Dynamic Holographic Medium Research Articles

Super-Fast Refresh Holographic Display Based on Liquid Crystal Films Doped With Silver Nanoparticles

Holography as a true three-dimensional (3-D) technique is thought to be the ultimate display technique. However, dynamic holographic materials still have problems such as slow refresh, high applied electrical field, etc., to be solved to realize video-rate 3-D display. We present silver nanoparticles (Ag NPs) doped liquid crystal films with very big birefringence and super-fast hologram refresh speed as real-time dynamic holographic medium, which can be applied in holographic video display. And the fastest response time of both build-up and self-erasure of hologram can be up to 0.1 ms, which means the material has the ability to realize a smooth holographic video-rate display. The holographic diffraction efficiency dependent on the intensity of recording light, applied electrical field, etc. is investigated. The maximum diffraction efficiency is measured up to 50% under a low applied electrical field of 0.12 V/$\mu$m. Real-time holographic videos at red, green, and blue colors are obtained and the process of hologram formation is analyzed, which shows that the LC films doped Ag NPs can be a good candidate for applications of large-size, real-time, color holographic true 3-D display in future.

Photorefractive moiré-like patterns for the multifringe projection method in Fourier transform profilometry.

In the present paper, the method of simultaneous moiré-like fringe pattern projection for Fourier transform profilometry is described. The photorefractive holographic interferometric process produces controlled moiré-like patterns with two or more different variation directions. Each low spatial frequency fringe pattern is experimentally obtained as a result of the superposition of two high spatial frequency sinusoidal gratings, with slightly different pitches, for each fringe variation direction. These dynamic moiré-like patterns are induced due to an optical holographic beating of the sinusoidal induced gratings in the volume of the photorefractive Bi₁₂TiO₂₀ (BTO) crystal sample used as dynamic holographic medium. Two or more moiré-like fringe patterns, with at least two different variation directions, simultaneously (or not), are projected onto the object surface. Thus, this is the 2D fringe projection stage of our proposed Fourier transform procedure to determine the profile of a simple object.

Single-mode output power enhancement of an extended cavity broad-area laser diode by an intracavity photorefractive crystal

Brightness improvement of high-power semiconductor lasers can be achieved by placing a broad-area laser diode, used as a pure optical amplifier, in an extended cavity. We report the modal behavior of an extended cavity which contains a photorefractive crystal. The dynamic holographic medium that plays the role of a spatial and spectral filter acts on the transverse and axial structure of the extended cavity modes and improves the single-mode operating range of the laser. An enhancement of the brightness in single-mode operation by a factor of seven has been experimentally observed thanks to the intracavity photorefractive crystal.

Enhanced detection of acousto-photonic imaging signals using a photorefractive crystal based system

Acousto-photonic imaging is a dual-wave sensing technique where diffuse coherent light wave interacts with a superimposed acoustic field. A phase-modulated light wave emanates from the interaction region and carries with it information about the local opto-mechanical properties of the insonated media. The coherent nature of the light produces a speckle field. The modulation of the speckle field is spatially incoherent, yielding a small modulation depth when collecting multiple speckles or extremely low light levels when detecting at the single speckle level. We report preliminary results from a new detection scheme where the scattered laser light is mixed with a reference beam in a photorefractive crystal. The crystal serves as a dynamic holographic medium and a photorefractive grating is formed from which the reference beam diffracts. The diffracted reference beam has the same spatial structure as the scattered light, and the two interfere at the photodetector where the phase modulation on the scattered beam is converted to an intensity modulation. Measurements of the signals are presented for gel phantoms seeded with suspended polystyrene beads. The results exhibit qualitative agreement with a simple theoretical model. [Work supported by the Center for Subsurface Sensing and Imaging Systems via NSF ERC Award No. EEC-9986821.]

Holographic scheme for independent component analysis

An optoelectronic analog feedback circuit uses dynamic holography to perform independent component analysis on an array of input signals imposed on a set of optical carriers. The requisite nonlinearity for achieving high-order statistical analysis is established with optical phase modulation in the feedback loop. The dynamics of the system is shown to be driven by the statistical characteristic functions of the input signals and reveals winner-takes-all dynamics when at least one of the signals is sub-Gaussian. Signal correlation and circuit adaptation is carried out by the dynamic holographic medium and therefore takes place in a parallel fashion. The allowed signal bandwidth is limited by the delay time of the feedback loop, thus signal bandwidths exceeding 100 MHz should be possible using relatively standard electronic and optoelectronic technology. Moreover, the scheme can directly accommodate microwave carrier frequencies to several tens of gigahertz.

Photorefractive Materials and Their Applications. Semiconductor Photorefractive Quantum Wells and Their Applications.

