The sensitivity of the FBG temperature sensor is sometimes referred to as the resolution or temperature sensitivity of the sensor, that is, the minimum temperature change that the sensor can measure.

To be precise, the sensitivity of each FBG fiber optic sensor temperature is different, mainly determined by the reflection wavelength of the FBG and the natural characteristics of the glass filament used in the fiber, and sometimes it is also related to the sensor packaging method. The method can sensitize or desensitize the FBG.

Types of FBG Temperature Sensor

FBG Temperature Sensor CT-01

Ceramic housed fiber optic temperature transmitter CT-01, high temperature and electric power resistance, makes it perfectly suitable for the temperature sensing in power station, oil & gas industries.

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FBG Temperature Sensor ST-01

Stainless Steel tube housed temperature sensor ST-01, waterproof, commonly used in long-time harsh environmental temperature sensing in railway, bridge, dam, tunnel, etc.

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Working Principle of FBG Temperature Sensor

Figure below shows a schematic diagram of a fiber core with a Fiber Bragg Grating (FBG) inscribed (the fiber core diameter is 9 microns). People use UV phase mask irradiation, Femtosecond laser point-by-point writing or other processing methods to form countless weakly reflective surfaces with the same spacing in the core part of the fiber, these weakly reflective surfaces are called fiber gratings, and the distance between each weakly reflective surface is called the grating pitch or grating period (we generally use the symbol Λ to denote it - remember this symbol, we will need it later).

The basic sensing measurement can be carried out using the above-mentioned fiber bragg grating, and its principle is shown in Figure below:

Shown in the above, the broadband incident light enters the fiber from one end of the grating sensor, and encounters the fiber bragg grating (FBG), most of the wavelength of light directly passes through the FBG as transmitted light, and a small part of the light of special wavelength is reflected back (please pay attention to this special wavelength, this is the object we want to detect every time, we use λB to represent it ). λB is directly related to the grating pitch (grating period) Λ we mentioned earlier. The mathematical expression to characterize the relationship is: λB = 2neffΛ, where λB is the reflected wavelength, neff is the refractive index of the fiber core, and Λ is the grating distance (grating period).

Next, the reflected light enters the FBG Interrogator (not marked on the figure), and the wavelength signal λB is demodulated. Because the FBG sensing device is connected, we can get a different test wavelength signal λB every moment. Through the previous mathematical expression, we can know why the wavelength signal λ returned at the next moment has changed (the amount of change is represented by ΔλB)? The fundamental reason is that the grating pitch (grating period) Λ of the fiber grating has changed (the amount of change is characterized by ΔΛ).

Well, at this point we can jump out of the mathematical notation, switch back to the real environment and imagine, what can cause the grating pitch (grating period) Λ to change?

The first possible reason is force: you give the FBG a tension, it will become longer, you give the FBG a pressure, it will become shorter;

The second possible reason is temperature: the FBG expands when heated, and it gets longer, and when the FBG contracts when it cools, it gets shorter.

In this process, people have linked the wavelength signal that can be accurately detected with three basic physical parameters, namely force (tension and pressure), length (longer and shorter) and temperature (heated and cold), Therefore, the basic physical parameters that can be directly measured by the FBG sensors include stress, strain and temperature.