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1.
The space-charge-field formation in a photorefractive crystal illuminated by a one-dimensional light pattern is studied when an alternating square-wave electric field is applied to the crystal. We derive the general nonlinear equation for the time-averaged distribution of the space-charge field and employ the simplified versions of this equation to analyze the space-charge-field formation in the case of a Gaussian beam and interference light pattern. The borders of the applicability of simplified equations and analytic expressions to describe a self-action of light beams and the large modulation effects under two-beam coupling are estimated from the numerical calculations. Received: 18 November 1998 / Revised version: 26 January 1999 / Published online: 12 April 1999  相似文献   

2.
The power of several coherent laser beams can be transferred into a single diffraction-limited signal beam by photorefractive coupling. The efficiency of the power transfer is investigated experimentally in the case of two pump beams. Two different geometrical arrangements, one with overlapping (simultaneous) and the other with separate (sequential) pump beams are compared in a BaTiO3:Ce crystal, with diffusion-driven photorefraction. We measured about the same power transfer efficiency in both arrangements for strong signal beams, but the efficiency was higher in the sequential arrangement for weak signals. A simple theoretical model of the power transfer process based on the standard linear two-wave-mixing theory is presented and the observed stationary beam-coupling behavior for different pump-to-signal intensity ratios is found to be in qualitative agreement with the theoretical predictions. Received: 10 November 1998 / Revised version: 22 December 1998 / Published online: 24 March 1999  相似文献   

3.
We present the numerical results of the photorefractive two-wave mixing in the presence of an incoherent beam by considering the influence of the ratios of the intensities and the light-excitation cross sections of the incoherent beam to the coherent beams on the space-charge field and the effective coupling coefficient. The results show that the incoherent beam can control the intensity of the coherent beams effectively. The experimental results agree with the theory well. One application is demonstrated to obtain the positive and negative coherent replicas of an incoherent image. The coherent images have a resolution of 80.6 line pairs/mm. Received: 2 March 1999 / Revised version: 26 July 1999 / Published online: 20 October 1999  相似文献   

4.
During holographic recording in lead germanate (Pb5Ge3O11) crystals two types of refractive-index gratings are observed. One has a very fast response whereas the second builds up comparably slowly. Measurements of diffraction efficiency and two-beam coupling are carried out to study the formation of both gratings and to obtain the relative phase between them. Differently doped and thermally treated samples are divided into four classes due to their different time evolution of diffraction efficiency and of the energy transfer direction during two-beam coupling. The classification depends on doping and treatment. For Ni-doped and thermally treated samples dark and photo conductivities corresponding to the slow grating are determined, indicating that Ni-doping combined with oxidation enhances the properties of the slow grating. Received: 18 November 1998 / Published online: 7 April 1999  相似文献   

5.
Suppression of an optical carrier from an RF modulated laser beam of wavelength 532 nm is performed using two-beam coupling in photorefractive barium titanate. A theoretical analysis reveals that perfect suppression can be achieved at a specific modulation strength, which depends on the gain and intensity ratio between the two beams. The experiments achieve a maximum of -72.9±2.4 dB carrier suppression at the theoretically ideal modulation strength, and -61.0±2.4 dB for small modulation strengths. Received: 29 November 2000 / Revised version: 2 February 2001 / Published online: 27 April 2001  相似文献   

6.
In this article, we demonstrate the capability of a two-beam coupling photorefractive optical novelty filter of detecting changes in the amplitude or phase of optical images. These changes may either be continuous or discrete in time. The performance of the two-beam coupling novelty filter is investigated and expressions for the output contrast corresponding to phase and amplitude changes based on a novel, simple interference model of two-beam coupling are derived. These expressions are verified by experimental results on the novelty contrast, revealing that the amplitude contrast is not described correctly by the commonly accepted coupled-wave theory. The novelty filter was applied to the detection of temporally continuous phase changes provided by a gas flow and moving microscopic objects. A novel scheme for image subtraction is also demonstrated, showing the novelty filter’s ability to detect temporally discrete changes. Received: 14 November 1998 / Revised version: 18 January 1999 / Published online: 12 April 1999  相似文献   

