Optical frequency combs, which provide hundreds of evenly spaced and mutually synchronized laser lines, are a crucial component of microwave photonics. They are extremely brief optical pulses with a steady repetition rate that matches comb line frequency spacing—the bursts’ photodetection results in the creation of a microwave carrier.
Chip-scale frequency combs made from nonlinear microresonators powered by continuous-wave lasers have advanced significantly in recent years. These frequency combs depend on generating ultra-short coherent light pulses called dissipative Kerr solitons, which circulate inside optical microresonators. As a result, these frequency combs are frequently referred to as “soliton microcombs.”
Nonlinear microresonators are needed to create soliton microcombs, and these can be made on-chip instantly using CMOS nanofabrication technology. The path to comb miniaturization is paved by co-integration with electronic circuitry and integrated lasers, opening up a variety of uses in communications, spectroscopy, and metrology.
Integrated soliton microcombs with repetition rates as low as 10 GHz have now been proven by a research team. This was accomplished by greatly reducing the optical losses of silicon nitride-based integrated photonic waveguides, a material used in CMOS micro-electronic circuits, and in the past ten years, to create photonic integrated circuits that direct laser light on-chip.
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