In-vivo imaging of the light-evoked responses of retinal cells in rodents can provide valuable insights into the correlation between optoretinography (ORG) signals and retinal degeneration. However, interpreting outer retina dynamics in rodents is challenging due to the limited resolution of optical coherence tomography, which often results in the superposition of outer retinal layers, such as the rod outer segment (ROS), retinal pigment epithelium (RPE), and Bruch’s membrane, within speckle patterns. Here, we present an automated, unbiased approach for extracting spatially-resolved outer retinal dynamics from complicated speckle patterns. Using this approach, we revealed the light-evoked dynamics of both ROS and RPE in rodents.
Previous studies in optoretinography often rely on AO-OCT systems to resolve individual cells or use intensity-based image processing to extract the dynamics of the retinal layer as an ensemble. For non-AO point-scan OCT, investigating tissue dynamics from obscure speckle patterns while preserving the spatial heterogeneity of signals remains challenging. Here, we developed phase-restoring subpixel image registration and unsupervised machine learning algorithms to accurately extract spatially-resolved OCT phase signals from the outer retina in rodents. In addition to observing light-evoked deformation of the photoreceptors outer segments, we discovered an optical signature of the retinal pigment epithelium (RPE) response to visual stimuli.
Small animals, such as rodents, are attractive options for investigating the intrinsic process of retinal degeneration. In this study, we used phase-sensitive optical coherence tomography to explore the comprehensive dynamics of rats’ outer retinas in response to visual stimuli. By calculating the temporal phase difference between different outer retinal bands, we revealed highly reproducible retinal dynamics, on the order of tens of nanometers, related to different parts of the outer retina. Our approach may pave the way for preclinical optoretinography study in small animals, facilitating clinical translations for the early detection of neurodegenerative diseases.
Needle insertion is a vital procedure in both clinical diagnosis and therapeutical treatment. To ensure the accurate placement of needle, ultrasound (US) imaging is generally used to guide the needle insertion. However, due to depthdependent attenuation and angular dependency, US imaging always face the challenge in consistently and precisely visualizing the needle, necessitating the development of reliable methods to track the needle. Deep learning, an advanced tool that has proven effective and efficient in addressing imaging challenges, has shown promise in enhancing needle visibility in US images. But the existing approaches often rely on manual annotation or simulated data as ground truth, leading to heavy human workload and bias or difficulties in generalizing to real US images. Recently, photoacoustic (PA) imaging has shown the capability of high-contrast needle visualization. In this study, we explore the potential of PA imaging as reliable ground truth for training deep learning networks, eliminating the need for expert annotation. Our network, trained on ex vivo image datasets, demonstrated the abilities of precise needle localization in US images. This research represents a significant advancement in the application of deep learning and PA imaging in clinical settings, with the potential to enhance the accuracy and safety of needle-based procedures.
Recently, there has been vast interest in probing photoreceptor dynamics using optical coherence tomography (OCT). Most successful demonstrations implemented adaptive optics or digital adaptive optics to resolve individual cones or rods in human subjects. Here we use phase information to trace the photoreceptor response in rodents using an ultrahigh-resolution, phase-sensitive, spectral-domain OCT. Brown Norway rats (six to 14 weeks) were sedated using a ketamine and xylazine cocktail. Repeated scans were registered by a phase-restoring subpixel motion correction algorithm to isolate the bulk motion, and two hyperreflective bands (inner segment/outer segment junction – IS/OS; outer segment tip + retinal pigment epithelium + Bruch's membrane) were segmented automatically. As a result, two types of nanoscale signals (biphasic Type-I and monophasic Type-II) were detected with a clear separation in depth. We tested the repeatability, scotopic stimulus strength dependency, and photopic background intensity dependency. Besides, we demonstrated enface mapping of the ORG signals in a wide field of 20°, analogous to the multifocal electroretinogram but with a much higher resolution, revealing the spatial distribution of the outer retina function. This method could be extended to study animal models with photoreceptor degeneration and clinical studies to investigate early photoreceptor dysfunction with high spatiotemporal resolution.
