Atmospheric transmission windows for high energy short pulse lasers

• Main Author: Chan, Mun Kit
• Other Authors: Walters, Donald L.
• Published: Monterey, California. Naval Postgraduate School 2012
• Online Access: http://hdl.handle.net/10945/6224

The US Navy is exploring the feasibility of using a high energy laser weapon as a ship-borne self-defense system against sea-skimming cruise missile attacks. Since the attenuation of laser energy by the atmosphere is the highest at low altitudes and varies with frequency, the selection of appropriate wavelengths becomes critical for a laser weapon to be effective. A high energy free electron laser (FEL) is suitable for employment in the envisaged role because it can be designed to operate at any desired frequency and, to a degree, is tunable in operation. This study aims to determine the optimal atmospheric windows for high energy, pico second, short pulse lasers. Computer simulations were carried out by convolving a pulse spectrum with the transmission spectrum and the absorption coefficient from the MODTRAN and FASCODE atmospheric codes. Transmission spectrum and absorption coefficient plots were compared to find the range of suitable wavelengths that give good transmittance and low absorption coefficient values. The molecular absorption spectrum was chosen over the extinction spectrum because of known limitations of the Navy Aerosol Model results incorporated into the MODTRAN and FASCODE calculations. Results showed that several suitable windows could be found within the 0.95 to 2.5 æm region that offer at least 90% transmittance with absorption coefficient values of not more than 0.02 per km. For 99% transmittance, optimal wavelengths are between 1.03 and 1.06 æm, and around 1.241 and 1.624 æm. However, the disadvantage of operating near the 1 æm region is the high aerosol extinction. The Navy Aerosol Model used in the atmospheric codes needs to be validated or replaced and actual aerosol data collected in the geographical areas of interest before a more accurate assessment of the optimal wavelengths can be made.