Guiding the Evolution of Microresonator Frequency Combs

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NontechnicalOptical frequency combs are a revolutionary form of broadband light consisting of a large number of colors that are precisely spaced in frequency. Since their development in the early part of the new century, optical combs have enabled remarkable advances in precision measurement of time and frequency, which are critical to position and inertial guidance, and in high-speed spectroscopy useful for applications such as environmental sensing and breath analysis. However, these combs are generated from specialized lasers, which are rather bulky, which still hinders their deployment for many real world applications. Furthermore, the pulses they emit are limited to relatively low repetition rates. Substantially higher repetition rates are needed for other promising applications, such as optical communications, photonics-based radio-frequency signal processing, and calibration of certain astronomical instruments. More recently, a new choice for generating optical frequency combs has emerged, based on the interaction of light with tiny "microresonator" structures on integrated photonic chips. These microresonator-based comb generators are fundamentally very small, which offers new opportunities for low cost deployment, and provide much higher repetition rate, which offers compatibility with new applications. Research at Purdue University will explore methods to overcome challenges in guiding the optical combs generated from microresonators into stable and repeatable low-noise states desirable for applications and will provide advanced training to students at the graduate and undergraduate level relevant to careers in high technology and science.TechnicalContinuous-wave pumping of a high quality factor microresonator can give rise to formation of combs of optical frequencies spaced by tens to hundreds of GHz. Such combs arise due to nonlinear wave mixing mediated by the optical Kerr effect and are frequently termed Kerr combs. In a particularly interesting state, the comb comprises cavity solitons (more formally called dissipative Kerr solitons), ultrashort pulses of light that propagate in the microresonator with remarkable stability, thanks to a double balance between loss and parametric gain and between dispersion and nonlinearity. However, controlling the nonlinear dynamics to achieve the desired stable states of operation is complex. One complication is that the comb evolves through a chaotic regime before it potentially switches into a stable cavity soliton state; the passage through chaos leads to strong indeterminacy in the state generated. Competition with thermo-optic nonlinearities that are exacerbated by power transients that occur upon transition into the soliton state cause further challenges. Although progress has been made, control strategies to deal with these issues are particularly challenging for desirable chip-scale microresonator devices. This project proposes to explore two new approaches to guiding the evolution of the comb to desired states, including single soliton states, within the rich, nonlinear dynamical phase space. One approach investigates the idea of a so-called chaos-avoiding trajectory involving coordinated manipulation of pump laser power and frequency to reach stable soliton states without passing through the chaotic operating regime usually encountered. A second complementary approach will explore a recently proposed passive scheme toward obtaining single solitons based on mode interactions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.