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Filamentation and pulse compression

 

Example: Filament-assisted Gas Laser

 

We investigate potential of a filament to provide conditions for stimulated emission from nitrogen and oxygen in the atmosphere.The goal of our research is a remote source of coherent backward propagating radiation created by a laser filament in the atmosphere. The development of such a source is envisaged in project CROSS TRAP (Coherently-enhanced Raman One-beam Standoff Spectroscopic TRacing of Airborne Pollutants), a STREP project funded by the EU Seventh Framework Program aimed at developing a highly sensitive and selective standoff diagnostics of atmosphere based on a symbiosis of the laser filamentation and CARS techniques.

Filamentation of high power femtosecond laser radiation in gases manifests itself in the formation of a self-guided high intensity field structure, accompanied by significant spectral broadening, supercontinuum generation and creation of a plasma channel in the wake of the pulse. Formation of plasma in the filament initiates a chain of plasma-chemical reactions in the atmosphere which lead to the appearance of a large variety of neutral and ionic species in rotationally, vibrationally and electronically excited states similar to a gas discharge. As one of the consequences, favorable conditions for population inversion and lasing between different electronic levels of nitrogen and oxygen might be created.  In our experimental research, we create a filament or multiple filaments in air or gas-filled cell under various pressures using our novel high power femtosecond laser sources at 1,03 μm and 3,9 μm wavelengths. We measure the spectrum, temporal and spatial profiles of fluorescence from neutral molecular nitrogen and atomic oxygen in the filament.

 

UV-lasing from nitrogen-argon mixture

In our recent experiments we demonstrate for the first time efficient lasing from the filament created by high power femtosecond laser pulse in a mixture of nitrogen and argon. To provide the lasing conditions we exploit the process of resonant excitation transfer from excited noble gas atoms to molecules (Bennett 1962). Excited metastable atoms of argon are produced as a result of two-step kinetic process in the filament plasma: three-body collisions Ar+ + 2Ar → Ar2+ + Ar and Ar2+ + e → Ar* + Ar. Population inversion in nitrogen is achieved due to second kind collisions Ar* + N2(X'Σg+) → Ar + N2(C3Πu). Thus excited argon atoms serve as a collisional pump source instead of electrons in a conventional discharge-pumped nitrogen laser.

 

Fig1. Spectrum of fluorescence without amplification (left panel) and when lasing at 337 nm is achieved (right panel). N2 pressure 1 bar, Ar pressure 5 bar. Under optimal conditions, lasing at 357nm line is about to begin.

 

In the experiment, 0.5 kHz, 200 fs, up to 6 mJ laser pulses at 1,03 μm wavelength from a home-made DPSS Yb:CaF2 laser system (Pugzlis 2009) are focused nto a gas-cell with Brewster-angle CaF2 windows through a dichroic mirror having high transmission at 1,03 μm and high reflection (>90 %) in the 300-450 nm spectral range. The gas-cell is filled by the nitrogen-argon mixture. The threshold on partial pressures of nitrogen and argon for lasing from the filament is measured as 0.6 bar and 3.5 bar correspondingly. Typical fluorescence spectrum of nitrogen below lasing threshold and under optimal lasing conditions is presented in Fig.1. The 2f-2f image from the cell input window of the beam and the pulse temporal profile of the generated UV emission are presented in Fig.2. Pulses with a duration 1.5 ns and energy 200 nJ were measured under optimal conditions.

 

Fig.2. Spatial profile of spontaneous fluorescence (left) and lasing beam (middle). Right panel presents temporal profile of the generated UV pulse. The red curve demonstrates response function of the photo-diode.

 

References

(Bennett 1962) - W.R. Bennett, W.L. Faust, R.A. McFarlane, and C.K.N. Patel, “Dissociative excitation transfer and optical maser oscillation in Ne-O2 and Ar-O2 rf discharges“, Phys. Rev. Lett. 8, p.470 (1962).

(Pugzlis 2009) - A. Pugžlys, G. Andriukaitis, A. Baltuška, L. Su, J. Xu, H. Li, R. Li, W. J. Lai, P. B. Phua, A. Marcinkevicius, M. E. Fermann, L. Giniunas, R. Danielius, and S. Ališauskas, “Multi-mJ, 200-fs, cw-pumped, cryogenically cooled, Yb,Na:CaF2 amplifier”, Opt. Lett. 34, 2075 (2009).