High efficiency, widely tunable, green-laser-pumped BiB ₃ O ₆ femtosecond OPO with noncollinear phase matching in the yz plane

A femtosecond optical parametric oscillator (fs-OPO) based on type-I (o→e + e) noncollinear phase matching in the yz plane of bismuth triborate (BIBO) crystal is demonstrated, which breaks the bandwidth (700-1900 nm) limit set by collinear phase matching. The pumping source is provided by the second-harmonic (SH, 520 nm) wave of a homemade 1040-nm Yb:fiber femtosecond laser with a pulse duration of 177 fs and a repetition rate of 100 MHz. Experimentally, the wavelengths can be tuned from 626 nm to 3051 nm (626-1026 nm for signal and 1067-3051 nm for idler) by adjusting the angle of the BIBO crystal, which, to the best of our knowledge, is the widest wavelength tuning range for any BIBO-based femtosecond OPO. At the wavelength of 713 nm, the highest average power of 1.3 W is obtained, with a conversion efficiency of 37.1%. If both signal and idler powers are counted in, the total efficiency is 52%, which is a new benchmark. In addition, the compressed pulse duration is measured to be 108 fs at 788 nm, with a time-bandwidth product (TBP) of 0.46. The beam quality factors M² of 788 nm laser are measured to be M<i>x</i>2=1.38 and M<i>y</i>2=1.18, respectively.