Publication

2018
Ki Youl Yang, Dong Yoon Oh, Seung Hoon Lee, Qi-Fan Yang, Xu Yi, Boqiang Shen, Heming Wang, and Kerry Vahala. 2018. “Bridging ultrahigh-Q devices and photonic circuits.” Nature Photonics, 12, 5, Pp. 297–302. Publisher's VersionAbstract
Optical microresonators are essential to a broad range of technologies and scientific disciplines. However, many of their applications rely on discrete devices to attain challenging combinations of ultra-low-loss performance (ultrahigh Q) and resonator design requirements. This prevents access to scalable fabrication methods for photonic integration and lithographic feature control. Indeed, finding a microfabrication bridge that connects ultrahigh-Q device functions with photonic circuits is a priority of the microcavity field. Here, an integrated resonator having a record Q factor over 200 million is presented. Its ultra-low-loss and flexible cavity design brings performance to integrated systems that has been the exclusive domain of discrete silica and crystalline microcavity devices. Two distinctly different devices are demonstrated: soliton sources with electronic repetition rates and high-coherence/low-threshold Brillouin lasers. This multi-device capability and performance from a single integrated cavity platform represents a critical advance for future photonic circuits and systems.Using silicon nitride waveguides processed by plasma-enhanced chemical vapour deposition, full integration of ultrahigh-Q resonators with other photonic devices is now possible, representing a critical advance for future photonic circuits and systems.
Xu Yi, Qi-Fan Yang, Ki Youl Yang, and Kerry Vahala. 2018. “Imaging soliton dynamics in optical microcavities.” Nature Communications, 9, 1, Pp. 3565. Publisher's VersionAbstract
Solitons are self-sustained wavepackets that occur in many physical systems. Their recent demonstration in optical microresonators has provided a new platform for the study of nonlinear optical physics with practical implications for miniaturization of time standards, spectroscopy tools, and frequency metrology systems. However, despite its importance to the understanding of soliton physics, as well as development of new applications, imaging the rich dynamical behavior of solitons in microcavities has not been possible. These phenomena require a difficult combination of high-temporal-resolution and long-record-length in order to capture the evolving trajectories of closely spaced microcavity solitons. Here, an imaging method is demonstrated that visualizes soliton motion with sub-picosecond resolution over arbitrary time spans. A wide range of complex soliton transient behavior are characterized in the temporal or spectral domain, including soliton formation, collisions, spectral breathing, and soliton decay. This method can serve as a visualization tool for developing new soliton applications and understanding complex soliton physics in microcavities.
Nicolas Volet, Xu Yi, Qi-Fan Yang, Eric J Stanton, Paul A Morton, Ki Youl Yang, Kerry J Vahala, and John E Bowers. 2018. “Micro-resonator soliton generated directly with a diode laser.” Laser and Photonics Reviews, 12, 5, Pp. 1700307. Publisher's Version
Daryl T Spencer, Tara Drake, Travis C Briles, Jordan Stone, Laura C Sinclair, Connor Fredrick, Qing Li, Daron Westly, Robert B Ilic, Aaron Bluestone, Nicolas Volet, Tin Komljenovic, Lin Chang, Seung Hoon Lee, Dong Yoon Oh, Myoung-Gyun Suh, Ki Youl Yang, Martin HP Pfeiffer, Tobias J Kippenberg, Erik Norberg, Luke Theogarajan, Kerry Vahala, Nathan R Newbury, Kartik Srinivasan, John E Bowers, Scott A Diddams, and Scott B Papp. 2018. “An optical-frequency synthesizer using integrated photonics.” Nature, 557, 7703, Pp. 81–85. Publisher's Version
Xinbai Li, Boqiang Shen, Heming Wang, Ki Youl Yang, Xu Yi, Qi-Fan Yang, Zhiping Zhou, and Kerry Vahala. 2018. “Universal isocontours for dissipative Kerr solitons.” Optics Letters, 43, 11, Pp. 2567–2570. Publisher's VersionAbstract
Dissipative Kerr solitons can be generated within an existence region defined on a space of normalized pumping power versus cavity-pump detuning frequency. The contours of constant soliton power and constant pulse width in this region are studied through measurement and simulation. Such isocontours impart structure to the existence region and improve understanding of soliton locking and stabilization methods. As part of the study, dimensionless, closed-form expressions for soliton power and pulse width are developed (including Raman contributions). They provide isocontours in close agreement with those from the full simulation, and, as universal expressions, can simplify the estimation of soliton properties across a wide range of systems.
