Transparent In-Line Optical Power Monitoring Using InP/Si Hybrid Waveguide Phototransistor

In-line optical power monitors are essential building blocks in a Si programmable photonic integrated circuit (PIC) for calibrating optical phase shifters and correcting phase errors inevitably induced in a PIC. Nevertheless, current state-of-the-art in-line optical power monitors struggle to simultaneously achieve low optical loss and high responsivity. Here, we demonstrate an in-line optical power monitor using a waveguide phototransistor with an ultrathin InP membrane. The trap-mediated photodetection with the photogating effect of a phototransistor enables the detection of a weak light down to −44 dBm with an optical insertion loss of < 0.25 dB at a wavelength of 1550 nm. We measured the switching speed to be approximately 100 μs, which is sufficiently fast for optical power monitoring applications. Furthermore, a Mach–Zehnder interferometer (MZI) with an energy-efficient InP/Si hybrid metal-oxide-semiconductor (MOS) optical phase shifter was monolithically integrated with the proposed InP/Si hybrid phototransistor using the same fabrication process. The photocurrent of the phototransistor increased with the input light power modulated by the InP/Si hybrid MOS phase shifter, demonstrating in-line optical power monitoring using an InP/Si hybrid phototransistor. The proposed InP/Si hybrid phototransistor is promising for transparent optical power monitoring in a Si programmable PIC by III–V/Si hybrid integration.