Physical Optics Model Research Articles

Enhanced Doppler reflectometry power response: physical optics and 2D full wave modelling

The power response of a Doppler reflectometer is investigated by means of the physical optics model; a simple model which considers basic scattering processes at the reflection layer. Apart from linear and saturated scattering regimes, non-linear regimes with an enhanced backscattered power are found. The different regimes are characterized and understood based on analytical calculations. The power response is also studied with two-dimensional full wave simulations, where the enhanced backscattered power regimes are also found in qualitative agreement with the physical optics results. The ordinary and extraordinary modes are compared for the same angle of incidence, with the conclusion that the ordinary mode is better suited for Doppler reflectometry turbulence level measurements due to the linearity of its response. The scattering efficiency is studied and a first approximation to describe it is proposed. At the end, the application of the physical optics results to experimental data analysis is discussed. In particular, a formula to assess the linearity of Doppler reflectometry measurements is provided.

Iterative physical optics model for electromagnetic scattering and Doppler analysis

An iterative physical optics (IPO) model is proposed to solve extra large scale electric electromagnetic (EM) scattering from randomly rough surfaces. In order to accelerate the convergence of the IPO model, the forward-backward methodology and its modification with underrelaxation iteration are developed to simulate the rough surface scattering; the local iteration methodology and the fast far field approximation (FaFFA) in the matrix-vector product are proposed to reduce greatly the computational complexity. These techniques make Monte Carlo simulations possible. Thus, the average Doppler spectra of backscattered signals obtained from the simulations are compared for different incident angles and sea states. In particular, the simulations show a broadening of the Doppler spectra for a more complicated sea state at a low grazing angle (LGA).

Scattering From Contaminated Rough Sea Surface by Iterative Physical Optics Model

This letter focuses on scattering from rough sea surface covered by insoluble oil. Oil film damps the capillary wave of the rough sea surface, which leads to a smooth profile of the contaminated sea. We investigate this phenomenon using a highly efficient iterative model based on physical optics (PO) and Huygens' equivalence principle. PO is used to calculate the scattering from the oil film and underlying rough sea surface. The Huygens equivalence principle and iterative strategy are employed to evaluate the coupled scattering between the oil film and rough sea surface. The scattering of contaminated rough sea surface based on the iterative model is compared with those using the method of moments as well as computational time and memory requirements. Then, the influence of some important parameters, such as wind speed, oil thickness, incident wave frequency, and oil coverage, on scattering is investigated and discussed in detail.

Analysis of High‐Frequency EM Scattering Properties on Time‐Varying Sea Surface

This paper proposes to solve the extralarge scale Electromagnetic (EM) scattering problem on a wind-driven sea surface at Low-grazing-angle (LGA) directed against a shore-based radar with an Iterative physical optics (IPO) model. Using Monte Carlo simulations, it compares the average Doppler spectra of backscattered signals at different angles and sea states. The methods are: the forward-backward methodology and its modification with under-relaxation iteration which improves convergence and stability of the IPO; the local iteration methodology and the Fast far-field approximation (FaFFA) in the matrix-vector product which reduce the computational complexity based on the scattering characteristics of sea surface. The simulations show a broadening effect of the Doppler spectra in a more complicated sea state at LGA.

An Accelerated Forward–Backward Iterative Physical Optics for Electromagnetic Scattering from Rough Sea Surface

This article proposes to solve the extra-large-scale (e.g., larger than 1,000 square lambda) electric electromagnetic scattering problem on a wind-driven sea surface at a low grazing angle directed against a shore-based radar with an iterative physical optics model. In order to accelerate the convergence of the iterative physical optics model and improve its robustness, the forward–backward methodology and its modification with under-relaxation iteration are developed first to simulate the rough sea surface scattering. Meanwhile, according to the scattering characteristics of rough sea surface, the local iteration methodology and the fast far-field approximation in the matrix-vector product are proposed to reduce greatly the computational complexity. Through these techniques, this model can solve effectively the extra-large-scale electric scattering problem from the randomly rough sea surfaces.

