Tailoring Optoacoustic Brillouin Interaction of Cylindrical Vector Modes in Annular-Core Photonic Crystal Fiber

Brillouin scattering in optical fibers has been widely studied over the last few decades and has given rise to numerous significant applications in optical sensing, laser technology, and optical communications. However, most Brillouin studies to date have focused on the fundamental modes; explorations of Brillouin scattering involving cylindrical vector modes, which possess diverse spatial information and polarization states, and have found applications in optical trapping, particle acceleration, and laser machining, still remain limited. Here, we report Brillouin scattering (including spontaneous and stimulated cases) of cylindrical vector modes in annular-core photonic crystal fiber and show robust nonreciprocal interaction between them via Brillouin-enhanced four-wave mixing, where two distinct Brillouin interactions involving different optical modes are coherently coupled through a common acoustic wave. Extending this principle to a resonant Brillouin laser cavity, we further demonstrate coherent oscillations of both optical and acoustic modes, leading to the stable emission of cylindrically vector beams with line widths of several kHz. These results pave the way for vector-mode Brillouin photonics, opening up opportunities for narrow-line-width structured light sources, reconfigurable nonreciprocal devices, and precision metrology.