Microresonator based Optical Frequency Combs: from Soliton Physics to Coherent terabit communications
Prof. Tobias Kippenberg, EPFL,
Optical frequency combs(1, 2) 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. Yet the transitions of laser frequency combs into widespread applications is hindered by their size, weight and lack of wafer scale integration. In 2007 a new method to generate optical combs was discoverer based on high Q optical microresonators(3, 4). Microresonator frequency combs have since then emerged as a new and widely investigated technology with which combs can be generated via parametric frequency conversion of a continuous wave (CW) laser inside a high Q resonator via the Kerr nonlinearity. These “Kerr combs” give access to repetition rates in the microwave domain, unattainable using conventional laser based frequency combs and moreover enable wafer scale photonic integration of frequency combs. Moreover the parametric gain is broadband enabling frequency combs that can extend over a full octave(5) without external broadening. In addition, micro-resonators are amenable to planar integration allowing further electronic and optical integration on a chip. The developments at EPFL will be reviewed, and results using silicon nitride photonic chip based resonators(6) and ultra high Q crystalline MgF2(7) resonators presented. In particular low noise broadband comb operation will be discussed, their use in coherent telecommunications(8) for terabit/second coherent datacommunication and the extension of these Kerr frequency combs to the mid-IR(9). Moreover the formation of dissipative temporal solitons discovered in microresonators will be discussed(10). By using solitoin formation and dispersion engineering it is possible to directly synthesize a spectrum that covers more than 2/3 of an octave(11), sufficient to establish an RF to optical link. Such chipscale frequency combs architectures can find use in massively parallel coherent optical telecommunication receives, as optical frequency synthesizers, compact atomic clocks, in microwave photonics applications for analog to digital conversion, as well as for compact spectroscopy and sensing devices.
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11. V. Brasch et al., Photonic chip–based optical frequency comb using soliton Cherenkov radiation. Science 351, 357 (2016)