Wavefront Imaging Research Articles

Polarization phase in aberration compensation

A phase/amplitude mask on the aperture of an imaging system results in a pupil function that is multiplicative with the lens function, resulting in a morphological transformation of the imaging wavefront. It was shown that such amplitude and phase functions can be implemented using polarization masks, with the advantage that the phase and amplitude can be controlled in real time and in some cases, independently of each other. The phase and amplitude variation over the mask can be controlled either by changing the polarization of the mask or by changing the input beam parameters. Wavefront tailoring using polarization-masked apertures is therefore feasible and may be utilized for focal shift and partial aberration compensation. For complete compensation of aberration, the phase distribution over the mask should be conjugate to that of the phase error of the aberrant wavefront, which necessitates the use of a continuously variable polarization mask. Since such a mask is difficult to implement, we have considered polarizing masks consisting of discrete polarized zones on the lens aperture, leading to polarization phase steps on the exit pupil of the imaging system. The simulation results presented in this paper show that effects of focal shift, partial compensation of primary spherical aberration and astigmatism can indeed be achieved by the proper use of polarization masked apertures.

Translational and rotational pupil tracking by use of wavefront aberration data and image registration techniques

We present a methodology with which to evaluate translations and rotations of wavefront aberration measurements of systems in which the exit pupil suffers displacements and rotations with respect to the reference frame of the measuring device. We propose to use image registration techniques to account for rotations, translations, and scale changes of the pupil. We present a proof of principle, using an artificial eye in addition to computer simulations. The method is software based and requires no additional hardware.

Ray hit analysis method and its application to complex upper crustal structure survey

Based on the results obtained from Pg wavefront imaging in active source deep seismic sounding, we propose a new ray hit analysis method for high-resolution seismic refraction profile data processing. This method can be used to further determine possible refraction interface, especially spatial location of basement and its pattern characteristics in complex upper crustal structure region, making data processing for high-resolution refraction profiles more fine. We use this method to study the crystalline basement structure of east part of A’nyemaqen suite zone at northeast side of Qinghai-Xizang Plateau and the basement patterns as well as its spreading features at the east part of Anemaqen suite zone and its adjacent region were determined.

Fine upper crustal structure of Jiashi strong earthquake swarm region in Xinjiang inferred from high resolution seismic refraction profile data

The data obtained from a high resolution seismic refraction profile, which was carried out in Jiashi, Xinjiang, strong earthquake swarm area, were processed with both finite difference inversion and Hagedoorn refractor wavefront imaging technique and the fine upper crustal structure was determined. The results show that the upper crustal structure is relatively well-distributed in laterally and obviously by layers vertically. From surface to 11.0 km depth, there are about four layers. The P wave velocity of top two layers range from 1.65 to 4.5 km/s and their bottom boundaries, the buried depths of which are 0.4, 2.96–3.0 km respectively, are almost horizontal; The third layer is comparatively complicated and its P wave velocity presents inhomogeneous in both laterally and vertically. The bottom boundary of third layer is crystalline basement and shows a little uplift, which seemly suggest that the upper crust had been resisted while the hard Tarim block inserting into Tianshan Mountain; The forth layer is relatively even and its P wave velocity is about 6.3 km/s. There are a lateral velocity variation at the depth of about 4.0 km, and suggest that it has something to do with the hidden Meigaiti fault and Meigaiti-Xiasuhong fault but there are no the structure features about these faults stretching to the surface and passing through the crystalline basement. The seismogenic tectonic of Jiashi strong earthquake swarm at least lies in middle or lower crust beneath 11.0 km depth.

Iris-Based Cyclotorsional Image Alignment Method for Wavefront Registration

In refractive surgery, especially wavefront-guided refractive surgery, correct registration of the treatment to the cornea is of paramount importance. The specificity of the custom ablation formula requires that the ablation be applied to the cornea only when it has been precisely aligned with the mapped area. If, however, the eye has rotated between measurement and ablation, and this cyclotorsion is not compensated for, the rotational misalignment could impair the effectiveness of the refractive surgery. To achieve precise registration, a noninvasive method for torsional rotational alignment of the captured wavefront image to the patient's eyes at surgery has been developed. This method applies a common coordinate system to the wavefront and the eye. Video cameras on the laser and wavefront devices precisely establish the spatial relationship between the optics of the eye and the natural features of the iris, enabling the surgeon to identify and compensate for cyclotorsional eye motion, whatever its cause.

