Sound makes light

World premiere: Researchers transmit frequency over 480 kilometer long fiber optic route

Over fiber, frequencies can now be passed on with unprecedented accuracy. © GasLINE
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If light serves as a transmitter of news, then one can fall back on the proven instruments of telecommunications. It becomes more difficult if the light itself - more precisely, its frequency - is the message and if this message is to be transmitted with extreme accuracy. Conventional amplifiers reach their limits. Researchers have now found the solution: They use so-called stimulated Brillouin scattering. That is, they send to the signal light so-called pump light with a well-defined frequency, which excites acoustic waves in the glass fiber.

In turn, the pump light is scattered at these acoustic phonons, with the few existing signal photons stimulating the emission of further signal photons. This creates a photon avalanche that is kept in motion by means of sound waves, bringing the frequency information to the other end of the fiber with extremely low losses. The researchers at the Physikalisch-Technische Bundesanstalt (PTB) have already demonstrated this on a fiber-optic link of 480 kilometers in length. The relative measurement uncertainty they achieved is one second in 16 billion years.

Now longer glass fiber lengths are to follow. The new technology simplifies the comparison of newly developed optical clocks whose high frequency stability is difficult to grasp with the usual satellite time and frequency transmission methods. But geodesy has also been approaching prospective PTB researchers. And even applications in radio astronomy make sense.

Transmission of frequencies by fiber

PTB physicists Harald Schnatz and Gesine Grosche have been leading the international market for the precise measurement and transmission of frequencies by fiber optics for years. The frequency of the light itself is the information: specifically about 195 x 1012 oscillations per second. A first application of the still young technology last year was the remote measurement of the so-called optical clock transition in a magnesium clock of the Leibniz Universität Hannover.

The scientists determined the characteristic frequency with which the atoms in the magnesium can be excited and which can therefore be used in principle to "generate" seconds - all via 73 kilometers of optical fiber from the PTB. "These measurements include femtosecond frequency comb generators at both ends, which establish a fixed phase relationship between the transmitted light and the local frequency standards, " explains Schnatz. display

The local frequency standards are the new magnesium clock in Hanover and an optical clock at PTB. The different frequencies of the two are synchronized by means of the femtosecond frequency comb generators, which could be compared to a transmission. Schnatz adds, "At first we were surprised how well this overall system works."

Cross over longer distances

Now the researchers wanted to bridge longer distances and establish a connection to the Max Planck Institute for Quantum Optics (MPQ) in Garching for joint experiments a distance of 900 kilometers of glass fiber, which, if not amplified, attenuates the unimaginable factor of 1, 020. The fiber has to be traversed twice because it is part of a huge interferometer; This stabilizes the entire length of the optical fiber in its optical length. Conventional amplifier techniques reach their limits.

"Our doctoral candidate Osama Terra had the initial idea of ​​using Brillouin amplification in the optical fiber itself, " says Grosche. "That brings us several advantages: First, it will also make it very weak Signals amplified; the signal power is multiplied by a factor of up to a million. So we need a lot less amplifier stations. In addition, very narrowband light signals can be targeted. This is very favorable for the investigation of the narrow-band clocks of optical clocks.

New concept in the test

The team immediately tested the concept on a laid fiber optic link: in cooperation with the German Research Network and GasLINE, which operate a Germany-wide fiber-optic network. With only one amplifier intermediate station, the highly stable frequency came to a

Track record of 480 kilometers of fiberglass and that with a relative accuracy of two parts in 1018, which corresponds to a deviation of one second in 16 billion years. Thus, even a connection to the PTB's French partner institute in Paris appears realistic in order to work together on the best watches in the future.

(idw - Physikalisch-Technische Bundesanstalt (PTB), 02.06.2010 - DLO)