Energy deposition in a telescopic laser filament for the control of fuel ignition.

The efficiency of energy coupled to plasma during femtosecond (fs) laser filamentation plays a decisive role in a variety of filament applications such as remote fabrication and spectroscopy. However, the energy deposition characterization in the fs laser filament formed by a telescope, which provides an efficient way to extend the filament distance, has not yet been revealed. In the present study, we show that when the distance between the two lenses in a telescope changes, i.e., the effective focal length changes, there exists an optimal plateau energy deposition region in which the energy deposited into the filament per unit length called the average lineic energy deposition (ALED) remains at high levels, exhibiting a remarkable difference from the monotonic change in a single-lens focusing system. As a proof of principle, we examined the influence of the energy deposition on the ignition of a lean methane/air mixture, and found that the use of the telescope can efficiently extend the ignition distance when compared with a single-lens focusing system under the same incident laser energy condition. Our results may help understand the energy deposition behaviors in a variety of telescopic filaments and provide more options to manipulating laser ignition at a desired distance.