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-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.

Optical-frequency synthesizers, which generate frequency-stable light from a single microwave-frequency reference, are revolutionizing ultrafast science and metrology, but their size, power requirement and cost need to be reduced if they are to be more widely used. Integrated-photonics microchips can be used in high-coherence applications, such as data transmission , highly optimized physical sensors and harnessing quantum states , to lower cost and increase efficiency and portability. Here we describe a method for synthesizing the absolute frequency of a lightwave signal, using integrated photonics to create a phase-coherent microwave-to-optical link. We use a heterogeneously integrated III-V/silicon tunable laser, which is guided by nonlinear frequency combs fabricated on separate silicon chips and pumped by off-chip lasers. The laser frequency output of our optical-frequency synthesizer can be programmed by a microwave clock across 4 terahertz near 1,550 nanometres (the telecommunications C-band) with 1 hertz resolution. Our measurements verify that the output of the synthesizer is exceptionally stable across this region (synthesis error of 7.7 × 10 or below). Any application of an optical-frequency source could benefit from the high-precision optical synthesis presented here. Leveraging high-volume semiconductor processing built around advanced materials could allow such low-cost, low-power and compact integrated-photonics devices to be widely used.

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.

Laser-based range measurement systems are important in many application areas, including autonomous vehicles, robotics, manufacturing, formation flying of satellites, and basic science. Coherent laser ranging systems using dual-frequency combs provide an unprecedented combination of long range, high precision, and fast update rate. We report dual-comb distance measurement using chip-based soliton microcombs. A single pump laser was used to generate dual-frequency combs within a single microresonator as counterpropagating solitons. We demonstrated time-of-flight measurement with 200-nanometer precision at an averaging time of 500 milliseconds within a range ambiguity of 16 millimeters. Measurements at distances up to 25 meters with much lower precision were also performed. Our chip-based source is an important step toward miniature dual-comb laser ranging systems that are suitable for photonic integration.

Exceptional points (EPs) are special spectral degeneracies of non-Hermitian Hamiltonians governing the dynamics of open systems. At the EP two or more eigenvalues and the corresponding eigenstates coalesce 1-3 . Recently, it has been proposed that EPs can enhance the sensitivity of optical gyroscopes 4,5 . Here we report measurement of rotation sensitivity boost by over 4× resulting from operation of a chip-based stimulated Brillouin gyroscope near an exceptional point. A second-order EP is identified in the gyroscope and originates from the dissipative coupling between the clockwise and counterclockwise lasing modes. The modes experience opposing Sagnac shifts under application of a rotation, but near the exceptional point new modal admixtures dramatically increase the Sagnac shift. Modeling confirms the measured enhancement. Besides the ability to operate an optical gyroscope with enhanced sensitivity, this result provides a new platform for study of non-Hermitian physics and nonlinear optics with precise control.High-Q optical microresonators have received considerable attention as sensors across a wide range of applications including biomolecule 6-8 and nanoparticle detection 9 , temperature measurement 10 , and rotation measurement [11][12][13][14][15] . In recent years, a new approach to enhance the sensitivity of microresonator sensors using the physics of exceptional points is being studied 4,5,16-20 . Traditionally, for precise sensing, a perturbation to an optical microcavity (or to its reference frame as in the case of a gyroscope) introduces either a linewidth change, a frequency shift, or a frequency splitting of a resonance that monotonically changes with the strength of the perturbation. However, operation of these systems near an exceptional point changes this situation by introduction of a square-root dependence into the transduction that can boost the sensor's ability to transduce perturbations 16 .In this work, we experimentally and theoretically demonstrate the existence of EPs in a microresonatorbased laser gyroscope, and then measure the enhanced rotation-rate transduction sensitivity near the EP. The laser gyroscope is described elsewhere 11 and uses counterpropagating Brillouin lasers in a high-quality-factor (Q ≈ 10 8 ) silica wedge resonator 21 . As shown in Fig. 1a pump light at frequencies ω pj (j = 1, 2) determined by radio-frequency modulation of a single laser (∼1552.5 nm) is coupled into the resonator from both ends of a fiber taper 22,23 . One of the pump frequencies is Pound-Drever-Hall locked to a resonator mode by feedback control to the laser. The second pump frequency is then varied to affect pump detuning change as described below. The two pump powers are stabilized via power feedback. Brillouin scattering causes a pump photon with frequency ω pj to scatter from a co-propagating acoustic phonon with frequency Ω phonon into a backwardpropagating Stokes photon with frequency ω sj . In the context of a resonator (and as illustrated in Fig. 1b), the associated phase matching con...