Vortex fibers for STED microscopy

Super-resolution imaging using the principles of stimulated emission depletion (STED) microscopy requires collinear excitation of a sample with a Gaussian-shaped excitation beam and a donut-shaped depletion beam whose spin (polarization) and orbital angular momentum (OAM) signs are aligned. We leverage recent advances in stable OAM mode propagation in optical fibers for telecom applications to design, fabricate, and validate the utility of a vortex fiber as the beam shaping device at visible and near-IR wavelengths for STED microscopy. Specifically, using compact UV-written fiber-gratings yielding high purity mode conversion (98.7%), we demonstrate the simultaneous generation of Gaussian and OAM beams at user-defined wavelengths. Point spread function measurements reveal a depletion beam with >17.5-dB extinction ratio, a naturally co-aligned Gaussian beam, and a setup in which these characteristics are maintained even as the fiber is bent down 6-mm radii. The proof-of-concept of all-fiber STED microscopy realized using this fiber device is used to image fluorescent bead samples yielding a sub-diffraction-limited resolution of 103 nm in the lateral plane. This opens the door to performing fiber-based STED microscopy with a setup that is not only resistant to environmental perturbations but also facilitates the development of endoscopic STED imaging.Super-resolution imaging using the principles of stimulated emission depletion (STED) microscopy requires collinear excitation of a sample with a Gaussian-shaped excitation beam and a donut-shaped depletion beam whose spin (polarization) and orbital angular momentum (OAM) signs are aligned. We leverage recent advances in stable OAM mode propagation in optical fibers for telecom applications to design, fabricate, and validate the utility of a vortex fiber as the beam shaping device at visible and near-IR wavelengths for STED microscopy. Specifically, using compact UV-written fiber-gratings yielding high purity mode conversion (98.7%), we demonstrate the simultaneous generation of Gaussian and OAM beams at user-defined wavelengths. Point spread function measurements reveal a depletion beam with >17.5-dB extinction ratio, a naturally co-aligned Gaussian beam, and a setup in which these characteristics are maintained even as the fiber is bent down 6-mm radii. The proof-of-concept of all-fiber STED microscopy ...