Optica 3, 678–681 (2016)
Laser filamentation, a nonlinear process that creates strands of laser light that maintain their spatial shape over long propagation distances, is a promising technology for remote sensing applications such as LIDAR. Houkun Liang and co-workers now report progress in the generation of laser filaments in air at mid-infrared (MIR) wavelengths, achieving filamentation at a kilohertz repetition rate. To this end, the team used a three-stage optical parametric chirped-pulse amplifier delivering 33-fs-long pulses at 1 kHz and a wavelength of 2.1 μm. They created filaments of about 6 cm in length when a CaF2 lens with a focal length f = 300 mm was used to focus the beam. The filamentation led to supercontinuum generation in the spectral range 1.5–3 μm, together with odd high harmonics up to the ninth order. The authors reported maximum efficiencies for the third- and fifth-harmonic generation of around 5 × 10−3 and 1.5 × 10−4, respectively. As a further test, the authors demonstrated MIR absorption spectroscopy of atmospheric CO2. In a 300-mm-long gas cell filled with CO2, the spectra of the MIR filaments clearly indicated two absorption lines at 2,692 nm and 2,768 nm.
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