Correction Of Phase Distortions Research Articles

Features of Adaptive Phase Correction of Optical Wave Distortions under Conditions of Intensity Fluctuations

Features of Adaptive Phase Correction of Optical Wave Distortions under Conditions of Intensity Fluctuations

Aberration correction by polynomial approximation for synthetic aperture ultrasound imaging

AbstractBackgroundPreviously, there has been some work in the field of optical imaging on phase compensation by employing Legendre polynomials as an expansion of the phase function, and this seems to be an appealing unstudied area in the field of ultrasound imaging.PurposeThe paper is devoted to solving one of the problems of enhancing the authenticity of diagnostics data obtained in ultrasound visualization systems and presents a novel approach to aberration correction, which ensures a reliable eliminating of phase distortions. The novelty of the proposed approach consists in the use of the decomposition of the wave front by Legendre polynomials to approximate aberrations of the phase front of ultrasonic waves propagating through distorting layers.MethodsThe phase aberrations are corrected using a single sector probe that captures echoes in synthetic aperture mode. The proposed method approximates the changes in the wave front by means of the Legendre polynomials. The performance was investigated by placing a 13‐mm‐wide ultrasonic probe with 64 elements operating at 2 MHz on the surface of a commercial quality control phantom ATS Model 539 through a distorting layer. The following metrics were measured: peak value, root mean square (RMS), full width at half maximum (FWHM), contrast‐to‐noise ratio (CNR). During the experiments, three different aberrators were used interchangeably as distorting layers, two of which were made of photopolymer resin with RMS values of 39 and 97 ns and a speed of sound close to that of a human temporal bone tissue, and the third was an ex vivo human temporal bone with the RMS value of 44 ns. To test the correction, three regions inside the phantom were examined. Two‐sided nonparametric Wilcoxon rank‐sum test was used for assessing the statistical significance. The significance level for this study was set at α ≤ 0.05. To counteract the problem of multiple comparisons, the p‐values were adjusted by the Holm–Bonferroni correction method.ResultsThe statistically significant results have demonstrated the possibility of increasing peak intensity value up to 3.08 times, reducing the RMS width and FWHM of the intensity angular distribution down to 60% and 82%, respectively, and increasing CNR up to 2.12 times by using the proposed phase distortion correction method, compared with the case without correction. The results show that the method can be used as an effective aberration correction technique for all tested regions inside the phantom and distorting layers. Its usage allowed correcting up to 97% of the aberrations caused by the ex vivo temporal bone model.ConclusionThe results show that the usage of the proposed method for aberration correction can successfully increase the intensity and reduce the angular width of the ultrasound wave scattered by the point targets inside the phantoms. The method works with different distorting layers and is capable of correcting phase aberrations in multiple sections of the sonogram. The limitation of the proposed method consists in the fact that it requires the use of aperture synthesis and access to raw radiofrequency data, which restricts its application in common scanners.

Greenhouse gases monitoring instrument on a GF-5 satellite-II: correction of spatial and frequency-dependent phase distortion.

Interference data obtained by a spatial heterodyne spectrometer (SHS) is subject to various error factors and suffers from complex phase distortion. Traditional phase correction methods, such as the Amplitude, Merzt, and Forman methods, extract phase distortion in the spectral domain and correct it, which cannot effectively correct spatial phase distortion. Through theoretical derivation and numerical simulation, the spatial phase distortion is firstly determined and corrected in the interference domain. The frequency-dependent phase distortion is then extracted in the spectral domain and corrected. This novel phase distortion correction method named the phase decomposition method was applied to the in-orbit interference data of Greenhouse gases Monitoring Instrument-II (GMI-II). Compared with traditional phase correction methods, the root-mean-square error of the spectrum corrected using the phase decomposition method is reduced by 81.37%.

Parallel array signal processing technology for spatial phase distortion correction in heterodyne detection.

The spatial phase distortion caused by a rough target causes a decoherence effect which, in turn, produces system sensitivity degradation. The decoherence phenomenon is the primary problem that restricts the application of active optical heterodyne detection, e.g., synthetic aperture radar and long-range coherent laser detection and ranging. By establishing a one-to-one correspondence between the combination of array signals and the system signal-to-noise ratio (SNR), a scheme for spatial phase distortion correction based on the intelligent optimization algorithm is proposed in this paper. The calculation of phase adjustments for each array signal is transformed into an optimization problem for the combination of array signals, experiments are conducted using rough target heterodyne images, and the parallel genetic algorithm (PGA) is used to calculate the phase adjustment of each array element. The results show that the spatial random phase distortion is corrected effectively without prior knowledge, and the PGA achieves an excellent computational performance which, along the efficiency of the proposed technology, has wide-scale implications for the application of active heterodyne detection and optical coherent communication.