KEYWORDS: Image restoration, Image quality, Image processing, Acquisition tracking and pointing, Reconstruction algorithms, Signal to noise ratio, Photoacoustic tomography, In vivo imaging, Spatial filtering, Brain
SignificanceIn photoacoustic tomography (PAT), numerous reconstruction algorithms have been utilized to recover initial pressure rise distribution from the acquired pressure waves. In practice, most of these reconstructions are carried out on a desktop/workstation and the mobile-based reconstructions are far-flung. In recent years, mobile phones are becoming so ubiquitous, and most of them encompass a higher computing ability. Hence, realizing PAT image reconstruction on a mobile platform is intrinsic, and it will enhance the adaptability of PAT systems with point-of-care applications.AimTo implement PAT image reconstruction in Android-based mobile platforms.ApproachFor implementing PAT image reconstruction in Android-based mobile platforms, we proposed an Android-based application using Python to perform beamforming process in Android phones.ResultsThe performance of the developed application was analyzed on different mobile platforms using both simulated and experimental datasets. The results demonstrate that the developed algorithm can accomplish the image reconstruction of in vivo small animal brain dataset in 2.4 s. Furthermore, the developed application reconstructs PAT images with comparable speed and no loss of image quality compared to that on a laptop. Employing a two-fold downsampling procedure could serve as a viable solution for reducing the time needed for beamforming while preserving image quality with minimal degradation.ConclusionsWe proposed an Android-based application that achieves image reconstruction on cheap, small, and universally available phones instead of relatively bulky expensive desktop computers/laptops/workstations. A beamforming speed of 2.4 s is achieved without hampering the quality of the reconstructed image.
KEYWORDS: Image registration, Optical coherence tomography, Image segmentation, Visualization, Tissues, Speckle pattern, Signal detection, Motion models, In vivo imaging, Image processing algorithms and systems
Phase-sensitive OCT can be used for imaging the photoreceptor deformations in response to the light stimulus or optoretinography (ORG). Here, we propose a phase-restoring subpixel image registration method and an automated signal extraction algorithm for optoretinography using phase-sensitive OCTs. We validated these methods in simulations, phantom experiments, and in-vivo optoretinogram imaging. Our image registration method yields better amplitude stability and higher phase accuracy compared with conventional approaches, and we found two types of signals (one monophasic and the other biphasic) simultaneously in rodent ORG imaging. These results can be beneficial to the ongoing preclinical/clinical ORG studies.
KEYWORDS: Optical coherence tomography, In vivo imaging, Adaptive optics, Signal detection, Optical signal processing, Human subjects, Head, Animal model studies
In recent years, there have been vast interests in probing photoreceptor dynamics using optical coherence tomography (OCT). Most of the successful demonstrations implemented adaptive optics or digital adaptive optics to resolve individual cones or rods in human subjects. Here we use phase information to trace the photoreceptor response in rodents using an ultrahigh-resolution, phase-sensitive, spectral-domain OCT. As a result, two types of nanoscale signals (monophasic and biphasic) were detected with a clear separation in depth. The monophasic signal is less susceptible to stimulus intensity and saturated from a 3% breach rate.
Phase-sensitive optical coherence tomography (OCT) enables label-free imaging of structural dynamics with nanoscopic sensitivity. However, the inevitable bulk tissue motions degrade the signal stability and introduce extra phase error. To suppress the motion-induced phase error, we propose a phase-restoring subpixel motion correction method for post-hoc motion correction in Fourier domain OCT, which enables translational shifts of complex-valued OCT images by arbitrary distance. Phantom and in-vivo rodent optoretinogram imaging experiments were conducted to demonstrate the advantages of the proposed method over conventional pixel-level method and the Fourier transform based method.
We present a method for determining the optical and thermal properties of layered materials, applicable to retinal laser therapy, using phase-resolved OCT. Transient heating of a tissue phantom is achieved by focusing a laser pulse onto a buried absorbing layer. Optical path length changes between the top of the phantom and the scattering absorbing layer induced by material expansion are extracted from the sequential B-scans. The absorption coefficient, heat conductivity and thermal expansion coefficient of the polymer are determined by matching the experimental data to a thermomechanical model of the tissue, yielding a temperature precision <2%, well below damage threshold.
