Sound creates light

When light is not used to transmit data (as it does in optical telecommunications), but when rather a property of light itself – specifically: its frequency – is to be transferred, and that with the highest possible precision, then the classical techniques of fiber optic telecommunications reach their limits. The novel system of signal amplification utilizes stimulated Brillouin scattering (fiber Brillouin amplification). A so-called pump light with a precisely defined frequency is injected into the far end of the optical fiber, so that the pump light travels in the opposite direction to the signal light, generating sound waves (acoustic phonons) in the glass fiber. The sound waves, in turn, scatter the pump light, enabling the few already existing signal photons to stimulate the emission of many more signal photons. Thus, a photon avalanche is created which is kept going by the sound waves and brings the frequency information to the remote end of the optical fiber with extremely small losses.

If the amplification is sufficient, the light can be reflected from the remote end point and returns to the transmitter by travelling along exactly the same optical fiber path as on the way out. With this two-way information, the optical link can be stabilized. Thus, a fixed phase relationship is created between two very remote sites; with the stabilized fiber connection, frequencies are transported across long distances with extreme accuracy.

Using this technique, it was possible to characterize the optical magnesium clock at the Leibniz University of Hanover across a 73 km long optical fiber link, with the aid of a PTB frequency standard as well as of femtosecond frequency comb generators. The PTB scientists then planned a direct fiber connection from PTB to the Max Planck Institute of Quantum Optics (MPQ) in Garching – a fiber link distance of approx. 900 km, which attenuates the light by the almost inconceivable factor of 1020, unless it is amplified. With the new system, even very weak signals are amplified; the signal power is increased by up to six orders of magnitude, so that only three amplifier stations are necessary instead of nine. Moreover, it is possible to selectively amplify very narrow-band light signals, which is advantageous for the testing of the narrow-band clock transitions of optical clocks. The method has already been tested on a deployed underground fiber link – in cooperation with the Deutsches Forschungsnetz (German National Research and Education Network) and the GasLINE company, which operate a German-wide fiber network. With only one intermediate amplification station, an ultra-stable frequency was transmitted across a 480 km long optical fiber link with a relative uncertainty of 2 · 10^–18.

Thus, the way is now open for a connection with MPQ in Garching in order to utilize the highly stable reference frequencies of PTB for joint experiments. Also, a connection with LNE-SYRTE, the French partner institute of PTB in Paris, now appears realistic, so that both institutes could work together on optical clocks in the future. Furthermore, applications in geodesy and also in radio astronomy are already on the horizon.