Microstructured Channel Waveguide Lasers in
KY(WO
4
)
2
:Gd
3+
,Lu
3+
,Yb
3+
D. Geskus, S. Aravazhi, C. Grivas, 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
d.geskus@ewi.utwente.nl
Abstract: Laser operation was achieved in microstructured channel waveguides of
KY(WO4)2:Gd3+, Lu3+, Yb3+, resulting in a threshold of only 5 mW, a slope efficiency of 62% versus launched pump power, and 76 mW output power.
OCIS codes: (140.3615) Lasers, Ytterbium; (230.7380) Waveguides, channeled
1. Introduction
The monoclinic double tungstate KY(WO4)2 (KYW) has been recognized as as excellent laser material, see Ref. [1] and Refs. therein. KYW strongly enhances the absorption and emission cross sections of rare-earth ions. In
KYW:Yb3+, planar waveguide lasing with 80% slope efficiency was demonstrated [2], however the low Yb3+ concentrations of 1-3 at.% induce a refractive index contrast between layer and substrate of only a few ×10−4, thus requiring a layer thickness in excess of 10 μm for single-mode waveguiding. Improved mode confinement was obtained by co-doping the active layer with large amounts of Gd3+ and Lu3+ ions, thereby increasing the refractive index contrast to ~7.5×10−3 and leading to few-µm-thin waveguide layers [3]. Such highly co-doped layers have recently enabled planar waveguide lasing with a record-high slope efficiency of 82.3% [4]. Furthermore, the much smaller layer thickness greatly facilitates microstructuring [3]. Channel waveguide lasing was previously achieved in bulk double tungstates by femtosecond-laser writing of refractive index changes [5], although the rather large mode size and considerable waveguide propagation losses diminished the laser efficiency. Here we demonstrate a microstructured channel waveguide laser with low loss and excellent mode confinement, resulting in a slope efficiency of 62% [6].
2. Device fabrication and laser experiments
By liquid phase epitaxy in a K2W2O7 solvent [7], a 2.4-μm-thick KYW:(43.3%)Gd3+,(15.0%)Lu3+,(1.7%)Yb3+ layer was grown onto an undoped KYW substrate. 7-µm-wide rib structures (Fig. 1a) were etched parallel to the Ng optical axis by transferring a lithographic mask of photoresist (Fujifilm OiR 908/35) to a depth of 1.4 µm into the active layer by Ar beam milling with an etch rate of 3 nm/min. The rib structures were overgrown by an undoped KYW overlay and endfacets were polished perpendicular to the waveguides. Dielectric mirrors were attached by use of a fluorinated oil. A continuous-wave Ti:Sapphire pump laser at 981 nm was coupled into a channel waveguide by a ×16 microscope objective. The outcoupled laser light with beam radii of 4.8 µm × 2.1 µm (Fig. 1b) was collimated by a ×20 microscope objective. A grating was used to separate the residual transmitted pump light from the laser emission. A schematic of the experimental setup is shown in Fig. 2a.
At the laser wavelength near 1028 nm the incoupling mirror had a reflectivity of 99.8%, while for the
outcoupling mirror transparencies of 2%, 5%, 10%, and 23% were tested. Fig. 2b shows the laser output power as a function of launched pump power. Laser oscillation commenced at a launched pump power as low as 4.5 mW. A slope efficiency of 62% was measured for 23% outcoupling efficiency. The maximum output power was 76 mW.
When analyzing the obtained slope efficiencies versus output coupling according to the theory for laser devices exhibiting reabsorption [2] an intracavity roundtrip loss of 11% is derived. This value includes the waveguide propagation losses and losses occurring due to butt-coupling of the mirror substrates to the endfacets. Therefore, the value of 11%, equaling 0.34 dB/cm, is the upper limit for the propagation loss at 1028 nm in the microstructured channel waveguide.
(a)
(b)
Fig. 1. (a) SEM micrograph of a microstructured KYW:Gd3+,Lu3+, Yb3+ channel waveguide before overgrowth and (b) measured mode profile of the laser emission (both to scale)
Objective Gr a tin g Objective 16X 20X Microstructured KYW:Gd , Lu , Yb TiSa Mirrors Position of Power Meters
3+ 3+ 3+ Lase r Em ission Resid ual Pum p
(a)
(b)
0 20 40 60 80 100 120 140 0 20 40 60 80 OC = 23%, η = 62% OC = 10%, η = 53% OC = 5%, η = 23% OC = 2%, η = 11% Output Power [mW]Launched Pump Power [mW]
Fig. 2. (a) Schematic of experimental setup; (b) measured output power as a function of launched pump power (approx. 99% of the launched pump power was absorbed) for different outcoupling (OC) values
3. Conclusions
We have demonstrated, to the best of our knowledge, the first microstructured double tungstate channel waveguide laser. Excellent control over the lateral waveguide dimensions results in tight confinement of pump and laser mode. Combination with the high absorption and emission cross-sections present in Yb3+-doped double tungstates and the tight confinement makes it an attractive high-gain laser device.
In the future, replacing the butt-coupled mirrors with an on-chip integrated cavity by etching Bragg reflectors or a distributed-feedback grating into the channel waveguide [7] or employing a ring-resonator configuration will make the device more robust and environmentally stable, such that real-world applications can seriously be envisaged.
This work was supported by the Dutch VICI Grant "Photonic integrated structures".
4. 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 (3), 661–671 (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] 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 (2007).
[4] D. Geskus, S. Aravazhi, E. Bernhardi, C. Grivas, S. Harkema, K. Hametner, D. Günther, K. Wörhoff, and M. Pollnau, “Low-threshold, highly efficient Gd3+, Lu3+ co-doped KY(WO
4)2:Yb3+ planar waveguide lasers,” Laser Phys. Lett. 6, 800 (2009).
[5 F. M. Bain, A. A. Lagatsky, R. R. Thomson, N. D. Psaila, N. V. Kuleshov, A. K. Kar, W. Sibbett, and C. T. A. Brown, “Ultrafast laser inscribed Yb:KGd(WO4)2 and Yb:KY(WO4)2 channel waveguide lasers,” Opt. Express 17, 22417 (2009).
[6] D. Geskus, S. Aravazhi, C. Grivas, K. Wörhoff, and M. Pollnau, “Microstructured KY(WO4)2:Gd3+, Lu3+, Yb3+ channel waveguide laser, ” Opt. Express 18, 8853 (2010).
[7] R. Solé, V. Nikolov, X. Ruiz, J. Gavaldà, X. Solans, M. Aguiló, and F. Díaz, “Growth of β-KGd1-xNdx(WO4)2 single crystals in K2W2O7 solvents,” J. Cryst. Growth 169, 600 (1996).
