Browsing by Author "Fang, Xiaojun"

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    Interferometric Model For Phase Analysis in Fiber Couplers
    Fang, Xiaojun; Claus, Richard O.; Indebetouw, Guy J. (Optical Society of America, 1996-07-01)
    An interferometric model is proposed to estimate the phase differences in lossless, strongly coupled biconical fiber couplers. This approximate method is simpler than the traditional s-parameter network theory-based analysis technique and minimizes the number of unknowns. The phase difference between the transmitted and coupled light fields is directly related to the field interaction and can be estimated by employing the energy conservation and mode orthogonality principles. The maximum coupling coefficient and dependence of phase difference on coupling conditions can be analyzed for multiport single-mode fiber couplers. (C) 1996 Optical Society of America
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    Nonreciprocal effects and their applications in fiber optic networks
    Fang, Xiaojun (Virginia Tech, 1996-02-15)
    Nonreciprocity is a fundamental property of networks. Unlike electronic networks theory, optical network theory is still a field to be investigated. Lightwave systems, including fiber optic and integrated optic, are becoming more and more complex, new function blocks ( or components) and networking strategies are very important for future highly integrated lightwave circuits. Several common nonreciprocal optical effects studied in this disseration and several basic applications to fiber components and fiber optic metrology systems analyzed. The common optical nonreciprocal phenomena include the Faraday effect, Sagnac effect, Fresnel drag effect, nonlinearity or asymmetric geometric structure-induced nonreciprocity, and some pseudo nonreciprocity. The best-known application of nonreciprocity to optical components is the isolator, and the known nonreciprocity-based fiber optic sensors are the fiber optic gyroscope and the fiber optic current sensor. The major difficulty in forming a general optical network theory is the complexity of optical signals compared to the electrical signal, because each light signal consists of four independent parameters, all of which changing during transmission. Fortunately, most optical signals can be classified into intensity-based and phase-based systems, and the Jones matrix technique is the ideal tool for describing the intensity-based system. Several reciprocity-insensitive structures designed and analyzed in chapter 3. The performance of the intensity-based reciprocity-insensitive structure (IRIS) was employed successfully in a fiber optic current sensor for stabilizing the signal from birefringence influences in chapter 5. A variable-loop Sagnac interferometer was designed and applied to distributed sensing in chapter 6, and the reciprocity-insensitive property of the Sagnac interferometer was preserved. Polarization independent isolators and wavelength division multiplexers were also realized by employing suitable nonreciprocal effects and were discussed in chapter 2 and chapter 4, and their feasibilities were verified by experiment. The primary contributions of this dissertation are the study of common nonreciprocal optical effects and demonstration of several basic applications to fiber components and fiber metrology systems.
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    Sapphire fiber based high temperature extensometer
    Fang, Xiaojun (Virginia Tech, 1994-05-05)
    New sapphire fiber based sensor structures for high temperature strain measurement are proposed and studied in this report. The self-mixing interferometry has been studied and tested. The advantage of this technique is the source coherence insensitivity and direction distinguishment capability. Fringes of the self-mixing interferometer built with standard multimode fiber and sapphire fiber were observed. The application of this technique to static strain rneasureament seems difficult to stabilize, whereas its implementation to acoustic sensing with single-mode fiber coil as the sensing component will be very interesting. The approach by Moire interferometry and equal-path coherence matching interferometer are two promising methods for high temperature displacement measurement. The advantages of the Moire interferometer are source coherence independence and large measurement range. The advantage of coherence matching absolute interferometer is its simplicity in sensor structure. The structures of coherence matching methods and multiplexing techniques are generalized. From this research, we can conclude that the sapphire fiber based high temperature extensometer may be realized by different methods.