by dichroic mirrors at each end of the final-stage amplification fiber.

With the use of the ML-VECSEL with longer seed pulses of 4. 6 ps, the fiber MOPA produces 5. 8 ps pulses with a high average power of 200 W, which corresponds to a peak power of 38 k W—low enough to minimize stimulated Raman scattering (SRS). Although pulse compression down to 430 ps is possible, the quality of the compressed pulse shows a pedestal due to the nonlinearities of the chirp, which is caused by self-phase modulation. Shorter seed pulses allow normal dispersion, gain, and self-phase modulation to interplay to create a linear chirp; this is the parabolic regime.

When seeding the MOPA with the 0.5 ps ML-VECSEL pulses, the researchers were concerned that shorter seed pulses would increase the peak power to a level that would make SRS a problem. However, the ML-VECSEL is capable of producing parabolic pulse amplification at gigahertz repetition rates. The 0.5 ps ML-VECSEL with 1. 1 GHz repetition-

Pump _{LD YDF}

^λ/2 YDF

1X or 2X

Output Compression

UNIVERSIT Y OF SOUTHAMPTON

Compared to the use of standard solid-state or diode seed-laser sources, an Yb-doped fiber amplification configuration seeded by a modelocked vertical-external-cavity surface-emitting laser (ML-VECSEL) produces ultrafast pulses with higher output power and near-transform-limited beam quality.

Modelocked VECSEL

975 nm pump LD

Unabsorbed pump

 

rate seed pulses produced parabolic output pulses with 53 W of average power and pulse durations of 4. 8 ps. These pulses could be further compressed to 110 fs with nearly transform-limited pulse shape. Further power scaling could be improved by using a ML-VECSEL source with an emission wavelength near 1070 nm to suit the ytterbium gain spectrum in the high-average-power regime.

“This project demonstrates the harmony of two advanced technologies developed in recent years: ultrafast VECSELs and high-power fiber amplifiers,” says

Pascal Dupriez, formerly a research student at the Optoelectronics Research Centre at the University of Southampton who is now at Fianium ( Southampton, England). “Future developments in novel fiber lasers could lead to the emergence on a commercial level of femtosecond sources with average powers reaching hundreds of watts.”

Gail Overton

REFERENCE

1. P. Dupriez et al., Optics Express 14( 21) 9611

(Oct. 16, 2006).

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