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Silicon photonics enables wafer-scale integration of optical functionalities on chip. Silicon-based laser frequency combs can provide integrated sources of mutually coherent laser lines for terabit-per-second transceivers, parallel coherent light detection and ranging, or photonics-assisted signal processing. We report heterogeneously integrated laser soliton microcombs combining both indium phospide/silicon (InP/Si) semiconductor lasers and ultralow-loss silicon nitride (Si3N4) microresonators on a monolithic silicon substrate. Thousands of devices can be produced from a single wafer by using complementary metal-oxide-semiconductor–compatible techniques. With on-chip electrical control of the laser-microresonator relative optical phase, these devices can output single-soliton microcombs with a 100-gigahertz repetition rate. Furthermore, we observe laser frequency noise reduction due to self-injection locking of the InP/Si laser to the Si3N4 microresonator. Our approach provides a route for large-volume, low-cost manufacturing of narrow-linewidth, chip-based frequency combs for next-generation high-capacity transceivers, data centers, space and mobile platforms.

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Continuous-wave-driven Kerr nonlinear microresonators give rise to self-organization in terms of dissipative Kerr solitons, which constitute optical frequency combs that can be used to generate low-noise microwave signals. Here, by applying either amplitude or phase modulation to the driving laser we create an intracavity potential trap to discipline the repetition rate of the solitons. We demonstrate that this effect gives rise to a novel spectral purification mechanism of the external microwave signal frequency, leading to reduced phase noise of the output signal. We experimentally observe that the microwave signal generated from disciplined solitons is injection-locked by the external drive at long time scales, but exhibits an unexpected suppression of the fast timing jitter. Counter-intuitively, this filtering takes place for frequencies that are substantially lower than the cavity decay rate. As a result, while the long-time-scale stability of the Kerr frequency comb's repetition rate is improved by more than 4 orders of magnitude, the purified microwave signal shows a reduction of the phase noise by 30 dB at offset frequencies above 10 kHz.

Soliton microcombs constitute chip-scale optical frequency combs, and have the potential to impact a myriad of applications from frequency synthesis and telecommunications to astronomy. The demonstration of soliton formation via self-injection locking of the pump laser to the microresonator has significantly relaxed the requirement on the external driving lasers. Yet to date, the nonlinear dynamics of this process has not been fully understood. Here, we develop an original theoretical model of the laser self-injection locking to a nonlinear microresonator, i.e., nonlinear self-injection locking, and construct state-of-the-art hybrid integrated soliton microcombs with electronically detectable repetition rate of 30 GHz and 35 GHz, consisting of a DFB laser butt-coupled to a silicon nitride microresonator chip. We reveal that the microresonator’s Kerr nonlinearity significantly modifies the laser diode behavior and the locking dynamics, forcing laser emission frequency to be red-detuned. A novel technique to study the soliton formation dynamics as well as the repetition rate evolution in real-time uncover non-trivial features of the soliton self-injection locking, including soliton generation at both directions of the diode current sweep. Our findings provide the guidelines to build electrically driven integrated microcomb devices that employ full control of the rich dynamics of laser self-injection locking, key for future deployment of microcombs for system applications.

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We demonstrate thermometry with a resolution of 80 nK= ffiffiffiffiffiffi Hz p using an isotropic crystalline whispering-gallery mode resonator based on a dichroic dual-mode technique. We simultaneously excite two modes that have a mode frequency ratio that is very close to two (AE0.3 ppm). The wavelength and temperature dependence of the refractive index means that the frequency difference between these modes is an ultrasensitive proxy of the resonator temperature. This approach to temperature sensing automatically suppresses sensitivity to thermal expansion and vibrationally induced changes of the resonator. We also demonstrate active suppression of temperature fluctuations in the resonator by controlling the intensity of the driving laser. The residual temperature fluctuations are shown to be below the limits set by fundamental thermodynamic fluctuations of the resonator material. DOI: 10.1103/PhysRevLett.112.160801 PACS numbers: 07.20.Dt, 42.60.Da, 42.62.Fi The high-resolution measurement of energy has long fascinated humans with its culmination seen in ultra-highsensitivity calorimeters [1,2] and bolometers [3]. These and related ideas have found a broad range of applications, including bolometric superconducting photon counters for quantum communication [4] and ultrasensitive radio astronomy [5,6]. The record for absolute thermometric sensitivity has been realized at cryogenic temperatures, achieving better than 100 pK= ffiffiffiffiffiffi Hz p [7]. In this Letter, we develop a new method to measure temperature based on excitation of two well-separated modes in a millimeter-scale whispering-gallery (WG) optical resonator. WG mode resonators have exceptionally high Q factors and can provide the potential of providing high-stability microwave and optical signals [8][9][10][11][12]. Recently, they have been applied to high-sensitivity label-free sensors for molecules and viruses [13,14] and for optical comb generation [15]. Nonetheless, an issue that afflicts all of these applications is the high temperature sensitivity of WG resonators [12,16], particularly when compared to conventional vacuum-spaced Fabry-Perot resonators [17][18][19][20][21]. In this Letter, we turn this problem to our advantage by using the WG resonator as an ultrasensitive thermometer.To suppress unwanted temperature fluctuations in WG resonators, several groups have demonstrated in situ thermometry by measuring the frequency difference between two orthogonally polarized modes. The best of these techniques have demonstrated a resolution of ∼100 nK= ffiffiffiffiffiffi Hz p [22], and subsequent temperature stabilization based on this sensing has resulted in improvement to the long-term frequency stability [23,24]. In contrast, we present a two-color approach to measure the resonator temperature with high resolution. In comparison to the birefringent dual-mode technique, our approach can be used in both anisotropic and isotropic resonators, which expands the range of material candidates. Isotropic materials have shown the highest Q facto...