Femtosecond fiber laser is energetic

Fiber lasers allow for high-power, diffraction-limited emission, the generation of short pulses, and multicolor emission.

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by Uwe Brinkmann

Fiber lasers allow for high-power, diffraction-limited emission, the generation of short pulses, and multicolor emission. Researchers at Friedrich-Schiller University (Jena, Germany) have raised the output of a femtosecond fiber laser by almost an order of magnitude. The diode-pumped system, which emits 350-fs pulses at 1064 nm, potentially can be shrunk to the size of a shoe box.

Output powers of more than 100 W have previously been achieved for diode-pumped systems in continuous-wave operation at diffraction-limited beam quality. The capability of fiber systems for extracting pulse energies as high as several millijoules at pulse durations in the range of 10 ns has been demonstrated recently.1 Now, by using sufficiently extended pulse stretching and chirped-pulse-amplification (CPA), nonlinear pulse distortions in the fiber can be reduced, enabling energies to be extracted out of rare-earth-doped fibers at close to the saturation fluence limit even in ultrashort-pulse operation.

The researchers, who collaborated with the Institut für Physikalische Hochtechnologie Jena, began with femtosecond pulses from a Nd:glass oscillator (1064-nm wavelength, 140-fs pulse duration, 74-MHz repetition rate, and 100-mW average power) and stretched them to a 200-ps pulse duration in a 400-m-long single-mode fiber (see figure). A diode-laser-pumped single-mode preamplifier boosted the seed pulses to 5-W average power, and a low-numerical-aperture large-mode-area (LMA) power amplifier raised the pulses up to 125-W average power.

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A diode-laser-pumped high-power 350-fs fiber laser system emits up to 125-W average power.
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The optical output of the 30-μm-core-diameter LMA fiber is diffraction limited (M2 less than 1.1) up to output powers of greater than 75 W. The slope efficiency of the diode-laser-pumped power amplifier is 75%. The chirped 200-ps high-power pulses are recompressible to 350 fs using a diffraction-grating compressor.

To handle such high average power, the compressor relies on the development of highly efficient transmission diffraction gratings fabricated in quartz glass, avoiding thermal distortions that are frequent problems with conventional gold-coated gratings. Investigations on such gratings with 1200 lines/mm manufactured by electron-beam lithography are currently in process in Jena. The group recently demonstrated such a grating with a diffraction efficiency of more than 95%. A CPA system containing the grating produced an output power enhanced by almost a factor of ten over previously published results. The footprint of the power amplifier is currently 30 x 60 cm, and that of the oscillator 20 x 60 cm. With proper engineering, those dimensions will be reduced.

REFERENCE
1.C. C. Renaud, et al, IEEE J. Quant. Electr. 37(2) 199.

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