Kerry J. Vahala

California Institute of Technology, Pasadena, CA, United States

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Publications (382)1250.25 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Optical frequency division by using frequency combs has revolutionized time keeping and the generation of stable microwave signals. We demonstrate optical frequency division and microwave generation by using a tunable electrical oscillator to create dual combs through phase modulation of two optical signals that have a stable difference frequency. Phase-locked control of the electrical oscillator by means of optical frequency division produces stable microwaves. Our approach transposes the oscillator and frequency reference of a conventional microwave frequency synthesizer. In this way, the oscillator experiences large phase noise reduction relative to the frequency reference. The electro-optical approach additionally relaxes the need for highly linear photodetection of the comb mode spacing. As well as simplicity, the technique is also tunable and scalable to higher division ratios.
    Science (New York, N.Y.). 07/2014; 345(6194):309-13.
  • Jiang Li, Scott Diddams, Kerry J Vahala
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    ABSTRACT: As thermo-optic locking is widely used to establish a stable frequency detuning between an external laser and a high Q microcavity, it is important to understand how this method affects microcavity temperature and frequency fluctuations. A theoretical analysis of the laser-microcavity frequency fluctuations is presented and used to find the spectral dependence of the suppression of laser-microcavity, relative frequency noise caused by thermo-optic locking. The response function is that of a high-pass filter with a bandwidth and low-frequency suppression that increase with input power. The results are verified using an external-cavity diode laser and a silica disk resonator. The locking of relative frequency fluctuations causes temperature fluctuations within the microcavity that transfer pump frequency noise onto the microcavity modes over the thermal locking bandwidth. This transfer is verified experimentally. These results are important to investigations of noise properties in many nonlinear microcavity experiments in which low-frequency, optical-pump frequency noise must be considered.
    Optics Express 06/2014; 22(12):14559-14567. · 3.55 Impact Factor
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    ABSTRACT: We demonstrate an optical clock based on stabilization of a microcomb to rubidium optical transitions. The clock’s output is the 33 GHz microcomb line spacing, which is a coherent, integer sub-division of the rubidium reference.
    CLEO: Science and Innovations; 06/2014
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    ABSTRACT: A low-loss silica spiral waveguide is used for demonstrating on-chip supercontinuum generation. The broadest measured spectrum spans an octave (936 - 1888 nm) at -50 dB from peak when 2.17 nJ pulses are launched.
    CLEO: Science and Innovations; 06/2014
  • Tong Chen, Hansuek Lee, Kerry J Vahala
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    ABSTRACT: Whispering gallery delay lines have demonstrated record propagation length on a silicon chip and can provide a way to transfer certain applications of optical fiber to wafer-based systems. Their design and fabrication requires careful control of waveguide curvature and etching conditions to minimize connection losses between elements of the delay line. Moreover, loss characterization based on optical backscatter requires normalization to account for the impact of curvature on backscatter rate. In this paper we provide details on design of Archimedean whispering-gallery spiral waveguides, their coupling into cascaded structures, as well as optical loss characterization by optical backscatter reflectometry.
    Optics Express 03/2014; 22(5):5196-208. · 3.55 Impact Factor
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    ABSTRACT: Supercontinuum generation is demonstrated in an on-chip silica spiral waveguide by launching 180 fs pulses from an optical parametric oscillator at the center wavelength of 1330 nm. With a coupled pulse energy of 2.17 nJ, the broadest spectrum in the fundamental TM mode extends from 936 to 1888 nm (162 THz) at -50 dB from peak. There is a good agreement between the measured spectrum and a simulation using a generalized nonlinear Schrödinger equation.
    Optics Letters 02/2014; 39(4):1046-8. · 3.39 Impact Factor
  • Jiang Li, Hansuek Lee, Kerry J Vahala
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    ABSTRACT: We demonstrate narrow-linewidth-stimulated Brillouin lasers at 1064 nm from ultra-high-Q silica wedge disk resonators on silicon. Fundamental Schawlow-Townes frequency noise of the laser is on the order of 0.1 Hz<sup>2</sup>/Hz. The technical noise spectrum of the on-chip Brillouin laser is close to the thermodynamic noise limit of the resonator (thermorefractive noise) and is comparable to that of ultra-narrow-linewidth Nd:YAG lasers. The relative intensity noise of the Brillouin laser also is reduced by using an intensity-stabilized pump laser. Finally, low-noise microwave synthesis up to 32 GHz is demonstrated by heterodyne of first and third Brillouin Stokes lines from a single resonator.
