In this paper, we present a novel technology for photonic cross-connect (PXC) in spatial mode domain for the realization of advanced and flexible optical transmission of spatial modes. The PXC is a kind of all -optical devices to switch highspeed optical signals for mode-division multiplexing (MDM) network and it is able to perform signal labeling in the spatial mode domain similar to current photonic switching in the wavelength domain. In addition, parallel and simultaneous mode conversion can be realized using multiplex holograms in a photorefractive crystal (PRC). In our experiment, during the recording process, a rewritable hologram is recorded in the PRC (LiNbO3) through the interference between the signal beam with certain input mode and the reference beam with the phase distribution of the desired output mode. Signal beams are generated by computer generated hologram (CGH) using a spatial light modulator (SLM) instead of an optical fiber emergent beam, and reference beams are generated by phase only modulation using another SLM. Subsequently, during the converting process, the input signal beam is converted into the desired output mode through the holographic diffract ion in the crystal and free-space propagation by an optical lens. By using phase code multiplexing method, parallel mode conversions can be realized. We performed an experiment on parallel mode conversions of several different conversion pairs. Signal beams and reference beams intersected in the PRC with an angle of 18.43 degree. The intensity distributions of converted modes were observed by CCD camera set on the Fourier plane. We confirmed that the two modes inter-conversion of LP11 with LP21 was successfully implemented.
Volume holographic recording is a promising solution for next- generation optical disc storage that has a high capacity more than 1 TB. This huge capacity is achieved by superimposing many holograms, each of which has millions of bits, at the same recording spot. We proposed a new technique, Spatial Spread Spectrum (SSS) multiplex recording. Unlike conventional multiplex holography based on Bragg effect of thick holograms, our technique utilizes spatial phase modulation and demodulation of the signal beam itself with a random diffuser to address the multiplexed page data. SSS multiplexing is additionally combined with other multiplexing methods, and provides further improvement of the total capacity of holographic storage. In this paper we experimentally verify the basic recording and readout feasibility, and investigate the shift selectivity and the aligning margin of the SSS holographic recording that are an important factor to determine the tolerance against vibration. It is shown that a clear 2-dimensional image is successfully reconstructed from the hologram even in the case the central part of the diffused signal beam is blocked in recording, and that a sharp shift selectivity about 5 microns was obtained by a diffuser with a diffusion angle of 15 degree, and the aligning margin for a sufficient SNR was approximately 1 micron.
In this paper, we propose a selective erasure method for multiplexed holograms in a photorefractive crystal using a phase conjugate mirror (PCM). In this method, for the accurate selective erasure, we use pai phase shift between the relative phase of the diffracted beam to the transmitted beam by the hologram in photorefractive crystal and the relative phase of the object beam to reference beam in recording process by setting the direction of c-axis to the appropriate direction. In the selective erasure process, the diffracted beam and the transmitted beam by the original hologram in main memory are returned to main memory by PCM. These returned beams automatically propagate through the strictly same pass as the diffracted beam and the transmitted beam. Furthermore, by these returned beams, the pi-phase shifted hologram is overwritten on the original hologram by setting c-axis of main memory to the appropriate direction. Therefore, the pi-phase shifted hologram is automatically overwritten on the strictly same position as the original hologram without the high precise alignment and the original hologram is selectively erased. We perform the analysis and the experiment on selective erasure and demonstrate that one of the multiplexed holograms is selectively erased with our method.
