Fiber Optic Sensor

Sensors are moving in the direction of sensitivity, precision, adaptability, compactness and intelligence. In this process, new members of the family of fiber-optic sensor sensors have become popular. Optical fiber has many excellent properties, such as: anti-electromagnetic and atomic radiation interference performance, fine diameter, soft, lightweight mechanical properties; insulation, non-inductive electrical properties; water resistance, high temperature resistance, corrosion resistance, chemical properties, etc. It can act as a human ear in areas that people cannot reach (such as high temperature areas) or areas that are harmful to humans (such as nuclear radiation areas), and it can also exceed the physiological limits of human beings and receive human senses. Unexpected outside information.

Features

1. Due to the prism used in the reflector, the detection performance is higher and more reliable than that of the general-purpose reflective optical sensor.

2. The circuit connection is simpler and easier than the discrete optical sensor.

3, Snap button embedded design, installation is more simple

use

1, used for telephone, network broadband and other digital models transmission.

2. Passage of banknotes, cards, coins, passbooks, etc. for devices related to vending machines, financial terminals, money counters, etc.

3, for product positioning, counting, identification on automation equipment.

The basic working principle of the fiber optic sensor is to send light from the light source to the modulator through the fiber, so that the measured parameter interacts with the light entering the modulation area, resulting in the optical properties of the light (such as light intensity, wavelength, frequency, phase, The change in polarization state, etc., is called the modulated signal light, and the measurement is performed using the influence of the measured light transmission characteristics.

The structure of the optical fiber

2. Optical transmission principle of optical fibers

3. Optical fiber sensor works

(1) Functional type - made using a sensitive characteristic or function of the optical fiber itself

(2) Light-transmitting type - The optical fiber only acts as transmitting light, and it adds other sensitive elements on the end face or in the middle of the fiber to sense the measured change. [2]

Optical fiber sensors have two measuring principles.

(1) Physical-type optical fiber sensor principle Physical-type optical fiber sensor utilizes the sensitivity of the optical fiber to environmental changes and converts the input physical quantity into a modulated optical signal. Its working principle is based on the optical modulation effect of the optical fiber, that is, when the optical fiber changes in external environment factors such as temperature, pressure, electric field, magnetic field, etc., its light transmission characteristics, such as phase and light intensity, will change.

Therefore, if the phase and intensity of the light passing through the fiber can be measured, the change in the measured physical quantity can be known. This type of sensor is also known as a sensitive element type or functional fiber sensor. The point source light beam of the laser is diffused into parallel waves and divided into two channels by the beam splitter. One is a reference light path and the other is a measuring light path. The external parameters (temperature, pressure, vibration, etc.) cause changes in the length of the optical fiber and phase changes in the optical phase, resulting in a different number of interference fringes, and counting its die movement to measure temperature or pressure.

(2) The principle of structured optical fiber sensors. Structured optical fiber sensors are measurement systems composed of optical detection elements (sensing elements) and optical fiber transmission circuits and measurement circuits. Wherein the optical fiber only serves as a light propagation medium, it is also called a light-transmitting or non-functional optical fiber sensor.

Optical fiber has many excellent properties, such as: anti-electromagnetic and atomic radiation interference performance, fine diameter, soft, lightweight mechanical properties; insulation, non-inductive electrical properties; water resistance, high temperature resistance, corrosion resistance, chemical properties, etc. It can play the role of human ears in places where people cannot reach people or in areas that are harmful to people (such as nuclear radiation areas). It can also surpass the physiological limits of human beings, and can not be perceived by human senses. Outside information. [3]

First, high sensitivity;

Second, the geometric shape has a variety of adaptability, can be made into any shape of fiber optic sensor;

Third, it can manufacture devices that sense various physical information (sound, magnetic, temperature, rotation, etc.);

Fourth, can be used for high pressure, electrical noise, high temperature, corrosion, or other harsh environments;

Fifth, but also with the inherent compatibility of optical fiber telemetry technology.

