Frequency-dependent Reflection Research Articles

Noncontact imaging of absorption and scattering in layered tissue using spatially modulated structured light

Sinusoidal patterns of spatially modulated near-infrared (650 nm) structured light are used to interrogate multilayer phantoms and tissue. Diffuse reflectance is imaged at multiple spatial frequencies from 0–0.3 mm−1. ac and dc components of the image are fit to a two layer model formulated from the diffusion approximation to the Boltzman transport equation. The two-layer model depends on optical properties (absorption, μa, and reduced scattering, μs′) in each layer and on top layer thickness (d). Layered tissue phantoms with variable optical properties in each layer (μa=0.006–0.034 mm−1 and μs′=0.89–1.45 mm−1) were constructed to test the accuracy of the model. Constraining top layer thickness to within 25% of the correct value in a four-parameter fit results in recovery of upper layer optical properties with average accuracies of ±2% for top layer μs′ and ±17% for top layer μa. Bottom layer μa can then be recovered to an average accuracy of ±25% with two parameter fits. Average accuracies of top and bottom layer absorption can further be improved to 12% and 18%, respectively, by fitting for each alone. Bottom layer scattering and top layer thickness do not vary significantly from initial guesses because of poor sensitivity to these parameters in frequency dependent reflectance data. Measurements of in vivo volar forearm optical properties at 650 nm produced spatially varying skin (d=2 mm) optical property maps that range from 0.025–0.045 and 1.7–2 mm−1 for upper layer μa and μs′ and 0.005–0.015 and 0.5–3 mm−1 for lower layer μa and μs′, respectively. These preliminary results suggest that spatial modulation of the source provides sufficient depth sensitivity to allow noncontact mapping and quantification of layered tissue optical properties using a wide-field, noncontact approach.

Predicting Microwave Digital Signal Integrity

High-speed digital signal integrity at data rates above 6 Gb/s is an obstacle to reliable serial link operation. Two signal integrity challenges include dispersion due to frequency-dependent losses and reflections created at impedance mismatches. Signal integrity analysis relies on time-domain simulation of pseudo-random data patterns. This paper explores a predictive method for interconnect eye closure caused by reflections at the transmitter and receiver and does not require extensive time domain simulation. Worst-case bounds on intersymbol interference and data-dependent jitter aid prediction for link budgets under channel variations. This method is applied to the design of a passive equalizer.

Correction for partial reflection in ultrasonic attenuation measurements using contact transducers

This research investigates the influence of partial reflection on the measurement of the absolute ultrasonic attenuation coefficient using contact transducers. The partial, frequency-dependent reflection arises from the thin fluid-layer interface formed between the transducer and specimen surface. It is experimentally shown that neglecting this reflection effect leads to a significant overestimation in the measured attenuation coefficient. A systematic measurement procedure is proposed that simultaneously obtains the ultrasonic signals needed to calculate both the reflection coefficient of the interface and the attenuation coefficient, without disturbing the existing coupling conditions. The true attenuation coefficient includes a correction based on the measured reflection coefficient--this is called the reflection correction. It is shown that including the reflection correction also reduces the variation (random error) in the measured attenuation coefficient. The accuracy of the proposed method is demonstrated for a material with a known attenuation coefficient. The proposed method is then used to measure the high attenuation coefficient of a cement-based material.

Computation of Casimir forces for dielectrics or intrinsic semiconductors based on the Boltzmann transport equation

The interaction between drifting carriers and traveling electromagnetic waves is considered within the context of the classical Boltzmann transport equation to compute the Casimir-Lifshitz force between media with small density of charge carriers, including dielectrics and intrinsic semiconductors. We expand upon our previous work (Phys. Rev. Lett. 2008, in press; arXiv:0805.1676) and derive in some detail the frequency-dependent reflection amplitudes in this theory and compute the corresponding Casimir free energy for a parallel plate configuration. We critically discuss the the issue of verification of the Nernst theorem of thermodynamics in Casimir physics, and explicity show that our theory satisfies that theorem. Finally, we show how the theory of drifting carriers connects to previous computations of Casimir forces using spatial dispersion for the material boundaries.

