Pump Excited-State Absorption at 442 ± 1 nm for Efficient Visible Fluoride Fiber Lasers

In the well-known excited-state absorption (ESA) process, population inversion of the laser transition can be achieved by a strong pump ESA involving a lower laser level. The exploitation of pump ESA has expanded from the visible to the near-infrared spectral range, based on various laser sources. Extending the operating wavelength of the pump ESA is greatly significant for the development of rare-earth-doped fluoride fiber lasers. Herein, we report, presumably, the shortest operating wavelength of pump ESA at 442 ± 1 nm for an efficient continuous-wave visible fluoride fiber laser. A novel scheme for direct blue diode pumping was proposed and demonstrated in an Ho
³⁺
-doped fluoride fiber. Utilizing a 442 ± 1 nm blue laser diode, single-wavelength ground-state absorption (GSA) and ESA pumping mechanism were investigated not only for double populating of the upper laser level by GSA and ESA, but also to depopulate the lower laser level by ESA. We report a slope efficiency of 50.3% with respect to the launched pump power of 442 ± 1 nm and an output power of up to 327 mW for a 750 nm fiber laser. This result is an order of magnitude higher than that previously reported for a directly blue-diode-pumped deep-red Ho
³⁺
-doped fluoride fiber laser. Moreover, the numerical modeling results were in good agreement with the experimental results. Our approach represents a significant advancement for blue pump ESA in visible fluoride fiber lasers, which opens new capabilities for blue-diode pumping for a wide range of future applications.