The advantages of fiber-optic sensors are that optical fiber sensors use light as a carrier for sensitive information, and optical fibers as medium for transmitting sensitive information have the characteristics of optical fiber and optical measurement, and have a series of unique advantages. Good electrical insulation, anti-electromagnetic interference, non-invasive, high sensitivity, easy to implement remote monitoring of the signal under test, corrosion resistance, explosion-proof, flexible optical path, easy to connect with the computer.

Sensors are developed in the direction of sensitivity, precision, adaptability, compactness and intelligence. They can serve as the eyes and ears of people where people cannot reach people (such as high temperature areas or areas that are harmful to people, such as nuclear radiation areas). It can also work beyond the physiological limits of human beings and receive external information that is not perceived by human senses.

Insulation in contamination, magnetism, sound, pressure, temperature, acceleration, gyro, displacement, liquid level, torque, photoacoustic, current, fiber optic sensors can be used for displacement, vibration, rotation, pressure, bending, strain, velocity, acceleration, current , magnetic field, voltage, humidity, temperature, sound field, flow, concentration, pH and strain, etc. Optical fiber sensors have a wide range of applications, covering almost all important fields of national economy and national defense and people's daily lives. They can be used safely and effectively in harsh environments, solving the technical problems that have existed in many industries for many years, and they have great potential. Market demand. Mainly in the following areas of application:

The application of interference gyroscopes and grating pressure sensors in bridges, dams, and oil fields in urban construction. Fiber-optic sensors can be embedded in concrete, carbon fiber-reinforced plastics, and composites to measure stress relaxation, construction stress, and dynamic load stress, thereby assessing the structural performance of bridges during short-term construction and long-term operating conditions.

In the power system, it is necessary to measure parameters such as temperature and current, such as temperature detection in stators and rotors of high-voltage transformers and large motors. Since electrical sensors are susceptible to electromagnetic field interference, they cannot be used in such applications and can only be used. Fiber Optic Sensor. Distributed fiber optic temperature sensor is a new technology developed in recent years for real-time measurement of spatial temperature field distribution. Distributed fiber optic temperature sensor system not only has the advantages of universal fiber optic sensor, but also has the temperature of each point along the fiber. The distributed sensing capability, using this feature, we can continuously measure the temperature of each point within a few kilometers along the optical fiber in real time, positioning accuracy can reach the order of meters, and the measurement accuracy can reach a level of 1 degree, which is very suitable for measuring the temperature of large-intersection points. Application occasions.

In addition, fiber-optic sensors can also be applied to railway monitoring, rocket propulsion systems, and well detection.

Optical fiber has the advantages of wideband, high capacity, long-distance transmission and multi-parameter, distributed, low-power sensing. Fiber-optic sensors can continue to capture new technologies and new devices for optical fiber communications. Various fiber-optic sensors are expected to be widely used in the Internet of Things. [4]

Applied to Civil Engineering

With the development of fiber-optic sensor technology, fiber-optic sensors in the field of civil engineering have been widely used to measure deformation and internal stress of concrete structures, and to detect large-scale structures and bridge health conditions. The most important ones are the fiber-optic sensors. A new type of strain sensor is used.

The optical fiber sensor can be attached to the surface of the structure for measurement, and can also be embedded to achieve the measurement of the internal physical quantity of the structure. By using a pre-embedded fiber-optic sensor, the internal strain of a concrete structure can be measured during internal damage. Based on the slope of the load-strain curve, the formation and propagation of internal damage can be determined. Concrete experiments show that the load-strain curve of the fiber test is higher than the linearity of the strain gauge test. [5]

Applied to detection technology

Optical fiber sensors in aerospace (pressure measurement, temperature measurement, gyroscope, etc.), navigation (sonar, etc.), oil production (level measurement, flow measurement, measurement of voidage in two-phase flow), power transmission (current measurement and voltage measurement of high-voltage transmission network), nuclear industry (radiation dose measurement, leakage dose monitoring of atomic power station), medical treatment (measurement of blood flow velocity, blood pressure and heart sound measurement), and scientific research (rotation of the earth) Widely used. [6]

