Study Of New Techniques Of Photorefractive Holographic Interferometry And Its Applications

Holographic interferometry (HI), witch permits whole-field and nondestructive optical testing, is one of the most powerful means of modern measurement and metrology techniques. It is employed in numerous scientific, technical, industrial, engineering and medical applications, such as displacement and deformation measurement, strain/stress analysis, defect/damage detection, resonance-mode visualization and measurement, monitoring of crystal/protein growth, density-field and convection-process observation in fluids, optical testing, among others. For coping with practical applications (especially in industrial and engineering applications) requirements, a holographic interferometer should be easy to use, rapidly reusable, and as fast as possible in giving results of quantitative measurement. The holographic recording materials must comply with these requirements. The ideal material should be self-developing in situ, reusable, highly sensitive, and capable of giving the largest diffraction efficiency. Currently, no recording medium has all these qualities, but some materials have most of them. The traditional silver halide emulsions have a high sensitivity but are not self-developing and are not reusable. Consequently it is inconvenient for them to realize real-time processing. For the purpose of real-time holography, several recording materials have been developed. Among them are photopolymer and thermoplastic materials. However, the former is not recyclable and usually needs post-processing of uniform exposure to make its refractive index modulation stable, while the latter suffers from the limited spatial resolution and the cumbersomeness of operation. All these disadvantages of the above materials limit their utilization in HI for applications where needing real-time or dynamic processing.Compared with the classical recording materials mentioned above, photorefractive crystals (PRCs) have been under extensive studies in recent decades and considered as promising holographic recording materials in HI owing to their high resolution, adequate sensitivity which is controllable by changing their compositions, large storage capacity, real-time processing ability and excellent reusability. Using PRCs as recording media inHI (referred to as Photorefractive Holographic Interferometry, PHI) can give full scope to advantages of both HI and PRCs, and can improve the real-time performance and flexibility of HI, and make the HI more suitable to the requirement of practical applications. The relatively slow response time of the PRCs has been usually considered as a drawback which makes them unsuitable for applications such as high-speed optical switching and high-speed optical write-and-read systems, but it is preferable in some real-time holographic testing applications in order to obtain a longer observation period. The aim of the presented dissertation is to construct a real-time PHI system with proper doped Ce:SBN under low power He-Ne laser light, give a systematic and comprehensive analysis on it, and study its new techniques, new methods and new applications in optical testing and image processing, theoretically and experimentally, with emphases on the following four respects:1. The fringes contrast of Photorefractive holographic interferoemtry, and its time-dependent propertiesFor real-time photorefractive holographic interferometry (with single exposure), the relations between the fringes and the recording time, and the observation/measurement time are derived, the methods of selecting the optimal observation/measurement time and the optimal capturing time of the fringes contrast are obtained. The analysis and numerical simulation results show that the time-dependence of fringes contrast is determined by the write/erase time constant, the placed direction of the used crystal, the light intensity ratio of the direct object wave and reconstructed object wave. These parameters should be considered in practical experiments and applications. For the double exposure photorefractive holographic interferometry, the relation between the two recording time is derived, and the optimal exposure time can be determined with this relation. This derived result accords with the results obtained by some other researchers with other methods. The above research results are important to instruct the experiments and applications of photorefractive holographic interferometer, to study its new techniques, new methods, and new applications.2. The experimental system is constructed with Ce:SBN as holographic recordingmedium and with low power He-Ne laser as light sourceThe holographic recording properties of Ce:SBN are measured, including spectrum response, diffraction efficiency, and write/erase time constant. The measurement results show that Ce:SBN is sensitive in red light spectrum area, presents good photorefractive response ability, and can be used as holographic recording medium under low power He-He laser. The experimental system with Ce:SBN as holographic recording medium and with low power He-Ne laser as light source is constructed. For verifying the effectivity of the system, some demonstrated experimental results are obtained, including the measurement and real-time analysis of the 3-D axisymmetric refractive index field, and the measurement of the tilt and in-plane displacement of object. These practical experimental results show that the constructed system is easy to use, rapidly reusable.3. A new technique, photorefractive wave-front shifting holographic interferometry,is proposed, and its some applications in optical testing are demonstrated.In the presented photorefractive wave-front shifting holographic interferometry, the wave-front shifting, including transverse and longitudinal shifting, is achieved by slightly moving the crystal before and after exposure. The principle and the realization method of the presented technique are systemically and comprehensively studied. Some applications of the presented technique in optical testing is demonstrated, such as the refractive index measurement of transparent materials and optical devices, simultaneously measurement of the wedge angle and refractive index of optical wedge plate, the 3-D displacement measurement. Based on the photorefractive wavefront shifting technique, a novel method of obtaining shearing interferogram is proposed. This method can measure the phase of an object itself instead of its diffraction field, and it is easy to realize continuously changeable shearing distance in any lateral direction and to introduce carrier fringes at the same time. Both the theoretical analysis and experimental verification are given.4. Novel methods for measuring the photorefractive phase shift and for optical amplitude-phase conversion, and their applications in optical image processing such as optical image subtraction and optical image encrypted storage.On the basis of photorefractive holographic interferometry, a new method for experimentally demonstrating and observing the photorefractive phase shift is proposed. Some experimental results are given for Ce:SBN crystal with no externally applied electric field. The method to realizing optical image subtraction using photorefractive phase shift is also demonstrated. A new method for conversing the spatial amplitude distribution of a given image into its spatially similar phase distribution is proposed. The implementation of the present method can be based on the double exposure photorefractive holographic phase-shifting interferometry, in which the optical 4-/ system with an amplitude-only spatial light modulator (SLM) as the image input device and with the photorefractive crystals as holographic recording medium is employed. We present the fundamental idea on which the method is based and verify its effectiveness and correctness by experimental result. A new method for optical encrypted image storage using the proposed optical amplitude-phase conversion is demonstrated theoretically and experimentally.