A 4 GHz adjustable CMOS 4-level modulator driving circuit for practical QKD applicaQtion

Quantum key distribution (QKD) theoretically achieves unconditional security for communication. However, the secret-key rate is limited, which can be enhanced by increasing the repetition rate of QKD. Additionally, to achieve a small form factor instead of a large one in a printed-circuit-board (PCB) solution, a novel integrated modulator driving circuit is proposed in this paper, which can realize adjustable amplitude and pulse width with a maximum repetition rate of 4 GHz. In the proposed architecture, four low-speed digital-to-analog converters (DACs) instead of one high-speed DAC are utilized to tune the pulse amplitude, so low cost and high-level output are achieved. The modulator driving circuit can output a random return-to-zero (RZ) pulse train with four different pulse amplitudes. Operated with 1.8 V supply, the four levels with a minimum 1.76 mV tunable accuracy cover a range of 0–225 mV, 225–450 mV, 450–900 mV, and 0.9–1.8 V, respectively. A 4-bit and a 6-bit controller are applied to the modulator driving circuit to achieve different pulse widths. Fabricated in 130-nm CMOS technology, the proposed modulator driving circuit occupies a core area of 7.6 mm² and consumes 612 mW. A repetition rate of 4 GHz is achieved and the measured pulse amplitude ranges from 77 mV to 1.67 V and the range of pulse width is 140 ps to 1049 ps.