The principle, classification, characteristics, and technical hotspots and types of fiber optic sensors

With the development of modern science and technology, the acquisition of information becomes more and more important. Sensors are an important technical means of sensing, detecting, monitoring and transforming information. Fiber optic sensors are new sensors that are integrated into optics and electronics.

A fiber-optic sensor is a sensor that converts measured light into light characteristics (intensity, phase, polarization state, frequency, wavelength) by utilizing the light-transmitting characteristics of the optical fiber. The light from the light source is sent to the modulator through the optical fiber, and the parameters to be measured interact with the light entering the modulation area, thereby causing the optical properties of the light (such as the intensity, wavelength, frequency, phase, partial normal state, etc.) of the light. The change, called the modulated signal light, is sent to the photodetector through the optical fiber, and after demodulation, the measured parameter is obtained.

Unlike conventional sensors, fiber optic sensors take the state of the signal under test as an optical signal. The optical signal can not only be directly perceived by humans, but also can be photoelectrically and electro-optically converted by using small simple components such as semiconductor diodes such as photodiodes, and is easily matched with some electronic assemblies. In addition, the optical fiber is not only a sensitive component but also a An excellent low loss transmission line. Therefore, fiber optic sensors can also be used for long distance measurements that are not suitable for conventional sensors.

Fiber optic sensing includes both the perception and transmission of external signals (measured). The so-called perception 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. The change of the measured optical parameter is the perception of the external signal. Variety. This perception is essentially an external signal that modulates the light waves propagating in the fiber. 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 optical wave. Demodulation technique (or detection technique) (measured).

The basic principle of fiber optic sensors

Optical fiber is an abbreviation for optical fiber. The main component of optical fiber is silicon dioxide, which consists of a core with high refractive index, a cladding with lower refractive index and a protective layer. The core is a fine glass filament having a diameter of about 0.1 mm, a waveguide structure in which light is enclosed and propagated in the axial direction. The discovery of fiber optic sensors originated from the practice of detecting external disturbances of optical fibers. In practice, it has been found that when the optical fibers are subjected to changes in the external environment, the optical wave parameters transmitted inside the optical fibers are changed, and these changes are in a regular pattern with external factors. Developed fiber optic sensing technology.

Fiber optics have a certain sensitivity to many external parameters. Studying the principle of optical fiber sensing is to study how to apply these effects of optical fiber, study the interaction of light with the measured parameters in the modulation area, and realize the function of “transmitting” and “sensing” to the measured parameters of the outside world. This is the optical fiber sensor. Core.

In optical communication systems, optical fibers are used as a medium for transmitting lightwave signals over long distances. Obviously, in such applications, the optical signal transmitted by the optical fiber is less affected by external interference as possible. However, in the actual optical transmission process, the optical fiber is susceptible to external environmental factors, such as changes in external conditions such as temperature, pressure, and electromagnetic field, which will cause changes in optical fiber parameters such as light intensity, phase, frequency, polarization, and wavelength. Therefore, it has been found that if the change in the parameters of the light wave can be measured, the magnitude of various physical quantities that cause changes in the parameters of the light wave can be known, and thus fiber-optic sensing technology is produced. Optical fiber sensing technology is a technology that uses optical fibers to sensitive to certain physical quantities and converts external physical quantities into signals that can be directly measured. Since the optical fiber can not only serve as a propagation medium for light waves, but also characterizes the characteristic parameters (amplitude, phase, polarization state, wavelength, etc.) of the light wave due to the propagation of the light wave in the optical fiber due to external factors (such as temperature, pressure, strain, magnetic field, electric field, displacement). The effect of the rotation, etc., changes directly or indirectly, so that the optical fiber can also be used as a sensing element to detect various physical quantities.

The figure above is the schematic structure of the fiber sensor. Fiber optic sensors are usually composed of a light source, a transmission fiber, a sensing element or a modulation area, and light detection. Parameters such as light intensity, wavelength, amplitude, phase, polarization state, and mode distribution may be affected by external influences during fiber transmission, such as temperature, pressure, acceleration, voltage, current, displacement, vibration, rotation, bending, When strains, as well as chemical and biochemical quantities, affect the optical path, these parameters change accordingly. The fiber optic sensor detects the magnitude of each corresponding physical quantity based on the relationship of these parameters with external factors.

Fiber optic sensor features

Different from traditional sensors, the excellent physical, chemical, mechanical and transmission properties of optical fibers make optical fiber sensors small in size, light in weight, resistant to electromagnetic interference, corrosion-proof, high in sensitivity, wide in measurement bandwidth, and capable of detecting electronic devices and sensors. The advantages of being very far apart and can constitute a sensor network.