Frequency-dependent reflection of spin waves from a magnetic inhomogeneity induced by a surface direct current

The reflectivity of a highly localized magnetic inhomogeneity is experimentally studied. The inhomogeneity is created by a direct current carrying wire placed on the surface of a ferrite film. The reflection of propagating dipole-dominated spin-wave pulses is found to be strongly dependent on the spin-wave frequency if the current locally increases the magnetic field. In the opposite case, the frequency dependence is negligible.

Three‐dimensional spatial coherence measurements: Vertical, longitudinal/transverse horizontal coherence.

In the Shallow Water ’06 experiment, two L‐shape arrays were deployed. Two horizontal components of these arrays were laid on the sea bottom. One was exactly perpendicular to the direction of sound propagation. The other was exactly parallel to the direction of sound propagation. This configuration offered an opportunity to directly measure the vertical, longitudinal horizontal, and transverse horizontal coherence. The results of spatial coherence were averaged over different pairs of hydrophones and over a frequency bandwidth of 100 Hz. The vertical coherence showed receiver depth dependence. When the source and the receivers were located below the thermocline, both the vertical and longitudinal horizontal coherence lengths (in units of wavelength) increased with increasing range and frequency. The longitudinal horizontal coherence length was much larger than the vertical coherence length. These results were similar to the predictions by Smith’s model [J. Acoust. Soc. Am. 60, 305–310 (1976)] with a frequency dependent bottom reflection loss [Zhou, Chin. Phys. 1, 494–504 (1981)]. The transverse horizontal coherence length/wavelength decreased with frequency. When the source depth was within the thermocline, the transverse horizontal coherence lengths exhibited weak range dependence in the 100–300 Hz range and was larger than 40 wavelengths. [Work supported by ONR.]

Estimation of the acoustic impedance of lung versus level of inflation for different species and ages of animals

In a previous study, it was hypothesized that ultrasound-induced lung damage was related to the transfer of ultrasonic energy into the lungs (W. D. O'Brien et al. 2002, "Ultrasound-induced lung hemorrhage: Role of acoustic boundary conditions at the pleural surface," J. Acoust. Soc. Am. 111, 1102-1109). From this study a technique was developed to: 1) estimate the impedance (Mrayl) of fresh, excised, ex vivo rat lung versus its level of inflation (cm H(2)O) and 2) predict the fraction of ultrasonic energy transmitted into the lung (M. Oelze et al. 2003, "Impedance measurements of ex vivo rat lung at different volumes of inflation." J. Acoust. Soc. Am. 114, 3384-3393). In the current study, the same technique was used to estimate the frequency-dependent impedance of lungs from rats, rabbits, and pigs of various ages. Impedance values were estimated from lungs under deflation (atmospheric pressure, 0 cm H(2)O) and three volumes of inflation pressure [7 cm H(2)O (5 cm H(2)O for pigs), 10 cm H(2)O, and 15 cm H(2)O]. Lungs were scanned in a tank of degassed 37 degrees C water. The frequency-dependent acoustic pressure reflection coefficient was determined over a frequency range of 3.5-10 MHz. From the reflection coefficient, the frequency-dependent lung impedance was calculated with values ranging from an average of 1.4 Mrayl in deflated lungs (atmospheric pressure) to 0.1 Mrayl for fully inflated lungs (15 cm H(2)O). Across all species, deflated lung (i.e., approximately 7% of the total lung capacity) had impedance values closer to tissue values, suggesting that more acoustic energy was transmitted into the lung under deflated conditions. Finally, the impedance values of deflated lungs from different species at different ages were compared with the thresholds for ultrasound-induced lung damage. The comparison revealed that increases in ultrasonic energy transmission corresponded to lower injury threshold values.

Characterization of Reflector Types by Phase-Sensitive Ultrasonic Data Processing and Imaging

Ultrasonic and radar imaging techniques are limited in resolution by the wavelength in the material, yet information beyond those limits is hidden in complex frequency dependent reflection coefficients. The analysis of the phase of complex reflection coefficients together with the properties of imaging algorithms can help to characterize and to classify indications of defects in concrete buildings. This paper describes a method to extract phase information from measurements and SAFT reconstructed images. The influencing factors like material properties, transducer characteristics, and imaging algorithms based on Born or Physical Optics approximations are elaborated. Simulated and experimental results are briefly discussed.