2017
Dong Yoon Oh, Ki Youl Yang, Connor Fredrick, Gabriel Ycas, Scott A Diddams, and Kerry J Vahala. 2017. “Coherent ultra-violet to near-infrared generation in silica ridge waveguides.” Nature Communications, 8, 1, Pp. 13922. Publisher's VersionAbstract
Short duration, intense pulses of light can experience dramatic spectral broadening when propagating through lengths of optical fibre. This continuum generation process is caused by a combination of nonlinear optical effects including the formation of dispersive waves. Optical analogues of Cherenkov radiation, these waves allow a pulse to radiate power into a distant spectral region. In this work, efficient and coherent dispersive wave generation of visible to ultraviolet light is demonstrated in silica waveguides on a silicon chip. Unlike fibre broadeners, the arrays provide a wide range of emission wavelength choices on a single, compact chip. This new capability is used to simplify offset frequency measurements of a mode-locked frequency comb. The arrays can also enable mode-locked lasers to attain unprecedented tunable spectral reach for spectroscopy, bioimaging, tomography and metrology.
Qi-Fan Yang, Xu Yi, Ki Youl Yang, and Kerry Vahala. 2017. “Counter-propagating solitons in microresonators.” Nature Photonics, 11, 9, Pp. 560–564. Publisher's VersionAbstract
Counter-propagating solitons are generated in microresonator systems, producing dual-soliton frequency-comb streams with different repetition rates but high relative coherence useful for spectroscopy and laser ranging systems.
Yu-Hung Lai, Ki Youl Yang, Myoung-Gyun Suh, and Kerry J. Vahala. 2017. “Fiber taper characterization by optical backscattering reflectometry.” Optics Express, 25, 19, Pp. 22312–22327. Publisher's VersionAbstract
Fiber tapers provide a way to rapidly measure the spectra of many types of optical microcavities. Proper fabrication of the taper ensures that its width varies sufficiently slowly (adiabatically) along the length of the taper so as to maintain single spatial mode propagation. This is usually accomplished by monitoring the spectral transmission through the taper. In addition to this characterization method it is also helpful to know the taper width versus length. By developing a model of optical backscattering within the fiber taper, it is possible to use backscatter measurements to characterize the taper width versus length. The model uses the concept of a local taper numerical aperture to accurately account for varying backscatter collection along the length of the taper. In addition to taper profile information, the backscatter reflectometry method delineates locations along the taper where fluctuations in fiber core refractive index, cladding refractive index, and taper surface roughness each provide the dominant source of backscattering. Rayleigh backscattering coefficients are also extracted by fitting the data with the model and are consistent with the fiber manufacturer&\#x02019;s datasheet. The optical backscattering reflectometer is also used to observe defects resulting from microcracks and surface contamination. All of this information can be obtained before the taper is removed from its fabrication apparatus. The backscattering method should also be prove useful for characterization of nanofibers.
Xu Yi, Qi-Fan Yang, Xueyue Zhang, Ki Youl Yang, Xinbai Li, and Kerry Vahala. 2017. “Single-mode dispersive waves and soliton microcomb dynamics.” Nature Communications, 8, 1, Pp. 14869. Publisher's VersionAbstract
Dissipative Kerr solitons are self-sustaining optical wavepackets in resonators. They use the Kerr nonlinearity to both compensate dispersion and offset optical loss. Besides providing insights into nonlinear resonator physics, they can be applied in frequency metrology, precision clocks, and spectroscopy. Like other optical solitons, the dissipative Kerr soliton can radiate power as a dispersive wave through a process that is the optical analogue of Cherenkov radiation. Dispersive waves typically consist of an ensemble of optical modes. Here, a limiting case is studied in which the dispersive wave is concentrated into a single cavity mode. In this limit, its interaction with the soliton induces hysteresis behaviour in the soliton's spectral and temporal properties. Also, an operating point of enhanced repetition-rate stability occurs through balance of dispersive-wave recoil and Raman-induced soliton-self-frequency shift. The single-mode dispersive wave can therefore provide quiet states of soliton comb operation useful in many applications.
