Third harmonics emission with nanosecond and femtosecond lasers in air: gamma radiation effects on optical fibers

Abstract

Harmonics generation in plasma by high-power pulsed lasers is an important task for plasma diagnosis. This is because of the strong relationship between the plasma characteristics and the properties of the harmonics radiation. Here we analyze the spatial and temporal properties of third harmonic (TH) radiation generated by focusing ns and fs laser pulses in air at intensities of the order of tens of TW/cm2. The air is considered a non-dispersive non-linear (NL) medium due to the low density ofN2 ions in the breakdown plasma produced under the laser pulses. We address the influence of the pump laser intensity on the properties of TH radiation. We find theoretically and experimentally that the intensity of TH radiation at the output of the NL medium increases linearly with third power of the pump peak intensity, both for ns and fs laser pulses. In case of fs pulses, we demonstrate that TH pulse duration decreases with pump intensity, in correlation to the air breakdown plasma properties. In case of ns laser pulses, we demonstrate that the TH beam diameter is two times smaller than the fundamental’s. The theoretical results on the dependence of the TH signal to the driving peak intensity are supported by experimental data. We further analyze the influence of the gamma radiation on the several commercially optical fibers that are usually employed in the laser-target interaction area. The radiation effects such as photo-darkening induced in optical fibers can severely decrease the performance of optical transmission. We investigated the change of optical transmission induced by gamma-ray radiation. Multimode optical fibers, with a core diameter of 200 μm (usable in UV, ultra-highvacuum and high temperatures conditions), were investigated in order to evaluate their possible use for diagnosis of plasmas produced by high power lasers. The optical properties were studied by analyzing the transmitted spectra of the fibers before and after irradiation, demonstrating a decrease of the measured spectrum of the optical fiber output when increasing gamma dose to 16 kGy. The results are important from both fundamental and practical points of view, providing an efficient tool for prediction of the non-linear optical phenomena in laser produced plasmas and for non-contact diagnosis of the harmonics-generating plasmas.