7.
We report on a numerical analysis of the temporal and spatial beam properties of nanosecond optical parametric oscillators (OPOs). The analysis is performed for a 355-nm-pumped critically phase-matched OPO of beta-barium borate. The calculations provide detailed information on the dependence of the OPO beam quality (measured by the quality factor M 2) on pump energy. An important result is the strong increase of the M 2 value for pump energies exceeding 1–2 times the energy at threshold. Furthermore, a temporal analysis of single OPO pulses indicates that the M 2 value strongly increases during the first few nanoseconds of the OPO oscillation. This increase is understood by considering the temporal dynamics of the spatial profiles of the OPO signal beam and the depleted pump radiation. Received: 1 April 1999 / Revised version: 26 July 1999 / Published online: 20 October 1999  相似文献   

8.
The dynamics of light-induced change in the refractive index of a resonant medium are examined. For illumination with weak fields, the two relevant relaxation times are T 1, the population lifetime and T 2, the dipole-dephasing time. The response time of the index change is determined by the slower relaxation time of the medium which is usually the time T 1 taken by the excited system to relax back to its thermal equilibrium value. Illumination with two beams of the same frequency that intersect within the medium leads to the formation of a volume grating in the medium that is spatially local. Hence there is no exchange of energy between the beams that write the grating, each beam merely reducing the absorption experienced by the other beam. Illumination with a moving, spatially periodic intensity pattern leads to gain for one beam and additional absorption for the other beam as they propagate through the medium. A complete set of coupled equations describing the intensities and phases of the beams undergoing non-degenerate two-wave mixing in the resonant media is derived using third-order perturbation theory, and the solutions are studied numerically. In particular, the two-beam coupling constant for intensity is shown to depend on the frequency difference between the two beams and on the pressure-induced collisional relaxations in the system.  相似文献   

9.
The third harmonic of 810-nm 100-fs pulses at 130 μJ is generated very efficiently when ultrashort pulses from two noncollinear beams interfere in an optical medium to create an instantaneous transient grating via the optical Kerr effect. The grating couples two pathways for third-harmonic generation, each taking two photons from one beam and one photon from the other beam, respectively. The coupling enables self-phase matching in the complete process, resulting in a conversion efficiency of ≈3%. Scattering an independent beam at the transient grating confirms a lifetime limited by the pulse duration, with a reaction on the order of one optical cycle. Using the second harmonic of a Ti-sapphire laser at 405 nm, it is shown that the generation of the transient Kerr grating is a general feature, requiring less than 20 μJ/pulse. By introducing a third femtosecond beam we are able to emulate various digital logic units with femtosecond response. Received: 16 October 2001 / Published online: 6 June 2002  相似文献   

10.
M.H. Majles Ara  R.S. Sirohi 《Optik》2007,118(9):445-451
Photorefractive crystals offer several attractive features such as high resolution and in situ processing. As the images are erasable, these crystals are suitable for read–write applications and hence find potential use in speckle photography, image processing and holography. The barium titanate (BaTiO3) crystal as recording medium has been extensively used as a novelty filter for real-time in-plane displacement measurements employing two-beam coupling configuration. This paper presents new optical configurations in speckle shear photography to measure in-plan displacement and the strain in real time using BaTiO3 crystal as recording medium. Speckle photography studies are made using a simple two-beam coupling configuration. In speckle shear photography, a diffused object illuminated with two parallel narrow laser beams is imaged inside the crystal, and a pump beam is added at this plane. The speckle patterns due to each beam and the pump beam produce index gratings. When the object is deformed, the speckle patterns shift consequently. We now have four speckle fields: two generated from the interaction of pump beam with the index gratings and two pertaining to deformed states directly transmitted through the crystal. Thus, the fields from respective points on the object interfere after passage through the crystal and produce the Young's fringe patterns. Due to strain, the fringes in each pattern are of different width and orientation, resulting in the generation of a Moiré pattern. The strain is obtained from the width and orientation of the fringes in the Moiré pattern. The experiments are conducted on a specimen with a notch, which is subjected to tensile loading. The in-plane displacement is measured separately in another experiment. The above studies are carried out at Nd–Yag laser.  相似文献   

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