Passive athermal fiber bragg grating strain gage

Abstract

Fiber Bragg gratings (FBGs) are particular suitable for measuring strain. However, a single parameter measurement is difficult to implement, since cross-sensitivity to temperature compels the use of an additional temperature reference, e.g. a strain-inactive FBG. The development of a passive athermal fiber Bragg grating strain gage is thus of particular interest since it renders optional the measurement of temperature, benefiting large scale system design and performance. In view of this need, a package for fiber Bragg gratings that enables strain measurements to be performed while canceling temperature sensitivity is disclosed. The proposed design is based on a structure composed of two parts, which can be made of the same material, having a defined length ratio that allows the adjustment of the temperature sensitivity to zero, providing athermal operation of the strain gage. Moreover, the disclosed passive athermal scheme is adjustable to further compensate for structural thermal expansion, enabling the load-induced strain component to be decoupled from the temperature-induced strain component.

Description

FIELD OF THE INVENTION [0001] The present invention relates to optical fiber sensors and, more particularly, pertains to fiber Bragg grating strain gages with adjustable compensation for thermal effects. BACKGROUND [0002] A fiber Bragg grating (FBG) is a periodic modulation of the refractive index of the core of a single mode optical fiber usually written by exposure to UV light. This periodic structure is characterized by a narrow reflective spectral response. The center wavelength λB of the reflection band matches the Bragg condition: λB=2neffΛ,  (1) where neff is the effective index of the guided mode and Λ is the period of the index modulation. The FBG resonance wavelength will vary accordingly with temperature and / or strain changes experienced by the fiber. For a temperature change ΔT; the corresponding wavelength shift is given by: ΔλB=λB⁡(1Λ⁢∂Λ∂T+1n⁢∂n∂T)⁢ ⁢Δ⁢ ⁢T=λB⁡(αF+ξ)⁢ ⁢Δ⁢ ⁢T=λB⁢βT⁢Δ⁢ ⁢T,(2) where αF is the fiber coefficient of thermal expansion (CTE) and ξ is the fibe...

AI Technical Summary

Benefits of technology

[0005] It is an object of the present invention to provide a package for fiber Bragg gratings that enables strain measurements to be performed while canceling temperature sensitivity, thus extending the range of applications of such components.

[0006] It is also an object of the present invention to provide an FBG strain gage comprising two structural elements, which can be made of the same material, having a defined length ratio that allows adjustment of the temperature sensitivity to zero, providing athermal operation of the strain gage.

[0008] It is also an object of the present invention to provide a method for measuring strain that does not require an independent measurement of temperature.

[0009] A strain gage according to the present invention can also be adjusted to compensate for structural thermal expansion, enabling load-induced strain-components to be isolated.

[0011] This method enables not only annulled temperature sensitivity, i.e., TS=0, but also enhanced positive temperature sensitivity, i.e., TS>βT, reduced positive temperature sensitivity, i.e., TS<βT, or even negative temperature sensitivity, i.e., TS<0. In the particular case of annulled temperature sensitivity, the exact balancing between the intrinsic FBG temperature sensitivity and temperature induced strain, βT=−βεε(T) / ΔT, provides so-called athermal operation of the strain gage.

Problems solved by technology

2226-2234 (2000)] have been demonstrated, but they are often too complex and difficult to implement in real world structures.

Moreover, besides being not required in all the strain monitoring cases, the measurement of temperature by the referred methods implies the allocation of additional bandwidth to each sensor, therefore limiting the total number of sensors in a given sensing network.

However, this restricts the adjustment of the FBG sensitivity to the set of discrete values that can be obtained employing available materials.

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