In the early 1990s, with the invention of Kerr Lens Clamping (KLM) Ti:Sapphire laser, femtosecond laser technology has been rapidly developed and widely used in strong field physics, ultra-fast phenomena, micro-nano processing, biomedicine, Broadband communications and other fields. At present, KLM Ti:Sapphire laser oscillator is the most mature femtosecond laser source, which can directly generate very short pulses close to a wholesale BBO nonlinear crystal (~2.7fs). However, because its center wavelength is in the near-infrared region, the tuning range is limited, so it is limited. Ultrafast spectroscopy research applications that can be performed over a wider range of wavelengths. In contrast, the Synchronous Pumped Femtosecond Optical Parametric Oscillator (OPO) has been favored by the ability to widen tuned femtosecond laser pulses, and has been the subject of KLM Ti:Sapphire lasers for more than 20 years. One of the hot topics of ultrafast laser research. However, compared with the titanium sapphire laser, the femtosecond OPO is limited by the power of the pump laser and the destruction threshold of the spherical/aspherical/cylindrical/paraboloid optical lens used, and the output power tends to be low, such as conventionally using quasi-phase matching crystals (such as PPLN, PPKTP, PPSLT). The output power of the femtosecond OPO is generally only about 100mW, which greatly restricts the practical application that can be carried out, and because the quasi-phase-matched crystal has a thick thickness, it not only causes the pulse to experience a large dispersion, but also causes the signal light and the pump light. There is a very large group speed mismatch (GVM) between them. In recent years, with the emergence and development of high-power all-solid femtosecond lasers, KTP, BBO, LBO and other crystals with high damage thresholds are used as parametric gain crystals, which provides a possibility for the development of high-power femtosecond OPO. In addition, since such block material crystals can be made thin, have small dispersion and group velocity mismatch, shorter pulses can also be obtained. But the new problem is the low efficiency of the parameters and the high oscillation threshold.

For the academic significance and wide application of femtosecond OPO, the L07 group led by Prof. Wei Zhiyi, Institute of Physics, Chinese Academy of Sciences/Beijing National Laboratory for Condensed Matter Physics, began research on this work many years ago and adopted self-design in 2007. The femtosecond Ti:Sapphire laser oscillator was built as a pump, and the femtosecond OPO output was realized in China for the first time [1, 2]. Since then, with the deepening of the research work, the research team and Xi'an University of Electronic Science and Technology have successively realized dual-wavelength femtosecond OPO[3], intracavity frequency-doubled femtosecond OPO[4], all-solid-state 1um-band femtosecond laser pump. OPO and intracavity frequency doubling results [5,6]. Recently, they have adopted the new nonlinear crystal bismuth borate (BIBO) as a parametric gain crystal in recent years, further realizing the femtosecond OPO output of 515 nm green light pump. Compared to BBO crystals, the crystal has a larger allowable angle, a wider angular tuning range, and a smaller spatial departure angle. The effective nonlinear coefficient in the visible-near-infrared band is 3.2 pm/V, which is BBO. 1.6 times and 4 times of LBO, and there are no two kinds of Ι phase matching conditions, so it is theoretically more suitable for high power green light pumped femtosecond OPO. In the experiment, they used a 1 mm long BIBO crystal, and under the 3.4-nm femtosecond 515-nm laser pump with an average power of 3.4 W, the signal light in the range of 688-1057 nm and the range of 1150-1900 nm were obtained by angle tuning. Frequency light output. The highest average power of the signal light is 1.09 W, and the power in the entire tuning range is greater than 450 mW, and the corresponding total light-to-light conversion efficiency exceeds 40%. Based on this, the SF6 prism is used to extract the signal light and idler light with the shortest pulse of 71 fs and 90 fs respectively. Compared with the existing OPO results using LBO and BBO crystals, BIBO is proved to be a new wholesale high efficiency laser grade optical filter. For femtosecond OPO, not only can higher light-light conversion efficiency be achieved, but also a femtosecond laser with a wider wavelength range can be produced. pulse.