Abstract
Phase coherently linking optical-to-radio frequencies with femtosecond mode-locked laser frequency combs has enabled counting the cycles of light and is the basis of optical clocks, absolute frequency synthesis, tests of fundamental physics, and improved spectroscopy. Using an optical microresonator frequency comb to establish a coherent link between optical and microwave frequencies will extend optical frequency synthesis and measurements to areas requiring compact form factor, on-chip integration, and comb line spacing in the microwave regime, including coherent telecommunications, astrophysical spectrometer calibration, or microwave photonics. Here we demonstrate a microwave-to-optical link with a microresonator. By using a temporal dissipative single soliton state in an ultrahigh- crystalline microresonator that is broadened in highly nonlinear fiber, an optical frequency comb is generated that is self-referenced, allowing us to phase coherently link a 190 THz optical carrier directly to a 14 GHz microwave frequency. Our work demonstrates that precision optical frequency measurements can be realized with compact high- microresonators.
© 2015 Optical Society of America
1. INTRODUCTION
The development of the optical frequency comb (OFC) based on femtosecond pulsed mode-locked lasers [1–3] in conjunction with nonlinear spectral broadening constituted a dramatic simplification over large-scale harmonic frequency chains [4]. In the frequency domain, OFCs give equidistant optical lines, where the frequency of each component obeys . The spacing between the lines is determined by the pulse repetition rate of the laser ( being an integer). Knowledge of the comb’s overall offset frequency (also referred to as the carrier envelope offset frequency) along with and allows linking the optical frequency to the electronically countable frequency and in the radio frequency or microwave domain. In this way the measurement of and corresponds to counting the cycles of light. Self-referencing of the comb, i.e., a self-contained measurement of and , has been achieved using nonlinear interferometers, whereby the comb’s bandwidth needs to be broadened to encompass typically two-thirds of or a full octave [5–8]. This measurement is a key prerequisite for many applications of OFCs. For example, referencing one of the comb lines of an OFC to an atomic frequency standard [9] allows the comb to function as a “gearbox,” realizing the next generation of atomic clocks based on optical transitions. Self-referenced frequency combs can also be used for optical frequency synthesis, and have enabled the most accurate frequency measurements [10,11]. Frequency combs with large mode spacings (), which are challenging to obtain via mode-locked lasers [12], can be used in a growing number of applications, including astronomical spectrometer calibration [13], dual-comb coherent Raman imaging [14], high-speed optical sampling or coherent telecommunications [15]. In each of these applications, having a line spacing is either beneficial or even required.
Different from mode-locked lasers, microresonator frequency combs (MFCs) [16,17] are generated using parametric frequency conversion [18,19], of a continuous-wave (CW) laser. This approach exhibits several attractive features that have the potential to extend further the use of OFCs to new areas in precision optical measurement, spectroscopy, astronomy, telecommunications, and industrial applications. Fundamentally different from mode-locked laser systems, MFCs offer high repetition rates (), compact form factors, broadband parametric gain that can be generated in wavelength regimes ranging from the visible [20] to the mid-infrared [21,22], CMOS-compatible microresonator platforms, and high power per comb line. Some unique characteristics of MFCs are that the pump laser constitutes a comb line and that there is no active laser gain medium in the system. These properties have encouraged in recent years the intense investigation of microresonator-based frequency combs with the ultimate goal of realizing a self-referenced system. Progress in recent years includes new microresonator platforms in crystalline materials [23], fused silica microtoriods [16], and photonic chips (based on silicon nitride [24,25], aluminum nitride [26], Hydex, [27,28] and diamond [29]). In addition, MFCs without self-referencing have been used for coherent telecommunications [15], compact atomic clocks [30], stabilized oscillators [31], and optical pulse generation [32–34]. The dynamics of microresonator frequency combs have been investigated, and regimes with low noise frequency comb operation have been identified based on intrinsic low phase noise regimes, or via tuning mechanisms such as matching [35], parametric seeding [36], and injection locking [36,37], or via the observation of phase locking [33]. Moreover recently, low noise frequency combs have been generated via temporal dissipative soliton formation [34,38–40] and numerical tools to simulate comb dynamics emerged based on the Lugiato–Lefever equation [41–43] and the coupled-modes equation [34,44].
