Fiber Optic Sensor

- Dec 28, 2020-

  The fiber optic sensor is a sensor that converts 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 to the modulator via the optical fiber, and interact with the external measured parameters in the modulator to make the optical properties of light such as light intensity, wavelength, frequency, phase, polarization state, etc. It changes and becomes 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. In 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 first to transmit the light beam, and secondly to act as an optical modulator.

Direction of development
Sensors are developing in the direction of sensitivity, precision, adaptability, compactness and intelligence. In this process, fiber optic sensors, a new member of the sensor family, are very popular. Optical fiber has many excellent properties, such as: anti-electromagnetic and atomic radiation interference performance, small diameter, soft, light weight 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 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 transcend human physiological boundaries and receive human senses. External information that you can't feel.

Features

1. Due to the use of prisms in the reflector, it has higher detection performance and more reliability than general reflective light control sensors.

2. Compared with a separate light control sensor, the circuit connection is simpler and easier.

3. The embedded design of the buckle makes the installation easier

use

1. Used for transmission of digital models such as telephone and network broadband.

2. Passing status of banknotes, cards, coins, passbooks, etc. used for vending machines, financial terminal-related equipment, and money counters

3. Used for product positioning, counting, and identification on automated equipment.

principle
The basic working principle of the optical fiber sensor is to send the light from the light source to the modulator through the optical fiber. After the parameter to be measured interacts with the light entering the modulation area, the optical properties of the light (such as light intensity, wavelength, frequency, phase, Polarization state, etc.) change, called modulated signal light, and then use the influence of the measured light transmission characteristics to complete the measurement.

There are two measurement principles for optical fiber sensors.

(1) The principle of the physical property type optical fiber sensor. The physical property type optical fiber sensor uses the sensitivity of the optical fiber to environmental changes to convert 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 external environmental factors such as temperature, pressure, electric field, magnetic field, etc. change, its light transmission characteristics, such as phase and light intensity, will change.

Therefore, if the light phase and light intensity change through the optical fiber can be measured, the change of the measured physical quantity can be known. This type of sensor is also called a sensitive element type or functional fiber optic sensor. The beam of the point light source of the laser diffuses into parallel waves and is divided into two paths by a beam splitter, one is the reference optical path and the other is the measurement optical path. External parameters (temperature, pressure, vibration, etc.) cause the length of the fiber to change and the light phase of the phase to change, thereby producing different numbers of interference fringes. By counting its mode movement, temperature or pressure can be measured.

(2) The principle of the structured optical fiber sensor. The structured optical fiber sensor is a measurement system composed of a light detection element (sensitive element), an optical fiber transmission loop and a measurement circuit. Among them, the optical fiber is only used as the transmission medium of light, so it is also called the light-transmitting or non-functional optical fiber sensor

performance
Optical fiber has many excellent properties, such as: anti-electromagnetic and atomic radiation interference performance, small diameter, soft, light weight 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 out of reach or harmful to humans (such as nuclear radiation areas), and it can also transcend human physiological boundaries and receive what cannot be felt by human senses. Outside information.

Features
1. High sensitivity;

2. The geometric shape is adaptable in many aspects and can be made into optical fiber sensors of any shape;

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

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

Fifth, and has the inherent compatibility with optical fiber telemetry technology.

The advantage of optical fiber sensor is that compared with traditional sensors, optical fiber sensor uses light as the carrier of sensitive information and optical fiber as the medium for transmitting sensitive information. It has the characteristics of optical fiber and optical measurement and has a series of unique advantages. Good electrical insulation performance, strong anti-electromagnetic interference ability, non-invasiveness, high sensitivity, easy to realize remote monitoring of the measured signal, corrosion resistance, explosion-proof, flexible light path, easy to connect with the computer.

The sensor is developing in the direction of sensitivity, accuracy, adaptability, compactness and intelligence. It can be used in places where people cannot reach (such as high temperature areas or areas harmful to humans, such as nuclear radiation areas). It can also transcend people’s physiological boundaries and receive external information that cannot be felt by human senses.

classification
According to the modulation mode of the object under test, it can be divided into: intensity modulation, polarization mode, phase mode, and frequency mode;

According to whether the light interferes, it can be divided into: interference type and non-interference type;

According to whether the measurement can be continuously monitored as the distance increases, it can be divided into: distributed and point-based;

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

Folding function type
The functional sensor uses the characteristics of the optical fiber itself to use the optical fiber as a sensitive element. The measured light modulates the light transmitted in the optical fiber to change the intensity, phase, frequency, or polarization state of the transmitted light. The signal is demodulated to obtain the signal under test.

The optical fiber is not only a light guide medium, but also a sensitive element. The light is measured and modulated in the optical fiber, and multimode optical fiber is mostly used.

Advantages: compact structure and high sensitivity.

Disadvantages: special optical fiber is required, high cost,

Typical examples: fiber optic gyroscopes, fiber optic hydrophones, etc.

Folded non-functional fiber type
Non-functional optical fiber sensors use other sensitive components to sense the changes being measured. Optical fibers are only used as information transmission media, and single-mode optical fibers are often used.

The optical fiber only plays a role of guiding light, and the light on the optical fiber-type sensitive element is modulated by the measurement.

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

Most of the practical ones are non-functional optical fiber sensors. AnyWay's variable-frequency voltage sensor, variable-frequency current sensor, and variable-frequency power sensor (a combination of voltage and current sensors) belong to non-functional optical fiber sensors, which have unique advantages in power measurement in complex electromagnetic environments.

Fiber optic sensors are a new technology that has emerged in recent years. It 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 to complete with existing measurement technologies. In a small space, in a strong electromagnetic interference and high voltage environment, fiber optic sensors have shown unique capabilities. There are more than 70 types of optical fiber sensors, which are roughly divided into optical fiber sensors and sensors using optical fibers.

