Polymer-Ge Hybrid Waveguide for Flexible Photonic Integration

Amorphous germanium nanostrip is embedded in polymer to form a waveguide. Such waveguides only transmit TM-polarized light while imposing a strong absorption on the TE-polarized light. Furthermore, this absorption is shown to saturate upon femtosecond pulse excitation. This property is used to construct on-chip saturable absorber by depositing a Ge strip onto a polymer waveguide. Such hybrid waveguide is sandwiched between an InP reflective gain chip and a fiber Bragg grating to construct a mode-locked laser. Experiments have shown a pulse train with a 147-ps pulse width at 50.58-MHz repetition rate. The electronic signal-to-noise ratio is larger than 50 dB.