However, despite the rapid progress in understanding, simulations, applications, and platforms, there is a milestone that has not yet been reached: a self-referenced microresonator system, capable of phase coherently linking the microwave and the optical frequency domain. So far, knowledge of the absolute frequency of all comb lines of a MFC has been achieved only by using an auxiliary self-referenced fiber laser frequency comb as a Ref. [45], as well as via locking of two comb teeth to a rubidium transition [30]. Self-referencing, however, has never been achieved. One reason for this is the high repetition rate of the microresonators, leading to correspondingly low pulse peak intensities. This makes external spectral broadening using nonlinear fiber, which is widely employed in mode-locked lasers, difficult to apply. While octave spanning combs [46,47] have been attained directly from MFCs, they have not been suitable for self-referencing techniques due to excess noise associated with subcomb formation [35].
Here, we demonstrate a coherent microwave-to-optical link using temporal dissipative soliton formation in a crystalline microresonator in conjunction with external spectral broadening, measuring simultaneously both and , which are necessary for linking the optical to the radio frequency domain. Our approach uses the newly discovered class of temporal dissipative cavity solitons in microresonators [34,39,48,49] and marks the first time that the carrier envelope offset frequency of such a soliton has been measured, to our knowledge. Our results demonstrate that MFCs generated by soliton formation are suitable for absolute frequency measurements. In particular, the pulse-to-pulse timing jitter is sufficiently low to enable precise measurement of the carrier envelope offset frequency via self-referencing. Although not demonstrated here, it has already been shown that microresonator comb parameters and can be stabilized by controlling the pump laser frequency and by actuating the resonator free spectral range (FSR) by heating or mechanical stress [45,50].
2. EXPERIMENT AND RESULTS
Optical microresonators support different azimuthal optical whispering gallery modes (WGMs). The FSR between modes of a particular mode family is determined by material and geometric dispersion. It has been shown that WGMs can have quality factors exceeding [51,52]. When a CW pump laser is coupled to a WGM, the resonator’s Kerr nonlinearity can lead to efficient nonlinear frequency conversion. The resonator used in this work is a crystalline ultrahigh- resonator made by polishing crystalline [53–55] (see Fig. 1), and can support WGMs [51] confined in one of the protrusions that extend around the circumference of the resonator. The mode used has a quality factor of and a FSR of 14.0939 GHz. Light from a CW fiber laser at 1553 nm with can be coupled into and out of the resonator via evanescent coupling using a tapered optical fiber [56]. To form the solitons in the cavity, the pump laser’s frequency is scanned over the resonance and stopped when the appropriate conditions are met [34]. The duration of the pulse inside the resonator depends mainly on the coupling and detuning of the pump laser to the resonator mode, and can be estimated from the bandwidth of the spectrum shown in Fig. 1, to be . The optical spectrum generated by the soliton is not yet sufficiently broad for self-referencing; however, the spectrum can be broadened via supercontinuum generation [57].
The experimental setup after the resonator is shown in Fig. 2. A portion of the pulse train produced by the resonator is sent to an optical spectrum analyzer (OSA). The rest of the pulse train then has the CW background, which consists of the residual pump laser [34] minimized by fiber optic filters. Then another small amount is sent to a photodetector and an electronic spectrum analyzer (ESA) to detect . The pulse is then preamplified with an erbium-doped fiber amplifier (EDFA). The main type of fiber in the experiment is SMF28, which has anomalous group velocity dispersion in the wavelength range of the soliton. Before being sent into a high power EDFA, the soliton pulse is prechirped using dispersion compensating fiber (DCF), which has normal group velocity dispersion, to minimize nonlinear effects in the EDFA. In this way the average power of the pulse train is increased to 2 W. After the EDFA, the pulse is recompressed using SMF28 fiber via the cut-back method to a duration of with an energy of . This pulse is subsequently sent through approximately 2 m of highly nonlinear fiber (HNLF) (Menlo Systems), where supercontinuum generation occurs. The resulting spectrum can be seen in Fig. 3. The blue trace shows the soliton spectrum after the optical microresonator, and the red after the HNLF fiber. The latter spectrum exceeds two-thirds of an octave, which is sufficient for self-referencing via a interferometer [5–8]. Importantly, the broadened spectrum is coherent. This is verified by using a heterodyne beat with additional external lasers at the two ends of the comb, as detailed below. Figure 3(b) shows a zoom into the spectrum taken after the HNLF fiber, where the individual comb lines are clearly visible, even with the limited resolution of the OSA (0.02 nm).
