A Silicon Photonic 32-Input Coherent Combiner for Turbulence Mitigation in Free Space Optics Links

Free space optical links promise high data rates for future wireless communications systems, but performance is degraded by turbulence, limiting the potential use cases. A photonic integrated circuit (PIC) for turbulence mitigation is reported and characterized, which coherently combines 32 input optical signals into a single output fiber. The PIC was fabricated using a low-loss, and high integration density, thick silicon-on-insulator (SOI) process and packaged with 32 input fibers and 1 output fiber. The basic building block is a $2\times 2$ Mach-Zehnder interferometer (MZI) with an external (to the MZI branches) and an internal thermal phase shifter, and a bandwidth in excess of 80 kHz. The PIC monolithically integrates 31 MZIs and 31 germanium photodetectors and is suitable in principle for turbulence mitigation in LEO-ground and horizontal free space optics links. Improvements to the device for the coherent combination of 64 inputs and for the reduction of insertion losses are also discussed.