We present a methodology that allows precise determination of the optical and thermal properties of layered materials using thermal perturbation and phase-resolved optical coherence tomography (pOCT). Following heating with a laser pulse of a buried absorbing layer, optical path length changes between the sample’s surface and the absorbing layer are monitored using a line-scan pOCT. Using an axisymmetric thermo-mechanical model for transversely uniform multi-layered media, we fit the absorption coefficient, the heat conductivity, and the thermal expansion coefficient of the polymer. We demonstrate that the temperature distribution can be determined with a precision under 0.1°C, after a single laser pulse.
Nanometer-scale deformations of the neuron accompany the action potential. These displacements are measured using a fast quantitative phase microscope and averaged in synchrony with optogenetic stimulation of cultured neurons. The phase movie is further processed by leveraging the spatial and temporal distribution of the spiking signal to detect and segment the separate action potentials in individual cells. An accompanying confocal fluorescence microscopy provides the 3-D cell shape for calibration of the refractive index to calculate the mechanical displacements from the optical phase. Together, these results illuminate the underlying mechanism of the cellular deformations and techniques for achieving all-optical single spike detection.
We report the full-field imaging of the mechanical deformations accompanying the action potential in primary cortical neurons using ultrafast quantitative phase imaging (QPI) with a temporal resolution of 0.1 ms and a membrane displacement sensitivity of <0.2 nm per pixel. The average displacements were ~0.7 nm on cell somas and ~0.5 nm on neurites. Finite element modeling based on the 3D shape extracted from confocal imaging and on scaling of the surface tension with trans-membrane voltage yielded the deformation map during action potential, which matched the features of the experimental results, including the displacement amplitude, time course, and spatial distribution.
We developed a high-speed adaptive optics, line-scan spectral domain OCT and used it to characterize stimulus-induced optical path length changes in cones with high spatiotemporal resolution. We find that individual cone outer segments exhibit a biphasic light-induced response—a rapid axial shrinkage followed by a gradual increase in optical path length, both increasing in magnitude with the stimulus intensity. AO line-scan OCT thus offers high-speed volume acquisitions, high phase stability, sub-ms temporal resolution and cellular-scale spatial resolution, that together enable imaging retinal structure and function in health and disease.
Nanometer-scale movements of the cell membrane associated with changes in cell potential can reveal the underlying electrical activity. Using wide-field quantitative phase imaging, we observed deformations of up to 3 nm (0.9 mrad) during the action potential in spiking HEK cells, and about 0.3 nm in neurons. The time course of the optically-recorded action potential is similar to intracellular recordings based on patch clamp, while time derivative of the rising edge of the optical spike matches the timing and duration of the extracellular electrical recording. Sufficiently fast QPI may enable non-invasive and label-free monitoring of cellular physiology.
Imaging of the optical phase changes induced by transient heating provides a sensitive measure of material properties associated with refractive index dependence on temperature and thermal expansion. Using fast (50 kHz) QPI, we demonstrate the shot-noise limited sensitivity of about 3.4 mJ/cm2 in a single pulse. Phase-resolved OCT can detect energy deposition of 4.7 mJ/cm2 between two scattering interfaces producing signals with about 45 dB SNR. Integration of the phase changes along the beam path helps increase temperature sensitivity during perturbation. For example, temperature rise of about 0.8 C can be detected in a single cell layer, while hundred times lower heating produces the same phase change in 100-fold thicker tissue layer. Time course of thermal relaxation in QPI can reveal the size and shape of the hidden objects.
Methods based on fast phase imaging may enable multiple applications, ranging from temperature control in retinal laser therapy to subsurface characterization of semiconductor devices.
