A widely tunable femtosecond light source based on green-pumped BBO optical parametric oscillator

Abstract

Femtosecond lasers in the 700–1000 nm spectral region hold irreplaceable research value in many frontier fields. In this work, to achieve tunable femtosecond laser output in the target spectral region, we first frequency-double a high-power Yb-doped all-fiber femtosecond laser at a repetition rate of 80 MHz to generate a high-quality femtosecond green laser. This green laser is then used as a light source to synchronously pump an optical parametric oscillator (OPO) based on a BBO nonlinear crystal. By optimizing the cavity parameters and pumping conditions, we successfully obtain continuous wavelength-tunable femtosecond laser output ranging from 697 nm to 1010 nm. The highest output power of 1.2 W is achieved at 854 nm, corresponding to an optical-to-optical conversion efficiency of 22.6 % from the green pump to the signal wave. The OPO output also exhibits excellent beam quality, with M 2 factors measured to be 1.18 in x direction and 1.11 in y direction, both very close to the diffraction limit. The power stability is measured to be about 0.7 % at an output power of 580 mW that is already sufficient for many applications. Across the entire tunable wavelength range, the pulse duration of the OPO output varies between 130 fs and 230 fs. Based on these performance characteristics, we believe that the tunable femtosecond laser developed in this work is well suited for various cutting-edge applications, including label-free biomedical imaging, nonlinear spectroscopy, and studies of ultrafast carrier dynamics.