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Photonic-chip-based soliton microcombs

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Optical frequency combs provide equidistant markers in the IR, visible and UV and have become a pivotal tool for frequency metrology and are the underlying principle of optical atomic clocks, but are also finding use in other areas, such as broadband spectroscopy or low noise microwave generation. In 2007 a new method to generate optical combs was discovered based on high-Q optical microresonators. Such microresonator frequency combs (microcombs) have since then emerged as a new and widely investigated method in which combs can be generated via parametric frequency conversion of a continuous wave laser inside a high Q resonator via the Kerr nonlinearity. Over the past years a detailed understanding of the comb formation process has been gained, and regimes identified in which dissipative temporal solitons (DKS) can be generated, that not only provide low noise optical frequency combs but moreover give access to femtosecond pulses. Such DKS have unlocked the full potential of soliton micro-combs by providing access to fully coherent and broadband combs and soliton broadening effects that we will describe. DKS have now been generated in a wide variety of resonators, including those compatible with photonic integration based on silicon nitride (Si3N4). However, in this platform, DKS have been only generated with repetition rates above 100 GHz, beyond the spectral bands targeted for easy signal processing by regular optoelectronic components. This was mainly due to the high optical loss in waveguides caused by the fabrication process when fabricating large geometries. In addition, the required pump power to generate the combs have been typically detrimental to the applications requiring full integration. We will discuss the recent progress achieved in the power requirements, form factor and microwave rate operation of the integrated microcombs. In particular we will report on the Photonic Damascene nanofabrication process that allows fabrication of large waveguides without stitching errors, hereby providing exquisite Q-factor beyond 2×107. This allowed the generation of microcombs operating in the K- and X- microwave bands that are used e.g. in radars and future 5G systems, with power threshold compatible with silicon-based integrated lasers. Moreover, the latest advances in the integration of the chip-scale lasers and nonlinear microresonators have led to impressive demonstration of integrated, low-power consumption microcombs that make these devices valid candidates for replacing the state-of-the-art of mode-locked laser-based optical frequency combs and truly provide ubiquitous chip-scale comb sources.

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