Monolithically Integrated Balanced Uni-Traveling-Carrier Photodiode with Tunable MMI Coupler for Microwave Photonic Circuits

A monolithically integrated balanced uni-traveling-carrier photodiode (UTC-PD) with a tunable 2times2 multimode interference (MMI) coupler has been fabricated and tested. Two waveguide UTC-PDs are electrically isolated using high-energy Helium implantation, and then connected in series using a monolithic metal interconnect. On chip metal-insulator-semiconductor (MIS) capacitors provide some DC decoupling. The tunable MMI coupler allows for tuning of the power balance in the PDs. Output saturation currents greater than 40 mA at 1 GHz are demonstrated for a single 10 mum times 150 mum UTC-PD. The third order intermodulation distortion (IMD3) is also measured and exhibits an output intercept point (OIP3) of 43 dBm at 20 mA and 34 dBm at 40 mA for this same UTC-PD. In the balanced configuration, the OIP3 values are therefore 49 dBm and 40 dBm. The balanced UTC-PD is also highly symmetric; the common mode rejection ratio (CMRR) was measured to be around 40 dB.