Efficient generation of narrowband picosecond pulses from a femtosecond laser.

In some applications of broadband ultrafast spectroscopy, such as surface sum frequency generation vibrational spectroscopy, femtosecond stimulated Raman spectroscopy (SRS), and coherent anti-Stokes Raman spectroscopy, a narrowband picosecond pulse is required to obtain a high spectral resolution. Here, we present a method to generate narrowband picosecond second harmonic (SH) and fundamental frequency (FF) pulses with high-conversion efficiency from a Ti:sapphire femtosecond laser amplifier. The narrowband picosecond SH pulse was generated based on the group velocity mismatch between the SH and FF pulses in a nonlinear crystal of β-barium borate (BBO). The small SH nonlinear optical coefficient was optimized by changing the azimuth angle of a thick BBO crystal, successfully avoiding the saturation effect in the SH generation process. The SH pulse was then used to pump an optical parametric amplifier to efficiently amplify the narrowband FF seed pulse, which was obtained with an etalon by spectrally filtering the output from the femtosecond laser amplifier. Dual-wavelength output, which could be very useful in femtosecond SRS, was also realized.