Qi-Fan Yang, Xu Yi, Ki Youl Yang, and Kerry Vahala. 2017. “Stokes solitons in optical microcavities.” Nature Physics, 13, 1, Pp. 53–57. Publisher's Version
Seung Hoon Lee, Dong Yoon Oh, Qi-Fan Yang, Boqiang Shen, Heming Wang, Ki Youl Yang, Yu-Hung Lai, Xu Yi, Xinbai Li, and Kerry Vahala. 2017. “Towards visible soliton microcomb generation.” Nature Communications, 8, Pp. 1295. Publisher's Version
2016
Myoung-Gyun Suh, Qi-Fan Yang, Ki Youl Yang, Xu Yi, and Kerry J. Vahala. 2016. “Microresonator soliton dual-comb spectroscopy.” Science, 354, 6312, Pp. 600-603. Publisher's VersionAbstract
Dual-comb spectroscopy is a powerful technique that uses the interference of two closely related combs to map spectroscopic features directly into a frequency domain that can be read by electronics. Suh et al. developed a dual-comb spectroscopy approach using combs produced by silica microresonators fabricated on a silicon chip. Perhaps high-resolution spectroscopy will soon be shrunk to the chip scale, doing away with the need for bulky spectrometers. Science, this issue p. 600 Dual-comb spectroscopy is demonstrated using a pair of silica microresonators. Measurement of optical and vibrational spectra with high resolution provides a way to identify chemical species in cluttered environments and is of general importance in many fields. Dual-comb spectroscopy has emerged as a powerful approach for acquiring nearly instantaneous Raman and optical spectra with unprecedented resolution. Spectra are generated directly in the electrical domain, without the need for bulky mechanical spectrometers. We demonstrate a miniature soliton-based dual-comb system that can potentially transfer the approach to a chip platform. These devices achieve high-coherence pulsed mode locking. They also feature broad, reproducible spectral envelopes, an essential feature for dual-comb spectroscopy. Our work shows the potential for integrated spectroscopy with high signal-to-noise ratios and fast acquisition rates.
Qi-Fan Yang, Xu Yi, Ki Youl Yang, and Kerry Vahala. 2016. “Spatial-mode-interaction-induced dispersive waves and their active tuning in microresonators.” Optica, 3, 10, Pp. 1132–1135. Publisher's VersionAbstract
The nonlinear propagation of optical pulses in dielectric waveguides and resonators induces a wide range of remarkable interactions. One example is dispersive-wave generation, the optical analog of Cherenkov radiation. These waves play an essential role in the fiber-optic spectral broadeners used in spectroscopy and metrology. Dispersive waves form when a soliton pulse begins to radiate power as a result of higher-order dispersion. Recently, dispersive-wave generation in microcavities has been reported by phase matching the waves to dissipative Kerr solitons. Here, it is shown that spatial mode interactions within a microcavity can be used to induce dispersive waves. The soliton self-frequency shift is also shown to enable fine tuning control of the dispersive-wave frequency. Both this mechanism and spatial mode interactions allow spectral control of these important waves in microresonators.
Xu Yi, Qi-Fan Yang, Ki Youl Yang, and Kerry Vahala. 2016. “Active capture and stabilization of temporal solitons in microresonators.” Optics Letters, 41, 9, Pp. 2037–2040. Publisher's VersionAbstract
Soliton mode locking and femtosecond pulse generation have recently been demonstrated in high-Q optical microcavities and provide a new way to miniaturize frequency comb systems, as well as create integrated comb systems on a chip. However, triggering the mode-locking process is complicated by a well-known thermal hysteresis that can destabilize the solitons. Moreover, on a longer time scale, thermal drifting of the cavity resonant frequency relative to the pumping frequency causes loss of mode locking. In this Letter, an active feedback method is used both to capture specific soliton states and to stabilize the states indefinitely. The capture and stabilization method provides a reliable way to overcome thermal effects during soliton formation and to excite a desired number of circulating cavity solitons. It is also used to demonstrate a low pumping power of 22 mW for generation of microwave-repetition-rate solitons on a chip.
Ki Youl Yang, Katja Beha, Daniel C Cole, Xu Yi, Pascal Del'Haye, Hansuek Lee, Jiang Li, Dong Yoon Oh, Scott A Diddams, Scott B Papp, and Kerry J Vahala. 2016. “Broadband dispersion-engineered microresonator on a chip.” Nature Photonics, 10, 5, Pp. 316–320. Publisher's VersionAbstract
The control of dispersion in fibre optical waveguides is of critical importance to optical fibre communications systems and more recently for continuum generation from the ultraviolet to the mid-infrared. The wavelength at which the group velocity dispersion crosses zero can be set by varying the fibre core diameter or index step. Moreover, sophisticated methods to manipulate higher-order dispersion so as to shape and even flatten the dispersion over wide bandwidths are possible using multi-cladding fibres. Here we introduce design and fabrication techniques that allow analogous dispersion control in chip-integrated optical microresonators, and thereby demonstrate higher-order, wide-bandwidth dispersion control over an octave of spectrum. Importantly, the fabrication method we employ for dispersion control simultaneously permits optical Q factors above 100 million, which is critical for the efficient operation of nonlinear optical oscillators. Dispersion control in high-Q systems has become of great importance in recent years with increased interest in chip-integrable optical frequency combs.
