# Why are FBG sensors recalibrated every 10 years?

So, I am working with FBG sensors and I have read that those sensors need to be re-calibrated every 10 years to get accurate measurements. But no where on the internet (AFAIK) is it explained why every 10 years it has to be re-calibrated and why.

• Is it a manufacturer recommendation? Have you contacted them? Aug 31, 2019 at 13:27
• Mechanical loading and thermal cycling may cause the material to creep and therefore change the grating line separation. Aug 31, 2019 at 13:33
• For Solar Mike. The sensors are glued on the road of steel in an bridge structure Aug 31, 2019 at 16:57
• @LinkCoder did you contact the manufacturer, glued or not? Sep 1, 2019 at 7:56
• @ Solar Mike ... Not ! just i would like to know what the reasons to re calibration .. the answer of alephzero was ok.. more information .. let me know .. thanks! Sep 1, 2019 at 13:38

Theory of Fiber Bragg Gratings

FBG technology is one of the most admired choices of optical fiber sensors due to their simple manufacturing and relatively strong reflected signal. FBGs are formed by a periodic modulation of the index of refraction of the fiber core along the longitudinal direction. They are the devices that are based on the principle of diffraction grating. By grating, it is meant that there is a periodic change in the core’s refractive index. When the light travels inside the grating structure, some portion of light gets reflected back from each grating plane. Figure 1 shows the structure of simple fiber Bragg grating. Every reflected portion of light combines to form one reflected beam of light, but this is possible only if Bragg condition is met. The Bragg condition is given as

$$\lambda_B = 2\Lambda n_{eff}$$

where Λ is the grating period that forms the distance between two adjacent grating planes, $$n_{eff}$$ is the effective core refractive index, and $$\lambda_B$$ is the Bragg wavelength. When Bragg condition is met, all the reflected light constructively adds up to form backward reflected peak whose central wavelength is given by $$\lambda_B$$. The grating structure that follows Bragg condition works as a mirror that reflects selected wavelength $$\lambda_B$$ and transmits the remaining. If the condition is not satisfied, all the reflections from the grating planes will go out of phase and cancels out, therefore no reflection can be observed.

Whenever dealing with a device that calculates "period" and measures "wavelength", the requirement on periodic "calibration" seems to be unavoidable.

Fiber Bragg grating sensors for monitoring of physical parameters: a comprehensive review

Fiber Bragg Grating sensors can reflect a particular wavelength of a light beam or a selection of wavelength in a fiber optic by having the sensors fiber refractive index grated according to specifications to reflect the particular wavelength.

They are used for many applications in fiber optics such as add drop multiplexing and precision measurement of heat and stress.

Because the are precision tools each manufacturer has its own criteria of calibration and set up.