Soliton Microcombs for Micro/MM-Wave Generation on Integrated Photonics Platforms

Microwave and millimeter waves are extensively used in radar, communications, and radio astronomy. Nowadays, these applications demand higher resolution, faster data transmission, and broader bandwidth, driving microwave technologies to higher carrier frequencies. However, electrical microwave oscillators at high-frequency bands experience increased noise and propagation loss, which degrade the precision and sensitivity in practical applications.
Overcoming the challenges of low-noise, high-frequency microwave generation, photonics microwave technologies stand out thanks to their inherent features of low loss at optical frequencies and high fractional frequency stability. Among these photonics oscillators, optical frequency division (OFD) technology, which leverages optical frequency combs and stable optical references, has set the record for spectral purity in micro/mm-wave generation. In optical frequency division systems, optical frequencies can be coherently divided down to microwave frequencies, with frequency noise scaling down as well. This frequency down-conversion is linked by optical frequency combs, a series of optical frequencies with uniform comb line spacing at microwave frequencies. Combined with stable optical references, optical frequency combs can transfer the reference stability to the comb spacing (repetition rate), enabling extraordinarily low-noise micro/mm-wave generation. OFD systems were first demonstrated in early 2011 and have been developed over many years, but mostly with table-sized bulk devices. Excitingly, recent rapid advances in integrated photonics have made the two key components, optical frequency combs and optical references, available on a chip scale. This innovation has initiated a transformation towards miniaturized OFD systems.
Aiming to generate low-noise micro/mm-wave on a chip scale, this thesis demonstrates integrated OFD systems on CMOS-compatible integrated photonic platforms. Here, a planar-waveguide-based optical reference coil cavity can provide frequency stability due to its low thermal refractive noise. The frequency stability is then transferred to a soliton frequency comb generated on a waveguide-coupled microresonator. As a result, these OFD demonstrations significantly miniaturize the systems and achieve state-of-the-art performance by setting a record for low noise in integrated photonic mmWave oscillators. Moreover, the waveguide-based devices can be heterogeneously integrated in photonic circuits, facilitating large-volume, low-cost manufacturing for mass-market applications.