Advanced device design for photonic optical phased arrays for laser communication systems

Raytheon Intelligence & Space (RIS) in El Segundo, CA is currently developing photonic integrated circuit (PIC) devices for the improved efficiency of photonics-based optical phased arrays (OPAs). The goal is to functionalize PIC OPAs as coherent transmit and receive terminals for high data rate, high bandwidth, free-space laser communication. RIS has built a testbed to simultaneously probe the near and far field of the PIC under test with plug and play installation of our different designs. Here we investigate a 1x4 silicon-on-insulator (SOI) OPA with microring resonator phasing elements. A tunable infrared laser couples the transverse electric mode (TE0) into the rectangular PIC waveguide via a polarization-maintaining tapered fiber. The waveguide is then power-split by low loss splitters and coupled to the microring resonators (one for each array antenna) via a pulley scheme with a gap of 170 nm. In this paper, we compare experimental transmission spectra of identical 1x4 OPAs with waveguide widths of either 340 nm or 360 nm. Combining measured spectra with Lumerical FDE mode analysis, we show that the 360 nm system does not support coupling conditions between the waveguide and resonator to satisfy both bandwidth and power consumption requirements. On the other hand, the 340 nm system supports one potentially strongly-overcoupled resonance at 1503 nm with minimal amplitude distortion (similar to 2 dB), similar to 42 GHz-wide bandwidth and Q similar to 10(3) that could be used for coherent beam combination. This effort will help refine resonator designs and coupling geometries of future larger-scale OPAs.