Here we propose a simple and effective system for conversion of 200-fs, 1030 nm pulses into few-optical-cycles pulses in the 3-6 um spectral range. The scheme works as follows. Small part of pump radiation is converted into supercontinuum and the radiation from the 700-900 nm spectral range is used as a seed in the first OPA. The remaining pump beam passes through a BBO crystal which converts less than 3% of the pulse energy into the second harmonic (515 nm). The green beam is used as a pump in first OPA based on 1 mm-think BBO crystal. Collinear geometry is used to generate idler pulses in the 1.2-1.6 um range without angular chirp. The idler beam is then used as a seed in the second collinear OPA, which is pumped by the unconverted 1030 nm beam. The resulting idler pulses in 3-6 um range are slightly chirped depending on the crystal used (LGS, GS, AGS, LiIO3) and are compressed in the bulk material (silicon or germanium).
Such scheme provides a passive stabilization of the carrier-to-envelope phase of the resulting pulses. Because of the lower group velocity mismatch (GVM) between interacting pulses in nonlinear infrared crystals, as compared to Ti:sapphire based pump lasers, bandwidths supporting few-optical-cycles pulses in the 3-6 um spectral range are easily obtained. In the experiment we have used 200 uJ pulses (1030 nm) at repetition rate 1-10 kHz and the total quantum conversion efficiency 25-35% was reached. The resulting mid-infrared pulses were fully characterized.
Michał Nejbauer, Marcin Pastorczak, Tomasz Kardaś, Yuriy Stepanenko, and Czesław Radzewicz, "Convenient scheme for efficient generation of mid-infrared CEP-stabilized femtosecond pulses using 1030-nm source (Conference Presentation)," Proc. SPIE 10516, Nonlinear Frequency Generation and Conversion: Materials and Devices XVII, 105160B (Presented at SPIE LASE: January 29, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2291468.5751428851001.
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