Applied to oil industry

In oil well logging technology, optical fiber sensors can be used to measure physical quantities such as downhole oil flow, temperature, pressure, and moisture content. The more mature application is to use an extrinsic fiber F-P cavity sensor to measure downhole pressure and temperature. The extrinsic optical fiber FP cavity sensor utilizes the principle of multi-beam interference of light. When the measured temperature or pressure changes, the interference fringe changes, and the cavity length of the F-P cavity of the optical fiber also changes, and the change of the cavity length is calculated. Temperature and pressure measurements. [7]

Applied to temperature measurement

Optical fiber sensing technology is a new technology formed with the rapid development of optical communication. In optical communication systems, optical fibers are long-distance transmission media for optical signals. When the light wave is transmitted in the optical fiber, characteristic parameters that characterize the phase, frequency, amplitude, and polarization of the light wave will change due to the influence of external factors such as temperature, pressure, magnetic field, and electric field, so that the optical fiber can be used as a sensor element. Detecting the size of various physical quantities that cause the change of the light wave signal, this is the optical fiber sensor. Devices that use external factors to detect changes in the phase of the optical fiber to detect physical quantities are called phase modulation sensing optical fiber sensors. Other optical fiber sensors include amplitude modulation sensing, polarization modulation, and light transmission.

Applicable to measuring the Young's modulus of wire

The use of a sensor measuring instrument instead of the optical lever mirror group constitutes a new Young's modulus measurement system, which not only has a short operation but also improves the accuracy and accuracy of the measurement results. The basic principle of the traditional wire drawing method is to convert the tiny elongated deformation variable of the wire after the code is sheared through the light path of the mirror group and enlarge it by a certain number of times so as to obtain a slight elongation, and then calculate. Obtain the Young's modulus value.
Since there are some sensors, we compared the new method of optical fiber sensor measurement with the above method. Optical fiber sensor measurements have improved in sensitivity, accuracy and accuracy. The basic principle of infrared light ranging system measurement is the use of infrared optical fiber sensors to directly measure small displacements. The linearity of infrared optical fiber sensors for measuring small distances within 3mm is very high. The system consists of a sensor measuring instrument and a reflective fiber displacement sensor.
The working principle of the reflective optical fiber displacement sensor is to use two multi-mode optical fibers, one end combined to form the optical fiber probe, and the other end is divided into two beams, respectively as the receiving optical fiber and the light source optical fiber. When the infrared light generated by the light emitter is irradiated to the reflector via the light source fiber, the reflected light passes through the receiving optical fiber and is transmitted to the photoelectric conversion element to convert the received light signal into an electric signal. There is a certain functional relationship between the light intensity of the output and the distance of the reflector from the optical fiber probe, so the displacement can be obtained by detecting the light intensity. A nickel-plated reflecting metal sheet was fixed on the cylinder at the metal wire of the Young's modulus meter by using a magnet so that it could move with the elongation of the wire. The infrared sensor is fixed on the support table, and then the potential difference obtained by the experiment is changed on the sensor measurement instrument. Through multiple tests, both the rotating micrometer equipped with the sensor measuring instrument and the probe and the metal plate are changed. At the distance and position, when the potential difference corresponding to the length of the wire length recorded during the experiment is recorded, the spiral micrometer reading at this time is recorded. Tests have shown that this method is simple to use with infrared light ranging. Just after the probe and the reflective sheet are installed, the actual measurement of the plus code on the tray can be directly started. The result of the side volume is obviously superior to the conventional test.

According to the modulation form of light subject to the measured object can be divided into: intensity modulation type, polarization state system, phase system, frequency system;

According to whether light interference occurs can be divided into: interference type and non-interference type;

According to whether it can be monitored continuously with increasing distance, it can be divided into: distributed and point-division;

According to the role of optical fiber in the sensor can be divided into: a type is a functional (Functional Fiber, abbreviated as FF) sensor, also known as sensor sensor; the other is a non-functional type (Non Functional Fiber abbreviated as NFF), in It is called a light-transmitting sensor.

