Temperature and vector bending sensing with a supermode fiber Bragg grating

Due to their multiple advantages, single-mode fiber Bragg gratings (FBGs) are widely used in a myriad of practical sensing applications. However, their concurrent sensitivity to strain and temperature makes a reference sensor necessary in several situations. Here, we demonstrate that a single supermode FBG can be used to measure bending and temperature. A specially designed two coupled-core optical fiber (TCCF) that supports two supermodes was fabricated in which gratings were inscribed with femtosecond or ultraviolet lasers. The interrogation of the gratings was carried out with a conventional FBG sensor interrogator. It was found that the reflection spectrum of the supermode grating depended on its inscription in the TCCF. So, it was possible to fabricate samples in which the reflection spectra exhibited two narrow peaks very close to each other with well-defined Bragg wavelengths and reflectivities. Cross sensitivities and polarization effects on the devices were studied. It was found that two Bragg wavelengths and two reflectivities provided abundant data to measure bending of the TCCF, temperature, and to discriminate in which direction the TCCF was bent. Thus, we believe that the results reported here can pave the way for next-generation grating-based specialty optical fiber devices that are capable of multi-parameter sensing. The results and approaches proposed here can also expand the use of Bragg grating technology.