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.
Highlights
Achieving subnanometric bandwidths and high extinction ratios is challenging, especially in low-footprint device formats
We present a comprehensive experimental study, supported by theoretical analyses, on sidewall modulated phase-shifted BG (PSBG) devices in LN on insulator (LNOI) nanophotonic wires, demonstrating ultranarrowband transmission bandwidths for devices operated at critical coupling in the telecom range
The full extent of the multipeaked spectral response measured in the long cavity PSBG devices is shown in Figure 5, where we show the evolution of the PSBG transmission spectra for four different cavity lengths δL, comprised between 100 and 400 μm
Summary
We reported a systematic study on phase shifted Bragg gratings (PSBG) for electrically tunable integrated ultranarrow bandpass filters in thin film lithium niobate, encompassing theory and experiments. These results hold promise for further applications of integrated LNOI photonic circuits to electro-optic switching and modulation in telecommunication systems as well as efficient photon manipulation in integrated quantum photonics They pave the way to the implementation of more advanced functionalities for spectral shaping and tuning, such as superstructured gratings for dispersion engineering and χ(2) nonlinearity enhancement for novel frequency comb or quantum sources.[12,40] The small footprints and low-voltage operation achieved with these devices and the scalability of their fabrication process might be advantageously exploited toward developments of microwave photonics and programmable nanophotonics for, for example, multispectral sensing, neuromorphic, and quantum computing.[4,41−43]
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