Tunable Ultranarrowband Grating Filters in Thin-Film Lithium Niobate

Abstract

Several applications in modern photonics require compact on-chip optical filters with a tailored spectral response. However, achieving subnanometric bandwidths and high extinction ratios is particularly challenging, especially in low-footprint device formats. Phase-shifted Bragg gratings implemented by the sidewall modulation of photonic nanowire waveguides are a good solution for on-chip narrowband operation with reasonable requirements in fabrication and scalability. In this work we report on their implementation and optimization in thin film lithium niobate, a photonic platform that affords reconfigurability by exploiting electro-optic effects. The phase-shifted Bragg grating filters have a footprint smaller than 1 μm × 1 mm and operate at telecom wavelengths, featuring extinction ratios up to 25 dB. We demonstrate transmission bandwidths as narrow as 14.4 pm (Q = 1.1 × 105) and 8.8 pm (Q = 1.76 × 105) in critically coupled structures and multiwavelength Fabry-Perot configurations, respectively, in full agreement with theoretical predictions. Moreover, by taking advantage of the strong electro-optic effect in lithium niobate, in combination with the tight light confinement of nanophotonic wires and the ultranarrow spectral resonances of optimized grating structures, we demonstrate an electric tunability in peak wavelength and transmission of 25.1 pm/V and 2.1 dB/V, respectively, and a 10.5 dB contrast at CMOS voltages. The results pave the way for reconfigurable narrowband photonic filters with a small footprint and low consumption, to be exploited toward on-chip quantum and nonlinear optics, as well as optical sensing and microwave photonics.