An improved image encryption/decryption approach is proposed. In the encryption system, a binary amplitude image is
encoded in the Fourier domain. Then, the encoded image is scrambled with a random binary phase image to produce the
encrypted image. Both the encoding and encryption processes are done electronically. The encrypted image is stored in
an encrypted phase mask of which the transmittance is proportional to the phase components of that image. In the
decryption process, the encrypted phase mask is optically descrambled with another phase mask which has the
transmittance identical to phase components of the random image used in the encryption process. The descrambled field
is inversely Fourier transformed by a Fourier lens in order to reproduce the original image. The simplicity and the
misalignment free characteristics are the most significant advantages of our method. The Fourier encoding process using
an optimization algorithm with iteration technique is discussed in detail. The effect of noise on the quality of the
decrypted image is investigated. The innovation of the system is demonstrated through simulations. The most critical
issue of our technique is that the encryption process requires an optimization search which consumes some computation
time so that the system is expected to be applied for unreal-time applications.
A new structure of polarization-selective elements consisting of two holographic gratings and a dove prism coupler is proposed. The absence of a multi-stage wave-guide, compact size, and lightweight volume are the outstanding features of the new structure. Based on the coupled-wave theory, the analysis and design of the structure are discussed in detail to calculate the required index modulation. Several parameters such as the recording intensity, the exposure time, and the recording angles for the fabrication of the proposed element are determined. Under the conditions the element is fabricated in Dupont photopolymer HRF-150-38 material and with the operating wavelength of 532nm. A simplified pick-up head is constructed to evaluate the performance of the fabricated element.
To reduce computation time, a new approach for designing computer-generated holograms is proposed. In this new method, we utilize artificial neural networks to initiate the genetic algorithm, so that the high computation cost of genetic algorithms for synthesizing holograms is significantly reduced, while the high diffraction efficiency and uniformity are still ensured. Designed binary phase holograms have a diffraction efficiency of 75.8% and uniformity of 5.8%, which are proven in computer simulation and experimentally demonstrated.
A Gaussian apodization technique applied to a transmission volume hologram for a holographic demultiplexer is proposed. A Gaussian apodized grating 15×11 mm in size, 38 µm thick, and with a 3.2-mm horizontal standard deviation of the modulating index profile is fabricated. A 22-channel demultiplexer based on that grating has been optically demonstrated. Channel spacing, interchannel crosstalk level, and channel uniformity of 0.8 nm, –30 dB, and 1.5 dB, respectively, are obtained. In addition, an insertion loss of 13 dB, which is caused by the primary limitation of this technique, is observed.
In order to reduce the computation time, a new approach for designing computer-generated holograms is proposed. In this new method, we utilize artificial neural network to initiate the genetic algorithm so that the high computation cost of genetic algorithm for synthesizing holograms is significantly reduced while the high diffraction efficiency and uniformity are still ensured. Designed binary phase holograms have the diffraction efficiency of 75.8% and uniformity of 5.8%, which are proven in computer simulation and experimentally demonstrated.
In this paper, the implementation of Gaussian apodized volume grating for a demultiplexer is presented. A 42-channel demultiplexer based on that grating is optically demonstrated. By using the Gaussian apodized grating, the crosstalk level between two channels is reduced down to -35dB. The bandwidth of a channel is 0.18 nm. The two adjacent fibers are seperated by 122.5-μm horizontal distance providing the wavelength spacing between each channel of 0.4 nm. Besides, for all 42 channels, the interchannel uniformity of 1.5 dB has been obtained.
A new approach to design the binary phase holograms, which is of less computation time and equal efficiency compared with the genetic algorithm method is proposed. Synthesized holograms having diffraction efficiency of 75.8% and uniformity of 5.8% are proven in computer simulation and experimentally demonstrated.
In this paper, Gaussian apodization technique applied to a transmission volume hologram for holographic demultiplexer is proposed. The Gaussian apodized grating of 15 mm x 11 mmsize, 38 μm thickness and 3.2 mm horizontal standard deviation of the modulating index profile was fabricated. A 22-channel demultiplexer based on that grating has been optically demonstrated. The channel spacing, the interchannel cross-talk level and the channel uniformity of 0.8 nm, -30 dB and 1.5 dB, respectively, were obtained.
A new beam steering scheme using computer-generated holograms(CGHs) is proposed. The steering devices in order to control the reference and object wave are necessary in various holographic multiplexing methods. The beam steering device using CGHs can be simultaneously processed the coarse address function controlling the beam up or down so as to select slice and the fine address function adjusting to the particular holographic page within the chosen layer. From the experimental results, we show that the beam steering can be easily implemented and so powerful to generate the electrically addressed reference wave in digital holographic memory system.
