Design of an equal-power dual-peak Brillouin gain spectrum based on M-shaped optical fibers transitioning from ring to circular core for temperature and curvature sensing

Here we design a simple M-shaped optical fiber to generate equal-power dual Brillouin gain peaks, and numerically simulate bending loss-resistant temperature and curvature sensing. By investigating the M-shaped fibers transitioning from ring-core to circular-core, we examine the Brillouin gain spectrum evolution from a single peak to dual peaks and back to a single peak. During this fiber transition and spectral evolution, we find that the calculated Brillouin frequency shift (BFS) and Brillouin gain exhibit unique developments based on acoustic-optic coupling theory, providing a methodology for designing and optimizing a desirable Brillouin gain spectrum in M-shaped optical fibers. These theoretical advances enable the generation of equal-power dual peaks with an appropriate peak frequency interval for temperature and curvature sensing. Furthermore, by simulation, this sensor has temperature sensitives of 1.086 and 1.100 MHz/°C and curvature sensitivities of 453.6153 and 459.5102 MHz/cm, with resistance to bending loss. By comparison, we improve the curvature sensitivity by more than 1.5-fold and reduce the bending-induced birefringence by more than 2.5-fold. Overall, the proposed design has advantages in curvature sensitivity, is more resistant to bending loss, and suffers less bending-induced birefringence, offering a competitive option for Brillouin-based temperature and curvature sensors.