    Optics Letters 01/2014; 39(2):287-90. · 3.39 Impact Factor
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    ABSTRACT: Frequency references are indispensable to radio, microwave and time keeping systems, with far reaching applications in navigation, communication, remote sensing and basic science. Over the past decade, there has been an optical revolution in time keeping and microwave generation that promises to ultimately impact all of these areas. Indeed, the most precise clocks and lowest noise microwave signals are now based on a laser with short-term stability derived from a reference cavity. In spite of the tremendous progress, these systems remain essentially laboratory devices and there is interest in their miniaturization, even towards on-chip systems. Here we describe a chip-based optical reference cavity that uses spatial averaging of thermorefractive noise to enhance resonator stability. Stabilized fibre lasers exhibit relative Allan deviation of 3.9 × 10(-13) at 400 μs averaging time and an effective linewidth <100 Hz by achieving over 26 dB of phase-noise reduction.
    Nature Communications 09/2013; 4:2468. · 10.02 Impact Factor
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    ABSTRACT: Optical-frequency combs enable measurement precision at the 20th digit, and accuracy entirely commensurate with their reference oscillator. A new direction in experiments is the creation of ultracompact frequency combs by way of nonlinear parametric optics in microresonators. We refer to these as microcombs, and here we report a silicon-chip-based microcomb optical clock that phase-coherently converts an optical-frequency reference to a microwave signal. A low-noise comb spectrum with 25 THz span is generated with a 2 mm diameter silica disk and broadening in nonlinear fiber. This spectrum is stabilized to rubidium frequency references separated by 3.5 THz by controlling two teeth 108 modes apart. The optical clocks output is the electronically countable 33 GHz microcomb line spacing, which features an absolute stability better than the rubidium transitions by the expected factor of 108. Our work demonstrates the comprehensive set of tools needed for interfacing microcombs to state-of-the-art optical clocks.
    09/2013;
  • Jiang Li, Hansuek Lee, Kerry J Vahala
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    ABSTRACT: Low-phase-noise microwave oscillators are important to a wide range of subjects, including communications, radar and metrology. Photonic-based microwave-wave sources now provide record, close-to-carrier phase-noise performance, and compact sources using microcavities are available commercially. Photonics-based solutions address a challenging scaling problem in electronics, increasing attenuation with frequency. A second scaling challenge, however, is to maintain low phase noise in reduced form factor and even integrated systems. On this second front, there has been remarkable progress in the area of microcavity devices with large storage time (high optical quality factor). Here we report generation of highly coherent microwaves using a chip-based device that derives stability from high optical quality factor. The device has a record low electronic white-phase-noise floor for a microcavity-based oscillator and is used as the optical, voltage-controlled oscillator in the first demonstration of a photonic-based, microwave frequency synthesizer. The synthesizer performance is comparable to mid-range commercial devices.
    Nature Communications 06/2013; 4:2097. · 10.02 Impact Factor
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    ABSTRACT: Using a wet etch process, optical resonators with quality factor as high as 875 million are demonstrated. These silicon-chip-based devices are fabricated without reflow, thereby expanding the range of integration opportunities and possible applications.
    CLEO: Science and Innovations; 06/2013
  • Jiang Li, Hansuek Lee, Kerry J. Vahala
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    ABSTRACT: An on-chip Brillouin microwave source is demonstrated. Phase noise of -106 dBc/Hz at 100kHz offset frequency (21.6 GHz carrier signal) is measured. A record low white phase noise floor for a microcavity-based source is demonstrated.
