Theoretical design of a single-mode fiber-based bi-order Bessel beam for a STED system

Stimulated emission depletion (STED) microscopy has attracted great research attention due to its applications to breaking diffraction limits for imaging and lithography. However, its implementation based on single-mode fibers often encounters challenges such as complex structural integration, costly fabrication processes, and the need for specific fiber designs. Herein, a low-cost bi-order Bessel beam based on one single-mode fiber integrated with a structurally simple wavelength-scale microstructure (WSM) on fiber end was proposed for STED system. Through simulation study for full-scale WSM optimization, we have successfully developed a bicolor laser beam (BLB) consisting of a zero-order Bessel beam at a wavelength of 405 nm and a donuts high-order Bessel beam at a wavelength of 532 nm. This fiber-based configuration allows us to achieve a diffraction-limited spot size with a working distance of 0.67λ_pump and a minimum FWHM of 0.395λ_pump. By combining wavelength division multiplexing technology with power modulation of the donuts beam, this work provides a promising way for achieving super-resolution imaging or lithography with only one single-mode fiber.