Ultrasound crack detection in a simulated human tooth.

OBJECTIVE: Currently, diagnosis of cracked teeth generally depends upon the overall clinical assessment, or on exclusion of other clinical possibilities, not primarily on the direct identification of cracks themselves. Owing to its short wavelength in hard tissues and associated high resolution, ultrasound has the potential to allow detection of cracks within tooth structure. However, ultrasound detection of dental cracks has not previously been achieved. The purpose was to determine if an ultrasound imaging system was capable of imaging cracks in simulated tooth structure. METHODS: A complete ultrasound system including a novel transducer made of PLZT-98, a novel gallium-indium alloy coupling agent, and customized electronic and digital signal processing (DSP) algorithms was developed for the specific application of optimizing crack detection within teeth. A simulated tooth with a known and uniform internal structure and acoustic properties similar to those of natural enamel and dentin was designed to model a human tooth with a crack located in dentin deep to the dentino-enamel junction (DEJ). The distance between the DEJ and a crack of the simulated tooth were calculated. RESULTS: The system unequivocally distinguished between areas with and without a simulated crack. CONCLUSION: A unique ultrasound dental crack detection system using a novel transducer; a novel coupling agent; and customized electronic and digital signal processing (DSP) algorithms has been validated in a simulated tooth.

High-efficiency nonvolatile holographic storage with two-step recording in praseodymium-doped lithium niobate by use of continuous-wave lasers.

We have observed efficient two-photon, two-step recording in a praseodymium-doped lithium niobate crystal by use of cw lasers. Single-photon erasure during the readout at near-infrared wavelengths was found to be negligible. Nonvolatile holographic image storage was demonstrated. This progress is an important step in the realization of an economically feasible nonvolatile read-write holographic recording system based on low-cost semiconductor diode lasers.

Intensity dependence and white-light gating of two-color photorefractive gratings in LiNbO(3).

Photorefractive gratings have been written in Pr:LiNbO(3) by use of a diode laser for writing and of filtered white light (390-520nm) as a gating source. The gating light increases the writing efficiency by more than 3 orders of magnitude, and the two-step writing process provides nondestructive readout. The material sensitivity for two-color writing rises strongly for gating wavelengths near the bandgap and approaches that of Fe-doped lithium niobate at power densities of a few watts per square centimeter. In addition, we show that the dynamic range of the recording process is strongly dependent on the writing intensity.

Digital holographic storage system incorporating optical fixing.

Digital holograms have been written in stoichiometric Pr:LiNbO(3) in a two-color recording scheme, demonstrating what is to our knowledge the first all-optical nondestructive readout of digital data. Using writing light at 800nm and gating light at 476 nm, we stored and retrieved 256-kbit digital data pages with a raw bit-error rate BER of <10(-4) .

Resonant two-photon photorefractive grating in praeseodymium-doped strontium barium niobate with cw lasers.

We report experimental observation of resonant two-photon photorefractive gratings in Pr-doped SBN:60. Long-lived 4f intermediate states significantly reduced intensity threshold, thus permitting use of cw lasers for the two-photon transitions. Both two-beam one-color two-photon gratings and three-beam two-color gratings were observed. We expect these results to be extended to other rare-earth dopants.

Edge enhancement of a phase-conjugate image with a mutually pumped phase conjugator.

Usually edge enhancement of optical images is produced by introduction of loss into the low spatial frequency components of the image-bearing beam in the Fourier plane of a lens. We report on edge-enhanced phase-conjugate images from a mutually pumped conjugator accomplished by spatial filtering in the Fraunhofer diffraction region of the input beams. High-resolution (128-160 lines/mm) edge-enhanced images are obtained through traditional spatial filtering in the Fourier plane and in the Fraunhofer regime. Amplification of these edge-enhanced images is observed with some loss of high spatial resolution (50 lines/mm).

High-resolution phase-conjugate imaging in double-pumped phase conjugators.

Phase-conjugate images with a resolution greater than 250 lines/mm are obtained through the use of a bridge, double-pumped phase conjugator. We demonstrate that this conjugator can carry out imageprocessing tasks, such as the addition and subtraction of complex spatial distributions, with a spatial resolution of >100 lines/mm. These results represent a significant improvement over previously reported resolutions obtained from photorefractive mutually pumped phase conjugators and approach the theoretical limit imposed by the grating spacing and cross talk.

Light-induced absorption in photorefractive strontium barium niobate.

Sr(0.59)Ba(0.41)Nb(2)O(6) and Sr(0.75)Ba(0.25)Nb(2)O(6) exhibit an intensity-dependent absorption in the visible spectral range. We perform pump (green)-probe (red) measurements and find variation as high as 0.45 cm(-1) in the absorption coefficient, which depends significantly on the polarization of the probe light beam. Our results indicate a nonexponential dark decay time of the induced absorption and suggest that the trapping potential of the shallow traps varies significantly from one trap to another (multiple isolated shallow traps).