Advanced fiber optic sensors are orders of magnitude higher than traditional sensors and can measure more than 70 physical quantities. To sum it up, it has several advantages:

1. High precision, fast response, wide linear range, good repeatability, high signal-to-noise ratio of detection signals. Due to the mass production of optical fibers, the price is low and can be widely used.

2. The optical fiber is made of dielectric material quartz, which transmits optical signals. Therefore, it has good safety and reliability, strong anti-electromagnetic interference capability, and can adapt to the inflammable, explosive or toxic environment of electricity, petroleum, chemical industry and metallurgy. Work under conditions.

3, anti-corrosion, strong anti-pollution ability, can be used in places with large temperature difference, excellent aging characteristics in time and long working life.

4, small size, light weight, easy to install, strong adaptability to the environment of the measured object.

5. The optical fiber is a passive device, and its independence is good, and it will not damage the measured state.

6, a wide range of measurement objects. At present, there are various optical fiber sensors that measure various physical quantities, chemical quantities, biomass, and the like, such as temperature, pressure, displacement, velocity, liquid level, and nuclear radiation.

7. It is convenient for multi-point multiplexing and small transmission loss. It is suitable for the measurement network and realizes multi-point real-time intelligent telemetry.

Classification and application principle of fiber optic sensors

According to the relationship between the modulation area and the optical fiber, the modulation can be divided into three categories:

Light-transmitting fiber sensor

The light-transmitting fiber sensor is also called a non-functional fiber sensor or an intensity-modulated fiber sensor. The fiber mainly functions to transmit light waves. The light-transmitting fiber sensor is mainly composed of a light source, an optical fiber, a light modulator, a sensitive component, a photodetector, and a detection. Circuit and other components. The basic principle of the optical transmission type optical fiber sensor is that the physical quantity to be measured causes a change in the transmitted light intensity I in the optical fiber, and the measurement of the physical quantity to be measured is realized by the detection of the light intensity I.

The intensity modulation is characterized by simplicity, reliability and economy. There are many ways to intensity modulation, mainly reflective intensity modulation (as shown in Figure 1) and transmission intensity modulation (as shown in Figure 2).

Figure 1 Reflective intensity modulation schematic

Figure 2 Transmission intensity modulation schematic

Sensing fiber optic sensor

Sensing fiber optic sensors are also called functional fiber optic sensors. Optical fibers are both transmitted and sensed, that is, they act as sensitive components. For a sensing fiber-optic sensor, when light propagates in the fiber, the measured physical quantity or external factors act on the fiber, so that the amplitude, phase, wavelength, polarization state, etc. of the transmitted light in the fiber change, and the process is light. The modulated and modulated light is transmitted to the photodetector through the optical fiber to be demodulated and converted into an electrical signal output.

The principle of the sensing fiber-optic sensor is much more complicated than that of the light-transmitting fiber-optic sensor. It uses an optical fiber (or special fiber) that is sensitive and capable of detecting external information as a sensing element, and combines "transmission" and "sensing". As a sensor. The optical fiber not only acts as a light transmission, but also uses the optical fiber to change the optical characteristics such as light intensity, phase, and polarization state under the action of external factors (bending, phase change) to achieve the functions of "transmission" and "sensation". In addition, the fiber in the sensor is continuous, and the sensitivity is improved because the fiber is continuous and its length is increased. The most widely used sensor fiber sensor is a phase modulation fiber sensor or an interference fiber sensor, that is, external factors cause the phase change of the transmitted light in the fiber, thereby changing the intensity variation of the outgoing light (interference light) to achieve the measurement purpose.

Commonly used interferometric fiber optic sensors include Michelson interferometric fiber optic sensors, Mach-Zehnder interferometric fiber optic sensors, Fabry-Perot (FP) interferometric fiber optic sensors, Sagnac interferometric fiber optic sensors, and Fizeau interferometric fiber optic sensors. Interferometric fiber optic sensors are the best choice for high precision fiber optic sensing and measurement technology.

Pick-up fiber optic sensor

Optical pickup type optical fiber sensor This type of sensor uses an optical fiber as a probe to receive light radiated by or reflected by the object to be measured. Typical examples thereof are a fiber laser Doppler speedometer, a radial fiber temperature sensor, and the like.

The fiber optic sensor can be divided into fiber temperature sensor, fiber displacement sensor, fiber concentration sensor, fiber current sensor, fiber flow sensor, etc. according to the object to be tested.

The fiber optic sensor can be divided into a light-emphasizing fiber sensor, a phase-modulating fiber sensor, a polarization-modulating fiber sensor, and a wavelength-modulating fiber sensor according to the modulated light wave parameters.

