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We propose and demonstrate a multistage design for microphotonic add-drop filters that provides reduced drop-port loss and relaxed tolerances for achieving high in-band extinction. As a result, the first microring-resonator filters with a rectangular notch stopband in the through port (to our knowledge) are shown, with extinctions exceeding 50 dB. Reaching 30 dB beyond previous results, without postfabrication trimming, such extinction levels open the door to microphotonic notch circuits for spectroscopy, wavelength conversion, and quantum cryptography applications. Combined with a low-loss, high-index-contrast electromagnetic design in SiN and frequency-matched microring resonators, this approach led to the first demonstration of flattop microphotonic filters meeting the stringent criteria for high-spectral-efficiency integrated add-drop multiplexers. The 40 GHz wide filters show a 20 nm free spectral range, 2 dB drop loss, and suppression of adjacent channels by over 30 dB.

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a b s t r a c tIn this work, 750 nm-thick BaTiO 3 thin films with highly (0 0 1) preferred orientation were grown on single crystal MgO substrates by RF-sputtering. Hydrogen silsesquioxane (HSQ) resist material based ridge waveguides, which were fabricated on BaTiO 3 thin films, were formed by e-beam lithography. Au electrodes were deposited on top of the BaTiO 3 films beside the waveguide. Propagation losses of the BaTiO 3 ridge waveguide were 3-5 dB/cm in transverse electric polarization. The measured electrooptic coefficient value (r 51 ) was 110 pm/V, which is three times larger than the electrooptic coefficient (r 33 = 30.8 pm/V) of single crystal LiNbO 3 . SiO 2 strip waveguide formed by HSQ exhibited light propagation with loss lower than 5 dB/cm. This result demonstrates potential possibility of creating highly oriented and/or epitaxially grown BaTiO 3 waveguides and optical components on oxide substrates.