Abstract
A long air plasma channel can be formed by filamentation of intense femtosecond laser pulses. However, the lifetime of the plasma channel produced by a single femtosecond laser pulse is too short (only a few nanoseconds) for many potential applications based on the conductivity of the plasma channel. Therefore, prolonging the lifetime of the plasma channel is one of the key challenges in the research of femtosecond laser filamentation. In this study, a unique femtosecond laser source was developed to produce a high-quality femtosecond laser pulse sequence with an interval of 2.9 ns and a uniformly distributed single-pulse energy. The metre scale quasi-steady-state plasma channel with a 60–80 ns lifetime was formed by such pulse sequences in air. The simulation study for filamentation of dual femtosecond pulses indicated that the plasma channel left by the previous pulse was weakly affected the filamentation of the next pulse in sequence under our experimental conditions.
Highlights
Close to the start and end positions of the plasma channel, the electron density produced by each pulse in sequence is very low, at only 1014cm−3
In the free period between pulses, the weak parts of the plasma channel decayed very slowly, and the electron density accumulated with the number of pulses
In the trailing edge of the pulse sequence, when the ionization could not compensate for the recombination, the electron density decreased with the envelope of pulse sequences
Summary
The interval between pulses (14 ns) was still much longer than the channel lifetime, and the uniformity of the pulse energy in sequence was rather poor. Under those conditions, a long-lifetime plasma channel could be generated only by the tight focusing of such laser pulse sequences in air, and the length of the plasma channel was only several mm. We used the output from a home-made 350 MHz repetition rate (corresponding only to a 2.9 ns interval) femtosecond oscillator as the seed and successfully generated an amplified pulse sequence with more than 20 pulses in 70 mJ total energy. The coupling between pulses in sequence during filamentation was studied by numerical simulations
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