homogeneous, polarized source may produce a beam in the far field that is unpolarized in the typical one-point sense but polarized in the two-point, mutual polarization sense.

RESUMEN

Three-dimensional displays have become increasingly present in consumer markets. However, the ability to capture three-dimensional images in space confined environments and without major modifications to current cameras is uncommon. Our goal is to create a simple modification to a conventional camera that allows for three dimensional reconstruction. We require such an imaging system have imaging and illumination paths coincident. Furthermore, we require that any three-dimensional modification to a camera also permits full resolution 2D image capture.Here we present a method of extracting depth information with a single camera and aberrated projected pattern. A commercial digital camera is used in conjunction with a projector system with astigmatic focus to capture images of a scene. By using an astigmatic projected pattern we can create two different focus depths for horizontal and vertical features of a projected pattern, thereby encoding depth. By designing an aberrated projected pattern, we are able to exploit this differential focus in post-processing designed to exploit the projected pattern and optical system. We are able to correlate the distance of an object at a particular transverse position from the camera to ratios of particular wavelet coefficients.We present our information regarding construction, calibration, and images produced by this system. The nature of linking a projected pattern design and image processing algorithms will be discussed.

Asunto(s)

RESUMEN

Imaging polarimeters infer the spatial distribution of the polarization state of the optical field as a function of time and/or wavelength. A polarimeter indirectly determines the polarization state by first modulating the intensity of the light field and then demodulating the measured data to infer the polarization parameters. This Letter considers passive Stokes parameter polarimeters and their inversion methods. The most widely used method is the data reduction matrix (DRM), which builds up a matrix equation that can be inverted to find the polarization state from a set of intensity measurements. An alternate strategy uses linear system formulations that allow band limited reconstruction through a filtering perspective. Here we compare these two strategies for overdetermined polarimeters and find that design of the null space of the inversion operator provides degrees of freedom to optimize the trade off between accuracy and signal-to-noise ratio. We further describe adaptive filtering techniques that could optimize the reconstruction for a particular experimental configuration. This Letter considers time-varying Stokes parameters, but the methods apply equally to polarimeters that are modulated in space or in wavelength.

RESUMEN

Data processing for sequential in time polarimeters based on the Data Reduction Matrix technique yield polarization artifacts in the presence of time varying signals. To overcome these artifacts, polarimeters are designed to operate at higher and higher speeds. In this paper we describe a band limited reconstruction algorithm that allows the measurement and processing of temporally varying Stokes parameters without artifacts. An example polarimeter consisting of a rotating retarder and polarizer is considered, and conventional processing methods are compared to a band limited reconstruction algorithm for the example polarimeter. We demonstrate that a significant reduction in error is possible using these methods.

Asunto(s)

RESUMEN

We consider target detection in images perturbed with additive noise. We determine the conditions in which polarimetric imaging, which consists of analyzing of the polarization of the light scattered by the scene before forming the image, yields better performance than classical intensity imaging. These results give important information to assess the interest of polarimetric imaging in a given application.

RESUMEN

Mueller matrix polarimeters (MMPs) are designed to probe the polarization properties of optical scattering processes. When using a MMP for a detection, discrimination, classification, or identification task, a user considers certain elements of the Mueller matrix. The usual way of performing this task is to measure the full Mueller matrix and discard the unused elements. For polarimeter designs with speed, miniaturization, or other constraints, it may be desirable to have a system with reduced dimensionality that measures only elements of the Mueller matrix that are important in a particular application as efficiently as possible. In this paper, we develop a framework that allows partial MMPs to be analyzed. Quantitative metrics are developed by considering geometrical relationships between the space spanned by a particular MMP and the space occupied by the scene components. The method is generalized to allow the effects of noise to be considered. The results are general and can also be used to optimize complete and overspecified MMPs for performing specific tasks, as well.

RESUMEN

Microgrid polarimeters operate by integrating a focal plane array with an array of micropolarizers. The Stokes parameters are estimated by comparing polarization measurements from pixels in a neighborhood around the point of interest. The main drawback is that the measurements used to estimate the Stokes vector are made at different locations, leading to a false polarization signature owing to instantaneous field-of-view (IFOV) errors. We demonstrate for the first time, to our knowledge, that spatially band limited polarization images can be ideally reconstructed with no IFOV error by using a linear system framework.