Due to the unparalleled advantages of fiber optic sensors and technology compared to other sensors, fiber optic sensor and measurement technology has become a new development direction in the field of instrumentation in recent years. New fiber optic sensors typically possess the following characteristics: excellent light transmission performance with very low light loss, currently reaching ≤0.2dB/km. Wide bandwidth, enabling ultra-high-speed measurements, and good sensitivity and linearity. Small size and light weight, allowing for non-contact, non-destructive, and long-distance measurements in harsh environments. High sensitivity, high reliability, abundant silicon resources, resistance to electromagnetic interference, corrosion resistance, high voltage resistance, good electrical insulation, flexibility, explosion-proof, wide bandwidth, and low loss. Furthermore, they are easy to connect to computers for intelligent and remote monitoring. They extend and improve upon traditional sensors, often accomplishing tasks that traditional sensors find difficult or impossible. Because of its many unique advantages, fiber optic sensors can solve many problems that traditional sensors cannot. Therefore, since their invention, they have been widely used in various fields such as medical care, transportation, power, machinery, petrochemicals, civil construction, and aerospace.
Fiber optic liquid level sensors, based on the principle of total internal reflection, can be designed as fiber optic liquid level sensors. A fiber optic liquid level sensor consists of three parts: a photosensitive element that detects the amount of light reflected after contact with the liquid; a dual-core optical fiber for transmitting the optical signal; and a receiving device for emitting light, receiving light, and processing the signal. Both the photosensitive element and the optical fiber for transmitting the signal are made of glass fiber, thus offering advantages such as good insulation and resistance to electromagnetic noise.
In a fiber optic liquid level sensor, the light emitted from the light-emitting device is transmitted through an optical fiber to the sensing element. A portion of the light passes through the spherical surface of the sensing element, while the remainder is reflected back. When the sensing element comes into contact with the liquid, compared to contact with air, the amount of light transmitted through the spherical surface increases, while the amount of light reflected decreases. Therefore, the amount of reflected light indicates whether the sensing element is in contact with the liquid. The amount of reflected light depends on the refractive index of the sensing element glass and the refractive index of the substance being measured. The higher the refractive index of the substance, the less reflected light. The reflected light from the sensing element is converted into photoelectric value by the phototransistor of the receiving device via the optical fiber and then output. The variation in the amount of reflected light from the sensing element, compared to the amount of light in air, is -6 to -7 dB in water and -25 to -30 dB in oil. This allows for the differentiation of substances such as water and oil with significantly different amounts of reflected light.
Distributed temperature sensing monitoring is crucial for detecting fires in power plants and substations. Cable trays, cable tunnels, cable interlayers, cable trenches, cable shafts, switchgear, transformers, resistor arrays, and other electrical equipment often malfunction due to overheating and aging under prolonged high voltage conditions. Years of fire investigation and research have shown that most fires are caused by excessively high temperatures. Therefore, timely detection of fire hazards during the slow, incubation period of a fire, allowing for appropriate preventative measures, represents the optimal time for prevention.
Traditional fire detection systems typically use electromagnetic components as their sensing elements. However, in applications in the power industry, the equipment operates under high voltage, resulting in strong electromagnetic interference. This often leads to false alarms and missed alarms from traditional detectors.
Distributed fiber optic temperature sensing technology is currently the most advanced temperature measurement technology internationally. The DTS (Distributed Fiber Optic Temperature Sensor) system can measure and display the temperature values at various points along a continuous fiber optic cable, as a continuous function of distance. This technology requires only a single fiber to measure temperature over a distance of ten kilometers. For temperature monitoring in power systems, the distributed fiber optic temperature sensing system is a highly effective monitoring device. It is based on the most advanced fiber optic, laser, and signal processing technologies. It uses fiber optics as the sensor for temperature information acquisition. By measuring the scattered waves generated at different distances along the fiber by incident laser light, the real-time temperature information of each point distributed along the fiber is determined. This system is specifically designed for regional (multi-point, linear, area) temperature measurement and can predict and alarm for overheating, overcooling, and fire hazards caused by temperature.
In conclusion, fiber optic sensing technology can comprehensively address the safety monitoring needs of all aspects of the power industry, from security precautions to safe production. It has gained widespread attention in the power industry and is beginning to be widely applied.
The application of fiber optic sensors in bridge health monitoring is increasingly important due to the growing importance of bridges in industrial and transportation development. The increasing span and structural complexity of bridges have brought greater attention to their safety hazards, necessitating improved bridge health monitoring. Bridge health monitoring involves non-destructive testing of the bridge structure, real-time monitoring of the overall structure, and diagnosis of the location and extent of damage, thus aiding in bridge maintenance and management.
With the development of fiber optic sensing technology for composite materials, fiber optic sensing systems embedded in concrete structures have been able to detect stress and fracture damage. Fiber optic sensors, due to their advantages such as light weight, small structure, large information capacity, immunity to electromagnetic interference, and ease of embedding in structures, can perform real-time monitoring and control of bridges, thereby achieving the goal of monitoring the health of bridge structures. Currently, the most widely used type is the fiber optic grating sensor, which is one of the important tools in bridge health monitoring systems.
