Fiber-optic sensors employ light propagating through an optical fiber to detect an environmental parameter. In principle, any property of the light, such as in-tensity, color, frequency, phase, or polarization state, can be used t. Fiber-optic sensors employ light propagating through an optical fiber to detect an environmental parameter. In principle, any property of the light, such as in-tensity, color, frequency, phase, or polarization state, can be used to sense a physical or chemical parameter if this parameter affects any property of the light. Fiber-optic sensors can be. Displacement measurement is an important topic in metrology. It is also essen-tial to the measurement of other parameters. Fiber-optic FMCW displacement sensors can measure not only relative displacement but also absolute distance and speed of motion.Figure 9.1 shows a reflectometric single-mode fiber FMCW displacement sen-sor. The sensor primarily consists of a Y-type single-mode fiber coupler (FC) stuck to a quarter-pitch gradient-index lens (GL) at the distant end of its output fiber. The outer planar surface of the gradient-index lens and the front surface of the object (O) to be measu. Strain and stress measurements are established, but active, topics in mechanical engineering. Many fields of industry, such as aerospace, aviation, and civil construction, employ various strain and stress sensors to ensure material and structural safety. However, the conventional electrical strain and stress sensors, such as metal-film strain senso. Fiber-optic temperature sensors are particularly useful in electrical transform-ers, jet engines, and certain medical treatments because they are isolative in electricity, passive in chemistry, small in size, and light in weight. In the fol-lowing subsections, we introduce a reflectometric single-mode fiber FMCW temperature sensor and a multiplexed.