Search

Learning occurs across distributed multimodal experiences. To accumulate knowledge one must integrate related infor-mation across different representational formats i.e. across text and photographs. We extended an established memoryintegration paradigm to test acquisition and integration of knowledge across different representational formats based onart history museum exhibits. Participants received integrable passage pairs in either text-text or text-text+photographformats. Even though the processing demands were higher with photographs, preliminary results indicate no significantdifferences between conditions. Future work will examine potential differences in integration across both context (class-room to museum) and representational formats (text and museum artifacts). We hypothesize that integration across contextand representational format will create higher cognitive demand than integration across representational formats; will thisbe offset by the higher information-value of museum exhibits? This research will provide key insights into multimodallearning and inform best practices for maximizing comprehension in informal learning settings such as museums.

Promoting STEM knowledge early in development helps prepare children for school success. Exposing children to STEMbooks may be a simple and effective means for promoting early STEM knowledge. However, whether preschool-agedchildrens STEM books are optimally designed is unknown. Children and adults learn new information more effectivelywhen there is support for encoding and demand for active processing. We have conducted a textual analysis of 50 STEMbooks designed for preschool-aged children. The books are coded for (a) support for encoding (narratively cohesive andtopic maintaining), and (b) demand of active processing (posing questions and including interactive prompts). Preliminarydata shows that on average the books include limited support for encoding and demand for active processing. This suggeststhat these books are not fulling their potential of promoting early STEM knowledge. Next steps in this research involveidentifying means for enhancing STEM childrens books efficacy.

The study of individuals with autosomal dominant Alzheimer's disease affords one of the best opportunities to characterize the biological and cognitive changes of Alzheimer's disease that occur over the course of the preclinical and symptomatic stages. Unifying the knowledge gained from the past three decades of research in the world's largest single-mutation autosomal dominant Alzheimer's disease kindred - a family in Antioquia, Colombia with the E280A mutation in the Presenilin1 gene - will provide new directions for Alzheimer's research and a framework for generalizing the findings from this cohort to the more common sporadic form of Alzheimer's disease. As this specific mutation is virtually 100% penetrant for the development of the disease by midlife, we use a previously defined median age of onset for mild cognitive impairment for this cohort to examine the trajectory of the biological and cognitive markers of the disease as a function of the carriers' estimated years to clinical onset. Studies from this cohort suggest that structural and functional brain abnormalities - such as cortical thinning and hyperactivation in memory networks - as well as differences in biofluid and in vivo measurements of Alzheimer's-related pathological proteins distinguish Presenilin1 E280A mutation carriers from non-carriers as early as childhood, or approximately three decades before the median age of onset of clinical symptoms. We conclude our review with discussion on future directions for Alzheimer's disease research, with specific emphasis on ways to design studies that compare the generalizability of research in autosomal dominant Alzheimer's disease to the larger sporadic Alzheimer's disease population.

The study of individuals with autosomal dominant Alzheimer's disease affords one of the best opportunities to characterize the biological and cognitive changes of Alzheimer's disease that occur over the course of the preclinical and symptomatic stages. Unifying the knowledge gained from the past three decades of research in the world's largest single-mutation autosomal dominant Alzheimer's disease kindred - a family in Antioquia, Colombia with the E280A mutation in the Presenilin1 gene - will provide new directions for Alzheimer's research and a framework for generalizing the findings from this cohort to the more common sporadic form of Alzheimer's disease. As this specific mutation is virtually 100% penetrant for the development of the disease by midlife, we use a previously defined median age of onset for mild cognitive impairment for this cohort to examine the trajectory of the biological and cognitive markers of the disease as a function of the carriers' estimated years to clinical onset. Studies from this cohort suggest that structural and functional brain abnormalities - such as cortical thinning and hyperactivation in memory networks - as well as differences in biofluid and in vivo measurements of Alzheimer's-related pathological proteins distinguish Presenilin1 E280A mutation carriers from non-carriers as early as childhood, or approximately three decades before the median age of onset of clinical symptoms. We conclude our review with discussion on future directions for Alzheimer's disease research, with specific emphasis on ways to design studies that compare the generalizability of research in autosomal dominant Alzheimer's disease to the larger sporadic Alzheimer's disease population.

