Core-cladding parallel FBG with enhanced SNR via direct femtosecond laser inscription

The signal-to-noise ratio (SNR) is a critical parameter influencing the performance of fiber Bragg grating (FBG)-based sensors, particularly in distributed sensing and harsh-environment applications. Enhancing the SNR is therefore essential for achieving repeatable, reliable and high-precision sensing. In this study, we present a novel approach based on evanescent wave coupling, wherein a cladding FBG is fabricated parallel to the primary core FBG at an offset of 2 µm from the core-cladding interface by utilizing the direct femtosecond (fs) laser inscription. This configuration significantly enhances the reflectivity of the sensing system. At the same time, it also preserves high spatial resolution and ensures minimal insertion loss (IL). The proposed method has demonstrated its effectiveness in enhancing reflectivity across FBGs with varying initial reflectivity levels, achieving a maximum improvement of approximately 700 % as reported in this study. This significant improvement is particularly beneficial for low-reflectivity FBGs utilized in a wider range of distributed sensing applications. Moreover, the sensor was subjected to temperature testing over a range of 25 °C–700 °C, followed by a long-term stability assessment conducted over an 18-hour period at 500 °C. The approach has also been successfully implemented in quasi-distributed sensing applications. Additionally, it has been demonstrated that the incorporation of cladding FBGs does not introduce any significant IL: four fabricated cladding FBGs showed maximum IL of ∼0.088 dB. This proposed method, therefore, holds significant potential and opens up a new avenue for improving the performance of low-reflectivity FBGs, enabling longer sensing ranges with minimal IL and improved measurement resolution, particularly in distributed sensing applications.