Highly sensitive and high speed, the principle and application analysis of fiber optic sensor

With the rapid development of optical fiber and optical fiber communication technologies, optical fiber sensing technology emerged. Since its birth, fiber optic sensors have developed rapidly due to their small size, light weight, high sensitivity, fast response, strong anti-electromagnetic interference capability and ease of use. Especially today, with the rapid development of the Internet of Things, the status of optical fiber sensing technology can not be ignored.

1. Basic principle and classification of fiber optic sensors

Optical fiber sensing technology is a new type of sensing technology developed in the 1970s. When light propagates through an optical fiber, it is reflected by light under the influence of external temperature, pressure, displacement, magnetic field, electric field and rotation. , refractive and absorption effects, optical Doppler effect, acousto-optic, electro-optic, magneto-optical and elastic effects, etc., can directly or indirectly change the amplitude, phase, polarization state and wavelength of the light wave, thus the fiber As a sensitive component to detect various physical quantities.

The fiber optic sensor is mainly composed of a light source, a transmission fiber, a photodetector, and a signal processing part. The basic principle is that the light from the light source is sent to the sensing head (modulator) through the optical fiber, so that the parameters to be measured interact with the light entering the modulation area, resulting in the optical properties of the light (such as the intensity, wavelength, frequency of the light, The phase, polarization state, etc. are changed to become modulated signal light, which is then sent to the photodetector through the optical fiber to convert the optical signal into an electrical signal, and finally the signal is processed to restore the measured physical quantity. Fiber optic sensors are generally classified into functional (sensing type) sensors and non-functional (light transmitting type) sensors.

The functional sensor is characterized by the ability of the optical fiber to be sensitive to the external information and the detection capability. When the optical fiber is used as a sensitive component, when measured in the optical fiber, the characteristics of the intensity, phase, frequency or polarization state of the light will change. The function of modulation is realized. Then, the signal to be measured is obtained by demodulating the modulated signal. In this kind of sensor, the optical fiber not only plays the role of light transmission, but also plays the role of "sense".

Non-functional sensors use other sensitive components to sense the measured changes. The optical fiber acts only as a transmission medium for information, that is, the optical fiber only acts as a light guide. Compared with traditional electric sensors, fiber optic sensors have strong anti-electromagnetic interference capability, good electrical insulation and high sensitivity, so they are widely used in various fields such as environment, bridges, dams, oil fields, clinical medical testing and food safety. Testing and other fields.

2. Development status of fiber optic sensors

Since the birth of the fiber sensor, its superiority and wide application have been closely watched and highly valued by all countries in the world, and it has been actively researched and developed. At present, optical fiber sensors have been measured for more than 70 physical quantities such as displacement, pressure, temperature, speed, vibration, liquid level and angle. Some countries such as the United States, Britain, Germany and Japan have focused on six aspects of fiber-optic sensor systems, modern digital fiber control systems, fiber optic gyros, nuclear radiation monitoring, aircraft engine monitoring and civil programs, and have achieved certain achievements.

The research work of fiber optic sensors in China began in 1983. The research on fiber optic sensors by some universities, research institutes and companies has led to the rapid development of fiber optic sensing technology. In 2010, Zhang Xuping, a professor at the School of Engineering and Management of Nanjing University, invented the "continuous distributed optical fiber sensing technology based on the Brillouin effect", which passed the expert appraisal organized by the Ministry of Education. The appraisal expert group unanimously believes that this technology has strong innovation, possesses a number of independent intellectual property rights, and has reached the domestic leading level and international advanced level in technology, and has a good application prospect. The essence of this technology is the use of the concept of the Internet of Things, which fills the gap in the Internet of Things in China.

The concept of the Internet of Things was proposed in 1999, and its English name is "The Internet of Things", which is "the network of things connected." The Internet of Things is based on the Internet and uses information technology such as RFID (radio frequency identification) technology, infrared sensors, global positioning system and laser scanner to connect items to the Internet to realize information exchange and communication, thus achieving intelligentization. A network that locates, intelligently identifies, tracks, monitors, and manages. The technical architecture of the Internet of Things consists of three levels: the perception layer, the network layer, and the application layer.

The sensing layer is mainly used to collect various data information of items in the physical world, mainly by temperature sensors, sound sensors, vibration sensors, pressure sensors, sensors, terminals, RFID tags and readers, and QR code tags. It consists of various types of acquisition and control modules such as readers and sensor networks.

