Masatoshi Bunsen

Method of Phase and Amplitude Modulation/Demodulation Using Datapages with Embedded Phase-Shift for Holographic Data Storage

A technique for the phase and amplitude detection of object beams with multivalued phase and amplitude modulation is proposed for holographic storage systems. Generally, the spatial distribution of the complex amplitude of the object beam can be precisely detected by phase-shifting interferometric measurements in which the phase of the reference wave for interferometry is temporally or spatially changed in the datapage retrieval process. On the other hand, our technique allows fast, accurate, and feasible phase and amplitude demodulations by preliminary embedding phase shift into the phase signal of the datapage during recording. This technique will significantly improve the data transfer rate and vibration tolerance of the holographic storage system because the complex amplitudes of the object beam carrying datapages can be detected by single-shot image capturing. The optical system for datapage replay will also be simplified because there is no need to use any phase-shifting device during data retrieval. The single-shot detection of the phase-modulated datapage is experimentally demonstrated.

Complexity and bandwidth enhancement in unidirectionally coupled semiconductor lasers with time-delayed optical feedback.

We numerically investigate the frequency bandwidth and the autocorrelation characteristics of chaotic temporal wave forms in unidirectionally coupled semiconductor lasers with time-delayed optical feedback. We evaluate the complexity of the chaotic temporal wave forms by using Lyapunov exponents. We found that larger maximum Lyapunov exponents can be obtained for smaller peak values of the autocorrelation function at the delay time of the optical feedback. On the contrary, the maximum Lyapunov exponent is independent from the frequency bandwidth of the chaotic temporal wave forms.

Theoretical and experimental studies of hologram multiplexing that uses a random wave front generated by photorefractive beam fanning

A hologram multiplexing technique-that uses random wave fronts generated by photorefractive beam fanning is investigated. A storage photorefractive crystal generates various random wave fronts to be used as reference beams without the external diffusers such as ground glass and multimode optical fiber that are generally employed. We experimentally demonstrate hologram multiplexing with six images and show that the stored holograms can be selectively retrieved. We also simulate photorefractive beam fanning inside a BaTiO3 crystal, in particular regarding the correlation properties of the fanning beams for the first time to our knowledge, and reveal the conditions of incidence of an object beam and a reference beam required for suppressing image degradation, implementing low-cross-talk retrieval, and producing a large number of stored holograms.

Symbol Error Characteristics of Hybrid-Modulated Holographic Data Storage by Intensity and Multi Phase Modulation

For the purpose of increasing the data density of holographic memories, we propose a novel signal modulation method named hybrid modulation method, which is a combination of multi level phase-modulated signals and intensity-modulated signals. As a first step, we show that the data density of holographic data storage is mainly determined by multiplexing numbers and by coding efficiency in input signals. In this study, we focus on increasing coding efficiency using the hybrid modulation method. From the results of recording and reading simulations based on the fast-Fourier transform beam propagation method, the hybrid modulation method achieves approximately 2 times higher data density than the conventional methods of two-level phase-only modulation and two-level intensity-only modulation.

Improved Holographic Recording Techniques for Data-Page Rewriting

Novel holographic recording configurations capable of selective erasure of multiplexed holograms for data-page rewriting in holographic storage are presented. By using a half wave plate and a polarizing beam splitter as the phase control device of the interference pattern in holographic recording/erasure procedure, comparatively large-area and finespun data-pages may be recorded, selectively erased, and rewritten in simple operations. We experimentally demonstrate the selective erasure of multiplexed data-pages in photorefractive iron-doped LiNbO3 crystals, and show the effectiveness of the configuration. We also propose a method to improve the in-plane uniformity of the erasure making use of the diffuser in the optical path of the object beam which is employed for dispersing the object image and the phase conjugate readout technique.

Selective Erasure of Holograms in Photorefractive and Photopolymer Materials for Rewritable and Secure Data Storage

A selective erasure/update method of multiplexed photorefractive and photopolymer holograms by use of the double Mach–Zehnder (DMZ) interferometric arrangement is proposed. The DMZ arrangement is used as the optical phase-control device for holographic recording/erasure/update operations, which can produce a pair of π-phase-shifted interference patterns with a simple optical setup. We intend to erase the holograms recorded in not only the inherently rewritable photorefractive crystal but also the photopolymer material generally used for the write-once (i.e., unerasable) holographic recording. It is experimentally confirmed that our method can erase the write-once type photopolymer holograms as well as the photorefractive holograms, although there will be a certain degree of restriction on the rewrite cycles limited by the monomer density, the degree of multiplexing and so on.

Selective erasure of speckle-multiplexed holograms by use of a double Mach-Zehnder interferometric arrangement.

A novel method to selectively erase and update speckle-multiplexed holograms in photorefractive crystals by use of a double Mach-Zehnder (DMZ) interferometric arrangement is presented. The DMZ arrangement automatically produces a pair of pi-phase-shifted interference patterns used for holographic recording, erasure, and update operations with a fairly simple optical configuration that consists of a commonly used dielectric multilayer beam splitter and two mirrors. The recording and the erasure conditions required for erasing a photorefractive hologram quickly and completely are discussed by calculating the temporal property of the hologram buildup and decay using the time-dependent coupled-wave equations. An experiment is also performed, in which arbitrary holograms in speckle-multiplexed holograms are selectively erased and updated with the simple DMZ optical configuration.

Experiment on selective erasure for multiplexed holograms by using photorefractive phase-shift

We propose a method of the selective erasure for multiplexed holograms by using photorefractive phase-shift. This method realizes the complete selective erasure with simple operation. We conduct an experiment on selective erasure with two LiNbO3:Fe.

Hologram multiplexing method with photorefractive beam-fanning speckle

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.

Fault-tolerant automatic routing method with two photorefractive double phase conjugate mirrors

We propose a fault-tolerant automatic routing method with two photorefractive double phase conjugate mirrors for a free space optical communication. This method offers automatic routing of a signal beam from the main line to the backup line for the cutoff of the main line by obstacles. The link between the transmitter and the receiver is kept efficiently without any complex optical configuration and electronic devices even if the optical path shifts or tilts. We analyze the temporal response of the signal beam intensity on the main line and the backup line to confirm the automatic routing function, and conduct a basic experiment with a BaTiO/sub 3/ crystal and Ar/sup +/ laser.