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Photonic integrated circuits are a paramount platform for optoelectronics and nonlinear optics, enabling high confinement nonlinear interactions, where advanced waveguide dispersion engineering can be leveraged to tailor broadband nonlinear processes, supercontinuum generation, soliton formation and joint spectral amplitudes of photon pairs. Accurately measuring dispersion is therefore crucial for achieving efficient nonlinear and quantum devices. Here we develop a dual comb spectroscopy methodology suitable for dispersion measurements on photonic integrated circuits at telecom wavelengths and apply it to the characterization of lithium niobate on insulator ridge waveguides, mapping group velocity dispersion for fundamental TE and TM modes as a function of waveguide top-width. This work paves the way for the broader deployment of dual comb spectroscopy as a characterization tool for a broad range of photonic integrated circuit components on different nanophotonic platforms., Comment: The following article has been submitted to APL Photonics. After it is published, it will be found at Link

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 {\mu}m x 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.

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.