Classification principle and characteristics of fiber optic sensors

Валаконна-аптычны датчык тэмпературы, Інтэлектуальная сістэма маніторынгу, Распаўсюджаны вытворца оптавалакна ў Кітаі

Валаконна-аптычныя датчыкі have the advantages of anti electromagnetic interference, strong accident resistance, miniaturization, and high accuracy. To promote the application of fiber optic sensors in more industries, the research progress of fiber optic sensors has been summarized. FJINO introduced the basic components and principles of fiber optic sensors, and summarized the classification characteristics of fiber optic sensors. According to the role of optical fibers in sensors, they can be divided into functional (all fiber) optical fiber sensors, transmission type optical fiber sensors, and pickup type optical fiber sensors. According to the modulation method of light waves, it can be divided into intensity modulation type fiber optic sensors, frequency modulation type fiber optic sensors, wavelength modulation type fiber optic sensors, phase modulation type fiber optic sensors, and polarization modulation type fiber optic sensors. According to the distribution of sensor targets, it can be divided into point type fiber optic sensors, quasi distributed fiber optic sensors, and distributed fiber optic sensors. Fiber optic sensors have a wide range of applications in temperature, displacement, and gas measurement, and the future development trend of fiber optic sensors shows a strong direction.

Sensors have been widely used in people’s daily lives. With the emergence of a new generation of transmission medium optical fibers, sensors have further developed and fiber optic sensors have emerged. The rapid development of fiber optic sensors began in 1977 and has become increasingly mature to this day. The impact of this new technology is currently very evident and is one of the important signs of human informatization process. Fiber optic is an excellent sensitive component that is suitable for remote transmission and has a wide monitoring range; Secondly, optical fibers have small volume, light weight, and can resist electromagnetic interference and radiation in optical signal transmission, making them suitable for monitoring in harsh environments such as flammable, explosive, and electromagnetic interference. Таму, fiber optic sensors have been researched and applied in various fields, becoming a leader in the sensor industry.

This article mainly introduces the basic components and principles of fiber optic sensors, and details the classification of fiber optic sensors according to different classification methods, as well as their applications in temperature, displacement, and gas. Finally, the future development trends of fiber optic sensors are discussed.

Principle of Fiber Optic Sensor

Fiber optic sensors are used to convert measured information into measurable optical signals, and their basic structure consists of a light source, sensitive components, fiber optic and photodetector, and signal processing system. Fiber optic sensors have information modulation and demodulation functions. The part where the optical parameters in the fiber optic sensor are modulated is called the modulation zone, and the photodetector and signal processing part are called the demodulation zone. When the light emitted by the light source is coupled into the optical fiber and enters the modulation zone through the fiber, its optical properties change (such as intensity, frequency, wavelength, phase, polarization state, etc.) due to the influence of the measured light in the modulation zone, becoming the modulated signal light. After being transmitted through optical fibers to the photodetector, the photodetector receives the incoming optical signal and performs photoelectric conversion to output an electrical signal. Finally, the signal processing system processes the electrical signal to obtain the relevant parameters being measured, which is demodulation.

Classification of Fiber Optic Sensors

Classified by the role of optical fibers in sensors

According to the role of fiber optic in sensors, fiber optic sensors can be divided into functional fiber optic sensors, transmission fiber optic sensors, and pickup fiber optic sensors.

① Functional fiber optic sensor.

Functional fiber optic sensors, also known as all fiber optic sensors, consist of a continuous optical fiber that couples the emitting fiber of the light source and the receiving fiber of the light detector. They have two functions: transmission and sensing. The measured parameters are modulated by directly changing certain transmission characteristic parameters of the optical fiber. This type of sensor has a compact structure and high sensitivity, but requires the use of special optical fibers and advanced sensing technologies, such as fiber optic gyroscopes, fiber optic hydrophones, г.д.

② Optical fiber sensor for transmitting light.

The optical fiber of the transmission type fiber optic sensor only plays the role of transmitting light waves, and its modulation region is outside the optical fiber. The transmitting fiber and the receiving fiber do not have continuity. The principle is that the light is modulated by the measured parameter when it is irradiated on the external modulation device (sensitive element). The advantages of this type of sensor are simple structure, low cost, and easy implementation, but its sensitivity is lower than that of functional fiber optic sensors. Most of the commercially available fiber optic sensors currently belong to the transmission type fiber optic sensors.

③ Pick up type fiber optic sensor.

The pickup type fiber optic sensor uses optical fibers as probes to receive light radiated by the measured object or light reflected or scattered by it and transmit it to the photodetector. After signal processing, the measured parameter is obtained. Fiber laser Doppler velocimeter is a typical pickup type fiber sensor. The turbidity of the fluid it can measure is at least 5 times higher than that of previous measured fluids, achieving the measurement of local velocity of opaque fluids. У дадатак, Doppler velocimeters are also used for measuring the speed of trains. Reflective fiber optic temperature sensors are also one of the representatives of pickup type fiber optic sensors.

Classified by light wave modulation method

According to the changes in the physical characteristic parameters of light waves modulated by external signals, fiber optic sensors can be divided into five types: intensity modulation fiber optic sensors, phase modulation fiber optic sensors, frequency modulation fiber optic sensors, wavelength modulation fiber optic sensors, and polarization modulation fiber optic sensors.

① Intensity modulated fiber optic sensor.