Functional sensors

The functional sensor uses the characteristics of the optical fiber to use the optical fiber as a sensing element and is used to modulate the light transmitted in the optical fiber so that the intensity, phase, frequency, or polarization state of the transmitted light is changed, and then the modulated light is modulated. The signal is demodulated to obtain the measured signal.

The optical fiber is not only a light guiding medium but also a sensitive element. Light is measured and modulated in the optical fiber, and multimode optical fibers are often used.

Advantages: compact structure, high sensitivity.

Disadvantages: special optical fiber is required, high cost,

Typical examples: Fiber Optic Gyroscopes, Fiber Optic Hydrophones, etc.

Fiber Optic Voltage/Current Sensors

Non-functional fiber sensor

Non-functional fiber-optic sensors use other sensitive elements to sense the changes that are measured. Optical fibers are used as information transmission media and often use single-mode fibers.

The optical fiber only plays the role of light, and the light is modulated by the fiber-based sensor.

Advantages: Optical fiber can be used for electrical isolation, has data transmission, and the signal transmitted by optical fiber is not affected by electromagnetic interference.

Most of the practical applications are non-functional optical fiber sensors. AnyWay's variable frequency voltage sensor, variable frequency current sensor, variable frequency power sensor (a combination of voltage and current sensors) is a non-functional optical fiber sensor, and has unique advantages in power measurement under complex electromagnetic environment.

Optical fiber sensors are new technologies that have emerged in recent years and can be used to measure a variety of physical quantities such as sound field, electric field, pressure, temperature, angular velocity, acceleration, etc., and can also complete measurement tasks that are difficult for existing measurement techniques to complete. In a confined space, fiber-optic sensors have demonstrated their unique capabilities in strong electromagnetic interference and high-voltage environments. There are more than 70 types of optical fiber sensors, which are roughly divided into optical fiber sensors and optical fiber sensors.

The sensor of the so-called optical fiber itself is the optical fiber that directly receives the outside world and is measured. The externally measured physical quantity can cause changes in the length, refractive index, and diameter of the measurement arm, so that the light transmitted in the optical fiber changes in amplitude, phase, frequency, polarization, and the like. The light transmitted by the measuring arm interferes with the reference light of the reference arm (comparison), and the phase (or amplitude) of the output light changes, and the measured change can be detected based on this change. The phase of the transmission in the optical fiber is highly sensitive to external influences, and the interferometric technique can detect the physical quantity corresponding to the slight phase change of 10 negative quadratic arc. Using the optical fiber's winding property and low loss, it can make long fiber plates into small diameter fiber rings to increase the utilization length and obtain higher sensitivity.

Optical fiber acoustic sensor is a kind of sensor using the optical fiber itself. When the optical fiber is subjected to a slight external force, micro-bending occurs, and its light transmission ability changes greatly. The sound is a kind of mechanical wave. Its effect on the optical fiber is to make the optical fiber be forced and bend, and the strength of the sound can be obtained by bending. Fiber optic gyro is also a kind of optical fiber sensor. Compared with laser gyro, fiber optic gyro has high sensitivity, small size, and low cost. It can be used in high-performance inertial navigation systems such as aircrafts, ships, and missiles. The figure shows the principle of an optical fiber sensor turbine flowmeter.

Fiber Bragg Grating Sensor

How Fiber Bragg Grating Sensors Work

The Fiber Bragg Grating Sensor (FBS) is an optical fiber sensor with the highest frequency and the widest range. The sensor can change the wavelength of the reflected light wave according to changes in ambient temperature and/or strain. A fiber Bragg grating is a holographic interferometer or phase mask method that exposes a short length of light sensitive fiber under a light wave with a periodic distribution of light intensity. In this way, the optical refractive index of the optical fiber will be permanently changed according to the intensity of the irradiated light wave. The periodic variation of the refractive index of light caused by this method is called fiber Bragg grating.

