Speckle reduction by combination of digital filter and optical suppression in a modified Gerchberg-Saxton algorithm computer-generated hologram.

A speckleless illuminated modified-Gerchberg-Saxton-algorithm-type computer-generated hologram, which adopts a lower frequency of the iterative algorithm and calculation time, is proposed to code a hologram with two signals and position a multiplexing phase-only function, which can reconstruct the left and the right viewing holograms on the pupillary-distance position after the decryption and still maintain the content with high contrast and definition. The reconstructed image quality presents root mean square error of 0.03, with a diffraction efficiency of 87%, and signal-to-noise ratio of 8 dB after the analysis. Furthermore, two denoising techniques for the digital filter and optical suppression are combined, in which the speckle suppression with pseudorandom phase modulation and a rotating diffuser are utilized for successfully reducing the speckle contrast, which was reduced to below 4%. The goal was to reduce visual fatigue for the viewers.

Lensless optical data hiding system based on phase encoding algorithm in the Fresnel domain.

A novel and efficient algorithm based on a modified Gerchberg-Saxton algorithm (MGSA) in the Fresnel domain is presented, together with mathematical derivation, and two pure phase-only masks (POMs) are generated. The algorithm's application to data hiding is demonstrated by a simulation procedure, in which a hidden image/logo is encoded into phase forms. A hidden image/logo can be extracted by the proposed high-performance lensless optical data-hiding system. The reconstructed image shows good quality and the errors are close to zero. In addition, the robustness of our data-hiding technique is illustrated by simulation results. The position coordinates of the POMs as well as the wavelength are used as secure keys that can ensure sufficient information security and robustness. The main advantages of this proposed watermarking system are that it uses fewer iterative processes to produce the masks, and the image-hiding scheme is straightforward.

Lensless multiple-image optical encryption based on improved phase retrieval algorithm.

A novel architecture of the optical multiple-image encryption based on the modified Gerchberg-Saxton algorithm (MGSA) by using cascading phase only functions (POFs) in the Fresnel transform (FrT) domain is presented. This proposed method can greatly increase the capacity of the system by avoiding the crosstalk, completely, between the encrypted target images. Each present stage encrypted target image is encoded as to a complex function by using the MGSA with constraining the encrypted target image of the previous stage. Not only the wavelength and position parameters in the FrT domain can be keys to increase system security, the created POFs are also served mutually as the encryption keys to decrypt target image from present stage into next stage in the cascaded scheme. Compared with a prior method [Appl. Opt.48, 2686-2692 (2009)], the main advantages of this proposed encryption system is that it does not need any transformative lenses and this makes it very efficient and easy to implement optically. Simulation results show that this proposed encryption system can successfully achieve the multiple-image encryption via fewer POFs, which is more advantageous in simpler implementation and efficiency than a prior method where each decryption stage requires two POFs to accomplish this task.

Lensless optical data embedding system using concealogram and cascaded digital Fresnel hologram.

A robust lensless optical data embedding system using both the computer-generated concealogram and the digital Fresnel hologram (DFH) is proposed. The concealogram and the DFH are cascaded as the input and filter planes, respectively. When the concealogram is illuminated by a plane wave, the hidden data can be extracted at the output plane without using any lenses. The longitudinal positions of the filter and the output planes, as well as the wavelength, can be used as the secret keys to enhance system security. Furthermore, the robustness of the proposed system against noise and distortion is also demonstrated.

Wavelength multiplexing multiple-image encryption using cascaded phase-only masks in the Fresnel transform domain.

We propose a method of wavelength multiplexing based on a modified Gerchberg-Saxton algorithm (MGSA) and a cascaded phase modulation scheme in the Fresnel transform domain to reduce the cross talk in the multiple-image-encryption framework. First, each plain image is encoded to a complex function by using the MGSA. Next, the phase components of the created complex functions are multiplexed with different wavelength parameters, and then they are modulated before being combined together as a phase-only function, which is recorded in the first phase-only mask (POM). Finally, the second POM is generated by applying the MGSA again on the amplitude derived from the summation of the total created complex functions. Simulation results show that the cross talk between multiplexed images has been significantly reduced compared with an existing similar method. Therefore, the multiplexing capacity in encrypting multiple gray-scale images can be increased accordingly.

Fast double-phase retrieval in Fresnel domain using modified Gerchberg-Saxton algorithm for lensless optical security systems.

A novel fast double-phase retrieval algorithm for lensless optical security systems based on the Fresnel domain is presented in this paper. Two phase-only masks are efficiently determined by using a modified Gerchberg-Saxton algorithm, in which two cascaded Fresnel transforms are replaced by one Fourier transform with compensations to reduce the consumed computations. Simulation results show that the proposed algorithm substantially speeds up the iterative process, while keeping the reconstructed image highly correlated with the original one.

Optical implementation of the complex Gabor-wavelet filter based on the Gaussian chirplet transform approach.

A 2D complex Gabor-wavelet filter (CGWF) optical architecture based on the proposed Gaussian chirplet transform approach is presented together with its mathematical derivation. Far from the conventional setup schemes in which only the real part of the CGWF (even-symmetrical GWF) can be implemented optically, the proposed optical scheme shows that the real and also the imaginary part of the CGWF (odd-symmetrical GWF) can be achieved. The computer application simulations to the oriented edge feature extraction are given to validate the feasibility of the proposed scheme.

Multiple-image encryption and multiplexing using a modified Gerchberg-Saxton algorithm and phase modulation in Fresnel-transform domain.

What we believe to be a new technique, based on a modified Gerchberg-Saxton algorithm (MGSA) and a phase modulation scheme in the Fresnel-transform domain, is proposed to reduce cross talks existing in multiple-image encryption and multiplexing. First, each plain image is encoded and multiplexed into a phase function by using the MGSA and a different wavelength/position parameter. Then all the created phase functions are phase modulated to result in different shift amounts of the reconstruction images before being combined together into a single phase-only function. Simulation results show that the cross talks between multiplexed images have been significantly reduced, compared with prior methods [Opt. Lett.30, 1306 (2005); J. Opt. A8, 391 (2006)], thus presenting high promise in increasing the multiplexing capacity and encrypting grayscale and color images.

Signal reconstruction algorithm based on a single intensity in the Fresnel domain.

A novel algorithm that can reconstruct a symmetrical signal (both the amplitude and the phase information) with only a single Fresnel transform intensity is proposed. A new complex-convolution method is introduced, which is needed in the algorithm. The essential properties of the discrete Fresnel transform are presented as well. Numerical results show that this method can successfully rebuild the signal from one signal intensity, which is more advantageous in speed and efficiency than the conventional method that requires two intensities to accomplish this task.

Fast algorithm of phase masks for image encryption in the Fresnel domain.

A high performance lensless optical security system based on the discrete Fresnel transform is presented. Two phase-only masks are generated with what we believe to be a novel and efficient algorithm. Their position coordinates and the wavelength are used as encoding parameters in the encryption process. Compared with previous studies, the main advantage of this proposed encryption system is that it does not need any iterative algorithms to produce the masks, and that makes it very efficient and easy to implement without losing the encryption security.