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Investigation of backward cladding-mode coupling in Bragg gratings implemented on a Si3N4 waveguide platform

We present experimental and simulation results of sidewall Bragg gratings (BGs) implemented on a silicon nitride waveguide platform, with emphasis on the appreciable drop of transmission on the high-frequency side of the grating stopband due to the coupling into backward cladding/radiative modes. We differentiate this from the coupling into backward slab modes as observed in BGs implemented in silicon-on-insulator ridge waveguides. The dimension of the device top cladding affects the shape of the drop pattern. Different approaches for circumventing this drop pattern are discussed. A better understanding of this physical phenomenon will help guide the design of BGs on integrated photonic platforms.