This article summarizes the presentations and recommendations of the tenth annual American Geriatrics Society and National Institute on Aging Bench-to-Bedside research conference, "Sensory Impairment and Cognitive Decline," on October 2-3, 2017, in Bethesda, Maryland. The risk of impairment in hearing, vision, and other senses increases with age, and almost 15% of individuals aged 70 and older have dementia. As the number of older adults increases, sensory and cognitive impairments will affect a growing proportion of the population. To limit its scope, this conference focused on sensory impairments affecting vision and hearing. Comorbid vision, hearing, and cognitive impairments in older adults are more common than would be expected by chance alone, suggesting that some common mechanisms might affect these neurological systems. This workshop explored the mechanisms and consequences of comorbid vision, hearing, and cognitive impairment in older adults; effects of sensory loss on the aging brain; and bench-to-bedside innovations and research opportunities. Presenters and participants identified many research gaps and questions; the top priorities fell into 3 themes: mechanisms, measurement, and interventions. The workshop delineated specific research questions that provide opportunities to improve outcomes in this growing population. J Am Geriatr Soc 66:2052-2058, 2018.

The experiments done in this grant can be broadly classified into two stages. The goal of both these stages was to demonstrate all-optical high-speed turbulence aberration correction through nonlinear optical phase conjugation. The first stage involved the development of a phase conjugate mirror (PCM) based on non-degenerate four-wave-mixiag (NDFWM). Coherent population trapping (CPT) in sodium vapor permitted the use of low intensity cw lasers. The second stage involve the development of a phase conjugate resonator (PCR) that could correct intracavity aberrations. In comparison to the PCM using CPT, the PCR was also based on CPT but was degenerate in optical frequency, with a simpler experimental set-up. A high conjugate power gain exceeding 35 was achieved in both the non-degenerate and degenerate FWM configurations. Hi gain is of importance in applications involving wide apertures. Experiments were demonstrated using a helium jet in air operated at a frequency of 18kHz with flow speeds up to 25Om/s. Substantial reduction in the modulation depth of temporal aberration is shown for both the PCM and PCR cases, along with high-fidelity spatial aberration correction.

We have proposed homodyne and heterodyne techniques for imaging in a scattering medium. The first technique is based on the principle of the Doppler shift difference of the light which is scattered from an object vs light scattered by the surroundings. We showed that the principle of Doppler shift difference, and the principle of first arriving light are similar. However, in contrast to first light, which requires ultrafast pulses, here modulated CW can be used. We also experimentally demonstrated the basic principle for a simple case of an object outside of the scattering medium. The second is an extension of the principle of photon density waves, using a two-dimensional (non-scanning) detection scheme.

In this work we presented a two port nonlinear joint transform correlator with two complimentary results. This correlator is based on a two port photorefractive limiting quadratic processor. In the limiting regime we demonstrated experimentally and by computer simulation, that the correlation operation is like a phase extraction correlator operation, regardless of the sign 0 the coupling coefficient. However, for positive coupling coefficients and intermediate beam ratios, one port performs as a phase-only filter and the other as a classical matched filter.

This research program resulted in the development of several new photorefractive devices, and an increase in basic understanding of photorefractive nonlinear interactions. We developed an interferometric phase conjugating ellipsometer which also led to a new highly accurate class of interferometer applicable to polarimetry and distance measurement. The effectiveness of photorefractive beam cleanup was investigated, and found to be most useful for highly aberrated beams. We investigated the feasibility of efficient self aligning coupling of multiple beams simultaneously into a single single mode fiber. This has applications to fiber optic communications. We studied methods of broadening the spectral bandwidth of volume holographic devices by using achromatic techniques. Applications are in self-pumped phase conjugation of trains of femtosecond pulses, and high data rate transmission in optical fibers. The photorefractive beam propagation method was developed as a tool to analyze photorefractive devices and effects involving large bandwidth signals. It was also applied to model amplified photorefractive scattering (fanning) and the double phase conjugate mirror. A successful statistical model of fanning was developed and tested. We have also begun to study photorefractive waveguides, both epitaxially grown and self induced. High resolution X-ray topography was used to investigate photorefractive crystals and waveguides. A new method for nonvolatile data storage in photorefractive crystals was discovered and theoretically analyzed. (MM)

