In this Letter, the authors design a new way of generating broadband mid-infrared femtosecond (fs) laser pulses at a central wavelength of 3.75 μm via two stages of non-collinear OPA (NOPA) in BBO and 4H-Silicon carbide crystals. First, a train of 52-fs pulses at a central wavelength of 800 nm (near infrared) is used to generate white light continuum (WLC) using a YAG crystal. Following WLC generation, in the first stage of NOPA the 1 μm component of the WLC is collimated and parametrically amplified within a thick BBO crystal pumped with a second harmonic of fs pulses (400 nm). Next the amplified WLC pumps the 4H-Silicon carbide crystal simultaneously with 800 nm fs pulses for the generation of the mid-infrared femtosecond pulses in the second stage of NOPA. Finally, the pulse duration of the generated mid-infrared fs pulses is confirmed via a sum frequency generation technique. During these two NOPA processes, the spatial and temporal overlaps of the pulses involved in nonlinear frequency conversion within the crystal are extremely important, since these significantly affect the efficiency of the processes. To achieve these overlap requirements, the authors carefully choose the phase matching angle where the walk off angle is close to zero, and control a non-collinear angle to make the group velocity mismatch zero, as well as compensating angular dispersion by using 4H-silicon carbide prisms.
In summary, the authors engineer the efficient way to generate 3.75 μm mid-infrared fs laser pulses by cleverly arranging the two stages of NOPA with BBO and 4H-Silicon carbide crystals. This technique attract a lot of attention in the mid-infrared ultrafast optics community, since, compared to the visible and the near infrared, the mid-infrared spectral region has not clearly explored in spite of many important applications such as molecular dynamics, remote sensing, and process monitoring.
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