Movements of the cell membrane accompanying action potentials have been detected by various methods, including reflection of a laser beam, atomic force microscopy and even bright-field microscopy. However, imaging of the entire cell dynamics during action potential has not been achieved, and the mechanism behind this phenomenon is still actively debated. Here we report full-field interferometric imaging of cellular movements during action potential by simultaneous quantitative phase microscopy (QPM) and multi-electrode array (MEA) recordings. Using spike-triggered averaging of the movies synchronized to electrical recording, we demonstrate deformations of up to 3 nm (0.9 mrad) during the action potential in spiking HEK-293 cells, with a rise time of 4 ms. The time course of the optically-recorded action potential is very similar to intracellular potential recorded with a whole-cell patch clamp, while the time derivative of the rising edge of the optical spike matches the timing and duration of the extracellular electrical recording on MEA. In some cells, phase increases at the center and decreases along the cell boundaries, while in others it increases on one side and decreases on the other. These findings suggest that optical phase changes during an action potential are due to cellular deformation, likely associated with changes in the membrane tension, rather than refractive index change due to ion influx or cell swelling. High-speed QPM may enable all-optical, label-free, full-field imaging of electrical activity in mammalian cells.
Wide-field interferometric imaging systems can detect mechanical deformations of a cell during an action potential (AP), such as in quantitative phase microscopy, which is highly sensitive to the changing optical path length. This enables non-invasive optophysiology of spiking cells without exogeneous markers, but high-fidelity imaging of such deformations requires averaging of a large number of spikes synchronized by electrical recordings. We have developed new iterative methods for detecting single APs from quantitative phase microscopy of spiking cells, enabling an all-optical detection system with high accuracy and good temporal resolution. We demonstrate performance of the method across multiple preparations of spiking HEK-293 cells and compare the outcomes of the all-optical measurements with the ground truth detected on a multi-electrode array. We initially use a spike-triggered average, synchronized to an electrical recording, to measure deformations during the AP in spiking cells, which reach up to 3 nm (0.9 mrad) with a rise time of 4 ms and fall time of about 120 ms. Based on this knowledge of the AP dynamics, optical data analysis can provide reliable spike detection, within a standard deviation of 11.6 ms (9.7% of the length of the action potential) with an 8.5% false negative detection rate. The method is robust to natural variations between cells and can be modified to function without any prior knowledge of the AP dynamics. Such a system could achieve high-throughput measurements of network activity in culture and help identify the mechanisms linking cell deformations to the changes of transmembrane potential.
Optical phase changes induced by transient perturbations provide a sensitive measure of material properties. One such measure is associated with the change in refractive index with temperature. Another - with thermal expansion. We demonstrate the high sensitivity and speed of such methods using two interferometric techniques: Quantitative Phase Imaging (QPI) in transmission, and phase-resolved Optical Coherence Tomography (OCT) in reflection. Camera frame rate in QPI varied from 10 to 50 kHz, exposure from 1 to 10 µs, and heating pulse – from 0.02 to 1 ms in duration. The phase-stabilized swept-source OCT was operating at 100 kHz repetition rate. Shot-noise limited QPI can resolve energy deposition of about 3.4 mJ/cm^2 in a single pulse, which corresponds to 0.8 ℃ temperature rise in a single cell. OCT can detect deposition of 24 mJ/cm^2 energy between two scattering interfaces producing about 30 dB SNR signals and 4.7 mJ/cm^2 with 45 dB. Finite element modeling of the phase changes in materials heated by laser and by electric current matched the experimental results very well. These techniques can be used for mapping absorption coefficients, electric current density, doping depth in semiconductors, and many other properties. Integration of the phase changes along the penetrating beam path helps increase sensitivity and reveals the size of the hidden objects by looking at the signal relaxation time. These methods may enable multiple applications, ranging from temperature control in retinal laser therapy and in gene expression to characterization of semiconductor devices.
The high precision ray tracing inside inertial confinement fusion (ICF) cryogenic targets plays an important role in the reconstruction of the three-dimensional density distribution by algebraic reconstruction technique (ART) algorithm. The traditional Runge-Kutta methods, which is restricted by the precision of the grid division and the step size of ray tracing, cannot make an accurate calculation in the case of refractive index saltation. In this paper, we propose an improved algorithm of ray tracing based on the Runge-Kutta methods and Snell's law of refraction to achieve high tracing precision. On the boundary of refractive index, we apply Snell's law of refraction and contact point search algorithm to ensure accuracy of the simulation. Inside the cryogenic target, the combination of the Runge-Kutta methods and self-adaptive step algorithm are employed for computation. The original refractive index data, which is used to mesh the target, can be obtained by experimental measurement or priori refractive index distribution function. A finite differential method is performed to calculate the refractive index gradient of mesh nodes, and the distance weighted average interpolation methods is utilized to obtain refractive index and gradient of each point in space. In the simulation, we take ideal ICF target, Luneberg lens and Graded index rod as simulation model to calculate the spot diagram and wavefront map. Compared the simulation results to Zemax, it manifests that the improved algorithm of ray tracing based on the fourth-order Runge-Kutta methods and Snell's law of refraction exhibits high accuracy. The relative error of the spot diagram is 0.2%, and the peak-to-valley (PV) error and the root-mean-square (RMS) error of the wavefront map is less than λ/35 and λ/100, correspondingly.
