Optical frequency combs allow for the precise measurement of optical frequencies and are used in a growing number of applications. The new class of Kerr-frequency comb sources, based on parametric frequency conversion in optical microresonators, can complement conventional systems in applications requiring high repetition rates such as direct comb spectroscopy, spectrometer calibration, arbitrary optical waveform generation and advanced telecommunications. However, a severe limitation in experiments working towards practical systems is phase noise, observed in the form of linewidth broadening, multiple repetition-rate beat notes and loss of temporal coherence. These phenomena are not explained by the current theory of Kerr comb formation, yet understanding this is crucial to the maturation of Kerr comb technology. Here, based on observations in crystalline MgF 2 and planar Si 3 N 4 microresonators, we reveal the universal, platformindependent dynamics of Kerr comb formation, allowing the explanation of a wide range of phenomena not previously understood, as well as identifying the condition for, and transition to, low-phase-noise performance.O ptical frequency combs [1][2][3][4] have revolutionized the field of frequency metrology and spectroscopy and are enabling components in a range of applications 5 . Recently, a novel class of frequency comb generators has been discovered 6 by coupling a continuous-wave (c.w.) laser to a high-finesse fused silica microcavity, where the Kerr nonlinearity enables (cascaded) fourwave-mixing (FWM), resulting in an optical frequency comb. These Kerr combs could complement conventional frequency combs in applications where high power per comb line (typically .100 mW) and high repetition rate (.10 GHz spacing between the comb lines) are desirable 7 , such as in astronomical spectrometer calibration [8][9][10] , direct comb spectroscopy 11 , arbitrary optical waveform generation 12,13 and advanced telecommunications. The creation of Kerr combs using microresonators has been demonstrated in crystalline CaF 2 (refs 14,15) and MgF 2 (refs 16-18) resonators, fused-silica microspheres 19 , planar high-index silica 20 and Si 3 N 4 ring resonators 21,22 , and compact fibre cavities 23 . Over recent years, a significant advance in Kerr comb technology has been achieved by demonstrating a single and well-defined radiofrequency (RF) beat note between adjacent comb lines (corresponding to the equidistant comb spacing and required for stabilizing the comb) 6,14,24 , a fully phase-stabilized Kerr comb 24 , the generation of octave-spanning spectra (required for self-referencing the comb using the f-2f scheme) 25,26 , the detection and shaping of pulses 13 , and extension of spectral coverage towards the visible 27 and midinfrared spectral regimes 18 . In addition to these experimental advances, theoretical work 28,29 has also enabled an explanation of the power distribution of Kerr combs, particularly the first comb modes appearing not necessarily adjacent to the pump. Despite these advances,...

No abstract

No abstract

Silicon photonics enables wafer-scale integration of optical functionalities on chip. Silicon-based laser frequency combs can provide integrated sources of mutually coherent laser lines for terabit-per-second transceivers, parallel coherent light detection and ranging, or photonics-assisted signal processing. We report heterogeneously integrated laser soliton microcombs combining both indium phospide/silicon (InP/Si) semiconductor lasers and ultralow-loss silicon nitride (Si3N4) microresonators on a monolithic silicon substrate. Thousands of devices can be produced from a single wafer by using complementary metal-oxide-semiconductor–compatible techniques. With on-chip electrical control of the laser-microresonator relative optical phase, these devices can output single-soliton microcombs with a 100-gigahertz repetition rate. Furthermore, we observe laser frequency noise reduction due to self-injection locking of the InP/Si laser to the Si3N4 microresonator. Our approach provides a route for large-volume, low-cost manufacturing of narrow-linewidth, chip-based frequency combs for next-generation high-capacity transceivers, data centers, space and mobile platforms.

We demonstrate dispersion engineering of integrated silicon nitride based ring resonators through conformal coating with hafnium dioxide deposited on top of the structures via atomic layer deposition (ALD). Both, magnitude and bandwidth of anomalous dispersion can be significantly increased. All results are confirmed by high resolution frequency-comb-assisted-diode-laser spectroscopy and are in very good agreement with the simulated modification of the mode spectrum.Silicon nitride (Si 3 N 4 ) integrated planar waveguide and ring resonator structures [1] are attractive platforms for resonant nonlinear frequency conversion [2]. Moreover Si 3 N 4 has been used for ultra-low loss integrated waveguides [3], particularly in the optical telecom band, where well established silicon on insulator waveguides [4,5] suffer from two-photon and free carrier absorption. Besides low absorption, the waveguide dispersion plays a central role for parametric frequency conversion. In particular this applies to microresonator based optical frequency comb generation via the χ (3) non-linearity ("Kerrcombs") [6], which has recently also been demonstrated in Si 3 N 4 [7]. In this scheme a set of equidistant optical frequencies is generated with a spacing corresponding to the free spectral range (FSR) of the resonator. Such integrated and CMOS-compatible microresonator frequency combs potentially offer a level of compactness and integration that is presently not attainable in mode-locked laser based combs and can moreover access GHz repetition rates. Microresonator based frequency combs are promising for applications like the calibration of astronomical spectrographs [8], on chip optical interconnects [2] and optical arbitrary waveform generation [9]. It is well known that the attainable spectral bandwidth of Kerr-combs is limited by the resonator dispersion, that leads to a wavelength dependent FSR of the resonator [10,11]. The dispersion may be described as, where ω l is the frequency of the fundamental resonance with azimuthal mode number l. The dispersion D 2 in microresonators is related to the group velocity dispersion of the structures via β 2 = −D 2 / 2πRD 3 1 , with D 1 = ω l+1 − ω l being the FSR of the resonator and R the ring radius. It has been shown that the phase noise characteristics of Kerrcomb generators is related to the ratio of cavity-decayrate and dispersion [12] and a sufficiently large anomalous dispersion is advantageous for low phase noise operation of Kerr-combs (intrinsically low phase noise combs are generated for (κ/D 2 ∼ 1). Dispersion in integrated ring resonators is composed of material dispersion (which is normal for Si 3 N 4 at all visible and nearinfrared wavelengths), geometric dispersion due to the waveguide cross-section [13], as well as an additional contribution from the finite resonator radius as observed in whispering-gallery-mode microcavities [14]. In previous numerical studies it has been shown that silicon (n = 3.5) waveguide dispersion can be decreased and flattened through a conformal coat...