Grating Coupler utilizing third-order Diffraction

A grating coupler [1,2] is useful for exciting a guided wave or taking out a guided wave into the air, and is one of the key components in constructing integrated optic devices [3]. A grating coupler utilizing higher-order diffraction has much longer grating period than that of a 1st-order grating coupler, and is attractive from a view point of fabrication and design flexibility especially for a case using a short wavelength or a high guided-mode index. However, a coupling efficiency of the usual higher-order diffraction is too low to be utilized in practical applications. We reported [4] a proposal and design consideration of a high-efficiency 3rd-order grating coupler for application to a GaAs/AlGaAs waveguide. In this report, an efficiency enhancement technique for 3rd-order grating coupler is demonstrated experimentally for the first time. Wavelength of 0.82μm and a glass waveguide are used for the demonstration.