Spectroscopy and ultra-narrow-linewidth lasers in rare-earth-doped amorphous $Al_2O_3$ waveguides on a silicon chip

Despite a strong fluorescence-quenching process that affects a significant fraction of erbium or ytterbium ions in amorphous Al2O3, channel waveguide lasers with 1.7 kHz linewidth at 1.5 µm or 55 mW output power at 1.0 µm, stable microwave beat-frequency generation, and intra-laser-cavity optical sensing is demonstrated on a silicon chip.