Amorphous Al2O3 channel waveguides with different Yb3+ concentrations are fabricated by reactive co-sputtering onto thermally oxidized silicon substrates and subsequent reactive ion etching. Pump transmission and bleaching of pump absorption, luminescence spectra, luminescence-decay curves, and the dependence of luminescence intensity on pump power are measured for the transition at 1 μm from the 2F5/2 first excited state of Yb3+ and for the green upconversion luminescence from the second excited state of the single quantum systems formed by Yb3+-Yb3+ pairs. The typical spectroscopic features of cooperative upconversion are observed. Analysis by rate equations reveals that only a microscopic two-ion-class model comprising single ions and ion pairs can quantitatively explain the measured spectroscopic data, i.e., the local chemical environment must be taken into account in the analysis. The fraction of quenched ions determined from the pump-absorption measurements and the fractions of ions forming Yb3+-Yb3+ pairs and producing cooperative upconversion (CU) are found to be equivalent and increase linearly with Yb3+ concentration. It suggests that the observed phenomena of non-saturable absorption and CU are linked to each other and evoked by the same class of ions. Internal net gain in these channel waveguides under the influence of cooperative upconversion is experimentally investigated and successfully modeled by use of the two-ion-class model. This analysis delivers important spectroscopic insight for optimizing the performance of distributed-feedback lasers in these waveguides. Particularly, it will provide a route towards optimizing the Yb3+ concentration for simultaneously ensuring sufficient pump absorption and minimizing non-saturable absorption by CU-quenched Yb3+-Yb3+ pairs that would otherwise significantly increase the laser linewidth, which is the most important performance parameter of such lasers.

Ridge waveguides in amorphous Al2O3:Yb3+ are produced by reactive co-sputtering and reactive-ion etching. Their spectroscopic properties, optical gain, and cooperative upconversion are studied and explained based on a model of distinct ion classes.

Ridge waveguides in amorphous Al2O3:Yb3+ are produced by reactive co-sputtering and reactive-ion etching. Their spectroscopic properties, optical gain, and cooperative upconversion are studied and explained based on a model of distinct ion classes. 

We investigate the spectral response of an unpumped distributed-feedback waveguide resonator, subject to thermal chirp. Experimental and simulated results indicate that such chirp causes an increase in outcoupling losses and, therefore, in the resonance linewidth.

The spectral response of a distributed-feedback resonator with a thermal chirp is investigated. An Al2O3 channel waveguide with a surface Bragg grating inscribed into its SiO2 top cladding is studied. A linear temperature gradient along the resonator leads to a corresponding variation of the grating period. We characterize its spectral response with respect to wavelength and linewidth changes of the resonance peak. Simulated results show good agreement with the experimental data, indicating that the resonance wavelength is determined by the total accumulated phase shift. The calculated grating reflectivities at the resonance wavelength largely explain the observed changes of the resonance linewidth. This agreement demonstrates that the linewidth increase is caused by the increase of resonator outcoupling losses.

We characterize experimentally and theoretically a distributed-feedback laser resonator subject to a thermal chirp. The total accumulated phase shift determines the resonance wavelength. The reflectivities (outcoupling losses) at the resonance wavelength govern the resonance linewidth.

We characterize the spectral response of a distributed-feedback resonator when subject to a thermal chirp. An Al2O3 rib waveguide with a corrugated surface Bragg grating inscribed into its SiO2 top cladding is experimentally investigated. We induce a near-to-linear temperature gradient along the resonator, leading to a similar variation of the grating period, and characterize its spectral response in terms of wavelength and linewidth of the resonance peak. Simulations are carried out, showing good agreement with the experimental results and indicating that the wavelength of the resonance peak is a result only of the total accumulated phase shift. For any chirp profile we are able to calculate the reflectivities at the resonance wavelength, and this information largely explains how the linewidth of the resonance changes. This result shows that the increase in linewidth is governed by the increase of the resonator outcoupling losses.