When a broad-spectrum light beam is propagated to a fiber Bragg grating, each small section of the fiber after the refractive index of the light is changed will reflect only a specific wavelength of light. This wavelength is called the Bragg wavelength. This characteristic makes Fiber Bragg gratings only reflect light waves of a specific wavelength, while light waves of other wavelengths are propagated.

According to the function of the optical fiber in the optical fiber sensor, it can be divided into two types: a sensor type and a light transmission type.

The optical fiber of the sensor type optical fiber sensor not only transmits light but also is an optical sensor. Due to the influence of the external environment on the optical fiber itself, the physical quantity to be measured acts on the sensor through the optical fiber, so that the properties of the optical waveguide (light intensity, phase, polarization, wavelength, etc.) are modulated. Sensor-type optical fiber sensors are further classified into optical emphasis, phase modulation, vibration modulation, and wavelength modulation. [8]

Transmissive optical fiber sensor

The optical fiber type optical fiber sensor measures the optical signal modulated by the measured object into the optical fiber, and performs optical signal processing at the output end. This type of sensor is sensitive to the physical quantity to be measured by the other photosensitive element. The optical fiber is only used as For the light-transmitting element, a sensing element capable of modulating the light transmitted by the optical fiber must be added to constitute the sensing element. Optical fiber sensors can be classified into point fiber sensors, integral fiber sensors, and distributed fiber sensors according to their measurement ranges. Among them, the distributed optical fiber sensor is used to detect the strain distribution of the large-scale structure, and it can quickly and non-destructively measure the structural displacement, internal or surface stress and other important parameters. The types of optical fiber sensors used in civil engineering include Math-Zender interferometric fiber sensors, Fabry-Pero cavity fiber sensors, and fiber Bragg grating sensors.

The light weight, durability and long-term stability of fiber optic sensors make them easy to apply to internal stress and strain detection of various building materials such as building steel structures and concrete. Achieve the healthy detection of building structures.

Another major category of fiber optic sensors is fiber optic sensors. The structure is roughly as follows: The sensor is located at the end of the fiber. The fiber is just the transmission line of light, and the physical quantity to be measured is transformed into the change of the amplitude, phase or amplitude of the light. In this sensor system, traditional sensors and fiber optics are combined. The introduction of the fiber allows the possibility of probed telemetry. This fiber-optic transmission sensor has a wide range of applications and is easy to use, but its accuracy is slightly lower than that of the first type of sensor.

Optical fiber is a rising star in the sensor family. It is widely used due to its excellent performance. It is a sensor that is worth noting in production practice.

Optical fiber sensors have become a rising star of the sensor family due to their large number of advantages, and they have played a unique role in various measurements and become an indispensable member of the sensor family.

With the continuous development of China's industrial automation application environment, the instrumentation industry is changing with each passing day, and the current instrumentation industry is facing new developments. The industry's 12th Five-Year Plan (Draft) also proposes several key developments in accordance with the requirements of the new period. Key technologies, which undoubtedly have important guiding significance for the future development of the industry. [9]

New sensor technologies include solid-state silicon sensor technology, fiber optic sensing technology, biochip technology, gene chip technology, image sensor technology, and all-solid-state inertial sensor technology. The "Twelfth Five-Year Plan" will focus on smart sensors and carry out key technologies. In the field of fiber-optic sensing, it focuses on the development of new principles, new-effect sensor technologies, sensor intelligence technologies, sensor network technologies, miniaturization and low-power technologies, and sensor arrays and multi-parameter multi-parameter design, manufacturing, and packaging technologies.

Hundreds of units currently work in this area, such as Tsinghua University, Fudan University, Tianjin University, Chongqing University, Beijing University of Aeronautics and Astronautics, etc. They have fiber optic temperature sensors, pressure gauges, flow meters, level gauges, and galvanometers. Many fields have been studied. In addition, in Wuhan, Shanghai, Guangdong, Shenzhen and other places, a number of manufacturers of fiber optic passive components have also been established, and the market size has reached more than 120 billion yuan.

