FIBER LASERS: VECSEL amplification yields femtosecond high-repetition-rate pulses

Ytterbium (Yb)-doped fiber amplifiers and fiber lasers that use solid-state lasers or gain-switched laser diodes as seed lasers in a master-oscillator power-amplifier (MOPA) configuration can produce picosecond and femtosecond pulses at high average power for materials-processing applications. However, these lasers often suffer from unwanted nonlinear effects that degrade output-pulse shape and duration and limit maximum output power. An alternative technique demonstrated by researchers at the University of Southampton (Southampton, England) uses a modelocked vertical-external-cavity surface-emitting laser (ML-VECSEL) source to produce near-transform-limited, 110 fs duration pulses with 53 W average power from a 500 fs seed pulse.¹

In their demonstration, the researchers used two different ML-VECSEL sources: one that produced 0.5 ps pulses at 1043 nm and another that produced 4.6 ps pulses at 1055 nm-to contrast average power and pulse-compression characteristics of the output pulses generated.

The pulses produced by the VECSEL are fed into an optical isolator and then preamplified by either one or two Yb-doped fiber amplifiers (YDFAs) to a power level that is sufficient to saturate a final-stage amplifier in the setup (see figure). After preamplification, the free-space beam is collimated, sent through a half-wave plate for partial polarization control, and into another isolator. It is then collimated again and launched into the final-stage power amplifier (a length of double-clad, Yb-doped fiber with a D-shaped 400-μm-thick inner cladding). This power-amplification fiber is pumped through its output end by a 975 nm diode-stack source; the pump beam is separated from the signal beam 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 kW-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-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.”

REFERENCE

1. P. Dupriez et al., Optics Express 14 (21) 9611 (Oct. 16, 2006).