ULTRAFAST LASERS: Femtosecond fiber CPA system puts out 830 W average power

A train of femtosecond pulses with an average power of nearly a kilowatt emitted from a fiber is now a reality, thanks to a femtosecond-fiber chirped-pulse-amplification (CPA) system created by researchers from Friedrich-Schiller-University Jena (Jena, Germany), NKT Photonics (Birkeroed, Denmark), JT Optical Engine (Jena, Germany), and the Fraunhofer Institute for Applied Optics and Precision Engineering (Jena, Germany).¹ The device emits 640 fs pulses at a 78 MHz repetition rate and a 1040 nm center wavelength, producing a peak power of 12 MW.

With such a high average power and repetition rate, the fiber CPA system is intended to replace many existing femtosecond lasers and decrease the measurement times in fundamental science experiments, or increase the processing speed in industrial applications, says Tino Eidam, one of the Friedrich-Schiller-University Jena researchers.

Water-cooled main amplifier

In the system, front-end oscillator produces 200 fs pulses at 78 MHz and a 1042 nm wavelength. The pulses are stretched to an 800 ps duration with an Öffner-type grating stretcher. The resulting 120 mW pulses are amplified in two stages, one 1.2 and the other 1.5 m long, each stage of which is a double-clad photonic-crystal fiber (PCF) with a 30 µm mode field and 170 µm pump-cladding diameter.

Next, the pulses enter the main amplifier fiber, which is water-cooled with a 27 µm mode-field diameter and 500µm air cladding. Pump light at 976 nm is used, and the fiber has a 1 m bending diameter to suppress higher-order modes while transmitting the fundamental mode at low loss (see figure).

Although the main amplifier fiber has a step-index core with no air holes, PCF technology was used in its fabrication, says Eidam. "Glass rods with different refractive indices are stacked and drawn until the diameter of each rod reaches a subwavelength (nanostructured) scale," he notes. "By this, the refractive index can be controlled much better than with standard technologies, resulting in a larger (single-mode) core diameter."

Interestingly, the small-mode-field-diameter amplifier fiber allows for a higher average power than would a larger-core rod-type fiber. The reason is that large-mode-area fibers have a threshold-like onset of mode instabilities at average-power levels of greater than on the order of a hundred watts; this occurs due to the fact that they carry higher-order modes, which, due to transversal spatial hole burning, are preferentially amplified.

Excellent beam quality

The uncompressed output power from the amplifying fiber is 950 W, for a pulse energy of 12.2 µJ; beam quality M² is less than or equal to 1.3. The pulses are then compressed with a pair of reflection gratings with 99% diffraction efficiency and a total compressor efficiency of 95%. The output spectrum is about 15 nm wide, with hard cuts at either edge of the spectrum that arise from the pulse stretcher and help the performance by limiting the pulse spectrum to wavelengths that are best compensated by the compressor. An autocorrelation measurement shows a pulsewidth of 880 fs; a deconvolution calculation results in the pulsewidth of 640 fs.

"The next steps are going beyond the kilowatt average-power level and increasing the pulse energy to 1 mJ," says Eidam. "This will require new fiber designs offering reduced nonlinearities due to short lengths and large cores with sufficient higher-order mode suppression." Eidam adds that one important application of such a high-power system will be the generation of high harmonics for use in spectroscopy, generating frequency combs in the XUV, lensless imaging, and generating attosecond pulses.

REFERENCE

1. T. Eidam et al., Optics Letters 35(2), p. 94, (Jan. 15, 2010).

Editor's Note: For more on ultrafast fiber lasers, please see "Fiber lasers bring femtoseconds to the masses".