Analysis of the impact of EDFA ASE noise on the outdoor free-space copropagation of WCPs and classical signals

The evaluation of interferences and noise in quantum key distribution (QKD) links that coexist with classical links is fundamental in the design of QKD deployments. In this work, we provide an experimental assessment of the effects of amplified spontaneous emission (ASE) noise resulting from the copropagation of weak coherent pulses (WCPs) and erbium-doped fiber amplifier (EDFA)-amplified classical signals through an outdoor free-space optical link. We achieve an average estimated lower bound for the quantum bit error rate (QBER) due to ASE noise below 4% in all the scenarios that we evaluate, thus making the QKD link feasible. Our study explores the need to take into consideration both the ASE noise and the interference from the amplified signal when designing a discrete variable (DV)-QKD scheme that coexists with the propagation of classical channels. By adding filters on the receiver side, the direct interference caused by the amplified classical signal can be reduced. At the transmitter side, ASE suppression filters must be used to address false detections caused by ASE noise, and the EDFA must operate at the best pump current possible to achieve the desired gain while minimizing noise. For a 10 dB increase in the EDFA’s gain, we have measured an increase in the estimated lower bound for the QBER of 1.71%. This establishes a tradeoff between the amount of launched power required and the amount of ASE noise that degrades the quality of the quantum signal.