Camera-based metrology of subwavelength scatterers in photonic integrated circuits

Photonic integrated circuits (PICs) are important to reduce the size and increase the capacity of optical systems. Testing of coupling loss, waveguide and bend loss, coupler splitting ratio, and polarization state are all needed for maintaining the quality of foundry-produced PICs. The use of photodetectors, loopbacks and grating couplers accomplishes some of these functions, but at the cost of chip real estate. Image capture from scattered light via a microscope set is possible but the light being emitted is random in nature thus not viable to accurately monitor light within the circuit. To solve this problem, we introduce deterministic, subwavelength scattering elements into the circuits for SWIR camera-based testing of PICs. These elements are designed for negligible footprint, foundry compatibility, and to produce little loss in the circuit while carrying polarization information in the scattered light. Finite-difference time-domain simulations were performed analyzing the use of these scattering element within the system, with subsequent numerical propagation to extend the fields into the microscope observation plane. Using PICs fabricated in the AIM Photonics foundry, we observe light from engineered scatterers that is greater than 20x brighter than the background scatter while also providing polarization sensitivity. Integration of these components with methods for circuit metrology will allow for faster processing of circuit layouts when packaging and distributing PICs.