(I) Analysis of favorable factors affecting the development of the industry

The white paper “Sensor Development Policy” formulated by the National Science and Technology Commission in April 1987 determined that it is necessary to develop sensor technology vigorously, and in particular to develop new sensor technologies as a priority area. Article 21 of the "Suggestions of the CPC Central Committee on Formulating the 10-year Plan for the National Economy and Social Development and the Plan for the Eighth Five-Year Plan" on December 30, 1991 made it clear that we must vigorously strengthen the development of sensors and their universal application in the national economy.

(II) Analysis of unfavorable factors affecting the development of the industry

Although fiber-optic sensor technology has achieved some applications in practical detection, there are still some problems, such as the process problems of fiber-embedding structures. Although it can be improved through installation methods, it also results in strains passing through the metal first, and then The strain is indirectly induced by the optical fiber, so it needs to be corrected experimentally to be able to measure accurately. At the same time, the output signal of the fiber-optic sensor will be affected by the fluctuation of the light source, the change of the optical fiber transmission loss, and the aging of the detector. These factors will reduce the accuracy of the optical fiber sensor measurement. Moreover, the practicality of fiber optic sensors has yet to be developed and their production costs are quite expensive. At present, a large part of optical fiber sensors are still in the laboratory. Therefore, it is necessary to put the experimental results into use as soon as possible.

With the gradual opening of the market and the improvement of China’s investment environment and the acceleration of the globalization process, sensor manufacturers in various countries have entered the Chinese market, which has exacerbated market competition. There is still a big gap between China's domestic sensor technology level and the world level. This gap is manifested on the one hand as the sensor's backwardness in the sense of information, and on the other hand as the technology behind the sensor itself in terms of intelligence and networking. Domestic enterprises have formed a situation where they are “foreigner and hard-working”, not only losing the mid-to-high end product market, but also directly leading to a single product variety that can be produced by themselves, and a serious homogeneity. The price advantage of domestic sensors is obvious, but the quality still exists in comparison with foreign products. Certain gaps are generally applied in areas where signal requirements are not high.

The basic working principle of the fiber optic sensor is to send light from the light source to the modulator through the fiber, so that the measured parameter interacts with the light entering the modulation area, resulting in the optical properties of the light (such as light intensity, wavelength, frequency, phase, The polarization state, etc.) changes to become the modulated signal light which is sent to the photodetector through the optical fiber and after demodulation, the measured parameter is obtained.

Due to the increased application possibilities in the traditional terminal market and the emerging opportunities in new application areas, the global fiber optic sensor market size in 2013 was 1.89 billion US dollars. It is estimated that by 2020, the global fiber optic sensor market is expected to reach 3.5 billion U.S. dollars (approximately RMB 21.72 billion).

Traditional end markets include aerospace, defense, oil and gas exploration, infrastructure development and telecommunications. The development of the traditional terminal market will continue to promote the growth of the global fiber optic sensor market. The continuous transition from the 3G to 4G networks in the communications industry, the growth in smart structures, the emerging growth in infrastructure construction, and the development in the oil and gas sector all provide important opportunities for market growth.

Especially in emerging markets such as China and India, increased manufacturing activity, rising auto demand, stable infrastructure construction activities, and increased defense spending have all become drivers of the development of the global fiber optic sensor industry.

Three Basic Compositions of Optical Fiber Sensor Networks

There are three basic types of fiber optic sensor networks, one of which is a single point sensor. An optical fiber only serves as a transmission here, and another type is called a multi-point sensor, where an optical fiber connects many sensors so that many sensors can share the light source to achieve network monitoring. Then there is the smart fiber sensor [10] .

The multi-point optical fiber sensor, which looks like a grating from the outside, has a periodic interval through ultraviolet radiation. When a fiber is incident, if the wavelength of the fiber is exactly equal to twice the interval, the light wave will be strongly reflected, and if the fiber is subjected to a temperature change or strain, etc., the reflection wavelength will change. This sensor There can be more than one fiber, and connecting it can be used for a variety of sensing applications.