We propose an optically controllable device Photorefractive Connection Module (PRCM) for free-space optical interconnection between boards. The optical signal composed of a large number of spatially divided channels passes through the photorefractive material which is core of PRCM. In the photorefractive material, diffraction gratings are induced only where the control beam and the pump beam are illuminated. The signal beams in desired spatial channels can be diffracted by setting an appropriate pattern of the control beam and extracted by separating from the control beam with a beam splitter. In this study, we use organic photorefractive polymer PATPD as the photorefractive material. Diffraction efficiency of PATPD is comparable with thick inorganic crystals although the thickness of PATPD is less than 1/10 of these crystals. Thin gratings have a large advantage for PRCM because the thickness of the gratings causes large crosstalk between spatial channels. In addition, the constraint of phase matching is reduced because the phase mismatch is proportional to the thickness of the material. The decrease of the phase mismatch provides large allowance for misalignment of incident angle of beams; therefore it becomes easy to adjust incident angles. We reveal the relation of diffraction efficiency to angular difference of PATPD by analysis and experiment. Diffraction efficiency decreases by half at the difference of 0.5o, which is about ten times larger than that of thick crystals. We demonstrate that it is possible to extract the signal beams in desired channels and reconfigure the extraction pattern according to optical control by using PATPD.
We propose an all-optical demultiplexer with grating array in a photorefractive crystal to achieve high density board-to-board optical interconnects with wavelength division multiplexing (WDM). WDM signal emitted from a board can be branched for each wavelength component and the components are connected to different spatial ports by this demultiplexer mounted on another board. Index gratings, which are induced by illumination of two control beams, are aligned spatially along the signal beam line in a photorefractive crystal. The wavelength component of WDM signal is diffracted by an index grating only when the phase-matching condition is satisfied. This condition is determined by incident angles of control beams and the wavelength of the signal beam. By setting incident angles of control beams appropriately, it can be determined whether a wavelength component of WDM signal is diffracted by the index grating or not. The component can be connected to the desirable output port by arraying the gratings with different grating vector. In our previous study, we have shown that this demultiplexer can branch WDM signal for each wavelength component. In this report, we show that the wavelength selectivity of 0.2nm can be achieved by optimizing the angles of input beams in this demultiplexer. Moreover, we propose a new system in which WDM signal is branched for each wavelength component by angularly multiplexed gratings. By using this multiple grating we can prevent demultiplexer module from growing in size with increasing the multiplicity of signal beam unlike grating array.
By irradiating alternative external light into the system with four-wave mixing in photorefractive polymers, the restriction on the generation of a phase conjugate beam or diffracted light will be demonstrated in this experiment. In addition, in order to verify the use of a two-dimensional array for simultaneous switching, the elimination of only part of diffraction grating recorded will be experimentally conducted.
The purpose of this study is to apply a free-space optical interconnection to a reconfigurable board-to-board connection where the wiring patterns connecting boards are optically formed without electrical-optical conversion. We regard a photorefractive bi-directional connection module (PBCM) based on a mutually pumped phase conjugate mirror as a key device to construct such a connection network and employ PBCMs at input/output interfaces of each board. Although optical behaviors of PBCM are influenced by the exposure conditions, we especially focus on the diameter of beams illuminating photorefractive media placed inside PBCM so as to find some geometrical restrictions in a design of networking system. Through numerical analyses, we show a sample configuration of PBCM for the board-to-board interconnection and present a conceptual design of input/output interface.
We have proposed a fault-tolerant automatic routing method with two photorefractive double phase conjugate mirrors (DPCMs) for free space optical communication by now. In this method, a signal beam can be all-optically and automatically switched from a main line to a backup line when the main line is shut off by obstacles. The optical link between a transmitter and a receiver is kept without any electronic devices and complex optical configuration because the adequate communication line is automatically selected by two DPCMs which are generated by the signal beam and support beams in one photorefractive crystal. In this report, we equalize the signal beam intensities on the main line and the backup line to increase the reliability of communication. If a coupling strength ratio between two DPCMs is inappropriate, the signal beam intensities on both lines become inequality and this induces the increase of the bit error rate in beam detection. Therefore, it is necessary to adjust the signal beam intensities by changing the coupling strength ratio between two DPCMs. We show that the signal beam intensities on both lines can be equalized completely by using the optimum coupling strength ratio between two DPCMs, e.g. about 1.28 in BaTiO3 crystal.
The volume holographic data storage has been attractive because of its large capacity due to the three-dimensional recording and the fast data transfer rate due to the two-dimensional retrieving. Among various holographic storage media, photorefractive crystals are expected because of rewritability. To enhance the storage density, several multiplexing techniques have already been suggested. Especially, the speckle multiplexing technique allows the easy implementation of the hologram multiplexing by the simple optical setup, in which only a diffuser such as the ground glass or the multimode optical fiber is embedded to generate the deterministic speckle-encoded reference beam.
