Abstract Designing and engineering microresonator dispersion are essential for generating microresonator frequency comb. Microresonator frequency combs (microcombs, Kerr frequency combs) offer the potential for various attractive applications as a new type of coherent light source that is power efficient and compact and has a high repetition rate and a broad bandwidth. They are easily driven with a continuous-wave pump laser with adequate frequency tuning; however, the resonators must have a high quality (Q) factor and suitable dispersion. The emergence of cavity enhanced four-wave mixing, which is based on third-order susceptibility in the host material, results in the generation of broadband and coherent optical frequency combs in the frequency domain equivalent to an optical pulse in the time domain. The platforms on which Kerr frequency combs can be observed have been developed, thanks to intensive efforts by many researchers over a few decades. Ultrahigh-Q whispering gallery mode (WGM) microresonators are one of the major platforms since they can be made of a wide range of material including silica glass, fluoride crystals and semiconductors. In this review, we focus on the dispersion engineering of WGM microresonators by designing the geometry of the resonators based on numerical simulation. In addition, we discuss experimental methods for measuring resonator dispersion. Finally, we describe experimental results for Kerr frequency combs where second- and higher-order dispersions influence their optical spectra.

Dispersion engineering and measurement of whispering gallery mode microresonator for Kerr frequency comb generation

The experimental realization of a Kerr frequency comb represented the convergence of research in materials, physics, and engineering. This symbiotic relationship continues to underpin efforts in comb innovation today. While the initial focus developing cavity-based frequency combs relied on existing microresonator architectures and classic optical materials, in recent years, this trend has been disrupted. This paper reviews the latest achievements in frequency comb generation using resonant cavities, placing them within the broader historical context of the field. After presenting well-established material systems and device designs, the emerging materials and device architectures are examined. Specifically, the unconventional material systems as well as atypical device designs that have enabled tailored dispersion profiles and improved comb performance are compared to the current state of art. The remaining challenges and outlook for the field of cavity-based frequency combs are evaluated.

Emerging material systems for integrated optical Kerr frequency combs

The on-chip creation of coherent light at visible wavelengths is crucial to field-level deployment of spectroscopy and metrology systems. Although on-chip lasers have been implemented in specific cases, a general solution that is not restricted by limitations of specific gain media has not been reported, to the best of our knowledge. Here, we propose creating visible light from an infrared pump by widely separated optical parametric oscillation (OPO) using silicon nanophotonics. The OPO creates signal and idler light in the 700 nm and 1300 nm bands, respectively, with a 900 nm pump. It operates at a threshold power of $ (0.9 \pm 0.1)\,\,{\rm mW} $(0.9±0.1)mW, over $ 50 \times $50× smaller than other widely separated microcavity OPO works, which have been reported only in the infrared. This low threshold enables direct pumping without need of an intermediate optical amplifier. We further show how the device design can be modified to generate 780 nm and 1500 nm light with a similar power efficiency. Our nanophotonic OPO shows distinct advantages in power efficiency, operation stability, and device scalability, and is a major advance towards flexible on-chip generation of coherent visible light.

Milliwatt-threshold visible-telecom optical parametric oscillation using silicon nanophotonics.

Here, we report $ {\chi ^{(3)}} $χ(3)-based optical parametric oscillation (OPO) with widely separated signal–idler frequencies from crystalline aluminum nitride microrings pumped at $ 2\,\,\unicode{x00B5}{\rm m} $2µm. By tailoring the width of the microring, OPO reaching toward the telecom and mid-infrared bands with a frequency separation of 64.2 THz is achieved. While dispersion engineering through changing the microring width is capable of shifting the OPO sideband by $ \gt {9}\;{\rm THz}$>9THz, the OPO frequency can also be agilely tuned in the ranges of 1 and 0.1 THz, respectively, by shifting the pump wavelength and controlling the chip’s temperature. At high pump powers, the OPO sidebands further evolve into localized frequency comb lines. Such large-frequency-shift OPO with flexible wavelength tunability will lead to enhanced chip-scale light sources.

Widely separated optical Kerr parametric oscillation in AlN microrings.

The development of ultrahigh-quality-factor (Q) microresonators has been driving such technologies as cavity quantum electrodynamics (QED), high-precision sensing, optomechanics, and optical frequency comb generation. Here we report ultrahigh Q crystalline microresonator fabrication with a Q exceeding 108, for the first time to our knowledge, achieved solely by computer-controlled ultraprecision machining. Our machining fabrication method readily achieves the dispersion engineering and size control of manufactured devices via programmed machine motion, both of which were not possible with the conventional manual polishing method. We can achieve an ultrahigh Q without the need for subsequent careful polishing that is generally required to ensure that surface integrity is maintained. We carefully addressed the cutting condition and crystal anisotropy to overcome the large surface roughness that has thus far been the primary cause of the low Q in the machining process. Our result paves the way for a reliable fabrication with a view to various photonic applications utilizing ultrahigh-Q crystalline microresonators.

All-precision-machining fabrication of ultrahigh-Q crystalline optical microresonators

This erratum presents the corrections to the Letter published in Opt. Lett.44, 3146 (2019)OPLEDP0146-959210.1364/OL.44.003146.

Octave-wide phase-matched four-wave mixing in dispersion-engineered crystalline microresonators

In this Letter, we report phase-matched four-wave mixing separated by over one-octave in a dispersion engineered crystalline microresonator. Experimental and numerical results presented here confirm that primary sidebands were generated with a frequency shift up to 140 THz, and that secondary sidebands formed a localized comb structure, known as a clustered comb in the vicinity of the primary sidebands. A theoretical analysis of the phase-matching condition validated our experimental observations, and our results good agree well with numerical simulations. These results offer the potential to realize a frequency tunable comb cluster generator operating from 1 um to mid-infrared wavelengths with a single and compact device.

Octave-wide phase-matched four-wave mixing in dispersion engineered crystalline microresonators

We demonstrated forward-error-correction (FEC)-free dense wavelength division multiplexing communication utilizing a Kerr soliton frequency comb generated from a high-Q crystalline microresonator with a 20-GHz free spectral range.

FEC-free dense WDM transmission with Kerr soliton microcombs in crystalline MgF2 microresonators

We investigated the feasibility of IM-DD communication in a microcomb. Turing pattern and soliton combs are ideal, though MI comb can transmit signals when the pump-detuning is well controlled and excessive noise is suppressed.

FEC-free optical data transmission with a chip-integrated microresonator frequency comb source

Erratum: Octave-wide phase-matched four-wave mixing in dispersion engineered crystalline microresonators (Optics Letters (2019) 44 (3146) DOI: 10.1364/OL.44.003146)

Shuya Tanaka

Papers

Octave-wide phase-matched four-wave mixing in dispersion-engineered crystalline microresonators

Octave-wide phase-matched four-wave mixing in dispersion engineered crystalline microresonators

FEC-free dense WDM transmission with Kerr soliton microcombs in crystalline MgF2 microresonators

FEC-free optical data transmission with a chip-integrated microresonator frequency comb source

Erratum: Octave-wide phase-matched four-wave mixing in dispersion engineered crystalline microresonators (Optics Letters (2019) 44 (3146) DOI: 10.1364/OL.44.003146)