RESUMEN
We report the study, both theoretical and experimental, of the finite-beam Bragg diffraction behavior of an electro-optic (EO) volume grating made of a periodically poled lithium niobate (PPLN) crystal. When a Gaussian laser beam is used, the experimental observations show that the diffraction characteristics of the PPLN EO Bragg device, including the diffraction mode pattern and diffraction efficiency, are closely related to the interaction beam size and applied voltage, which cannot be modeled properly by a simplified theory using the plane-wave approximation. In this work, we have developed a theoretical model for describing the diffraction behavior of a PPLN EO Bragg device based on the coupled-wave theory with the aid of the plane-wave decomposition method. Specifically, we found that it is the angular distribution (or the dephasing bandwidth) of the plane wave elements decomposed from the incident Gaussian beam and grating strength that determine the Bragg coupling behavior of the device. We also identified some other electro-optically induced effects in the PPLN grating as an important mechanism in affecting the diffraction performance of the present device, especially at high working voltages.
RESUMEN
A novel confocal microscope designed for use with turbid media is proposed. We use a Zeeman laser as the light source. Based on the properties of two-frequency polarized photon-pairs and the common-path feature of polarized photon-pairs with heterodyne detection employed in the proposed confocal microscope, three gatings (spatial filtering gating, polarization gating and spatial coherence gating) are thus simultaneously incorporated in the microscope. Experimental results for the angular distribution of polarized photon-pairs in a scattering medium indicate that polarization gating and spatial coherence gating preclude the detection of multiply scattered photons, whereas the pinhole selects the least scattered photon-pairs. Thus, better performance for axial resolution than can be obtained with a conventional confocal microscope is demonstrated experimentally. In addition, the proposed microscope is able to either look deeper into a turbid medium or work with a denser medium; furthermore, the axial resolution is improved.
RESUMEN
We have demonstrated experimentally the simultaneous self-conjugation of extraordinarily and ordinarily polarized light waves (lambda=514 nm), with the polarization state preserved, in a Ce:BaTiO(3) crystal. Conjugate o waves originate from coupling between o and e waves by means of a circular photogalvanic effect. The conjugator also takes advantage of the Ce:BaTiO(3) crystal, for which it is easy to generate an extraordinary conjugate beam by photorefractive backscattering. This beam of light is necessary for double phase conjugation.
RESUMEN
We demonstrate experimentally a setup with which a two-dimensional picture can be transmitted from one plane to another at a signal request of the latter plane and the original picture continues to propagate unhindered along its original direction to the next stage. The setup makes use of photorefractive four-wave mixing and the self-pumped phase conjugation for its operation. Some means for increasing the contrast of the final transmitted image have been discussed.
RESUMEN
A pentagon-shaped 0 degrees-cut undoped BaTiO(3) crystal was used to construct a high-quality self-pumped phase conjugator. The conjugator shows a steady phase-conjugate output with a high reflectivity (as high as approximately 68%), a fast rising time, and a large angular range of acceptance. It also shows a good fidelity in the phase-conjugate reconstructed image; the resolution reaches 17 microm.
RESUMEN
Self-pumped phase conjugation is generally believed to be attributed to degenerate four-wave mixing. Accordingly, the phenomenon requires the existence of three waves at certain interaction regions for the excitation of the fourth one. Although this condition is not satisfied by a train of femtosecond pulses with a repetition rate of 110 MHz, we have observed self-pumped conjugate pulses retroref lected by a BaTiO(3) crystal. The conjugate character of the pulses is demonstrated with the distorter method.
RESUMEN
Experimental results of phase-only circular harmonic filtering implemented in a standard Fourier transform correlator are presented. The filter is a computer generated hologram which contains only the phase information of a single circular harmonic component of the target. The result with simple binary objects bears out the simulation result published earlier [H. F. Yau and C. C. Chang, Appl. Opt. 28, 2070-2074 (1989)] in that the filter is indeed shift- and rotational-invariant and that the correlation peak is more prominent than that produced by ordinary circular harmonic filtering.
RESUMEN
The idea of a phase-only matched filter [J. L. Horner and P. D. Gianino, Appl. Opt. 23, 812-816 (1984)] is incorporated in the scheme of circular harmonic component matched filtering [Y. N. Hsu and H. H. Arsenault, Appl. Opt. 21, 4016-4019 (1982)]. Computer simulation showed that the phase-only circular harmonic matched filter possesses the rotational invariant property for 2-D recognition. Furthermore, it does a better job in recognizing closely placed targets compared with traditional circular harmonic filters. The simulation also demonstrated that a pure phase circular harmonic filter is less liable to give rise to false alarms in recognition because the sidelobe produced by it is much less pronounced and much less extensive than that produced by a traditional circular harmonic filter.
RESUMEN
Photosynthetic reaction centers from Rhodopseudomonas spheroides were prepared with the detergent lauryl dimethylamine oxide (LDAO). In contrast to reaction centers made with Triton X-100, these contained no cytochromes and little or no ubiquinone (UQ). The reduction of P-870, after its photochemical oxidation, was studied in these materials with the following results. In reaction centers made with Triton X-100, slow kinetic components (seconds to minutes) could be attributed to secondary electron acceptors or traps. In reaction centers made with LDAO the kinetics were predominantly fast (half-times, 100 msec or less); slower components could be introduced by adding UQ. Added UQ appeared to become bound to reaction centers made with LDAO, but the binding might have meant only that both components were trapped within detergent micelles. Ferricyanide could retard the reduction of oxidized P-870, apparently by capturing electrons from the reducing side of the photochemical system. Under conditions in which the participation of secondary electron acceptors seemed to have been eliminated, the recovery of P-870 was mainly by a first-order process with a half-time of about 60 msec at room temperature and 20-30 msec at about -80 degrees C and below. The transition with decreasing temperature suggested the presence of a mixed population, exhibiting both the 60 and 20 msec components, but variations in the absorption spectra with temperature did not suggest the presence of a mixed population. Absorption difference spectra in the ultraviolet were compatible with the idea that UQ added to reaction centers became reduced in the light.
