Novel Design of Polarization Independent Multi-layer Diffraction Grating with High Angular Dispersion

摘要

Diffraction grating is a key component for the multiplexing / demultiplexing in the WDM optical communication systems. To reduce device size and prevent signal deterioration, the realization of a grating with high angular dispersion, high diffraction efficiency, and low polarization dependence is strongly desired. The authors conducted a study on the polarization independent design of a transmission grating with sub-wavelength period, using optical simulation (RCWA method) and computational optimization (SA method). First of all, using the conventional surface-relief grating, the authors show that reducing the reflection at upper/bottom surfaces of a grating is important, in order to realize high optical performance. And next, to control the reflection, the authors propose a novel grating with multi-layer structure, where a silica glass substrate is coated with Ta_2O_5 and SiO_2 layers, both of which are etched at the same time. A good design, which provides high optical performance in the broad wavelength band, is successfully obtained. Furthermore, in order to verify the effect of multi-layer structure, the authors fabricated this 2-layer grating. In spite of the high angular dispersion, good optical performance with high diffraction efficiency ( > 95%) and low polarization dependence ( < 2%) around C-band is confirmed in the experiment.