The human eye wavefront aberrator based on the Shack-Hartmann wavefront sensor (SHWFS) has become a common device for detecting eye aberrations in modern ophthalmology clinics. In order to eliminate the problem of spot and subaperture matching in traditional methods, we use deep learning method to directly map Hartmann spot pattern and corresponding Zernike coefficient, so as to expand the dynamic range of measurement. The lightweight network realizes to fully extract high dimensional feature information and achieves high precision measurement of diopter and astigmatism. The experimental results show that the proportion of the network falling into the tolerant error range (±0.25D) in diopter and astigmatism measurement reaches 94.2% and 100%. This method can measure the low order aberrations of human eyes effectively without changing the SHWFS setting, and at the same time ensure the accuracy and dynamic range, which has been verified by the real machine.
Interferometric imaging technology requires accurate acquisition of the modulus, phase, and spatial frequency coordinate information of the complex coherence when reconstructing the target image. Due to the inherent jitter of optical fibers, it is relatively difficult to accurately obtain the phase information. Using the continuous hybrid input–output algorithm, the target image can be reconstructed when only the modulus and spatial frequency coordinate information of the complex coherence are known. However, in the actual imaging process, there are always certain errors in the measurement of the modulus and spatial frequency coordinate information, which affects the quality of the reconstructed image. Based on the theoretical calculation formula of the complex coherence modulus, we analyzed the factors affecting the measurement results of the complex coherence modulus and built an indoor testbed for experimental verification. Finally, through theoretical simulation, the impact of the error in the measurement of the complex coherence modulus on the quality of the reconstructed image was analyzed, proving the necessity of accurate measurement of the complex coherence modulus. The experimental and simulation results have important guiding significance for the design of subsequent interferometric imaging systems and experimental schemes.
There are more than 40 years history of adaptive optics (AO) in Institute of Optics and Electronics (IOE), Chinese Academy of Sciences since 1980. The research concern all the aspects including the theories study, devices manufacture, and system development. The recent advances on astronomical AO are reported in this presentation. The recent AO systems developments for 4-meter night-time optical telescope, 1.8-meter solar telescope CLST and the 1-m New Vacuum Solar Telescope at Fuxian Lake Solar Observatory are presented respectively. The Deformable Secondary Mirror advancement is also introduced.
Segmented telescope is an effective way to realize high-resolution observations in astronomy. An important work for high-resolution observations using segmented telescopes is phasing the segmented primary mirror. Modified Shack-Hartmann sensor. Is proposed for piston error detection. The interference pattern created by a circular lens placed across two adjacent mirrors in exit pupil plane is used as the signal of the modified Shack-Hartmann sensor. Piston errors need to be extracted from the interference pattern. The offset of lens and gap error of adjacent mirrors causes the distortion of interference pattern, and leads to a reduction in the detection accuracy of existing piston error extraction techniques. In this paper, we propose to replace the circular lens with a square lens and the mathematical model of the corresponding interference pattern is modeled by Fourier optics, including the one-dimensional and two-dimensional analytical solution of the interference pattern. The simulation results show that the proposed analytical solution can effectively characterize the interference pattern in the ideal situation and in the presence of lateral offset of the lens and the gap error of the adjacent mirrors situation. The results presented here give a deeper insight into the interference pattern of modified Shack-Hartmann sensor, and are of great help for developing new piston error detection techniques based on modified Shack-Hartmann sensor.
An important technique in hyperspectral unmixing is collaborative sparse regression. It improves the unmixing results by solving a joint sparse regression problem, where the sparsity is simultaneously imposed to all pixels in the data set. Now it is well known that introducing weighted factors to enforce sparseness becomes a necessary process in sparse unmixing methods. In this paper, considering the desirable performance of reweighted minimization, a double reweighted collaborative sparse regression (DR-CLSUnSAL) has been utilized. The proposed method enhances the sparsity of abundance factions in both the spectral sparsity (column sparsity of the fractional abundances in the sense) and the spatial sparsity (row sparsity of the fractional abundances in the sense). Then the optimization problem was simply solved by the variable splitting and augmented Lagrangian algorithm. Our experimental results with simulated data sets generated by randomly extracting from the United State Geological Survey(USGS) library demonstrate that the proposed method is an effective and accurate sparse unmixing algorithm compared with other advanced hyperspectral unmixing methods.
