Gain dynamics in a highly ytterbium-doped potassium double tungstate epitaxial layer

Active media with high rare-earth concentrations are essential for small-footprint waveguide amplifiers. When operating at high population inversion, such devices are often affected by undesired energy-transfer processes and thermal effects. In this work, we study a 32-μm-thick epitaxial layer of KGd_0.43Yb_0.57WO₄₂, representing an Yb³ concentration of ∼3.8 × 10²¹ cm⁻³, grown on an undoped KYWO₄₂ substrate. The pump absorption, luminescence decay, and small-signal gain are investigated under intense pumping conditions. Spectroscopic signatures of an energy-transfer process and of quenched ions, as well as thermal effects, are observed. We present a gain model which takes into account excessive heat generated due to the abovementioned experimental observations. Based on finite-element calculations, we find that the net gain is significantly reduced due to, first, a fraction of Yb³ ions not contributing to stimulated emission, second, a reduction of population inversion owing to a parasitic energy-transfer process and, third, degradation of the effective transition cross-sections owing to device heating. Nevertheless, a signal enhancement of 8.1 dB was measured from the sample at 981 nm wavelength when pumping at 932 nm. The corresponding signal net gain of ∼800 dB∕cm, which was achieved without thermal management, is promising for a waveguide amplifier operating without active cooling.