Because the fiber is soft, it can be two-dimensional and three-dimensional, so the horizontal axis is the position of the space, and the vertical axis is the measurement object. What problem does such a sensor network solve? It solves what happened at what position, and how many intensity problems there are, that is, it provides two-dimensional information. This is the problem that the intelligent optical fiber sensor needs to solve. It has very prominent characteristics requirements, including small size, high strength, good stability, and implantable materials. Anti-electromagnetic interference, environmental resistance.

Optical fiber sensors have been successfully applied to aircraft structure monitoring. We see that the A-380 and the Boeing 787 are characterized by more than half of the carbon fiber content. For example, carbon fiber has several defects in the resin, and one is the layer-to-layer peeling. Because this material is relatively strong, it is difficult to Carbonation tests are performed like aluminum alloys, so researchers are now working on embedding fiber-optic sensors in composites. Because the material is about 125 microns thick, this fiber-optic sensor must be a very small fiber-optic sensor. About 50 microns in diameter.

We say that optical fiber sensor networks can become a nerve network for a safe and secure society. Fiber optic sensor networks can be termed diagnostic techniques for fiber optic communication networks. Optical fiber sensor network has many applications in security, and many domestic enterprises have carried out effective work in this area.

Optical fiber sensor development status

In the domestic market, the most widely used optical fiber sensing technology is a Bragg fiber grating and a distributed sensor based on optical time domain reflection. This technology can basically meet the needs of the low-end market. Nowadays, single-frequency fiber lasers whose spectral line width is narrow to 2 kHz and its latest generation of optical sensor technology are very different from traditional optical fiber sensors. It can carry out ultra-long-distance transmission with accuracy and sensitivity. The higher requirements can be achieved. This has a great demand in the high-end market. At the beginning of the 21st century, this technology is still in the stage of establishment and pre-research. There are mainly four types of fiber optic sensor applications in the domestic market: fiber optic gyroscopes, fiber grating sensors, fiber optic current sensors, and fiber optic hydrophones. The following describes the four products separately.

First, the fiber optic gyro. The principle of FOG can be divided into interference type, resonance type, and Brillouin type. This is the representative of the third generation fiber optic gyro. The first generation of interferometric fiber optic gyros, 21 in the initial stage, the technology is mature, suitable for mass production and commercialization; the second generation of resonant fiber optic gyroscopes is temporarily still in the stage of development from laboratory research to practical application; The three-generation Brillouin type is still in the stage of theoretical research. Fiber Optic Gyro Structures There are three ways to implement optical components depending on the type of system used: small discrete component systems, all-fiber systems, and integrated optical component systems. At the beginning of the 21st century, discrete optical element technology has basically been withdrawn. All-fiber systems are used in open-loop low-precision, low-cost fiber optic gyroscopes. The integrated optics gyro is highly accurate due to its simple process, good overall repeatability, and low cost. Fiber optic gyros are very popular and are their main implementation method.

Second, fiber grating sensors. One of the research hotspots in the sensor field at home and abroad is fiber Bragg grating sensor. Traditional fiber optic sensors can be basically divided into two types: light intensity type and interference type. The disadvantage of the light intensity sensor is that the light source is unstable, and the optical fiber loss and the detector are prone to aging. The interference type sensor requires the same light intensity of the two interference light sources, so the fixed reference point is required and the application is inconvenient. Fiber grating sensors based on fiber Bragg gratings developed at the beginning of the 21st century can avoid the above two situations. The sensing signals are wavelength modulation and multiplexing capability. In applications such as building health detection, impact detection, shape control and vibration damping detection, fiber grating sensors are the most ideal sensitive components. Fiber grating sensors are widely used in geodynamics, spacecraft, power industry and chemical sensing.

