1.55 GHz balanced homodyne detector with high gain flatness based on low noise amplifier

In this paper, a model for simulating the shot noise power and the electronic noise power of a balanced homodyne detector (BHD) using cascaded low noise amplifiers (LNAs) is presented. Moreover, the factors influencing the enhancement of BHD gain flatness are analyzed. Based on these theories, a BHD with a large clearance between shot noise and electronic noise, along with a flat broadband frequency response, is designed using LNAs and an optimized printed circuit board design. According to the experimental measurements, the bandwidth for 1 dB flatness reaches 1.4 GHz with the -3 dB bandwidth extending up to 1.55 GHz. With a 4 mW optical signal input, a signal-to-noise ratio of 12 dB is obtained at 1 GHz. The BHD exhibits excellent linearity for shot noise output up to an 8 mW optical signal input and a tested common mode rejection ratio of 63 dB. This developed BHD is well-suited for applications in high-speed continuous variable quantum key distribution and quantum random number generation.