Wavelength tunable ultra-short pulses based on a flat broadband spectrum generated in a nonlinear ytterbium-doped fiber amplifier**Project supported by the National Basic Research Program of China (Grant No. 2013CB922404), the National Scientific Research Project of China (Grant No. 61177047), and the National Natural Science Foundation of China (Grant No. 61575011).

Abstract

We present a nonlinear ytterbium-doped fiber amplifier based on enhanced nonlinear effects that can produce a flat broadband spectrum ranging from 1050–1225 nm with a maximum average output power of 7.8 W at 14 W pump power. Its repetition rate is 89 MHz. Using a pair of gratings and two knife edges as a filter, wavelength tunable picosecond pulses of tens to hundreds of milliwatts can be obtained in the broadband spectrum range. The output power, pulse width, and spectrum (center wavelength and linewidth) are adjusted by tuning the distance of the grating pair and/or the knife edges. Fixing the distance between the two gratings at 15 mm and keeping the output spectrum linewidth at approximately 20 nm, the shortest pulse width obtained is less than 1 ps centered at 1080 nm. The longest wavelength of the short pulses is around 1200 nm, and its output power and pulse width are 40 mW and 5.79 ps, respectively. The generation of a flat broadband spectrum is also discussed in this paper.