Integrated SOI Stagger-Tuned Optical Filter With Flat-Top Response

We propose and demonstrate a stagger-tuned optical filter (STOF), offering a new synthesis method for flat-top optical filters based on the mapping electrical synthesis approach to the optical domain. In the design, light is guided through a series of offset stages in the STOF, where each stage consists of one add-drop microring resonator as the basic building block. With identical coupling coefficients specified across every building block, the transfer function of the STOF is solely dependent on the free spectral range and resonance frequency offset of each building block. Thus, various orders of optical filters with maximum flatness can be synthesized as a number of stagger-tuned stages. As a proof of concept, a two-stage STOF with 50-GHz bandwidth is fabricated on a CMOS compatible platform and measured experimentally. The measured results show a flat-top response of less than 0.2-dB peak degradation with a bandwidth discrepancy of 8% from the desired bandwidth. An error tolerance analysis shows that the device performance is exceptionally insensitive to the mismatch in the coupling coefficients arising from fabrication imperfections, showing less than 1-dB distortion in the peak despite up to 20% variation in the coupling power ratio.