Abstract
This paper discusses a novel detection scheme to measure strain and temperature simultaneously using a section of specially designed few mode fibre (FMF). The work shows that the propagation constant difference between LP01 and LP02 modes, Δβ, has a maximum at the critical wavelength (CWL) in the transmission spectrum. Theoretical analysis and experimental verification indicate that the peaks located on each side of the CWL shift to opposite directions under strain and temperature variations. The two peaks located closest to the CWL from both sides, Left Peak 1 and Right Peak 1, shift linearly with indeed different and high strain and temperature sensitivities, when an appropriate length of FMF is chosen, allowing such a device to be used to simultaneously measure strain and temperature over a known range.
Highlights
Optical fibre sensor devices to monitor strain and temperature simultaneously have been intensively studied because their potential applications in key industrial sectors
An alternative and simpler sensor design is preferred and this is the approach used in this work, using a simple length of few mode fibre (FMF) as the basis of the device
∆φ= ∂φ ∆λ+ ∂φ ∆χ ∂λ ∂χ where ∆ε =∆L / L is the variation of axial strain. ∂φ / ∂L and ∂φ / ∂T are the change in phase difference produced by per unit increase in the actual length of the FMF, and. per degree change of temperature, respectively
Summary
Optical fibre sensor devices to monitor strain and temperature simultaneously have been intensively studied because their potential applications in key industrial sectors. Monitoring two different interference fringes with different responses to strain and temperature is an alternative approach Such a device uses the transmission spectra of one single interferometer, for example constructed using a suspended core photonic crystal fibre [3], or a fibre taper and lateral-shifted junction [4] with single mode fibre (SMF). An in-line Mach-Zehnder Interferometer (MZI) is used where, based on the interference between the LP01 and LP02 modes, there is an identifiable critical wavelength (CWL) in the transmission spectra [57] Such a device can be constructed as follows: a piece of few-mode fibre (FMF) is spliced between two pieces of SMF to create what is termed a SFS structure and in which the FMF supports only two modes, LP01 and LP02 [8], as a result of its design.
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