Joint Motion Compensation and Distortion Correction for Maneuvering Target Bistatic ISAR Imaging Based on Parametric Minimum Entropy Optimization

Bistatic inverse synthetic aperture radar (Bi-ISAR) can obtain complementary information of moving targets and overcome the inherent imaging limitations of monostatic ISAR. However, the complex motion of maneuvering targets invalidates the assumption that the imaging projection plane (IPP) is constant in conventional Bi-ISAR imaging. The 2-D spatial variant phase errors would be induced. Moreover, the phase errors have a high-order form due to the time-varying bistatic angle and the high maneuvering characteristics of the target. Meanwhile, the linear geometric distortion induced by the bistatic configuration seriously challenges target identification and classification. In this paper, we propose a novel method to compensate for the 2-D spatial variant phase errors and correct the geometric distortion simultaneously for Bi-ISAR imaging based on parametric minimum entropy optimization. First, the signal mode for maneuvering target in the bistatic configuration is developed. Second, based on the developed signal model, we analyze the coupling relationship between the 2-D high-order spatial variant phase errors and the bistatic angle, and establish a parametric minimum entropy optimization model for high-order spatial variant phase errors compensation. Then, an efficient Broyden–Fletcher–Goldfarb–Shanno (BFGS) method is adopted to obtain the optimal solution of spatial variant coefficients. Finally, with the estimated optimal parameters, the integrated processing of 2-D spatial variant phase errors compensation and distortion correction can be realized. This method can simultaneously obtain well-focused and restored Bi-ISAR images of maneuvering targets without selecting prominent scatterers. Experiments based on scattering point simulation data and electromagnetic data verify the effectiveness of the proposed method.

Phase-modulated rapid-scanning fluorescence-detected two-dimensional electronic spectroscopy.

We present a rapid-scanning approach to fluorescence-detected two-dimensional electronic spectroscopy that combines acousto-optic phase-modulation with digital lock-in detection. This approach shifts the signal detection window to suppress 1/f laser noise and enables interferometric tracking of the time delays to allow for correction of spectral phase distortions and accurate phasing of the data. This use of digital lock-in detection enables acquisition of linear and nonlinear signals of interest in a single measurement. We demonstrate the method on a laser dye, measuring the linear fluorescence excitation spectrum as well as rephasing, non-rephasing, and absorptive fluorescence-detected two-dimensional electronic spectra.

Calibration of Mars Reconnaissance Orbiter Shallow Radar (SHARAD) data for subsurface probing and surface reflectivity studies

The Shallow Radar (SHARAD) instrument on the Mars Reconnaissance Orbiter (MRO) has been operating since 2006, revealing the detailed layered structure of the polar caps, mid-latitude glacial deposits, and a range of volcanic features. Here we address the major sources of change in the effective gain and signal-to-noise ratio of data collected by SHARAD: (1) the altitude and background noise level, (2) the configuration of the solar arrays (SA) and high-gain antenna (HGA), (3) ionospheric attenuation, and (4) the roll angle of MRO. Background noise fluctuations have a range of ~5 dB, and electromagnetic interference may significantly affect comparisons among limited bands of the full spectrum. We use the dense spatial coverage of sounder data to define a model for gain as a function of SA and HGA orientations, and demonstrate its predictive capability over a 4-dB range with radargrams collected under different configurations. Ionospheric attenuation as a function of the phase distortion correction used in radargram processing is refined through a larger dataset than available in earlier studies. Gain improvements due to MRO rolls up to 28o are also a function of the SA-HGA configuration, such that the maximum span of combined SHARAD gain contributions is about 7 dB. After calibration, Planum Boreum in the north polar layered deposits is ~3 dB less reflective than Planum Australe, and multi-band analysis suggests destructive interference occurs in the more densely layered shallow structure of the northern cap. The new calibration model allows targeting of future observations during optimum conditions over features of interest.

