Fabrication of all-fiber gas cavity by hollow-core fiber micro-channel using femtosecond laser

Mid-infrared fiber lasers are critically important for communications, remote sensing and defense applications. Hollowcore fiber (HCF) gas lasers offer constitute a promising route to mid-infrared lasers; however, their integration is hindered by bulky gas cells that require free-space optical coupling. This impedes achieving all-fiber integrated laser systems. To overcome this limitation, we propose a novel technique for fabricating micro-channels in HCFs via femtosecond laser direct writing. A 515 nm femtosecond laser (repetition rate: 100 kHz) was focused through a high-NA objective onto the cladding of a triple-nested HCF. Precise axial translation under three-axis motion control enables material ablation, before which hydrofluoric acid etching applied to reduce cladding diameter and enhance processing efficiency. Debris generated by femtosecond-laser ablation of the HCF is removed by compressed-air purging, while CMOS imaging ensures accurate localization between capillaries to preserve structural integrity. The fabricated micro-channels function as gas inlets/outlets, obviating conventional gas cells. Both fiber end faces were fusion-spliced to solid-core fibers, eliminating free-space interfaces. Red-light testing confirmed full cladding penetration of the micro-channel. Optical characterization using a supercontinuum source and an optical spectrum analyzer demonstrated negligible additional transmission loss after processing, proving compatibility with low-loss laser operation. This method establishes hermetic gas pathways without compromising optical performance. This work pioneers a robust route to monolithic all-fiber gas laser systems by integrating gas delivery directly into the HCF architecture. The precision and scalability the technique address a critical bottleneck in compact mid-infrared laser development, and are directly applicable to quantum optics and sensing technologies.