Intensity modulation is a relatively simple and widely used modulation method in fiber optic sensing. The basic principle is that the measured parameter modulates the transmitted light in the optical fiber to change the light intensity, and then measures the measured parameter by detecting the change in light intensity (i.e. demodulation). Most intensity modulated fiber optic sensors are based on reflective intensity modulation. This type of sensor has a simple structure, low cost, and is easy to implement, but it is susceptible to fluctuations in light source intensity. The development and application of intensity modulated fiber optic sensors started earlier, and in recent years, research has been constantly breaking through and innovating.

② Frequency modulated fiber optic sensor.

Optical frequency modulation refers to the modulation of the frequency of the transmitted light in an optical fiber by the measured parameter, and the detection of the measured parameter through frequency offset. At present, frequency modulated fiber optic sensors are mostly used to measure displacement and velocity.

③ Wavelength modulated fiber optic sensor.

The measured signal changes the wavelength of the transmitted light through frequency selection, filtering, and other methods, which is called optical wavelength modulation. Traditional optical wavelength modulation fiber sensors include cascaded fiber interferometer temperature sensors based on the cursor effect. Although the temperature measurement range of this sensor is small, its enormous sensitivity enables it to meet certain special requirements, such as scientific instruments that require precise temperature control. The performance of light sources and spectrum analyzers greatly affects fiber optic wavelength detection technology. The rapid development of fiber Bragg grating sensors in recent years has expanded the application range of functional optical wavelength modulation sensors.

④ Phase modulated fiber optic sensor.

The basic principle of phase modulation fiber optic sensors is that under the action of the measured parameter on the sensitive element, the refractive index or propagation constant of the sensitive element changes, causing a change in the phase of the transmitted light. Then, an interferometer is used to detect this phase change to obtain the measured parameter. This type of sensor has the advantages of high sensitivity, fast response, and large dynamic measurement range, but it requires high precision for the light source and detection system.

⑤ Polarization modulation type fiber optic sensor.

Polarization modulation fiber optic sensors are not affected by light source intensity, have a simple structure, and high sensitivity. The current sensor utilizing Faraday effect is one of its main application areas.

Classify by target distribution

According to the distribution of detection targets, sensors can be divided into point type fiber optic sensors, quasi distributed fiber optic sensors, and distributed fiber optic sensors.

① Point type fiber optic sensor.

Point type fiber optic sensors can only sense physical quantities at a certain point, and only one extremely small sensitive element is connected to the fiber optic sensor, usually a transmission type fiber optic sensor. The advantages and disadvantages of this type of sensor are obvious, with high detection performance, but it cannot perform multi-point distribution detection on the object being tested.

② Quasi distributed fiber optic sensor.

Quasi distributed fiber optic sensors can perform multi-point detection on targets simultaneously, with multiple point type fiber optic sensors connected to the fiber optic cable. Typical application cases of quasi distributed fiber optic sensors include fiber optic hydrophone arrays [24] and fiber optic grating array sensors.

The advantage of quasi distributed fiber optic sensors is the ability to perform simultaneous multi-point sensing, which is an important development trend in fiber optic sensing. Аднак, the number of points that can be simultaneously sensed by current quasi distributed fiber optic sensors is limited.

③ Distributed fiber optic sensors.

The entire fiber optic cable of a distributed fiber optic sensor is a sensitive component. The fiber optic cable is both a sensor and a medium for transmitting signals, and is suitable for detecting the strain distribution of structures. For example, in large structures applied in civil engineering, important parameters such as displacement, internal or surface stress of the structure can be measured quickly and non destructively. Based on the distribution of optical wave parameters along the fiber optic cable, and by simultaneously obtaining the distribution information of the measured signal that varies with time and space in the sensing fiber region, continuous sensing over a large range, long distance, and long time can be achieved.

At present, the main types of distributed fiber optic sensors include interferometric fiber optic sensors, fiber Bragg grating sensors, г.д. In engineering applications, distributed fiber optic sensing technology can continuously and dynamically monitor the force changes of target objects without interruption, with high accuracy and strong anti-interference ability in monitoring results.

Application of Fiber Optic Sensors

Fiber optic sensors can achieve a wide range of sensing physical quantities and can measure many external parameters. Among them, the measurement applications for temperature, displacement, and gas are particularly prominent and widely used in various industries, with a very broad market and prospects.

1. Валаконна-аптычны датчык тэмпературы

2. Fiber optic displacement sensor.

3. Fiber optic gas sensor.

In the past two decades, with the development of technological facilities, fiber optic sensors based on the inherent characteristics of optical fibers have been widely used. Fiber optic sensors will continue to develop, affecting social development and various aspects of people’s lives. У дадатак, the key technologies and applications of fiber optic sensors in special fields should be given attention. In the 1990s, foreign countries conducted research on the application of fiber optic sensors in nuclear power plants. Fiber optic sensors can cover the measurement of various parameters in nuclear power plants. In theory, an all fiber optic sensing system can be constructed to fully leverage the advantages of fiber optic sensors, such as high accuracy, miniaturization, distributed anti electromagnetic interference, and intrinsic safety. In the future, in order to meet the requirements of automation applications, fiber optic wireless sensor network technology is the key to achieving automation. It can achieve real-time monitoring, which is conducive to achieving automated production. Fiber optic wireless sensors will have broad application prospects.