Photorefractive time differentiation of coherent optical images

Up-converted photorefractive optical transient detection with femtosecond laser pulses

We report on experimental demonstration of optical transient detection (OTD) based on photorefractive two-wave mixing of femtosecond pulses. The demonstrated technique also combines nonlinear-crystal-based OTD with up-conversion from infrared into the visible range. The approach enables measurement of phase changes of a dynamic signal in the infrared using GaP- or Si-based detectors while suppressing stationary background. Experimental results reveal existence of the relation between input phases in the infrared and output phases in the visible wavelength range. We further present experimental evidence of additional merits of up-converted transient phase analysis under noisy conditions, such as residual continuous-wave emission affecting the ultrashort pulses from the laser.

Interferometric measurement of complex-field changes in transient detection imaging

We report an experimental method that combines nonlinear-crystal-based transient detection imaging (TDI) with interferometric complex-field retrieval. The system allows measuring both phase and amplitude of a dynamic scene while suppressing stationary background. Theoretical and experimental results prove the linear relation existing between input and output phases, as well as the benefits of phase analysis for both detection and measurement with high resolutions of λ/30, even under noisy conditions. Additionally, we present experimental evidence of the remarkable ability of the technique to detect phase sign changes in the scene -what we call differential-phase TDI- showing great detection sensitivity and no calibration requirements.

Time-bandwidth problem in room temperature slow light

For many applications of slow or stopped light, the delay-time-bandwidth product is a fundamental issue. So far, however, slow-light demonstrations do not show a large delay-time-bandwidth product, especially in room temperature solids. Here we demonstrate that the use of artificial inhomogeneous broadening has the potential to solve this problem. A proof-of-principle experiment is done using slow light produced by two-beam coupling in a photorefractive crystal Ce:BaTiO3 where Bragg selection is used to provide the artificial inhomogeneity. Examples of how to generalize this concept for use with other room temperature slow-light solids are also given.

Relationship between parameters of bacteriorhodopsin film and behavior of optical novelty filters

To continue our earlier research on novelty filters in a system of incoherent light [Opt. Lett. 30,81 (2005)], we discuss the relationship between parameters of a bacteriorhodopsin film and the quality of a novelty filter image. For both fixed and moving velocities of the input image, differences in the novelty filter's image as a function of thickness, lifetime of the M state, and molecular concentration are displayed, and the optimal ranges of parameters of the bR film that correspond to the entire novelty filter image and obvious gray-level differences in the image are given. The method can be used to design high-quality novelty filter images in incoherent light systems.

Optical novelty filter using bacteriorhodopsin film

A new kind of optical novelty filtering by an incoherent light system is presented that is based on a special property of dynamic complementary suppression modulated transmission in bacteriorhodopsin film. By simplifying the energy system, we establish a theoretical model, and our experimental results are compared with those reached by numerical simulation of the novelty filter.

Tracking novelty filter at 780 nm based on a photorefractive polymer in a two-beam coupling geometry

We have constructed an all-optical tracking novelty filter based on the dynamic holographic properties of an efficient and fast infrared-sensitive photorefractive polymer. The photorefractive polymer was used in a two-beam coupling geometry. The polymer had a gain coefficient of 175 cm(-1) at a wavelength of 780 nm and an applied field of 72 V/microm. In contrast to what has been observed in photorefractive crystals, the gain coefficient and the filter contrast are largely independent of the writing beam's intensity ratio. We show images of a swinging pendulum observed through the novelty filter.

Holographic interferometers with photorefractive recording media

We discuss two types of holographic interferometer that contain photorefractive recording media. The first type contains two beams interacting in a photorefractive medium. The second type utilizes a single beam and relies on self-pumped phase conjugation from a photorefractive crystal to make phase changes appear as intensity changes. We show both theoretically and experimentally that the first type can be analyzed in a straightforward manner; however, the second type cannot be approximated as simply a special case of the first type, as one may näively suspect.

Incoherent-to-coherent conversion by use of the photorefractive fanning effect

A dynamic incoherent-to-coherent converter using the photorefractive fanning effect is demonstrated in a BaTiO(3):Ce crystal, and high-quality positive replicas of the incoherent image are obtained with a resolution of 32line pairs/mm. In this method the incoherent image-bearing beam modulates the transmission intensity of the coherent beam directly, and no readout beam is required. When the intensity ratio of the incoherent beam to the coherent beam is smaller than 5, the transmission intensity of the coherent beam increases almost linearly with increased intensity of the incoherent beam.

Novelty filter that uses a bacteriorhodopsin film

We propose a new novelty optical filter that uses a bacteriorhodopsin film. This filter is based on the time-dependent nonlinear diffraction efficiency of real-time holograms recorded in the film. As soon as the signal beam carrying a pattern is diffracted by the polarization hologram recorded in the bacteriorhodopsin film, it begins to erase the hologram and suppresses the diffraction of the beam at the position of the stationary part of the pattern. This filter enhances only leading edges of moving patterns. In this system undesired scattered light, which is orthogonally polarized to the diffracted beam, is discriminated by a polarizer.

Photon-echo novelty filter for measuring a sudden change in index of refraction

An optical novelty filter with phase-conjugate photon echoes is constructed to measure a sudden change in the index of refraction of a transparent medium. This novelty filter is set up as a Michelson interferometer and employs a europium-doped crystal as a phase-conjugate mirror. The filter is sensitive to the sudden change in index of refraction when the phase-conjugate echo experiences a different path length from the first laser excitation pulse in a photon-echo experiment. Using a Pockels cell as a pure phase modulator, we demonstrate that the filter can resolve a sudden phase change occurring on a nanosecond time scale with an accuracy and resolution close to 5°.