High-resolution synchrotron x-ray diffraction imaging was used to examine the state of crystal perfection in the photorefractive ferroelectric materials barium titanate and strontium barium niobate. Ferroelectric 180 degree domains, dislocations, striations, faceted growth remnants, and other lattice features were identified. The technique was also used to image the photorefractive space charge field profile in-situ. Measurements of the field were compared with numerical simulations of the standard model of the photorefractive effect, and simulations of the x-ray image formation process.

Many optical image processing operations such as edge enhancement may be performed by suitable linear spatial filters. A broader class of operations is made available by extending this technique to nonlinear filters. An example of this is the use of a logarithmic nonlinearity for conversion of multiplicative to additive noise. In this work, we demonstrate that phase preserving thresholding in the Fourier plane which linearly transmits signals above a certain intensity, reduces additive signal dependent noise, such as noise from coherent artifacts, image defects and uniformity noise. Coherent artifact noise, defect noise and film grain noise are often important limitations in the operation of coherent optical processing devices. It can be shown that both of these noise sources contribute a similar spectral effect in the Fourier transform plane: addition of signal dependent noise to the signal transform. This enables a nonlinear optical thresholding mechanism operating in the Fourier plane to separate the relatively intense signal peaks from the noise related smear. This.noise reduction even works with input amplitudes which have a small signal relative to noise. We use the thresholding mechanism of photorefractive two-beam coupling to reduce strong artifact noise in plane wave illumination of a bar chart transparency. There is no noticeable reduction in the resolution of the signal in our measurements. Furthermore, no reference signal is used in this noise reduction technique., This article is from 'Organization of the Optical Society of America Photonic Science Topical Meeting Series. Volume 14. Photorefractive Materials, Effects, and Devices Held in Beverly, Massachusetts on 29-31 July 1991,' AD-A253 001, p400-403.

In photorefractive two-beam coupling the interference of two optical beams creates a refractive index grating resulting in energy transfer between the two beams. In models describing this effect it is usually assumed that both beams have perfect spatial coherence. For some applications such as achromatic volume, holography, multimode fiber gyroscopes and optical phase conjugation through turbulence, spatial coherence effects may become important in the coupling process. Reduced spatial coherence leads lower contrast interference fringes in an interference fringe region smaller than that of the perfectly coherent case. In recent work, we have made a quantitative study of the influence of these effects on coupling under the assumption that the coherence properties of the interacting beams do not change during the coupling. In the present work, we include the influence that beam coupling has on spatial coherence of the interacting beams. It is worth noting that the problem of spatial coherence in nonlinear wave mixing has been studied earlier. However, all of these studies referred to the case of fast Kerr medium. Photorefractive materials are much slower and cannot follow fast changes of the phases of interacting beams. Thus description of the mixing process will be different., This article is from 'Organization of the Optical Society of America Photonic Science Topical Meeting Series. Volume 14. Photorefractive Materials, Effects, and Devices Held in Beverly, Massachusetts on 29-31 July 1991,' AD-A253 001, p101-104.

