What is a Fiber Optic Sensor?

23 July 2020

7042

Fiber optic sensor is a sensor that converts the state of the measured object into a measurable optical signal. The fiber optic sensor is to send the light beam incident from the light source to the modulator through the optical fiber, and interact with the external measured parameters in the modulator so that the optical properties of the light such as light intensity, wavelength, frequency, phase, polarization state, etc. will be changed.

Catalog

Ⅰ Introduction

A fiber optic sensor is a sensor that converts the state of the measured object into a measurable optical signal. The working principle of the fiber optic sensor is to send the light beam incident from the light source to the modulator through the optical fiber, and interact with the external measured parameters in the modulator so that the optical properties of the light such as light intensity, wavelength, frequency, phase, polarization state, etc. will be changed. The light beam becomes the modulated optical signal, which is sent to the photoelectric device and the modulator through the optical fiber. The measured parameter will be obtained after being demodulated. In the whole process, the light beam is introduced through the optical fiber and then exits after passing through the modulator. The function of the optical fiber is first to transmit the light beam, and secondly to function as the optical modulator.

Ⅱ Working Principle 

The basic working principle of the fiber optic sensor is to send the light from the light source to the modulator through the fiber, so that the measured parameter interacts with the light entering the modulation area, resulting in the optical properties of the light (such as light intensity, wavelength, frequency, phase, the polarization state, etc.) changes, called the modulated signal light. And then uses the influence of the measured on the light transmission characteristics to complete the measurement. There are two measuring principles for fiber optic sensors.

Fiber optic sensor

1.The principle of physical fiber sensor

Physical fiber sensor uses the sensitivity of optical fiber to environmental changes and converts 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 change of the optical phase and light intensity through the optical fiber can be measured, the change of the measured physical quantity can be known. This type of sensor is also known as a sensor-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 the beam splitter, one is the reference optical path and the other is the measuring optical path. External parameters (temperature, pressure, vibration, etc.) cause changes in the length of the fiber and changes in the optical phase of the phase, thereby generating different amounts of interference fringes. Counting its mode movement, we can measure temperature or pressure.

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, optical fiber is only used as the propagation medium of light, so it is also called light transmission type or non-function type optical fiber sensor.

Ⅲ Performance and features

Optical fiber has many excellent properties, such as resistance to electromagnetic and atomic radiation interference, mechanical properties of small diameter, soft quality, and lightweight; insulation, electrical performance without induction; chemical resistance to water, high temperature, and corrosion, etc. It can play the role of human ears in places where people can't reach (such as high-temperature areas), or areas that are harmful to people (such as nuclear radiation areas). It can also exceed human physiological boundaries that receive the external information you can't feel.

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

Features of fiber optic sensors are as following:

1. High sensitivity

2.The geometric shape has many aspects of adaptability and can be made into any shape of a fiber optic sensor.

3.It can be manufactured 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.

5. It is also inherently compatible with fiber optic telemetry.

6. Because of the use of prisms in the reflector, the detection performance is higher and more reliable than the general reflective light control sensor.

7. Compared with the separated light control sensor, the circuit connection is simpler and easier.

8. The embedded design makes installation easier

9. It is used for the transmission of digital models such as telephone and Internet broadband.

10. Passage of banknotes, cards, coins, passbooks, etc. used in vending machines, equipment related to financial terminals, and banknote counters

11. It is used for product positioning, counting, and identification on automated equipment.

Ⅳ Classification

 

According to the modulation form of the subject to be measured, it can be divided into intensity modulation type, polarization state type, phase-type, frequency type; According to whether light interferes, it can be divided into interference type and non-interference type; According to whether it can continuously monitor the measurement with the increase of distance, it can be divided into distributed and point-split; According to the role of optical fiber in the sensor, it can be divided into Functional Fiber (abbreviated as FF); Non-Functional Fiber ( abbreviated as NFF), and it is called a light-transmitting sensor.

1.Functional fiber sensor

The functional fiber sensor uses the optical fiber as a sensitive element by using the characteristics of the optical fiber itself. It is measured to modulate the light transmitted in the fiber to change the intensity, phase, frequency, or polarization state of the transmitted light. The demodulated signal is used to obtain the measured signal. The optical fiber is not only a light-guiding medium but also a sensitive component. The light is measured and modulated in the optical fiber, and multi-mode optical fiber is mostly used.

Advantages: compact structure and high sensitivity.

Disadvantages: special fiber is required, the cost is high,

Typical examples: fiber optic gyro, fiber optic hydrophone, etc.

2.Non-functional fiber sensor

Non-functional optical fiber sensors use other sensitive components to sense the measured changes. Optical fibers are only used as information transmission media, and single-mode optical fibers are often used. The optical fiber only plays the role of a light guide, and the light irradiated on the optical fiber type sensitive element is modulated by the measurement.

Advantages: The optical fiber can be used for electrical isolation and data transmission, and the signal transmitted by the optical fiber is not affected by electromagnetic interference. Most of the practical ones are non-functional optical fiber sensors.

Fiber optic sensors are new technologies that have emerged in recent years and can be used to measure a variety of physical quantities, such as sound fields, electric fields, pressures, temperatures, angular velocities, accelerations, etc., and can also complete measurement tasks that are difficult to achieve 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 kinds of optical fiber sensors, which are roughly divided into optical fiber sensors and sensors using optical fibers.