Amplitude Modulation Sensing Fiber Sensor

A sensor that detects various parameters such as physical quantities by utilizing changes in light intensity in an optical fiber caused by external factors is called an amplitude modulation sensing type optical fiber sensor. There are many ways to change the light intensity in an optical fiber, and changing the microbend state of the optical fiber is one of them. The fiber microbend sensor uses the microbend loss in the fiber to detect changes in the physical quantity of the outside world. It is a principle that uses a multimode fiber to convert a part of the core mode energy into a cladding mode energy when subjected to microbending. The displacement or vibration is measured by measuring the change of the cladding mode energy or the core mode energy. The schematic diagram is shown in the figure.

The laser beam is expanded and focused into a multimode fiber. The non-guided mode is removed by the impurity mode filter and then displaced by the deformer. When the degree of microbending of the fiber is different, the energy converted into the cladding mode also changes. The deformer is adjusted by the micrometer to a certain constant deformation; the alternating displacement to be measured is given by the piezoelectric ceramic. Experiments show that the sensitivity of the device is 0.6μV/A (it strongly depends on the guided mode distribution in multimode fiber, the more high-order modes, the easier it is to convert into a cladding mode, the higher the sensitivity), which is equivalent to the minimum testable The displacement is 0.01 nm and the dynamic range is expected to exceed 100 dB.

Phase modulation sensing fiber sensor

A sensor that detects various parameters such as physical quantities by using changes in the phase of light waves in an optical fiber caused by external factors is called a phase modulation sensing type optical fiber sensor. This type of sensor is mainly used to make interferometers, and the fiber Sagnac interferometer is one of them.

The basic principle of the fiber-optic Sagnac interferometer is that two light waves traveling in the opposite direction in the fiber ring wound by the same fiber produce different phase shifts under the influence of external factors. Then, the detection is performed by the interference effect. The most typical applications are rotary sensing and fiber optic gyroscopes. Since it has no moving parts, no nonlinear effects and a locked area of ​​the laser gyro at low speeds, it is highly promising to make high-performance, low-cost devices.

The figure below is a schematic diagram of a fiber-optic Sagnac interferometer. A fiber of length L is wound into a fiber loop of radius R. A laser beam is split into two beams by a beam splitter, which are input from both end faces of the fiber and then output from the other end face. The superposition of the two output lights will produce an interference effect which is detected by the photoreceiver.

1 is a laser; 2 is a photodetector; 3 is a fiber optic ring

Polarization modulated optical fiber sensor

1 is a laser; 2 is a polarizer; 3 is an objective lens; 4 is a transmission fiber; 5 is a sensing fiber; 6 is a current wire;

7 is a photodetector; 8 is a polarizing prism; 9 is a signal processing unit

Wavelength modulated fiber optic sensor

A sensor that detects various parameters such as physical quantities by using a change in the wavelength of light waves in an optical fiber caused by external factors is called a wavelength modulation sensing type optical fiber sensor. The fiber Bragg grating sensor is a typical wavelength modulation type fiber sensor.

The fiber optic sensor can be divided into fiber temperature sensor, fiber displacement sensor, fiber concentration sensor, fiber current sensor, fiber flow sensor, etc. according to the object to be tested.

Fiber optic sensor technology hotspot

Current research interests in fiber optic sensors focus on two major segments: fiber grating (FBG and LPG) sensors and distributed fiber sensing systems.

The FBG fiber optic sensor has gone through the principle research and experimental demonstration stage since the invention. At present, the mature FBG production process can form a small batch production capacity, and the research focus has also turned to solving high-precision applications, perfecting demodulation and multiplexing technology, and reducing costs. On the other hand, since the optical fiber sensor has the characteristics of combining the transmission and the sensing medium, the optical fibers along the routing path can all become sensitive components, and thus distributed sensing becomes an inherent advantage of the optical fiber sensor.

Fiber Bragg grating

The fiber Bragg grating FBG was introduced in 1978. This simple intrinsic sensing element can be written into the fiber core by the ultraviolet sensitivity of the silicon fiber. The following figure shows the basic principle of the fiber grating.

A common FBG sensor achieves the detection of the measurement by measuring the drift of the Bragg wavelength. When a broad spectrum source is incident on the fiber, the grating will reflect a narrow spectrum component with the Bragg wavelength as the center wavelength. In addition to all the advantages of fiber optic sensors, fiber Bragg gratings feature self-calibration and the ability to integrate multiple sensor multiplexes within the same fiber.

There are many applications in the field of grating sensors. For example, the distributed fiber Bragg grating sensor is embedded in the material to form a smart material, which can monitor the parameters of load, stress, temperature and vibration of large components in real time. The grating can also replace other types of structures. Fiber optic sensors for chemical, pressure and acceleration sensing.

Long-period gratings are gratings with a period greater than 100 mm and are another important branch of fiber grating sensors after FBG. Since the measurement utilizes the principle of the coupling of the cladding film, it has the advantages of excellent sensitivity and ease of fabrication.

Other branches of fiber gratings also include chirped gratings, oblique gratings, and the like.

