Nonlinear effects channel high-peak-power laser beams through turbulent air

Laser weapons, free-space optical communications, and various types of laser remote sensing all depend on the ability to transmit a laser though the Earth’s atmosphere without the beam becoming too broken up by atmospheric turbulence. Adaptive optical (AO) systems, in which the wavefront of the laser beam is modified by a deformable mirror to compensate for wavefront distortions introduced by the atmosphere, are often used to alleviate this problem. However, high-power laser pulses introduce nonlinear atmospheric distortions such as filamentation that are not easily compensated by AO, which is a linear-optical process.

Researchers at the U.S. Naval Research Laboratory (Washington, DC), University of Alabama (Huntsville, AL), University of Rochester Laboratory for Laser Energetics (Rochester, NY), and Georgetown University (Washington, DC) have shown that for nonlinear high-peak-power beam propagation in which the beam size is smaller than the inner scale size of the turbulence (the smallest eddies produced, limited by the viscosity of air) or the transverse coherence length, the beam remains self-channeled and resists turbulence-induced beam spreading. A computer model, along with experiments using a Ti:sapphire laser emitting 7 mJ pulses at a 1 kHz repetition rate, a 35 fs duration, and an 800 nm center wavelength, showed that the laser beam spot size of 14 mm (smaller than the measured inner scale size) was maintained to within 33% over an 825 m propagation distance, even under very turbulent conditions (a scintillation index >7). Thus, via nonlinear self-channeling, a laser beam can be made to propagate without much beam spreading over long distances through high levels of random turbulence in the atmosphere. Reference: M. H. Helle et al., arXiv:1905.08668v1 [physics.optics] (May 21, 2019).