Weimin Zhou, James Cahill, Jimmy Ni, Andrew DeLoach, Tanvir Mahmood, Sang-Yeon Cho, Jason Sun, and Stephen Anderson, US Army CCDC-ARL; Logan Courtright, Curtis Menyuk, University of Maryland Baltimore County; John Bowers, University of California at Santa Barbara; Lue Wu, and Kerry Vahala, California Institute of Technology

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We report our R&D efforts on developing chip-scale optical long-holdover oscillators and clocks. Photonic-based oscillators and clocks are well positioned for future communication, data processing/computing, RF wireless and PNT applications due to their capabilities of providing ultra-low phase noise/jitter high-frequency/high-data-rate signals for future electronic systems. Since these future systems will have combined electronic and photonic circuits, optical oscillators/clocks have an advantage to provide output signals in both electronic and photonic domains. However, the current state-of-the-art precision photonic clock is an optical-frequency-comb-based oscillator/clock that has large size, weight and power consumption, as well as high cost (SWaP-C). It uses a high-Q, ultra-low-expansion-cavity laser to produce a “narrow-linewidth” optical frequency which is divided by the frequency comb to provide the microwave frequency. The clock is also locked to an atomic transition to achieve long-term stability. The high-Q device is naturally sensitive to the environment; therefore, a temperature-controlled, EM shielded, and vibration/acoustic reduction measures must be used to isolate the environmental effects, which increases the SWaP-C. In addition, the atomic source adds significant cost and is hard to integrate in a chip-scale semiconductor integrated circuit. To achieve maximum reduction of the SWaP-C of an optical clock, we introduce a new approach: First, we use a self-referenced interferometric locking circuit to stabilize the optical frequency comb, which eliminates the need of ultra-low-expansion-cavity laser. Second, we use an epsilon-near-zero (ENZ) metamaterial to design an environment-insensitive cavity/resonator to replace the atomic cell and the environment isolation housing for long-term stability. In order to make it a true chip-scale clock, we designed each of the parts with standard semiconductor fabrication techniques. These include an Si3N4 micro-ring resonator comb that is pumped by a self-injection locked InGaAs laser, a low-loss Si3N4 waveguide self-stabilization circuit that is based on a delay-line interferometer, and a waveguide ring resonator with an air core and an ENZ indium-tin-oxide (ITO) metamaterial cladding. We are also investigating other material systems, including AlGaAs for the micro-resonator, and ITO/SiO2 multilayer-ENZ metamaterial for the cladding of environment-insensitive resonator. These devices cannot be monolithically fabricated at present, but each part can be fabricated by existing integrated-photonics foundries and can be integrated with commercial techniques for integration of III-V-based devices to Si-based devices. Therefore, our chip-scale optical clock has the potential to be commercialized with extreme low SWaP-C.