Ultralow-LO-Power Silicon Photonic Coherent Receiver with Balanced Ge/Si Avalanche Photodiodes

A silicon photonic coherent receiver with balanced Ge/Si avalanche photodiodes (APDs) is proposed and demonstrated, facilitating the realization of coherent detection with a low-power local oscillator (LO). The Ge/Si APDs used here are realized with a high responsivity for the C-band, enabling a primary responsivity of 0.84 A/W and a maximum gain of ∼289 for a −30 dBm input optical power. The present APD-based silicon photonic coherent receiver works well for 25 Gbaud Quadrature Phase-Shift Keying (QPSK) signals even with a low LO optical power of −11 dBm, and high-sensitivity data-receiving of the QPSK signals with a low optical power of −22 dBm is demonstrated successfully even without any equalization. The minimum signal power that satisfies the BER less than the hard-decision forward error correction (HD-FEC) limit is estimated to be −25 dBm by fitting the BER curve based on the equalization-free experimental results.