The precise generation of spectrally pure microwave signals plays a pivotal role in both fundamental research and practical applications such as metrology and communications. Optical frequency combs have emerged as a powerful tool for generating high-quality microwave oscillations through the technique of optical frequency division (OFD). However, current OFD implementations involve multiple lasers, necessitating complex optical stabilization and electronic feedback components. These requirements lead to devices with footprints that are impractical for integration into compact and robust photonic platforms.
Microwave Signal Generation
In a groundbreaking development, researchers have successfully demonstrated all-optical OFD on a photonic chip. This achievement involves synchronizing two distinct dynamical states of Kerr microresonators using a single continuous-wave laser. The key innovation lies in exploiting the inherent stability of the terahertz beat frequency between the signal and idler fields of an optical parametric oscillator. This stability is transferred to a microwave frequency of a Kerr soliton comb. Crucially, synchronization is accomplished through a coupling waveguide, eliminating the need for electronic locking.
The experimental setup achieved OFD factors of N = 34 and 468 for 227 GHz and 16 GHz soliton combs, respectively. Notably, OFD enabled an impressive 46 dB reduction in phase noise for the 16 GHz soliton comb. This represents the lowest microwave noise observed in an integrated photonics platform, showcasing the effectiveness of the proposed approach.
The significance of this work extends beyond the immediate achievement of all-optical OFD on a photonic chip. The demonstrated technique provides a straightforward and efficient pathway for implementing OFD, paving the way for chip-scale devices capable of generating microwave frequencies comparable to those produced in advanced metrological laboratories.
Traditional OFD methods rely on the use of multiple lasers, leading to challenges in terms of space requirements, energy consumption, and the overall complexity of the system. In contrast, the newly demonstrated approach simplifies the process by leveraging the distinct dynamical states of Kerr microresonators. By using a single continuous-wave laser, the researchers achieved synchronization of these states without the need for intricate electronic feedback mechanisms.
The stability of the terahertz beat frequency in the optical parametric oscillator plays a crucial role in ensuring the reliability of the OFD process. This stability is harnessed to create a microwave frequency in the Kerr soliton comb, offering a novel and efficient way to transfer stability across different frequency regimes.
The achieved OFD factors demonstrate the efficacy of this approach in generating high-quality microwave signals. The 46 dB reduction in phase noise for the 16 GHz soliton comb underscores the potential for significant improvements in signal quality and precision, especially in integrated photonics platforms.
Overall, this work represents a major step forward in the field of microwave signal generation. The ability to perform all-optical OFD on a photonic chip not only addresses existing challenges but also opens up new possibilities for the development of compact, integrated devices with applications in metrology, communications, and beyond. The simplicity and effectiveness of the proposed approach position it as a promising candidate for advancing the capabilities of chip-scale devices in generating microwave frequencies with unprecedented purity and precision.
