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. / Ph. D.