Self-referencing is achieved by measuring and of the generated frequency comb. By picking off a small portion of the light after it leaves the resonator and sending it to a photodetector, can be directly measured (see Fig. 2). The broadened spectrum is sufficiently wide and allows employing a interferometer [5] to determine . This is traditionally implemented by frequency tripling a component of the low-frequency part of the spectrum using a combination of second-harmonic and sum-frequency generation to give , where is an integer. In addition, a component of the higher frequency part of the spectrum is frequency doubled using second-harmonic generation to give , where is an integer. Mixing the doubled and the tripled light and detection on a photodetector gives the offset frequency , granted , i.e., necessitating a spectrum that covers two-thirds of an octave. Here a scheme involving two transfer lasers is implemented, which has the advantage of allowing independent verification of the coherence of the supercontinuum generation at the two ends of the spectrum. The coherence of the generated broadband OFC is verified by optically heterodyning the two reference lasers at (external cavity diode laser) and (thulium fiber laser) with the supercontinuum and detecting the optical heterodyne beat signal on a photodetector (see Fig. 3). The beatnote frequencies can be written in terms of the frequency comb parameters and an offset as
and where and are integers and and are the frequency offsets () of the transfer lasers from the generated OFC (see Fig. 3). The offset frequencies and are chosen to both be positive by convention. The interferometry is constructed with reference lasers in which one arm of the interferometer serves for second-harmonic generation of light at 1272 nm to give light at 636 nm, written as (see Fig. 2) The other interferometer arm serves for frequency tripling of the light at 1908 nm via second-harmonic generation to create light at 954 nm followed by sum-frequency generation of the 954 and 1908 nm light to give light at 636 nm, and the frequency can be written as The doubled light and tripled light are mixed and detected on a photodetector, giving an optical heterodyne signal at the difference frequency: The offset frequency is related to the beats of the transfer lasers with the generated frequency comb: The transfer laser at 1272 nm is phase locked to a frequency comb component with offset frequency. The frequency tripled 1908 nm transfer laser is phase locked via the interferometer signal to 20 MHz below the frequency doubled 1272 nm transfer laser at . Both phase locks are referenced to a commercial atomic clock. For (which can readily be achieved by locking the 1272 nm transfer laser to the appropriate comb line), the beat between the 1908 nm transfer laser and the OFC corresponds to With this measurement technique, we measure an offset frequency of (see Fig. 4). The signal-to-noise ratio (SNR) of of in a 100 kHz resolution bandwidth (RBW), as well as a SNR of in a 100 Hz RBW of , is sufficient for accurate, i.e., real-time counting of the cycles of the two radio frequency beats (and making the use of, e.g., tracking oscillators unnecessary). This, along with the knowledge of the comb teeth number, provides the ability to directly count the cycles of the pump laser as well as the other comb teeth.It should be noted that the determination of the comb line index with a mode-locked laser frequency comb with repetition rates can be challenging. Here the much denser comb spectrum does not normally permit resolving the comb line with a grating-based OSA, and moreover, the sensitivity to drifts in repetition rate is significantly higher, making it necessary to fully phase stabilize the comb in order to determine the comb line index. In contrast, the comb line index can be obtained in the present work with a low-resolution wavemeter without requiring locking of either or . In this way, the comb line number of the pump laser was determined to be . Self-referencing, along with knowledge of the comb line numbers, implies that the absolute laser frequency of the pump laser (, as well as any other comb teeth) can be directly determined via the repetition rate beatnote and the recorded beatnote , since the absolute laser frequency is related to the two quantities via
establishing the ability to count the cycles of light via the crystalline microresonator.3. CONCLUSIONS
Our results constitute the first phase-coherent link from the microwave to the optical domain using a microresonator, by demonstrating measurement of the carrier envelope frequency of a temporal dissipative soliton in a microresonator. These results demonstrate that microresonator-based frequency combs can provide accurate and precise absolute optical frequency standards for a wide range of applications in optical frequency metrology, optical atomic clocks, optical frequency synthesis, or low-noise microwave generation by frequency division. In terms of soliton dynamics, this demonstration constitutes the first measurement of the carrier envelope offset frequency of a temporal dissipative Kerr cavity soliton, to our knowledge. This opens a new route to studying nonlinear dynamics of solitons. A further important step in the future will be to phase lock both and to an external radio frequency reference, and thereby achieve a phase-stabilized self-referenced microresonator frequency comb. This can be achieved by controlling both and independently via changing the pump frequency detuning along with either the pump power [45] or by applying a stress to the resonator with a piezoelectric crystal [50]. The crystalline-microresonator-based microwave-to-optical link can be made more compact with almost all optical components being fiber optic based, and aside from the optical microresonator, the non-fiber-based components (filters and sum-frequency generation stages) can be replaced with fiber-based components in the future. Finally, the external broadening stage itself, required in the present case, can, in suitably dispersion engineered optical microresonators, be avoided when making use of soliton-induced higher order spectral broadening [42,47,58]. Eventually, this provides a path to counting the cycles of light using chip-scale microresonators.
