Jizhao Zang, Su-peng Yu, Time and Frequency Division, National Institute of Standards and Technology, University of Colorado; David R. Carlson, Travis C. Briles, Time and Frequency Division, National Institute of Standards and Technology; Yan Jin, Haixin Liu, and Scott B. Papp, Time and Frequency Division, National Institute of Standards and Technology, University of Colorado

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Over the past few decades, optical frequency combs (OFCs) have proved to be a remarkable tool for widespread applications, such as spectroscopy, metrology, optical clocks, frequency synthesis and calibration of astronomical spectrograph. The conventional generation of coherent and equidistant OFCs relies on the mode-lock lasers, which are well-developed but their practical applications are still limited in some research areas because of the large footprints, high power consumption and low repetition rate. Recently microresonators have merged as a promising platform for chip-scale OFC generators. The microresonators, consisting of a whispering gallery mode resonator (WGMR) coupled to an external bus waveguide, offer revolutionary advantages over conventional OFC sources, including compactness, low power consumption, high repetition rate and flexible dispersion engineering. In those microresonators, light is highly confined in the micrometer-scale cavity, resulting as enhanced light intensity and extremely low threshold power (sub milliwatt level) for nonlinear optical effects. By pumping the high-Q microresonator with a continuous-wave (CW) laser, OFCs are generated through parametric four-wave mixing. Under suitable conditions, self-sustaining optical wavepackets, or Kerr solitons, can be excited in microresonators. The Kerr solitons provide miniaturized highly coherent frequency combs and are expected to accelerate the development of OFC-based technologies, especially those need low-size, weight, and power (SWaP) comb sources. Since the core function of microresonators is to convert a CW pump into a series of equidistant comb lines, the pump-to-comb conversion efficiency is an important parameter to evaluate their performance and also a key consideration in power-sensitive applications. High conversion efficiency enables higher comb line power when only limited pump power is available. For example, in wavelength-division multiplexing (WDM) systems, higher comb line power results as higher signal-to-noise ratio (SNR) at the receiver, allowing for higher data rates, longer transmission distance and lower bit error rate. In OFC-based microwave and millimeter-wave signal generators, high comb line power is beneficial to the SNR of generated RF signals and potentially eliminates the optical or electrical amplifiers. Comb flatness is another important figure of merit. A flat comb can dramatically improve the system performance in some specific applications, such as microwave photonic filters and coherent optical communications. The dark soliton pulses formed in normal group velocity dispersion (GVD) microresonators usually provide better flatness than those in anomalous regime, but special techniques are required to fulfill the phase-matching condition, such as mode interactions or photonic crystal induced more split. We implement the latter approach by inscribing uniform shape oscillation in the inner wall of Kerr microresonators. This kind of resonators, named as photonic crystal ring resonators (PhCRs), have shown rich dynamical behavior, including spontaneous pulse formation and a continuum of bright and dark-pulse states. To improve the pump-to-comb conversion efficiency, we optimize the coupling, mode split and dispersion of our PhCRs. Both simulation and experiment show that the conversion efficiency could be improved by over-coupling the ring, but the highest conversion efficiency is limited to 50% due to the pump power distribution in PhCRs. We modify the pump power distribution by designing a photonic crystal reflector on the downstream side of the microresonator. The additional reflector provides several benefits: turn-key soliton generation, lower threshold power and higher efficiency compared with the PhCRs without reflectors. We optimize the relative phase between the optical field inside the ring and the reflected pump by rotating the ring. The variation of conversion efficiency with reflection phase agrees well with the simulation results. We experimentally demonstrate up to 86% pump-to-comb conversion efficiency in a PhCR with free spectral range (FSR) of 500 GHz. On a single chip of 200 GHz FSR PhCRs, 11 out of 42 devices (26.2%) can achieve 47% to 65% conversion efficiency and the required on-chip pump power for soliton generation is only 30-40 mW. Our work represents an important step towards near-unit efficiency in microresonators. The minimized pumping power and high conversion efficiency relax the requirement of pump lasers in monolithic integration. This research could greatly benefit power-sensitive OFC applications, such as WDM transmitters, integrated optical clocks and chip-scale microwave/millimeter-wave synthesizers.