    CLEO: Science and Innovations; 06/2013
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    ABSTRACT: High-Q disk resonators are used to frequency stabilize two fiber lasers. The improved phase noise of the devices is measured by heterodyne detection and compared to theoretical limits set by thermo-refractive noise.
    CLEO: Science and Innovations; 06/2013
  • Tong Chen, Hansuek Lee, Kerry J. Vahala
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    ABSTRACT: The thermal expansion mismatch of thermal grown silica on a silicon wafer is well known to induce compressive stress upon cooling from the growth temperature to room temperature. In this Letter, we investigate how this stress impacts silica disk structures by comparison of measurements with both a finite element and an analytical model. The disk structures studied are also whispering gallery optical resonators, and proper control of stress is critical to obtain high-Q resonances. Based on our analysis, thicker oxide layers and proper control of undercut enable ultra-high-Q optical performance and mechanical stability.
    Applied Physics Letters 01/2013; 102(3). · 3.79 Impact Factor
  • K. Vahala, H. Lee, Jiang Li, Tong Chen
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    ABSTRACT: form only given. High-Q performance in microcavities relies upon use of low absorption dielectrics and creation of smooth dielectric interfaces. For chip-compatible devices, silica has the lowest intrinsic material loss [1]. Microtoroid resonators combine this low material loss with a reflow technique in which surface tension is used to smooth lithographic and etch-related blemishes [2]. At the same time, reflow smoothing makes it very challenging to fabricate larger diameter UHQ resonators and likewise to leverage the full range of integration tools and devices available on silicon. The devices reported here attain Q factors as high as 875 million using only conventional semiconductor processing methods [3]. Moreover, the best Q performance occurs for diameters greater than 500 microns, a size range that is difficult to access using microtoroids on account of the limitations of the reflow process. Fabrication control of the free-spectral range to 1:20,000 is also demonstrated, opening the possibility of precision repetition rate control in microcombs [4] or precision spectral placement of modes in certain nonlinear oscillators [3,5,6]. As an application, the use of these devices in microcombs [5] and also for microwave generation by stimulated Brillouin scattering will be described [6]. For the latter, a microwave synthesizer with performance comparable to mid-range commercial systems is demonstrated. Finally, the resonator micro-fabrication process also lends itself to creation of record low loss waveguides on chip. Delay lines as long as 27 meters will be described in which waveguide loss is less than 0.1 dB/m [7].
    Photonics Conference (IPC), 2013 IEEE; 01/2013
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    Jiang Li, Hansuek Lee, Tong Chen, Kerry J Vahala
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    ABSTRACT: Microresonator-based frequency combs (microcombs or Kerr combs) can potentially miniaturize the numerous applications of conventional frequency combs. A priority is the realization of broadband (ideally octave spanning) spectra at detectable repetition rates for comb self-referencing. However, access to these rates involves pumping larger mode volumes and hence higher threshold powers. Moreover, threshold power sets both the scale for power per comb tooth and also the optical pump. Along these lines, it is shown that a class of resonators having surface-loss-limited Q factors can operate over a wide range of repetition rates with minimal variation in threshold power. A new, surface-loss-limited resonator illustrates the idea. Comb generation on mode spacings ranging from 2.6 to 220 GHz with overall low threshold power (as low as 1 mW) is demonstrated. A record number of comb lines for a microcomb (around 1900) is also observed with pump power of 200 mW. The ability to engineer a wide range of repetition rates with these devices is also used to investigate a recently observed mechanism in microcombs associated with dispersion of subcomb offset frequencies. We observe high-coherence phase locking in cases where these offset frequencies are small enough so as to be tuned into coincidence. In these cases, a record-low microcomb phase noise is reported at a level comparable to an open-loop, high-performance microwave oscillator.
    Physical Review Letters 12/2012; 109(23):233901. · 7.73 Impact Factor
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    Jiang Li, Hansuek Lee, Ki Youl Yang, Kerry J Vahala
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    ABSTRACT: The measurement of dispersion and its control have become important considerations in nonlinear devices based on microcavities. A sideband technique is applied here to accurately measure dispersion in a microcavity resulting from both geometrical and material contributions. Moreover, by combining the method with finite element simulations, we show that mapping of spectral lines to their corresponding transverse mode families is possible. The method is applicable for high-Q, micro-cavities having microwave rate free spectral range and has a relative precision of 5.5 × 10<sup>-6</sup> for a 2 mm disk cavity with FSR of 32.9382 GHz and Q of 150 milllion.