A Wideband Resistive Beam-Splitter Screen

We present the design, construction, and measurements of the electromagnetic performance of a wideband space beam splitter. The beam splitter is a sheet in free space that is designed to divide incident radiation into reflected and transmitted components for interferometer measurement of spectral features in the mean cosmic radio background. Analysis of a two-element interferometer configuration with a vertical beam splitter between a pair of antennas leads to the requirement that the beam splitter be a resistive sheet with sheet resistance $\eta_o/2$ , where $\eta_o$ is the impedance of free space. The transmission and reflection properties of such a sheet are computed for normal and oblique incidences and for orthogonal polarizations of the incident electric field. We have constructed such an electromagnetic beam splitter as a square-soldered grid of resistors of value 180 Ohms (approximately $\eta_o/2$ ) and a grid size of 0.1 m. We measured the reflection and transmission coefficients over a wide frequency range between 50 and 250 MHz in which the wavelength well exceeds the mesh size. Measurements of the coefficients for voltage transmission and reflection agree to within 5% with physical optics modeling of the wave propagation, which takes into account edge diffraction.

Comparison of microfacet BRDF model to modified Beckmann-Kirchhoff BRDF model for rough and smooth surfaces.

A popular class of BRDF models is the microfacet models, where geometric optics is assumed. In contrast, more complex physical optics models may more accurately predict the BRDF, but the calculation is more resource intensive. These seemingly disparate approaches are compared in detail for the rough and smooth surface approximations of the modified Beckmann-Kirchhoff BRDF model, assuming Gaussian surface statistics. An approximation relating standard Fresnel reflection with the semi-rough surface polarization term, Q, is presented for unpolarized light. For rough surfaces, the angular dependence of direction cosine space is shown to be identical to the angular dependence in the microfacet distribution function. For polished surfaces, the same comparison shows a breakdown in the microfacet models. Similarities and differences between microfacet BRDF models and the modified Beckmann-Kirchhoff model are identified. The rationale for the original Beckmann-Kirchhoff F(bk)(2) geometric term relative to both microfacet models and generalized Harvey-Shack model is presented. A modification to the geometric F(bk)(2) term in original Beckmann-Kirchhoff BRDF theory is proposed.

The GO4 Model in Near-Nadir Microwave Scattering From the Sea Surface

We introduce a practical and accurate model, referred to as “GO4,” to describe near-nadir microwave scattering from the sea surface, and at the same time, we address the issue of the filtered mean square slope (mss) conventionally used in the geometrical optics model. GO4 is a simple correction of this last model, taking into account the diffraction correction induced by the rough surface through what we call an effective mean square curvature (msc). We evaluate the effective msc as a function of the surface wavenumber spectrum and the radar frequency and show that GO4 reaches the same accuracy as the physical optics model in a wide range of incidence and frequency bands with the sole knowledge of the mss and msc parameters. The key point is that the mss entering in GO4 is not the filtered but the total slope. We provide estimation of the effective msc on the basis of classical sea spectrum models. We also evaluate the effective msc from near-nadir satellite data in various bands and show that it is consistent with model predictions. Non-Gaussian effects are discussed and shown to be incorporated in the effective msc. We give some applications of the method, namely, the estimation of the total sea surface mss and the recalibration of relative radar cross sections.

Modeling physical optics phenomena by complex ray tracing

Physical optics modeling requires propagating optical wave fields from a specific radiometric source through complex systems of apertures and reflective or refractive optical components, or even complete instruments or devices, usually to a focal plane or sensor. The model must accurately include the interference and diffraction effects allowed by the polarization and coherence characteristics of both the initial optical wave field and the components and media through which it passes. Like a spherical wave and a plane wave, a Gaussian spherical wave (or Gaussian beam) is also a solution to the paraxial wave equation and does not change its fundamental form during propagation. The propagation of a Gaussian beam is well understood and easily characterized by a few simple parameters. Furthermore, a paraxial Gaussian beam can be propagated through optical systems using geometrical ray-trace methods. The decomposition of arbitrary propagating wave fields into a superposition of Gaussian beamlets is, thus, an alternative to the classical methods of propagating optical wave fields. This decomposition into Gaussian beamlets has been exploited to significant advantage in the modeling of a wide range of physical optics phenomena.

Enhancing the performance of the light field microscope using wavefront coding.

