Kelvin H. Wagner

Adaptive optical networks using photorefractive crystals

The capabilities of photorefractive crystals as media for holographic interconnections in neural networks are examined. Limitations on the density of interconnections and the number of holographic associations which can be stored in photorefractive crystals are derived. Optical architectures for implementing various neural schemes are described. Experimental results are presented for one of these architectures.

Multilayer optical learning networks

A new approach to learning in a multilayer optical neural network based on holographically interconnected nonlinear devices is presented. The proposed network can learn the interconnections that form a distributed representation of a desired pattern transformation operation. The interconnections are formed in an adaptive and self-aligning fashioias volume holographic gratings in photorefractive crystals. Parallel arrays of globally space-integrated inner products diffracted by the interconnecting hologram illuminate arrays of nonlinear Fabry-Perot etalons for fast thresholding of the transformed patterns. A phase conjugated reference wave interferes with a backward propagating error signal to form holographic interference patterns which are time integrated in the volume of a photorefractive crystal to modify slowly and learn the appropriate self-aligning interconnections. This multilayer system performs an approximate implementation of the backpropagation learning procedure in a massively parallel high-speed nonlinear optical network.

Pulsed ultrasound-modulated optical tomography using spectral-hole burning as a narrowband spectral filter

We applied a submegahertz nonlinear optical filter afforded by a cryogenically cooled spectral-hole burning crystal to ultrasound-modulated optical tomography. Our experimental results show that this technique, having the largest etendue among all available ultrasound-modulated optical tomography techniques and being immune to speckle decorrelation, offers potential for imaging in vivo and forming high resolution optical tomograms in real time. It opens an opportunity for the development of a clinically applicable high resolution optical imaging modality.

Holographic diffraction in photoanisotropic organic materials

A theoretical study is presented that describes the recording and the readout of transmission volume holograms in dynamic photoanisotropic organic materials. The stationary coupled differential equations of two recording beams that are polarized perpendicular to the plane of incidence are derived and solved analytically for the beam intensities and the phase changes. These equations self-consistently satisfy the recording dynamics of the hologram formation including the effects of fringe curvature and energy transfer. Analytic formulas for the diffraction efficiency are obtained from the readout equations under the assumption of a Bragg-matched weak probe beam of arbitrary polarization and in the small fringe curvature regime. Previously measured parameters of Methyl Orange–polyvinyl alcohol holograms indicate that the optimal hologram thickness is approximately 0.2 mm and predict the decrease of diffraction efficiency with an increase of beam ratio.

Time-of-flight cross correlation on a detector array for ultrafast packet detection

We describe and demonstrate an interferometric technique for measuring the first-order cross correlation of ultrafast optical pulses. This technique may permit single-shot pulse detection and is applicable to receivers for time-domain optical communications.

Photoanisotropic incoherent-to-coherent optical conversion

A new approach to incoherent-to-coherent optical conversion based on a real-time five-wave-mixing technique in photoanisotropic organic film is presented. A uniform grating is written holographically in the sample and then erased locally by an incident white-light image. Subsequent coherent diffraction of the spatially modulated grating imposes the incoherent image upon the reading laser beam, permitting subsequent coherent optical processing. A theoretical analysis of the holographic recording and erasing mechanism in these photoanisotropic materials is presented, and the saturation is shown to be responsible for the grating intermodulation that produces the incoherent-to-coherent conversion. Experimental results of white-light images converted to inverted coherent images in real time are presented, and the resolution is shown to exceed 28 line pairs/mm.

Spectral hole burning for wideband, high-resolution radio-frequency spectrum analysis

We present experimental results for what is to our knowledge the first spectral-hole-burning based rf spectrum analyzer to cover 10 GHz of rf analysis bandwidth. The rf signal of interest is modulated onto an optical carrier, and the resultant optical sidebands are burned into the inhomogeneously broadened absorption band of a Tm3+:YAG crystal. At the same time a second, frequency-swept laser reads out the absorption profile, which is a double-sideband replica of the rf spectrum, and thus the rf spectrum can be deduced after spectral calibration of the nonlinear readout chirp. This initial demonstration shows spectral analysis covering 10 GHz of bandwidth with >5500 spectral channels and provides 43 dB of dynamic range.

Diffraction analysis of photoanisotropic holography: an anisotropic saturation model

The photoisomerization of a highly anisotropic dye molecule depends not only on the intensity but also on the polarization state of the acting light with respect to the molecular axis. Polarization-dependent isomerization of uniformly oriented distributions of anisotropic dye molecules produces an orientational distribution of photoexcited states. Saturation occurs when all the molecules aligned in a particular orientation are isomerized so further photoexcitation cannot occur. This anisotropic saturation occurs first in the direction aligned with the incident polarization. We investigate this anisotropic saturation behavior and present a theoretical model that incorporates intensity saturation for describing holographic diffraction in dynamic photoanisotropic organic materials. Numerical simulations of diffraction versus intensity and polarization are provided and compared with the experimental results.

Coupled mode analysis of polarization volume hologram

We theoretically analyze polarization holographic recording based on photoinduced dichroism and birefringence. Such vectorial recording is achieved by preferentially transforming dye molecules aligned with the incident polarization in dynamic photoanisotropic organic materials. Orthogonal linear and orthogonal circular recording geometries, which produce only pure polarization modulations, are considered. The analytic solutions of the intensity and phase of the recording beams at steady state are derived. The theoretical results indicate the dependence of diffraction efficiency on the photoinduced dichroism and birefringence and on the polarization state of the probe beam. Results of numerical simulation are provided and compared with the analytic solutions. >

Asymmetric spatial soliton dragging

A new low-latency, cascadable optical logic gate with gain, high contrast, and three-terminal input-output isolation is introduced. The interaction between two orthogonally polarized spatial solitons brought into coincidence at the boundary of a saturating nonlinear medium and propagating in different directions results in the phase-insensitive spatial dragging of a strong pump soliton by a weaker signal. As a result, the strong pump is transmitted through an aperture when the weak signal is not present, and it is dragged to the side by more than a beam width and blocked in the presence of the weak signal, thus implementing an inverter with gain. A multi-input, logically complete NOR gate also can be implemented in a cascaded system.