    Optics Express 11/2012; 20(24):26337-44. · 3.55 Impact Factor
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    Tong Chen, Hansuek Lee, Jiang Li, Kerry J Vahala
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    ABSTRACT: Single-mode waveguide designs frequently support higher order transverse modes, usually as a consequence of process limitations such as lithography. In these systems, it is important to minimize coupling to higher-order modes so that the system nonetheless behaves single mode. We propose a variational approach to design adiabatic waveguide connections with minimal intermodal coupling. An application of this algorithm in designing the "S-bend" of a whispering-gallery spiral waveguide is demonstrated with approximately 0.05dB insertion loss. Compared to other approaches, our algorithm requires less fabrication resolution and is able to minimize the transition loss over a broadband spectrum. The method can be applied to a wide range of turns and connections and has the advantage of handling connections with arbitrary boundary conditions.
    Optics Express 09/2012; 20(20):22819-29. · 3.55 Impact Factor
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    Jiang Li, Hansuek Lee, Tong Chen, Kerry J Vahala
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    ABSTRACT: Recently, a high efficiency, narrow-linewidth, chip-based stimulated Brillouin laser (SBL) was demonstrated using an ultra-high-Q, silica-on-silicon resonator. In this work, this novel laser is more fully characterized. The Schawlow Townes linewidth formula for Brillouin laser operation is derived and compared to linewidth data, and the fitting is used to measure the mechanical thermal quanta contribution to the Brillouin laser linewidth. A study of laser mode pulling by the Brillouin optical gain spectrum is also presented, and high-order, cascaded operation of the SBL is demonstrated. Potential application of these devices to microwave sources and phase-coherent communication is discussed.
    Optics Express 08/2012; 20(18):20170-80. · 3.55 Impact Factor
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    ABSTRACT: Quantum control of strong interactions between a single atom and a single photon has been achieved within the setting of cavity quantum electrodynamics (cQED). To move beyond proof-of-principle experiments involving one or two conventional optical cavities to more complex scalable systems that employ many microscopic resonators requires localization of atoms on distance scales ˜100 nm from a resonator's surface. A single atom trapped near the surface of a fiber-coupled microtoroidal resonator provides a promising system that allows access to a new regime of cQED. Here, due to its proximity to the surface of the resonator, an atom experiences both strong 1-photon and surface interactions [1]. To advance beyond transient observations [1], we are currently working to trap single atoms within the evanescent field of a microtoroidal resonator using a single tapered fiber to provide both optical coupling and a dipole trap for the atoms [2-4]. Our goal is to realize a flexible experimental platform for investigations of small quantum networks using strong interactions of single atoms and photons. [4pt] [1] D. J. Alton, et al., Nature Phys. 7, 159 (2011).[0pt] [2] V. I. Balykin et al. Phys. Rev. Lett., 60, 2137 (1988).[0pt] [3] E. Vetsch et al., Phys. Rev. Lett., 104, 203603 (2010).[0pt] [4] C. Lacroute et al., arXiv:1110.5372.
    06/2012;

Publication Stats

10k Citations
1,250.25 Total Impact Points

Institutions

  • 1983–2013
    • California Institute of Technology
      • Department of Applied Physics
      Pasadena, CA, United States
  • 2010
    • University of New Mexico
      • Department of Electrical and Computer Engineering
      Albuquerque, NM, United States
  • 2006–2010
    • Max Planck Institute of Quantum Optics
      Arching, Bavaria, Germany
  • 2008–2009
    • University of Michigan
      • Department of Electrical Engineering and Computer Science (EECS)
      Ann Arbor, MI, United States
  • 1998
    • Fondazione Ugo Bordoni
      Roma, Latium, Italy
  • 1993
    • University of Oregon
      Eugene, Oregon, United States