Volumetric Imaging and Characterization of Focusing Waveguide Grating Couplers

Volumetric imaging of focusing waveguide grating coupler emission with high spatial resolution in the visible ( ${{\lambda \,= \,637.3\,\text{nm}}}$ ) is demonstrated using a scanning near-field optical microscope with long z -axis travel range. Stacks of 2-D images recorded at fixed distance from the device are compiled to yield 3-D visualization of the light emission pattern and enable extraction of parameters, such as spot size, angle of emission, and focal height. Measurements of such parameters are not prevalent in the literature yet are necessary for efficacious design and integration. It is observed that finite-difference time-domain simulations based on fabrication layout files do not perfectly predict in-hand device behavior, underscoring the merit of experimental validation, particularly for critical application.