Eric Lee with the Free Space Laser Comunication gear.

Subject: Laser communications experiment performed between Fremont Peak State Park and Lick Observatory on Mount Hamilton

Lasers used:  20 milliwatt diode laser, 685 nm
              30 milliwatt diode laser, 830 nm
              beam divergence 1 milli radian  
                                              
propagation path length  48 kilometers          D
visual range  20 - 100 km                       V
beam power                                      P
tangent(divergence angle)                       T
Rayleigh scattering coefficient                 R
Mie scattering coefficient                      M
wavelength                                      l
                                                 
                                                 
                                                 
Geometric divergence:

power density = 4P/(3.14(T^2)(D^2))

              = 1.1 * 10^-9 watts/cm2   at 685 nm

              = 1.6 * 10^-9 watts/cm2   at 830 nm

Atmospheric path loss:

Rayleigh coefficient  R = 0.00833*(l/600)^-4.08

                        = 4.9 * 10^-3  km^-1  at 685 nm

                        = 2.2 * 10^-3  km^-1  at 830 nm

Mie coefficient       M = (3.91/V)*(l/550)^((-0.585)*(V^0.3))

                        = 0.14 to 0.023   km^-1   at 685 nm

                        = 0.11 to 0.015   km^-1   at 830 nm

visual range of 20 to 100 km, considered to correspond to

"clear" and "extremely clear" transparency
conditions

percent transmission = e^-(M+R)D

                     = 9.5 * 10^-4  to  2.6 * 10^-1   at 685 nm

                     = 4.6 * 10^-3  to  4.3 * 10^-1   at 830 nm

Maximum power density at Lick Observatory:
(power density)*(percent transmission) =
                     
		     = 2.9 * 10^-10 watts/cm2    at 685 nm

                     = 6.9 * 10^-10 watts/cm2    at 830 nm