Multichannel Lithium-Niobate-On-Insulator Photonic Filter for Dense Wavelength-Division Multiplexing

A multichannel lithium-niobate-on-insulator (LNOI) photonic filter for dense wavelength-division multiplexing (DWDM) is proposed and realized for the first time by introducing high-order 1 × 2 Fabry–Perot (FP) cavity filters in cascade. These 1 × 2 FP cavity filters are developed with a high-order FP cavity consisting of three multimode waveguide gratings (MWGs) and a mode (de) multiplexer for achieving flat-top spectral responses. Such an MWG-based LNOI photonic filter is circulator-free and convenient to be cascaded for working with multiple channels. In addition, there are no bends introduced in the cavities, which enables an extensive free spectral range (FSR) owing to the ultracompactness of the cavity. As an example, a 6-channel LNOI photonic filter is designed and fabricated for DWDM with a channel spacing of 3.2 nm. It shows that the measured 3 dB bandwidth is ∼1.4 nm, and the extinction ratio is up to 38 dB, while the excess loss is ∼0.5 dB, and the interchannel crosstalk is <−25 dB. Besides, an 8 nm × 1.6 nm LNOI DWDM filter is also developed, exhibiting an excess loss of ∼0.5 dB and interchannel crosstalk of <−18 dB. The present devices show great potential as a promising option for realizing future high-capacity optical interconnects.