Optical 101: FBG

FBG is an abbreviation for (Fiber Bragg Grating), which is a refractive grating (diffraction grating) installed in the core of an optical fiber and constructed with short segments .

In FBG, among the light of various wavelengths traveling inside the optical fiber, only the light having the wavelength matching the period (refractive index) of the FBG fringes is reflected.

Since the period of this fringe changes due to distortion due to applied pressure and expansion and contraction due to temperature, the wavelength of reflected light also changes accordingly, so by using it together with an interrogator, a sensor that measures pressure, distortion, temperature, etc. It is used as.

Usage of FBG

FBG is one of the effective alternatives even in the measurement environment where the conventional sensor technology cannot be applied, and it is very effective even in the harsh measurement environment such as high temperature, high radiation and high vacuum.

FBG can be used, for example, for vibration and temperature control in high voltage generators, high temperature monitoring in transformers, blade monitoring in windmills, load monitoring in aircraft fuel tanks, strain, temperature, and displacement in nuclear reactors. Accurate measurement is possible even in harsh environment applications such as surveillance and spacecraft surveillance.

In addition, measurement using FBG is one of the effective means even when the number of sensors required is very large or when installation over an ultra-long distance is required.

FBG principle

According to DZOptics.com, The FBG sensor measures and quantifies changes in distortion due to temperature and pressure through measurement of light, but in reality, by detecting the intensity of each wavelength to see changes in reflected light, light information is converted into temperature and distortion. , It is possible to detect by recalculating as pressure.

Therefore, it can be said that FBG functions as a direct element that directly detects distortion and temperature.

The flow of detection by FBG is as follows.

First, the incident light is incident on the optical fiber from the interrogator.

This incident light travels through the core while repeating total internal reflection, and eventually reaches the FBG.

Part of the incident light that reaches the FBG becomes "reflected light" that is reflected in the opposite direction by the FBG, and the remaining light becomes "transmitted light" that passes through as it is.

This "reflected light" travels in the opposite direction in the core and eventually returns to the interrogator.

The interrogator measures and quantifies this "wavelength intensity of the returned reflected light".

From the obtained wavelength information, "temperature", "strain", and "pressure" are calculated using a predetermined coefficient.

In this way, the temperature, strain, and pressure can be derived in the end, but the information directly obtained by the FBG, which is the detection element, is the "wavelength of reflected light."

However, the wavelength of the reflected light changes as the distance between the FBGs changes due to temperature, strain, and pressure.

The wavelength of the reflected light of FBG is called "Bragg wavelength", and the change of wavelength is called "wavelength shift".

The interrogator measures the Bragg wavelength shift by measuring the intensity of these wavelengths.

Since the Bragg wavelength changes in response to pressure, strain, and temperature, it can be said that the strain dependence and temperature dependence of the reflected light can be used, and the pressure, strain, and temperature are calculated from the obtained wavelength information. FBG can be used as a detection element for sensors.


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