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 microresonator-based Kerr frequency comb (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.
Highlights
The synthesis of ultralow-noise microwaves is of both scientific and technological relevance for timing, metrology, communications and radio-astronomy
An additional approach is based on optical frequency division, which makes use of a self-referenced fs-laser comb optically locked to an ultrastable laser (USL) with a typical linewidth at the Hz-level[5,6,7,8]
A microresonator pumped by a sub-Hz-linewidth USL at frequency νUSL generates a soliton-Kerr comb with a GHz-range repetition rate frep that is set by the resonator free spectral range (FSR)
Summary
The synthesis of ultralow-noise microwaves is of both scientific and technological relevance for timing, metrology, communications and radio-astronomy. 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 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. We show how to extend the transfer oscillator technique to exploit this salient feature of microcombs (or equivalently of electro-optic combs[25]) In this proof-of-principle demonstration, we achieved a measured single-sideband phase noise of −110 dBc/Hz at 200 Hz offset from the 14.09-GHz carrier, which is 15 dB below the lowest phase noise microresonator-based photonic oscillator reported so far[26], demonstrating the potential of this approach
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