Scattering of Rayleigh wave by microcrack with interacting faces

Scattering of short Rayleigh pulse by a partially closed surface-breaking microcrack is studied both experimentally and numerically. The microcrack was generated by a nonlinear surface wave with shock. In practice such type of cracks corresponds to fatigue or thermally generated ones. The crack was irradiated by a probe laser-generated Rayleigh. The scattered acoustic field at the surface, monitored by means of optical detector, consisted of the reflected and transmitted Rayleigh waves, longitudinal and shear acoustic waves excited by mode-conversions at the crack. Characteristic size of the crack was 50-100 μm, experimental bandwidth was 5-200 MHz. Scattering was simulated numerically in finite differences. For modeling the partially closed crack we proposed a simple one-parameter model, which takes into account weak interaction between the crack faces. It was shown, that interaction changes the predicted frequency-dependent transmission and reflection coefficients significantly. On the basis of fitting the simulated acoustic field to the measured one, the evaluation of the crack size has been performed. In this work frequency dependence of the following three parameters were employed for that purpose: transmission and reflection coefficients, and the time delay of the transmitted pulse. Obtained estimates were compared to the direct microscopic measurement of the crack.

The Impact of Reflections From Stratified Building Materials on the Wave Propagation in Future Indoor Terahertz Communication Systems

In order to derive reliable propagation models for future terahertz indoor pico-cellular communication systems, accurate reflectivity data of building materials is necessary. Here we present reflection terahertz time domain spectroscopy (THz-TDS) measurements and matching transfer matrix simulations of the frequency dependent reflection coefficient of multi layer building materials in the frequency range from 100 to 500 GHz for a set of angles, both in TE- and TM-polarization. Two prominent stratified structures, a double pane window and white paint on plaster are investigated as they usually account for large areas in indoor environments. Communication systems located above 100 GHz are expected to be strongly affected by the variations of the reflectivity over the frequency and incident angle of such stratified materials as they will rely both on line of sight (LOS) and non line of sight (NLOS) propagation. We discuss this impact on the power distribution in a sample scenario employing the ray-tracing method.

Casimir energies with finite-width mirrors

We use a functional approach to the Casimir effect in order to evaluate the exact vacuum energy for a real scalar field in $d+1$ dimensions, in the presence of backgrounds that, in a particular limit, impose Dirichlet boundary conditions on one or two parallel surfaces. Outside of that limit, the backgrounds are described by a nonlocal effective action and may be thought of as modelling finite-width mirrors with frequency-dependent transmission and reflection coefficients. We obtain formal expressions for the Casimir energy in general backgrounds, and provide new explicit results in some particular cases.

Extraction of frequency-dependent reflection, transmission, and scattering parameters for short metal reflectors from FEM-BEM simulations

Reflectors comprised of only a single or a few electrodes provide controllable, weak reflectivity essential for surface acoustic wave (SAW) radi -frequency identification (RFID) tags. The reflection, transmission, and scattering parameters of such reflectors must be known as a function of frequency in order to be able to control the amplitudes of tag responses and to use phase-based encoding reliably. In this work, we present a method of extracting the main reflection, transmission, and scattering parameters for short metal reflectors as a function of frequency. We use test device S parameters obtained through finite- and boundaryelement method (FEM-BEM)-based simulations and, as an example, determine the reflection and transmission coefficients (their absolute values and phase angles) and the energy scattered into bulk for a few different single electrode reflectors. We compare these parameter values to earlier results. Although only used for simulated data in this work, the same method can be applied to measured data as well. Assuming the S parameters available, this method is very fast and does not require any heavy calculation or special software.

