High field nonlinear interaction of femtosecond pulses with gases

Non-linear propagation of ultra-short, high energy pulses with a central wavelength of 1240 nm was experimentally investigated in the gaseous medium at high pressures. Experimental conditions were optimized to generate the widest output spectra through a nonlinear interaction of 110 fs Cr: Forsterite impulses in a high-pressure chamber filled with pure xenon or nitrogen. Record spectral widths for Cr: Forsterit pulses with milijoule energies were achieved using nitrogen and shaping the input beam profile exhibiting significant spectral intensities in the range of 1100 to 1600 nm. During the spectral broadering study, simultaneously the output beam profile and chamber throughput were monitored, revealing different nonlinear propagating modes of filamentation formation. Conditions of single filament generation have been proven to be most effective for generating a wide spectrum with energies above 1.0 mJ and excellent beam quality. We present an alternative and, in many ways, an attractive approach to efficiently generate pulses with mJ energy compressible for the duration of only a few oscillations of the light field. Cr: Forsterite lasers are able to routinely provide light pulses with lengths of 30 fs at output from the main oscillator by mode locking synchronization. Amplify technologies for Cr: Forsterite systems allow to achieve a milijoule energy intensity with less than 100 fs. In this work, for the first time, was shown the spectral broadening in area of pulse energies instrumental for pulse compression over one octave. Numerical simulations based on the general Schroedinger equation illustrate the basic pulse propagation dynamics by reproducing key trends in experimental data, and indicate the time compressibility of the emerging supercontinuum over the pulse width of several cycles. One way how to create a multi-cycle impulse is non-linear self-compression. Nonlinear mixing of the second harmonic frequency with the fundamental laser frequency in the laser filamentation process was used for complex characterization of the output pulse. The fundamental wavelength of 1240 nm from the amplified Cr: Forsterlite femtosecond laser system was mixed with its second harmonic in a high-pressure gas chamber filled with SF₆ gas, where generation of filamentation was achieved by intense fs pulses in the SF₆ gas. The recorded spectral interference was evaluated using spectral phase interferometry for direct electric-field reconstruction (SPIDER) to determine the complex characteristics of the output impulses. Experimental results supported by numerical simulations also indicated a self-compression in the filaments where the original 3.5 mJ pulse with a length of 112 fs was shortened to 6 times, via passing through a high pressure gas filled SF₆ gas chamber. The presented method is original not only from the point of view of generating very short pulses in milijoule energies, but also from the point of view of complex pulse characterization without the use of special diagnostic apparatuses. (author)