Customized femtosecond laser-inscribed superstructure fiber Bragg grating: A novel approach to decoupling temperature and strain.

Abstract

This paper introduces a novel technique to simultaneously measure temperature and strain using a single 5 mm femtosecond laser-inscribed superstructure fiber Bragg grating (SFBG). This SFBG enables improved spectral capabilities, resulting in side-resonances within the optical spectrum. The characteristics of the device depend on three key factors: the length of the SFBG, the inscribed FBG’s periodic perturbations, and the length of the uninscribed sections that compose the entire structure. Initially, mathematical expressions were derived to calculate the separation between the central peak and a resonance on each side, which plays a crucial role in determining the sensor’s response. Then, a series of complete simulations using OptiGrating software, and a theoretical analysis were performed to comprehend the functioning of this superstructure. Subsequently, the fabrication process employed a femtosecond laser following a point-by-point inscription. Finally obtaining a comprehensive study on the principle, design, simulation, and implementation of the proposed SFBG. The sensitivities of the central Bragg wavelength and the distance between central peak and first-order resonance when applying temperature and strain were calibrated. The results yielded sensitivities for the central Bragg peak of 10.58 pm/°C and 1.1598 pm/µɛ, while the peak-to-resonance distance had sensitivities of 0.3495 pm/°C and 0.03052 pm/µɛ. The different response of both magnitudes makes it possible to distinguish the contributions of strain and temperature within short distances, having the possibility to improve measurements in sensing applications. This study finally provides a critical insight into the design process, as it aligns with the theoretical expectations.