Electric-field multiplexing/demultiplexing of volume holograms in photorefractive media.

We propose a new method of volume hologram multiplexing/demultiplexing in noncentrosymmetric media. Volume holograms may be multiplexed by tuning the material parameters of the recording medium (such as refractive index or lattice parameters) while keeping the external parameters (wavelength and angles) fixed. For example, an external dc electric field alters the index of refraction through the electro-optic effect, effectively changing the recording and reconstruction wavelengths in the storage medium. Then the storage of holograms at different fields, hence different indices of refraction, is closely related to wavelength multiplexing. We demonstrate this concept in a preliminary experiment by electrically multiplexing two volume holograms in a strontium barium niobate crystal.

Mutually pumped phase conjugator as a moving-object correlator.

Cross talk is observed in a photorefractive bridge mutually pumped phase conjugator during the transient time of photorefractive grating formation and is utilized to construct a moving-object correlator. The correlation of various input images has been demonstrated and compared with calculated results. The device is currently capable of a resolution of approximately 4 to 6 lines/mm.

Electrical fixing of photorefractive holograms in Sr(0.75) Ba(0.25)Nb(2)O(6).

Photorefractive holograms stored in Sr(0.75)Ba(0.25)Nb(2)O(6) crystals are electrically fixed at room temperature. The fixed holograms can be read out directly or after a positive-voltage pulse is applied that can dramatically enhance the diffraction efficiency. Single gratings as well as images are recorded and fixed.

Improved photorefractive time response using a cylindrical lens.

Experimental observations are reported which demonstrate that the response time for beam fanning, self-pumped phase conjugation, and double phase conjugation can be shortened by more than an order of magnitude without a significant reduction in coupling strength by using a cyclindrical lens to focus incident laser light into a photorefractive crystal. These results are compared with those obtained using a spherical lens where a reduction in the photorefractive response time is accompanied by a corresponding reduction in coupling strength. It is shown that the fidelity of a phase conjugate beam is not degraded when cylindrical optics are used.

Order-of-magnitude reduction of the photorefractive response time in rhodium-doped Sr(0.6)Ba(0.4)Nb(2)O(6) with a dc electric field.

We present what is to our knowledge the first report on the photorefractive properties of Rh-doped Sr(0.6)Ba(0.4)Nb(2)O(6) and experimental results showing a reduction of the photorefractive two-beam coupling response time by more than an order of magnitude with an external dc field of 10 kV/cm.

Self-pumped phase conjugation with nanosecond pulses in strontium barium niobate.

We report the observation of self-pumped phase conjugation by means of internal reflection in a photorefractive medium produced by a series of intense nanosecond pulses. Nanosecond pulses from a YAG laser ranging in intensity from 9 x 10(4) to 9 x 10(5) W/cm(2) were used. The crystal was rhodium-doped strontium barium niobate. The conjugate signal began with the first pulse, and the time to reach 63% of its equilibrium value scaled as I(-2). The equilibrium reflectivity was 29%. A similar cw experiment with the 514-nm line of an argon laser produced a response time that scaled as I(-1.1).

Double phase conjugation in tungsten bronze crystals.

In this paper we report a new method for double phase conjugation particularly suited to the tungsten bronze crystal strontium barium niobate. It has also been observed to produce conjugate waves in BaTiO(3) and BSKNN. This new arrangement is called the bridge conjugator because the two beams enter opposing [100] crystal faces and fan together to form a bridge without reflection off a crystal face. Our measurements indicate that the bridge conjugator is competitive with previously reported double phase conjugate mirrors in reflectivity, response time, ease of alignment, and fidelity.

Enhanced photorefractive beam fanning due to internal and external electric fields.

Significant increases (x10) in both speed and gain of the beam fanning process were obtained via three different methods in SBN and BSKNN. These methods involve the creation of a dc electric field either (1) externally, (2) by the pyroelectric effect, or (3) by thermally cycling the crystal and the presence of laser radiation. The enhanced effects were observed for both ordinary and extraordinary polarized light.

Photorefractive dark conductivity in Cr-doped strontium barium niobate.

We present the theoretical and experimental results of photorefractive two-beam coupling in Cr-doped strontium barium niobate:60, using thermal excitation (i.e., dark conductivity) to model the experimentally observed temperature-dependent behavior of the two-beam coupling constant and response time.

Time response of a cerium-doped Sr(0.75)Ba(0.25)Nb(2)O(6) self-pumped phase-conjugate mirror.

Self-pumping in cerium-doped strontium barium niobate has been observed with phase-conjugate reflectivities near 6% and a formation time of 8 sec for a 200-mW/cm(2)beam at 442 nm. The time response for asymmetrical self-defocusing was also measured, and the observed transmissions through the crystal at normal incidence were limited to about 1.5% of the incident radiation.