The so-called optical fiber sensor means that the optical fiber itself directly receives the external measurement. The external measured physical quantity can cause the length, refractive index, and diameter of the measuring arm to change, so that the light transmitted in the optical fiber changes in amplitude, phase, frequency, polarization, etc. The light transmitted by the measuring arm and the reference light of the reference arm interfere with each other (compared), so that the phase (or amplitude) of the output light changes, and the measured change can be detected based on this change. The phase transmitted in the optical fiber is highly sensitive to external influences, and the physical quantity corresponding to the small phase change of 10 minus 4 radian can be detected by using the interference technology. Using the winding properties and low loss of the optical fiber, a very long optical fiber can be coiled into a small diameter optical fiber coil to increase the utilization length and obtain higher sensitivity.

Optical fiber acoustic sensor is a sensor that uses optical fiber itself. When the optical fiber is subjected to a very small external force, it will be slightly bent, and its light transmission ability will change greatly. Sound is a kind of mechanical wave, and its effect on the optical fiber is to force the optical fiber and produce bending. The strength of the sound can be obtained by bending. Fiber optic gyroscope is also a kind of fiber optic sensor. Compared with laser gyroscope, fiber optic gyroscope has high sensitivity, small size and low cost. It can be used in high-performance inertial navigation systems of aircraft, ships, missiles, etc. The figure shows the principle of fiber optic sensor turbine flowmeter.

Folded Bragg grating
Fiber Bragg Grating Sensor (FBS) is a kind of fiber optic sensor with the highest frequency and widest range. This kind of sensor can change the wavelength of the reflected light wave according to the change of environmental temperature and/or strain. Fiber Bragg gratings are used to expose a small section of light-sensitive fiber to light waves with a periodic distribution of light intensity through holographic interferometry or phase masking. 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 changes in the refractive index of light caused by this method are called fiber Bragg gratings.

When a broad-spectrum beam is propagated to the fiber Bragg grating, each small segment of the fiber after the refractive index is changed will only reflect 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 will be propagated.

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

The optical fiber of the sensing type optical fiber sensor not only transmits light, but also acts as a photoelectric 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 state, wavelength, etc.) are modulated. Sensor-type optical fiber sensors are further divided into light-emphasizing type, phase modulation type, vibration state modulation type and wavelength modulation type.

Folded light transmission fiber
The light transmission type optical fiber sensor inputs the optical signal modulated by the measured object into the optical fiber, and then performs the measurement by processing the optical signal at the output end. This type of sensor has another photosensitive element sensitive to the physical quantity to be measured, and the optical fiber is only used as The light transmission element must be attached with a sensitive element capable of modulating the light transmitted by the optical fiber to form a sensor element. Fiber optic sensors can be divided into three types: point fiber optic sensors, integral fiber optic sensors, and distributed fiber optic sensors according to their measuring range. Among them, distributed optical fiber sensors are used to detect the strain distribution of large structures, and can quickly and non-destructively measure the displacement, internal or surface stress and other important parameters of the structure. The types of fiber optic sensors used in civil engineering mainly include Math-Zender interferometric fiber optic sensors, Fabry-pero cavity fiber optic sensors, and fiber Bragg grating sensors.

The lightness, durability and long-term stability of the optical fiber sensor enable it to be easily applied to the internal stress and strain detection of various building materials such as building steel structures and concrete. Realized health inspection of building structure.

Another major category of fiber optic sensors is the use of fiber optic sensors. Its structure is roughly as follows: The sensor is located at the end of the optical fiber, and the optical fiber is just a transmission line of light, which converts the measured physical quantity into the amplitude, phase or amplitude of the light. In this sensor system, the traditional sensor and optical fiber are combined. The introduction of optical fiber makes it possible to realize probe-based telemetry. This optical fiber transmission sensor has a wide application range 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 the excellent performance of optical fiber. It is a kind of sensor worth noting in production practice.

Fiber optic sensors have become a rising star in the sensor family due to their many advantages, and play their own unique role in various measurements, becoming an indispensable member of the sensor family.

application
Insulated from dirt, magnetism, sound, pressure, temperature, acceleration, gyro, displacement, liquid level, torque, photoacoustic, current, optical fiber sensor 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 measurement. 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 lives. They can be used safely and effectively in harsh environments. They solve many technical problems that have existed in many industries for many years. Market demand. Mainly manifested in the following applications:

Application of interferometric 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 the long-term operating state.

In the power system, it is necessary to measure temperature, current and other parameters, such as the temperature detection in the stator and rotor of high-voltage transformers and large motors. Since electrical sensors are susceptible to electromagnetic field interference, they cannot be used in such occasions. Fiber Optic Sensor. Distributed optical fiber temperature sensor is a high-tech developed in recent years for real-time measurement of spatial temperature field distribution. Distributed optical fiber temperature sensing system not only has the advantages of common optical fiber sensors, but also has the ability to measure the temperature of various points along the optical fiber. With its distributed sensing capability, we can continuously measure the temperature of various points within a few kilometers along the optical fiber in real time. The positioning accuracy can reach the order of meters, and the measurement accuracy can reach the level of 1 degree. It is very suitable for large-scale intersection temperature measurement. Application occasions.

In addition, optical fiber sensors can also be used in railway monitoring, rocket propulsion systems, and oil well detection.

Optical fiber has the remarkable advantages of broadband, large capacity, long-distance transmission, and multi-parameter, distributed, and low-energy-consumption sensing. Optical fiber sensing can continue to absorb new technologies and devices for optical fiber communication, and various optical fiber sensors are expected to be widely used in the Internet of Things.

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