Femtosecond laser-based generation of radio frequency signals has produced astonishing improvements in achievable spectral purity, one of the basic features characterizing the performance of an radio frequency oscillator. Kerr frequency combs hold promise for transforming these lab-scale oscillators to chip-scale level. In this work we demonstrate a miniature 10 GHz radio frequency photonic oscillator characterized with phase noise better than −60 dBc Hz−1 at 10 Hz, −90 dBc Hz−1 at 100 Hz and −170 dBc Hz−1 at 10 MHz. The frequency stability of this device, as represented by Allan deviation measurements, is at the level of 10−10 at 1–100 s integration time—orders of magnitude better than existing radio frequency photonic devices of similar size, weight and power consumption.

Advanced applications in optical metrology demand improved lasers with high spectral purity, in form factors that are small and insensitive to environmental perturbations. While laboratory-scale lasers with extraordinarily high stability and low noise have been reported, all-integrated chip-scale devices with sub-100 Hz linewidth have not been previously demonstrated. Lasers integrated with optical microresonators as external cavities have the potential for substantial reduction of noise. However, stability and spectral purity improvements of these lasers have only been validated with rack-mounted support equipment, assembled with fibre lasers to marginally improve their noise performance. In this work we report on a realization of a heterogeneously integrated, chip-scale semiconductor laser featuring 30-Hz integral linewidth as well as sub-Hz instantaneous linewidth.

Ultrastable high-spectral-purity lasers have served as the cornerstone behind optical atomic clocks, quantum measurements, precision optical microwave generation, high-resolution optical spectroscopy, and sensing. Hertz-level lasers stabilized to high-finesse Fabry-Pérot cavities are typically used for these studies, which are large and fragile and remain laboratory instruments. There is a clear demand for rugged miniaturized lasers with stabilities comparable to those of bulk lasers. Over the past decade, ultrahigh-Q optical whispering-gallery-mode resonators have served as a platform for low-noise microlasers but have not yet reached the stabilities defined by their fundamental noise. Here, we show the noise characteristics of whispering-gallery-mode resonators and demonstrate a resonator-stabilized laser at this limit by compensating the intrinsic thermal expansion, allowing a sub-25 Hz linewidth and a 32 Hz Allan deviation. We also reveal the environmental sensitivities of the resonator at the thermodynamical noise limit and long-term frequency drifts governed by random-walk-noise statistics.

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We demonstrate experimentally, and explain theoretically, generation of a wide, fundamentally phase locked Kerr frequency comb in a nonlinear resonator with a normal group velocity dispersion. A magnesium fluoride whispering gallery resonator characterized with 10 GHz free spectral range and pumped either at 780 nm or 795 nm is used in the experiment. The envelope of the observed frequency comb differs significantly from the Kerr frequency comb spectra reported previously. We show via numerical simulation that, while the frequency comb does not correspond to generation of short optical pulses, the relative phases of the generated harmonics are fixed.A nonlinear monolithic optical resonator pumped with continuous wave (cw) light can produce a broad optical frequency (Kerr) comb and a train of ultrashort optical pulses [21,22], generated due to resonant modulation instability effect [25][26][27][28][29][30]. The process is phase matched in a broad range of parameters if the group velocity dispersion (GVD) of the resonator modes is anomalous [31,32]. However, phase matching is compromised in the case of purely normal GVD. While modulation instability [26,27,30,[33][34][35][36] as well as mode locking [37,38] is still possible under this condition, generation of a broad frequency comb has not been previously demonstrated under net normal GVD. Short optical pulses can be created in a Kerr frequency comb system if the optical loss of a nonlinear ring microresonator has specific frequency dependence [39]. The loss dependence modifies the GVD of the resonator in a way similar to conventional mode locked lasers that can operate at any GVD. This method, though, is not easily utilizable for a large variety of the broadband monolithic microresonators.In this Letter we report on observation of a stable normal GVD Kerr frequency comb. The shape of the frequency envelope of the comb differs significantly from the earlier predictions and observations [34,37,38]. By demodulating the comb on a fast photodiode and observing the phase noise of the generated radio frequency (RF) signal we prove that the frequency harmonics are phase locked [3]. To ensure generality of the phenomenon we performed the experiment at two different wavelengths, 780 nm and 795 nm, using several different resonators and confirmed generation of the mode locked combs with similar properties.Using numerical simulations, we reproduce the frequency comb envelope and show that the comb does not correspond to generation of a short optical pulse in the resonator; rather, the pulses are "dark," i.e. they have lower power as compared to the DC background in the resonator. The frequency comb produces bright pulses at the resonator output, due to the interference with the pump light. It is worth noting that generation of bright pulses inside the resonator usually leads to generation of "dark" pulses at the resonator output [40]. The pump light has to be filtered out to enable observation of the bright pulses [22].Numerical simulations are particularly useful for vali...