Terahertz-Rate Kerr-Microresonator Optical Clockwork

Abstract

Kerr microresonators generate interesting and useful fundamental states of electromagnetic radiation through nonlinear interactions of continuous-wave (CW) laser light. With photonic-integration techniques, functional devices with low noise, small size, low-power consumption, scalable fabrication, and heterogeneous combinations of photonics and electronics can be realized. Kerr solitons, which stably circulate in a Kerr microresonator, have emerged as a source of coherent, ultrafast pulse trains and ultra-broadband optical-frequency combs. Using thef−2ftechnique, Kerr combs can support carrier-envelope-offset phase stabilization to enable optical synthesis and metrology. Here, we introduce a Kerr-microresonator optical clockwork, which is a foundational device that distributes optical-clock signals to the mode-difference frequency of a comb. Our clockwork is based on a silicon-nitride (Si3N4) microresonator that generates a Kerr-soliton frequency comb with a repetition frequency of 1 THz. We measure our terahertz clockwork by electro-optic modulation with a microwave signal, enabling optical-based timing experiments in this wideband and high-speed frequency range. Moreover, by EO phase modulation of our entire Kerr-soliton comb, we arbitrarily generate additional CW modes between the 1-THz modes to reduce the repetition frequency and increase the resolution of the comb. Our experiments characterize the absolute frequency noise of this Kerr-microresonator clockwork to one part in1017, which is the highest accuracy and precision ever reported with this technology and opens the possibility of measuring high-performance optical clocks with Kerr combs.

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Plain Language Summary

Optical-frequency combs are versatile precision measurement tools, suitable for measuring time, identifying chemicals, sensing distance, and generating quantum states. The output of a frequency comb is a train of ultrafast pulses, each composed of up to millions of equally spaced laser frequencies. An important use of the optical-frequency comb is down-conversion of an optical atomic clock, which is a laser stabilized to an ultraprecise atomic transition, to a microwave output that can be read electronically without any loss of precision. Here, we demonstrate such a down-conversion using a Kerr-microresonator frequency comb. These “microcombs” are a new technology that forms an optical frequency comb within a nanofabricated dielectric microring or microtoroid. Since they can be built with integrated photonics on silicon chips, they offer a path to make frequency-comb technology chip scale, cost effective, and available to a wider range of users.

We design a silicon-nitride photonic-integrated circuit to generate a microcomb with more than an octave of bandwidth through excitation by a continuous laser. Our microcomb consists of a soliton pulse that circulates through the silicon-nitride resonator once every picosecond, effectively down-converting the laser to a terahertz signal. The terahertz regime is interesting in its own right; however, we also introduce an electro-optic sampling scheme to fully convert the optical clock to less than 10 GHz, which can be used for conventional electronic timekeeping.

With this system, we show how to operate a microcomb clockwork through precision carrier-envelope phase stabilization, and we assess its additive frequency noise to be less than one part in1017, a record coherence for microcombs.