Thermally tunable high-order microring filters with large free-spectral-range

Abstract

We propose a high-order microring filter structure based on the Silicon-On-Insulator (SOI) platform for flattening filtering. This filter structure can achieve a box-like spectral response with large free-spectral-range (FSR) and high out-of-band extinction ratio (ER), which can be widely used in various fields, especially in the wavelength division multiplexing. The filter consists of two or more rings which are connected in series, with Ti electrodes added on them for thermal tuning. According to the proposed requirements of spectral bandwidth and FSR, the physical dimensions of the high-order microring filter structure can be calculated based on theoretical formulas, and we optimize them for better performance. The relationship of the coupling coefficients of each coupling zone is calculated based on the Butterworth spectral responses. Result shows that the higher-order filter has steeper rise-fall edges and a flatter top, and can be designed to achieve a larger FSR. The FSR of the fourth-order microring filter can be designed to 78.4 nm, with ER of 12.6 dB, and the insertion loss of 1.1 dB. The fluctuations at the top of the box-like spectra caused by the fabrication deviations can be compensated by applying voltages to Ti electrodes, which can change the effective refractive index of the waveguide.