Efficient continuous wave laser operation of Tb3+-doped fluoride crystals in the green and yellow spectral regions

In this work Tb3+ is revisited as a laser ion for efficient visible laser operation. In detailed spectroscopic investigations of absorption and fluorescence properties we reveal, that neither the spin‐forbidden transitions nor the widespread belief of excited state absorption or upconversion into 4f75d1‐states ultimately prevent efficient visible laser operation in Tb3+‐doped fluorides. In contrast, the rise of blue semiconductor‐based pump sources enabled us to achieve slope efficiencies up to 58% around 545 nm in the green spectral region in highly Tb3+‐doped LiLuF4, LiYF4, KY3F10, β‐BaLu2F8 and LaF3 crystals. In addition, we obtained laser emission from Tb3+ in the yellow spectral region around 585 nm with slope efficiencies approaching 20%. To the best of our knowledge, these results represent the first continuous wave laser operation of Tb3+‐doped crystals and demonstration of laser oscillation on the 5D4 → 7F4‐transition in this ion. A thorough evaluation of laser relevant spectroscopic parameters like ground state absorption, excited state absorption, and stimulated emission enabled highly efficient continuous wave laser operation in the visible in various Tb3+‐doped fluoride crystals up to the Watt‐level‐regime under pumping in the blue. Slope efficiencies exceeding 50% are achieved around 545 nm. Furthermore, output powers exceeding 100 mW at nearly 20% of slope efficiency are demonstrated at 585 nm.