The so-called optical fiber's own sensor means that the optical fiber itself directly receives the measurement from the outside world. The external measured physical quantity can cause changes in the length, refractive index, and diameter of the measuring arm, thereby causing changes in the amplitude, phase, frequency, and polarization of light transmitted in the fiber. The light transmitted by the measuring arm interferes (compares) with the reference light of the reference arm, causing the phase (or amplitude) of the output light to change. Based on this change, the measured change can be detected. The phase of the transmission in the fiber is highly sensitive to external influences, and the interference can be used to detect the physical quantity corresponding to a small phase change of 10 minus 4 radians. Using the winding properties and low loss of the optical fiber, a very long optical fiber can be coiled into a fiber coil with a small diameter to increase the utilization length and obtain higher sensitivity.

An optical fiber acoustic sensor is a kind of sensor using optical fiber itself. When the fiber is subjected to a very small external force, it will be slightly bent, and its light transmission capacity will change greatly. Sound is a kind of mechanical wave, and its effect on the optical fiber is to make the optical fiber force and bend, and the strength of the sound can be obtained by bending. Fiber optic gyros are also a type of optical fiber sensor. Compared with laser gyros, fiber optic gyros have high sensitivity, small size, and low cost. They can be used in high-performance inertial navigation systems such as aircraft, ships, and missiles.

3.Fiber Bragg Grating Sensor

 

The working principle of fiber Bragg grating sensor

Fiber Bragg Grating Sensor (FBS) is a fiber sensor with the highest frequency and widest range. This sensor can change the wavelength of the reflected light wave according to the change of ambient temperature and strain. Fiber Bragg Grating uses a holographic interference method or a phase mask method to expose a small length of light-sensitive fiber to a light wave with a periodic intensity distribution. 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 change of light refractive index caused by this method is called fiber Bragg grating.

When a broad-spectrum beam is propagated to a fiber Bragg grating, each small length 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 the Fiber Bragg gratings only reflect light waves of a specific wavelength, while light waves of other wavelengths are propagated.

Structure of fiber Bragg grating sensor

According to the role of optical fiber in fiber optic sensors, it can be divided into two types: sensing type and light transmission type. The optical fiber of the sensing type not only plays the role of transmitting light, but also is a photoelectric sensitive element. 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. Sensing type fiber optic sensors are divided into the light-emphasis systems, phase modulation type, vibration state modulation type, and wavelength modulation type.

The light transmission type fiber optic sensor measures the optical signal modulated by the measured object after input into the optical fiber, and then performs optical signal processing at the output end to measure. This type of sensor has an additional photosensitive element that is sensitive to the physical quantity to be measured. The light-transmitting element must be attached with a sensitive element that can modulate the light transmitted by the optical fiber to form a sensing element.

The light-weight, durability, and long-term stability of the fiber optic sensor make it easy to apply to the internal stress and strain detection of various building materials such as building steel structures and concrete.

Another major category of fiber optic sensors is sensors that use optical fibers. The structure is roughly as follows: the sensor is located at the end of the optical fiber, and the optical fiber is only a transmission line of light, which converts the measured physical quantity into a change in the amplitude, phase, or amplitude of the light. In this sensor system, traditional sensors are combined with optical fibers. The introduction of optical fibers makes it possible to realize probe-based telemetry. This kind of optical fiber transmission sensor has a wide range of applications and is easy to use, but its accuracy is slightly lower than that of the first type of sensor.

Ⅴ Applications

Insulated in dirt, magnetism, sound, pressure, temperature, acceleration, gyro, displacement, liquid level, torque, photoacoustic, current, fiber optic sensor can be used for displacement, vibration, rotation, pressure, bending, strain, speed, acceleration, current , magnetic field, voltage, humidity, temperature, sound field, flow rate, concentration, PH value and strain measurement of physical quantities. The application range of fiber optic sensors is very wide, involving almost all important fields in the national economy and national defense, and people's daily lives. It can be used safely and effectively in harsh environments. It solves the technical problems that have existed in many industries for many years. The application is mainly manifested in the following aspects:

The application of interferometric gyroscopes and grating pressure sensors in bridges, dams, oil fields, etc. in urban construction. Fiber optic sensors can be pre-buried in concrete, carbon fiber reinforced plastics, and various composite materials to test stress relaxation, construction stress, and dynamic load stress to evaluate the structural performance of the bridge during the short-term construction phase and long-term operation.

Fiber Bragg Grating Pressure Sensor

In the power system, parameters such as temperature and current need to be measured, such as temperature detection in the stator and rotor of high-voltage transformers and large motors. Because electrical sensors are susceptible to interference from electromagnetic fields, they cannot be used in such occasions and can only be used fiber optic sensor. Distributed optical fiber temperature sensor is a new technology developed in recent years for real-time measurement of spatial temperature field distribution. Distributed optical fiber temperature sensor system not only has the advantages of a universal optical fiber sensor, but also has the temperature sensing capability of various points along with the fiber. Using this feature, we can continuously measure the temperature of various points within a few kilometers along with the fiber in real time. The positioning accuracy can reach the order of meters, and the measurement accuracy can reach the level of 1 degree.

In addition, fiber optic sensors can also be used in railway monitoring, rocket propulsion systems, and oil well detection. Optical fiber also has the significant advantages of broadband, large capacity, long-distance transmission, and multi-parameter, distributed, and low energy consumption sensing. Optical fiber sensing can continuously absorb new technologies and new devices of optical fiber communication, and various fiber optic sensors are expected to be widely used in the Internet of Things.

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