Third, fiber optic current sensor. The rapid development of the power industry has led to an increase in the capacity of power transmission systems, higher and higher operating voltage levels, and increasing currents. This makes it difficult to measure. This shows the advantages of fiber-optic current sensors. In the power system, the traditional sensor for measuring current is based on electromagnetic induction. This has the following disadvantages: it is prone to explosion and even causes catastrophic accidents; large fault currents cause magnetic saturation of the iron core; resonance of the iron core occurs. Frequency response is slow; Measurement accuracy is low; Signals are susceptible to interference; Volume weight, price is expensive, etc., it is difficult to meet the development needs of a new generation of digital power networks. At this time, fiber-optic current sensors came into being.

Fourth, fiber optic hydrophone. Fiber-optic hydrophones are mainly used to measure underwater acoustic signals. They use high-sensitivity optical fiber coherent detection to convert underwater acoustic signals into optical signals and transmit them through optical fibers to signal processing systems for identification. Compared with traditional hydrophones, fiber optic hydrophones have the characteristics of high sensitivity, wide response bandwidth, and freedom from electromagnetic interference. They are widely used in military and petroleum exploration, environmental testing and other fields, and have great potential for development. Optical fiber hydrophones can be classified into interference type, intensity type, and grating type according to the principle. The key technologies of interferometric fiber optic hydrophones have gradually matured and formed products in some fields. Optical fiber grating hydrophones are currently the focus of research. The key technologies studied include light sources, fiber optic devices, probe technology, anti-polarization fading technology, and resistance. Phase fading technology, signal processing technology, multiplexing technology and engineering technology.

Optical fiber sensor technology is based on optical fiber, optical communication and optoelectronic technology. Electromagnetic interference and corrosion have little effect on it. It can also adapt to a variety of harsh meteorological environments. No additional power supply is required. Transmission over long distances has become a research hotspot in the sensor industry.

Sensors have always evolved in the direction of sensitivity, precision, adaptability, compactness and intelligence. In this process, the new member of the fiber-optic sensor family of sensors is twice as popular.
Optical fiber has many excellent properties, such as: anti-electromagnetic interference and atomic radiation properties. Optical fiber sensors are used to measure the magnetic, acoustic, pressure, temperature, acceleration, gyro, displacement, liquid level, torque, photoacoustic, current, and strain. Its application range is very extensive. Therefore, we can say that optical fiber sensors have a large market demand, not to mention long, at least in the next five years, fiber optic sensors will have broad prospects for development.

The rapid development of fiber-optic sensing technology and related technologies has met the higher requirements of various types of control devices and systems for the acquisition and transmission of information, making the degree of automation in various fields more and more high, as the core of system information acquisition and transmission. The research of the fiber optic sensor of the device is very important. The main directions of the development of optical fiber sensor technology are: (1) Multi-purpose. That is, a kind of optical fiber sensor not only aims at one kind of physical quantity, but also can simultaneously measure a variety of physical quantities. (2) Improve the spatial resolution and sensitivity of distributed sensors, reduce their cost, and design complex sensor network engineering. Pay attention to the parameters of the distributed sensor, namely the pressure, temperature, especially the influence of chemical parameters (hydrocarbons, some pollutants, humidity, pH, etc.) on the fiber. (3) Development of new sensing materials and sensing technologies. (4) Development and application of low-cost sensors (brackets, connections, and mountings) under harsh conditions (high temperature, high pressure, chemical corrosion). (5) Optical fiber connectors and micro-optical technologies combined with other micro technologies.

Optical fiber sensing applications are divided into five major directions:

(1) Oil and gas - Reservoir monitoring downhole P/T sensing, seismic arrays, energy industry, power plants, boilers and steam turbines, power cables, turbine transport, oil refineries;

(2) Aerospace - jet engines, rocket propulsion systems, airframes;

（3）民用基础建设——桥梁、大坝、道路、隧道、滑坡；

（4）交通运输——铁路监控、运动中的重量、运输安全；

（5）生物医学——医用温度压力、颅内压测量、微创手术、一次性探头。