Optical link techniques involve transferring the signal from an ultra-stable laser through optical fiber, where the laser’s frequency serves as the metrological signal. This laser emits at a wavelength of 1.55 μm, which lies within the frequency band used by long-distance optical telecommunications networks. The laser frequency is stabilized using an ultra-stable Fabry–Perot cavity and can be referenced to the optical and microwave clocks at LTE using a frequency comb. As a result, the laser frequency can be controlled with a relative accuracy better than 10⁻¹⁵.
Part of the light from the ultra-stable laser is injected into the optical fiber. However, thermal fluctuations and acoustic noise disturb the propagation time of the light in the fiber, inducing fluctuations in phase and consequently in the transmitted frequency. To compensate for this noise, the phase fluctuations of the transmitted signal are measured after a round-trip through the fiber and are corrected using an electronic system (see figure).
The beat is performed between the injected signal and the return signal. This gives the accumulated phase error. A correction, equal to half of the measured phase error (with the opposite sign), is then applied to ensure that the signal at the end of the fiber is free from noise induced during the one-way propagation. This method assumes the absence of non-reciprocal noise (not identical in the forward and return paths), which cannot be corrected. Finally, to characterize the frequency arriving at the remote laboratory, it is sent back to the initial laboratory (via a second fiber stabilized using the same method) to be compared with the initial signal. In this transfer method, the bandwidth of the servo system is limited by the round-trip time of the light in the fiber. For a 100 km fiber, it is on the order of a kHz.
This phase noise compensation technique was initially developed for optical links using dedicated fibers, both in France and abroad. However, these developments were hindered by the very high cost of accessing fibers, which made it impossible to consider building metrological fiber networks on a national or international scale. We therefore proposed to exploit the fiber infrastructure that already connects all of our laboratories through the national academic network RENATER. The principle is to wavelength-multiplex the metrological signal with the optical signals that carry telecommunications data. These optical signals are transmitted on different channels, i.e., at close but distinct frequencies, so their propagations remain independent and do not interfere with each other. This approach follows the classic principles of wavelength division multiplexing in optical telecommunications networks. We thus established a collaboration with RENATER and demonstrated that it is indeed possible to transport the metrological signal and digital data in the same fiber in parallel, without interference between these signals.