Signal processing in photorefractive crystals is complicated by photorefractive amplified scattering (fanning). The scattering centers responsible for seeding the fanning noise may be presumed to be traceable to crystal lattice irregularities. These defects have not yet been examined in any great detail, and the relative importance of various specific imperfections and the role they play in the crystal optics are not known. An understanding of these defects should lead to improvement in signal processing through growth of more satisfactory photorefractive crystals. We have set out to address these problems through direct, in situ observation of -photorefractive crystals and their gratings in barium titanate by high resolution monochromatic synchrotron x-ray diffraction imaging. The results that we report here indicate the nature of the principal limiting imperfections and demonstrate the feasibility of direct observation of photorefractive gratings and scattering centers., This article is from 'Organization of the Optical Society of America Photonic Science Topical Meeting Series. Volume 14. Photorefractive Materials, Effects, and Devices Held in Beverly, Massachusetts on 29-3 1 July 1991,' AD-A253 001, p162-165.

This program succeeded in developing several photorefractive devices. One is a distortion compensating phase conjugate interferometer for the measurement of thin film thickness, refractive index and absorption coefficients. Also, we have investigated achromatic volume holography for adaptive optics applications. We have been working also on a device for passive mode locking of semiconductor lasers and have shown self pumped phase conjunction at 1.32 micrometers, the longest wavelength to date at which self- pumped phase conjugation has been observed. An understanding now exists of temporal instabilities in photorefractive phase conjugate mirrors. We are beginning to understand the effects of reduced spatial coherence in photorefractive devices, and using concepts developed in that work, we have analyzed our observations of high diffraction efficiency femtosecond two beam coupling in photorefractive crystals.

Subjects who had undergone complete surgical division of the forebrain commissures for treatment of intractable epilepsy were tested on a variety of cognitive and perceptual tasks. It was found that the right hemisphere performs as well as the left on a test of abstract concept comprehension when the stimulus materials are presented in a non-verbal format. In light of evidence of a selective right hemisphere deficiency for processing abstract words, this result is taken to imply a dissociation of language and cognition at a high level. A second experiment involved the nature of information which can cross subcortically between the cerebral hemispheres. With stimuli presented to opposite visual hemi-fields for prolonged durations, three commissurotomy subjects were able to make matches which convincingly demonstrated interhemispheric transfer and integration of cognitive information, including concrete and abstract concepts. Transfer between the hemispheres was equally successful in the two directions, though the pathway originating in the right and terminating in the left hemisphere may be more sensitive to some affective and semantic components of the stimulus. The information relayed subcortically is neither verbal nor imagic in nature, but appears to involve contextual or connotative associations of the stimulus. Implications for the evolution and development of non-verbal thought include the possible existence of a common bilateral cognitive system which permits interhemispheric communication of complex, if imprecise, associations that are distinct from the more specific verbal and visuospatial constructs of the left and right hemispheres, respectively. Finally, differences in the ability of the two hemispheres to perceive figure and background were described for four commissurotomy subjects. While the left hemisphere preferentially identified figures from briefly-presented picture compositions, the right hemisphere was equally adept at recognizing both figur

Subjects who had undergone complete surgical division of the forebrain commissures for treatment of intractable epilepsy were tested on a variety of cognitive and perceptual tasks. It was found that the right hemisphere performs as well as the left on a test of abstract concept comprehension when the stimulus materials are presented in a non-verbal format. In light of evidence of a selective right hemisphere deficiency for processing abstract words, this result is taken to imply a dissociation of language and cognition at a high level. A second experiment involved the nature of information which can cross subcortically between the cerebral hemispheres. With stimuli presented to opposite visual hemi-fields for prolonged durations, three commissurotomy subjects were able to make matches which convincingly demonstrated interhemispheric transfer and integration of cognitive information, including concrete and abstract concepts. Transfer between the hemispheres was equally successful in the two directions, though the pathway originating in the right and terminating in the left hemisphere may be more sensitive to some affective and semantic components of the stimulus. The information relayed subcortically is neither verbal nor imagic in nature, but appears to involve contextual or connotative associations of the stimulus. Implications for the evolution and development of non-verbal thought include the possible existence of a common bilateral cognitive system which permits interhemispheric communication of complex, if imprecise, associations that are distinct from the more specific verbal and visuospatial constructs of the left and right hemispheres, respectively. Finally, differences in the ability of the two hemispheres to perceive figure and background were described for four commissurotomy subjects. While the left hemisphere preferentially identified figures from briefly-presented picture compositions, the right hemisphere was equally adept at recognizing both figur

