Wavelength tuning of polymer optical fibre Bragg grating at longer wavelengths permanently

Permanent Bragg wavelength tuning of polymer optical fibre Bragg grating (POFBG) at longer wavelengths has been demonstrated for the first time utilising the thermal annealing process. In general, exposing the polymer material above its β-Transition temperature, the fibre shrinks in length, the Bragg grating period becomes shorter and the Bragg wavelength shifts permanently to shorter wavelengths. In this work, a positive tuning of Bragg wavelength has been shown to be feasible when the polymer fibre is stretched during its thermal exposure. The results show that the degree of Bragg wavelength tuning strongly depends on the applying fibre strain or equivalently stress. The work presented in this paper can be used to multiplex POFBGs at any desirable wavelength

Utilising thermal annealing for multiplexing and sensitivity enhancement of polymer optical fibre sensors

Thermal annealing was initially introduced for multiplexing purposes, since it can induce a permanent negative Bragg wavelength shift for polymer fibre grating sensors. At a later stage, it is shown that annealing can also provide additional benefits, such as strain and humidity sensitivity enhancement and augmented temperature operational range. In this paper, we report additional usage of thermal annealing on PMMA fibre Bragg grating sensors. We show the possibility to tune Bragg wavelengths to longer wavelengths permanently by stretching the polymer optical fibre during the thermal annealing process. An array of sensors fabricated with only one phase-mask, demonstrates the concept by having Bragg wavelengths below and above the original inscribed spectral position. In addition, we report that thermal annealing can be also used to enhance the performance of sensors when used for stress and force monitoring