Single-frequency and near-single-mode fiber laser breaks kilowatt barrier for the first time
image: Figure | Design and performance of the bat-type refractive index fiber. a, theoretical calculation results based on the thermal optimized SBS and TMI theory. b, schematic diagram of optical fiber research and development. c, fiber with bat-type refractive index profile and it’s performance schematic diagram.
Credit: Pengfei Ma et al.
High-power single-frequency fiber lasers with diffraction-limited spots are indispensable for a wide range of photonic applications and are particularly in advanced detection and sensing technologies. However, the simultaneous achievement of kilowatt-level output power and diffraction-limited beam quality has remained elusive in all reported single-frequency fiber laser systems to date, primarily due to limitations imposed by the stimulated Brillouin scattering effect (SBS) and transverse mode instability effect (TMI). And the output power of all-fiber linearly-polarized single-frequency lasers with near-single-mode beam quality still remained at 400 W level—a plateau unbroken for nearly ten years.
Tackling this enduring challenge, a research team from the National University of Defense Technology in China, analyzed the optic-acoustic-thermodynamic coupling effects within optical fibers and proposed a novel approach utilizing regional refractive-index design to comprehensively suppress both SBS and TMI. Specifically, the research team demonstrated a bat-type refractive-index profile design, which offers advantages including high gain coefficient, large mode field area, and large higher-order mode loss coefficient. Further, they precisely engineered this novel fiber design using the modified chemical vapor deposition (MCVD) in conjunction with the chelate gas deposition technique. This bat-type refractive index fiber could deliver a remarkable 1015 W single-frequency laser output. Crucially, this high power was achieved while maintaining exceptional beam quality (M2 < 1.2) with ~94% fundamental mode content showcasing true near-single-mode performance.
This landmark work shatters a critical barrier, achieving the world's first kilowatt-level output from an all-fiber, single-frequency, near-single-mode laser---a transformative tool for the next-generation gravitational wave detection and ultra-long-range communication/imaging.