Optical frequency combs [1,2] provide a series of equidistant laser lines and have revolutionized the field of frequency metrology within the last decade. Originally developed to achieve absolute optical frequency measurements, optical frequency combs have enabled advances in other areas [3] such as molecular fingerprinting [4,5], astronomy [6], range finding [7] or the synthesis of low noise microwave signals [8]. Discovered in 2007[9, 10], microresonator (Kerr) frequency combs have emerged as an alternative and widely investigated method to synthesize optical frequency combs offering compact form factor, chipscale integration, multi-gigahertz repetition rates, broad spectral bandwidth and high power per frequency comb line. Since their discovery there has been substantial progress in fundamental understanding [11][12][13], theoretical modeling [14][15][16], on-chip planar integration [17,18] and resulting applications [19][20][21]. Yet, in no demonstration could two key properties of optical frequency combs, broad spectral bandwidth and coherence, be achieved simultaneously. Here we overcome this challenge by accessing, for the first time, soliton induced Cherenkov radiation [22,23] in an optical microresonator. By continuous wave pumping of a dispersion engineered, planar silicon nitride microresonator [17,18], continuously circulating, sub-30 fs short temporal dissipative Kerr solitons [24][25][26] are generated, that correspond to pulses of 6 optical cycles and constitute a coherent optical frequency comb in the spectral domain. Emission of soliton induced Cherenkov radiation caused by higher order dispersion broadens the spectral bandwidth to 2/3 of an octave, sufficient for self referencing [1,2], in excellent agreement with recent theoretical predictions [16] and the broadest coherent microresonator frequency comb generated to date. Once generated it is shown that the soliton induced Cherenkov radiation based frequency comb can be fully phase stabilized. The overall relative accuracy of the generated comb with respect to a reference fiber laser frequency comb is measured to be 3 · 10 −15 . The ability to preserve coherence over a broad spectral bandwidth using soliton induced Cherenkov radiation marks a critical milestone in the development of planar optical frequency combs, enabling on one hand application in e.g. coherent communications [19], broadband dual comb spectroscopy [27] and Raman spectral imaging [28], while on the other hand significantly relaxing dispersion requirements for broadband microresonator frequency combs [29] and providing a path for their generation in the visible and UV. Our results underscore the utility and effectiveness of planar microresonator frequency comb technology, that offers the potential to make frequency metrology accessible beyond specialized laboratories.Optical solitons are propagating pulses of light that retain their shape due to a balance of nonlinearity and dispersion [24][25][26]30]. In the presence of higher order dispersion optical solitons can emit solito...

The formation of temporal dissipative solitons in optical microresonators enables compact, high repetition rate sources of ultra-short pulses as well as low noise, broadband optical frequency combs with smooth spectral envelopes. Here we study the influence of the resonator mode spectrum on temporal soliton formation. Using frequency comb assisted diode laser spectroscopy, the measured mode structure of crystalline MgF2 resonators are correlated with temporal soliton formation. While an overal general anomalous dispersion is required, it is found that higher order dispersion can be tolerated as long as it does not dominate the resonator's mode structure. Mode coupling induced avoided crossings in the resonator mode spectrum are found to prevent soliton formation, when affecting resonator modes close to the pump laser. The experimental observations are in excellent agreement with numerical simulations based on the nonlinear coupled mode equations, which reveal the rich interplay of mode crossings and soliton formation.Temporal dissipative solitons [1][2][3] can be formed in a Kerr-nonlinear optical microresonator [4] with anomalous dispersion that is driven by a monochromatic continuous wave pump laser. These temporal solitons are sech 2 -shaped ultra-short pulses of light circulating inside the microresonator, where the temporal width of the solitons is fully determined by the resonator dispersion and nonlinearity as well as the pump power and pump laser detuning [4,5]. It has been shown that the pump laser parameters can be used to control the number of solitons circulating in the microresonator. In particular the single soliton state, where one single soliton is circulating continuously inside the resonator, is of high interest for applications. In the time domain soliton formation in microresonators allows for the generation of periodic ultrashort femto-second pulses, which in the frequency domain correspond to a frequency comb spectrum with smooth sech 2 -shaped spectral envelope. The free spectral range (FSR) of the resonator, typically in the range of tens to hundreds of GHz, determines the pulse repetition rate (equivalent to the frequency comb line spacing). Soliton formation is related to four-wave mixing based frequency comb generation in microresonators [6][7][8][9][10][11][12][13][14][15], where low and high noise operating regimes [12,16,17] have been identified. Here, techniques such as δ − ∆-matching [17], self-injection locking [18,19] or parametric seeding [20] can be used to achieve low noise operation. In contrast to these low noise four-wave mixing based combs (also termed Kerr combs), the transition to the soliton regime [17] offers a unique combination of features, such as intrinsic low noise performance, direct pulse generation in the microresonator [4,21,22], and smooth spectral envelope as shown in Figure 1. These properties are critical to applications in e.g. telecommunications [23][24][25], low phase noise microwave generation [18,26]

We investigate soliton generation dynamics with the influence of thermal effects. Either soliton annihilation or survival can occur in different trials with the same tuning method, and a spontaneous route to soliton formation is observed.