Semiconductor photorefractive quantum well (PR-QW) devices have high sensitivity and fast response because of large excitonic electrooptic effects and large carrier mobilities. Taking advantages of these attractive features, interesting applications using PR-QW devices as dynamic holographic media have been demonstrated. Here basic features of PR-QW devices, materials, and device configurations are reviewed. In addition, two important applications of PR-QW devices, adaptive vibration measurement and optical imaging through scattering media, are also discussed.

Rotationally invariant pattern recognition in a photorefractive joint transform correlator using circular harmonic filters and the wavelet transform

A photorefractive joint transform correlator using circular harmonic filters and the wavelet transform for high discriminant and rotationally invariant pattern recognition is reported. In the experimental implementation, a photorefractive crystal, BSO, is used as the dynamic holographic medium and a bank of wavelet functions, derived from a mother Mexican-hat wavelet have been selected to achieve a band-pass operation in the Fourier domain. The zero-order and second-order circular harmonic components are used in the investigation. It is shown that, with the proper dilation factors, there is a trade-off between peak correlation intensity and signal-to-noise ratio (SNR) with the wavelet transform filters. With the second-order circular harmonic, an improvement of 63% in the SNR is achieved with only a 4% loss in the peak correlation intensity.

Image feature extraction with various wavelet functions in a photorefractive joint transform correlator

The wavelet transform has found a lot of uses in the field of optics. We present an experimental realization of employing variant wavelet filters into the object space of a photorefractive joint transform correlator to realize image feature extraction. The Haar’s wavelet, Roberts gradient and Mexican-hat wavelet are employed in the experiment. Because of its good optical properties, the photorefractive crystal Bi 12SiO 20 is used as the dynamic holographic medium in the Fourier plane. Both scene and reference have been detour-phase encoded in a liquid crystal television in the input plane. Computer simulations, experimental results and analysis are presented.

Optical implementation of a photorefractive joint transform correlator with wavelet filters

We have investigated a real-time photorefractive joint transform correlator (JTC) with a bank of wavelet filters. In the experimental set-up, a photorefractive crystal BSO has been used as the dynamic holographic medium and a bank of discrete wavelet functions, derived from a mother Mexican-hat wavelet have been selected to achieve band-pass operations in the JTC Fourier domain. With the proper dilation factors, shaper correlation peaks, higher discrimination ability and higher noise robustness have been obtained. Experimental results and analysis are presented.

Implementation of a photorefractive binary joint transform correlator

The use of photorefractive materials such as Bi12SiO20 as dynamic holographic media is becoming an interesting alternative to that of the current liquid-crystal-based modulators in real-time optical image processing. We present an experimental realization of optical correlation for pattern recognition by means of a photorefractive joint transform correlator. The correlator operates with a liquid-crystal television as the input and a photorefractive crystal at the recording plane. We consider two possible ways of registering the Fourier plane information: conventional detection of the joint power spectrum, and utilization of only phase information at the Fourier plane by suitable preprocessing of the scene and the reference at the object plane. We compare the latter case with the performance of a binary joint transform correlator. Analysis, simulations, and experimental results are presented.

Time-domain image processing using dynamic holography

Coded ultrafast optical pulses can be treated as one-dimensional (1-D) images in the time domain. We have converted spare-domain images into time-domain images using diffraction from dynamic holograms inside a Fourier pulse shaper, with photorefractive quantum wells (QW's) used as the dynamic holographic medium. We present several examples, in which amplitude or phase modulation of the hologram writing beams modifies the complex spectrum of the femtosecond output, resulting in a time-domain image. Both storage and processing of time-domain images can be achieved, depending on the hologram writing geometry and power densities. Time-domain processing operations such as edge enhancement, Fourier transform, and correlation are demonstrated.

New azo-dye-doped polymer systems as dynamic holographic recording media

New azo-dye-doped polymer systems as dynamic holographic recording media

OPTICAL PROCESSING WITH PHOTOREFRACTIVE COMPOUND SEMICONDUCTORS

Photorefractive compound semiconductors are attractive for optical 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 a collection of recent experimental results on optical processing using photorefractive GaAs and InP. The results demonstrate the feasibility of using photorefractive compound semiconductors as dynamic holographic interaction media for optical processing applications.

Real-time Optical Intensity Correlator Using Photorefractive BSO and a Liquid Crystal Television

Abstract We demonstrate the implementation of an optical intensity correlator using Bi12SiO20 (BSO) as a dynamic holographic medium. The advantages of this correlator are large tolerance in the placement of input objects and BSO crystal, and insensitivity to phase inhomogeneities in the readout portion. This allows readout to be conducted via a commercial liquid crystal television.

Optical tracking filter using transient energy coupling

Transient energy coupling between two coherent beams occurs in dynamic holographic media with local and noninstantaneous responses. The physical origin of the effect is described, and an optical tracking filter based on the effect is demonstrated in a photorefractive Bi12SiO20 crystal.