Hybrid fiber MOPA-bulk amplifier system for frequency conversion

Abstract

ABSTRACT We report on a hybrid fiber MOPA + solid-state amplifier for frequency conversion and compare a hybrid scheme versus all- fiber MOPA. Using a thoroughly designed master oscillator and optimized fiber amplifiers we were able to achieve 15-30ns long pulses at average powers above 20 W with a good spectral quality and suppressed SBS. Bulk Vanadate amplifiers boosted the 1064nm output power to greater 65W at pulse repetition rates of 300-500 kHz. More than 35 W in green and 20 W of UV light has been obtained at pulse repetition rates above 300 kHz and pulse energies of 30-100 µJ. Keywords : Fiber amplifiers, nanosecond pulse, stimulated Raman scattering, four-wave mixing, harmonic generation, Vanadate amplifier 1. INTRODUCTION In material processing a general trend is to increase the processing speed by implementing systems with higher pulse repetition rates while maintaining constant pulse energies. Modern solid-state Q-switched lasers have continuously expanded their operation range offering puls e repetition rates (PRR) above 100 kHz with average powers at 355nm of over 20 W. At present typical PRR for Q-switched lasers are in the range of 10-250 kHz, but even higher repetition rates of up to 500 kHz have been demonstrated. However, there are some drawbacks in using solid state-lasers at higher repetition rates due to the decrease in available gain. The low gain results in lower harmonic conversion efficiencies and increased rms noise. Figure1 shows a typical behavior of Avia-355-23 W solid-state laser MOPA with external third harmonic generation. Once the pulse repetition ra te increases the optical power in the harmonics reduces. While the pulse length at repetition rates of ~ 25-70 kHz is in the range of 15-30 ns, it becomes longer ~ 50-60 ns at PRR of greater 100 kHz.