Photonic integrated circuits (PICs) are today acknowledged as an effective solution to fulfill the demanding requirements of many practical applications in both classical and quantum optics. Phase shifters integrated in the photonic circuit offer the possibility to dynamically reconfigure its properties in order to fine tune its operation or to produce adaptive circuits, thus greatly extending the quality and the applicability of these devices. In this paper, we provide a thorough discussion of the main problems that one can encounter when using thermal shifters to reconfigure photonic circuits. We then show how all these issues can be solved by a careful design of the thermal shifters and by choosing the most appropriate way to drive them. Such performance improvement is demonstrated by manufacturing thermal phase shifters in femtosecond laser written PICs (FLW-PICs), and by characterizing their operation in detail. The unprecedented results in terms of power dissipation, miniaturization and stability, enable the scalable implementation of reconfigurable FLW-PICs that can be easily calibrated and exploited in the applications.Index Terms-Femtosecond laser writing, integrated photonics, reconfigurable optical circuits, thermal phase shifters.This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/

Several applications in modern photonics require compact on-chip optical filters with a tailored spectral response. However, achieving sub-nanometric bandwidths and high extinction ratios is particularly challenging, especially in low-footprint device formats. Phase shifted Bragg gratings implemented by 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 electrooptic effects. The phase-shifted Bragg grating filters have a footprint smaller than 1 μm× 1mm 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 x 10 5 ) and 8.8 pm (Q = 1.76 x 10 5 ) in critically coupled structures and multi-wavelength Fabry-Perot configurations, respectively, in full agreement with theoretical predictions. Moreover, by taking advantage of the strong electrooptic 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 a tunability of 25.1pm/V and a record modulation of the filter transmission amounting to 1.72 dB/V at CMOS voltages. The results pave the way for reconfigurable narrowband photonic filters in lithium niobate with small footprint and low consumption, to be exploited towards on-chip quantum and nonlinear optics, as well as optical sensing and microwave photonics.

We demonstrate integrated resonators with phase-shifted Bragg gratings operating at telecom wavelengths in 500 nm-thick LNOI waveguides, exhibiting transmission bandwidths of 59 pm and 75 pm in single and multi-resonance configurations, respectively.

Lithium niobate has been for decades the material of election for integrated nonlinear and electro-optics. Its recent availability in thin films affording subwavelength confinement of light and nanostructuring capabilities has led to ground-breaking results in numerous applications, ranging from ultrafast signal processing to efficient nonlinear optics, where electro-optic (EO) and thermo-optic (TO) functionalities can be further leveraged for enhanced tunability and reconfigurability.This work provides a consistent comparison between these two approaches in the most widely used configuration in LiNbO3 nanophotonics at telecom wavelengths. Using state of the art Bragg grating technology for high precision index measurements, we evaluate the guided-wave EO and TO tunability to be 3×10 -5 V -1 and 3.6×10 -3 W -1 , respectively, and study further operation and design tradeoffs, cross-talk effects and long-term stability. The results provide useful insights to identify the most appropriate strategies for implementing reconfigurable integrated photonic circuits effectively leveraging the unique features of LiNbO3.

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