Within optical microchips, light finds its way through channels, waveguides, made of silicon. Light from a glass fiber, for example, is led through a structure of optical channels with splitters and couplers. Silicon is the workhorse for this, but it is still passive conduction of light, with some losses as well. To be able to amplify the signal, or even to include a light source on the chip, extra steps are necessary. Other types of semiconductors, like Gallium Arsenide, are an option. But materials doped with the rare earth material erbium have good amplification properties as well.