In engineering applications and scientific research, temperature is a very important reference quantity, occupying a pivotal position. Traditional temperature sensors are mainly composed of thermocouples, semiconductors, and platinum alloys, which have simple measurement principles and low costs. Therefore, they are widely used in practical applications. Compared with traditional temperature sensors, fiber optic temperature sensors utilize the absorption spectrum of some substances to change with temperature, and then analyze the spectrum transmitted by fiber optic to measure temperature changes in real time. Its measurement accuracy is higher and its sensitivity is adjustable in real time.
Fiber optic sensors have the characteristics of small size, light weight, high sensitivity, no electromagnetic interference, and corrosion resistance compared to other sensors, making their application range wide, involving national defense, national economy, and people’s daily life. There are many types of fiber optic temperature sensors, mainly including fiber optic grating temperature sensors, fiber optic FabryPerot interferometric temperature sensors, fiber optic fluorescence temperature sensors, Raman fiber optic temperature sensors, etc.
The characteristics and advantages of fiber optic sensors
Fiber optic sensors are devices based on optical fibers that are used to detect quantities, usually temperature or mechanical strain, but sometimes also include displacement, vibration, pressure, acceleration, rotation (measured using optical gyroscopes based on the Sagnac effect), or concentration of chemical substances. The general principle of this device is that light from a laser (usually a single frequency fiber laser) or from a superlight source is sent through a fiber, undergoes subtle changes in its parameters in the fiber or one or more fiber Bragg gratings, and then reaches the detector device for measuring these changes.
A sensor that distinguishes between internal and external factors. The inherent sensor is the fiber optic itself (which may be a modified form, such as containing a Bragg grating) as the sensor. External sensors only use optical fibers to transmit light to or from the actual sensor.
Many fiber optic sensors are based on a single fiber, while other fiber optic sensors are made using fiber bundles. For example, there are external sensors where some illumination light is sent to the sample through some optical fibers of the beam, and the reflected light or used fluorescence is sent back through other optical fibers.
Compared to other types of sensors, fiber optic sensors have many advantages:
They are composed of electrical insulation materials (without the need for cables), which makes them suitable for use in high voltage environments, for example.
They can be safely used in explosive environments because there is no risk of electric sparks even in the event of defects.
They are not affected by electromagnetic interference (EMI), and even nearby lightning strikes will not cause electrical shock to other devices.
Their materials can be chemically passive, meaning they do not pollute their surrounding environment and are not subject to corrosion.
They have a very wide operating temperature range (much wider than many electronic devices may have).
They have multiplexing function: multiple sensors in a single fiber optic line can be queried through a single light source.
fiber bragg grating sensor
Fiber optic sensors are typically based on fiber Bragg gratings. The basic principle of many fiber optic sensors is that the Bragg wavelength (i.e. the maximum reflection wavelength) of the fiber Bragg grating depends not only on the Bragg grating period, but also on temperature and mechanical strain.
For optical strain sensors based on silica fibers, the fractional response of Bragg wavelength to strain is about 20% smaller than the strain itself, because the direct effect of strain is reduced to some extent by the reduction of refractive index. The temperature effect is close to the expected temperature effect of individual thermal expansion. The effects of strain and temperature can be distinguished through various techniques (such as using reference gratings that are not affected by strain, or combining different types of fiber Bragg gratings) to obtain both quantities simultaneously.
What can be the resolution for pure strain sensing μ The range of s (i.e., the relative length variation is several times 10-6), and the accuracy may not be much lower. For dynamic measurements (such as acoustic phenomena), it is possible to achieve better performance than 1n within a 1Hz bandwidth ε Sensitivity of.
There is also a Bragg grating laser sensor, which implements a fiber laser consisting of two gratings and a rare earth doped fiber between them. Alternatively, there can be an FBG and a broadband reflector on the other side. When pump light is provided, this device produces an output with a wavelength close to the Bragg wavelength.
Quasi distributed sensing
A single fiber optic can contain many series connected grating sensors (see above) to monitor the temperature and strain distribution along the entire fiber optic. This is called quasi distributed sensing. There are different techniques to solve individual gratings (as well as certain positions along the fiber):
In a technique called wavelength division multiplexing (WDM) or optical frequency domain reflectometer (OFDR), gratings have slightly different Bragg wavelengths. The interrogator unit in a wavelength tunable laser can be tuned to a specific grating wavelength, and the maximum wavelength reflectivity represents the influence of strain or temperature, for example. Alternatively, broadband light sources (such as superemitters) can be used in conjunction with wavelength scanning photodetectors (such as fiber optic Fabry P é rot based) or CCD based spectrometers.. In any case, the maximum number of gratings is usually between 10 and 50, limited by the tuning range or bandwidth of the light source and the wavelength interval required for each fiber grating.
Another technique called Time Division Multiplexing (TDM) uses the same weak reflection grating to interrogate with short light pulses. Then, the reflections from different gratings are distinguished by their arrival times. Time division multiplexing is usually combined with wavelength division multiplexing to multiply the number of different channels by hundreds or even thousands.
Optical switches allow people to choose between different fiber optic circuits, further increasing the number of possible sensors.
Distributed sensing
Other fiber optic sensors do not use fiber Bragg gratings as sensors, but instead use the fiber itself. Then, the sensing principle can be based on Rayleigh scattering, Raman scattering, or Brillouin scattering. For example, an optical time-domain reflectometer is a method that uses pulse detection signals to locate weak reflections. For example, the temperature or strain dependence of Brillouin frequency shift can also be utilized.
In some cases, the measured quantity is the average of the entire fiber length. The same applies to some temperature sensors, but also to Sagnac interferometers used as gyroscopes. In other cases, measure position related quantities (such as temperature or strain). This is called distributed sensing.
For more detailed information, please refer to the articles on optical temperature sensors and optical strain sensors
Other methods
In addition to the above methods, there are many alternative technologies. Some examples are:
Fiber Bragg gratings can be used for interferometric fiber optic sensors, where they are only used as reflectors and the measured phase shift is generated by the fiber span between them.
There is a Bragg grating laser sensor, where the sensor grating forms the end mirror of the fiber laser resonator, which includes, for example, some erbium-doped fibers that receive 980nm pump light through fiber lines. The Bragg wavelength depends on factors such as temperature or strain, determining the wavelength of laser emission. This method has many further changes, due to the small linewidth of this fiber laser, it has very high resolution and high sensitivity.
In some cases, paired Bragg gratings are used as fiber optic Fabry Perot interferometers, which can respond particularly sensitive to external influences. Fabry Perot interferometers can also be manufactured using other methods, such as having variable air gaps in optical fibers.
Long period fiber Bragg gratings are particularly useful for multi parameter sensing, such as temperature and strain, and also for strain sensing with very low sensitivity to temperature changes.
application
Even after years of development, fiber optic sensors have not yet achieved significant commercial success because even with certain limitations, they are difficult to replace mature technologies. However, for certain application areas, fiber optic sensors are increasingly recognized as a technology with very interesting possibilities. This is particularly suitable for harsh environments, such as sensing in high-pressure and high-power machinery or microwave ovens. Bragg grating sensors can also be used to monitor conditions in aircraft wings, wind turbines, bridges, large dams, oil wells, and pipelines. Buildings with integrated fiber optic sensors are sometimes referred to as “smart structures”;