Qu Wang

Multiple-image encryption based on interference principle and phase-only mask multiplexing in Fresnel transform domain

In this article, a multiple-image encryption method based on the optical interference principle and phase-only mask (POM) multiplexing is proposed. During the encryption process, each secret image is encoded into two analytically obtained POMs and one computer-generated random POM, in which no iterative computation is required. The analytically obtained POMs taken from different secret images are then synthesized by POM multiplexing and further encoded into two complex ciphertext images. The silhouette problem that exists in the earlier interference principle-based encryption approaches is totally resolved by the proposal. Both digital and optical means can be used for decryption. The crosstalk effect between the secret images will not appear in the decrypted results by using the proposed system. Numerical simulations have been given to verify the performance and feasibility of the proposal. We also discuss briefly the influence of information compression on the quality of decrypted images.

Optical image encryption based on joint fractional transform correlator architecture and digital holography

Abstract. We present a hybrid configuration of joint transform correlator (JTC) and joint fractional transform correlator (JFTC) for encryption purpose. The original input is encoded in the joint fractional power spectrum distribution of JFTC. In our experimental arrangement, an additional random phase mask (master key) is holographically generated beforehand by a Mach–Zehnder interferometer with a JTC as the object arm. The fractional order of JFTC, together with the master key, can remarkably strengthen the safety level of encryption. Different from many previous digital-holography-based encryption schemes, the stability and alignment requirement for our system is not high, since the interferometric operation is only performed in the generation procedure of the master key. The advantages and feasibility of the proposed scheme have been verified by the experimental results. By combining with a multiplex technique, an application for multiple images encryption using the system is also given a detailed description.

Multiple-image encryption using polarized light encoding and the optical interference principle in the Fresnel-transform domain

We propose a multiple-image encryption scheme, based on polarized light encoding and the interference principle of phase-only masks (POMs), in the Fresnel-transform (FrT) domain. In this scheme, each secret image is converted into an intensity image by polarized light encoding, where a random key image and a pixilated polarizer with random angles are employed as keys. The intensity encrypted images produced by different secret images are convolved together and then inverse Fresnel-transformed. Phase and amplitude truncations are used to generate the asymmetric decryption keys. The phase-truncated inverse FrT spectrum is sent into an interference-based encryption (IBE) system to analytically obtain two POMs. To reduce the transmission and storage load on the keys, the chaotic mapping method is employed to generate random distributions of keys for encryption and decryption. One can recover all secret images successfully only if the corresponding decryption keys, the mechanism of FrTs, and correct chaotic conditions are known. The inherent silhouette problem can be thoroughly resolved by polarized light encoding in this proposal, without using any time-consuming iterative methods. The entire encryption and decryption process can be realized digitally, or in combination with optical means. Numerical simulation results are presented to verify the effectiveness and performance of the proposed scheme.

Double image encryption using phase-shifting interferometry and random mixed encoding method in fractional Fourier transform domain

Based on the two-step phase-shifting interference (PSI) tech- nique in fractional Fourier transform (FRT) domain and random mixed encoding, we present a new scheme for double image encryption. In the proposed scheme, information of each primitive image is recorded in two intensity interference patterns of FRT spectra by PSI technique, from which an encrypted image for each primitive image can be digitally derived. Random mixed encoding is then employed to divide and recom- bine both encrypted images into a single synthetic encrypted image. During the mixed encoding process, repositioning operations based on shift-variance of FRT are performed on the encrypted images to realize the spatial separation of decoded results in the output plane. By inverse FRT with correct fractional order, any of the primitive images can be easily retrieved directly from the synthetic encoded image with the corresponding phase encoding key. Crosstalk effect due to the overlapping of decoded images is alleviated for their spatial separation. Computer simulation and experimental results are presented to verify the validity and efficiency of our scheme.

THE AMPLITUDE AND PHASE MEASUREMENTS OF FEMTOSECOND PULSE USING FRINGE FREE INTERFEROMETRY OF SPECTRAL HIGH-RESOLUTION METHOD

In this paper, we present analytical and experimental results of a novel method which enables significant simplifications in spectral shearing interferometry as applied for ultrashort optical pulse measurements. The method attempts to improve the superiority in conventional spectral phase interferometry for direct electric-field reconstruction (SPIDER). It does not split a test pulse into two parts, but compensate for the delay time between two quasi-monochromatic frequency components in a chirped pulse with spectral and temporal shear to produce an interferogram without fringe. Fringe free interferogram could simplify data processing procedure and reduce the restriction on resolution of spectrometer. The experimental setup avoids the influence of material dispersion with real-time and single-shot nature, and could be made up as a miniaturization measurement device. Although being short of the beauties in SPIDER, it still could be an optional femtosecond pulse measurement method in certain condition.

