Optical encryption system is generally vulnerable to attacks based on phase retrieval algorithm. In this paper, an improved encryption scheme is proposed based on the coherent 4-f system and complementary masks. This pair of irregular shaped complementary masks are introduced to cut the ciphertext and regroup them. The stitched ciphertext has high immunity to ciphertext attacks based on phase retrieval algorithms. We have done numerically simulation to demonstrate the feasibility and validity of the proposed technique.
A method of measuring water colority based on HSV chromaticity (H hue, S saturation, V value) system is proposed. The measurement system is composed of halogen lamp, sample cell and spectrometer. The spectrum data of transmission light captured by spectrometer is used to calculate the XYZ tristimulus values which is then converted to HSV chromaticity. The colority and saturation value shows a good functional relationship which is calibrated in the experiment. Therefore the water colority can be calculated by the saturation in the HSV chromaticity. Since the hue value is acquired at the same time, the method can be adopted to test water sample with different hue. Moreover, the V value is an independent component, so the instability of light source has no influence on the measurement. The colority obtained by the calibrated function coincides with the standard solution.
A Fizeau interferometer based on the self-collimation effect of two-dimensional air-cylinder photonic crystal is discussed theoretically. The Fizeau interferometer includes one total reflection photonic crystal mirrors and two splitters. Compared with non-common optical path interferometer (e.g. Michelson and Sagnac), Fizeau is a type of common optical path interferometer, therefore the machining errors of optical paths lead to little influence on the interference signals. The equi-frequency contours are calculated by using the plane-wave expansion method, and interference properties are investigated by using the finite-difference time-domain method. The curves of the interference signals at output port are determined by the optical path difference of the interferometer. The peaks of the curves are sensitive to the refraction index changes in the optical path, which make it possible to be applied in future high-density photonic integrated circuits, such as air-sensors.
KEYWORDS: Imaging systems, Sensing systems, Objectives, Genetic algorithms, Error analysis, Contrast transfer function, System on a chip, Optical transfer functions, Americium, Physics
The phase diversity wave-front sensing (PDWFS) technique is a posteriori image-based wave-front sensing method which utilizes two images collected simultaneously whose pupil phase differs from each other in a known manner, typically the defocus phase diversity. Here, we present a new method of implementing phase diversity on the sparse aperture imaging system that adds an intentional piston phase to one subaperture. The objective function is firstly derived for the sparse aperture imaging system, then the genetic algorithm is used to minimize the objective function to estimate the piston errors of the subapertures. Digital simulations are conducted for varying amounts of piston phase diversity and levels of noise, the performance of sub-aperture phase diversity is evaluated by comparing with the conventional defocus phase diversity. The results show that the conventional defocus phase diversity performs better than the sub-aperture phase diversity when there is no noise, while the sub-aperture phase diversity outperforms the conventional defocus phase diversity when the noise strength increases. Sub-aperture phase diversity may be an useful alternative if the conventional defocus phase diversity method fails.
KEYWORDS: Wavefront sensors, Imaging systems, System on a chip, Monte Carlo methods, Numerical simulations, Error analysis, Americium, Fourier transforms, Zernike polynomials, Deconvolution
The phase diversity wavefront sensing (PDWFS) technique is an a posteriori image-based wavefront sensing technique which has been successfully implemented to the Hubble Space Telescope. The analytical form for the phase diversity Cramér-Rao lower bound(CRLB) of Golay3 aperture is firstly derived. Monte Carlo analysis of the PDWFS CRLB is used due to the dependence of CRLB on the true values of aberration parameters being estimated. Then the ensemble average of mean-squared errors(MSE) quantities of CRLB is used to evaluate the performance of imaging schemes with different photon distributions and different amounts of defocus. The numerical simulation shows that for a point source target, if a third image implies the inclusion of extra photons, the MSE would be reduced to a degree in accordance with the amount of the extra photons, the MSE remains nearly unchanged if the totoal photons is finite, no matter for a two-channel or a three-channel system. We also find that varying the defocus of one image becomes meaningless if the defocus of the other image is at a high level.
In order to eliminate the noise in images acquired by the sparse aperture system, the modeling and filtering of electrical and optical noise are analyzed by the case of three-mirror aperture optical system. The study shows that the median filter can be applied to remove Gauss and salt & pepper noise, meanwhile high-pass filter with Gauss function can eliminate the influence of non-uniform illuminating on imaging. The Lucy-Richardson algorithm is used to restore the image, by which the resolution is heightened.
A method based on white-light Michelson interferometer for measuring the retardation of multiple-order wave plates is
presented. The linear polarized white-light splits into o-beam and e-beam by the test multiple-order wave plate which
introduces retardation between the two beams, and then they are divided by a beam splitter and reflected by two plane
mirrors in the Michelson interferometer respectively. Finally three white-light interference packets are formed.
According to the optical path between the center packet and one of the side packets, the retardation can be obtained. The
retardation of a multiple-order wave plate is measured in the experiment, whose result (2990.6nm) coincides with the
one obtained by spectroscopic method (2992.8nm).
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