Generation of Stable and High Extinction Ratio Light Pulses for Continuous Variable Quantum Key Distribution

We propose and demonstrate an approach to generate stable and high extinction ratio light pulses with an extinction ratio 80 dB. To this end, a high stable bias locking technique is proposed, and a small size, easily integrated, cost-effective pulse generator based on delay line chips is designed and constructed. The pulse generator can generate voltage >12.5 V at a load impedance of 50 Ω, which is enough to drive the lithium niobate-based Mach-Zehnder (LNMZ) intensity modulator with a half wave voltage ~6-8 V. The pulse width and delay time can be programmed directly via digital I/O ports. The bias locking technique utilizes a successive scanning and fitting method which overcomes the sensitivity limit of the photodetector and can achieve the bias voltage accurately and quickly. By this method, the bias points of two or more cascaded LNMZ intensity modulators can be locked stably. The presented system can be easily integrated into the continuous variable quantum key distribution system.