Extremely short laser pulses – the duration of only one cycle of light – have been generated at the 1.5-µm wavelength used to transmit data, an achievement that could benefit frequency metrology and the ultrafast sciences such as ultrafast optical imaging. Members of the Konstanz team in front of their 4-fs fiber laser system are (l-r): Alfred Leitenstorfer, Stefan Eggert, Günther Krauss, Rupert Huber and Alexander Sell. (Photos: University of Konstanz) Obtaining short laser pulses is very difficult because two sets of laser pulses must be coherently combined, requiring extremely careful synchronization and manipulation. Any time variation in the signal between the two pulse streams ruins the process. Scientists at the Center of Applied Photonics of the University of Konstanz generated the extremely short pulses – just 4.3 femtoseconds long – by deriving the two pulses from an existing technology, a single erbium-doped fiber laser source. Close-up view of fs erbium fiber system with the near-infrared beam converted into the visible regime via frequency doubling (yellow spot). The Konstanz research group, led by professor Alfred Leitenstorfer, used that single source to dramatically reduce the timing jitter between the two pulse trains, allowing most of the energy of the flash of light to be concentrated in a single cycle of the electric field – pulses that are close to the shortest possible value for a data bit of information transmitted in the telecommunications wavelength of 1.5 µm. Pulsed light is key for many applications in data transmission and for experiments with extremely high temporal resolution. The shorter the pulse duration, the higher the flux of information or time resolution that may be reached. The ultimate limit for the duration of a flash of light is given by the oscillation period of the underlying optical frequency. Shorter pulses are impossible even in theory since they would lose the oscillatory character of light and would be unable to propagate in space. Temporal oscillations of the electric field of two ultrashort light pulses with different center frequencies (left, green and red graphs). The transients are combined in space and time such that the central field maxima are exactly in sync with each other. In this way, these regions get amplified. Due to the different frequencies, destructive interference sets in already during the oscillation cycles before and after the central maximum. Because the new pulse represents the sum of both fields, it contains only a single cycle of light (right, black graph). Reaching this frontier represents a formidable technological challenge since one cycle of light in the telecom wavelength range is as short as 4 fs (1 fs = 10-15 sec). One femtosecond represents the millionths part of a billionths second, corresponding to 0.000,000,000,000,001 seconds. It takes light one tenth of a second to surround the earth while in 4 fs it only propagates a distance corresponding to approximately one hundredth of the diameter of a human hair. Red and green outputs from a two-color femtosecond fiber laser system similar to the one used in the single-cycle experiment. The near infrared pulse trains have been frequency doubled in nonlinear-optical crystals in order to make them visible for the human eye. Leitenstorfer's group has looked into new laser concepts for several years, especially in the field of compact fiber lasers. Besides their role in fundamental physics, the lasers from Konstanz are finding applications in a number of other areas, such as precision metrology and cancer research. Commercial products by TOPTICA Photonics AG near Munich and by Carl Zeiss AG in Oberkochen and Jena are already based on this new technology. The work is featured in the January issue of Nature Photonics. For more information, visit: www.uni-konstanz.de