This paper outlines the basic principles and practical applications of fiber optic grating (FBG) sensors, introducing their applications in geodynamics, spacecraft and shipbuilding, civil engineering structures, the power industry, medicine, and chemical sensing. I. Introduction In 1978, K.O. Hill et al. at the Ottawa Communications Research Centre in Canada first discovered the photosensitivity of optical fibers in germanium-doped silica fibers and fabricated the world's first FBG using the standing wave writing method. In 1989, G. Meltz et al. at the United States Joint Technologies Research Center achieved UV laser side writing technology for fiber Bragg gratings (FBGs), marking a breakthrough in FBG fabrication technology. With the continuous improvement of FBG manufacturing technology, its applications are increasingly numerous. The entire field, from fiber optic communication and fiber optic sensing to optical computing and optical information processing, will undergo revolutionary changes due to the practical application of FBGs. FBG technology is another major technological breakthrough in fiber optic technology after erbium-doped fiber amplifiers (EDFAs). FBGs are made using the photosensitivity of optical fibers. The photosensitivity in optical fibers refers to the characteristic that the refractive index of a doped fiber changes accordingly with the spatial distribution of light intensity when laser light passes through it. The spatial phase grating formed within the fiber core essentially functions as a narrow-band (transmission or reflection) filter or mirror. This characteristic allows for the fabrication of many unique fiber optic devices. These devices possess a wide reflection bandwidth, low additional loss, small size, easy coupling with optical fibers, compatibility with other optical devices, and immunity to environmental dust, among other excellent properties. There are many types of fiber optic gratings, mainly divided into two categories: Bragg gratings (also known as reflection or short-period gratings) and transmission gratings (also known as long-period gratings). Structurally, fiber optic gratings can be divided into periodic and aperiodic structures; functionally, they can be classified into filtering gratings and dispersion-compensating gratings. Dispersion-compensating gratings are aperiodic gratings, also known as chirped gratings. Currently, the applications of fiber optic gratings are mainly concentrated in the fields of optical fiber communication and optical fiber sensors. In the field of fiber optic sensors, fiber Bragg grating (FBG) sensors have a very broad application prospect. Due to their advantages such as resistance to electromagnetic interference, small size (standard bare fiber is 125µm), light weight, good temperature resistance (operating temperature upper limit can reach 400℃-600℃), strong multiplexing capability, long transmission distance (sensor to demodulation end can reach several kilometers), corrosion resistance, high sensitivity, passive device nature, and easy deformation, FBG sensors were successfully used as an effective non-destructive testing technology in the aerospace field as early as 1988. FBG sensors can also be applied in various fields such as chemical and pharmaceutical industries, materials industry, water conservancy and hydropower, shipbuilding, and coal mining. Furthermore, in the field of civil engineering (such as buildings, bridges, dams, pipelines, tunnels, containers, highways, airport runways, etc.), they are used to determine the structural integrity and internal strain state of concrete components and structures, thereby establishing flexible structures and further realizing intelligent buildings. II. Working Principle of Fiber Bragg Grating Sensors We know that the Bragg wavelength λB of a grating is determined by the following formula: λB=2nΛ (1) where n is the effective refractive index of the core mode and Λ is the grating period. When the temperature, stress, strain, or other physical quantities of the environment in which the fiber grating is located change, the period of the grating or the refractive index of the fiber core will change, thereby changing the wavelength of the reflected light. By measuring the change in the wavelength of the reflected light before and after the change in the physical quantity, the change in the physical quantity to be measured can be obtained. For example, by utilizing the different refractive index changes of left-hand and right-hand polarized waves induced by a magnetic field, the magnetic field can be directly measured. In addition, through specific techniques, stress and temperature can be measured separately or simultaneously. By coating the grating with specific functional materials (such as piezoelectric materials), the indirect measurement of physical quantities such as electric fields can also be achieved. 