1) Intensity Modulated Fiber Optic Sensor

The basic principle is that the physical quantity to be measured causes a change in the light intensity of the transmitted light in the optical fiber, and the measurement to be measured is realized by detecting the change in the light intensity. The light of intensity I emitted by the constant light source is injected into the sensor head. Within the sensor head, the intensity of the light changes under the action of the measured signal, that is, it is modulated by the external field, so that the envelope of the output light intensity and The shape of the signal to be measured is the same. The output current measured by the photodetector is also modulated in the same way. The signal processing circuit then detects the modulated signal to obtain the signal to be measured.

The advantages of this type of sensor are simple structure, low cost, and easy implementation. Therefore, it was developed earlier and has been successfully applied to displacement, pressure, surface roughness, acceleration, gap, force, liquid level, vibration, radiation, etc. measuring. There are many ways of intensity modulation, which can be roughly divided into reflection intensity modulation, transmission intensity modulation, light mode intensity modulation, refractive index and absorption coefficient intensity modulation, and so on. Generally, reflection-type intensity modulation, transmission-type intensity modulation, and refractive index intensity modulation are called external modulation types, and the light mode is called internal modulation type. However, due to the limitation of the principle, it is susceptible to the influence of light source fluctuations and connector loss changes, so this sensor can only be used in places with small interference sources.

2) Phase modulation fiber optic sensor

The basic principle is: under the action of the measured energy field, the phase of the light wave in the optical fiber changes, and then the interferometric measurement technology is used to convert the phase change into a change in light intensity, thereby detecting the physical quantity to be measured. The advantages of phase-modulated fiber-optic sensors are extremely high sensitivity, large dynamic measurement range, and fast response speed. The disadvantages are high requirements on the light source and high requirements on the precision of the detection system, so the cost is relatively high. .

At present, the main application fields are: acoustic, pressure or vibration sensors using photoelastic effect; current and magnetic field sensors using magnetostrictive effect; electric field and voltage sensors using electrostrictive force; rotational angular velocity sensors using Segnak effect (Fiber Optic Gyro) and so on.

3) Frequency modulation fiber optic sensor

The basic principle is to use the Doppler frequency shift effect of the reflected or scattered light of a moving object to detect its moving speed, that is, the light frequency is related to the movement state between the light receiver and the light source. When they are relatively stationary, they receive the oscillation frequency of light; when there is relative movement between them, the received light frequency undergoes a frequency shift with its oscillation frequency, and the magnitude of the frequency shift is related to the magnitude and direction of the relative motion speed.

Therefore, such sensors are mostly used to measure the speed of object movement. There are some other methods of frequency modulation. For example, the absorption and fluorescence of certain materials also change in frequency with external parameters, and Brillouin and Raman scattering caused by quantum interaction are also a kind of frequency modulation phenomenon. Its main application is to measure fluid flow. Others include gas sensors that measure the concentration of gas or monitor atmospheric pollution by using Raman scattering when a substance is illuminated by strong light; temperature sensors that use photoluminescence.

4) Polarization Modulation Fiber Optic Sensor

The basic principle is to use the change of the polarization state of light to transmit the information of the measured object.

A light wave is a transverse wave whose light vector is perpendicular to the direction of propagation. If the direction of the light vector of a light wave is always the same, but its size changes with the phase, such light is called linearly polarized light. The plane composed of the light vector and the light propagation direction is the vibration plane of linearly polarized light. If the size of the light vector remains the same and its direction rotates uniformly about the direction of propagation, the trajectory at the end of the light vector is a circle. Such light is called circularly polarized light. If the size and direction of the light vector change regularly, and the end of the light vector rotates along an ellipse, such light is called elliptically polarized light.

Using the polarization properties of light waves, polarization-modulated fiber sensors can be made. In many fiber optic systems, especially those containing single-mode fibers, polarization plays an important role. Many physical effects can affect or change the polarization of light, and some effects can cause birefringence. The so-called birefringence phenomenon is a phenomenon in which a beam of incident light is often decomposed into two beams of refracted light for some crystals whose optical properties vary with direction. The phase delay of light through a birefringent medium is a function of the polarization state of the input light.

The polarization-modulated fiber-optic sensor has high detection sensitivity, which can avoid the influence of light source intensity changes, and the relative phase-modulated fiber-optic sensor has a simple structure and is easy to adjust. Its main application fields are: current and magnetic field sensors that use the Faraday effect; electric and voltage sensors that use the Bower effect; pressure, vibration, or acoustic sensors that use the photoelastic effect; temperature, pressure, and vibration sensors that use birefringence. At present, it is mainly used for monitoring strong current.

5) Wavelength Modulated Fiber Optic Sensor

The traditional wavelength-modulated fiber-optic sensor is realized by utilizing the property that the spectral characteristics of the sensing probe change with external physical quantities.

Such sensors are mostly non-functional sensors. In a wavelength-modulated fiber-optic probe, the fiber is simply used as a light guide, that is, the incident light is sent to the measurement area, and the returned modulated light is sent to the analyzer. The key to fiber wavelength detection technology is the good performance of the light source and spectrum analyzer, which has a decisive influence on the stability and resolution of the sensing system.

Optical fiber wavelength modulation technology is mainly used in medical, chemical and other fields. For example, human blood gas analysis, pH detection, chemical analysis of indicator solution concentration, phosphorescence and fluorescence analysis, blackbody radiation analysis, and Fabry-Perot filters. The so-called wavelength-modulated fiber-optic sensors are mainly fiber Bragg grating sensors (FBG).

Finally, I will introduce three kinds of optical fiber sensors with various functions imported from the mining industry from overseas. The first is the optical fiber temperature sensor-FOT-L-SD, which is a type of optical fiber temperature sensor that is very suitable for measuring temperature in extreme environments. Such extreme environments include low temperature, nuclear environment, microwave, and high-intensity RF. FOT-L combines all the excellent features you expect from an ideal sensor body. Therefore, this type of sensor can still provide high-precision and reliable temperature measurement even under extreme temperature and adverse environment.

Then there is the fiber optic pressure sensor-FOP-M, FOP-M is specially designed for the high temperature field of aerospace and defense. FOP-M can withstand high temperatures of 150 ° C (302 ° F). Unlike most traditional pressure sensor designs, FOP-M uses a unique silicon crystal diaphragm deflection design. FOP-M is the best choice for harsh environmental pressure measurement. The sensor is completely immune to electromagnetic and radio frequency interference, and its inherent reliability is suitable for hazardous and high temperature environments.

Finally, the optical fiber refractive index sensor-FRI, is a unique sensor design based on the length change of the liquid Fabry-Perot optical cavity to accurately determine the refractive index of the fluid. The length of the Fabry-Perot optical cavity is proportional to the refractive index of the fluid sample. Therefore, the refractive index can be measured by measuring the length of the Fabry-Perot cavity using white light interference technology. The fiber optic signal conditioner has the ability to measure the refractive index even in harsh temperature, EMI, humidity environments, and variable calibration conditions. The FRI optical fiber refractive index sensor provides better and more reliable refractive index measurement for existing applications in the industry. At the same time, the sensor also has the new expansion capability of continuous online monitoring of the refractive index of fluids under harsh conditions.

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