Abstract

A femtosecond laser direct writing system was developed to explore the fabrication of long-period fiber gratings (LPFGs) in SMF28 fibers. The LPFGs, showing the mode LP1,6 at 1500 nm, were exposed to high-temperature annealing up to 950 °C. Modifications in the refractive index (RI) modulation are observed through a blue-shift in the LPFG attenuation bands and above 850 °C, the mode LP1,7 appear at 1600 nm. The wavelength sensitivity to external RI from 1.300 to 1.452 was estimated for both modes before and after annealing. Greater sensitivity was found for the higher order mode in the entire range reaching 2400 nm/RIU around 1.440.

Highlights

  • Long-period fiber gratings (LPFGs) are applied in order to couple light between the core and copropagating cladding modes in standard single-mode optical fibers (SMF28, Corning) [1]

  • The femtosecond laser inscribed LPFG can be reliably used for various applications but temperature annealing is necessary, after which the LPFG reaches the regime of spectral stability

  • The annealing process leads to modifications in the LPFG spectral response affecting its sensitivity to the surrounding refractive index (SRI)

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Summary

Introduction

Long-period fiber gratings (LPFGs) are applied in order to couple light between the core and copropagating cladding modes in standard single-mode optical fibers (SMF28, Corning) [1]. This property turns the LPFGs intrinsically sensitive to external perturbations, such as mechanical stress, temperature, or refractive index (RI) changes in the surrounding medium. Changes in the grating period, in the RI modulation affects the coupling conditions leading to spectral variations and sensitivity to parameter to be measured [2]. The femtosecond laser inscribed LPFG can be reliably used for various applications but temperature annealing is necessary, after which the LPFG reaches the regime of spectral stability. Experimental characterization of the spectral response of the femtosecond LPFGs, to variations of the surrounding refractive index (SRI) before and after temperature annealing was carried out

Material and Methods
Results and Discussion
Spectral Tunning of LPFG through Thermal Annealing
LPFG Refractive Index Characterization
Conclusions
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