Highly efficient, variable pulse format, 770-nm source based on powerful seeded fiber amplifier and second-harmonic generation in periodically poled KTP

Summary form only given. Recent developments in sources based on fiber amplifiers have allowed the achievement of high peak powers over a wide spectral range. Parallel progress in the fabrication of periodically poled crystals allows high conversion efficiencies in the visible to be obtained. The key requirements for practical visible and ultraviolet sources based on fiber lasers and frequency upconversion in periodically poled crystals are the wavelength stability, narrow linewidth, and high intensity of fundamental radiation, which implies using pulsed format fiber sources. Several periodically poled lithium niobate (PPLN)-based schemes demonstrated before have reduced scalability to higher average powers because of the low damage threshold of PPLN. One particularly flexible and efficient configuration, that of the seeded fiber amplifier, allows significantly wider diversity in the output parameters of the source such as narrow linewidth, ultrashort pulses at selectable repetition rates and high average saturated powers over an expansive wavelength range. We report on highly (66%) efficient peak power conversion efficiency of second-harmonic (769-nm) generation in PPKTP based on a seeded high-power Yb/Er fiber source. The control over wavelength and pulse parameters was achieved using direct gain switching of the seeding semiconductor laser, which allowed variation of pulse duration from 50 ps to 2 ns and repetition rates from 2 MHz to 25 MHz.