A lateral shearing interferometer based on randomly encoded hybrid grating (REHG) is proposed to measure the optical system aberrations. According to the theory of Fraunhofer diffraction, the REHG is designed to be a combination of a randomly encoded binary amplitude grating and a phase chessboard. Compared with the conventional cross-grating lateral shearing interferometer, the REHG is more suitable for the general aberration testing since no order selection mask is needed. Collimated beam for aberration measurement will converge after passing through the optics system under test. Then the quadriwave lateral shearing interferogram containing the wave-front aberration information is then recorded by the CCD. By selecting its +1 order of the Fourier spectrum in both X and Y directions, the shearing wavefronts in both two orthogonal directions can be obtained employing phase unwarping algorithm. Zernike polynomials are used as basic functions for the original wave-front, and the coefficients of Zernike polynomials can be obtained with shearing wave-fronts. In the experiment, we employed a REHG with a grating pitch of 240μm to test a cemented doublet optics with an aperture of 50mm and a focal lengths of 90mm. The test results showed the peak-to-valley (PTV) aberration is 0.242λ while the root-mean-square (RMS) is 0.064λ. The test results by the REHG are very close to the results by the ZYGO GPI interferometer while the error of PTV is 0.003λ and the error of RMS is 0.007λ. The measurement of optical system aberrations by REHG can reach high precision and exhibit good immunity to environmental disturbance. The REHG can be applied to the optical testing of beam quality, optical system aberration and biomedical research.
A compact wavefront diagnosis system with nice repeatability based on the randomly encoded hybrid grating (REHG) is proposed. The REHG comes from the ideally calculated grating for quadriwave lateral shearing interference, and it consists of a binary amplitude grating and a phase chessboard. The phase chessboard simulates the phase modulation of the ideally calculated grating, while the binary amplitude grating is designed and fabricated based on the randomly encoding method. In this method, the amplitude distribution on the ideally calculated grating is firstly divided into discrete grids. And the radiant flux in each grid is quantized into several quantization levels. The binary amplitude grating is then generated by encoding the pixels in the grids with 1 and 0, which stands for whether the light can pass through or not, so that the total radiant flux in each grid on this grating approximate to the flux in the corresponding grid on the ideally calculated quadriwave grating. In addition, random pattern is employed in the encoding process to avoid introducing extra diffraction orders. The far-field diffractions of the REHG only contain the ±1 orders in two orthogonal directions, and no order selection mask is needed for quadriwave lateral shearing interference. Due to the common-path configuration, the wavefront testing results obtained by the REHG lateral shearing interferometer exhibits nice repeatability and good suppression over environmental noise and vibration.
A wavefront retrieval method for the quadriwave lateral shearing interferogram obtained by randomly encoded hybrid grating (REHG) is proposed. The REHG consists of a binary amplitude grating and a phase chessboard, and the Faunhofer diffractions of this grating only contain the ±1 orders in two orthogonal directions. As a result, no order selection mask is ever needed by the REHG for quadriwave lateral shearing interference. To retrieve the phase distributions from the REHG interferograms, fast Fourier transform (FFT) technique is employed at first to get the frequency spectrum. By performing inverse fast Fourier transform (IFFT) of the +1 order spectrum in the x and y directions, it is possible to extract shearing wavefronts from the interferogram in both two orthogonal directions. Using the translation property of Fourier transform, the relationship between the Fourier spectrum of the shearing wavefronts and the Fourier spectrum of the wavefront under test is deduced. The wavefront under test is retrieved by establishing an evaluation function firstly and finding the minimum value with least-square-solution. Analysis and compensations are made to reduce the errors in the testing results. Simulation experiments have shown that this method can retrieve different phase distributions without losing high-frequency information.
