While soliton microcombs offer the potential for integration of powerful frequency metrology and precision spectroscopy systems, their operation requires complex startup and feedback protocols that necessitate difficult-to-integrate optical and electrical components. Moreover, CMOS-rate microcombs, required in nearly all comb systems, have resisted integration because of their power requirements. Here, a regime for turnkey operation of soliton microcombs co-integrated with a pump laser is demonstrated and theoretically explained. Significantly, a new operating point is shown to appear from which solitons are generated through binary turn-on and turn-off of the pump laser, thereby eliminating all photonic/electronic control circuitry. These features are combined with high-Q Si3N4 resonators to fully integrate into a butterfly package microcombs with CMOS frequencies as low as 15 GHz, offering compelling advantages for high-volume production.

†All three authors contributed equally to this work pg. 2 Recent advances in nonlinear optics have revolutionized the area of integrated photonics, providing on-chip solutions to a wide range of new applications. Currently, the state of the art integrated nonlinear photonic devices are mainly based on dielectric material platforms, such as Si3N4 and SiO2. While semiconductor materials hold much higher nonlinear coefficients and convenience in active integration, they suffered in the past from high waveguide losses that prevented the realization of highly efficient nonlinear processes on-chip. Here we challenge this status quo and demonstrate an ultra-low loss AlGaAs-on-insulator (AlGaAsOI) platform with anomalous dispersion and quality (Q) factors beyond 1.5 × 10 6 . Such a high quality factor, combined with the high nonlinear coefficient and the small mode volume, enabled us to demonstrate a record low Kerr frequency comb generation threshold of ~36 µW for a resonator with a 1 THz free spectral range (FSR), ~100 times lower compared to that in previous semiconductor platform. Combs with >250 nm broad span have been generated under a pump power lower than the threshold power of state of the art dielectric micro combs. A soliton-step transition has also been observed for the first time from an AlGaAs resonator. This work is an important step towards ultra-efficient semiconductor-based nonlinear photonics and will lead to fully integrated nonlinear photonic integrated circuits (PICs) in near future. pg. 3 The extensive research on integrated nonlinear photonics in the last few years, driven by the breakthrough of the microcomb and other on-chip nonlinear devices, has opened up many new opportunities for on-chip integrated photonics, ranging from spectroscopy to atomic clock applications [1-3]. The demand to construct efficient nonlinear devices has motivated the development of different material platforms in nonlinear photonics. A common endeavor of those efforts is the reduction of the waveguide propagation loss, which is essential to enable high Q cavities so as to enhance the build-up power in the resonators and therefore increase the efficiency of the nonlinear optical processes [4]. In this regard, silica on silicon resonators [5-7] have long been dominant offering Q factors as high as 1 billion [6]. These devices can access a wide range of nonlinear effects including microwave rate soliton microcombs [8].However, over the last 5 years, there has been remarkable progress to significantly improve the Q factors of resonators in many other nonlinear integrated optical material platforms. One example is the Si3N4 platform, which delivers high performance in Kerr comb generation on chip [9][10][11]. The Si3N4 micro-resonators have enabled the generation of efficient frequency combs with repetition rates from microwave to THz frequencies [12] and improved Q factor of beyond 30 million [13,14]. Another material, which recently attracted attention, is LiNbO3. It offers additional opportunities for integrated nonlinear...

Silicon photonics is advancing rapidly in performance and capability with multiple fabrication facilities and foundries having advanced passive and active devices, including modulators, photodetectors, and lasers. Integration of photonics with electronics has been key to increasing the speed and aggregate bandwidth of silicon photonics based assemblies, with multiple approaches to achieving transceivers with capacities of 1.6 Tbps and higher. Progress in electronics has been rapid as well, with state-of-the-art chips including switches having many tens of billions of transistors. However, the electronic system performance is often limited by the input/output (I/O) and the power required to drive connections at a speed of tens of Gbps. Fortunately, the convergence of progress in silicon photonics and electronics means that co-packaged silicon photonics and electronics enable the continued progress of both fields and propel further innovation in both.

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.

Silicon nitride (Si 3 N 4 ), as a complementary metal-oxide-semiconductor (CMOS) material, finds wide use in modern integrated circuit (IC) technology. The past decade has witnessed tremendous development of Si 3 N 4 in photonic areas, with innovations in nonlinear photonics 1 , optical sensing 2 , etc. However, the lack of an integrated laser with high performance prohibits the large-scale integration of Si 3 N 4 waveguides into complex photonic integrated circuits (PICs). Here, we demonstrate a novel III-V/Si/Si 3 N 4 structure to enable efficient electrically pumped lasing in a Si 3 N 4 based laser external cavity. The laser shows superior temperature stability and low phase noise compared with lasers purely dependent on semiconductors. Beyond this, the demonstrated multilayer heterogeneous integration provides a practical path to incorporate efficient optical gain with various low-refractive-index materials. Multilayer heterogeneous integration could extend the capabilities of semiconductor lasers to improve performance and enable a new class of devices such as integrated optical clocks 3 and optical gyroscopes.

Silicon nitride (SiN) waveguides with ultra-low optical loss enable integrated photonic applications including low noise, narrow linewidth lasers, chip-scale nonlinear photonics, and microwave photonics. Lasers are key components to SiN photonic integrated circuits (PICs), but are difficult to fully integrate with low-index SiN waveguides due to their large mismatch with the high-index III-V gain materials. The recent demonstration of multilayer heterogeneous integration provides a practical solution and enabled the first-generation of lasers fully integrated with SiN waveguides. However, a laser with high device yield and high output power at telecommunication wavelengths, where photonics applications are clustered, is still missing, hindered by large mode transition loss, non-optimized cavity design, and a complicated fabrication process. Here, we report high-performance lasers on SiN with tens of milliwatts output power through the SiN waveguide and sub-kHz fundamental linewidth, addressing all the aforementioned issues. We also show Hertz-level fundamental linewidth lasers are achievable with the developed integration techniques. These lasers, together with high-Q SiN resonators, mark a milestone towards a fully integrated low-noise silicon nitride photonics platform. This laser should find potential applications in LIDAR, microwave photonics and coherent optical communications.

Significant improvements in III–V/Si epitaxy have pushed quantum dots (QDs) to the forefront of Si photonics. For efficient, scalable, and multifunctional integrated systems to be developed, a commercially viable solution must be found to allow efficient coupling of the QD laser output to Si waveguides. In this work, the design, fabrication, and characterization of such a platform are detailed. Record‐setting evanescent QD distributed feedback lasers on Si with a 3 dB modulation bandwidth of 13 GHz, a threshold current of 4 mA, a side‐mode‐suppression‐ratio of 60 dB, and a fundamental linewidth of 26 kHz, are reported. The maximum temperature during the backend III/V process is only 200 °C, which is fully compatible with CMOS process thermal budgets. The whole process is substrate agnostic and hence can leverage previous development in QD lasers grown on Si and benefit from the economy of scale. The broadband and versatile nature of the QD lasers and the Si‐on‐insulator low‐loss waveguiding platform can be expanded to build fully functional photonic integrated circuits throughout the O band.

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