Collection of remote optical signals by air waveguides
  Collection of weak signals from remote locations is the primary goal and the primary hurdle of optical stand-off detection schemes. Typically, the measured signal is enhanced using large numerical aperture collection optics and high gain detectors. We show that the signal in remote detection techniques can be enhanced by using a long-lived air waveguide generated by an array of femtosecond filaments. We present a proof of principle experiment using an air plasma spark source and a ~1 m air waveguide showing an increase in collected signal of ~50%. For standoff distances of 100 m, this implies that the signal-to-noise ratio can be increased by a factor ~10$^4$. 
E. W. Rosenthal, N. Jhajj, J. K. Wahlstrand, and H. M. Milchberg
physics.optics