Recent trends of interferometric wavefront sensing tend to focus on high precision, anti-vibration, compact, along with much more involved of electric and computer technology. And the optical principles employed not only limit to interference but also include diffraction, scattering, polarization, etc. In this paper, some selected examples basing on the research works in our group will be given to illustrate the trends mentioned above. To achieve extra high accuracy, phase-shifting point diffraction interferometry (PS-PDI) is believed to be a good candidate as it employs a nearly perfect point diffraction spherical wavefront as the reference and also takes advantage of the high precision of phase-shifting algorithms. Cyclic radial shearing interferometry (C-RSI) successively demonstrate the anti-vibration characteristic and can diagnose transient wavefront with only one single shot by employing a three-mirror common-path configuration and a synchronizing system. In contrast sharply with those early interferometers, interferometers with very compact configuration are more suitable to develop portable wavefront sensing instruments. Cross-grating lateral shearing interferometer (CG-LSI) is a very compact interferometer that adopts a cross-grating of millimeters to produce lateral shearing of the diffraction wave of the test wavefront. Be aware that, computer technique has been used a lot in all of the above interferometers but the non-null annual sub-aperture stitching interferometer (NASSI) for general aspheric surface testing mostly relies on the computer model of the physical interferometer setup and iterative ray-tracing optimization. The principles of the above mentioned interferometric wavefront sensing methods would be given in detail.
When using interference wave front to detect density field, it is better to have an interference system which is small and
compact so that different directions of wave fronts can be obtained to reconstruct the density field to be detected. A twodimensional
CGLSI (Cross Grating Lateral Shearing Interferometer) system which consists of a two-dimensional cross
grating and a two-dimensional order-selecting window used as a filter is presented in this paper. Lateral shearing
interferogram of two orthogonal directions (X and Y) each other can be obtained by using this system. With the
advantage of anti-vibration and no reference surface, lateral shearing interferometer is suitable to inhibit external
environment disturbance. In this paper, analysis and simulations have been conducted on grating constant from
geometrical optics and physical optics using Fresnel approximation method respectively based on lateral shearing rate,
windows’ distance in two-dimensional order-selecting window and layout of the system which concludes the best option
for grating constant is d = 25μm . The most optimized design of size and distance for windows in two-dimensional
order-selecting window has been carried out on the basis that complex amplitude distribution can go through the filter so
that there is no distortion on wave front. All designs have gone through computer simulation and fit into the requirements
for the designs.
High spectral resolution lidars (HSRLs) are becoming more and more important in profiling atmospheric aerosols as accurate measurement can be achieved by employing the high-spectral-resolution spectroscopic filter. A field-widened Michelson interferometer (FWMI) is specially designed to be the spectroscopic filter for a near-infrared HSRL. The FWMI is superior to other commonly used filters, such as F-P interferometric filters and atomic/molecular absorption filters for its large angular acceptance, high photon efficiency and wavelength flexibility. It consists of a cubic beam splitter and two orthogonal arms. In this paper, the designing procedure of determining the materials and dimensions of the arms is described in detail. The result designed to the spectral line of 1064nm is presented and is analyzed to have good working performance. A tolerance evaluation model is also established to assist the design process.
High spectral resolution lidars (HSRLs) have shown great advantages for the measurement of backscatter and extinction coefficients of aerosols and clouds due to its spectral discrimination process, which brings about more straightforward and accurate retrieval without additional assumptions in contrast to standard backscatter lidars. We have developed a tilted, field-widened Michelson interferometer (FWMI) to obtain this spectral discrimination. The interferometer is composed of a cubic beam splitter, a solid arm and an air-solid blending arm, and will be employed as the spectroscopic filter of an HSRL to block the aerosol signals but transmit the molecular backscattered photons optimally. In this paper, a comprehensive radiometric model is developed to evaluate the spectral discrimination performance of the FWMI, especially under varies of practical imperfections, such as fabrication errors and utility defections. The principle of the modeling for tilted FWMI as the spectroscopic filter of the HSRL has been presented, through which, the effects on transmittance characteristics of the FWMI from these practical imperfections are analyzed in detail. The model can be used to evaluate the machinery tolerance budgets for new FWMI designs and decide optimal state of usage for FWMI appliance.
