Out of the blue: semiconductor laser pumped visible rare‐earth doped lasers

The rise of semiconductor‐based pump sources such as In_xGa_1‐xN‐laser diodes or frequency‐doubled optically pumped semiconductor lasers with emission wavelengths in the blue encourages a revisitation of the rare‐earth ions Pr³⁺, Sm³⁺, Tb³⁺, Dy³⁺, Ho³⁺ and Er³⁺ with respect to their properties as active ions in crystalline solid‐state laser materials with direct emission in the visible spectral range. Nowadays, some of these blue‐pumped visible lasers compete with Nd³⁺‐lasers in terms of efficiency and direct lasing at various colors from the cyan‐blue to the deep red can be addressed in very simple and compact laser setups. This paper highlights the spectroscopic properties of suitable rare‐earth ions for visible lasing and reviews the latest progress in the field of blue‐pumped visible rare‐earth doped solid‐state lasers.

     

Efficient laser operation of diode-pumped Pr3+,Mg2+:SrAl12O19

We report on diode-pumped laser operation of Pr³⁺,Mg²⁺:SrAl₁₂O₁₉ at lasing wavelengths of λ _L = 724.4 nm, λ _L = 643.5 nm, and λ _L = 622.8 nm. Furthermore, the laser threshold could be reached in the green spectral range. By pumping the crystal longitudinally from each side with two polarization beam combined InGaN laser diodes, a total pump power of ≈4 W was available. In the deep red spectral range, a maximum output power of 564 mW was achieved with a maximum slope efficiency of 50 % with respect to the absorbed pump power. The maximum possible internal losses were estimated to ≈1 %. Beam quality factors M ² were in the range of 1.2–1.5.     

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.