Thermal response of Bragg gratings in PMMA microstructured optical fibers

Abstract

Abstract:We report on the thermal characteristics of Bragg gratings fabricated in polymer optical fibers. We have observed a permanent shift in the grating wavelength at room temperature which occurs when the grating has been heated above a threshold temperature. This threshold temperature is dependent on the thermal history of the grating, and we attribute the effect to a shrinking of the fiber. This effect can be avoided by annealing the fiber before grating inscription, resulting in a linear response with temperature and an increased linear operating temperature range of the grating.

Improved thermal and strain performance of annealed polymer optical fiber Bragg gratings

“…Taking into account the refractive index change by UV-induced polymerization, UV-induced heating should increase the local temperature of POFs by approximately 15°C in order to have the -0.15 nm blue shift of the resonance wavelength after 120 minutes of UV irradiation which is observed in Fig.1 (b). This scale of temperature increase is still far below the shrinking threshold [22] and will not result in any irreversible shrinking effect. The lack of further blue shift of the resonance wavelengths after 120 minutes can be explained by the POF reaching thermal equilibrium.…”

“…UVinduced heating in the irradiated region during focused laser inscription may contribute to a permanent index increase [5], but this happens only when the local temperature is beyond some shrinking threshold, which is determined by the thermal history of the POF. At the threshold temperature, the length of the fiber starts to shrink, which is mainly due to the release of the frozen-in stress induced in the fiber during the drawing process [22]. This shrinking is an irreversible process [22].…”

“…At the threshold temperature, the length of the fiber starts to shrink, which is mainly due to the release of the frozen-in stress induced in the fiber during the drawing process [22]. This shrinking is an irreversible process [22]. Temperature increases below the threshold can only result in a reversible decrease of the refractive index due to the negative thermo-optic coefficient (TOC, -1.1×10 -4 /°C) of PMMA [24], which can be reversed to the original state once the temperature decreases [22].…”

“…This shrinking is an irreversible process [22]. Temperature increases below the threshold can only result in a reversible decrease of the refractive index due to the negative thermo-optic coefficient (TOC, -1.1×10 -4 /°C) of PMMA [24], which can be reversed to the original state once the temperature decreases [22]. Our experiments show that the blue shift of the resonance wavelengths, as shown in Fig.1 (b), was indeed reversible once the writing was stopped.…”

“…The widely accepted point is that the principle mechanism of index change is an increase due to the photo-induced polymerization of the unreacted monomers [5,[18][19][20], while laser-induced heating in the irradiated region during the inscription may also contribute to the index change [5]. Previous reports indicated that annealing of the POF before FBG inscription can relieve the frozen-in stress induced by the fiber drawing process [21] and increase the linear operation temperature range of FBGs [22]. However, the effect of annealing on the strain sensitivity performance was not yet considered.…”