Standard backscatter lidars encounter problems when solving the two unknowns (aerosol backscatter coefficient and extinction coefficient) from the only one recorded lidar equation. With the help of the high-spectral-resolution filter, high spectral resolution lidars (HSRLs) can provide unambiguous retrieval without critical assumptions. Spectral discrimination between scattering from molecules and aerosols or cloud particles is the basis of the HSRL technique, and several lidar approaches have been developed to obtain this discrimination. Iodine cell filter, which is a kind of atomic/molecular absorption filter, is robust, stable, and can achieve very good separation of aerosol Mie scattering from atmosphere molecular Cabannes scattering. However, absorption filters are lossy and gaseous absorption lines do not exist at many convenient laser wavelengths. Fabry-Perot interferometers are simple and can be tuned to any wavelength, but are limited by acceptance angle. Field-widened Michelson interferometer (FWMI) is considered to have the ability to overcome the deficiencies of the aforementioned filters as it can perform well at relatively large off-axis angles, is nearly lossless, and can be built to any wavelength. In this paper, the development process of an FWMI that is introduced to be the spectroscopic filter for a polarized near-infrared HSRL instrument will be present. The retrieval process of the aerosol optical properties, the design requirements with special focus on the selection of the free spectral range (FSR) of the FWMI, as well as the design result will be described in detail.
In experiments of inertial confinement fusion (ICF), the thickness uniformity of capsule and the density uniformity of
deuterium-tritium (DT) ice are both key to successful ignition, while the cross-grating lateral shearing interferometer
(CGLSI), which is accurate and insensitive to disturbance, can be employed to test the density distribution of DT ice
precisely. In this paper, a wavefront retrieval method for CGLSI based on differential Zernike polynomial fitting is
presented. Fast Fourier Transform technique (FFT) is employed to get the frequency spectrum of the interferogram
obtained by CGLSI. By performing Inverse Fast Fourier Transform (IFFT) of the +1 order spectrum in both X and Y
directions, it is possible to extract shearing wavefronts from the interferogram in both two orthogonal directions.
Utilizing differential Zernike polynomial fitting method, we are capable of integrating two shearing wavefronts in both X
and Y directions together and retrieving the wavefront under testing. In the process of solving Zernike coefficients, the
characteristics of differential Zernike orthogonal polynomials should be taken fully into account in mathematical
modeling. To avoid the retrieval error introduced due to matrix mutation, the determination of discrete grid number and
aperture shape must be in line with the theory that Zernike polynomials are orthogonal over a unit circle as well. The
result of simulation analysis shows that the wavefront retrieval method for CGLSI based on differential Zernike
polynomial fitting is correct and accurate, and the root-mean-square error of this method is less than λ/15.
In this article, an interference digital testing method for measuring spatial density distribution of transmissive objects is
presented. This method applies a radial shearing interferometer to test the density field from 8 projections in the same
plane. By taking advantage of the regularized phase-tracking technique (RPT), the single interferogram will be
demodulated to two-dimensional phase distribution of the corresponding projection beam. Then the phase data on one
given cross-section of every projection is selected to form 8 curves, which describe one-dimensional phase variation on
the given cross-section from each projection. Regarding these curves as computer tomography projection data, the
refractive index distribution of the given cross-section can be reconstructed utilizing the algebraic reconstruction
technique (ART). Thus, a three-dimensional distribution of refractive index can be obtained by applying the method
above to different cross-sections in order. Finally, we are capable of calculating the spatial density distribution with the
relation between density and refractive index of the substance tested. In addition, the density field testing for hypersonic
flow field is investigated as an example in this article. Considering the fact that the target model in the optical window
center of a wind tunnel will inevitably block some testing beams, which will lead to the sharp decline in accuracy of the
testing results, a modified algebraic reconstruction technique which improves accuracy by introducing biharmonic
spline interpolation is presented. In simulation, an error less than 3% in non-block situation is reached while an error less
than 8% in small-area-block situation is also obtained.
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