Ultrastable Synchronization of Mode-Locked Laser and Microcomb

Microcombs [1] have garnered significant interest over the past decade due to their capability to produce high repetition-rate, low-jitter pulse trains, which are beneficial for generating high-frequency microwave/mm-wave signals and pulse streams with precise timing. For precision applications, it is beneficial that microcombs are referenced their timing to optical or RF references. In conventional mode-locked lasers (MLLs), the frequency stability of optical references can be transferred to the repetition-rates by stabilizing their carrier-envelope offset frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathrm{f}_{\text{ceo}})$</tex>, but the low pulse energy of microcombs challenges <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{f}_{\text{ceo}}$</tex> stabilization. Dispersion-controlled, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$&gt; 100$</tex> GHz free spectral range (FSR) silicon nitride resonators can generate <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{f}_{\text{ceo}}$</tex>-stabilized microcombs [2], but applying this method to microwave-rate microcombs is difficult. To address this, an intermediate phase-locking between the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{f}_{\text{ceo}}$</tex>-stabilized 1- THz repetition-rate comb and the microwave-rate microcomb has been proposed to link the frequency of optical reference to the timing of the microwave-rate microcomb [3]. However, the intermediate phase-locking lacks the stability needed for sub-10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−14</sup>-level references. Additionally, transferring the stability of ultrastable RF references, such as cryogenic Sapphire oscillator (CSO), to microcombs also requires high stability phase-locking. In this work, we demonstrated an MLL-microcomb timing synchronization facilitated by an electro-optic sampling-based timing detector (EOS-TD), which previously enabled 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−20</sup> -level timing synchronization between a fiber-MLL comb and a microwave oscillator [4], to achieve an ultrastable intermediate phase-locking.