Pyramid wavefront sensor is a promising sensor technology based on the beam splitting in the focal plane. Due to its advantages of adjustable gain and variable spatial sampling, the pyramid wavefront sensor has been successfully applied in many large telescopes. In recent years, we have carried out the related research of this sensor. Firstly we studied the adaptive optical closed-loop system based on the liquid crystal spatial light modulator and the pyramid wavefront sensor. Subsequently, the adaptive optical system based on the pyramid wavefront sensor and the deformable mirror is studied in our lab. Currently the experiment on the 1.8-m telescope at Yunnan observatory has been successfully carried out and the high resolution images of the natural stellar star have been obtained. The experiment results are present in this paper.
The AO progresses for astronomy in the Key Laboratory of Adaptive Optics, Chinese Academy of Sciences are reported in this presentation. For night-time astronomical observations, the recent AO technological developments, such as Laser Guide Star, Pyramid Sensor and Deformable Secondary Mirror, are introduced. The solar AO researches are also presented for day-time astronomical observations. Furthermore, we will show the on-sky high resolution observational results in the 1.8m telescope at Gaomeigu site, Yunnan Observatory and the 1-m New Vacuum Solar Telescope (NVST) at Fuxian Lake Solar Observatory respectively.
Images are one of vital ways to get information for us. However, in the practical application, images
are often subject to a variety of noise, so that solving the problem of image denoising becomes
particularly important. The K-SVD algorithm can improve the denoising effect by sparse coding atoms
instead of the traditional method of sparse coding dictionary. In order to further improve the effect of
denoising, we propose to extended the K-SVD algorithm via group sparse representation. The key point
of this method is dividing the sparse coefficients into groups, so that adjusts the correlation among the
elements by controlling the size of the groups. This new approach can improve the local constraints
between adjacent atoms, thereby it is very important to increase the correlation between the atoms. The
experimental results show that our method has a better effect on image recovery, which is efficient to
prevent the block effect and can get smoother images.
The limitation of star detection in daytime is poor contrast of image. Compare with the strong sky background light, star signal is so weak that can’t be detected. There are some solutions to find star signal out from strong sky background, spectral filter is selected in this article. According to the different spectrum distributions of stars and sky background, different dead wavelengths is chosen to reduce the sky background light and increase the contrast between star object and sky background. Different kinds of spectral filters were placed between adaptive optical system and secondary mirror of 1.8m telescope in Lijiang observation station. The RMS value of wavefront correction residual error and the FWHM value of far-field light spot are chosen to evaluate the correction capability of AO system. Applying spectral filter, the RMS value is improved from 0.25λ to 0.17λ and the FWHM value is improved from 0.46 arc sec to 0.34 arc sec. Both of the evaluation criterions show that spectral filter is effective for detecting stars in daytime. Furthermore, it also makes it possible to apply our adaptive optics system in daytime in future.
The pyramid wavefront sensor is an innovative device with the special characteristics of variable gain and adjustable
sampling in real time to enable an optimum match of the system performance, which make it an attractive option for next
generation adaptive optics system compared with the Shack-Hartmann. At present most of the pyramid wavefront sensor
are used with modulation based on oscillating optical component in order to give a linear measurement of the local tilt,
but the PWFS without modulation would greatly simplify the optical and mechanical design of the adaptive optics
system and also give highest sensitivity as expected to be achieved. In this paper we describe the optical setup of our
adaptive optics system with nonmudulated pyramid wavefront sensor. In this system, the pyramid wavefront sensor with
8×8 sub-apertures in the pupil diameter has been designed, and the deformable mirror with 61 actuators based on the
liquid-crystal spatial light modulator is used to introduce aberrations into the system, as well as to correct them
afterwards. The closed-loop correction results of single order Zernike aberrations and the Kolmogorov turbulence phase
screen are given to show that the PWFS without modulation can work as expected for closed-loop system.
