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Femtosecond fiber lasers get magnitude boost

29 Aug 2007

The performance of femtosecond fiber lasers now matches bulk oscillators. Optics.org speaks to the team about its result.

Researchers in Germany have unveiled a mode-locked high-energy fiber laser with a performance that they say rivals state-of-the-art bulk femtosecond oscillators for the first time. The laser produces 265 nJ ultrashort pulses and the team believes that power scaling beyond the 1µJ-level is possible with the current design. (Optics Express 15 10725)

"Bulk femtosecond oscillators are expected to offer a few 100 nJ of energy, a few Watts of average power and a pulse duration of a few 100 femtoseconds and that is exactly the range we have with our fiber laser," Bulend Ortac from the Friedrich-Schiller University in Jena told optics.org. "Our pulse energy is one order of magnitude higher than so far reported for fiber oscillators in the 1 micron wavelength region."

The peak power and pulse energy scaling of mode-locked fiber lasers has to date been limited by nonlinear effects (such as Kerr-nonlinearity) that occur as light propagates along the fiber. To overcome this, Ortac and colleagues have enlarged the fiber's mode area which in turns reduces the nonlinearity.

"Low-nonlinearity large-mode-area fibers open the possibility of energy scaling," explained Ortac. "A newly designed low-nonlinearity single-transverse mode ytterbium-doped photonic crystal fiber and a laser design that better controls the nonlinear effects has allowed our laser system to reach this performance level."

The team's passively mode-locked large-mode-area fiber laser uses a sigma cavity configuration and a 976 nm pump diode laser. At this wavelength, the fiber has an absorption of 30 dB/m, which reduces the required length of fiber to just 51 cm.

"Passive mode-locking is achieved using a real saturable absorber based on a multi-layer GaAs/AlAs Bragg mirror and a low-temperature MBE-grown InGaAs quantum well structure in front of the mirror," explained Ortac. "The saturable absorber mirror (SAM) has high nonlinear modulation depth with fast relaxation time. Self-consistent intra-cavity pulse evolution is obtained by the combined action of the SAM and the active fiber so dispersion compensation is not needed."

Above the mode-locking threshold of around 2W average output power, the laser delivered a single-pulse train with a repetition rate of 10.18 MHz. The group recorded a maximum average output power of 2.7W, corresponding to a pulse energy of 265 nJ. The laser's central wavelength was 1031.7 nm and pulses were compressed by an external transmission-grating pair down to a duration of 400 fs.

"The next steps of our research are to develop even higher pulse energy with better control of nonlinear effects and a design a new real saturable absorber with a high optical damage threshold," commented Ortac. "We have had contact with industrial partners but there are no fixed plans as yet to commercialize this laser."

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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