1. Working Principle of Chirped Fiber Bragg Grating Sensors The grating sensor system described above, with its uniform geometry, is effective for point-to-point measurement of a single parameter. However, it falls short when simultaneously measuring strain and temperature, or measuring the distribution of strain or temperature along the grating length. A better approach is to use chirped fiber Bragg grating sensors. Chirped fiber Bragg gratings are used in high-bit-rate long-distance communication systems due to their excellent dispersion compensation capabilities. The working principle is basically the same as that of fiber Bragg grating sensors. Under the influence of external physical quantities, the chirped fiber Bragg grating, in addition to changes in ΔλB, also causes spectral broadening. This type of sensor is very useful in situations where both strain and temperature are present. The strain effect of the chirped fiber Bragg grating leads to broadening of the reflected signal and a shift in the peak wavelength, while temperature changes, due to the temperature dependence of the refractive index (dn/dT), only affect the position of the centroid. By simultaneously measuring the spectral shift and broadening, strain and temperature can be measured simultaneously. 2. Working Principle of Long-Period Fiber Bragg Grating (LPG) Sensors The period of a long-period fiber grating (LPG) is generally considered to be several hundred micrometers. The LPG couples light from the fiber core into the cladding at a specific wavelength: λi = (n0 - niclad)·Λ. Where n0 is the refractive index of the fiber core, and niclad is the effective refractive index of the i-th order axisymmetric cladding mode. Light in the cladding will rapidly attenuate due to losses at the cladding/air interface, leaving a loss band. A single LPG may have many resonances over a wide wavelength range. The center wavelength of the LPG resonance mainly depends on the refractive index difference between the core and cladding. Any change caused by strain, temperature, or external refractive index variations can produce a large wavelength shift in the resonance. By detecting Δλi, information about changes in external physical quantities can be obtained. The response of the LPG resonance band at a given wavelength typically has different amplitudes, thus making LPG suitable for multi-parameter sensors. III. Applications of Fiber Bragg Grating Sensors 1. Applications in Geodynamics In geodynamics, such as earthquake detection, the principles and hazard assessment and prediction of sudden surface changes are highly complex. Stress and temperature changes in volcanic areas are currently the most effective means to reveal volcanic activity and the evolution of its key activity range. Fiber Bragg grating sensors are mainly used in rock deformation, vertical seismic wave detection, and as topographic detectors and optical seismometers. Strain in active zones typically includes both static and dynamic components. Static strain (including static deformation caused by volcanoes) is generally located close to the geological deformation source; while dynamic strain, represented by seismic waves from the earthquake source, can be detected in the surrounding environment of the Earth, farther from the source. To obtain a fairly accurate location of the earthquake or volcanic source and better describe the geometry and evolution of the source region, densely packed stress-strain measuring instruments are required. Fiber Bragg grating sensors are broadband, highly networked sensors capable of long-distance and densely packed multiplexed sensing, meeting the requirements of earthquake detection and other applications. Therefore, they undoubtedly have significant potential applications in geodynamics. Reports indicate that fiber Bragg grating sensors have successfully detected dynamic strain in rocks and land surfaces with frequencies ranging from 0.1 to 2 Hz and strain values ​​of 10⁻⁹ ε. 2. Applications in Spacecraft and Ships: Advanced composite materials exhibit good fatigue and corrosion resistance and can reduce the weight of hulls or spacecraft, which is crucial for rapid shipping or flight. Therefore, composite materials are increasingly used in the manufacture of aviation and marine vehicles (such as aircraft wings). To comprehensively assess the condition of a ship's hull, it is necessary to understand the deformation moments, shear pressures, and impact forces on the deck at different locations. For a typical ship hull, approximately 100 sensors are required. Therefore, fiber Bragg grating sensors, with their high wavelength multiplexing capabilities, are best suited for ship hull inspection. Fiber Bragg grating sensing systems can measure the bending stress of the hull and the impact force of waves on a wet deck. A 16-channel fiber Bragg grating multiplexing system with interferometric detection capabilities has successfully achieved dynamic strain measurement within a bandwidth of 5 kHz and a resolution of less than 10 nε/(Hz)¹/². Furthermore, to monitor the strain, temperature, vibration, takeoff and landing status, ultrasonic fields, and acceleration of an aircraft, over 100 sensors are typically required. Therefore, the sensors must be as lightweight and small as possible, making the most agile fiber Bragg grating (FBG) sensor the best choice. Additionally, aircraft composite materials exhibit strain in two directions, making FBG sensors embedded in the material ideal intelligent components for multi-point, multi-axial strain and temperature measurement. 