Enhancement of interference-based image encryption using chaotic permutation and phase-blend operation

In conventional interference-based optical encryption schemes, a potential cracker can retrieve partial information (silhouette) of the secret image using only one phase-only mask (POM). We resolve this drawback using a phase-blend operation and piecewise linear chaotic map (PWLCM) to further encode the POMs. One cannot recover a secret image visibly when inverse phase-blend operation and inverse chaotic permutation are not carried out with the correct decryption keys. Chaotic parameters of PWLCM, and random phase-angle function in the phase-blend operation enlarge the key space and improve the security of the proposed system greatly. Numerical simulations and optoelectronic experiments are performed to verify the effectiveness of the proposed scheme.

Color holographic image by using digital lensless Fourier transform holography with optical fiber

A new method of lensless Fourier transform holography for color reconstruction image is presented. With this method, the recording distance can be kept invariant in recording process. The resolutions of reconstruction images can be adjusted in the same recording distance. Three lasers with different wavelengths are used in an experimental setup to synthesize a color image in a reconstruction process. An optical fiber is used effectively, and the quality of the reconstruction image is improved after filter operation.

Nonlinear joint fractional Fourier-transform correlator based on the threshold-decomposition preprocessing

The morphological joint fractional Fourier transform correlator (MJFTC), as a modified nonlinear joint fractional Fourier transform correlator (JFTC), is introduced in this paper. An optoelectronic setup that can implement such a nonlinear JFTC is advised and constructed. We investigate the correlation properties of the MJFTC using computer simulation and optical experiments. Numerical and experimental results show that the MJFTC exhibits remarkable improvement in terms of discriminability and peak sharpness compared with the classic JFTC. Moreover, the MJFTC provides better noise tolerance to the salt-and-pepper noise or normally distributed white additive noise than the JFTC and the other nonlinear processing or fractional-order filtering JFTC.

Suppression of imaging crack caused by the gap between micromirrors in maskless lithography

Abstract. The digital micromirror device (DMD) is the key device in maskless lithography. However, because of the machinery manufacturing limit of DMDs, the gap between the micromirrors may destroy the continuity of the graphic. This work presents a simple way to fill the imaging crack by controlling the partial coherence factor σ of the light source. A crack can be regarded as the image of a dark space. By considering the resolving power for such cracks under partially coherent illumination, the images of such dark spaces can be covered, preventing them from being imaged on the substrate. By using mathematical derivations of the light intensity distribution exposed to the substrate, and by utilizing the diffraction effect induced by the finite aperture of the optical projection system, an appropriate σ value can be determined for eliminating the image of the crack in an actual scene. The numerical simulation results demonstrate that this method can ensure the continuity of the graphic at the critical partial coherence factor σc regardless of the shape of the target graphic.

Shadow extraction for urban area based on hyperspherical color sharpening information distortion

A shadow extraction method for urban area is presented based on the hyperspherical color transform (HCT) fusion information distortion. We use the near-infrared band of WorldView-2 data to detect the shadow, because the near-infrared band as the long-wave band is more sensitive to shadow comparing to the short-wave band. In the hyperspherical color sharpening (HCS), n input bands are transformed from an n-dimensional Cartesian space to an n-dimensional hyperspherical color space to generate a single intensity component and n-1 angles, and then the intensity component is replaced with the adjusted panchromatic (Pan) image. After HCT, the information amount of the intensity is larger than that of the Pan band. When using the Pan to replace the intensity to get the fused multispectral (MS) image, the information amount is lost. To assess the information distortion of the fusion result, it is found that the shadow is sensitive to the difference index. Hence, the relative difference index is constructed to enhance the shadow information. More specifically, the relative difference index values are made high for shadow area while they are made low for non-shadow area. However, for the original MS image, the digital number values are low for the shadow area while they are high for non-shadow area. Then, by thresholding, the possible shadow area is separated from the non-shadow area. The experimental results show that this shadow extraction method is simple and accurate; not only the shadow of high building but also the little shadows of low trees and between buildings are all detected.