The Effects of Automated Registration on Aberration Measurement With the LADARWave Aberrometer

To evaluate the effects of automating the registration process for wavefront measurement using the LADARWave aberrometer. Measures of interest include accuracy, repeatability, and speed of measurement. Five separate registration images were collected for each of eight eyes of four individuals with varying iris color using the LADARWave aberrometer. Technicians manually aligned the images and the autoregistration feature was used to align the same images. Time, accuracy, and repeatability of the alignment were measured using objective techniques. The potential effect on positional error was evaluated using a retrospective reprocessing of LADARWave data from an existing sample dataset. Autoregistration was the fastest method of alignment, from five to seven times faster than manual registration. It was also the most consistent, with the lowest pupil/limbus offset error (45 microm). Technicians averaged 48 to 91 microm. The differences in wavefronts used to generate the composite wavefront image for surgery export were 18% lower with autoregistration. The autoregistration feature of the LADARWave aberrometer not only removes inter-technician variability, but reduces the time to perform image alignments and improves the consistency of the alignments. This helps reduce errors related to wavefront position, potentially improving outcomes.

Monochromatic ocular wavefront aberrations in the awake-behaving cat

Measurement of wavefront aberrations in human eyes has become a reliable, quantitative way of assessing the optical impact of experimental and corrective ocular manipulations. Wavefront measures have also been performed in several other species, but never in cats, an animal model of choice for many ocular studies. Our goal in this study was to measure wavefront aberrations reliably in live, awake-behaving cats in a manner that is directly comparable to that used in human subjects. Six adult cats ( felis cattus) were trained to fixate small targets on a computer screen. A compact Shack–Hartmann wavefront sensor was aligned with each animal's pupil center and line of sight during fixation. Wavefront images were then collected from which the cats' ocular aberrations were measured up to tenth order Zernike polynomials over a 6 mm pupil. Results show that cat and human ocular wave aberrations were very similar. Second order Zernike modes accounted for more than 90% of the total wave aberration. In agreement with our observation that cat ocular optics were comparable with those of humans, the half height width of both the cat and human higher order point spread function was about 0.95°. These results form a solid basis for future wavefront sensing studies aiming to quantify the effects of ocular manipulations in experimental animals.

Monochromatic ocular wavefront aberrations in the awake-behaving cat

Measurement of wavefront aberrations in human eyes has become a reliable, quantitative way of assessing the optical impact of experimental and corrective ocular manipulations. Wavefront measures have also been performed in several other species, but never in cats, an animal model of choice for many ocular studies. Our goal in this study was to measure wavefront aberrations reliably in live, awake-behaving cats in a manner that is directly comparable to that used in human subjects. Six adult cats ( felis cattus ) were trained to fixate small targets on a computer screen. A compact Shack–Hartmann wavefront sensor was aligned with each animal's pupil center and line of sight during fixation. Wavefront images were then collected from which the cats' ocular aberrations were measured up to tenth order Zernike polynomials over a 6 mm pupil. Results show that cat and human ocular wave aberrations were very similar. Second order Zernike modes accounted for more than 90% of the total wave aberration. In agreement with our observation that cat ocular optics were comparable with those of humans, the half height width of both the cat and human higher order point spread function was about 0.95°. These results form a solid basis for future wavefront sensing studies aiming to quantify the effects of ocular manipulations in experimental animals.

Image transfer through cirrus clouds. II. Wave-front segmentation and imaging.

A hybrid technique to simulate the imaging of space-based objects through cirrus clouds is presented. The method makes use of standard Huygens-Fresnel propagation beyond the cloud boundary and a novel vector trace approach within the cloud. At the top of the cloud, the wave front is divided into an array of input gradient vectors, which are in turn transmitted through the cloud model by use of the Coherent Illumination Ray Trace and Imaging Software for Cirrus. At the bottom of the cloud, the output vector distribution is used to reconstruct a wave front that continues propagating to the ground receiver. Images of the object as seen through cirrus clouds with different optical depths are compared with a diffraction-limited image. Turbulence effects from the atmospheric propagation are not included.

Wave-front images of acoustic waves in the (100) and (001) surfaces ofTeO2

Based on the finite-difference time-domain (FDTD) method, we study theoretically the wave-front images of acoustic waves propagating on the (100) and (001) surface of a highly anisotropic tetragonal ${\mathrm{TeO}}_{2}$ crystal for which imaging experiments have recently been conducted. The theoretical images well reproduce characteristic features observed experimentally. The group-velocity calculations of both surface and pseudosurface acoustic waves account for the shapes and locations of the major wave fronts obtained with the FDTD method. Additional weak wave-front structures that disappear rather quickly in time are attributed to bulk acoustic waves.

Visualization of Shear Waves Radiated from Pseudocapillary Mode on Agarose Gel by Stroboscopic Photoelasticity Technique

We have demonstrated the existence of shear waves which were predicted to radiate from the pseudocapillary surface mode propagating on agarose gel. The technique of stroboscopic photoelasticity imaging was used to visualize the shear waves propagating in the bulk of the gel. The shear-wave velocity was obtained from the wavefront image, and was found to agree with that obtained from the surface-wave measurements previously reported.

