Single-crystalline KY 1-x-y-z Gd x Lu y Yb z (WO 4 ) 2 layers are grown onto undoped KY(WO 4 ) 2 substrates by liquid-phase epitaxy. The purpose of co-doping the KY(WO 4 ) 2 layer with suitable fractions of Gd 3? and Lu 3? is to achieve lattice-matched layers that allow us to engineer a high refractive-index contrast between waveguiding layer and substrate for obtaining tight optical mode confinement and simultaneously accommodate a large range of Yb 3? doping concentrations by replacing Lu 3? ions of similar ionic radius for a variety of optical amplifier or laser applications. Crack-free layers, up to a maximum lattice mismatch of *0.08 %, are grown with systematic variations of Y 3? , Gd 3? , Lu 3? , and Yb 3? concentrations, their refractive indices are measured at several wavelengths, and Sellmeier dispersion curves are derived. The influence of co-doping on the spectroscopy of Yb 3? is investigated. As evidenced by the experimental results, the lattice constants, refractive indices, and transition crosssections of Yb 3? in these co-doped layers can be approximated with good accuracy by weighted averages of data from the pure compounds. The obtained information is exploited to fabricate a twofold refractive-index-engineered sample consisting of a highly Yb 3? -doped tapered channel waveguide embedded in a passive planar waveguide, and a cladding-side-pumped channel waveguide laser is demonstrated.

Neodymium-doped aluminum oxide films with a range of Nd 3+ concentrations are deposited on silicon wafers by reactive co-sputtering, and single-mode channel waveguides with various lengths are fabricated by reactive ion etching. Photoluminescence at 880, 1060, and 1330 nm from the Nd 3+ ions with a lifetime of 325 µs is observed. Internal net gain at 845-945 nm, 1064, and 1330 nm is experimentally and theoretically investigated under continuous-wave excitation at 802 nm. Net optical gain of 6.3 dB/cm at 1064 nm and 1.93 dB/cm at 1330 nm is obtained in a 1.4-cm-long waveguide with a Nd 3+ concentration of 1.68 × 10 20 cm −3 when launching 45 mW of pump power. In longer waveguides a maximum gain of 14.4 dB and 5.1 dB is obtained at these wavelengths, respectively. Net optical gain is also observed in the range 865-930 nm and a peak gain of 1.57 dB/cm in a short and 3.0 dB in a 4.1-cm-long waveguide is obtained at 880 nm with a Nd 3+ concentration of 0.65 × 10 20 cm −3 . By use of a rate-equation model, the gain on these three transitions is calculated, and the macroscopic parameter of energy-transfer upconversion as a function of Nd 3+ concentration is derived. The high internal net gain indicates that Al 2 O 3 :Nd 3+ channel waveguide amplifiers are suitable for providing gain in many integrated optical devices.

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Laser experiments were performed on buried, ridge-type channel waveguides in an 8 at. % thulium-doped, yttrium-gadolinium-lutetium codoped monoclinic double tungstate. A maximum slope efficiency of 70% and output powers up to 300 mW about 2.0 μm were obtained in a mirrorless laser resonator, by pumping with a Ti:sapphire laser near 800 nm. To the best of our knowledge, this result represents the most efficient 2 μm channel waveguide laser to date. Lasing is obtained at various wavelengths between 1810 nm and 2037 nm.

Laser experiments were performed on buried, ridge-type channel waveguides in 8 at.% thulium-doped, yttrium-gadolinium-lutetium co-doped potassium double tungstate. By pumping with a Ti:sapphire laser at 794 nm, 1.6 W of output power at 1.84 μm with a maximum slope efficiency of ∼80% was obtained in a laser resonator with a high output-coupling degree of 89%. To the best of our knowledge, this result represents the most efficient 2-μm channel waveguide laser to date.