3. Applications in Civil Engineering Structures Structural monitoring in civil engineering is the most active field for FBG sensors. Measuring mechanical parameters is crucial for the maintenance and condition monitoring of bridges, mines, tunnels, dams, and buildings. By measuring the strain distribution of these structures, local loads and conditions can be predicted. FBG sensors can be attached to the surface of the structure or pre-embedded within it, simultaneously performing impact detection, shape control, and vibration damping detection to monitor structural defects. Furthermore, multiple FBG sensors can be cascaded into a sensor network for quasi-distributed structural monitoring, allowing for remote control of the sensor signals via computer. Bridges are one of the building structures that FBG sensors can detect. In application, a set of fiber Bragg gratings (FBGs) is bonded to the surface of the bridge composite reinforcement, or a small groove is cut into the surface of the beam to embed the bare fiber core of the grating for protection. For more comprehensive protection, it is best to embed the gratings into the composite reinforcement during bridge construction. Since it is necessary to correct for strain caused by temperature effects, separate stress and temperature sensing arms can be used, and these two arms are installed on each beam. Two fiber Bragg gratings with the same center wavelength replace the mirrors of a Fabry-Perot interferometer, forming an all-fiber Fabry-Perot interferometer (FFH). This method utilizes low coherence to minimize phase noise in the interference, achieving highly sensitive dynamic strain measurement. By combining an FFPI with two other FBGs, one grating is used to measure strain, while the other is protected from stress to measure and correct for temperature effects. Therefore, the FFP-FBG method simultaneously measures three quantities: temperature, static strain, and instantaneous dynamic strain. This method combines the coherence of an interferometer with the advantages of fiber Bragg grating sensors. Within a measurement range of 5 mε, static strain measurement accuracy of less than 1 µε, temperature sensitivity of 0.1℃, and dynamic strain sensitivity of less than 1 nε/(Hz) 1/2 have been achieved. 4. Applications in the Power Industry: Fiber optic sensors are ideal for power industry applications due to their immunity to electromagnetic interference and ability to achieve long-distance, low-loss transmission. The load on power lines, the temperature of transformer windings, and high currents can all be measured using fiber optic grating sensors. In the power industry, current converters can convert current changes into voltage changes. These voltage changes cause deformation in piezoelectric ceramics (PZTs). By utilizing the wavelength shift of the fiber optic grating attached to the PZT, the deformation can be easily determined, thus revealing the current intensity. This is a relatively inexpensive method and does not require complex electrical isolation. Furthermore, excessive pressure on power lines, such as from heavy snow, can lead to dangerous incidents; therefore, online monitoring of power line pressure is crucial, especially for power lines in mountainous areas where detection is difficult. Fiber Bragg grating (FBG) sensors can measure the load on a wire. The principle is to convert changes in load into changes in stress on a metal plate attached to the wire. This stress change is detected by the FBG sensor bonded to the metal plate. This is an example of using FBG sensors for long-distance measurements in harsh environments. In this case, the spacing between adjacent gratings is large, so rapid modulation and demodulation are not required. 5. Applications in Medicine: Sensors used in medicine are mostly electronic sensors, which are unsuitable for many internal medicine procedures, especially in high-frequency microwave (radiation), ultrasonic fields, or excessively high-temperature treatments involving laser radiation. The metal conductors in electronic sensors are easily affected by electromagnetic fields such as current and voltage, causing thermal effects around the sensor head or tumor, leading to erroneous readings. To determine the safety of high-frequency radiation or microwave fields, ultrasonic sensors are needed to test the performance of a range of ultrasound diagnostic instruments used in medical procedures (including ultrasound surgery, excessively high-temperature treatments, and lithotripsy). In recent years, the use of high-frequency current, microwave radiation, and lasers for hyperthermia as an alternative to surgery has garnered increasing attention in the medical community. Furthermore, the small size of sensors is crucial in medical