Photorefractive two-wave mixing in the presence of high-speed optical phase modulation

Equations that describe the steady-state dependence of the coherent-coupling properties of photorefractively induced refractive-index gratings on high-speed periodic biphase, sinusoidal, and triangular phase modulation impressed on one of the input optical beams are found and solved for both depleted and undepleted pump conditions. The period of the phase modulation wave form was kept short compared with the grating-formation time but did not cause significant spectral broadening. The results obtained were verified with data obtained from measurements of two-wave mixing in the photorefractive material, InP:Fe.

Applications of binary and analog hydrogenated amorphous silicon/ferroelectric liquid-crystal optically addressed spatial light modulators

Analytical and experimental results that show novelty filtering, optical phase conjugation, and real-time edge enhancement by using optically addressed spatial light modulators that comprise amorphous silicon photodiodes and analog and binary ferroelectric liquid-crystal modulators are presented. The advantages of these devices for the above applications include high-speed, low-power operation and high spatial resolution.

Fiber-optic acoustic Fourier transducer for audio sound processing

We demonstrate a fiber-optic acoustic transducer operating in the audio-frequency regime. The device is made of an array of 120 multimode optical fibers and a photorefractive novelty filter. Each fiber in the array acts as a cantilevered mechanical resonator. The resonant frequencies of the fibers logarithmically sample the acoustic spectrum from approximately 100 Hz to 5 kHz. Laser light is injected into all fibers simultaneously and is reflected from the end of each fiber. An optical novelty filter extracts the acoustic information from the reflected light. The output of the novelty filter is essentially a Fourier transform of the acoustic signal. The background intensity in the transducer output corresponds to a driving amplitude of approximately 50 A. We describe holographic storage of complex sound patterns that use a LiNbO(3) crystal and an acoustic transducer.

Novelty imaging system with a desired long-time scale using BaTiO(3) and a controlled shutter sequence

A novelty imaging system using photorefractive crystal BaTiO(3) and a controlled shutter sequence, which can be applied to the slow-motion detection of the object, is proposed. In this system the observation time scale is set to the desired long value uninfluenced by the time constant of the crystal that is used. This system utilizes a photorefractive beam-fanning effect in the BaTiO(3) crystal, so it is constructed simply by using only the object beam without suffering from the external disturbance that is intrinsic when the reference beam is used. Moreover, the visibility for an opaque object is also maintained by the use of an additional random-phase plate. We examine the basic characteristics and determine the appropriate conditions of the system.

Noise reduction using adaptive spatial filtering in photorefractive two-beam coupling

We present an optical signal-processing technique for additive noise reduction that uses the noisy signal and a Gaussian reference beam to produce an adaptive Wiener filter. We experimentally demonstrate an improvement from 1 to 8 in the signal-to-noise ratio by using nonlinear gain in two-beam coupling in barium titanate to transmit 50% of the signal and 6% of the noise.

Noise suppression in photorefractive image amplifiers

We propose and experimentally demonstrate a simple technique capable of significantly enhancing the signal-to-noise ratio of photorefractive amplifiers. The optical noise due to amplified scattered light and multiple interface reflections is removed by performing two-wave mixing in off-axis-rotating BaTiO(3) and Bi(12)SiO(20) crystals. A 20-fold improvement of the signal-to-noise ratio is achieved, and virtually noise-free image amplifiers are demonstrated.

Enhanced photorefractive performance from 45 degrees -cut BaTiO(3)

We have characterized the photorefractive performance of a special 45 degrees -cut crystal of BaTiO(3) and compared it with the conventional 0 degrees -cut material. An increased peak gain coefficient of 26 cm(-1) was measured using near normal beam incidence. Response times measured were ~1 order of magnitude faster than similar 0 degrees -cut crystals. The effects of an externally applied electric field parallel to the grating wave vector on the gain and response time were investigated. As an application example, the crystal was used to produce efficient and stable self-pumped image phase conjugation.

Application of the BaTiO(3) beam-fanning optical limiter as an adaptive spatial filter for signal enhancement in pulsed infrared laser-excited photothermal spectroscopy

A method for obtaining a high-contrast visible-light signal from infrared absorption in low-absorbance samples is described. This method is based on a beam-fanning optical limiter in photorefractive BaTiO(3). The resulting signal is not linear but does exhibit an enhanced signal-to-background ratio that is 4 to 6 orders of magnitude better than that of conventional infrared absorption spectrophotometry in the shot-noise limit. A simple model for the beam-fanning, optical-limiter-based, pulsed-laser-excited photothermal spectroscopy detector is found to describe the experimental data adequately. This technique using photothermal spectroscopy detection may have advantages for rapid signal analysis and for two-dimensional visible imaging of infrared absorption.

Time-integrating interferometry using photorefractive fanout

We have experimentally demonstrated a single-beam interferometer that effectively subtracts an exponentially weighted history of the input from the current value, thus functioning as a novelty filter. The single-beam interferometer uses signal depletion due to noise amplification (fanout) in a specially cut crystal of photorefractive BaTiO(3). To demonstrate its real-time operation we used a Hughes liquid-crystal light valve to convert a video image into a phase- and/or amplitude-modulated input signal. Potential applications of this interferometer include image-clutter removal, motion detection and tracking, edge enhancement, and image time differentiation.