Turbulent fluctuations in the thermodynamic propeties of the atmosphere, and therefore the refractive index, degrade the ability of laser systems to accurately and precisely put energy onto a target at long ranges. To study these complex effects, we generate repeatable turbulent flows with prescribed intensity distributions and propagate lasers through these flow fields within the 400-meter-long Ballistic, Aero-Optics, and Materials (BAM) Range at the George H.W. Bush Combat Development Complex. Within this experimental environment, we are able to perform fundamental studies of laser propagation with unprecedented control over atmospheric conditions, compare with and validate theory and simulation used with real-world systems, and develop novel diagnostic techniques for atmospheric laser propagation.
Figure 1: Laser propagation through artificially generated buoyancy-driven turbulence within the BAM range. Images of fluctuating beam profiles are being captured using a fluorescent imaging panel (FLIP) attached to the mobile imaging platform at the far end of the range.
Atmospheric Laser Propagation – Thermal Blooming
As a laser propagates through the atmosphere, absorption of light by molecules and aerosols in the laser path can heat the air and cause a defocusing of the laser that may also skew the beam if there is a relative windspeed. In the Subscale Atmospheric Facility (SAF) in the ALLEMO, we can simulate different atmospheric compositions at up to an 80-meter path length. In this facility we can introduce different gas mixtures with varying levels of humidity, as well as solid and liquid aerosols, at pressures ranging from above 1 atm to below 1 Torr. When combined with high-speed imaging, the timescales of thermal blooming can be characterized from the initial heating of the air out to stabalization of buoyancy effects. More complex processes may also be studied over the extended path length, including pulsed laser vaporization of particulate and nonlinear optical processes.
Figure 2: Co-propagating CW HeNe and pulsed Nd:YAG lasers (top right) and reference HeNe path (bottom left) within particulate laden atmosphere inside of SAF. White colored particulate field visible around top beam path is created by incandescence and vaporization of soot particulate by a laser pulse.