The recently emerged photonic integration technology based on thin-film lithium niobate (LN) have been regarded as a very promising candidate for advanced photonic integrated circuits (PICs) due to its attractive nonlinear properties, wide-spread use in electro-optic applications, and etc. Generally, the thin-film LN optical waveguide used in PICs is sub-micrometer scale. Mode mismatch between fiber and sub-micrometer LN waveguide in chip is the main factor of increasing the fiber-to-chip coupling loss and the total insertion loss of LN PICs. Therefore, for practical applications, low-loss mode size converter for coupling between fiber and sub-micrometer LN waveguide is essential. In this paper, an efficient and novel fiber-to-chip mode size converter for thin-film LN PICs was designed and fabricated. The converter consists of a LN nano-taper and a cantilevered SiO2 waveguide. The nano-taper is embedded in the center of SiO2 waveguide. Laterally connected SiO2 cantilever beams are fabricated to provide structural support for the cantilevered SiO2 waveguide. Our work provides an efficient way to realize low-loss fiber-to-chip interface for thin-film LN PICs.
A flat band-pass photonic filter is experimentally demonstrated on a silicon-on-insulator (SOI) substrate. The filter is composed of a micro-ring resonator assisted with an asymmetric Mach-Zehnder interferometer (MZI). By thermal tuning the ring, the device achieves a rapid roll-off on the band edges, a wide 3-dB bandwidth of 0.95 nm and a low crosstalk of 14 dB. The filter can be used in the WDM network and integrated microwave photonic signal processing.
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