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Register a new userCascaded optical fiber link using the Internet network for remote clocks comparison
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We report a cascaded optical link of 1100 km for ultra-stable frequency distribution over an Internet fiber network. The link is composed of four spans for which the propagation noise is actively compensated. The robustness and the performance of the link are ensured by five fully automated optoelectronic stations, two of them at the link ends, and three deployed on the field and connecting the spans. This device coherently regenerates the optical signal with the heterodyne optical phase locking of a low-noise laser diode. Optical detection of the beat-note signals for the laser lock and the link noise compensation are obtained with stable and low-noise fibered optical interferometer. We show 3.5 days of continuous operation of the noise-compensated 4-span cascaded link leading to fractional frequency instability of 4x10-16 at 1-s measurement time and 1x10-19 at 2000 s. This cascaded link was extended to 1480-km with the same performance. This work is a significant step towards a sustainable wide area ultra-stable optical frequency distribution and comparison network at a very high level of performance.
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We developed an all-optical link system for making remote comparisons of two distant ultra-stable optical clocks. An optical carrier transfer system based on a fiber interferometer was employed to compensate the phase noise accumulated during the propagation through a fiber link. Transfer stabilities of $2times10^{-15}$ at 1 second and $4times10^{-18}$ at 1000 seconds were achieved in a 90-km link. An active polarization control system was additionally introduced to maintain the transmitted light in an adequate polarization, and consequently, a stable and reliable comparison was accomplished. The instabilities of the all-optical link system, including those of the erbium doped fiber amplifiers (EDFAs) which are free from phase-noise compensation, were below $2times10^{-15}$ at 1 second and $7times10^{-17}$ at 1000 seconds. The system was available for the direct comparison of two distant $^{87}$Sr lattice clocks via an urban fiber link of 60 km. This technique will be essential for the measuring the reproducibility of optical frequency standards.
In long-haul optical continuous-wave frequency transfer via fiber, remote bidirectional Er$^+$-doped fiber amplifiers are commonly used to mitigate signal attenuation. We demonstrate for the first time the ultrastable transfer of an optical frequency using a remote fiber Brillouin amplifier, placed in a server room along the link. Using it as the only means of remote amplification, on a 660 km loop of installed underground fiber we bridge distances of 250 km and 160 km between amplifications. Over several days of uninterrupted measurement we find an instability of the frequency transfer (Allan deviation of $Lambda$-weighted data with 1 s gate time) of around $1times10^{-19}$ and less for averaging times longer than 3000 s. The modified Allan deviation reaches $3times10^{-19}$ at an averaging time of 100 s, corresponding to the current noise floor at this averaging time. For averaging times longer than 1000 s the modified Allan deviation is in the $10^{-20}$ range. A conservative value of the overall accuracy is $1times10^{-19}$.
Phase compensated optical fiber links enable high accuracy atomic clocks separated by thousands of kilometers to be compared with unprecedented statistical resolution. By searching for a daily variation of the frequency difference between four strontium optical lattice clocks in different locations throughout Europe connected by such links, we improve upon previous tests of time dilation predicted by special relativity. We obtain a constraint on the Robertson--Mansouri--Sexl parameter $|alpha|lesssim 1.1 times10^{-8}$ quantifying a violation of time dilation, thus improving by a factor of around two the best known constraint obtained with Ives--Stilwell type experiments, and by two orders of magnitude the best constraint obtained by comparing atomic clocks. This work is the first of a new generation of tests of fundamental physics using optical clocks and fiber links. As clocks improve, and as fiber links are routinely operated, we expect that the tests initiated in this paper will improve by orders of magnitude in the near future.
We have explored the performance of two dark fibers of a commercial telecommunication fiber link for a remote comparison of optical clocks. The two fibers, linking the Leibniz University of Hanover (LUH) with the Physi-kalisch-Technische Bundesanstalt (PTB) in Braunschweig, are connected in Hanover to form a total fiber length of 146 km. At PTB the performance of an optical frequency standard operating at 456 THz was imprinted to a cw trans-fer laser at 194 THz, and its frequency was transmitted over the fiber. In order to detect and compensate phase noise related to the optical fiber link we have built a low-noise optical fiber interferometer and investigated noise sources that affect the overall performance of the optical link. The frequency stability at the remote end has been measured using the clock laser of PTBs Yb+ frequency standard operating at 344 THz. We show that the frequency of a frequency-stabilized fiber laser can be transmitted over a total fiber length of 146 km with a relative frequency uncertainty below 1E-19, and short term frequency instability given by the fractional Allan deviation of sy(t)=3.3E-15/(t/s).
The frequency stability of lasers is limited by thermal noise in state-of-the-art frequency references. Further improvement requires operation at cryogenic temperature. In this context, we investigate a fiber-based ring resonator. Our system exhibits a first-order temperature-insensitive point around $3.55$ K, much lower than that of crystalline silicon. The observed low sensitivity with respect to vibrations ($<5cdot{10^{-11}},text{m}^{-1} text{s}^{2}$), temperature ($-22(1)cdot{10^{-9}},text{K}^{-2}$) and pressure changes ($4.2(2)cdot{10^{-11}},text{mbar}^{-2}$) makes our approach promising for future precision experiments.
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