Samenvatting

A laboratory test setup is built to transfer and actively stabilize an ultrastable signal, obtained from a laser with sub-Hz linewidth, trough a 5 km long fiber link. The fiber noise is measured and compensated via a feedback loop acting on an AOM that can change the frequency (and therefore the phase) of the light using a beat note between a local oscillator and the round-trip signal as input. The performance of the active noise compensated fiber link is characterized by the beat note between the remote optical frequency and the ultra-stable optical source frequency. The fractional frequency instability of the actively stabilized 5 km link is 1x10^(-18) at 1 s, reaching 4x 10^(-20) after 10^3 s of averaging.

Commercial fiber-optic networks for long-distance telecommunication employ optical amplifiers to compensate for attenuation of optical signals. This is typically done using erbium-doped fiber amplifiers (EDFAs) which, however are equipped with optical isolators and cannot be used for bidirectional round trip signals. In this project semiconductor optical amplifiers (SOAs) are studied as an alternative to EDFAs, which can operate outside the gain bandwidth of EDFAs. Inclusion of a bidirectional SOA in the link adds to the frequency instability, but the increase remains less than one order of magnitude over the full range of averaging times. The SOAs are also tested in a quasi-bidirectional configuration where the light coming from different directions are amplified by different SOAs, which introduces 6 m of uncompensated fiber. Nevertheless, the current quasi-bidirectional setup results in a relative frequency instability of 3x10^(-17) at 1 s, reaching 4x10^(-18) after 8x10^3 s of averaging.

For all the configurations tested here, the performance of the active noise compensated fiber link is compared with the instability of a comparison between two high-accuracy optical Al+ clocks (2.0x10^(-15)tau^(-1/2)) which are currently the most accurate clocks ever built. The 5 km link will not add a significant amount of noise to the comparison of the Al+ clocks, since the fractional frequency instability of the 5 km link is 2-3 orders of magnitude less than the instability of the Al+ clocks on a time scale of 1-14x10^5 s for the unamplified and bidirectional amplified links. Even in the sub-optimum case of a quasi-bidirectional amplifier, the link stability remains well below the Al+ clock stability for averaging times 1-10^4 s.


The use of SOAs, here demonstrated for the first time, offers the possibility to use wavelengths and amplifiers outside the C-band which is commonly used in commercial optical telecommunication systems. Thus, no valuable C-bandwidth needs be sacrificed for ultrastable frequency transfer, and bidirectional out-of-band 'bypass' amplifiers are made feasible. The results of this project may therefore be useful for future implementations of 'SuperGPS' time and frequency transfer via optical networks.