A European-based research collaboration between the Institute of Photonic Sciences (ICFO) in Spain and the Max Planck Institute for the Science of Light (MPL) in Germany have reported the development of a hollow-core photonic crystal fiber system producing 9.6-W, mid-IR pulses at a repetition rate of 160 kHz by using an innovative fiber geometry and parametric amplifier together.

Each pulse consists of a single cycle of the optical wave generated from a gas-filled, hollow-core photonic crystal fiber that does not require external compression, an external signal processing that other systems typically require to produce such clean pulses. A vital part of producing such short pulses involves their broadening and precise compression. In order to properly overlap the spectrum of frequencies, the team worked to produce the final optical pulse wave.

"The significance of our work is the achievement of pulse generation at the ultimate physical limit of one oscillation of the electric field in the mid-IR and with unprecedented power,” said Ugaitz Elu, a doctoral student at ICFO and member of the research team. “The electric field is reproducible, carrier-to-envelope-phase stable and the application to strong-field physics and high-harmonic generation should lead to the first isolated waveforms in the hard x-ray and zeptosecond range.”

Chirped mirrors, which consist of multiple stacked coatings to reflect each part of the spectra separately, are often used in fiber laser systems to achieve this compression externally after broadening in the fiber’s gas-filled core. In the mid-IR region, however, the fiber would absorb the energy of the pulses before achieving any type of spectral broadening and destroy it. The geometry implemented by Elu and his collaborators skips this use of chirped mirrors altogether and achieves both broadening and compression in the fiber.

“Here, we used a specifically designed photonic bandgap fiber whose geometry avoids such absorption,” said Elu. “We can achieve broadening and compression in the same fiber without any chirped mirrors.”

The energy and time regimes this optical table-top configuration demonstrates allow for a wide array of applications, most notably those stemming from the coherent hard x-rays that they make achievable. Having a tool to capture dynamics with such precision would open a window to watching, in real time, the subatomic processes of electrons absorbing and emitting energy during chemical reactions.

“Our system is amazingly versatile,” Elu said. “For instance, we use it for electron self-diffraction with which we could resolve all the atoms within a molecule while one of its bonds broke.”