Fibre top-loaded channel waveguide laser in
KY(WO
4)
2:Yb
3+D.Geskus, J.D.B. Bradley, S. Aravazhi, K. Wörhoff and M. Pollnau
Integrated Optical MicroSystems Group, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
phone: +31-53-4894440, e-mail: d.geskus@ewi.utwente.nl
A channel waveguide laser has been demonstrated by top-loading of a KY(WO4)2:Yb3+
planar waveguide with a piece of standard optical fibre in combination with contact fluid. Laser emission with a threshold of 85 mW and a slope efficiency of 30% has been obtained.
Introduction
Monoclinic potassium yttrium double tungstate, KY(WO4)2, hereafter abbreviated as
KYW, has proven to be an excellent host for solid-state [1] and waveguide [2] lasers. Rare-earth ions incorporated into KYW exhibit very high absorption and emission cross-sections. In particular, KYW:Yb exhibits an absorption maximum at 981 nm with a cross-section, for polarization parallel to the Nm principal optical axis, which is ~15 times larger than that of YAG:Yb. This short absorption length, in combination with a quantum defect as small as 1.6% [3], leads to minimal heat generation and makes KYW:Yb an excellent candidate for thin-disk lasers [4]. Its high refractive index (2.0-2.1) makes it suitable for highly integrated optical devices [5] and potentially allows for side-pumped channel waveguide lasers. Here we present laser emission from KYW:Yb layers grown onto undoped KYW substrates by liquid phase epitaxy with butt-coupled cavity mirrors, where the lateral optical confinement is created by a short piece of standard fibre with a contact fluid forming a top-loaded channel waveguide.
Experiment
We use a 17-μm-thick layer of KYW:(1.2 at%)Yb grown onto an undoped KYW substrate. The doping causes a refractive index increase of 4×10-4, thereby creating a planar waveguide. Dielectric mirrors are butt-coupled to the parallel polished end-faces, while the in-plane confinement is created by placing a bare fibre in contact fluid (Fluorinert) on top of the planar waveguide, which induces a top-loaded channel waveguide (Fig. 1a).
Fig. 1. (a) Photograph of the waveguide in lasing condition. The channel waveguiding achieved by the fibre can be clearly indentified; (b) out-coupled pump and laser wavelengths
(a) (b) 0 0.5 1 1.5 2 970 980 990 1000 1010 1020 1030 1040 Wavelength (nm) In tensity (a.u.)
The waveguide is pumped by a Ti:Sapphire laser operating CW at 980 nm. Because of the low numerical aperture of the waveguide in the horizontal direction and the correspondingly low magnification of the incoupling microscope objective of 2.5× the pump laser spot was somewhat larger than the geometrical waveguide dimensions and only a rather small fraction of the incident pump power of ~33% was estimated to be launched into this channel waveguide.
Results and discussion
The wavelength of the laser emission could be tuned between 1025 nm (Fig. 1b) and 1040 nm. The laser emission terminates as the contact fluid evaporates, but is restored after reapplying the contact fluid to the mirrors. As the bare fibre did not fully span the distance between the mirrors, no fresh contact fluid reached this strip. The horizontal confinement was apparently induced by the residual of the evaporated Fluorinert, which had concentrated along the fibre due to surface tension, as removal of the bare fibre did not affect the lasing properties. The laser emission at 1025 nm was measured after separation of pump and laser wavelengths by a grating, resulting in a strongly elliptical beam profile (Fig. 2a), an output power of 14 mW, and a slope efficiency of 30% (Fig. 2b). The pump threshold was 82 mW.
(a) (b) 0 2 4 6 8 10 12 14 16 0 20 40 60 80 100 120 140
Absorbed pump power (mW)
Laser e m is sion (mW) 30% slope efficiency Laser threshold 82mW
Fig. 2. (a) Measured output beam profile at 1025 nm; (b) input-output curve of the KYW:Yb channel waveguide laser
Conclusions
These experiments show that horizontal confinement by top-loaded structures on a slab waveguide can be applied to obtain channel waveguide laser emission.
References
[1] M. Pollnau, Y.E. Romanyuk, F. Gardillou, C.N. Borca, U. Griebner, S. Rivier, and V. Petrov, “Double tungstate lasers: From bulk toward on-chip integrated waveguide devices”, IEEE J. Select. Topics Quantum Electron. 13, 661 (2007).
[2] Y.E. Romanyuk, C.N. Borca, M. Pollnau, S. Rivier, V. Petrov, and U. Griebner, “Yb-doped KY(WO4)2 planar waveguide laser”, Opt. Lett. 31, 53 (2006).
[3] P. Klopp, V. Petrov, and U. Griebner, “Potassium ytterbium tungstate provide the smallest laser quantum defect”, Jpn. J. Appl. Phys. 42, L246 (2003)
[4] F. Brunner, T. Südmeyer, E. Innerhofer, F. Morier-Genoud, R. Paschotta, V.E. Kisel, V.G. Shcherbitsky, N.V. Kuleshov, J. Gao, K. Contag, A. Giesen, and U. Keller, “240-fs pulses with 22- W average power from a mode-locked thin-disk Yb:KY(WO4)2 laser”, Opt. Lett. 27, 1162 (2002).
[5] F. Gardillou, Y.E. Romanyuk, C.N. Borca, R.P. Salathé, and M. Pollnau, “Lu, Gd co-doped KY(WO4)2:Yb epitaxial layers: Towards integrated optics based on KY(WO4)2”, Opt. Lett. 32, 488
