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Analysis of Novel Alignment Method for Fabricating Three-Dimensional Photonic Crystal

We analyze a novel alignment method of fabricating three-dimensional photonic crystals (3D PhCs) by using an absolute shift measurement system (ASMS). The ASMS functions by measuring the transmittance of a slab waveguide integrated into the substrate layer during the fabrication process together with distributed-feedback (DFB) and phase-shifter (PS) elements. The working principle of the ASMS has been confirmed by means of rigorous simulations based on the coupled-wave analysis. As compared with conventional techniques using a diffraction grating and its far-field pattern for alignment purposes, we found that the ASMS provides both a high resolution and a large dynamic shift range by breaking the DFB periodicity with the PS. Our method is considered suitable for fabricating 3D PhCs with arbitrary structures (e.g., photonic integrated circuits) more accurately.