Superconducting nanowire single-photon detectors for laser communication

Single Quantum developed a free-space coupled 6x6 squared array of Superconducting Nanowire Single-Photon Detectors (SNSPDs) for Deep Space Optical Communication (DSOC). This multipixel architecture, along with the sub-50ps timing jitter provided by NbTiN nanowires and readout electronics, is estimated to achieve data-rates beyond several hundreds of Megabits per second (Mbits) depending on the condition set by the communication protocol. The 6x6 array with 60x60 μm² active area and 10x10 μm² pixel size, demonstrated a system detection efficiency around 55% at 1550 nm and a dark count-rate around 150 counts per second per pixel owing to an effective black-body radiation cryogenic filtering. Single Quantum also developed a signal conditioning and summing board which first converts the long decaying SNSPD pulses into 300 picosecond FWHM wide pulses that are Pulse-Position Modulation (PPM) compatible. These pulses are then divided per quadrant and summed to distinguish the arrival of multiple events. The processed information is fed to a demodulator for message decoding and a tracking module to maintain contact with the satellite. The whole signal conditioning chain including the SNSPD array achieves a timing jitter ⪅50 ps FWHM. Such system will support tracking and communication with the NASA's transmitter from the Psyche Mission at distances ⪆ 235 Million kilometers and could also be exploited for lower range missions. To assess the potential of the multipixel architecture, a DSOC link based on PPM was mimicked in the lab. We implemented the protocol and open sourced it. In this work, we used increasing modulation orders and varied the mean photon number per pulse with a fixed bin size of 1.133 ns. Finally, the Bit-Error-Rate (BER) before and after iterative decoding was evaluated. The proof-of-concept experiment was performed with a fiber-coupled 4-pixel SNSPD supporting count-rates ⪆1 Giga counts per second (-3dB point) and System Detection Efficiency (SDE) of 65%.