What Is a Fiber-Optic Sensor?

Optical fiber sensor is a sensor that changes the state of the measured object into a measurable light signal. The working principle of the optical fiber sensor is to send the light beam incident from the light source into the modulator through the optical fiber, and interact with the measured parameters inside the modulator to make the optical properties of the light such as light intensity, wavelength, frequency, phase, polarization state, etc It changes into a modulated optical signal, which is then sent to the optoelectronic device through the optical fiber, and the measured parameter is obtained after the demodulator. During the whole process, the light beam is introduced through the optical fiber, and then emitted after passing through the modulator. The function of the optical fiber is to transmit the light beam first, and then to act as an optical modulator. ^[1]

Sensors are moving in the direction of sensitivity, precision, adaptability, compactness and intelligence. in this process,

The basic working principle of a fiber optic sensor is to send light from a light source through an optical fiber into

1. High sensitivity;

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

3. It can manufacture devices that sense a variety of different physical information (sound, magnetism, temperature, rotation, etc.);

4. Can be used in high voltage, electrical noise, high temperature, corrosion, or other harsh environments;

Five, and has the same

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

It can be divided into interference type and non-interference type according to whether light interferes;

According to whether it can continuously monitor the measured distance as the distance increases, it can be divided into: distributed and point type;

According to the role of optical fiber in the sensor, it can be divided into: one type is Functional Fiber (abbreviated as FF) sensor, also known as sensor type sensor; the other is non-functional (Non Functional Fiber abbreviated as NFF), and It is called a light-transmitting sensor. ^[3]

Insulated from pollution, magnetic, acoustic, pressure, temperature, acceleration, gyro, displacement, liquid level, torque, photoacoustic, current, fiber optic sensors can be used for displacement, vibration, rotation, pressure, bending, strain, speed, acceleration, current , Magnetic field, voltage, humidity, temperature, sound field, flow, concentration, PH value and strain and other physical quantities. Optical fiber sensors have a wide range of applications, involving almost all important areas of the national economy and national defense and people's daily life. They can be used safely and effectively in harsh environments. Market demand. The main applications are as follows:

The application of interference gyroscopes and grating pressure sensors in bridges, dams, oil fields, etc. in urban construction. Optical fiber sensors can be embedded in concrete, carbon fiber reinforced plastics and various composite materials to test stress relaxation, construction stress and dynamic load stress, so as to evaluate the structural performance of the bridge in the short-term construction phase and long-term operating state.

In power systems, parameters such as temperature and current need to be measured.

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 development. The twelfth five-year plan (draft) of this industry also proposes several key developments according to the requirements of the new period. Key technologies, which undoubtedly have important guiding significance for the future development of the industry.

New type

The basic working principle of a fiber optic sensor is to send light from a light source through an optical fiber into

Three basic components of fiber optic sensor networks.

Optical fiber sensor network has three basic components, one of which is called a single-point sensor. One optical fiber only plays a role of transmission here. The other is called a multi-point sensor. Here, one optical fiber connects many sensors, so that many sensors can share the light source for network monitoring. Then there are smart fiber optic sensors.

Multi-point fiber optic sensor, from the appearance is a section

In the domestic market, the most widely used fiber sensing technology is the Bragg fiber grating and distributed sensors based on optical time domain reflection. This technology can basically meet the needs of the low-end market. Now the single-frequency fiber laser with a narrow spectral line width to 2kHz and its latest generation of optical sensing technology are very different from the traditional fiber sensing. It can perform ultra-long-distance transmission, accuracy and sensitivity. It can meet higher requirements, which is in great demand in the high-end market. At the beginning of the 21st century, this technology is still in the stage of project establishment and preliminary research in China. Fiber optic sensor applications in the domestic market are mainly in the following four types: fiber optic gyroscope, fiber grating sensor, fiber current sensor, and fiber hydrophone. The four products are introduced below.

First, fiber optic gyroscope. The fiber optic gyro can be divided into interference type, resonance type and Brillouin type according to the principle. This is the representative of the three generations of fiber optic gyro. The first generation of interference fiber optic gyro, at the early stage of 21, this technology has matured and is suitable for mass production and commercialization; the second generation of resonance fiber optic gyro is still in the development stage of laboratory research to practicality; Three generations of Brillouin type, it is still in the theoretical research stage. There are three ways to implement the fiber optic gyro structure according to the optical components used: small discrete component systems, all-fiber systems, and integrated optical component systems. At the beginning of the 21st century, the discrete optical element technology has basically withdrawn. All-fiber systems are used in open-loop, low-precision, low-cost fiber-optic gyroscopes. Integrated optics gyroscopes have high accuracy due to their simple process, good overall repeatability, and low cost Fiber optic gyroscopes are very popular and their main implementation.

