50 GHz on-chip germanium waveguide photodetector for the 2 µm wavelength band

The 2 µm wavelength band is emerging as a promising alternative spectral window for free-space and fiber-optic communications. As a proven photonic integration platform, silicon photonic components can operate in this band; however, on-chip photodetection remains a major challenge. Although germanium is widely used in silicon photonic photodetectors, its material bandgap is larger than photon energies in the 2 µm spectral range, resulting in limited absorption. Here, we demonstrate a germanium waveguide photodetector with dual sidewall doping, which significantly strengthens the electric field within the active region. The fabrication is fully compatible with standard silicon photonic processes. The device is measured with 1.9 A/W responsivity and a record large bandwidth of 50.3 GHz. Up to 112 Gbaud four-level pulse amplitude modulated signal transmission has been achieved using this photodetector, offering a silicon photonic solution for receiving single lane 200 Gb/s signals at 2 µm waveband.