Giant gain in a rare-earth-ion-doped waveguide

Nowadays two types of optical amplifiers are widely used: rare-earth (RE) doped fiber amplifiers (RDFAs) and semiconductor optical amplifiers (SOAs). RDFAs are not suitable for micro-scale on-chip integration, partly because their gain per unit length is hampered by the combination of small transition cross-sections in RE ions and the restriction to low doping concentrations. SOAs can deliver high gain over short distances, which makes them suitable for providing on-chip gain on silicon wafers. Despite the extraordinarily high material gain in the nm-sized recombination region of a III–V semiconductor, the usually μm-sized confinement of the signal beam results in a poor overlap factor with the active gain region, reducing accordingly the modal gain to a few hundred dB/cm. On the other hand, the typical gain per unit length reported so far for RE-doped integrated waveguides has hardly exceeded a few dB/cm [1, 2], almost two orders of magnitude less than in SOAs.