Superposition of Polarized Doughnut Beams Using Acoustically Induced Fiber Grating

We demonstrate an acoustically induced fiber grating (AIFG) modulated by acoustic flexural wave propagating along polarization-maintaining fiber (PMF). At the wavelength of dispersion turnaround points (TAPs), the linearly polarized LP₀₁ mode converts to four non-degenerate LP₁₁ modes (LP_11ax, LP_11bx, LP_11ay, and LP_11by) with a 3-dB bandwidth spanning from 37.7 to 50.1 nm. Furthermore, the tunable wavelength range extends beyond 300 nm. Our investigation explores the correlation between acoustic polarization and the coupling strength of orthogonal modes (LP_11a and LP_11b). Optimized configurations of AIFG structure facilitate efficient excitation of both LP_11a and LP_11b modes concurrently. Through manipulating optical polarization direction of the input light and the application of dual acoustic optical interactions (AOIs) in the PMF, the incoherent superposition between LP_11a and LP_11b modes generates doughnut-shaped hollow beams with polarization superposition. This approach holds potential applications in particle manipulation, material processing, and communication systems.