**1. Introduction**

Fiber sensors have several advantages compared to some conventional sensors. They are lightweight and have a small size, high resolution, and good stability; fiber sensors not only are insensitive to electromagnetic interference but also can withstand high temperature and radiation. A variety of linear and nonlinear optical transduction mechanisms have been studied in the last 30–40 years, dealing with the conversion from all kinds of measurands to local measurable optical effects in the fiber. There are previous works, for instance, that designed temperaturecompensated fiber Bragg grating (FBG) sensor for monitoring the stress [1], FBG-integrated spherical-shape structure for refractive index sensing [2], and D-shaped fiber combined with a FBG for refractive index and temperature sensing [3]. Fiber sensor can measure and/or monitor many parameters such as strain, weight, temperature, speed, pressure, and so on. Moreover, fiber sensor can also measure the variation of light intensity, wavelength, frequency, phase, and polarization by combining other detectors with optical fiber. Firstly, optical fiber sensors for temperature and pressure have been developed for measurement in oil wells. For example, a precise and real-time ammonia monitoring technique is important especially for gas sensing [3]. Once the gas leakage happens, an immediate alarm is helpful to prevent danger. Secondly, fiber sensing is also used to make a hydrogen sensor. Temperature can be measured by using a fiber that has evanescent loss with various temperature ranges or by analyzing the Brillouin scattering in the optical fiber. Thirdly, angle measurement sensors can be designed based on the Sagnac effect. In recent years, various sensing materials are available for biosensor fabrication, so various fiber-optic biosensors have been proposed and demonstrated. Finally, optical fiber sensors have been developed to simultaneous measurement of temperature and strain with very high accuracy by using fiber Bragg gratings.