Pyramid wavefront sensor (PWFS) without modulation is prevailing over one with modulation. So far how to describe
measured signals of non-modulation PWFS needs deeply research. In this paper, the theory of the non-modulation PWFS
is briefly presented according to wave optics. This paper analyses the existing four approaches in theory. By numerical
simulation this paper further verifies the performance of four approaches under the experiment condition. The result
shows that the approach with total intensity of pixels conjugate to the same spot in the pupil as signal denominator is the
best choice for the non-modulation PWFS in closed-loop correction.
For the optical system of the telescope, with the increase in telescope size, the manufacture of monolithic primary
becomes increasingly difficult. Instead, the use of segmented mirrors, where many individual mirrors (the segments)
work together to provide an image quality and an aperture equivalent to that of a large monolithic mirror, is considered a
more appropriate strategy. But with the introduction of the large telescope mirror comprised of many individual
segments, the problem of insuring a smooth continuous mirror surface (co-phased mirrors) becomes critical. One of the
main problems is the measurement of the vertical displacement between the individual segments (piston error), for such
mirrors, the segment vertical misalignment (piston error) between the segments must be reduced to a small fraction of
the wavelength (<100nm) of the incoming light. The measurements become especially complicated when the piston error
is in order of wavelength fractions. To meet the performance capabilities, a novel method for phasing the segmented
mirrors optics system is described. The phasing method is based on a high-aperture Michelson interferometer. The use of
an interferometric technique allows the measuring of segment misalignment during the daytime with high accuracy,
which is a major design guideline. The innovation introduced in the optical design of the interferometer is the
simultaneous use of monochromatic light and multiwavelength combination white-light source in a direct method for
improving the central fringe identification in the white-light interferometric phasing system. With theoretic analysis, we
find that this multiwavelength combination technique can greatly increase the visibility difference between the central
fringe and its adjacent side fringes, and thus it offers an increased signal resolution. So make the central fringe
identification become easier, and enhance the measure precision of the segment phasing error. Consequently, it is
suitable for high-precision measurement purpose and application in the segment piston error phasing system. The
description about the expected interferograms and the feasibility of the phasing method are presented here.
The quest for higher angular resolution in astronomy will inevitably require the telescope with large aperture. However,
the primary mirror diameter is limited by the fabrication problems, and the cost of monolithic optics increases faster than
diameter squared. The Fizeau interferometer imaging system represents a promising new technology to overcome the
above-mentioned problems. The sub-apertures of the system are phased together to synthesize an imaging system that
has a large effective aperture than any of the independent sub-apertures. Due to discontinuation of pupil plane, the lost of
circular symmetry and the increase of secondary peak indicate that the standard Rayleigh resolution criterion for the
single-aperture system is not appropriate for the Fizeau interferometer imaging system. On the other hand, in order to
obtain diffraction-limited performance, the sub-apertures of the system must be coherently phased together to much less
than the wavelength of light, so the optical aberrations of sub-apertures can rapidly degrade system performance. The
purpose of this paper is to analyze the resolution performance and the effect of aberrations for a Fizeau interferometer
imaging system that consists of four identical sub-apertures. The resolution based on the threshold and contracted with
directional property is deduced. The relationship between the system's Strehl ratio and the sub-apertures' aberrations is
given with the exponential approximation, which implies that Strehl ratio contacts not only with each sub-aperture's
aberrations, but also with the interference between sub-aperture's aberrations.
With the increase of telescope size, the manufacture of monolithic primaries becomes increasingly difficult. Instead, the
use of segmented mirrors, where many individual mirrors (the segments) work together to provide good image quality
and an aperture equivalent to that of a large monolithic mirror, is considered a more appropriate strategy. But, with the
introduction of large telescope mirror comprised of many individual segments, the problem of insuring a smooth
continuous mirror surface (co-phased mirrors) becomes critical. One of the main problems arising in the co-phasing of
the segmented mirrors telescope is the problem of measurements of the vertical displacements between the individual
segments (piston errors). Because of such mirrors to exhibit diffraction-limited performance, a phasing process is
required in order to guarantee that the segments have to be positioned with an accuracy of a fraction of a wavelength of
the incoming light.The measurements become especially complicated when the piston error is in order of wavelength
fractions. To meet the performance capabilities, a novel method for phasing the segmented mirrors optical system is
described. The phasing method is based on a high-aperture Michelson interferometer. The use of an interferometric
technique allows the measurement of segment misalignment during daytime with high accuracy, which is a major design
guideline. The innovation introduced in the optical design of the interferometer is the simultaneous use of both
monochromatic and white-light sources that allows the system to measure the piston error with an uncertainty of 6nm in
50µm range. The description about the expected monochromatic and white-light illumination interferograms and the
feasibility of the phasing method are presented here.
