The vertically coupled microring resonator filter is an attractive add-drop wavelength filter due to its functionality and compactness and the possibility of dense integration resulting from the cross-grid configuration. However, in the concatenated 1 × 8 filter array in which the ring radii are changed with the increments of 50 nm, the wavelength channel spacing was limited to 5.7 nm due to the resolution limit of photomask (50 nm). Therefore, we developed a new technique to control precisely the channel spacing by an ultraviolet (UV)-trimming technique using polysilane as the overcladding layer. As a result, we successfully decreased the channel spacing to 0.5 nm, and also controlled the channel spacing precisely to 1.0 nm using the same trimming technique.
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