Four-Mode Coupling in Fiber Bragg Grating Sensors Using Full Fields of the Fundamental Modes

Abstract The four-mode coupling in shear-strain-loaded fiber Bragg grating sensors has been demonstrated to have influence on the measured spectra and, thus, on the application of embedded fiber Bragg gratings for strain field measurements. In a recent work, these coupling coefficients were derived using a set of assumptions on the properties of the fiber's modes. However, the validity of these approximations was not explicitly tested. Here, the required coupling coefficients are computed, making use of the full-field components. It is demonstrated that additional terms will appear; however, they are negligible in practical fibers, and thus, the made approximations are valid.