Funding
Defense Advanced Research Projects Agency (DARPA), PULSE program (W31PQ-13-1-0016); European Space Agency (ESA); Eurostars Program; Marie Curie IEF; Marie Curie IIF; Swiss National Science Foundation.
REFERENCES
1. D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, and S. T. Cundiff, “Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis,” Science 288, 635–639 (2000). [CrossRef]
2. S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, T. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84, 5102–5105 (2000). [CrossRef]
3. J. Ye, H. Schnatz, and L. Hollberg, “Optical frequency combs: from frequency metrology to optical phase control,” IEEE J. Sel. Top. Quantum Electron. 9, 1041–1058 (2003). [CrossRef]
4. K. M. Evenson, J. S. Wells, F. R. Petersen, B. L. Danielson, and G. W. Day, “Accurate frequencies of molecular transitions used in laser stabilization: the 3.39-μm transition in CH4 and the 9.33- and 10.18-μm transitions in CO2,” Appl. Phys. Lett. 22, 192 (1973). [CrossRef]
5. J. Reichert, R. Holzwarth, T. Udem, and T. W. Hänsch, “Measuring the frequency of light with mode-locked lasers,” Opt. Commun. 172, 59–68 (1999). [CrossRef]
6. H. R. Telle, G. Steinmeyer, A. E. Dunlop, J. Stenger, D. H. Sutter, and U. Keller, “Carrier-envelope offset phase control: a novel concept for absolute optical frequency measurement and ultrashort pulse generation,” Appl. Phys. B 69, 327–332 (1999). [CrossRef]
7. U. Morgner, R. Ell, G. Metzler, T. Schibli, F. Kärtner, J. Fujimoto, H. Haus, and E. Ippen, “Nonlinear optics with phase-controlled pulses in the sub-two-cycle regime,” Phys. Rev. Lett. 86, 5462–5465 (2001). [CrossRef]
8. S. Diddams, A. Bartels, T. Ramond, C. Oates, S. Bize, E. Curtis, J. Bergquist, and L. Hollberg, “Design and control of femtosecond lasers for optical clocks and the synthesis of low-noise optical and microwave signals,” IEEE J. Sel. Top. Quantum Electron. 9, 1072–1080 (2003). [CrossRef]
9. S. A. Diddams, T. Udem, J. C. Bergquist, E. A. Curtis, R. E. Drullinger, L. Hollberg, W. M. Itano, W. D. Lee, C. W. Oates, K. R. Vogel, and D. J. Wineland, “An optical clock based on a single trapped 199Hg+ ion,” Science 293, 825–828 (2001). [CrossRef]
10. T. Rosenband, D. B. Hume, P. O. Schmidt, C. W. Chou, A. Brusch, L. Lorini, W. H. Oskay, R. E. Drullinger, T. M. Fortier, J. E. Stalnaker, S. A. Diddams, W. C. Swann, N. R. Newbury, W. M. Itano, D. J. Wineland, and J. C. Bergquist, “Frequency ratio of Al+ and Hg+ single-ion optical clocks; metrology at the 17th decimal place,” Science 319, 1808–1812 (2008). [CrossRef]
11. N. Hinkley, J. A. Sherman, N. B. Phillips, M. Schioppo, N. D. Lemke, K. Beloy, M. Pizzocaro, C. W. Oates, and A. D. Ludlow, “An atomic clock with 10−18 instability,” Science 341, 1215–1218 (2013). [CrossRef]
12. A. Bartels, D. Heinecke, and S. A. Diddams, “10-GHz self-referenced optical frequency comb,” Science 326, 681 (2009). [CrossRef]
