Synchro-pumped femtosecond optical parametric oscillator (OPO) is very popular because of its ability to broaden the tuned femtosecond laser pulse. It has been one of the hot topics in ultrafast laser research for more than 20 years since KLM titanium gem laser appeared. However, due to the limited power of the pump laser and the failure threshold of the nonlinear crystal used, the femtosecond OPO output power is often very low, and due to the thick thickness of the quasi phase matching crystal, not only the pulse experiences large dispersion, but also leads to a very large group velocity mismatch (GVM) between the signal light and the pump light. In recent years, KTP, BBO, LBO and other crystals with high failure threshold have been used as parametric gain crystals. Because such block material crystals can be made very thin and have small dispersion and group velocity mismatch, short pulses can be obtained, but with the new problems of low parametric efficiency and high oscillation threshold.
Recently, the L07 group, led by Zhiyi Wei from the Institute of Physics, Chinese Academy of Sciences/Beijing National Laboratory of Condensed Matter Physics (C), adopted a novel nonlinear crystal Bismuth borate (BIBO) as a parametric gain crystal to further realize femtosecond OPO output by 515nm green light pump (see experimental optical path diagram). Compared with BBO and LBO crystals, BIBO crystals have the advantages of ultraviolet transmittance, high damage threshold, large group velocity dispersion and so on. Theoretically, Bibo crystals are more suitable for femtosecond OPO pumped by high power green light. In the experiment, they used 1 mm long BIBO crystal pumped by femtosecond 515nm frequency-doubling mode-locking Yb:KGW laser with an average power of 3.4 W, and obtained signal light with wavelength covering 688-1057 nm and idle light output in the range of 1150-1900 nm through Angle tuning. The highest average power of signal light is 1.09W, and the power in the whole tuning range is greater than 450 mW, and the corresponding total optical conversion efficiency is more than 40%. On this basis, the signal light and idle light with the shortest pulse of 71 fs and 90 fs are obtained by using the outside cavity compression technique of SF6 prism. Compared with the existing OPO results using LBO and BBO crystals, it is proved that BIBO, as a new crystal, can not only achieve higher optical conversion efficiency, but also generate femtosecond laser pulses with a wider wavelength range.
The results are in the newly published Optics Letters (Vol.41, Issue 21, pp. 4851-4854 (2016)).
The article links: https://doi.org/10.1364/OL.41.004851