Determination of conditions for small residual errors in the correction of atmospheric phase distortions, taking into account the space-time constraints of the adaptive optical system

Determination of conditions for small residual errors in the correction of atmospheric phase distortions, taking into account the space-time constraints of the adaptive optical system

Lidar model of the formation of a sodium laser star when space objects are being observed and angularly tracked

This paper discusses a lidar model of the formation of a sodium laser star for the measurement and adaptive correction of atmospheric phase distortions in a ground-based optical system when space objects are being observed and angularly tracked. The energy characteristics of the laser star are studied when it occupies various angular positions in the visibility zone of a ground-based optical system. Requirements are formulated on the energy characteristics of a laser for forming a laser star in order to implement adaptive phase-matched correction of atmospheric phase distortions when low-orbit space objects are being observed.

Phase distortion correction in dual-comb ranging system

We systematically discussed the principle of the dual-comb ranging system and built a theoretical model. To correct the phase distortion caused by femtosecond frequency comb noise, a correction approach is proposed and numerical simulations are conducted subsequently. In the simulations, the performance of the method under noise of repetition rate and offset frequency is analyzed respectively. The results indicate that ranging accuracy can be effectively improved by the method. It is verified by the experiments.

Gerchberg–Saxton algorithm based phase correction in optical wireless communication

Optical wireless communication (OWC) enables to establish the backhaul in the fifth generation (5G) wireless communication networks. The air turbulence, however, could distort the phases of optical signals, thus limit transmission capacity. In this paper, we study the correction of phase distortions by using the Gerchberg–Saxton (GS) algorithm. The air turbulence-induced phase is generated by the Monte-Carlo phase screen method, which characterizes the realistic air turbulence effect on the optical signals. The numerical results reveal that the GS algorithm is able to retrieve the phase information with fast convergence by adopting a proper initial condition. Also, the GS algorithm based phase correction in OWC is confirmed.

Joint Correction of Ionosphere Noise and Orbital Error in L-Band SAR Interferometry of Interseismic Deformation in Southern California

The accuracy of L-band synthetic aperture radar (SAR) differential interferometry (InSAR) on crustal deformation studies is largely compromised by ionosphere path delays on the radar signals. The ionosphere effects cause severe ionospheric distortion such as azimuth streaking and long wavelength phase distortion similar to orbital ramp error. Effective detection and correction of ionospheric phase distortion from L-band InSAR images are necessary to measure and accurately interpret surface displacement. In this paper, we investigate the performance improvement of L-band InSAR interseismic deformation measurements in southern California through the joint correction of both ionosphere noise and orbital error. Our results show that this method can effectively remove orbit and ionosphere phase distortions. In comparison with in situ GPS measurements, the achieved InSAR measurement accuracy is improved from ~ 30 mm to ~ 10 mm by the proposed joint correction method. We show that, after the joint correction, the remaining atmosphere noise can be further mitigated through stacking, leading to an RMS error of ~ 4.7 mm/year in resultant line-of-sight velocity, as compared with ~ 11.3 mm/year before the correction. Our results demonstrate that the proposed joint correction technique provides a promising way to jointly correct orbital and ionospheric artifacts in L-band InSAR studies of crustal deformation.

High resolution T2*-weighted Magnetic Resonance Imaging at 3 Tesla using PROPELLER-EPI

We report the application of PROPELLER-EPI for high resolution T2*-weighted imaging with sub-millimeter in-plane resolution on a clinical 3 Tesla scanner. Periodically rotated blades of a long-axis PROPELLER-EPI sequence were acquired with fast gradient echo readout and acquisition matrix of 320×50 per blade. Images were reconstructed by using 2D-gridding, phase and geometric distortion correction and compensation of resonance frequency drifts that occurred during extended measurements. To characterize these resonance frequency offsets, short FID calibration measurements were added to the PROPELLER-EPI sequence. Functional PROPELLER-EPI was performed with volunteers using a simple block design of right handed finger tapping. Results indicate that PROPELLER-EPI can be employed for fast, high resolution T2*-weighted imaging provided geometric distortions and possible resonance frequency drifts are properly corrected. Even small resonance frequency drifts below 10Hz as well as non-corrected geometric distortions degraded image quality substantially. In the initial fMRI experiment image quality and signal-to-noise ratio was sufficient for obtaining high resolution functional activation maps.

Beam phase-distortion correction in a high power laser based on the stochastic parallel gradient descent technique

The feasibility of the closed-loop adaptive optic system based on laser far-field information was experimentally demonstrated, which can significantly increase the working efficiency of the multi-beamline high power solid laser facility. A stochastic parallel gradient descent algorithm with mean-radius cost function was applied in the adaptive optics that was installed in one chosen beamline in the SG-III laser facility. The far-field diameter from a reference laser was corrected from about 275 to 100 μm, very close to the ideal limit of 75 μm. Multiple-wavefront correction can be performed with this far-field detection based adaptive optics system, and the experimental result validates the practicability of the proposed system in the multi-beamline high power solid laser facilities.