Switched-Beam Slot Antenna over Electromagnetic Band-Gap Reflector

This paper presents the switched-beam slot antenna over the electromagnetic band-gap (EBG) reflector. This antenna is composed of two slot elements fed with a phase difference and the EBG reflector, which is used in order to realize a low profile structure. The radiation characteristics of this antenna are calculated using the FDTD method. Calculations show that the height of the antenna using the EBG reflector is 60 % lower than that of the antenna using a perfect electric conductor (PEC) reflector. The radiation characteristics at the center of the operating frequency band in the EBG reflector are equivalent to that in the PEC reflector. It is shown that the tilt angle of the main beam in the elevation plane varies with the operating frequency, and the variation in the case of the EBG reflector is caused by its frequency-dependent reflection phase. Moreover, the radiation pattern of the fabricated antenna is measured. The results demonstrate that the low profile design can be achieved by using the EBG reflector, and reveal the influence of the EBG reflector on the antenna efficiency.

Non-invasive VHF injected signal monitoring in atmospheric pressure plasma and axial DC magnetron

A universal non-invasive real-time frequency domain reflectometry plasma process measurement that is independent of impedance matching network is described. The measurement is deployed on the main power transmission line of the plasma tool to detect both hardware and reflections form plasma. An illustration for atmospheric pressure plasma and axial DC magnetron plasma is given. Power handling capability is 20 (2 A×10 V) and 100 W (250 mA×400 V), respectively. Four simple equivalent electrical models in terms of transmission-line theory and frequency dependent reflection modulus (gain) are employed to analyze the DC magnetron measurement results. It is shown that the electrical length between the directional device measurement plane and the plane at which the transmission line is termination by the hardware and plasma determines the frequency response signal. The strength of the signal (depth of the zero) is found to be inversely related to the termination resistance. Information on hardware, and dynamic process-induced changes due to physical sputtering/erosion are accessible.

Design of High-Performance Millimeter Wave and Sub-Millimeter Wave Quasi-Optical Isolators and Circulators

Faraday rotators using permanently magnetized ferrite materials are used to make quasi-optical isolators and circulators at millimeter wave and sub-millimeter wave frequencies that have far higher performance than their waveguide equivalents. This paper demonstrates state-of-the-art performance for four-port quasi-optical circulators with 60-dB isolation, 0.2-dB insertion loss, and better than 80-dB return loss for devices centered at 94 GHz. A method is presented for the accurate characterization of the complex permeability and permittivity of permanently magnetized ferrites via a series of frequency and polarization dependent transmission and reflection measurements. The dielectric and magnetic parameters for the sample are determined by fitting theoretical curves to the measured data. These fitted parameters are then used in a model for a complete quasi-optical Faraday rotator, including matching layers, allowing the accurate design and fabrication of these devices for any specific operational frequency band in the millimeter wave and sub-millimeter wave regime. Examples are given showing typical results and demonstrating how temperature cycling can significantly improve the temperature stability of these devices, while allowing fine tuning of the center frequency. We also indicate the performance possible at higher frequencies to above 1 THz and outline performance of truly planar isolators where lossy polarizer material is built into the Faraday rotator matching structure

Frequency-dependent attenuation analysis of ground-penetrating radar data

In the early 1990s, it was established empirically that, in many materials, ground-penetrating radar (GPR) attenuation is approximately linear with frequency over the bandwidth of a typical pulse. Further, a frequency-independent [Formula: see text] parameter characterizes the slope of the band-limited attenuation versus frequency curve. Here, I derive the band-limited [Formula: see text] function from a first-order Taylor expansion of the attenuation coefficient. This approach provides a basis for computing [Formula: see text] from any arbitrary dielectric permittivity model. For Cole-Cole relaxation, I find good correlation between the first-order [Formula: see text] approximation and [Formula: see text] computed from linear fits to the attenuation coefficient curve over two-octave bands. The correlation holds over the primary relaxation frequency. For some materials, this relaxation occurs between 10 and [Formula: see text], a typical frequency range for many GPR applications. Frequency-dependent losses caused by scattering and by the commonly overlooked problem of frequency-dependent reflection make it difficult or impossible to measure [Formula: see text] from reflection data without a priori understanding of the materials. Despite these complications, frequency-dependent attenuation analysis of reflection data can provide valuable subsurface information. At two field sites, I find well-defined frequency-dependent attenuation anomalies associated with nonaqueous-phase liquid contaminants.

