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 the technique, 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 () 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 in , 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.
- Received 17 October 2018
- Revised 11 March 2019
DOI:https://doi.org/10.1103/PhysRevX.9.031023
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular 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 in , a record coherence for microcombs.