The effect of optical turbulence along the slant paths

Authors:

  • Ove Gustafsson

Publish date: 2015-06-08

Report number: FOI-R--4074--SE

Pages: 43

Written in: Swedish

Keywords:

  • Atmosphere Turbulence
  • structure function of the refractive index
  • optical
  • turbulence
  • CN2
  • laser
  • lidar.

Abstract

This report summarizes a study of optical turbulence as a function of altitude and its effect on laser propagation along slant paths, specifically with regard to laser safety. The work is conducted on behalf of the FMV in accordance with FMV's order 380301- LB871360, "Turbulenseffekter för sneda banor". The work includes analyzing and summarizing relevant literature for assessment of the turbulence effect on the laser beams along the slant paths, collecting information and briefly describing relevant models of the structure parameter of the refractive index Cn 2, and gathering information about and briefly describing models for calculation of turbulence effects on risk assessment and risk area for laser radiation. Structure function of refractive index is the single most important parameter in the description of turbulence effects on the propagation of electromagnetic radiation. The report discusses the importance of optical turbulence and reports the most common models for Cn 2 as function of height and a number of methods of measuring Cn 2. Furthermore, calculations are carried out with a new propagation model, WONAT, implemented with so-called CUDA technology for a laser pointer from a high altitude and long distance down to a target at the ground. The probability of laser damage is calculated based on the example of a designator laser and slant path at long distance but with more traditional computing method. The probability of exceeding the maximum permissible exposure level (MPE) is given as a function of distance in comparison with NOHD for three adopted levels of turbulence. The used example shows that there is an 8% risk of exceeding the MPE at a distance 1 km beyond the NOHD due to turbulence level of 2·10-15 m-2/3 at ground height.