13. T. Steinmetz, T. Wilken, C. Araujo-Hauck, R. Holzwarth, T. W. Hänsch, L. Pasquini, A. Manescau, S. D’Odorico, M. T. Murphy, T. Kenitscher, W. Schmidt, and T. Udem, “Laser frequency combs for astronomical observations,” Science 321, 1335–1337 (2008). [CrossRef]
14. T. Ideguchi, S. Holzner, B. Bernhardt, G. Guelachvili, N. Picque, and T. W. Hansch, “Coherent Raman spectro-imaging with laser frequency combs,” Nature 502, 355–358 (2013). [CrossRef]
15. J. Pfeifle, V. Brasch, M. Lauermann, Y. Yu, D. Wegner, T. Herr, K. Hartinger, P. Schindler, J. Li, D. Hillerkuss, R. Schmogrow, C. Weimann, R. Holzwarth, W. Freude, J. Leuthold, T. J. Kippenberg, and C. Koos, “Coherent terabit communications with microresonator Kerr frequency combs,” Nat. Photonics 8, 375–380 (2014). [CrossRef]
16. P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, and T. J. Kippenberg, “Optical frequency comb generation from a monolithic microresonator,” Nature 450, 1214–1217 (2007). [CrossRef]
17. T. J. Kippenberg, R. Holzwarth, and S. A. Diddams, “Microresonator-based optical frequency combs,” Science 332, 555–559 (2011). [CrossRef]
18. T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Kerr-nonlinearity optical parametric oscillation in an ultrahigh-Q toroid microcavity,” Phys. Rev. Lett. 93, 083904 (2004). [CrossRef]
19. A. A. Savchenkov, A. B. Matsko, D. Strekalov, M. Mohageg, V. S. Ilchenko, and L. Maleki, “Low threshold optical oscillations in a whispering gallery mode CaF2 resonator,” Phys. Rev. Lett. 93, 243905 (2004). [CrossRef]
20. A. A. Savchenkov, A. B. Matsko, W. Liang, V. S. Ilchenko, D. Seidel, and L. Maleki, “Kerr combs with selectable central frequency,” Nat. Photonics 5, 293–296 (2011). [CrossRef]
21. C. Y. Wang, T. Herr, P. Del’Haye, A. Schliesser, J. Hofer, R. Holzwarth, T. W. Hänsch, N. Picqué, and T. J. Kippenberg, “Mid-infrared optical frequency combs at 2.5 μm based on crystalline microresonators,” Nat. Commun. 4, 1345 (2013). [CrossRef]
22. A. G. Griffith, R. K. Lau, J. Cardenas, Y. Okawachi, A. Mohanty, R. Fain, Y. H. D. Lee, M. Yu, C. T. Phare, C. B. Poitras, A. L. Gaeta, and M. Lipson, “Silicon-chip mid-infrared frequency comb generation,” Nat. Commun. 6, 6299 (2015). [CrossRef]
23. A. A. Savchenkov, A. B. Matsko, V. S. Ilchenko, I. Solomatine, D. Seidel, and L. Maleki, “Tunable optical frequency comb with a crystalline whispering gallery mode resonator,” Phys. Rev. Lett. 101, 093902 (2008). [CrossRef]
24. M. A. Foster, J. S. Levy, O. Kuzucu, K. Saha, M. Lipson, and A. L. Gaeta, “Silicon-based monolithic optical frequency comb source,” Opt. Express 19, 14233–14239 (2011). [CrossRef]
25. J. S. Levy, A. Gondarenko, M. A. Foster, A. C. Turner-Foster, A. L. Gaeta, and M. Lipson, “CMOS-compatible multiple-wavelength oscillator for on-chip optical interconnects,” Nat. Photonics 4, 37–40 (2009). [CrossRef]
26. H. Jung, C. Xiong, K. Y. Fong, X. Zhang, and H. X. Tang, “Optical frequency comb generation from aluminum nitride microring resonator,” Opt. Lett. 38, 2810–2813 (2013). [CrossRef]
