0555126 - ÚPT 2023 RIV US eng J - Journal Article
Čížek, Martin - Pravdová, Lenka - Pham, Minh Tuan - Lešundák, Adam - Hrabina, Jan - Lazar, Josef - Pronebner, T. - Aeikens, E. - Premper, J. - Havliš, O. - Velc, R. - Smotlacha, V. - Altmannová, L. - Schumm, T. - Vojtěch, J. - Niessner, A. - Číp, Ondřej
Coherent fibre link for synchronization of delocalized atomic clocks.
Optics Express. Roč. 30, č. 4 (2022), s. 5450-5464. ISSN 1094-4087
R&D Projects: GA ČR(CZ) GA19-14988S; GA MŠMT(CZ) EF16_026/0008460
EU Projects: European Commission(XE) 17FUN07 - CC4C; European Commission(XE) 20FUN01 - TSCAC
Institutional support: RVO:68081731
Keywords : optical frequency * transfer * fibre link * stability * metrology * hydrogen maser * synchronization
OECD category: Optics (including laser optics and quantum optics)
Impact factor: 3.8, year: 2022 ; AIS: 0.781, rok: 2022
Method of publishing: Open access
Result website:
https://opg.optica.org/oe/fulltext.cfm?uri=oe-30-4-5450&id=469194DOI: https://doi.org/10.1364/OE.447498

Challenging experiments for tests in fundamental physics require highly coherent optical frequency references with suppressed phase noise from hundreds of kHz down to mu Hz of Fourier frequencies. It can be achieved by remote synchronization of many frequency references interconnected by stabilized optical fibre links. Here we describe the path to realize a delocalized optical frequency reference for spectroscopy of the isomeric state of the nucleus of Thorium-229 atom. This is a prerequisite for the realization of the next generation of an optical clock the nuclear clock. We present the established 235 km long phase-coherent stabilized cross-border fibre link connecting two delocalized metrology laboratories in Brno and Vienna operating highly-coherent lasers disciplined by active Hydrogen masers through optical frequency combs. A significant part (up to tens of km) of the optical fibre is passing urban combined collectors with a non-negligible level of acoustic interference and temperature changes, which results in a power spectral density of phase noise over 10(5) rad(2).Hz(-1). Therefore, we deploy a digital signal processing technique to suppress the fibre phase noise over a wide dynamic range of phase fluctuations. To demonstrate the functionality of the link, we measured the phase noise power spectral density of a remote beat note between two independent lasers, locked to high-finesse stable resonators. Using optical frequency combs at both ends of the link, a long-term fractional frequency stability in the order of 10(-15) between local active Hydrogen masers was measured as well. Thanks to this technique, we have achieved reliable operation of the phase-coherent fibre link with fractional stability of 7 x 10(-18) in 10(3) s.
Permanent Link: https://hdl.handle.net/11104/0332560