Stochastic parallel gradient descent optimization based on decoupling of the software and hardware

We classified the decoupled stochastic parallel gradient descent (SPGD) optimization model into two different types: software and hardware decoupling methods. A kind of software decoupling method is then proposed and a kind of hardware decoupling method is also proposed depending on the Shack–Hartmann (S–H) sensor. Using the normal sensor to accelerate the convergence of algorithm, the hardware decoupling method seems a capable realization of decoupled method. Based on the numerical simulation for correction of phase distortion in atmospheric turbulence, our methods are analyzed and compared with basic SPGD model and also other decoupling models, on the aspects of different spatial resolutions, mismatched control channels and noise. The results show that the phase distortion can be compensated after tens iterations with a strong capacity of noise tolerance in our model.

Adaptive correction of distortions in a multichannel optical system

Results of modern investigations of multichannel laser systems are analyzed; sources of aberrations in such systems and the methods of their active and passive compensation are described. Simulation results of combined beam propagation are also presented; a possibility of long-range energy transfer in systems of such a type is considered. Effects of random phase incursions and atmospheric turbulence on beam parameters in the plane of observations are studied. The use of the multidither algorithm for correction of random phase distortions in multichannel systems is considered.

An evaluation of motion compensation strategies and repeatability for abdominal 1H MR spectroscopy measurements in volunteer studies and clinical trials

Increased expression of choline kinase has frequently been shown in tumours and is thought to be associated with disease progression. Studies using magnetic resonance spectroscopy have shown an increase in total choline-containing metabolites (tCho) in tumour compared with healthy tissue. Subsequent reductions in tCho following successful treatment support the use of tCho as a biomarker of disease and response. However, accurate measurement of tCho using MRS in abdominal tumours is complicated by respiratory motion, blurring the acquisition volume and degrading the lineshape and signal-to-noise ratio (SNR) of metabolites. Motion compensation using prospectively gated acquisitions or offline correction of phase and frequency distortions can help restore the SNR and linewidth of metabolites. Prospectively gated acquisitions have the advantage of confining the volume of acquisition to the prescribed volume but are constrained by the repetition time (TR) of the respiratory motion. In contrast, data acquired for offline correction may use a shorter repetition time and therefore yield an increased SNR per unit time. In this study abdominal spectra acquired from single-voxel 'free-breathing' measurements in liver of healthy volunteers and in abdominal tumours of cancer patients were compared with those of prospective gating and with an implementation of offline correction. The two motion compensation methodologies were assessed in terms of SNR, linewidth and repeatability. Our experiments show that prospective gating and offline correction result in a 12-22% reduction in median tCho linewidth, while offline correction also provides a significant increase in SNR. The repeatability coefficient (the expected interval for 95% of repeat measurements) for tCho/water ratio was reduced by 37% (prospective gating) and 41% (offline correction). Both methods of motion compensation substantially improved the reproducibility of the tCho/water measurement and the tCho linewidth. While offline correction also leads to a significant improvement in SNR, it may suffer more from out-of-voxel contamination.

A Novel Phase Distortion Correction Method Based on Temporal Correlation Matrix

Since the existence of the ionosphere contamination, results of high-frequency radar has been greatly affected. This paper addresses a new method based on temporal correlation matrix to correct phase distortion for HF sky-wave radars. Strong point echoes, such as target, clutter, transponder and so on, which are highly temporal correlated, can be used to structure temporal correlation matrix. And the matrix contains angles of echoes and phase distortions information. In this paper, we use two-dimension FFT to estimate angle of strong point echo, and then obtain the corrections. Computer simulations are conducted to show the validity of the new method, and some of performances of PGA were compared.

畸变波前相位校正效果分析

畸变波前相位校正效果分析

Correction of phase distortions of laser beams by the backscattering signal in the atmosphere

The results of a numerical simulation of adaptive compensation for distortions of partially coherent laser radiation using the backscattering signal are presented. A combined optical scheme of a transceiver channel with annular transmitter and circular receiving apertures is considered. In this scheme, initial field fluctuations are caused by a phase screen with a Gaussian correlation function which is placed near the source aperture. The effect of the finite response rate and limitations caused by the wavefront corrector on the system control efficiency was determined.