Generation of an air laser at extended distances by femtosecond laser filamentation with telescope optics

We present the generation of self-induced-white-light-seeded lasing action of nitrogen molecules in air by a Ti:sapphire femtosecond laser (800nm, 5.5mJ) and demonstrate that such lasing action is strongly influenced by external focusing conditions. It is found that the self-seeded lasing signal of N ₂⁺ at ∼391 nm decreases dramatically by orders of magnitude and ultimately disappears when the focal length of an external lens increases from 0.5 m to 1 m. By using a telescope, it is shown that such limitation can be overcome and the 391 nm lasing can be controlled to occur at remotely designated distance, providing a possibility for practical applications in standoff spectroscopy.