Interface waves propagated along a fracture

The existence of interface waves along fractures depends on the specific stiffness of the fracture. With increasing fracture stiffness, the velocity of the interface wave increases and the spectral content of the signal is altered. We used interface wave measurements and wavelet analysis to examine the effect of normal and shear stresses on fracture shear stiffness. Fracture shear stiffness is more sensitive to changes in shear stress than to normal stress. Compressional waves propagated along a fracture appear to contain a delayed wave that is sensitive to changes in the fracture stiffness. Using acoustic wavefront imaging, we are able to visualize the delayed produced in the compressional-wave by the fracture from the spatial distribution of the arriving energy.

Three-dimensional wavefront imaging by near-field scanning optical microscopy

We have designed a near-field scanning optical microscope head compatible with an existing commercial scanning probe system. Near-field images of coherent scattering from a 500 nm fused silica grating are collected in transmission mode as a function of tip-sample separation, wavelength, and incident polarization. The images are compared with numerical simulations of scattering using both scalar wave and vector dipole approximations. The simulations suggest that the procedure can be inverted, allowing one to measure the dielectric constant ε(x,y,z) of an embedded three-dimensional object using information obtained in the near field.

Acoustic wavefront imaging

AbstractWe introduce a new class of experiments which provide graphic insights into the propagation of acoustic waves in anisotropic media. Simply stated, we have devised a means of observing the expanding acoustic wavefront from a point disturbance in a solid. The data may be viewed as a movie or a series of snapshots. The observed wavefronts represent the group‐velocity surfaces of acoustic waves, which reflect the basic elastic anisotropy of the solid. The technique has been applied to coherent acoustic waves with frequencies in the megahertz range (at ambient temperatures) and to incoherent heat pulses in the hundred‐gigahertz range (at liquid‐helium temperatures). In this article, we first provide a pedagogical introduction to wave propagation in elastically anisotropic media, reviewing some early methods for visualizing acoustic waves. Next, we describe the “acoustic wavefront imaging” method and give representative results in crystals and composite materials. Finally, we show how this method relates to recent advances in phonon imaging and internal diffraction of ultrasound.

Wavefront amplitude distortion and image sidelobe levels. I. Theory and computer simulations

The quality of an imaging system is degraded by propagation anomalies that distort wavefronts propagating through the medium. Adaptive phase-deaberration algorithms compensate for phase errors in the wavefront. The algorithms suffer, however, when the wavefront is also significantly distorted. A theory which shows that the rise of image background level, which is the average sidelobe floor (ASF), in a single point-like source image is proportional to the amplitude distortion of the wavefront and inversely proportional to the effective number of array elements is derived. From the theory, the tolerance to the amplitude distortion, after the phasefront has been corrected by a deaberration algorithm, can be calculated based on the design requirement of the sidelobe floor for a given array. Computer simulations show good agreement with the theory.

Wavefront amplitude distortion and image artifacts: Qing Zhu and Bernard D. Steinberg, Valley Forge Research Center, The Moore School of Electrical Engineering, Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104

Wavefront amplitude distortion and image artifacts: Qing Zhu and Bernard D. Steinberg, Valley Forge Research Center, The Moore School of Electrical Engineering, Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104

Image synthesis from wave-front sensor measurements of a coherent diffraction field

We demonstrate that images of laser-illuminated objects can be formed from measurements of the wave-front slope (gradient) associated with the backscattered, coherent laser-speckle field. A digital wave-front recovery and image synthesis procedure is described, and the results of computer-simulation experiments are presented in which coherent images are reconstructed from digitally simulated Fourier-plane laser-speckle measurements. Images are recovered from noisy wave-front-difference data to illustrate the effect that measurement noise has on recovered image quality.

Wavefront imaging of plasma by means of computerized tomography

A simple implementation of well-resolved computerized tomographical imaging was designed using external and periodic excitation of a wave packet, and applied to the ionization wave in a helium positive column of a glow discharge. The wavefront revealed by pursuing the temporal evolution of the reconstructed image was convex such that the rays converged as the wave propagated in the direction of its phase velocity.

Visualization of the strain wave front of a progressive acoustic wave based on the Talbot effect

How to visualize a strain wave front of a progressive acoustic wave by means of conventional Fresnel imaging is shown. The theory suggests that for a Raman-Nath parameter of 0.1 ≤ υ ≤ 1, the stroboscopic illumination automatically produces the strain wave front image in a limited detection zone, because the spatial harmonic images of the original sinusoidal field coming from the interference among the higher-order diffracted light, are appropriately superimposed. The experiment showed good coincidence with theory and its usefulness in observing surface acoustic waves.

Formation of the Wavefront Image of Progressive Acoustic Wave by Self-Imaging Optical Method with a Diode-Laser Strobe

Theory holds that when a progressive acoustic wave not as intense as Raman-Nath parameter v≤1 is illuminated by an optical plane wave, its strain wavefront image appears alone in a limited region of the diffractied field. This imaging is based on the generation and superposition of the diffracted lights due to optical phase modulation by the acoustic wave. The experiment revealed that distinct and sharp wavefront fringes of the wave were formed in such a diffraction zone and these were helpful in our understanding of the vibrational mechanism of an energy-trapping transducer.