Subjects who had undergone complete surgical division of the forebrain commissures for treatment of intractable epilepsy were tested on a variety of cognitive and perceptual tasks. It was found that the right hemisphere performs as well as the left on a test of abstract concept comprehension when the stimulus materials are presented in a non-verbal format. In light of evidence of a selective right hemisphere deficiency for processing abstract words, this result is taken to imply a dissociation of language and cognition at a high level. A second experiment involved the nature of information which can cross subcortically between the cerebral hemispheres. With stimuli presented to opposite visual hemi-fields for prolonged durations, three commissurotomy subjects were able to make matches which convincingly demonstrated interhemispheric transfer and integration of cognitive information, including concrete and abstract concepts. Transfer between the hemispheres was equally successful in the two directions, though the pathway originating in the right and terminating in the left hemisphere may be more sensitive to some affective and semantic components of the stimulus. The information relayed subcortically is neither verbal nor imagic in nature, but appears to involve contextual or connotative associations of the stimulus. Implications for the evolution and development of non-verbal thought include the possible existence of a common bilateral cognitive system which permits interhemispheric communication of complex, if imprecise, associations that are distinct from the more specific verbal and visuospatial constructs of the left and right hemispheres, respectively. Finally, differences in the ability of the two hemispheres to perceive figure and background were described for four commissurotomy subjects. While the left hemisphere preferentially identified figures from briefly-presented picture compositions, the right hemisphere was equally adept at recognizing both figur

We report the results of a theoretical and experimental investigation of a passive (self-pumped) phase conjugate mirror. This device is based on real time holography in materials which allow a spatial phase shift between the refractive index grating and the light interference pattern. An imaging experiment is reported showing the phase conjugating nature of the device. The holographic medium used was a single crystal of barium titanate.

We describe semi-self-pumped photorefractive phase-conjugate mirrors based on the unidirectional ring resonator and the double phase-conjugate mirror. Their applicability to thresholding phase conjugation and one-way imaging through distortions is discussed.

Expressions for the phase of reflection from a photorefractive phase-conjugate mirror are obtained as a function of the intensity and phase of the pump and the probe beams. The phase is independent of these parameters in common photorefractive conditions in which the index grating is spatially shifted 90 degrees with respect to the light-interference pattern. Multiple solutions exist for the phase and intensity of the reflection at large coupling strength. Oscillation conditions involving frequency detuning are obtained for the double phase-conjugate resonator (resonator formed with two phase-conjugate mirrors).

We present an exact solution of a nonlinear vector analysis of two-beam coupling and forward four-wave mixing in photorefractive media.

A theory of phase conjugation in asymmetric materials that allow a phase shift between the grating and the light-interference pattern is developed. We find that when this phase is nonzero, maximum phase-conjugate reflectivity occurs for unequal pump intensities. The conditions for self-oscillation are studied.

Frequency detuning effects in photorefractive oscillators are used in a new type of (passive) interferometry which converts optical path length changes to frequency shifts. Such an interferometer is potentially more accurate than conventional interferometers which convert optical path length changes to phase or intensity changes.

We describe the experimental measurement of phase-conjugate reflectivity versus various ratios of input-beam intensities in photorefractive barium titanate and strontium barium niobate crystals. The experimental results are compared with the theoretical prediction from the coupled-wave theory. Three different methods to measure the nonlinear coupling constant of the crystal are also presented and compared.

An exact solution of a nonlinear model of holographic four-wave mixing is derived. An expression for the reflectivity of a phase-conjugate mirror with depleted pumps is presented. We find that such a phase-conjugate mirror may exhibit bistability.