Light field microscopy has been proposed as a new high-speed volumetric computational imaging method that enables reconstruction of 3-D volumes from captured projections of the 4-D light field. Recently, a detailed physical optics model of the light field microscope has been derived, which led to the development of a deconvolution algorithm that reconstructs 3-D volumes with high spatial resolution. However, the spatial resolution of the reconstructions has been shown to be non-uniform across depth, with some z planes showing high resolution and others, particularly at the center of the imaged volume, showing very low resolution. In this paper, we enhance the performance of the light field microscope using wavefront coding techniques. By including phase masks in the optical path of the microscope we are able to address this non-uniform resolution limitation. We have also found that superior control over the performance of the light field microscope can be achieved by using two phase masks rather than one, placed at the objective's back focal plane and at the microscope's native image plane. We present an extended optical model for our wavefront coded light field microscope and develop a performance metric based on Fisher information, which we use to choose adequate phase masks parameters. We validate our approach using both simulated data and experimental resolution measurements of a USAF 1951 resolution target; and demonstrate the utility for biological applications with in vivo volumetric calcium imaging of larval zebrafish brain.

Compact-range accuracy improvement: ground-reflection-error mitigation using surface patterning [AMTA corner

Compact-range measurement facilities have been used successfully for many years to characterize antenna performance as well as radar signatures. This paper investigates strategies for improving compact-range-measurement accuracy by mitigating errors associated with ground reflections inherent in most range designs. A methodology is developed for strategically modifying, or patterning, the surface between the range's source antenna and the reflector to reduce error terms, thereby increasing measurement accuracy. Candidate patterns were evaluated using a full-wave computational Finite-Difference Time-Domain (FDTD) model at VHF/UHF frequencies to determine baseline performance, and to develop trade rules for more advanced designs. Physical Optics (PO) models were used to analyze the final design at the frequencies of interest.

The adaptive x-ray optic project at the Lawrence Livermore National Laboratory

Deformable mirrors (DMs) have been successfully used in astronomical adaptive optics at near-infrared wavelengths, greatly reducing atmospheric-induced aberrations [1]. Extending this capability to the soft and hard x-ray regime is now required in order to take full advantage of the beam quality characteristic of new generation synchrotron facilities and X-ray Free Electron Lasers (XFELs). Achieving this goal challenges both current mirror manufacturing techniques and physical optics modeling. The Lawrence Livermore National Laboratory (LLNL) is currently developing an x-ray DM to correct wavefront aberrations introduced along the beam path of a typical x-ray beam-line [2]. To model the expected performance of such a mirror, we have also developed simulation code based on the wavefront propagation library of functions PROPER [3]. Here we present the current status of the project, including metrology done on the mirror substrate. Additionally we report on results from our wavefront simulation code, which have proven very useful in predicting technical aspects of mirror deployment at a typical x-ray facility.

Modeling the SAR backscatter of linear dunes on Earth and Titan

We modeled the Synthetic Aperture Radar (SAR) response of linear dunes of the Great Sand Sea in Egypt using a single surface scattering term, based on Integral Equation and Physical Optics Models. Using multi-frequency SIR-C/X-SAR radar scenes and topography obtained from the Shuttle Radar Topography Mission (SRTM), we were able to estimate reasonable values for the parameters describing the surface roughness of the dunes. As the linear dunes of the Great Sand Sea are relevant analogs for the linear dunes observed on Titan by the Cassini Radar instrument, these results were thus used as a starting point to simulate the radar response of Titan’s dunes, as imaged by the Radar instrument onboard the Cassini spacecraft during the T8 flyby in October 2005. We show that a single surface scattering term is not sufficient to simulate the radar signal backscattered by the dunes on Titan: one has to add a diffuse scattering term, indicating that Titan’s dunes are likely to have somewhat inhomogeneous internal structures related to porosity and/or internal layering. Our results also indicate that the dunes on Titan should be close to perfectly smooth, possibly because of the formation of smaller ripples than on Earth, plus smoothing resulting from precipitation events.

Near Field Physical Optics modelling for Concealed Weapon Detection

A modelling technique is presented for simulating the received signal in a body scanner for concealed weapon detection (CWD) at millimeter wavelengths. The technique is based on a physical optics (PO) near field refinement and encompasses dielectric and perfect electric conductor (PEC) targets, thus reproducing the behaviour of human tissues and of possible concealed threats. Simulated data can be used for testing near field imaging algorithms without resorting to costly measurement experiments. In this communication the original far field formulation and the near field refinement are compared by using simulated data. The model is then validated using single and multi-frequency measured scattered fields by a metallic cylinder acquired in an anechoic chamber.

