112 Gb s⁻¹ Germanium Photodetector at 2 µm Enabled by a 3D Integrated Waveguide Loop

Recently, the 2 µm wavelength band has attracted increasing interest because of its potential prospects for free-space and fiber optical communications. Silicon photonic integrated circuits can operate in this band, but a major challenge exists for integrated photodetectors because the photon energies in this wavelength band are less than the energy bandgap of germanium which is the material used for in silicon photonic photodetectors for the conventional optical communications bands. While it is possible to have weak optical absorption at the 2-µm wavelength band using strained germanium and defect states, the need for a long absorption length can degrade the resistance-capacitance (RC) limited bandwidth. To address this problem, we proposed a compact 3D waveguide loop to improve both the responsivity and the bandwidth. A closed loop for light-matter interaction enhancement is demonstrated, enabling the responsivity of the detector to reach 0.11 A W⁻¹ at wavelengths of 1955 nm. The measured 3 dB bandwidth of the photodetector is 36 GHz and is sufficient for high-speed detection of 112 Gb s⁻¹ PAM-4 signals. The device is fabricated in a commercial foundry and extends the high-speed communications capabilities of silicon photonics to the 2 µm wavelength band.