In this paper, we propose a hologram multiplexing method with the reference beam speckled by the photorefractive beam-fanning effect. Compared with the conventional speckle multiplexing method, this method realizes the facile implementation of the speckle multiplexing with the simple and compact optical setup because the external apparatus for generating the speckled reference beam is not required. A bulk photorefractive crystal takes the role of generating various speckle fields as well as storing holograms. All-optically controlled hologram multiplexing and retrieving would also be realized because the beam-fanning effect can be controlled only by light. We demonstrate the hologram multiplexing and the selective retrieving with the speckled reference beam generated by the beam-fanning effect in a photorefractive BaTiO3 crystal. The speckle field of the reference beam to record multiple holograms is varied by changing the boundary condition of the incident beam such as the incident position and the spatial phase distribution. We can successfully reconstruct the desired image from four multiplexed holograms with the reference beam speckled by the photorefractive beam-fanning effect in both procedures.
We propose an all-optical free-space interconnection with a photorefractive grating array for wavelength division multiplexing (WDM) signal. Input WDM signal propagating on free-space can be branched for each wavelength component and the components are connected to different spatial ports by this interconnection. The wavelength component of WDM signal is diffracted by one index grating, which is induced by two control beams, only when the phase-matching condition is satisfied. The phase-matching condition is determined by incident angles of control beams and the wavelength of the signal beam. By setting incident angles of control beams appropriately, it can be determined whether a wavelength component of WDM signal is diffracted by the index grating or not. The component can be connected to the desirable spatial output port by arraying the gratings with different grating vector. We analyze the diffraction efficiency to investigate the wavelength selectivity of this interconnection and investigate the influence of the multiplicity of the signal beam on the diffraction efficiency. As the results, the wavelength selectivity 0.7[nm], which is equivalent to that of the conventional arrayed-waveguide grating, is achieved when the waveband is 500-520[nm]. Moreover the diffraction efficiency 0.055 is obtained when the degree of multiplexing is 50.
In this paper, we propose a photorefractive disk-type memory with cross polarized four-wave mixing, and suggest a method to enhance the storage density for this disk-type memory. The shift multiplexing technique, in which the feasible recording of the angularly multiplexed holograms can be implemented by means of the spherical reference beam and the rotation of the disk-shaped crystal, is applied for this memory. We analytically investigate the amounts of shift required for multiplexing differential holograms in this memory, and we carry out an experiment to verify the analytical results. Also we suggest a logical multilayer recording method as the novel scheduled recording method, and analyze the temporal property of cross polarized four-wave mixing to estimate the diffraction efficiencies from the multiplexed holograms with the method. From those analyses, it is revealed that the diffraction efficiency and the storage density of the photorefractive disk-type memory are significantly improved by this method.
We propose an all-optical encryption memory using the photorefractive four wave mixing and random phase masks. In our encryption method, the image data are encrypted to white noise by the phase shift patterns added on the signal beam with the image data and the reference beam through the random phase masks. Only reading beam with the phase-conjugated distribution of the reference beam can restore the encrypted data. If the encrypted data are read out by incorrect phase pattern, the output data can't be obtained because of the wavefront mismatch between the recorded hologram and the reading beam. We evaluate the encryption and decryption process by analyzing the diffraction efficiency with consideration of the angular spectrum. We show that high performance encryption that high contrast and high gradation image data can be decrypted accurately and the output data are perfectly dark by using the incorrect decryption key can be realized theoretically.
In this paper, we propose the enhancement technique of the storage density in the disk-type memory with photorefractive cross polarized four-wave mixing. We calculate the amounts of shift that enable us to distinguish adjacent signals in the directions of the circle and the radius of the disk, and we carry out experiments to verify the analytical results. Also we investigate the temporal property of cross polarized four-wave mixing to estimate the optimum exposure time.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.