Towards milli-Hertz laser frequency noise on a chip

Narrow-linewidth lasers are important to many applications spanning precision metrology to sensing systems. Their miniaturization in the form of on-chip lasers is receiving increasing attention. Here, a noise level that is consistent with a fundamental frequency noise of 9 mHz⋅Hz/Hz linewidth (60 mHz linewidth) is measured in a Brillouin laser. The results leverage ultra-high-Q silica-on-silicon resonators and point towards a new performance target for chip-based laser platforms

Towards milli-Hertz laser frequency noise on a chip

Narrow-linewidth lasers are important to many applications spanning precision metrology to sensing systems. Their miniaturization in the form of on-chip lasers is receiving increasing attention. Here, a noise level that is consistent with a fundamental frequency noise of 9 mHz⋅Hz/Hz linewidth (60 mHz linewidth) is measured in a Brillouin laser. The results leverage ultra-high-Q silica-on-silicon resonators and point towards a new performance target for chip-based laser platforms

Linewidth enhancement factor in a microcavity Brillouin laser

The linewidth of regenerative oscillators is enhanced by amplitude–phase coupling of the oscillator field [Phys. Rev. 160, 290 (1967)]. In laser oscillators, this effect is well known for its impact on semiconductor laser performance. Here, this coupling is studied in Brillouin lasers. Because their gain is parametric, the coupling and linewidth enhancement are shown to originate from phase mismatch. The theory is confirmed by measurement of linewidth in a microcavity Brillouin laser, and enhancements as large as 50× are measured. The results show that pump wavelength and device temperature should be carefully selected and controlled to minimize linewidth. More generally, this work provides a new perspective on the linewidth enhancement effect

Quantum diffusion of microcavity solitons

Coherently pumped (Kerr) solitons in an ideal optical microcavity are expected to undergo random quantum motion that determines fundamental performance limits in applications of the soliton microcombs. Here this random walk and its impact on Kerr soliton timing jitter are studied experimentally. The quantum limit is discerned by measuring the relative position of counter-propagating solitons. Their relative motion features weak interactions and also presents common-mode suppression of technical noise, which typically hides the quantum fluctuations. This is in contrast to co-propagating solitons, which are found to have relative timing jitter well below the quantum limit of a single soliton on account of strong correlation of their mutual motion. Good agreement is found between theory and experiment. The results establish the fundamental limits to timing jitter in soliton microcombs and provide new insights on multisoliton physics

Interleaved difference-frequency-generation for mid-infrared microcomb spectral densification

Generation of mid-infrared combs (3.3 micron band) with GigaHertz line spacing is demonstrated by interleaved difference-frequency-generation. The method, applied to a 22 GHz repetition-rate microcomb, is useful for spectral densification of sparse microcomb spectra