A reduced physical optics model for electromagnetic scattering problem

The ultimate simplified point scatterer model contains no object's geometry informations, so a more complex prediction model containing geometrical informations is needed. The Physical Optics (PO) approximation has been widely used and considered as a good approximation of the far field electromagnetic scattering. Here, a forward model of bistatic scattering by PEC infinite elliptic cylinder based on the Physical Optics approximation is presented. The aim is to develope a simple scattering model for 2D targets illuminated by a monochromatic wave bistatic Radar. The reduced model is obtained by applying the stationary phase method to the PO integrals in both TE and TM wave cases. A parameter estimation procedure is also presented in order to examinate the efficiency of the presented model.

On Airborne Doppler Navigation System Application for Measurement of the Water Surface Backscattering Signature and Estimation of the Near-Surface Wind

Abstract: A method for measuring the water surface backscattering signature and estimating the near-surface wind vector over water using the airborne Doppler navigation system in addition to its standard navigational application is discussed. A case of an airplane circle flight measurement of the azimuth normalized radar cross section curve of the water surface in the range of middle incidence angles is considered. The system op-erates in the scatterometer mode and uses a fore-beam directed to the right side at a typical mounting angle in the vertical plane that is not so far from nadir at a straight flight. Wind vector is recovered from the azimuth normalized radar cross section curve obtained. Algorithms for measuring the water surface backscattering signature and extracting the wind speed and direction are proposed. Keywords: Doppler navigation system, Scatterometer, Water surface backscattering signature, Sea wind, A lgorithm. INTRODUCTION On the global scale, the information about sea waves and wind, in general, could be obtained from a satellite using active microwave instruments: Scatterometer, Synthetic Aperture Radar and Radar Altimeter [1]. However, for the local numerical weather and wave models as well as for a pilot on an amphibious airplane to make a landing decision, the local data about wave height, wind speed and direction are required. Many researchers have been investigating the microwave backscattering signatures of the water surface and solving the problem of remote measuring of the wind speed and direction over water [2-13]. However, the mechanics of interactions between water surfaces and microwaves have not been well studied in detail. The typical method for describing sea clutter is in the form of the normalized radar cross section (NRCS), the statistical distribution of the NRCS, the amplitude correlation and the spectral shape of the Doppler returns [14]. To describe the radar backscatter from the water surfaces, three major scattering models are used: the Kirchhoff or physical optics model, the composite-surface or two-scale model, and the Bragg model. The Kirchhoff model assumes a perfectly conducting surface (unless it is modified to include the Fresnel reflection coefficient) and applies from small to intermediate incidence angles without shadowing effects. Apart from the implicit dependence on the Fresnel

Planckearly results. IV. First assessment of the High Frequency Instrument in-flight performance

The Planck High Frequency Instrument (HFI) is designed to measure the temperature and polarization anisotropies of the Cosmic Microwave Background and galactic foregrounds in six wide bands centered at 100, 143, 217, 353, 545 and 857 GHz at an angular resolution of 10' (100 GHz), 7' (143 GHz), and 5' (217 GHz and higher). HFI has been operating flawlessly since launch on 14 May 2009. The bolometers cooled to 100 mK as planned. The settings of the readout electronics, such as the bolometer bias current, that optimize HFI's noise performance on orbit are nearly the same as the ones chosen during ground testing. Observations of Mars, Jupiter, and Saturn verified both the optical system and the time response of the detection chains. The optical beams are close to predictions from physical optics modeling. The time response of the detection chains is close to pre-launch measurements. The detectors suffer from an unexpected high flux of cosmic rays related to low solar activity. Due to the redundancy of Planck's observations strategy, the removal of a few percent of data contaminated by glitches does not affect significantly the sensitivity. The cosmic rays heat up significantly the bolometer plate and the modulation on periods of days to months of the heat load creates a common drift of all bolometer signals which do not affect the scientific capabilities. Only the high energy cosmic rays showers induce inhomogeneous heating which is a probable source of low frequency noise.

