Experimental Research On Some Problems In Photorefractive Effects

Photorefractive effects can result in the change of refractive index of Photorefractive media under the illumination of very weak light, which may be widely used in future optical information technology. The application study on photorefractive materials has attracted more and more people's attention. With the development of material technology, the photorefractive materials have been developed from nonlinear crystals to the easily integrated materials, such as semiconductor, thin film and liquid crystal, etc. The new photorefractive materials offer a good foundation for future application. So it is very necessary for the exploratory development of photorefractive effect in new application field. For this purpose, we develop the correlation experimental study on some problems of photorefractive effects in this dissertation.The forming mechanism of space-charge field and its influence on the refractive index of nonlinear materials are first derived based on the Kukhtarev-Vinestskii model. We discuss the interaction of two laser beams inside a photorefractive medium, analyze the intensity and phase coupling process, and investigate qualitatively the role of photorefractive volume phase grating on the intensity coupling process. Then, relevant theories of photorefractive spatial solitons are presented. Spatial solitons generally can be classified into two generic types: The first type is originated from the self-phase modulation self-focusing mechanism, and the second type is originated from the mechanism that arises from the nonlinear phase coupling. Normally, we call the spatial solitons arises from the nonlinear phase coupling as holographic solitons. The holographic solitons investigated here is based on two-wave mixing, it can be divided into Hamitonian holographic (HH) solitons and dissipative holographic (DH) solitons.Basing on CCD technology and software processing, a new method, namely top- and side-view method, is first presented for the observation of spatial optical solitons in photorefractive crystals. This method has an ability to show the horizontal and vertical outline of the beam at any location of the crystal very intuitively. The max intensity, full wave at half maximum (FWHM) and total energy of the beam at any location of the crystal can be obtained by this means. By use of this method, we have experimentally observed the two-dimension bright photovoltaic solitons in a KNSBN crystal. Because there exists intensity coupling, the DH solitons can't be observed directly by this means. We have experimentally observed the DH bright photovoltaic solitons in a KNSBN crystal by this method combined with optical shutter. The correlated character of DH solitons is analyzed from experimental data. Experimental results prove the existence of holographic waveguide.The influence of infinitesimal disturbance on the phase shift between the incident fringes pattern and the volume phase grating is discussed based on two-wave mixing theory. The change of the phase shift will immediately result in the intensity fluctuation of two output beams. As soon as the volume phase grating forms under two incident beams with the same intensity, the phase grating is almost unaffected under the influence of infinitesimal disturbance, the disturbance only changes the phase shift, and the volume phase grating avoids reforming, so the volume phase grating can implement signal modulation quickly.It can be seen from the experiments, the influence of infinitesimal disturbance on the intensity of two output beams is very tiny. Compare with the total energy, the modulated energy is very small. The modulated signal is the alternating current (AC) portion of the composite signal generated by photoelectric detector. By use of special amplification circuit, the modulated signal can be obtained. Two modulated signals of the two output beams own the character of differential signals. High speed optical differential modulation device can be made by utilizing this character of volume phase grating. In the sensor domain, this differential modulation device can be used to detect the air disturbance, environmental vibration, transient pressure, etc. For space optic communication, this differential modulation device can implement differential space optic communication, which can eliminate the common-mode disturbance effectively.