Ultra-Sensitive Optical Receivers

This thesis covers research on highly sensitive optical receivers in the field of direct detection (DD) and coherent detection (CD) systems. The project USPAR was launched to reduce the gap to the quantum limit (QL) in DD systems, which sets the fundamental sensitivity limit for optical reception. USPAR targeted to develop simpler and more energy-efficient highly sensitive receiver architectures than state-of-the-art single-photon avalanche photodiode (SPAD) receivers.The approach is based on capacitive transimpedance amplifier (CTIA) frontends, which use a tiny parasitic layout capacitance for photo charge integration. If the layout parasitics can be kept low, relatively large voltage signals can be achieved from a photo charge quantum. USPAR frontends need operating point (OP) control for continuous operation. Two different methods were investigated. The reset-approach works by resetting the integration capacitance periodically and the input current control (ICC) approach uses a feedback loop to source DC-current to the input node. In total, five receivers were designed in 350nm, 180nm and 55nmCMOS. The best measured distance to the quantum limit (dQL) by a P-i-N photodiode (PIN-diode) receiver was 15.2dB at 100Mbps, which matches the sensitivity of state-of-the-art SPAD receivers. In avalanche photodiode (APD)-mode the dQL could be closed to 10.7dB, which is the smallest dQL for any published DD receiver results so far. In contrast to DD, CD systems can be operated at the QL if the local oscillator (LO) shot noise dominates the system noise. In case of sub-optimal electronic noise, high LO power is required to establish shot noise limitation. The project EQUINOX targeted on enhancing a continuous variable quantum key distribution (CV-QKD) system with regards to the optical and noise domain. For the latter, a custom noise-optimized transimpedance amplifier (TIA) was designed in 55nm CMOS to improve clearance and linearity for balanced homodyne detection (BHD). For a bandwidth of 1.4GHz and a transimpedance of 9.2kΩ, the input referred rms noise current was 110nA and the sensitivity resulted in −27.45dBm at 2Gbps and BER=1·10−9. These results make it one of the best CV-QKD TIAs among state-of-the-art designs while consuming orders of magnitude less area and power.