Abstract
Kerr microresonators generate interesting and useful fundamental states of electromagnetic radiation through nonlinear interactions of continuous-wave (CW) laser light. Using 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 f-2f technique, Kerr combs support carrier-envelope-offset phase stabilization for optical synthesis and metrology. In this paper, we introduce a Kerr-microresonator optical clockwork based on optical-frequency division (OFD), which is a powerful technique to transfer the fractional-frequency stability of an optical clock to a lower frequency electronic clock signal. The clockwork presented here is based on a silicon-nitride (Si$_3$N$_4$) microresonator that supports an optical-frequency comb composed of soliton pulses at 1 THz repetition rate. By electro-optic phase modulation of the entire Si$_3$N$_4$ comb, we arbitrarily generate additional CW modes between the Si$_3$N$_4$ comb modes; operationally, this reduces the pulse train repetition frequency and can be used to implement OFD to the microwave domain. Our experiments characterize the residual frequency noise of this Kerr-microresonator clockwork to one part in $10^{17}$, which opens the possibility of using Kerr combs with high performance optical clocks. In addition, the photonic integration and 1 THz resolution of the Si$_3$N$_4$ frequency comb makes it appealing for broadband, low-resolution liquid-phase absorption spectroscopy, which we demonstrate with near infrared measurements of water, lipids, and organic solvents.
Highlights
Optical-atomic clocks [1], which are among the most precise metrological instruments currently available, provide continuous-wave (CW) laser radiation that is stabilized to a narrow-linewidth atomic transition
To implement optical-frequency division (OFD), mode n of an optical frequency comb is phase locked to an optical reference such that frep 1⁄4 ðνn − fceoÞ=n, dividing the optical clock carrier frequency to a lower frequency by a factor of n and concurrently reducing the phase noise by approximately a factor 20 log10ðnÞ
We show that the residual noise of the microcomb clockwork is 1 part in 1015 for 1-s measurement durations and reduces to 1 part in 1017 after 2 h of measurement [Fig. 2(b)], which, to our knowledge, is the initial demonstration of complete optical frequency division with a microcomb and is a factor of 30 improvement over the most precise microcomb measurements reported to date [19]
Summary
Optical-atomic clocks [1], which are among the most precise metrological instruments currently available, provide continuous-wave (CW) laser radiation that is stabilized to a narrow-linewidth atomic transition. Opticalfrequency combs provide a clockwork to coherently transfer the stability of an optical clock to all the comb modes and to a microwave signal derived from the comb repetition frequency The latter capability, called optical-frequency division (OFD), was developed for optical timekeeping [2,3], and it leverages the frequency multiplication inherent in the comb’s mode spectrum [4], namely, νn 1⁄4 fceo þ nfrep, where fceo is the carrier-envelope offset frequency. We verify the fractional-frequency accuracy and precision of our Kerr-microresonator clockwork to the 10−17 level after continuous, glitch-free operation for two hours This verification experiment is enabled by terahertz-rate frequency metrology with respect to a separate clockwork system, which is based on a f–2f self-referenced electro-optic (EO) modulation comb (hereafter referred to as the reference comb) [29]. Our work both explores a new range of accuracy and precision with Kerrsoliton comb technology and highlights the advantages of terahertz-rate measurements
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