Quasi-steady-state air plasma channel produced by a femtosecond laser pulse sequence

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.

Figure 1. The scheme of the laser system for the generation of a femtosecond pulse sequence and the photodiode signals of the pulse sequence at different stages.

(a) From oscillator, (b) after 1st preamplifier, (c) after 2nd preamplifier, (d) final output optimized for maximum total energy, and (e) final output optimized for best uniformity.

Figure 2. Setup of electric diagnostics of the plasma channel and an example of the electric signal of the plasma channel generated by a single femtosecond laser pulse.

Figure 3. Electric signal of the plasma channel under f = 2 m external focusing at different distances.

(a) z = 0 cm, (b) z = 5 cm, (c) z = 10 cm, (d) z = 20 cm, (e) z = 25 cm, and (f) z = 35 cm.

Figure 4. Electric signal of the plasma channel under f = 4 m external focusing at different distances.

(a) z = 0 cm, (b) z = 25 cm, (c) z = 37.5 cm, (d) z = 60 cm, (e) z = 85 cm, and (f) z = 112.5 cm.

Figure 5

Peak electric signal (left vertical axis) and electron density (right vertical axis) of the plasma channel under initial focusing conditions f = 2 m (open circle) and f = 4 m (square).

Figure 6

The electron density distribution of the plasma channel at different points in time: (a) after the first pulse; (b) just before the second pulse; and (c) after the second pulse.

Figure 7

On-axis electron density of the plasma channel at different points in time: red line – after the 1st pulse, green line – before the 2nd pulse, and blue line – after the 2nd pulse.