Bending-Insensitive Doughnut Beam Generation Using a Polarization-Maintaining Fiber Mode-Selective Coupler

High-stability doughnut beams propagating in the fibers are characterized as high-order modes, which have been widely exploited for optical tweezers, high-capacity fiber communication, and precision metrology. We experimentally demonstrate bending-insensitive doughnut beam generation based on a polarization-maintaining mode-selective coupler (PM-MSC). Efficient LP01-LP11 mode conversion (>95%) at 532 nm is achieved with a polarization extinction ratio (PER) of >17 dB and LP11a/bx/y modes are separated with different fiber coupler configurations. Two all-fiber mode schemes for generating doughnut beams, including orbital angular momentum (OAM) modes through 45° fusion splicing and bending-insensitive doughnut beams via incoherent superposition of LP11a/bx/y modes, are investigated based on the proposed PM-MSCs. Stable doughnut intensity profiles are maintained under different bending radii with strong environmental stabilization. The bending-insensitive doughnut beams are desired due to their transversal intensity, phase, and polarization distributions, which provide attractive solutions for free-space communications, quantum entanglement distribution, and adaptive microscopy, particularly in polarization-sensitive scenarios.