Holmium monolithic fiber lasers: an investigation of the fiber Bragg grating spatial dimensions on fiber laser performance

We have investigated how the physical dimensions of a fiber Bragg grating (FBG) may affect the performance of a monolithic fiber laser system. In particular, we perform a series of direct laser inscriptions in a Holmium-doped single mode fiber using the plane-by-plane femtosecond laser inscription method, using exactly the same inscription conditions but having different grating widths. Specifically, the gratings were divided in to three groups; group 1, gratings inscribed with planes having widths smaller of the core size but in the center of the fiber; group 2, gratings inscribed with plane widths similar to the core diameter and group 3, gratings with plane widths larger the core diameter that were extended uniformly in to cladding region. All the gratings were characterized in a fiber laser configuration and their performance were analyzed using as metrics the threshold power, the effective length and the power slope efficiency. We note that all the gratings were designed to have a resonance Bragg wavelength at 2.1 μm, having the same length and inscribed directly through the fiber coating. The monolithic fiber laser strands were pumped using Thulium-doped fiber laser operating at 1.95 μm. The results clearly show that the spatial dimensions of the FBGs are certainly an important parameter that is required for consideration during the development of the monolithic fiber lasers, especially for medium and high-power fiber laser systems.