We present a discussion of the physics and nonlinear optical properties of liquid crystals in processes and phenomena where photorefractive crystals are currently being applied. The broadband birefringence and nonlinearities of liquid crystals spanning the visible to the far infrared, and recent successful observation of novel nonlinear optical phenomena in bulk liquid crystalline film or optical fiber, show that they are potentially very useful materials for technological applications.
Volume phase gratings in photorefractive crystals have nonuniform amplitude and phase due to the energy exchanged by the writing beams within the material. Analytic expression is obtained for the diffraction efficiency of a weak reading beam that does not perturb the grating and has a different polarization from the write beams. For a read beam of arbitrary intensity, the diffraction efficiency is a nonlinear function of the read beam intensity and is nonreciprocal with respect to readout from the two input ports. These properties of photorefractive gratings are studied for arbitrary phase shifts of the index grating from the intensity pattern.
The photorefractive and electrical properties of ferroelectric tungsten bronze (T.B.) materials are reviewed with respect to optical data storage applications. Our work indicates that both the photorefractive properties (speed and coupling) and dark conductivity are sensitive to the type of T.B. host, dopants, and structural defects. The filled-lattice T.B. crystals exhibit faster response and lower dark conductivities, which are needed for data storage applications, as compared to unfilled T.B. crystals. Concepts for 3-D holographic memory applications using these materials arc discussed.
In this invited paper we discuss recent advances in holographic data storage using ferroelectric SBN fibers as the recording medium. An optical architecture involving an array of SBN fibers is discussed that potentially allows 2-3 orders of magnitude faster access times than for conventional magnetic data storage devices of Gbyte-size . To realize this potential we are studying the interplay between fundamental materials issues related to fiber growth and photorefractive processes underlying storage and readout processes in SBN.
Two key obstacles to the development of high-capacity three-dimensional information mass storage systems using photorefractive crystals are: (1) readout of holograms stored in the conventional manner is destructive, and (2) undoped photorefractive crystals are typically quite insensitive to red and infrared write light from diode lasers (the preferred source). As a consequence, bulky, expensive ion lasers have been necessary for hologram writing. Thus hologram fixing and high infrared sensitivity in these materials are both important if practical systems are to be built. We present our own high-temperature (80-110°C) photorefractive grating writing results for a variety of pure and doped barium titanate (BaTiO3) crystals. We found strong fixable secondary gratings that correlated with increasing levels of Fe and Ni in doped crystals. Fixing was not observed in our pure crystals or those doped with V, Rh, or Co. Fixing was enhanced in an iron-doped crystal reduced by high-temperature annealing at low oxygen partial pressures. We also found that the readout diffraction efficiency of a fixed grating in BaTiO3 is a rapidly increasing function of readout temperature above a threshold temperature, which depends upon the crystal orientation. Lower temperatures lead to longer storage times, as would be expected if the carriers forming the fixed grating have a thermally activated mobility. We also report preliminary experimental results on blue Rh-doped BaTiO3 crystals with fast and high-gain infrared (840 nm) response.
Proc. SPIE 10270, Holographic storage and interconnection using a (Ce:Fe: doped LiNbO3) photorefractive crystal fiber with a tunable visible-light diode laser, 102700A (28 April 1994); doi: 10.1117/12.178631
In this paper, we shall briefly review some recent works done at the Pennsylvania State University on the photorefractive (PR) fibers and their applications. Firstly, a laser heated pedestal growth (LHPG) technique to grow single crystal PR fiber is briefly described. A technique of constructing phase conjugate PR fiber hologram is introduced. Angular and wavelength selectivities for the fiber holograms are evaluated, in which we have shown that the reflection-type wavelength-multiplexed fiber hologram offers a uniform and higher selectivity, as in contrast with the transmission-type. Cross-talk noise for reflection-type wavelength-multiplexed fiber hologram is also calculated, in which we have shown that the noise level would be lower by using a narrower linewidth of the light source. An application of the PR fiber hologram to optical interconnection is advocated, for which a 5-dimensional holographic memory is addressed. Finally, a simple experimental demonstration using a Ce:Fe: doped LiNb03 PR fiber with a tunable visible-light diode laser is given, in which we have expressed that the potential application to large capacity holographic memory, reconfigurable interconnects, and many others are possible.
Periodic copying and electrical fixing are presented as two practical methods for controlling the decay rate of volume photorefractive holograms in the implementation of optical image processing systems.
Photorefractive materials are useful for optical interconnection and data storage in optoelectronic computing systems because of their unique properties (e.g., writelreadlerase capability, real time wave mixing, and high storage capacity). In this paper we will review the recent advances in these two application areas that make use of these properties and the techniques for improving their real-time and storage performance.
Heavily iron doped lithium niobate photorefractive crystals are grown and characterized. The crystals have high diffraction efficiency and fast temporal response and hence are suitable for real-time image processing. Three applications of thin Fe: LiNb03 plates are demonstrated: the joint-transform correlator, the optical perception, and the hybrid image classifier. In the first and third systems, the heavily doped (0.5% mole) crystals are used. These crystals have fast temporal response (300-400 msec) and thus provide a means for real time optical processing. In the second system, a slightly light doped (0.1% mole) crystal is used. It has relatively long (> 1 minute) response time and is used for recording the dynamic memory in a neural network. Image recognition by these three systems are presented.
After a rapid introduction that indicates the main reasons for the renewal of interest for photorefractive holographic memories, we will first present and compare both the coding techniques and recording procedures used for storage of superimposed images. Potentialities and limitations that are relevant to existing photorefractive crystals will be then discussed. Refreshing procedures that allow to operate the dynamic memory without loss of information will be also described. We will then devote the major part of the article to discussing applications we envision for photorefractive holographic memories.
Photorefractive semiconductors are attractive for information processing, because of fast material response, compatibility with semiconductor lasers, and availability of cross polarization diffraction for enhancing signal-to-noise ratio. This paper presents recent experimental results on information processing using photorefractive GaAs, InP and CdTe, including image processing with semiconductor lasers. The results demonstrate the feasibility of using photorefractive compound semiconductors as dynamic holographic interaction media for information-processing applications in low-power, compact configurations.
This paper surveys various approaches that have been proposed or demonstrated for optical interconnections using photorefractive holograms. Some material issues, technical challenges, and competing technologies are discussed.
Holographic implementations of neural networks using photorefractive crystals and vertical-cavity surface-emitting microlaser arrays are described. This paper concentrates on the nonlinear thresholding elements (optical neurons), an associative memory using time-division multiplexing, and image transmission through a single mode fiber.
Applications of photorefractive devices for radio frequency signal processing problems are reviewed. The problems of signal correlation, filtering and adaptive filtering are addressed using the time integrating property of grating formation in photorefractive crystals. The dynamic range of photorefractive crystals is sufficiently large for critical applications such as adaptive signal processing. New system architectures that use photorefractive two-beam coupling to substantially simplify overall complexity are shown and demonstrated for the problems of tunable notch filtering and adaptive notch filtering.