Abstract

We demonstrate an efficient amplification of the broadband tunable (from 3.8 µm up to 4.8 µm) mid-IR femtosecond (~200 fs) seed pulse in optically pumped Fe²⁺:ZnSe amplifier [1]. The experimental setup is sketched in Fig. 1a. Seed radiation was generated in two-stage AGS-based optical parametric amplifier pumped by Cr:forsterite laser [2]. Mid-IR output radiation with energy up to 1 μJ was then directed into a multipass amplifier. Iron-doped ZnSe crystal was pumped by Q-switched Cr:Yb:Ho:YSGG laser with 40 ns pulse duration, total energy of 20 mJ, operating at 2.85 μm wavelength [3]. In order to prevent carbon dioxide absorption, the last OPA stage and multi-pass amplifier was placed in the chamber, where atmospheric air was substituted by noble gas. Prior to the main experiments, gain evolution measurements were carried out. They have shown that the lifetime of population inversion of Fe²⁺:ZnSe strongly depends on the temperature. It increases from 350 ns (at room temperature) up to 1 µs (at 7 degrees Celsius) paving the way for multi-pass amplification. As a result, femtosecond seed radiation with energy 0.7 µJ centered at 4.3 µm was amplified in six-pass Fe²⁺:ZnSe amplifier up to the energy 70 μJ (Fig.1b), which corresponds to 0.5 GW peak power. Decreasing of the input energy down to 40 nJ did not prevent to decrease in output energy. Therefore the total gain was about 2000. Inset in fig. 1 (b) presents far-field beam profile of amplified pulses. Substitution of air by argon removes spectral distortions in the laser pulse caused by the absorption by atmospheric CO₂ molecules. It does not radically improve the amplification process in Fe²⁺:ZnSe but stabilizes the operating regime (Fig. 1c).

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