K. van Dalfsen

Thulium channel waveguide laser in a monoclinic double tungstate with 70% slope efficiency

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.

Highly efficient Yb 3+ -doped channel waveguide laser at 981 nm

Channel waveguide lasers operating at 981 nm are demonstrated in KY(1-x-y)Gd(x)Lu(y)(WO4)2:Yb3+ waveguides grown by liquid phase epitaxy onto undoped KY(WO4)2 substrates and microstructured by Ar+ beam etching. Under pumping at 934 nm of samples with different waveguide geometry and outcoupling degree, a record-high slope efficiency of 76% versus absorbed pump power and a record-high output power of 650 mW for rare-earth-ion-doped microstructured channel waveguide lasers is achieved. The laser performance is compared to that of the same devices when pumping at 981 nm and lasing near 1025 nm.

Efficient KY 1-x-y Gd x Lu y (WO 4 ) 2 :Tm 3+ channel waveguide lasers

Laser experiments on 1.5at.%, 5at.%, and 8at.% thulium-gadolinium-lutetium-yttrium co-doped, buried, ridge-type channel waveguides in a monoclinic potassium double tungstate demonstrate a maximum slope efficiency of 70% and output powers of 300 mW at ~1.9 μm.

Thulium-doped channel waveguide laser with 1.6 W of output power and exceeding 80% slope efficiency

Dielectric channel waveguide lasers are of great interest because of their low pump thresholds and, even more so, high slope efficiencies resulting from the excellent overlap between pump and laser modes and the small transverse channel cross-sections which reduce the volume of active medium that needs to be pumped. Slope efficiencies up to 75% in a femtosecond-written YAG:Yb3+ channel waveguide with an output power of 800 mW [1], 76% in a LiYF4:Tm3+ planar waveguide with an output power of 560 mW [2], as well as 70% and 76% in Tm3+ - and Yb3+ -doped potassium double tungstate channel waveguides with output powers of 300 mW [3] and 650 mW [4], respectively, are state of the art.

Highly efficient solid-state waveguide lasers

This paper reviews our recent results on highly efficient rare-earth-ion-doped planar and channel waveguide lasers in crystalline potassium double tungstates and amorphous aluminum oxide on silicon chips.

On the efficiency of Tm-doped 2-µm lasers

A potassium double tungstate layer with the composition KY0.40Gd0.29Lu0.23Tm0.08(WO4)2 was grown onto a pure KY(WO4)2 substrate by liquid-phase epitaxy, microstructured by standard lithography and Ar-ion etching, and overgrown by a pure KY(WO4)2 layer. The end-facets were polished. Laser experiments were performed on these buried, ridge-type channel waveguides in a resonator with one butt-coupled mirror and Fresnel reflection from the other end-facet, resulting in a high output-coupling degree of 89%, compared to intrinsic round-trip losses of only 2%. 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. To the best of our knowledge, this result represents the most efficient 2-μm channel waveguide laser to date. We determined the optimum Tm3+ concentration in double tungstate channel waveguides to be at least 8at.% for efficient lasing. The theoretical limit of the slope efficiency depends on the Stokes efficiency which here is 43.2%, the outcoupling efficiency which here is 99%, and the pump quantum efficiency. The pump quantum efficiency of a 2-μm Tm3+ laser pumped around 800 nm hinges on the efficiency of its cross-relaxation process. By fitting the macroscopic cross-relaxation parameter which linearly depends on the Tm3+ concentration to concentration-dependent luminescence- decay data, calculating the overall decay rate of the pump level, and deriving the concentration-dependent pump quantum efficiency, we obtain a theoretical limit for the slope efficiency of 83% for the chosen Tm3+ concentration. The experimental slope efficiency of ~80% closely approaches this limit.

Highly efficient channel waveguide lasers at 2 µm

Laser experiments on thulium-gadolinium-lutetium-yttrium co-doped monoclinic potassium double tungstates in buried, ridge-type channel-waveguide geometry demonstrate maximum slope efficiencies approaching twice the Stokes limit at ~1.9 μm, when pumping with a Ti:Sapphire laser at 800 nm.

Thulium channel waveguide laser with 69.7% efficiency

Laser experiments on thulium-gadolinium-lutetium-yttrium-co-doped, buried, ridge-type channel waveguides in a monoclinic potassium double tungstate demonstrate a maximum slope efficiency of 69.7% and output powers up to ~300 mW at ~1.9 µm.

Efficient Gd 3+ , Lu 3+ co-doped KY(WO 4 ) 2 :Tm 3+ channel waveguide lasers

Integrated planar and channel waveguide lasers in the KY(WO4)2 (= KYW) host material is a subject of heavy investigation. Waveguide lasers based on Yb3+-doped KYW with slope efficiencies as high as 82.3% and output powers of several hundreds of milli-Watts have demonstrated the potential for highly efficient integrated lasers in this material, as well as low thresholds [1, 2]. Recently, lasers based on Tm3+-doped KYW with planar and channel geometries reported maximum slope efficiencies of 13% and maximum output powers of 32 mW [3, 4]. The higher efficiencies of the lasers based on Yb3+ compared to the ones based on Tm3+ can partly be attributed to the absence of detrimental up-conversion effects in Yb3+. However, slope efficiencies of 44% and 68% in thulium-doped bulk KGd(WO4)2 crystals and also fiber lasers suggest that there is significant room for improvement in terms of slope efficiency in thulium-doped KYW [5, 6].

Growth and characterization of highly Yb 3+ -doped KY(WO 4 ) 2 thin layers

Crystals of the monoclinic double tungstates KY(WO<inf>4</inf>)<inf>2</inf> (= KYW), KGd(WO<inf>4</inf>)<inf>2</inf>, and KLu(WO<inf>4</inf>)<inf>2</inf> are excellent laser host materials due to their high refractive indices on the order of 2, good thermal conductivity, and large transition cross-sections of rare-earth ions, in particular Yb³⁺, doped into these crystals. Yb³⁺-doped double tungstate thin layers enable lasing with a small quantum defect [1] and high slope efficiency [2], making them well suited for high-power lasers [3] and waveguide lasers [4]. Co-doping KYW:Yb³⁺ layers with appropriate amounts of Lu³⁺ and Gd³⁺ increases the refractive index contrast with respect to the undoped KYW substrate, thus reducing the required layer thickness for waveguiding, while simultaneously providing lattice matching between layer and substrate [5]. On the other hand, for thin-disk laser applications highly Yb³⁺-doped KYW layers are needed. This can be achieved by either growing KLu(WO<inf>4</inf>)<inf>2</inf>:Yb³⁺ layers on undoped KLu(WO<inf>4</inf>)<inf>2</inf> substrates because of the similar ion radii of Yb³⁺ and Lu³⁺ [3] or co-doping a KYW:Yb³⁺ layer with Gd³⁺ for compensating the induced lattice mismatch with respect to the undoped KYW substrate.