Agazzi, L.

Aravazhi, S.

Bernhardi, Edward

García Blanco, Sonia Maria

Geskus, D.

Pollnau, Markus

English

150 dB/cm gain over 55 nm wavelength range between 977-1032 nm is obtained in a 47.5% Yb-doped potassium double tungstate waveguide amplifier. The dependence of luminescence lifetime and gain on Yb concentration is investigated

University of Twente Research Information

150 dB/cm gain over 55 nm wavelength range near 1 µm  in an Yb-doped waveguide amplifier  D. Geskus, S. Aravazhi, E. H. Bernhardi, L. Agazzi, S. M. García-Blanco, and M. Pollnau Integrated Optical MicroSystems Group, MESA+ Institute for Nanotechnology, University of Twente, 7500 AE, Enschede, The Netherlands e-mail: m.pollnau@utwente.nl  Abstract: 150 dB/cm gain over 55 nm wavelength range between 977-1032 nm is obtained in a 47.5% Yb-doped potassium double tungstate waveguide amplifier. The dependence of luminescence lifetime and gain on Yb concentration is investigated. ©2011 Optical Society of America OCIS codes: (130.3130) Integrated optics materials; (140.4480) Optical amplifiers  1. Introduction On-chip photonic integration requires the regeneration of optical signals at very high rates. Semiconductor optical amplifiers operate in the saturated regime, hence their gain recovery time limits the maximum achievable data rates. Rare-earth-ion-doped fiber amplifiers provide superior characteristics for high-speed amplification, but their gain of ~30-50 dB requires employing several meters of fiber length, making this solution unsuitable for on-chip integration. Recently, we have demonstrated that Yb3+-doped potassium double tungstate can provide a modal gain up to ~1000 dB/cm in channel-waveguide and thin-film geometry [1], making this material highly suitable for amplification and lasing in micro- and nano-structured optical devices. Here we investigate the Yb3+-concentration and wavelength dependence of modal gain around 1 µm in microstructured KGdxLuyYb1-x-y(WO4)2 channel waveguides. 2. Sample fabrication Yb3+-doped KY(WO4)2 layers were grown by liquid-phase epitaxy onto undoped, (010)-oriented, laser-grade-polished, 1-cm2-sized KY(WO4)2 substrates in a K2W2O7 solvent at temperatures of 920–923°C [2]. Co-doping the layers with appropriate percentages of optically inert Gd3+ and Lu3+ ions enables the growth of lattice-matched layers with enhanced refractive-index difference between layer and substrate, allowing for thinner active layers [3]. Partial replacement of Lu3+ by Yb3+ ions of similar ionic radius resulted in up to 47.5at.% Yb3+-doped  KGdxLuyYb1-x-y(WO4)2 layers with a refractive-index contrast between layer and substrate of up to ~2 × 10-2. The samples were between 180–320 µm long and the layer surface was polished down to 2.2–3.5 µm thickness. Microstructuring by Ar-beam etching [4] provided 1.4-µm-deep, 6-µm-wide ridge waveguides along the Ng optical axis. The microstructured samples were overgrown by a layer of undoped KY(WO4)2. 3. Amplifier performance We performed small-signal-gain measurements using a pump-probe-beam set-up with a Ti:Sapphire pump laser operating at 932 nm, a broadband probe-beam source polarized parallel to the Nm optical axis, mechanically modulated with a frequency of 142 Hz, and a monochromator selecting the signal wavelength S. Pump and signal light were combined and coupled into the channel waveguide by an objective lens. Light emitted from the opposite waveguide end was collimated by an objective lens and residual pump light was suppressed by a dichroic filter and a spectrometer set to 980.6 nm. The amplified signal was detected by an InGaAs detector and discriminated from luminescence at this wavelength by lock-in amplification. Using a spatially resolved rate-equation model, we determined the pump power establishing transparency at S. Relative to this 0-dB transmitted signal intensity, transmitted signal intensities at other pump powers were investigated and the modal gain was determined [1]. Results for different Yb3+ concentrations of 15at.%, 20at.%, and 47.5at.