Physical quantities like pressure and temperature, when acting upon an optical fiber, can directly induce phase changes in the light wave within the fiber. This phenomenon enables phase modulation in fiber vibration/acoustic sensors, pressure sensors, temperature sensors, or fiber rotation sensors (fiber gyros). Among these, the fiber optic gyro and the fiber optic hydrophone are the most representative and have been successfully commercialized in the evolution of fiber optic sensor technology. Nevertheless, certain physical quantities that do not directly affect the fiber can still induce changes in stress and/or temperature within the fiber through the interaction with sensitive materials. This, in turn, causes a phase change in the transmitted light wave within the fiber, such as fiber current sensor, fiber gas sensor, etc. This chapter focuses on problems and available solutions in typical several applications.