Abstract A10: A new in silico physical optical model (iPOM) for the study of cervical intraepithelial neoplasia

Abstract Background: Cervical cancer is the second leading cause of deaths in women worldwide. More than 80% of cervical cancers and deaths occur in the developing world. New optical and imaging technologies (confocal imaging, fluorescence/absorbance spectroscopy, OCT, etc…) are being developed and tested as potential keys components of a necessary global solution to this problem. The success of these new technologies requires a better understanding of the precise interactions between the incident light, the underlying biological events and the physical structure of the cervical tissue. Herein, we present a new platform based on an in silico Physical Optical Model (iPOM), which is designed to simulate, analyze, and quantify the cell-and tissue-level biological changes that are associated with cervical preneoplastic growths. By reproducing these features, iPOM also gives insights into the ways in which dysplastic changes will be detected by different optical technologies being developed for cervical cancer screening, detection, diagnosis or treatment. Methods: The physical module of this platform is based on a database of more than 4000 cervical histological intraepithelial neoplastic specimens, collected from about 1800 women. As part of previous multicenters clinical trials (NIH funded Po1 project), Feulgen-stained sections were quantitatively analyzed by an high-resolution imaging software. Features describing the nuclear phenotype such as shape, size, orientation, nuclear membrane irregularities, DNA amount and DNA chromatin organization (ratio and spatial organization of euchromatin and heterochromatin) were calculated for each nucleus and the position of each of them relatively to the basal membrane automatically recorded. At the tissue level, epithelial thickness, cell density and tissue organization were assessed using sophisticated graph-theory algorithms measuring the intrinsic order/disorder and the loss of differentiation, trademarks of the dysplastic growth process. On a subset of 350 biopsies representing the entire spectrum of dysplastic and non-dysplastic abnormalities, p16 protein and Ki-67 protein expression were precisely evaluated by measuring the spatial distribution of Ki67-positive and p16-positive cells in the each individual epithelial layer. HPV typing and ploidy analysis were also performed for each patient. All these data were extrapolated to build a realistic (real metric dimension) cell-based physical model of the stratified normal cervical epithelium and of epithelium from each pathological grade. In the optical module, we used Finite-Difference Time-Domain (FDTD) modeling to simulate light scattering from normal, hyperplastic, metaplastic, CIN 1, CIN2, and CIS cervical cell nuclei at different epithelial depths and we constructed models of basal, parabasal, and other layers nuclei up to the last superficial layer for computational analysis. Simulation results give significant insights into the depth-dependent scattering profile of cervical epithelium as dysplasia progresses. We also intend to carry out meticulous investigation that will also enable mapping of azimuthal asymmetry in angle-dependent scattering patterns; this may lead to observation of fine pattern changes that can further be linked to precancer progression. Results: We will be presenting this novel conceptual framework illustrated by some preliminary data of our ongoing analyses using iPOM predicted results to guide acquisition of confocal imaging as well as spectroscopy probes from clinical cervical tissues. Ultimately, this strategy will allow us to identify the most robust in vivo macroscopic imaging approaches for detection of cervical neoplasia and aid in determining which image features must be acquired to achieve accurate diagnoses (e.g. variations in excitation and emission wavelength, numerical aperture, polarization, etc.). Citation Information: Cancer Prev Res 2011;4(10 Suppl):A10.

Statistical Analysis of VHF-Band Tree Backscattering Using Forest Ground Truth Data and PO Scattering Model

This paper analyzes the statistical properties of the very high frequency (VHF)-band radar backscattering from coniferous trees by incorporating forest ground truth data into a physical-optics (PO) model that assumes horizontally transmit and receive polarizations and dominant double-bounce scattering from vertical stems standing on an undulating ground surface. The analysis shows that a statistically adequate model for the tree backscattering amplitude can be presented as a mixture of generalized gamma or lognormal distribution, and the mixture model can be reduced to a single density model if the trees with trunk volumes exceeding an appropriate threshold are to be taken into account. The generalized gamma density is shown to provide an appreciably better fit to the exceedance functions associated with the PO model data than that for the lognormal density. The results can be used to design statistically adequate models of forest clutter for VHF synthetic aperture radar systems.

Improved time-domain physical optics for transient scattering analysis of electrically large conducting targets

An improved time-domain physical optics model (TDPO) is proposed to determine the transient response of electromagnetic scattering of electrically large conducting targets modelled with triangular facets. A formula for the improved TDPO is derived, in which a time-domain convolution product between the incident plane wave and the time-domain physical optics (PO) integral is included. The time-domain PO integral is evaluated with a closed-form expression based on a Radon transform interpretation. Since the tedious and time-consuming numerical integration utilised in the conventional TDPO is avoided, the improved TDPO has a significant superiority over the conventional TDPO in terms of speed and stability, which is of great value to the transient response analysis of electrically large targets. Numerical results demonstrate the efficacy of the new method.