Densely Integrated Phase Interrogators for Low-Complexity On-Chip Calibration of Optical Phased Arrays

Optical phase errors are inherent to practical waveguides and will degrade the beam quality of optical phased arrays. This paper demonstrates a scalable approach to on-chip phase calibration of optical phased arrays using a compact phase interrogator structure. This phase interrogator is designed for optical phased arrays with a $2.5 \,{\mu }\mathrm{m}$ pitch and demonstrated in a 64-channel optical phased array realized in a Si/SiN photonic stack. The optical phased array exhibits $38 \,\mathrm{^{\circ }}$ of unambiguous beam steering range and a side lobe level of at most $-9.8 \,\mathrm{d}\mathrm{B}$ after calibration. The calibration algorithm is model-based and does not rely on any iterative optimization algorithm. This enables calibration with linear scaling of both the computational complexity and the area of the calibration structures. By including wavelength dependent effects, the model allows for 2D beam steering without any need for re-calibration.