In this paper, we demonstrate the holographic smart card system using digital holographic memory technique that uses reference beam encrypted by the random phase mask to prevent unauthorized users from accessing the stored digital page. The input data that include document data, a picture of face, and a fingerprint for identification is encoded digitally and then coupled with the reference beam modulated by a random phase mask. Therefore, this proposed system can execute recording in the order of MB~GB and readout all personal information from just one card without any additional database system. Also, recorded digital holograms can't be reconstructed without a phase key and can't be copied by using computers, scanners, or photography.
A 42-channel demultiplexer using a volume holographic grating formed in photopolymer medium for dense wavelength division multiplexing (DWDM) applications is experimentally demonstrated for the first time. A 3dB bandwidth of 0.18nm and channel crosstalk of -20dB for a 50GHz channel spacing, is reported.
Holographic multi-channel demultiplexer for the wavelength division multiplexing optical transmission system has been experimentally demonstrated by using photorefractive lithium niobate crystals doped with iron. As a proof of our scheme, we have designed the 8-channel demultiplexer with a channel spacing of 0.8 nm, and some preliminary experimental results are presented and discussed.
A novel photorefractive demultiplexer for dense wavelength division multiplexing (DWDM) applications has been experimentally demonstrated for the first time, by using the narrow-band filtering and demultiplexing properties of a volume holographic gratings formed in lithium niobate crystal. For the multiple recording of the 16-wavelength channels with equal diffraction efficiency, the rotation multiplexing and the exposure time schedule are used. The 1x16 photorefractive DMUX is designed to work with a channel spacing of 0.5nm range from 670nm to 677.5nm and a bandwidth of 0.16nm. From the experimental results, the measured diffraction efficiency of each channel is 8.3+/- 0.62%. The 3dB bandwidth of 0.16+/- 0.005nm and the channel spacing of 0.46 approximately 0.5nm, with optical loss from Fresnel reflection and absorption in the crystal of 2.22dB/cm, are measured.
The independent characteristic of the angular selectivity for the writing conditions was presented in photorefractive grating. In general, the incident angle of writing beam and the interaction length determine the angular selectivity of the grating. When the focused probe beam is incident on the grating, we obtain a new relation that the angular selectivity of the photorefractive grating depends on the probe beam incident conditions and is independent of the writing conditions such as writing beam incident angle, wavelength, and interaction length. These characteristics are experimentally investigated as a function of the following parameters; writing beam incident angle, probe beam width and lens focal length. In the experimental results, it has demonstrated that the control parameters are not the incident angles of writing beam but the probe beam width and focal length of the convex lens. Another property of this configuration is to have same value within large angular range that is also determined by the readout conditions.
A new tuning scheme of the photorefractive filter is presented using LiNbO3 crystal doped with 0.015Wt.% Fe. Analyzing the filter bandwidth property from the geometrical method, a new wavelength selectivity theory is derived. The center wavelength can be tuned by the real-time received beam angle control using the spatial light modulator. As tuning time depends on the response time of the spatia light modulator, it has high tuning speed. The use of thermally fixed single grating makes the uniform diffraction property over all filtering range. Designed tunable filter consists of the three channel with 4 nm bandwidth and 10 nm spacing. From the optical experiment, we get the real-time tunable filter which has 4.5 nm, 4.25 nm, 4 nm bandwidth and 1530.5 nm, 1540.5 nm, 1549.5 nm center wavelength, respectively. Demonstrated filter has a switching time of several tens milli-second and a full tuning range of 20 nm.
In the photorefractive wavelength selective filter, a new tuning scheme is proposed and optically demonstrated. Center wavelength of photorefractive optical filter can be tuned in an electronics speed with a special optical system having LC-SLM and positive lens. Preliminary experimental results in proposed scheme are given, and compared with theoretical values.
The interaction between two incident beams in a photorefractive Cu-doped (Cu-KNSBN) crystal is investigated at 632.8nm He-Ne laser wavelength. In addition, the coherent two-wave coupling properties of a photoinduced volume refractive-index phase grating in the presence of amplitude modulation on the signal beam are also experimentally investigated as a function of the intensity ratio of the two incident beams. Some preliminary experimental result are presented for use as a dynamic photorefractive beam-splitter and pulse shaping element in coherent optical communication systems and in optical signal processing.
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