Channel waveguide lasers and amplifiers in single-crystalline ytterbium-doped tungstates

Abstract

In this work the technology of Gd3+ and Lu3+ co-doped KY(WO4)2:Yb3+ thin films grown onto pure KY(WO4)2 substrates by liquid phase epitaxy (LPE) is explored. The co-doping enables lattice matching of the grown layer with the substrate and enhances the refractive index difference between the grown layer and the undoped KY(WO4)2 substrate. This technology enabled the demonstration of waveguide lasers with tight pump and laser mode confinement, resulting in excellent slope efficiencies in Yb3+-doped planar and microstructured channel waveguide lasers of 82.3% and 72%, respectively. Fabrication of double tungstate microstructured channel waveguides with further enhanced refractive index contrast of 1.5×10-2 by completely interchanging the Y3+ component of the KY(WO4)2 host material with Gd3+, Lu3+, and Yb3+ doping allows for the demonstration of a laser with an output power of 418 mW at 1023 nm and slope efficiency of 71% versus launched pump power. In addition, in two other resonator configurations these channels showed broad tunability of the laser wavelength from 980 to 1045 nm as well as a low quantum defect of 0.7% when pumping at 973 nm and lasing at 980 nm, thereby minimizing heat dissipation in the device. Laser operation at 980 nm is achieved in an open cavity configuration, which allows for optimal extraction of the laser power, resulting in a total extracted emission of 650 mW. Moreover, Laser emission has been demonstrated by an on-chip resonator structure fabricated using a deep Bragg-reflector milled by a focused ion beam (FIB) through the complete waveguide structure, similar to the approach demonstrated in a semiconductor gain material. Micro-chip optically active devices, currently developed in the well-established semiconductor gain material platform, can be possibly being fabricated in the discussed rare earth ion doped material platform. This is highlighted by the last outcome of this project; demonstrating similar modal gain compared to state of the art semiconductor waveguide amplifiers of nearly 1000 dB/cm. To obtain this remarkable result the developed technology is used for the fabrication of nearly 50% Yb3+-doped channel waveguide structures. This result is beyond the common expectation, as rare earth ions are regarded as impurities providing low gain.