27. M. Peccianti, A. Pasquazi, Y. Park, B. E. Little, S. T. Chu, D. J. Moss, and R. Morandotti, “Demonstration of a stable ultrafast laser based on a nonlinear microcavity,” Nat. Commun. 3, 765 (2012). [CrossRef]
28. L. Razzari, D. Duchesne, M. Ferrera, R. Morandotti, S. Chu, B. E. Little, and D. J. Moss, “CMOS-compatible integrated optical hyper-parametric oscillator,” Nat. Photonics 4, 41–45 (2009). [CrossRef]
29. B. J. M. Hausmann, I. Bulu, V. Venkataraman, P. Deotare, and M. Lončar, “Diamond nonlinear photonics,” Nat. Photonics 8, 369–374 (2014). [CrossRef]
30. S. B. Papp, K. Beha, P. Del’Haye, F. Quinlan, H. Lee, K. Vahala, and S. A. Diddams, “Microresonator frequency comb optical clock,” Optica 1, 10–14 (2014). [CrossRef]
31. A. A. Savchenkov, D. Eliyahu, W. Liang, V. S. Ilchenko, J. Byrd, A. B. Matsko, D. Seidel, and L. Maleki, “Stabilization of a Kerr frequency comb oscillator,” Opt. Lett. 38, 2636–2638 (2013). [CrossRef]
32. F. Ferdous, H. Miao, D. E. Leaird, K. Srinivasan, J. Wang, L. Chen, L. T. Varghese, and A. M. Weiner, “Spectral line-by-line pulse shaping of on-chip microresonator frequency combs,” Nat. Photonics 5, 770–776 (2011). [CrossRef]
33. K. Saha, Y. Okawachi, B. Shim, J. S. Levy, M. A. Foster, R. Salem, A. R. Johnson, M. R. E. Lamont, M. Lipson, and A. L. Gaeta, “Modelocking and femtosecond pulse generation in chip-based frequency combs,” Opt. Express 21, 1335–1343 (2013). [CrossRef]
34. T. Herr, V. Brasch, J. D. Jost, C. Y. Wang, N. M. Kondratiev, M. L. Gorodetsky, and T. J. Kippenberg, “Temporal solitons in optical microresonators,” Nat. Photonics 8, 145–152 (2013). [CrossRef]
35. T. Herr, K. Hartinger, J. Riemensberger, C. Y. Wang, E. Gavartin, R. Holzwarth, M. L. Gorodetsky, and T. J. Kippenberg, “Universal formation dynamics and noise of Kerr-frequency combs in microresonators,” Nat. Photonics 6, 480–487 (2012). [CrossRef]
36. P. Del’Haye, K. Beha, S. B. Papp, and S. A. Diddams, “Self-injection locking and phase-locked states in microresonator-based optical frequency combs,” Phys. Rev. Lett. 112, 043905 (2014). [CrossRef]
37. J. Li, H. Lee, T. Chen, and K. J. Vahala, “Low-pump-power, low-phase-noise, and microwave to millimeter-wave repetition rate operation in microcombs,” Phys. Rev. Lett. 109, 233901 (2012). [CrossRef]
38. S. Wabnitz, “Suppression of interactions in a phase-locked soliton optical memory,” Opt. Lett. 18, 601–603 (1993). [CrossRef]
39. F. Leo, S. Coen, P. Kockaert, S.-P. P. Gorza, P. Emplit, and M. Haelterman, “Temporal cavity solitons in one-dimensional Kerr media as bits in an all-optical buffer,” Nat. Photonics 4, 471–476 (2010). [CrossRef]
40. P. Grelu and N. Akhmediev, “Dissipative solitons for mode-locked lasers,” Nat. Photonics 6, 84–92 (2012). [CrossRef]
41. L. A. Lugiato and R. Lefever, “Spatial dissipative structures in passive optical systems,” Phys. Rev. Lett. 58, 2209–2211 (1987). [CrossRef]