Distributed optical fiber sensing system

Worldwide, the demand for integrated safety inspection systems has gradually increased due to the increasing requirements for safety and efficiency of civil and industrial construction and industrial facilities. A distributed fiber optic sensing system with continuous, uninterrupted, long-distance measurement and strong affinity to the measured medium seems to be tailor-made for this purpose.

Distributed fiber optic sensing systems are typically available in three types: Raman, Brillouin, and FBG.

The Raman-type distributed optical fiber sensing system is a continuous sensor based on the Raman scattering effect of the fiber. The working principle is shown in Fig. 6. The application of three types of sensing systems has been reported. Among them, the Raman-type distributed sensing system is the most mature and has been successfully loaded on the A340 transport aircraft.

The FBG distributed sensing system has unique advantages in stress multi-point distributed measurement, and can simultaneously perform temperature and stress double-parameter measurement, which opens up a broader prospect for FBG applications.

Fiber Optic Sensor Application Technology Type

The application development of fiber optic sensors can be roughly divided into four major directions according to the current application hotspots and technology types: optical (fiber) tomography analysis technology OCT, fiber optic smart material (SMART MATERIAL), fiber optic gyroscope and inertial navigation system, And conventional industrial engineering sensors.

Optical tomography

Optical fiber tomography analysis technology can be divided into optical coherence tomography (OCT) and optical process tomography (OPT) according to different principles and applications.

Optical tomography is derived from X-ray tomography (CT). When X-rays or light are transmitted through the sample to be tested, different sample materials have different absorption characteristics of the rays. Therefore, the rays or rays passing through the sample are measured, analyzed, and solved according to a predetermined topology and design. Get the required sample parameters.

Optical fiber coherence tomography (OCT) is mainly used in biological, medical, chemical analysis and other fields, such as retinal scanning, gastrointestinal endoscopy and color Doppler (CDOCT) blood flow imaging. Its working principle is based on the principle of coherent detection of light, and the basic system structure is shown in the figure.

OCT provides an effective way to detect the activity of biological cells and organisms, and many countries in the world have developed corresponding products. Figure 11 is a CT scan image of the retina. German scientists have recently launched an OCT device that can be used for the diagnosis of skin cancer. In addition, OCT can be used to achieve the advantage of depth measurement (~1mm), and existing examples have been applied to the observation and monitoring of growing cells.

OPT is aimed at industrial engineering - oil wells, pipelines and other places, to solve the process measurement problems of fluids with high precision. Since OPT is concerned with the integration process on the ray path, the related system integration design, unit segmentation and signal processing in measurement theory analysis are all key. OPT has an excellent means for the safe measurement of industrial processes because of its many advantages for narrow or irregular spaces, high safety, electromagnetic interference in the measurement area and the ability to form a measurement network.

Smart Materials

The introduction and research of smart materials has been around for a long time and is familiar to the industry. Smart materials refer to the embedding of sensitive components into the body and materials of the component under test, so that real-time monitoring of safe operation, failure, etc. is achieved while the component or material is working normally. Among them, the effective combination of optical fibers and electrical wires with a variety of materials is one of the key issues, especially the automatic weaving with textile materials.

Intelligent materials as monitoring systems for large concrete buildings such as bridges and dams have been installed and tested in many foreign projects. In addition, the application of smart materials in the aerospace field is also becoming more and more extensive, especially the use of fiber grating and fiber distributed stress, temperature measurement system for harsh environmental conditions - high temperature, deformation of multi-parameter monitoring has achieved significant results.

Fiber Optic Gyro and Inertial Navigation System

In the decade from 1980 to 1990, significant progress has been made in the study of systematic error factors and fiber optic devices. The use of new SLD sources, polarization-maintaining fibers and couplers, and special winding techniques have become practical for gyros. Paved the way. In the 1990s, the mid-level I-FOG achieved cost reduction, volume reduction and performance improvement by adopting new fiber optic devices and technologies such as depolarization structure, 3-axis I-FOG, and EDFA light source, and was the first in aerospace and The military field has gained application. For example, the three-axis inertial navigation system for helicopters manufactured by Honeywell Corporation of the United States for the US military is only 86 mm in diameter. Some high-performance fiber optic gyroscopes in the world have reached the drift index of 0.001 ° / hr, many products have been put into the civil aircraft and automotive industry. In the future, fiber optic gyroscopes will have a broader world in the industrial field.

Industrial engineering sensor

Traditional industrial engineering sensors include large voltage and current sensors for the power industry that measure the electro-optical and magneto-optical effects of optical fibers. The use of fiber optic bounce and FBG device stress sensors have been widely used in stress monitoring. Monitoring sensing systems are used in many special applications - nuclear industry, chemical and oil drilling. Fiber optic sensor systems are increasingly maturing and gradually integrated into everyday production and life.