Pascal Del'Haye, Aurélien Coillet, Tara Fortier, Katja Beha, Daniel C Cole, Ki Youl Yang, Hansuek Lee, Kerry J Vahala, Scott B Papp, and Scott A Diddams. 2016. “Phase-coherent microwave-to-optical link with a self-referenced microcomb.” Nature Photonics, 10, 8, Pp. 516–520. Publisher's Version
Xu Yi, Qi-Fan Yang, Ki Youl Yang, and Kerry Vahala. 2016. “Theory and measurement of the soliton self-frequency shift and efficiency in optical microcavities.” Optics Letters, 41, 15, Pp. 3419–3422. Publisher's VersionAbstract
Dissipative Kerr cavity solitons experience a so-called self-frequency shift (SFS) as a result of Raman interactions. The frequency shift has been observed in several microcavity systems. The Raman process has also been shown numerically to influence the soliton pumping efficiency. Here, a perturbed Lagrangian approach is used to derive simple analytical expressions for the SFS and the soliton efficiency. The predicted dependences of these quantities on soliton pulse width are compared with measurements in a high-Q silica microcavity. The Raman time constant in silica is also inferred. Analytical expressions for the Raman SFS and soliton efficiency greatly simplify the prediction of soliton behavior over a wide range of microcavity platforms.
2015
Xu Yi, Qi-Fan Yang, Ki Youl Yang, Myoung-Gyun Suh, and Kerry Vahala. 2015. “Soliton frequency comb at microwave rates in a high-Q silica microresonator.” Optica, 2, 12, Pp. 1078–1085. Publisher's VersionAbstract
Frequency combs are having a broad impact on science and technology because they provide a way to coherently link radio/microwave-rate electrical signals with optical-rate signals derived from lasers and atomic transitions. Integrating these systems on a photonic chip would revolutionize instrumentation, time keeping, spectroscopy, navigation, and potentially create new mass-market applications. A key element of such a system-on-a-chip will be a mode-locked comb that can be self-referenced. The recent demonstration of soliton mode locking in crystalline and silicon nitride microresonators has provided a way to both mode lock and generate femtosecond time-scale pulses. Here, soliton mode locking is demonstrated in high-Q silica resonators. The resonators produce low-phase-noise soliton pulse trains at readily detectable pulse rates&\#x2014;two essential properties for the operation of frequency combs. A method for the long-term stabilization of the solitons is also demonstrated, and is used to test the theoretical dependence of the comb power, efficiency, and soliton existence power on the pulse width. The influence of the Raman process on the soliton existence power and efficiency is also observed. The resonators are microfabricated on silicon chips and feature reproducible modal properties required for soliton formation. A low-noise and detectable pulse rate soliton frequency comb on a chip is a significant step towards a fully integrated frequency comb system.
2014
Dong Yoon Oh, David Sell, Hansuek Lee, Ki Youl Yang, Scott A. Diddams, and Kerry J. Vahala. 2014. “Supercontinuum generation in an on-chip silica waveguide.” Optics Letters, 39, 4, Pp. 1046–1048. Publisher's VersionAbstract
Supercontinuum generation is demonstrated in an on-chip silica spiral waveguide by launching 180&\#xA0;fs pulses from an optical parametric oscillator at the center wavelength of 1330&\#xA0;nm. With a coupled pulse energy of 2.17&\#xA0;nJ, the broadest spectrum in the fundamental TM mode extends from 936 to 1888&\#xA0;nm (162&\#xA0;THz) at &\#x2212;50&\#x2009;&\#x2009;dB from peak. There is a good agreement between the measured spectrum and a simulation using a generalized nonlinear Schr&\#xF6;dinger equation.
2012
Hansuek Lee, Tong Chen, Jiang Li, Ki Youl Yang, Seokmin Jeon, Oskar Painter, and Kerry J Vahala. 2012. “Chemically etched ultrahigh-Q wedge-resonator on a silicon chip.” Nat. Photonics, 6, 6, Pp. 369–373. Publisher's VersionAbstract
Ultrahigh-Q optical resonators are being studied across a wide range of fields, including quantum information, nonlinear optics, cavity optomechanics and telecommunications. Here, we demonstrate a new resonator with a record Q-factor of 875 million for on-chip devices. The fabrication of our device avoids the requirement for a specialized processing step, which in microtoroid resonators8 has made it difficult to control their size and achieve millimetre- and centimetre-scale diameters. Attaining these sizes is important in applications such as microcombs and potentially also in rotation sensing. As an application of size control, stimulated Brillouin lasers incorporating our device are demonstrated. The resonators not only set a new benchmark for the Q-factor on a chip, but also provide, for the first time, full compatibility of this important device class with conventional semiconductor processing. This feature will greatly expand the range of possible `system on a chip' functions enabled by ultrahigh-Q devices.

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