Properties of Planar Electric Metamaterials for Novel TeraHertz Applications

Planar electric metamaterials are experimentally studied in transmission and reflection utilizing terahertz time-domain spectroscopy. Electrically resonant behavior is observed and provides an estimate of the frequency-dependent transmissivity, reflectivity, and absorptivity of metamaterial composites. Numerical simulations are in good agreement with the measured results and provide additional information helpful in understanding their electromagnetic response. Our results and approach help define the boundaries of a metamaterials-based design paradigm and should prove beneficial in future terahertz applications, particularly with respect to novel filtering, modulation, and switching devices. In addition, this work clarifies some of the mechanisms that limit efficient metamaterials operation at higher-frequencies.

Local seismic quantification of gas hydrates and BSR characterization from multi-frequency OBS data at northern Hydrate Ridge

High-resolution multi-frequency and multi-component seismic data were acquired at northern Hydrate Ridge in the accretionary complex of the Cascadia subduction zone to quantify the amount of hydrate and free gas in the sediment. We present a detailed local analysis of four component (4C) ocean bottom seismometer (OBS) data and show the importance of multi-frequency and shear wave data for determining hydrate reservoir properties. A detailed model of the elastic parameters at the bottom simulating reflector (BSR) is developed by using synthetic seismogram modelling. The main focus in this study is an amplitude-versus-offset (AVO) analysis of shear waves, which originate by PS-conversion at the BSR in 73 m below the seafloor (bsf). The AVO analysis enables the determination of the shear wave velocity above the BSR. A velocity of 400 m/s indicates that the presence of gas hydrate in the pore space significantly increases the shear modulus of the sediment above the BSR. Information about the attenuation and the shape of the BSR transition zone is obtained from the frequency-dependent reflection amplitudes of the BSR. The BSR is shown to be a gradual type transition zone of 1.5–2.5 m thickness. Average Q factors of Q p = 150 for P-waves and of Q s = 35 for S-waves are determined within the gas hydrate stability zone (GHSZ). The low Q s factor points to a pronounced attenuation of S-waves in the uppermost sediments. From rock physics modelling, the hydrate concentration is estimated to vary locally between 3–12% of the pore space. Below the BSR, free gas concentrations of 0.5% and 8% are determined for homogeneous and patchy distributed gas, respectively.

FDTD Analysis of a Novel 8mm E-Plane T-Junction Waveguide Circulator

A novel 8 mm E-plane T-junction waveguide circulator with off-center ferrite discs is developed. Detailed analysis of it using the three-dimensional finite-difference time-domain (FDTD) method is presented. The results of experiment show that the method to widen frequency band is useful. Frequency dependent insertions loss, isolation and reflection loss of the circulator are calculated over a wide band of frequencies with a single FDTD run, good agreement is found between theoretical analysis and results of experiment.

Elastodynamic wave propagation in graded materials: Simulations, experiments, phenomena, and applications

A two-dimensional numerical simulation model for the elastodynamic wave propagation in two linear elastic, isotropic, joint half-spaces is presented. The border between the two half-spaced is graded in a way, that the values of the elastic properties and the densities vary smoothly (sinusoidally) from the values of one continuum to the values of the other continuum within a transition zone of a defined thickness. It is demonstrated, that a graded layer leads to a frequency and wavelength dependent refraction and reflection behavior of elastodynamic waves. Numerical results show that wavelengths which are long compared with the transition layer thickness are dominantly reflected whereas short waves are dominantly transmitted, a phenomena which does not occur in the case of an infinitely thin transition layer. Furthermore the frequency dependent reflection and transmission behavior of elastodynamic waves is verified experimentally. There the interface between two vapor deposited films is graded due to intermetallic diffusion effects. These graded microstructures are analyzed with a short-pulse-laser-acoustic set-up. The corresponding frequencies of the elastodynamic waves which are filtered with these functionally graded microstructures are in the range of 0.5 THz.