Sound creates light

When light is used to transmit information, well established techniques of optical telecommunications are available: modulated light pulses travel along optical fibers, become weaker due to optical attenuation in the fiber and are "refreshed" in signal regeneration stations along the way, where the signals are amplified and filtered. This goal becomes more demanding when the light itself - or more precisely, its optical frequency - is the information, and when this information is to be transmitted with extreme precision. Here conventional amplification techniques reach their limits. Three researchers at Physikalisch-Technische Bundesanstalt (PTB) have now found an elegant solution: they employ so-called fiber Brillouin amplification, which is closely related to stimulated Brillouin scattering. The researchers inject pump light with a well-defined frequency into the far end of the 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 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 and very high precision. The PTB researchers have already demonstrated this technique on a 480 km optical fiber link: the relative measurement uncertainty they achieved is equivalent to a deviation of one second in 16 billion years. Now they plan to span even larger distances. The new method simplifies the comparison of newly developed optical clocks, which possess such a high frequency stability that traditional methods for frequency and time comparison via satellite are no longer sufficient. The technique is likely to have applications in other areas where precise synchronisation is needed, for example in radio astronomy. Experts in geodesy have already approached the PTB researchers with suggestions for joint projects.

The PTB physicists Harald Schnatz and Gesine Grosche are internationally leading experts in the precise measurement and transmission of frequencies via optical fibers. They use the optical frequency of the light, with some 195 · 1012 cycles per second, as the information. A first application of this new method was the remote measurement, conducted last year, of the so-called optical clock transition in a magnesium clock at the Leibniz University of Hannover. The scientists determined the characteristic frequency with which very cold magnesium atoms can be excited to a particular long-lived state, by a measurement from PTB via 73 km optical fiber. It is important to measure such frequencies accurately as they can in principle be used to "generate" seconds. "For such measurements, there are femtosecond frequency comb generators at both ends, which produce a fixed phase relationship between the transmitted light and the frequency standards on site", explains Harald Schnatz. The frequency standards on site are the new magnesium clock in Hanover and an optical clock at PTB. The different frequencies of the two are synchronized with the aid of femtosecond frequency comb generators, which can be compared to a gear mechanism. Schnatz adds: "At first we were astonished how well this complete system works." ...

via Physikalisch-Technische Bundesanstalt (PTB).