The quest for higher angular resolution in astronomy will inevitably require the telescope with large aperture. However,
the diameter of primary mirror is limited by the fabrication problems as well as the scaling laws of manufacturing costs.
Optical synthetic aperture telescopes represent a promising new technology to overcome the above-mentioned problems.
Three incoherent imaging techniques based on optical synthetic aperture including Fourier telescopy, Michelson
interferometer and Fizeau interferometer are described in the paper. Fourier telescopy is an active imaging technique
combined with the advantages of synthetic aperture measurement. Michelson interferometers measure spatial Fourier
transform of objects, while Fizeau interferometers produce direct images with full instant frequency coverage. We give
an overview of the basic aspects and the differences of these techniques.
This paper describes a novel approach to multisensor image fusion using a new mathematical transform: the curvelet
transform. The transform has shown promising results over wavelet transform for 2-D signals. Wavelets, though well
suited to point singularities have limitation with orientation selectivity, and therefore, do not represent two-dimensional
singularities (e.g. smooth curves) effectively. Curvelet improves wavelet by incorporating a directional component. This
paper employs the curvelet transform for image fusion. Based on the local energy of direction curvelet subbands, we give
the definition of local band-limited contrast and use it as one of the fusion rules. The local band-limited contrast can
reflect the response of local image features in human visual system truly. When used to image fusion in noiseless
circumstance, it is effective. But in noisy circumstance, it is not always robust. According to the different characteristics
between image features and noise, the local directional energy entropy is proposed. It can distinguish the noise and local
image features. In this paper, the combination of local band-limited contrast and local directional energy entropy is used
as image fusion. Experimental results show that it is robust in noisy and noiseless image fusion system.
The optical correlation technique based on volume holographic storage has a more obvious advantage on rapid processing, for the input image to be compared is computed with all stored templates simultaneously. Thus it is promising in application for pattern recognition, content addressing and associative recall. As the continuing progress in volume holographic storage, the volume holographic optical correlation technique will draw more and more attention. The volume holographic correlators always use Fe:LiNbO3 as their storage materials, in which a nonuniform photovoltaic field was created during hologram recording exposure. As we have discussed before, the field not only reduced the dynamic range of the crystal, but also degraded the volume holograms. Furthermore, the field debases the recognition rate. In this paper, we analyze the influence of the field on holographic correlation recognition according to theories and experiments. A sodium chloride solution is used to short-circuit the crystal and suppress the influence of the photovoltaic dc field. 1000 holograms, which have been stored in a common volume of the crystal placed in the sodium chloride solution, have been reorganized with the rate of more than 84%.
Diarylethene is one of photochromic material with many advantages and one of the most promising recording materials for huge optical data storage. Diarylethene has two forms, which can be converted to each other by laser beams of different wavelength. The material has been researched for rewritable optical disks. Volatile data storage is one of its properties, which was always considered as an obstacle to utility. Many researches have been done for combating the obstacle for a long time. In fact, volatile data storage is very useful for anti-piracy optical data storage. Piracy is a social and economical problem. One technology of anti-piracy optical data storage is to limit readout of the data recorded in the material by encryption software. By the development of computer technologies, this kind of software is more and more easily cracked. Using photochromic diarylethene as the optical recording material, the signals of the data recorded in the material are degraded when it is read, and readout of the data is limited. Because the method uses hardware to realize anti-piracy, it is impossible cracked. In this paper, we will introduce this usage of the material. Some experiments are presented for proving its feasibility.
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