The network layer is divided into two parts: the access layer and the bearer network. This layer can realize a wide range of information communication. Through the existing communication systems such as mobile networks and the Internet, the data information obtained by the sensing layer is transmitted to various parts of the earth to realize the earth. Long-range communication within range.

The application layer is composed of various application servers. The main task of this layer is to summarize all the information about the items after the work of the sensing layer and the network layer is completed, and then re-process the information to further improve the comprehensive utilization of the information. This layer is a bridge between the Internet of Things and various industries. It can realize the application of IoT technology to various industries to meet the needs of the industry and realize the intelligence of the industry.

According to the basic knowledge of the Internet of Things mentioned above, a large number of sensors are needed in the Internet of Things to perceive various environmental parameters, provide the most original data information for the Internet of Things, and then obtain the desired results by processing, so the sensor is The core of the Internet of Things.

1. Application in infrastructure engineering internet of things

During the construction and use of major projects such as roads, bridges, tunnels and buildings, local collapse, leakage and fires of the tunnels, local cracks and collapses of the bridges often occur, which not only seriously threaten the lives and property of the people, but also affect them. The rapid and stable development of the national economy. Embedding fiber optic sensors in these buildings or public facilities, you can feel the structural deformation, structural dynamic characteristics and traffic loads of the bridge. At the same time, the tension sensor can be used to feel the local deformation of the tunnel. This information can be compared with the Internet. Combined, a "fiber optic Internet of Things" is formed to achieve long-term stable real-time monitoring of these infrastructures and reduce accidents.

2. Application in modern agricultural internet of things

The development of the Internet of Things and sensing technology has promoted the advancement of agricultural modernization technology. The application of sensing technology to the breeding, growth, harvesting and storage of crops and the formation of an Internet of Things system can achieve high production and low consumption of agricultural production and management.

In the process of seedling growth and growth of crops, physical quantities such as temperature, relative humidity, light intensity, nutrient, pH and carbon dioxide concentration in the crop growth environment are detected by fiber temperature sensor, fiber optic humidity sensor and fiber carbon dioxide sensor. And feedback to the management center in real time, and then adjust the corresponding parameters through the automatic control device, so that the crop has an optimal growth environment to achieve rapid growth and high yield.

In addition, in the processing and storage of agricultural products, fiber temperature sensors and fiber optic humidity sensors can also be used to obtain environmental information of the granary and to guide the operation of ventilation and lighting. Storage of fruits, vegetables, etc. requires a corresponding fiber optic gas sensor to measure the concentration of gases such as ethylene, oxygen, carbon dioxide, ammonia, and freon. This information can be fed back to the monitoring center through the IoT system for real-time monitoring and remote control.

3. Application in intelligent perimeter intrusion prevention system

The perimeter intrusion prevention system based on optical fiber sensing technology is a major application highlight in the industrial field with the development of optical fiber sensing technology in recent years. At present, there are many kinds of sensors applied in the perimeter intrusion prevention system, so the technical solutions of the system are also very different, but in general, the fiber perimeter intrusion sensor is used to feel the external intrusion, and the alarm is timely. Can be widely used in important places such as airports, power stations and military bases.

4. Other applications

Because of its anti-electromagnetic interference, fiber-optic sensors can be used in applications where electrical sensors are not easy to use. In national defense, fiber-optic sensors can be used for underwater acoustic exploration (fiber hydrophone), fiber guidance, attitude control, and spacecraft structures. Damage detection and detection of the battlefield environment (electromagnetic environment, biochemical environment, etc.); in the power system, it can be used to measure the current, voltage and temperature inside the rotor, stator and high voltage transformer of large motors; in addition, fiber optic sensors can also be used In the process of oil and gas exploitation, the monitoring of parameters such as pressure and temperature in the underground, leakage monitoring of the oil pipeline, etc., can be combined with the Internet to form a powerful Internet of Things network to achieve safe production and management.

Fiber optic sensors have attracted more and more attention because of their various advantages, and have been widely used in different fields of national life. Sensors are the core of the Internet of Things. With the rapid development of sensing technology and the Internet of Things, the close integration of optical sensing technology and the Internet of Things will become the focus of attention and will become more and more important in people's production and life. The role.