Telecommunication fiber link for the remote characterization of a magnesium optical frequency standard

Abstract

We have characterized the 24 Mg optical frequency standard at the Institute of Quantum Optics (IQ), Hanover, using a clock laser at the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, via a noise compensated 73 km fiber link and present preliminary results for the stability of the Mg standard. The stability of the clock laser (λ = 657 nm) is transferred with a femtosecond frequency comb to a telecommunication laser at λ = 1542 nm. The signal is then transmitted from PTB through the fiber link to IQ. A second comb at IQ (the remote end) is used to compare the transmitted laser frequency with that of the Mg clock laser λ = 914 nm. The frequency ratio of the clock lasers ν Mg /ν Ca shows a relative instability < 10 -14 at 1 s. The upper limit for the contribution of the fiber link to the frequency instability is measured by connecting another optical fiber following the same 73 km route at Hanover computer center. The comparison performed at PTB between the local and the transmitted signal after a round trip length of 146 km showed a relative uncertainty below 1 x 10 -19 and a short term instability σ y (τ)= 3.3 x 10 -15 / (τ/s).

Optoelectronic oscillations for thermo-optic coefficient measurement of optical fibers

“…In recent years, optical fiber networks have replaced the old network of copper cables since it provides higher bandwidth transmission and lower losses. Optical fiber links over hundreds of kilometers are developed, not only for high speed communications but also for ultra-stable optical frequency transfer [1,2]. However, there are several impairments that must be considered before reaching speeds beyond 10 Gbit s −1 or having ultra-stabilized optical frequency links.…”

“…Thermally-induced change in the refractive index of optical fibers, which is given by the thermo-optic coefficient (TOC), can significantly shift the zero-dispersion wavelength and the dispersion slope, which can degrade the high-speed communication systems [3]. It is also a major challenge for the ultra-precise optical frequency transfer over optical fibers [2]. Although TOC induced shifts can be compensated actively during the optical frequency transfer or digitally during the next-generation coherent communications, a well-known value for the TOC is required for the efficient compensation for these parameters; for example, to avoid the unnecessary number of filters applied to compensate chromatic dispersion digitally which can lead to failure of the system [4].…”