Narrow-bandwidth picosecond pulses by spectral compression of femtosecond pulses in second-order nonlinear crystals

We introduce a simple approach for the efficient generation of tunable narrow-bandwidth picosecond pulses synchronized to broadband femtosecond ones. Second harmonic generation in the presence of large group velocity mismatch between the interacting pulses transfers a large fraction of the energy of a broadband fundamental frequency pulse into a narrowband second harmonic one. Using a periodically poled stoichiometric lithium tantalate crystal coupled to an infrared optical parametric amplifier, we generated 200-nJ pulses with spectral width lower than 8.5 cm-1 and tunability from 720 to 890 nm. Energy scaling and extension of the tuning range are straightforward.