Shorted Saturable Absorbers to Enhance Extinction Ratio in QKD Transmitter Chips

The extinction ratio of modulators in quantum key distribution (QKD) transmitters has a significant influence on the quantum bit error rate. In simulations and measurements, we demonstrate that adding an electrically shorted saturable absorber (SA) behind the modulator of a photonic-integrated QKD transmitter chip can increase the extinction ratio of the modulator by up to 10 dB. This paves the way for significantly lower quantum bit error rates when using these transmitters. Because it is shorted, the SA becomes a simple building block, with no need for additional signal inputs. The extra losses in the signal induced by the absorber are irrelevant to the QKD transmitter performance (for both continuous variable and discrete variable QKD transmitters) because the pulses will be attenuated to low photon numbers after the modulation.