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Er3+- and Nd3+-doped Al2O3 amplifiers on a silicon chip

Dielectric Er3+- and Nd3+-doped waveguide materials offer broad gain in the third, second, and first telecommunication windows. Rare-earth-ion-doped Al2O3 has broad emission spectra for gain over a wider wavelength range, a high refractive index contrast which allows tighter bend radii and more compact devices, and can be deposited on a number of common substrates, including thermally oxidized Si wafers. This opens the possibility for integration of Al2O3 directly with photonic materials which are optimized for passive waveguiding functions. In this contribution we present recent results demonstrating the high performance of Er3+- and Nd3+-doped Al2O3 integrated optical amplifiers. Rare-earth-ion-doped Al2O3 layers were deposited onto thermally oxidized Si wafers by reactive co-sputtering and channel waveguides were microstructured by chlorine-based reactive ion etching. We developed Al2O3:Er3+ amplifiers with up to 2.0 dB/cm net gain at 1533 nm and gain over an 80-nm bandwidth, among others enabling loss-less power splitters over the entire telecom C-band, and demonstrated 170-Gbit/s signal transmission in an integrated Al2O3:Er3+ amplifier. In Al2O3:Nd3+ amplifiers, net gain of 6.3 dB/cm at 1064 nm and 1.93 dB/cm at 1330 nm, as well as in the range 865-930 nm with a peak gain of 1.57 dB/cm at 880 nm was obtained. Monolithic integration of Al2O3:Er3+ amplifiers with passive silicon-on-insulator waveguides was demonstrated and a signal enhancement of >7 dB at 1533 nm wavelength was obtained. The straightforward wafer-scale fabrication process allows for parallel integration of multiple amplifier and laser sections with silicon or other photonic circuits on a chip. Furthermore, a solution for compensating losses in optical interconnects is provided. Large-core Al2O3:Nd3+ channel waveguide amplifiers were tested in combination with passive polymer waveguides. Coupling between the two waveguide types was optimized and tapering the Al2O3:Nd3+ waveguide improved the pump intensity in the active region. 0.21 dB net gain at 880 nm was demonstrated in a structure with an Al2O3:Nd3+ waveguide coupled between two polymer waveguides. In spectroscopic investigations, the influence of energy migration and energy-transfer upconversion (ETU) among neighboring Er3+ ions on luminescence decay in Al2O3:Er3+ was investigated. We observed a fast quenching process induced by, e.g., active ion pairs and clusters, undesired impurities, or host material defects such as voids. This process was verified by pump-absorption experiments, but was not revealed by any particular signature in the luminescence decay curves. This result highlights the fact that spectroscopic processes can be – and in many cases probably are – present in optical materials, although these processes are invisible in any kind of luminescence measurements, because they do not lead to the emission of a photon. On the other hand, we show that these processes strongly affect device performance as an amplifier.