A GaAlAs semiconductor laser with feedback from a barium titanate photorefractive ring passive phase-conjugate mirror can be made to perform repeating or nonrepeating frequency scans over a 10-nm range toward either the blue or the red. The direction of scanning and whether the scans repeat may be controlled by adjusting the overlap of the interaction beams in the crystal. This overlap region may be adjusted so that the diode frequency spectrum, originally occupying about 10 longitudinal modes, scans and narrows as the conjugate signal builds up, coming to rest often in one, but sometimes two or three, longitudinal modes as a result of self-generated distributed-feedback effects. We also report similar effects caused by feedback from the total-internal-reflection passive phase-conjugate mirror. The alignment-control mechanism of the ring mirror is, however, not available in this case.

We present a plane-wave theory of nondegenerate oscillation in the linear passive (self-pumped) phase-conjugate mirror (PPCM). The circumstances under which the plane-wave theory permits spontaneous nondegenerate oscillation in this and other PPCM's are discussed.

We demonstrate one-way optical field imaging through a distorting medium using a four-wave mixing implementation of real-time holography. Information can be transmitted at an arbitrarily fast rate as long as the mixing medium can respond to changes in the distortion.

The development of a theory of four-wave mixing in photo-refractive crystals is described. This theory is solved in the undepleted pumps approximation with linear absorption and without using the undepleted pumps approximation for negligible absorption. Both the transmission and reflection gratings are treated individually. The results are used to analyze several photorefractive phase conjugate mirrors, yielding reflectivities and thresholds. The use of photorefractive crystals as optical distortion correction elements and experimental demonstrations of several of the passive phase conjugate mirrors are described.

We report here a novel laser resonator with the ability to correct for intracavity phase distortions. The optical cavity employs a passive (self-pumped) phase conjugate reflector to provide this capability.

Passive phase conjugation of up to six lines (457, 476, 488, 496, 501, and 514 nm) of the all lines output of an argon ion laser is reported. Imaging characteristics and reflectivity measurements are given. In general, multiline operation results in some loss in both reflectivity and image resolution. This work opens the possibility for passive phase conjugation of full color images.

A passive phase conjugate mirror based on four-wave mixing in an optical ring cavity is described. Unlike previously demonstrated passive phase conjugate mirrors it generates only one of its pumping beams by nonlinear optical interactions, the other being provided by feedback of the probe after transmission through the nonlinear medium. The results of a theory yielding phase conjugate reflectivity and oscillation thresholds are presented together with an experimental demonstration of phase conjugation in barium titanate and strontium barium niobate. The device is self-starting by four-wave mixing, and has an oscillation threshold lower than that of other previously demonstrated passive phase conjugate mirrors with similar ease of alignment. The operation of a device which generates nonconjugate oscillation beams is also reported.

Bistability and hysteresis have been observed in a photorefractive passive phase conjugate mirror. A threshold basis for the effect is presented and the results of an experimental demonstration of the device are given.

We review theories and experimental demonstrations of oscillation with photorefractive gain. The unidirectional ring resonator; the linear passive phase conjugate mirror, a phase conjugate resonator (the semilinear passive phase conjugate mirror), and the double phase conjugate resonator are treated, the applications in path-length-to-frequency converting interferometers and one-way wavefront converters are described.

We measure the spectral characteristics of an external cavity semiconductor laser which uses a phase conjugate mirror for its external reflection. This device has significant advantages over the conventional external cavity system owing to the self-aligning nature of the phase conjugate mirror. The fiber delay line self-heterodyne technique is used to measure the fundamental linewidth for single mode operation of this device. It shows the linewidth to be at least as narrow as the instrumental resolution of 100 kHz.

A theory of phase conjugation in asymmetric materials that allow a phase shift between the grating and the light-interference pattern is developed. We find that when this phase is nonzero, maximum phase-conjugate reflectivity occurs for unequal pump intensities. The conditions for self-oscillation are studied.

We present an exact solution of a nonlinear vector analysis of two-beam coupling and forward four-wave mixing in photorefractive media.

