Engineering lattice matching, doping level, and optical properties of KY(WO4)2:Gd, Lu, Yb layers for a cladding-side-pumped channel waveguide laser

Abstract

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.

Potassium double tungstate waveguides with high ytterbium concentration for optical amplification

“…) and/or lutetium (Lu 3+ ), the refractive index contrast of the layer with respect to the substrate can be enhanced for waveguiding purpose [64,65]. Epitaxial layers grown using this technique had been used to demonstrate continuous wave (CW) planar waveguide laser with slope efficiency as high as 82.3% [66] as well as channel waveguide laser with 71% slope efficiency and high CW output power of 418 mW [67].…”

“…Thirdly, the use of co-doped Gd 3+ and Lu 3+ in the layer allows the growth of high Yb 3+ concentration layer with refractive index contrast of ~0.01-0.02 with respect to the substrate [65]. Furthermore, with appropriate micro-structuring of the highly Yb 3+ -doped KRE(WO4)2 layer, the resulting channel waveguide allow for high optical intensity to be maintained over a rather long interaction length, hence greatly enhances the light-matter interaction within a limited physical device length.…”

“…KYb(WO4)2, can also be realized [104]. Many reports had shown that concentration dependent lifetime quenching on these high Yb 3+ concentration crystals is very limited [65,106,107]. In addition, the KRE(WO4)2:Yb 3+ has absorption band at ~932 nm and ~981 nm, hence permitting the use of high brightness, high wall-plug efficiency InGaAs pump lasers [75].…”

“…The unit cell parameters of α-KRE(WO4)2 can be expressed in either C2/c or I2/c space group, both belong to the centrosymmetric 2/m point group [65,103] proper choice of the composition epitaxial layer or the substrate will ensure that the lattice matching condition between the layer and the substrate is fulfilled. This will be further discussed in Section 3.3.…”

“…Among these double tungstate crystals, the KY(WO4)2 exhibits the lowest refractive index value while the KYb(WO4)2 has the highest value, with two of them differ by ~0.029 at 1000 nm wavelength. With a proper choice of material composition, KY1-x-y-zGdxLuyYbz(WO4)2 epitaxial layers with Yb 3+ concentration up to 47.5 at.% grown on KY(WO4)2 substrate can result in refractive index contrast of ~0.012 at Nm [65] which permits optical waveguiding in the layer. …”