Optical Fiber Sensing System
The fiber-optic sensing system began in 1977 and developed rapidly with the development of optical fiber communication technology. The optical fiber sensing system is an important indicator to measure the degree of informationization in a country. From the Hangzhou Internet of Things and Sensing System Application Forum, the optical fiber sensing system has been widely used in military, defense, aerospace, industrial and mining enterprises, energy and environmental protection, industrial control, medical and health, measurement and testing, construction, household appliances and other fields. a vast market. There are hundreds of fiber-optic sensing systems in the world, such as temperature, pressure, flow, displacement, vibration, rotation, bending, liquid level, velocity, acceleration, sound field, current, voltage, magnetic field and radiation. Sensing.
Optical fiber working frequency bandwidth, large dynamic range, suitable for telemetry remote control, is an excellent low-loss transmission line; under certain conditions, the optical fiber is particularly easy to accept the measurement or field loading, is an excellent sensitive component; the optical fiber itself is not Charged, small size, light weight, easy to bend, anti-electromagnetic interference, good radiation resistance, especially suitable for use in harsh environments such as flammable, explosive, space constrained and strong electromagnetic interference. Therefore, fiber optic sensing technology has received great attention since its inception, and has been researched and applied in almost all fields, becoming the forerunner of sensing technology and promoting the vigorous development of sensing technology.
Optical fiber sensing, including the perception and transmission of external signals (measured). The so-called perception (or sensitivity) refers to the physical characteristic parameters of the light wave transmitted by the external signal according to its changing law, such as the intensity (power), wavelength, frequency, phase and polarization state, and the change of the measured optical parameter is "Perceive" changes in external signals. This "perception" is essentially an external signal that modulates the light waves propagating in the fiber in real time. The so-called transmission means that the optical fiber transmits the optical wave modulated by the external signal to the photodetector for detection, extracts the external signal from the optical wave and performs data processing as needed, that is, demodulation. Therefore, fiber-optic sensing technology includes both modulation and demodulation techniques, namely, how the external signal (measured) modulates the optical wave parameters in the optical fiber (or loading technique) and how to extract the external signal from the modulated light wave ( Demodulation technique (or detection technique) that is measured).
The portion of the external signal that modulates the optical parameters in the sensing fiber is called the modulation region. According to the relationship between the modulation area and the optical fiber, the modulation can be divided into two categories. One type is functional modulation, and the modulation area is located in the optical fiber. The external signal modulates the optical wave by directly changing certain transmission characteristic parameters of the optical fiber. Such a fiber-optic sensor is called a functional type (Functional Fiber, FF type for short) or an intrinsic type fiber-optic sensor, and also becomes an internal modulation type sensor, and the optical fiber has the functions of "transmission" and "sensing". The receiving fiber coupled to the light source and the receiving fiber coupled to the photodetector are a continuous fiber called a sensing fiber, so the functional fiber sensor is also called an all-fiber or sensing fiber sensor. The other type is non-functional modulation. The modulation area is outside the optical fiber. The external signal is modulated by the external modulation device to the optical wave entering the optical fiber. This type of optical fiber sensor is called Non-Functional Fiber (NFF) or extrinsic. The type of fiber-optic sensor, the transmitting fiber and the receiving fiber only serve to transmit light waves, which are called light-transmitting fibers, and have no continuity. Therefore, the non-functional fiber-optic sensor is also called a light-transmitting fiber sensor or an externally-modulated fiber sensor.
According to the change of the physical characteristic parameters of the light wave modulated by the external signal, the modulation of the light wave can be divided into five types: light intensity modulation, optical frequency modulation, optical wavelength modulation, optical phase modulation, and polarization modulation.
Since any existing photodetector can only respond to the intensity of light, and cannot directly respond to the frequency, wavelength, phase, and polarization modulation signal of the light, it must be converted into an intensity signal by some conversion technique. Receive and implement detection.
Optical fiber sensing technology application classification
Light intensity modulation
Light-emphasis is a relatively simple and widely used modulation method in fiber-optic sensing technology. The basic principle is to use the disturbance of the external signal (measured) to change the intensity (ie modulation) of the light (wide spectrum light or specific wavelength light) in the fiber, and then measure the output light intensity (demodulation) to achieve the external signal. Measurement.
Optical phase modulation refers to the change of the phase of the light wave propagating in the optical fiber according to a certain law of the external signal (measured), and the amount of change of the optical phase reflects the measured external quantity.
There are generally three types of optical phase modulation used in fiber optic sensing technology. One type is functional modulation, and the external signal changes the geometrical size and refractive index of the sensing fiber through the force strain effect, thermal strain effect, elastic light effect and thermo-optic effect of the optical fiber, thereby causing the optical phase change in the optical fiber. To achieve modulation of the optical phase. The second type is the Sagnac effect modulation. The external signal (rotation) does not change the parameters of the fiber itself. Instead, the circular fiber in the inertial field is rotated to generate the corresponding optical path difference between the two beams that are propagating in opposite directions. Modulation of the optical phase. The third type is non-functional modulation, that is, modulation of the optical phase in the optical fiber by changing the optical wave path difference into the optical fiber outside the sensing fiber.
Polarization modulation means that the external signal (measured) causes a regular deflection (optical rotation) or birefringence of the plane of polarization of the light wave in the fiber in a certain way, thereby causing a change in the polarization characteristic of the light, by detecting a change in the polarization state of the light. It can be measured that the outside world is measured.
The external signal (measured) changes the wavelength of the transmitted light in the optical fiber by means of frequency selection, filtering, etc., and the measured wavelength change can be detected and measured. This type of modulation is called optical wavelength modulation.
Current methods for optical wavelength modulation are mainly optical frequency selection and filtering. Conventional optical wavelength modulation methods mainly include external modulation techniques such as F-P interferometric filtering, Riot polarization birefringent filtering, and various displacement spectral selection. In the past 20 years, especially the fiber grating filtering technology developed rapidly in recent years has opened up new prospects for functional optical wavelength modulation technology.
Frequency modulation type
Optical frequency modulation means that the external signal (measured) modulates the frequency of the optical wave transmitted in the optical fiber, and the frequency offset reflects the measured. At present, more modulation methods are used in the Doppler method, that is, the external signal modulates the frequency of the optical wave in the receiving fiber through the Doppler effect, which is a non-functional modulation.
High sensitivity and immunity to electromagnetic interference. Because the fiber sensor detection system is difficult to be interfered by the external field, and the optical signal does not interact with the electromagnetic wave during transmission, and is not affected by any electrical noise. Due to this feature, the fiber sensor is obtained in the detection of the power system. widely used.
The fiber has good flexibility and toughness, so the sensor can be made into different shapes according to the needs of on-site inspection.
The measured frequency bandwidth and dynamic response range are large.
It has strong portability and can be made into sensors of different physical quantities, including sound field, magnetic field, pressure, temperature, acceleration, displacement, liquid level, flow, current, radiation and so on.
It is highly embeddable and easy to connect with computers and fiber optic systems, making it easy to implement telemetry and control of the system.
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