Frequency comb formation and transition to chaos in microresonators with near-zero dispersion
Spotlight summary: Imagine an optical synthesizer the size of a small Lego piece converting an input radio-frequency signal into a user-defined optical waveform with Hertz level precision. Imagine the device consuming less than 1 Watt of power. Such a synthesizer does not exist yet, but it would be a disruptive technology for applications in broadband coherent communications, LIDAR, optical atomic clocks and high-precision spectroscopy. It may sound like science fiction, but actually the Defense Advanced Research Projects Agency (DARPA) in the US is now soliciting innovative research proposals to accomplish this goal within the next 4 years, in the so-called Direct On-chip Digital Optical Synthesizer (DODOS) program.
One of the core technologies in the above Lego-size synthesizer is likely to be a microresonator frequency comb (also called a Kerr comb or microcomb). In order to achieve the desired performance, DARPA foresees significant efforts towards having a highly efficient, fully stabilized Kerr comb with a bandwidth spanning over an octave (~ 130 THz). This is indeed one of the greatest challenges in today’s microresonator comb research.
The conventional way to understand the frequency comb generation process in microresonators is quite different to that of femtosecond mode-locked lasers. In a Kerr comb, a continuous-wave laser pumps an ultrahigh-Q microresonator. The power builds up, new frequency components are generated and the comb forms by a cascade of nonlinear mixing processes. Various groups have shown a transition to a locked state, where the time-domain waveform consists of a femtosecond pulse (a soliton) circulating in the microresonator.
The theoretical work of Rogov and Narimanov suggests a new approach for the comb generation process. They consider the case in which the microresonator has no dispersion. This assumption prevents efficient phase matching in the aforementioned picture, yet their simulations show broadband frequency comb generation. Instead, the underlying mechanism consists of a combination of nonlinear switching and self-phase modulation. To understand the beauty of this finding, remember that in a linear resonator the power buildup is proportional to the finesse. In the nonlinear regime however, there could exist multiple steady-state powers in the resonator. What Rogov and Narimanov show is that the microresonator can instantaneously switch between these allowed power levels within a roundtrip time. A nonzero dispersion would limit the spectrum, leading to finite rise and fall times.
Will this frequency comb mechanism be underlying in the final DODOS prototype? Probably not, since, as the authors point out, the amount of power required to enter into this regime is quite high (at least for the nonlinear microresonator materials for which Kerr combs have been reported). However, having a flat, close to zero dispersion profile is desirable for the generation of temporal solitons in microresonators.
What this work nicely illustrates is that microresonator frequency combs offer an unprecedented opportunity to explore the physics of nonlinear resonators. This implies scientific discoveries as interesting as their envisioned applications. These are exciting times for microresonator comb research.
Technical Division: Light–Matter Interactions
ToC Category: Nonlinear Optics
|OCIS Codes:||(190.0190) Nonlinear optics : Nonlinear optics|
|(190.3100) Nonlinear optics : Instabilities and chaos|
|(230.4555) Optical devices : Coupled resonators|
|(320.6629) Ultrafast optics : Supercontinuum generation|
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