We report on the experimental realization of optical frequency comb (OFC) generation in a doubly-resonant cavity second harmonic generation (SHG) system. OFCs continue to attract significant interest, offering a wealth of potential applications beyond frequency metrology. Continuously-driven Kerr microresonators, whose nonlinear response is dominated by the third-order nonlinearity, have proven to be viable alternatives to comb sources based on femtosecond mode-locked lasers. Recently, OFCs have also been directly generated through second-order nonlinear interactions in cw-pumped resonators namely, a singly-resonant cavity SHG system and a nearly-degenerate optical parametric oscillator. Theoretical studies have also predicted OFCs in doubly-resonant cavity SHG systems with a much lower threshold with respect to the singly-resonant configurations. Here we report on the first observations of OFCs in such a doubly-resonant system. The experiment is based on a periodically poled lithium niobate crystal, placed in a traveling-wave optical cavity, pumped by a cw Nd:YAG laser emitting 0.5 W at 1064 nm. The cavity is resonant for frequencies around both the fundamental pump and its second harmonic at 532 nm, and an intracavity adjustable silica window is used to separately set the detunings of the pump and its second harmonic. Stable cavity locking to the pump laser is achieved via the Pound-Drever-Hall offset locking technique, thanks to a counterpropagating orthogonally polarized auxiliary beam. We measured a power threshold for comb formation as low as 5 mW, reduced by more than one order of magnitude with respect to singly-resonant configurations. The locking system permitted to explore frequency detunings up to several cavity linewidths, and to correspondingly observe a large variety of comb regimes, with different teeth spacing and spectral span, as well as the contribution of photothermal effect to the whole dynamics. In this regard, we developed an extended theoretical model that includes thermo-optical nonlinearities.
Second order nonlinearities are inhibited in centrosymmetric crystals, like silicon. However, in the last ten years many attempts have been carried out to induce second order nonlinear susceptibility applying a stressing layer of silicon nitride on the top of a silicon waveguide. Succesful experiments showed both Second Harmonic Generation (SHG) or electro-optic modulation in strained silicon waveguide. In order to develop new devices, a full comprehension of the origins of such a nonlinearity is needed. In fact, a lot of estimations of the second order nonlinear coefficient have been given, all different from each other and, in some cases, even contradictory.
In this work, we perform SHG in multimodal phase-matched silicon waveguides. We propose a way to individuate the origin of the nonlinearity, discriminating among the break of the centrosymmetry, the presence of charged states at the interfaces between silicon and silicon nitride and the overlap of the optical mode with the silicon nitride. We estimated a value of the second order nonlinear coefficient of 0.5 pm/V, demonstrating that it results from the coupling of the silicon third order nonlinear coefficient with the electric field induced by the presence of the trapped charges at the core/cladding interface.
We also show preliminary results on SHG in strained silicon microring resonators. Our results open the door to interesting applications, going from broad frequency conversion, to generation of quantum states of light, up to the generation of octave spanning frequency comb based on second order nonlinearities.
We demonstrate optical frequency comb generation in a continuously pumped optical parametric oscillator, in the parametric region around half of the pump frequency. We also model the dynamics of such quadratic combs using a single time-domain mean-field equation, and obtain simulation results that are in good agreement with experimentally observed spectra. Moreover, we numerically investigate the coherence properties of simulated combs, showing the existence of correlated and phase-locked combs. Our work could pave the way for a new class of frequency comb sources, which may enable straightforward access to new spectral regions and stimulate novel applications of frequency combs.
Optical frequency combs currently represent enabling components in a wide number of fast-growing research fields, from frequency metrology to precision spectroscopy, from synchronization of telecommunication systems to environmental and biomedical spectrometry. As recently demonstrated, quadratic nonlinear media are a promising platform for optical frequency combs generation, through the onset of an internally pumped optical parametric oscillator in cavity enhanced second-harmonic generation systems. We present here a proposal for quadratic frequency comb generation in AlGaAs waveguide resonators. Based on the crystal symmetry properties of the AlGaAs material, quasi-phase matching can be realized in curved geometries (directional quasi-phase matching), thus ensuring efficient optical frequency conversion. We propose a novel design of AlGaAs waveguide resonators with strongly reduced total losses, compatible with long-path, high-quality resonators. By means of a numerical study, we predict efficient frequency comb generation with threshold powers in the microwatt range, paving the way for the full integration of frequency comb synthesizers in photonic circuits.