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The high-order microring filter has been proposed for a larger free spectral range (FSR) compared with the single microring filter; therefore, it has great potential to be used in wavelength division multiplexing (WDM) systems. In this article, we have designed and fabricated a second-order microring filter made up of two rings connected in series with two Ti thermal heaters deposited above them. The large FSR of 56.8 nm is obtained by decreasing the difference of the radii between the two series rings, achieving similar FSRs to that of higher-order filters but with a simpler and more compact design. The average electrical tuning efficiencies of the two heaters are 0.186 nm/mW and 0.207 nm/mW, and the center wavelength of the filter can be tuned over the entire FSR with an applied electrical power of less than 40 mW. [ABSTRACT FROM AUTHOR]

Broadband filters with sharp transition edges are important elements in diverse applications, including Raman and fluorescence spectral analysis, wideband wavelength-division multiplexing (WDM), and multi-octave interferometry. While the multi-layer thin-film interference broadband filter has been widely applied in various free-space optical systems, its integrated counterpart is still far from mature, which is also highly desired for constructing chip-scale miniature optical modules. In this article, we design, fabricate, and characterize an integrated broadband filter with sharp transition edges. An adiabatic coupler based on silicon nitride (SiN) and silicon oxynitride (SiON) composite waveguide is employed here. Long-pass, short-pass, band-pass, and band-stop filters can be realized in a single design of the composite waveguide coupler for a specific wavelength range, with only a difference in the SiN taper waveguide width. Experimental results with a roll-off value of larger than 0.7 dB nm−1 and a 15 dB extinction ratio (ER) are presented. [ABSTRACT FROM AUTHOR]