Jinyun Zhou

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.

Design of an Optical Power and Wavelength Splitter Based on Subwavelength Waveguides

A plasmonic T-shaped metal-insulator-metal waveguide followed by two or four output waveguides with ultranarrow silica slits is proposed to serve as an optical power and wavelength splitter in compact footprint. When the lengths of all silica slits are identical, uniform transmittances and identical center wavelengths are observed at all the output waveguides, and they can be tuned by changing the positions and the lengths of the slits, respectively. In addition, the proposed structure can also be used as a multichannel wavelength splitter by employing the resonance characteristics of the slits. When the silica slits are different in length, a transmission peak with a specific center wavelength will be achieved at the corresponding output waveguide, such that different wavelengths are splitted, respectively. The performances of the proposed power/wavelength splitter are demonstrated by using the finite-difference time-domain method.

Two-Layer Dielectric Grating as Two-Port Beam Splitter

A two-port beam splitter is presented to obtain the 50/50 output based on the two-layer grating. The previously reported beam splitter grating can work only at the special prescribed incident angle. The deviation of incident angle introduces the effect on the splitting ratio uniformity. The proposed two-layer beam splitter grating is aimed to obtain the broad incident angular bandwidth. To optimize the beam splitter, a simplified modal method and rigorous coupled-wave analysis are employed to analyze the propagation process and to calculate the efficiency. Based on the optimized grating parameters, the 50/50 output can be obtained with the polarization-independent property. Most importantly, good splitting ratio uniformity can be exhibited within the broad angular bandwidth for both TE and TM polarizations. The polarization-independent property and the broad incident angular bandwidth are significant for practical applications.

Multiple Plasmon-Induced Transparency Responses in a Subwavelength Inclined Ring Resonators System

On the basis of single metal-insulator-metal (MIM) ring resonator (RR) structure, which acts as a band-stopped filter, a dual RR (DRR) system is proposed to obtain the plasmon-induced transparency (PIT) effect. By rotating the second RR an angle, double PIT windows are achieved due to the dual interference effects, which are attributed to different excitation pathways from the first RR to the second RR. In addition, triple PIT peaks are also achieved by adding an extra inclined RR to the DRR system. Phase shifts, which will occur at each transparency window, are also achieved and analyzed. These compact MIM waveguide structures may be used in the highly integrated optical circuits for biochemical sensors optical signal processing, and optical data storage.

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.

Enhancement of the Efficiency Based on a Sandwiched Grating for Separating Polarized Beams

We describe a novel polarizing beam splitter (PBS) based on a sandwiched grating, where a binary grating is covered by a layer. A modal method is employed to optimize the grating period with the duty cycle of 0.5 by the excited modes in the grating region. The grating depth and thickness of the covering layer are accurately optimized using a rigorous coupled-wave analysis by the numerical calculation. The sandwiched PBS grating can show high efficiency for both transverse electric polarization in the -1st order and transverse magnetic polarization in the zeroth order. The significant wide fabrication tolerances are given for mass production during fabrication.

Plasmonic-induced absorption in an end-coupled metal-insulator-metal resonator structure

An end-coupled composite-slot-cavity resonator (CSCR) is proposed based on the subwavelength metal-insulator-metal (MIM) waveguides. When compared with the perfect slot cavity, which acts as a Fabry–Perot resonator, plasmonic-induced absorption effect is achieved in the CSCR system. Single or dual absorption windows will arise at the former transmission peaks by arranging the position of vertical-slot cavity in the CSCR. According to the analyses on the phase responses, abnormal dispersions will be achieved inside the windows. Therefore, one can manipulate the fast-light applications in the nano-scale integrated circuits. Furthermore, based on the same interference effect, plasmonic-induced transparency response with normal dispersion is also obtained by changing the end-coupled CSCR system to a side-coupled one. The performances of the proposed structure are analyzed and investigated using the coupled mode theory and the finite-difference time-domain method, respectively.

Plasmonic-Induced Absorption and Transparency Based on a Compact Ring-Groove Joint MIM Waveguide Structure

Through adding a groove to an end-coupled perfect ring (PR) resonator, a ring-groove (RG) joint metal-insulator-metal (MIM) structure is proposed. Destructive interference for the expected surface plasmon mode will occur due to the phase differences between two optical paths, leading to the plasmonic-induced absorption response with abnormal dispersion, which is analogous to the electromagnetically induced absorption in the three-level atomic system. A transmission dip is achieved at the former-peak wavelength of the PR resonator, while two transmission peaks arise around the window. The proposed structure, which benefits from -0.3 ps group delay time, will be preferred in the ultrafast-light applications. Due to the same interference effect, plasmonic-induced transparency response with slow-light characteristic is also investigated by arranging the RG joint resonator to be a side-coupled configuration. Therefore, a new approach for on-chip light-speed control can be developed by using the proposed structures, whose performances are investigated by the finite-difference time-domain (FDTD) method and the coupled mode theory.

Reflection-Reduced Two-Layer Grating With Nearly 100% Diffraction Efficiency

A novel reflection-reduced two-layer grating is described with nearly 100% diffraction efficiency. The efficiency of the conventional grating should be improved, or some reported high-efficiency grating can work only for one polarization. The reflection-reduced grating can achieve high efficiency with a covering layer on the surface-relief grating. In addition, the two-layer grating can have wideband property compared with the single-layer grating. The presented novel grating can have merits of high efficiency of reflection-reduced grating and wideband property of two-layer grating. The modal method is applied to analyze the physical mechanism of high efficiency for both TE and TM polarizations. With the optimized grating parameters, efficiencies of 99.69% and 99.64% can be diffracted into the first order for the TE and TM polarizations, respectively. The moderate tolerance should make it possible to be fabricated easily for such a reflection-reduced two-layer grating, which can achieve nearly 100% diffraction efficiency for both the TE and TM polarizations.