Gd3+, Lu3+ co-doped KY(WO4)2:Yb3+ planar waveguide lasers at 1025 and 980 nm

Single-crystalline layers of KY(WO4)2:Yb3+ co-doped with Gd3+ and Lu3+ have been grown onto pure (010) oriented KY(WO4)2 (KYW)substrates by vertical liquid phase epitaxy. The Yb3+ concentration is optimized to 1.2-2.4at.% for application of these layers as planar waveguide lasers near 1 μm, yielding a refractive index contrast between layer and\ud substrate of 6×10-4. Both co-dopants, Gd3+ and Lu3+, are optically inert but possess higher electron densities than Y3+, thus co-doping a total of 40% of these ions significantly increasing the refractive index contrast by an order of magnitude to 7.5×10-3 without affecting the optical properties of the layer.\ud This allows for a significant reduction of the layer thickness to 2-4 μm for single-mode guiding, thus facilitating micro-structuring and making the layer\ud suitable for active integrated optical devices. Two main advantages of this method are its nearly constant refractive index contrast over a wide range of Yb3+ doping level, because Yb3+ can now replace Lu3+, and the ability to\ud engineer good lattice matching by adjusting the amounts of Gd3+ and Lu3+ ions in the KYW:Yb3+ layer. The grown layers resulted in a planar waveguide laser with butt-coupled mirrors operating at 1025 nm with a record-high slope\ud efficiency of 82.3% versus absorbed pump power, and an output power of 195 mW. Another laser experiment without the use of an outcoupling mirror revealed laser emission at the zero-phonon line at 981 nm, with a slope efficiency of 71% versus absorbed pump power

Low-threshold laser in a high-index-contrast double tungstate waveguide

The paper reports the laser emission from an enhanced-index-contrast KYW waveguide fabricated by co-doping the active layer with Lu and Gd ions. Both, Lu3+ and Gd3+ possess higher electron densities than Y3+, thus increasing the refractive index. The emission wavelength varied from 1010 nm to 1040 nm, strongly depending on the alignment, which was probably caused by the etalon effects of the gaps between the mirrors and the endfaces

Growth, characterization, and waveguide lasing of Yb3+, Lu3+, Gd3+ co-doped KY(WO4)2 thin layers

Monoclinic crystals of KY(WO4)2 (KYW) doped with different rare-earth ions are among the highly promising materials for building compact solid-state lasers. \ud We report the liquid phase epitaxy (LPE) growth of 3-5 µm thick KYW:Gd3+, Lu3+, Yb3+ layers for Yb3+ concentrations of 1.2, 1.7, and 2.4 mol% and 30 to 40-µm-thick KYW:Gd3+, Yb3+ (20 mol%) layers. The concentration of the dopants Yb3+, Lu3+, and Gd3+ in the grown film were determined by laser ablation inductively coupled plasma - mass spectrometry (LA-ICP-MS). The growth conditions were optimized, leading to crack-free layers for all investigated Yb3+ concentrations. X-ray investigations have confirmed the high crystallinity of the films.\ud Based on the Gd3+, Lu3+ co-doped thin films, planar waveguide lasers operating on the Yb3+ transition at 1025 nm were demonstrated. Due to the co-doping, resulting in high refractive-index difference between film and substrate, very thin waveguides with strong light confinement were obtained, thus allowing for a pump threshold of laser operation as low as 18 mW. The highest slope efficiency versus absorbed pump power and output power were 82.3% and 195 mW, respectively

Giant optical gain in a rare-earth-ion-doped waveguide amplifier

For optical amplification, typically rare-earth-ion (RE) doped fiber amplifiers (RDFA) or semiconductor optical amplifiers (SOAs) are selected. Despite the weak transition cross-sections of RE ions and their low doping level in silica fibers, resulting in very low gain per unit length, the extremely long interaction lengths realized in fibers can lead to significant overall gain. SOAs can deliver similarly high overall gain over much shorter distances, which makes them suitable for providing on-chip gain. Very high material gain in the nanometer-wide recombination region of a III-V semiconductor, but small overlap with the usually µm-sized signal beam results in a modal gain of several hundred dB/cm. In contrast, the gain per unit length in RE-doped integrated waveguides has hardly exceeded a few dB/cm. Here we demonstrate an ultra-high modal gain of 950 dB/cm in a RE-doped waveguide amplifier, comparable to the modal gain reported for SOAs. The potassium double tungstates KGd(WO4)2, KY(WO4)2, and KLu(WO4)2 are excellent host materials for RE-doped lasers, partly thanks to the high transition cross-sections of RE ions in these hosts. In 2006, the first planar KY(WO4)2:Yb3+ waveguide laser was demonstrated. Co-doping the layer with Gd3+ and Lu3+ ions offers the possibility for lattice matching with the undoped KY(WO4)2 substrate and a significantly enhanced refractive index contrast, hence improved mode confinement. Microstructuring by Ar+ beam etching resulted in channel waveguides, in which lasing with 418 mW output power at 1023 nm and 71% slope efficiency vs. launched pump power was demonstrated. Replacing Y3+ in the layer completely by Gd3+ and Yb3+ ions results in highly doped channel waveguides with a refractive-index contrast of >2 x 10-2. These novel dielectric micro-structures combine a high dopant concentration, large transition cross-sections, and strong light confinement, all features that are crucial for achieving high optical gain, in a single device. When pumping such a KGd0.447Lu0.078Yb0.475(WO4)2 channel waveguide with a 932-nm Ti:Sapphire laser via a microscope objective, high inversion of the Yb3+ system is obtained. Signal light at the zero-phonon line at 980.6 nm, which is the wavelength of highest absorption and emission cross-section, exhibits a small-signal modal gain of 950 dB/cm, exceeding the gain per unit length previously reported in RE-doped materials by two orders of magnitude, thus paving the way for applications of on-chip integrated RE-doped amplifiers

Microstructured KY(WO4)2:Gd3+, Lu3+, Yb3+ channel waveguide laser

Epitaxially grown, 2.4-μm-thin layers of KY(WO4)2:Gd3+, Lu3+, Yb3+, which exhibit a high refractive index contrast with respect to the undoped KY(WO4)2 substrate, have been microstructured by Ar beam milling, providing 1.4-μm-deep ridge channel waveguides of 2 to 7 μm width, and overgrown by an undoped KY(WO4)2 layer. Channel waveguide laser operation was achieved with a launched pump power threshold of only 5 mW, a slope efficiency of 62% versus launched pump power, and 76 mW output power

Lattice matching and microstructuring of Gd3+, Lu3+ co-doped KY(WO4)2:Tm3+ channel waveguide lasers

Lattice-matched KY(WO4)2:Gd3+,Lu3+,Tm3+ layers with a thickness of 6 μm have been grown onto pure KY(WO4)2 substrates. Channel waveguides of 7.5 μm to 12.5 μm width have been microstructured to a depth of 1.5 μm using Ar+ beam milling. Laser experiments with buttcoupled mirrors demonstrate laser oscillation near 1844 nm while pumping at 792 nm