  Tunable optical filter at the chip scale plays a crucial role in fulfilling
the need for the reconfigurability in channel routing, optical switching, and
wavelength division multiplexing systems. In this letter, we propose a tunable
double notch filter on thin-film lithium niobate using dual micro-ring
architecture. This unique integrated filter is essential for complex photonic
integrated circuits, along with multiple channels and various frequency
spacing. With only one loaded voltage, the device demonstrates a wide frequency
spacing tunability from 16.1 GHz to 89.9 GHz by reversely tunning the
resonances of the two micro-rings while the center wavelength between the two
resonances remains unaltered. Moreover, by utilizing the pronounced
electro-optic properties of lithium niobate, associated with the tight light
confinement nanophotonic waveguides, the device demonstrates a spacing
tunability of 0.82 GHz/V and a contrast of 10~16 dB. In addition, the device
has an ultracompact footprint of 0.0248 mm2.