applications because smaller sizes minimize damage to human tissue. Fiber Bragg grating (FBG) sensors are currently the smallest sensors achievable. FBG sensors can measure precise local information about temperature, pressure, and acoustic fields within human tissue with minimal invasiveness. To date, FBG sensing systems have successfully detected the temperature and ultrasound fields of diseased tissue, achieving measurement results with a resolution of 0.1℃ and an accuracy of ±0.2℃ within the range of 30℃–60℃. The ultrasound field measurement resolution is 10⁻³ atm/Hz¹/², providing valuable information for the study of diseased tissue. FBG sensors can also be used to measure cardiac efficiency. In this method, a doctor inserts a thermodilution catheter embedded with a fiber Bragg grating into the right atrium of the patient's heart and injects a cold solution. The temperature of the blood in the pulmonary artery can be measured, and combined with pulse power, the cardiac output can be determined, which is crucial for cardiac monitoring. 6. Applications in Chemical Sensing: Fiber Bragg grating sensors can be used for chemical sensing because the center wavelength of the grating changes with the refractive index. Interactions between grating stray waves and changes in the concentration of chemicals in the environment both cause changes in the refractive index. Long-period fiber gratings (LPFGs), like Bragg gratings, are formed by periodic refractive index modulation along the fiber axial direction, with a period typically greater than 100µm. Their coupling mechanism involves the forward-propagating core fundamental mode being coupled with several forward-propagating cladding modes of specific wavelengths. These cladding modes are quickly lost, so LPFGs have virtually no back reflection, resulting in absorption peaks of several specific wavelengths in their transmission spectrum. LPFGs are more sensitive to changes in the refractive index of the fiber cladding material than Bragg gratings. Any change in the cladding material's refractive index alters the characteristics of the transmission spectrum, causing changes in the absorption peaks. Therefore, long-period grating refractive index measurement systems can achieve a sensitivity of 10⁻⁷. Currently, long-period gratings have been used to measure the concentration of many chemical substances, including sucrose, ethanol, hexanol, hexadecane, CaCl2, and NaCl. In principle, any chemical substance with an absorption peak spectrum and a refractive index between 1.3 and 1.45 can be detected using long-period gratings. IV. Conclusion Besides the applications mentioned above, fiber optic grating sensors have also been applied in other fields, and in many aspects, their performance is more stable, reliable, and accurate than traditional electromechanical sensors. Fiber optic grating sensors can be used to sense and measure physical quantities such as stress, strain, or temperature, exhibiting high sensitivity and measurement range. By writing fiber optic gratings with different pitches into several parts of an optical fiber, the corresponding physical quantities and their changes at several locations can be measured simultaneously, realizing quasi-distributed fiber optic sensing. In conclusion, the application of fiber optic grating sensors is a burgeoning field with very broad development prospects. Current research on fiber Bragg grating (FBG) sensors mainly focuses on three aspects: first, research on the sensor itself and sensors capable of lateral strain sensing with high sensitivity, high resolution, and simultaneous sensing of strain and temperature changes; second, research on grating reflection or transmission signal analysis and testing systems, aiming to develop low-cost, miniaturized, reliable, and sensitive detection technologies; and third, research on the practical applications of FBG sensors, including packaging technology, temperature compensation technology, and sensor network technology. The most significant obstacle limiting the application of FBG sensors is the demodulation of the sensing signal. Many demodulation methods are under research, but few demodulation products are practically applicable, and they are expensive. Secondly, other issues in FBG sensor applications are also crucial, such as: 1. Due to the limited bandwidth of the light source, and the requirement that the reflection spectra of the gratings generally do not overlap, the number of reusable gratings is limited; 2. How to simultaneously measure multi-axial strain in composite materials to reproduce the multi-axial strain morphology of the measured object; 3. How to achieve large-scale, high-precision, and rapid real-time measurement; 4. How to correctly distinguish whether the grating wavelength change is caused by temperature changes or by stress-induced strain, etc. Effectively solving the above problems is of great significance for realizing a low-cost, stable, high-resolution, large-measurement-range, multi-grating multiplexing sensing system, and these aspects still need to be developed.