% versus launched pump power are displayed in Fig. 1(a). With increasing dopant concentration the gain increases because of an accordingly increasing absorbed pump power density and Yb3+ excitation density. On the other hand, also lifetime-quenching processes induced by increased energy migration among Yb3+ ions and subsequent energy transfer to impurities or cooperative upconversion can diminish the achievable excitation density. Luminescence-decay measurements show an increasing lifetime with increasing Yb3+ concentration, see the measured data and fit in Fig. 1(b), which is an effect of radiation reabsorption. Using the formalism presented in Ref. [5], we corrected the measured lifetime for this effect (blue line), resulting in a lifetime quenching from  ~270 µs to slightly less than 200 µs at 47.5at.% Yb3+ concentration. Apparently this small amount of quenching does not affect the achievable gain significantly, i.e., even higher gain may be obtained with further increased Yb3+ concentrations.  (a) (b) 0 10 20 30 40 50050100150200250300350400450500Data: Data1_TauModel: reabsorptionWeighting: y No weighting  Chi^2/DoF = 5.43344R^2 =  0.99912  Tw 263.58362 ±1.53909A 0.0296 ±0.0008N0 84.09354 ±1.89803Lifetime (s)Yb conc. (mol%)deconvolution Fig. 1. (a) Modal gain versus launched pump power for different Yb3+ concentrations. (b) 2F5/2 lifetime as a function of Yb3+ concentration. The red line is the fit with the equation given in [5], while the blue line is the deconvolution of the fit that shows the self-quenching contribution. For the 47.5at.% sample we investigated the gain at different signal wavelengths experimentally and theoretically, exploiting the transition cross-sections determined for this mixed compound, see Fig. 2(a). While the measured peak gain amounted to 935 dB/cm at the zero-phonon line of 980.6 nm, we could achieve gain exceeding 150 dB/cm at several other wavelengths spanning a large wavelength range, indicated by the symbols in Fig. 2(b). Comparison with the calculated data (lines) shows that this gain level of > 150 dB/cm can be achieved continuously over a wavelength range of 55 nm, between 977 nm and 1032 nm.  (a)024681012900 920 940 960 980 1000 1020 1040Cross-section (10-20cm-2)Wavelength (nm)Emission Cross-sectionAbsorption Cross-section(b)  Fig. 2. Spectral information on KGd0.447Lu0.078Yb0.475(WO4)2: (a) Emission and absorption cross-sections, derived as the weighted average of spectroscopic data from the compounds KGd(WO4)2:Yb3+, KLu(WO4)2:Yb3+, and KYb(WO4)2 [17,18,9]. (b) Waveguide modal gain for different launched pump powers, calculated (lines) from the absorption and emission cross-sections presented in part (a) and measured (symbols); indication (arrow) of the spectral range with gmod > 150 dB/cm. 4. Conclusions Ultra-high gain has been demonstrated in KGdxLuyYb1-x-y(WO4)2 channel waveguides over a wide wavelength range around 1 µm wavelength. Further improvement seems feasible with higher dopant concentrations. These results suggest that Yb3+-doped waveguides may set a new wavelength standard at 1 µm for on-chip signal amplification. 5. References [1] D. Geskus, S. Aravazhi, S. M. García-Blanco, and M. Pollnau, "Giant optical gain in a rare-earth-ion-doped microstructure," Adv. Mater., accepted (2011). [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 codoped KY(WO4)2:Yb epitaxial layers: towards integrated optics based on KY(WO4)2," Opt. Lett. 32, 488 (2007). [4] 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 (April 2010). [5] F. Auzel, G. Baldacchini, L. Laversenne, and G. Boulon, "Radiation trapping and self-quenching analysis in Yb3+, Er3+, and Ho3+ doped Y2O3," Opt. Mater. 24, 103 (2003). -1200-1000-800-600-400-20002004006008001000960 970 980 990 1000 1010 1020 1030 1040Gain (dB/cm)Wavelength (nm)P = 60 mWP = 30 mWP = 18 mWP = 12 mWP = 9 mWP = 6 mWP = 3 mWP = 0 mWP = 60 mWP = 48 mWP = 36 mWP = 24 mWP = 19.2 mWP = 14.4 mWP = 12 mWP = 9.6 mWP = 7.2 mWP = 2.4 mWP = 0 mWExperimentaldataModeled data

150 dB/cm gain over 55 nm wavelength range near 1 µm in an Yb<sup>3+</sup> waveguide amplifier

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150 dB/cm gain over 55 nm wavelength range near 1 µm in an Yb<sup>3+</sup> waveguide amplifier

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