Second, the fiber grating sensor . Fiber Bragg grating sensor is one of the research hotspots in the field of sensors at home and abroad. Traditional optical fiber sensors can be basically divided into two types: light intensity type and interference type. The shortcomings of the light intensity sensor are that the light source is unstable, and the optical fiber loss and the detector are easy to age; the interference sensor requires the same light intensity of the two interference light, so it needs to fix the reference point, which makes the application inconvenient. The fiber Bragg grating sensors based on fiber Bragg gratings developed in the early 21st century can avoid the above two situations, and their sensing signals are wavelength-modulated and have strong multiplexing capabilities. In applications such as building health testing, impact testing, shape control, and vibration damping testing, fiber Bragg grating sensors are the most ideal sensitive components. Fiber grating sensors have a wide range of applications in geodynamics, spacecraft, the power industry, and chemical sensing.

Third, the fiber optic current sensor. The rapid development of the power industry drives the capacity of power transmission systems to increase continuously, the operating voltage level is getting higher and higher, and the current is getting larger and larger, so it is very difficult to measure, which shows the advantages of fiber optic current sensors. In power systems, the traditional sensors used to measure current are based on electromagnetic induction, which has the following disadvantages: it is easy to explode and cause catastrophic accidents; large fault currents can cause the core to saturate magnetically; the core has a resonance effect Frequency response is slow; measurement accuracy is low; signals are susceptible to interference; large volume and weight, expensive, etc., it is difficult to meet the development needs of the new generation of digital power networks. At this time, fiber-optic current sensors came into being.

Fourth, fiber optic hydrophone. Optical fiber hydrophone is mainly used to measure underwater acoustic signals. It uses high-sensitivity optical fiber coherent detection to convert underwater acoustic signals into optical signals and transmits them to the signal processing system for identification through optical fibers. Compared with traditional hydrophones, fiber-optic hydrophones have the characteristics of high sensitivity, wide response bandwidth, and immunity to electromagnetic interference. They are widely used in military and petroleum exploration, environmental detection and other fields, and have great development potential. Optical fiber hydrophones can be divided into interference type, intensity type, and grating type according to the principle. The key technologies of interference fiber optic hydrophones have gradually matured and formed products in some fields; 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 anti-polarization technology. Phase fading technology, signal processing technology, multiplexing technology, and engineering technology.

Optical fiber sensor technology is developed on the basis of optical fiber, optical communication and optoelectronic technology. The influence of electromagnetic interference and corrosion on it is small. It can also adapt to various harsh weather environments. Without additional power supply, Transmission over long distances has become a research hotspot in the sensor industry.

Sensors have been developing in the direction of sensitivity, precision, adaptability, compactness and intelligence. In this process, fiber optic sensors, a new member of the sensor family, have become much more popular.
Optical fiber has many excellent properties, such as resistance to electromagnetic interference and atomic radiation. Optical fiber sensors are used to measure physical quantities such as magnetic, acoustic, pressure, temperature, acceleration, gyroscope, displacement, liquid level, torque, photoacoustic, current and strain. Its application range is very wide. Therefore, we can say that fiber optic sensors have great market demand, not to mention long-term, at least in the next 5 years, fiber optic sensors will have broad development prospects.

The rapid development of optical fiber sensing technology and related technologies has met the higher requirements of various types of control devices and systems for information acquisition and transmission, making the degree of automation in various fields increasingly higher, 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 fiber optic sensor technology development are: (1) Multi-purpose. That is to say, an optical fiber sensor is not only aimed at one physical quantity, but also can measure multiple physical quantities simultaneously. (2) Improve the spatial resolution and sensitivity of distributed sensors, reduce their costs, and design complex sensor network projects. Pay attention to the influence of the parameters of the distributed sensor, that is, the pressure, temperature, and especially the 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 (bracket, connection, installation) under harsh conditions (high temperature, high pressure, chemical corrosion). (5) Optical fiber connector and micro-optical technology combined with other micro-technology.

The application of optical fiber sensing is mainly divided into five directions:

(1) Oil and gas-P / T sensing in reservoir monitoring wells, seismic arrays, energy industry, power plants, boilers and steam turbines, power cables, turbine transportation, oil refineries;

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

(3) Civil infrastructure construction-bridges, dams, roads, tunnels, landslides;

(4) Transportation-railway monitoring, weight during movement, transportation safety;

(5) Biomedicine-medical temperature and pressure, intracranial pressure measurement, minimally invasive surgery, disposable probes. ^[4]