42. S. Coen, H. Randle, T. Sylvestre, and M. Erkintalo, “Modeling of octave-spanning Kerr frequency combs using a generalized mean-field Lugiato-Lefever model,” Opt. Lett. 38, 37–39 (2013). [CrossRef]
43. M. R. E. Lamont, Y. Okawachi, and A. L. Gaeta, “Route to stabilized ultrabroadband microresonator-based frequency combs,” Opt. Lett. 38, 3478–3481 (2013). [CrossRef]
44. Y. K. Chembo and N. Yu, “Modal expansion approach to optical-frequency-comb generation with monolithic whispering-gallery-mode resonators,” Phys. Rev. A 82, 033801 (2010). [CrossRef]
45. P. Del’Haye, O. Arcizet, A. Schliesser, R. Holzwarth, and T. J. Kippenberg, “Full stabilization of a microresonator-based optical frequency comb,” Phys. Rev. Lett. 101, 053903 (2008). [CrossRef]
46. P. Del’Haye, T. Herr, E. Gavartin, M. L. Gorodetsky, R. Holzwarth, and T. J. Kippenberg, “Octave spanning tunable frequency comb from a microresonator,” Phys. Rev. Lett. 107, 063901 (2011). [CrossRef]
47. Y. Okawachi, K. Saha, J. S. Levy, Y. H. Wen, M. Lipson, and A. L. Gaeta, “Octave-spanning frequency comb generation in a silicon nitride chip,” Opt. Lett. 36, 3398–3400 (2011). [CrossRef]
48. N. N. Akhmediev, V. M. Eleonskii, and N. E. Kulagin, “Exact first-order solutions of the nonlinear Schrödinger equation,” Theor. Math. Phys. 72, 809–818 (1987). [CrossRef]
49. N. Akhmediev and A. Ankiewicz, Dissipative Solitons: From Optics to Biology and Medicine (Springer, 2008).
50. S. B. Papp, P. Del’Haye, and S. A. Diddams, “Mechanical control of a microrod-resonator optical frequency comb,” Phys. Rev. X 3, 031003 (2013).
51. V. B. Braginsky, M. L. Gorodetsky, and V. S. Ilchenko, “Quality-factor and nonlinear properties of optical whispering-gallery modes,” Phys. Lett. A 137, 393–397 (1989). [CrossRef]
52. I. Grudinin, A. B. Matsko, A. A. Savchenkov, D. Strekalov, V. S. Ilchenko, and L. Maleki, “Ultra high Q crystalline microcavities,” Opt. Commun. 265, 33–38 (2006). [CrossRef]
53. J. Hofer, A. Schliesser, and T. J. Kippenberg, “Cavity optomechanics with ultrahigh-Q crystalline microresonators,” Phys. Rev. A 82, 031804 (2010). [CrossRef]
54. V. S. Ilchenko, A. A. Savchenkov, A. B. Matsko, and L. Maleki, “Nonlinear optics and crystalline whispering gallery mode cavities,” Phys. Rev. Lett. 92, 043903 (2004). [CrossRef]
55. T. Herr, V. Brasch, J. D. Jost, I. Mirgorodskiy, G. Lihachev, M. L. Gorodetsky, and T. J. Kippenberg, “Mode spectrum and temporal soliton formation in optical microresonators,” Phys. Rev. Lett. 113, 123901 (2014). [CrossRef]
56. S. M. Spillane, T. J. Kippenberg, O. J. Painter, and K. J. Vahala, “Ideality in a fiber-taper-coupled microresonator system for application to cavity quantum electrodynamics,” Phys. Rev. Lett. 91, 043902 (2003). [CrossRef]
57. J. M. Dudley and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78, 1135–1184 (2006). [CrossRef]
58. V. Brasch, T. Herr, M. Geiselmann, G. Lihachev, M. H. P. Pfeiffer, M. L. Gorodetsky, and T. J. Kippenberg, “Photonic chip based optical frequency comb using soliton induced Cherenkov radiation,” arXiv:1410.8598 (2014).