Versatile Integrated Brillouin-Kerr Frequency Combs for On-Chip Photonic Systems

Project Information

Veritas

Grant agreement ID: 101157112

DOI

10.3030/101157112

Project closed

EC signature date 16 February 2024

Start date 1 April 2024

End date 30 September 2025

Funded under

• European Research Council (ERC)

Total cost

No data

EU contribution

€ 150 000,00

Coordinated by

UNIVERSITEIT TWENTE
Netherlands

Periodic Reporting for period 1 - Veritas (Versatile Integrated Brillouin-Kerr Frequency Combs for On-Chip Photonic Systems)

Reporting period: 2024-04-01 to 2025-09-30

Summary of the context and overall objectives of the project

Lasers and optical frequency combs are ever present in our lives, serving as the backbone of the internet and underpinning critical technologies, such as telecommunications, microwave photonics, high-precision sensing, as well as quantum computing. Developing a versatile on-chip light source offers significant advantages for those applications in size, weight, power consumption, and cost. Nevertheless, current designs are hindered by high phase noise, poor stability, and lack of integration density. Leveraging stimulated Brillouin scattering (SBS), an optomechanical effect involving the interaction between optical and acoustic waves, these lasers can efficiently filter out high-frequency noise by coupling to acoustic phonons. In this way, Brillouin lasers can achieve sub-Hertz linewidths, but they operate only at single specific wavelength. In contrast, Kerr frequency combs, typically realized in dispersion-engineered, low-loss microresonators with third-order Kerr nonlinearity, can achieve stable, phase-locked, multiwavelength emission. Very recently, the combination of Brillouin lasing with Kerr frequency combs has been hinted as a promising pathway to realize narrow-linewidth broadband frequency combs Realizing Brillouin-Kerr frequency combs on a photonic integrated platform can potentially unlock compact and ultra-low noise multiwavelength coherent light sources with enormous technological impacts. But currently, this pathway is blocked by the lack a scalable and low-loss platform that can provide both strong Brillouin and Kerr nonlinearities.

The Veritas project aims to develop a versatile, low-noise, and stable on-chip light source by leveraging Brillouin-Kerr frequency combs in the tellurium oxide (TeO2)-covered silicon nitride (Si3N4) photonic integrated platform.The TeO2-covered Si3N4 platform presents itself as an ideal platform for integrated Brillouin-
Kerr frequency combs due to recent demonstration of both a high Brillouin gain coefficient as well as a high Kerr nonlinearity. The focus is on both realizing a stimulated Brillouin laser (SBL) as well as a Brillouin-Kerr frequency comb and demonstrate the use for wireless data transport, e.g. 5/6G radios.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The project involved the following activities. First, tellurite-covered silicon nitride waveguides have been characterized for optical loss and Brillouin gain coefficient. It is the balance between these two parameters that determines the usefulness of the platform for applications. These parameters have been measured to be 0.3 to 0.7 dB/cm and about 81 1/(m W), respectively. Further waveguide and cladding engineering shows in simulations that the gain could be improved to above 100 1/(m W). This signifies a significant boost in gain coefficient compared to our previous work on symmetric double stripe (SDS) silicon nitride waveguides. We have also investigate alternative material platforms to find the most promising candidate for the actual application. Improvement in process technology has lowered the propagation loss of the SDS silicon nitride waveguide, down to 0.07 dB/cm, while the Brillouin gain coefficient remained at around 0.2 1/(m W). Although the gain coefficient is much lower than found for the telluride-coverred silicon nitride waveguide, the much lower loss and higher optical damage threshold of the uncovered silicon nitride platform allows for creating a stimulated Brillouin laser (SBL)that is more efficient than the one created using tellurite-covered silicon nitride waveguides. In parallel we also have investigated an alternative material, thin-film lithium niobate and found a direction dependent gain coefficient due to the crystalline structure of the material. The maximum value and optical loss is comparable to that of tellurite-covered silicon nitride. The higher optical damage threshold of SDS silicon nitride allowed us to incease the pump power of a SBL to a level where nonlinear four-wave mixing resulted in the creation of a Brilouin-Kerr frequency comb. Stimulated Brillouin lasing was also observed in tellurite-covered silicon nitide and thin-film lithium niobate (first demonstrations) and we observed the onset of four-wave mixing in both platforms. However, the pumping levels were too low for a complete development of the comb. A second activity involved the constructing of a prototype for a standalone SBL or SBL-Kerr comb light source. Such a prototype is ready and consists of a tuneable pump laser that is amplified using an erbium-doped fiber amplifier. The output of the amplifier is connected to the packaged PIC via a circulator. The SBL light, together with the reflected pump light is separated from the incident pump light through the circulator. The current prototype is lacking an on-board power monitoring and wavelength measurement. The prototype is awaiting the photonic chip that is expected to arrive soon. Third and last activity involves the use of the Brillouin-Kerr frequency comb to demonstrate a coherent optical link and frequency up- and down conversion in 5/6G radios. The currently used maximum pump power for the tellurite covered silicon nitride waveguides allowed for successful SBL, however, these were insufficient for fully developing a Brillouin-Kerr frequency comb using the available designs. In the thin-film lithium niobate platform, we also could not observe fully developed Brillouin-Kerr frequency combs with the maximum on-chip pump powers and photonic circuits available and investigated. However, the low-loss SDS waveguide with the higher optical damage threshold could be pumped harder and we did observe fully developed Brillouin-Kerr frequency combs. However, due to delays in the manufacturing of these chips, we could not yet implement these two demonstrators, which are foreseen to be done soon after the end of this project.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The requirements for the Vertias project has stimulated and driven process development to improve the quality of integrated circuits for all three platforms investigated, SDS silicon nitride, tellurite-covered silicon nitride and thin-film lithium niobate. The project has resulted in the first demonstration ever of stimulated Brillouin lasing in all three platforms and also demonstrated the first Brillouin-Kerr frequency combs in SDS ring resonators. The demonstrated performances formed the foundation for successful grant applications, both national (NL) and European (EU), which ensures that the research directions pursued within Veritas are continued and extended to further develop the goals of Veritas and beyond. Finally, the outcome of Veritas contributed to the foundation of two new start-up companies, one Sabratha (https://sabratha.ai) is focusing on thin-film lithium niobate while the soon to be established company Temporal will focus on using SBL and Brillouin-Kerr frequency combs to miniaturize atomic clocks. These two companies will cover the market analysis and take up commercialization for their respective application domains. The remainder of the Veritas research team will engage in further development of the science, technology and applications via research grants and demonstrator projects.

Stimulated Brillouin scattering in tellurite-covered silicon nitride waveguides