Ultra-high gain and highly efficient lasing in rare-earth-ion-doped double tungstate channel waveguides

The potassium double tungstates KY(WO4)2, KGd(WO4)2, and KLu(WO4)2, are excellent host materials for rare-earth (RE) doped waveguide amplifiers and lasers, because RE ions exhibit very large absorption and emission cross-sections in these materials. Already the first planar waveguide laser demonstrated in KY(WO4)2:Yb3+ showed high performance. Co-doped layers of KY1-x-y-zGdxLuyREz(WO4)2 grown onto undoped KY(WO4)2 substrates allow for increased refractive-index contrast between layer and substrate, high RE concentrations, and simultaneous lattice matching. With this approach we demonstrated, among others, Yb3+-doped channel waveguide amplifiers with ~1000 dB/cm gain, two orders of magnitude higher than previous reports on RE-doped amplifiers and comparable to semiconductor waveguide amplifiers, as well as Tm3+-doped fundamental-mode channel waveguide lasers exploiting the well-known cross-relaxation process, thereby obtaining 1.6 W of output power at 1.84 µm and 80-85% slope efficiency, thus approaching the absolute theoretical limit of the slope efficiency for this transition.