We present a plane-wave theory of nondegenerate oscillation in the linear passive (self-pumped) phase-conjugate mirror (PPCM). The circumstances under which the plane-wave theory permits spontaneous nondegenerate oscillation in this and other PPCM's are discussed.

An exact solution of a nonlinear model of holographic four-wave mixing is derived. An expression for the reflectivity of a phase-conjugate mirror with depleted pumps is presented. We find that such a phase-conjugate mirror may exhibit bistability.

A GaAlAs semiconductor laser with feedback from a barium titanate photorefractive ring passive phase-conjugate mirror can be made to perform repeating or nonrepeating frequency scans over a 10-nm range toward either the blue or the red. The direction of scanning and whether the scans repeat may be controlled by adjusting the overlap of the interaction beams in the crystal. This overlap region may be adjusted so that the diode frequency spectrum, originally occupying about 10 longitudinal modes, scans and narrows as the conjugate signal builds up, coming to rest often in one, but sometimes two or three, longitudinal modes as a result of self-generated distributed-feedback effects. We also report similar effects caused by feedback from the total-internal-reflection passive phase-conjugate mirror. The alignment-control mechanism of the ring mirror is, however, not available in this case.

We demonstrate one-way optical field imaging through a distorting medium using a four-wave mixing implementation of real-time holography. Information can be transmitted at an arbitrarily fast rate as long as the mixing medium can respond to changes in the distortion.

The development of a theory of four-wave mixing in photo-refractive crystals is described. This theory is solved in the undepleted pumps approximation with linear absorption and without using the undepleted pumps approximation for negligible absorption. Both the transmission and reflection gratings are treated individually. The results are used to analyze several photorefractive phase conjugate mirrors, yielding reflectivities and thresholds. The use of photorefractive crystals as optical distortion correction elements and experimental demonstrations of several of the passive phase conjugate mirrors are described.

Frequency detuning effects in photorefractive oscillators are used in a new type of (passive) interferometry which converts optical path length changes to frequency shifts. Such an interferometer is potentially more accurate than conventional interferometers which convert optical path length changes to phase or intensity changes.

Bistability and hysteresis have been observed in a photorefractive passive phase conjugate mirror. A threshold basis for the effect is presented and the results of an experimental demonstration of the device are given.

We review theories and experimental demonstrations of oscillation with photorefractive gain. The unidirectional ring resonator; the linear passive phase conjugate mirror, a phase conjugate resonator (the semilinear passive phase conjugate mirror), and the double phase conjugate resonator are treated, the applications in path-length-to-frequency converting interferometers and one-way wavefront converters are described.

We describe semi-self-pumped photorefractive phase-conjugate mirrors based on the unidirectional ring resonator and the double phase-conjugate mirror. Their applicability to thresholding phase conjugation and one-way imaging through distortions is discussed.

Expressions for the phase of reflection from a photorefractive phase-conjugate mirror are obtained as a function of the intensity and phase of the pump and the probe beams. The phase is independent of these parameters in common photorefractive conditions in which the index grating is spatially shifted 90 degrees with respect to the light-interference pattern. Multiple solutions exist for the phase and intensity of the reflection at large coupling strength. Oscillation conditions involving frequency detuning are obtained for the double phase-conjugate resonator (resonator formed with two phase-conjugate mirrors).

We report the results of a theoretical and experimental investigation of a passive (self-pumped) phase conjugate mirror. This device is based on real time holography in materials which allow a spatial phase shift between the refractive index grating and the light interference pattern. An imaging experiment is reported showing the phase conjugating nature of the device. The holographic medium used was a single crystal of barium titanate.

Passive phase conjugation of up to six lines (457, 476, 488, 496, 501, and 514 nm) of the all lines output of an argon ion laser is reported. Imaging characteristics and reflectivity measurements are given. In general, multiline operation results in some loss in both reflectivity and image resolution. This work opens the possibility for passive phase conjugation of full color images.

We report here a novel laser resonator with the ability to correct for intracavity phase distortions. The optical cavity employs a passive (self-pumped) phase conjugate reflector to provide this capability.