Broadband quasi-phase-matching in dispersion-engineered all-optically poled silicon nitride waveguides

Quasi-phase-matching (QPM) has become one of the most common approaches for increasing the efficiency of nonlinear three-wave mixing processes in integrated photonic circuits. Here, we provide a study of dispersion engineering of QPM second-harmonic (SH) generation in stoichiometric silicon nitride (Si3N4) waveguides. We apply waveguide design and lithographic control in combination with the all-optical poling technique to study the QPM properties and shape the waveguide dispersion for broadband spectral conversion efficiency inside Si3N4 waveguides. By meeting the requirements for maximal bandwidth of the conversion efficiency spectrum, we demonstrate that group-velocity matching of the pump and SH is simultaneously satisfied, resulting in efficient SH generation from ultrashort optical pulses. The latter is employed for retrieving a carrier-envelope-offset frequency of a frequency comb by using an f - 2f interferometric technique, where supercontinuum and SH of a femtosecond pulse are generated in Si3N4 waveguides. Finally, we show that the waveguide dispersion determines the QPM wavelength variation magnitude and sign due to the thermo-optic effect. (C) 2020 Chinese Laser Press

Photonic Damascene process for integrated high Q microresonator based nonlinear photonic devices

Martin Hubert Peter Pfeiffer

Chip-based optical waveguides formed by a silicon nitride (Si3N4) core and a silicon dioxide (SiO2) cladding are an important platform for low loss waveguides and nonlinear optics. High Q microresonators based on this material system have become a standard platformfor Kerr frequency comb generation. After their discovery in 2007, Kerr frequency combs have attracted significant interest due to their novel properties, featuring repetition rates from the microwave to the terahertz range, and the potential to realize mass-producible, compact frequency comb systems. The excitation of dissipative Kerr solitons (DKS) has quickly become the preferred operational state, allowing generation of fully coherent frequency combs with designable properties. However, the design and fabrication of microresonators that allow for DKS excitation has remained challenging. In this thesis, a novel fabrication process, the photonic Damascene process, is introduced that solves problems of previous fabrication processes. Inverting the process order and depositing the Si3N4 thin film onto a pre-patterned substrate allows for wafer-scale, crack-free fabrication of low loss photonic waveguides. Through process optimization, high dimensional accuracy similar to existing process schemes and good process stability is achieved. This enables a significant advance in the sample design and understanding of the important design elements required for microresonators with excellent linear and nonlinear performance. Especially, the design of the coupling between the microresonator and the bus waveguide is found to be critical. Together, the advances in design and fabrication allow for the demonstration of octave spanning DKS frequency combs in microresonators with 1-THz free spectral range. Finally, a systematic study of the loss processes in the photonic waveguides is presented and allows for the first time a quantification of the scattering and absorption loss rates. The ultra-smooth sidewalls of waveguides fabricated with a novel reflow process lead to dominant absorption losses. Material analysis techniques reveal for the first time the critical role of transition metal impurities, found to be present in significant amounts, for integrated photonic waveguides. The findings of this thesis provide the technological basis for design and fabrication of chipscale microresonator based Kerr frequency comb generators. In the future it will enable custom-designed and application-specific nonlinear waveguide devices that drive the further proliferation of Kerr comb technology.

EPFL2018

Difference frequency generation in optically poled silicon nitride waveguides

Camille Sophie Brès, Edgars Nitiss, Ezgi Sahin, Ozan Yakar, Boris Zabelich

All-optical poling leads to an effective second-order nonlinearity (chi((2))) in centrosymmetric materials without the need for sophisticated fabrication techniques or material processing, through the periodic self-organization of the charges. The absence of the inherent chi((2)) in prevailing silicon-based platforms can be surmounted through all-optical poling. Using the induced effective chi((2)) in silicon nitride (Si3N4) waveguides, nonlinear frequency up-conversion processes, such as second-harmonic generation, were previously demonstrated on Si3N4. Here, we report near- and non-degenerate difference-frequency generation in all-optically poled Si3N4 waveguides. We show the agreement between the theory and the measurements and optimize achievable QPM bandwidth range, reaching conversion efficiency of 1 %/W.

SPIE-INT SOC OPTICAL ENGINEERING2021

Platicon microcomb generation using laser self-injection locking

Miles Henry Anderson, Jijun He, Tobias Kippenberg, Junqiu Liu, Yang Liu, Rui Ning Wang, Wenle Weng

The past decade has witnessed major advances in the development and system-level applications of photonic integrated microcombs, that are coherent, broadband optical frequency combs with repetition rates in the millimeter-wave to terahertz domain. Most of these advances are based on harnessing of dissipative Kerr solitons (DKS) in microresonators with anomalous group velocity dispersion (GVD). However, microcombs can also be generated with normal GVD using localized structures that are referred to as dark pulses, switching waves or platicons. Compared with DKS microcombs that require specific designs and fabrication techniques for dispersion engineering, platicon microcombs can be readily built using CMOS-compatible platforms such as thin-film (i.e., thickness below 300 nm) silicon nitride with normal GVD. Here, we use laser self-injection locking to demonstrate a fully integrated platicon microcomb operating at a microwave K-band repetition rate. A distributed feedback (DFB) laser edge-coupled to a Si3N4 chip is self-injection-locked to a high-Q ( > 10(7)) microresonator with high confinement waveguides, and directly excites platicons without sophisticated active control. We demonstrate multi-platicon states and switching, perform optical feedback phase study and characterize the phase noise of the K-band platicon repetition rate and the pump laser. Laser self-injection-locked platicons could facilitate the wide adoption of microcombs as a building block in photonic integrated circuits via commercial foundry service. 'Here the authors provide the demonstration of platicon comb generation in an integrated photonic chip using laser self-injection locking, They take advantage of platicons generation in normal GVD resonators, which significantly relaxes the material and geometry design restrictions