摘要
全息干涉技术可做到全场、定量、直观的无损检测,是现代光学检测计量的重要手段之一,在科学研究、工业检测、生物医疗及国防军事技术等领域具有广泛的应用,如:位移及形变测量、应变与应力分析、缺陷或损伤探测、振动模式可视化及测量、晶体和蛋白质生长过程监测、流体中密度场和热对流场的观察与测量、以及光学材料与器件的参数测量等。实际应用(尤其是工业及工程应用)要求全息干涉系统应具备以下性能:实时处理能力,能够重复使用,操作方便,能快速定量地给出测量数据。全息记录介质是决定全息干涉系统性能(如实时性、灵活性、所采用的光源、系统复杂程度及操作难易程度等)及其应用范围的主要因素。传统全息记录介质(如卤化银乳胶等)需要化学显影、定影处理,需要精确复位调整,无法做到实时处理,且不能重复使用。已有的一些实时记录材料也各有其局限性,例如:光导热塑材料的空间分辨率较低,不能连续多次曝光,高质量的胶片不易获得;光色材料的感光灵敏度较底,易疲劳。这些都限制了它们在全息干涉中的应用。
相比之下,光折变晶体作为全息干涉的记录介质则有其明显优点,如:可实时记录/读出和擦除、可无限制地重复使用、信息存储容量大、具有较高的灵敏度和分辨率等。在全息干涉中采用光折变晶体作为全息记录介质(光折变全息干涉),可充分发挥全息干涉与光折变晶体两方面的优点,可大大提高全息干涉系统的实时性、灵活性,使全息干涉技术更好地满足实用要求。在已有的关于光折变全息干涉研究的文献报导中,对光折变全息干涉的时间变化特性没有作系统深入的研究;在方法上以散斑干涉居多,在一些散斑干涉方法中还需附加空间光调制器及光学相关检测系统,使得装置复杂化而测量精度不高;在所用材料上,以铋硅族氧化物(BSO等)和LiNbO_3晶体居多,受材料光谱特性、灵敏性及衍射效率等方面的限制,光源多采用大功率激光光源,有时需要外加电场,使得系统体积较大、成本较高;在结构设计及应用研究范围等方面还有很大局限性,没能充分体现光折变全息干涉的优点和特点。因此,选用新的光折变晶体,采用新的系统结构和技术,探索新的应用领域,从理论与实践两方面进一步深化光折变全息干涉技术的研究,是十分必要的。本论文的研究工作,从理论与实践两方面对光折变全息干涉技术进行了深入研究,系统地研究了光折变全息干涉条纹衬比度的时间变化特性,提出了一系列新
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 in
HI (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 recording
medium 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.
引文
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