# Ultralow-Noise Photonic Microwave Synthesis using a Soliton Microcomb-based Transfer Oscillator

## TL;DR

This paper demonstrates a method to generate ultralow-noise microwave signals using a soliton microcomb and transfer oscillator technique, achieving phase noise levels comparable to the best optical frequency division methods.

## Contribution

It introduces a transfer oscillator approach with a soliton microcomb at 14 GHz, enabling ultralow-noise microwave synthesis without requiring high repetition rates or tight comb stabilization.

## Key findings

- Achieved phase noise below -60 dBc/Hz at 1 Hz offset
- Demonstrated transfer oscillator noise cancellation
- Highlighted potential for integrated microcombs in future applications

## Abstract

The synthesis of ultralow-noise microwaves is of both scientific and technological relevance for timing, metrology, communications and radio-astronomy. Today, the lowest reported phase noise signals are obtained via optical frequency-division using mode-locked laser frequency combs. Nonetheless, this technique ideally requires high repetition rates and tight comb stabilisation. Here, a soliton microcomb with a 14 GHz repetition rate is generated with an ultra-stable pump laser and used to derive an ultralow-noise microwave reference signal, with an absolute phase noise level below -60 dBc/Hz at 1 Hz offset frequency and -135 dBc/Hz at 10 kHz. This is achieved using a transfer oscillator approach, where the free-running microcomb noise (which is carefully studied and minimised) is cancelled via a combination of electronic division and mixing. Although this proof-of-principle uses an auxiliary comb for detecting the microcomb's offset frequency, we highlight the prospects of this method with future self-referenced integrated microcombs and electro-optic combs, that would allow for ultralow-noise microwave and sub-terahertz signal generators.