## Wednesday, June 17, 2020

### Free Space Laser Communications 35 Miles - June 1993 - AM Video and PWM Audio

Eric Lee with home made Free Space Laser Communication gear.

35MM camera lenses used to collimate IR laser diodes and Telescopes with Avalanche photodiodes to receive data.

The Link was actually established June of 1993 between Mt Hamilton and Fremont Peaks at total of 35 Miles, Both Infra-red and Visible laser diodes were uses to transmit PWM voice and attempted video.

I actually got my Amature Radio licence just to be able to officially log our communication.  KE6BYN.

### 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

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
```

Sadly Eric Passed away May, 2018 .

https://quelab.net/19535/memorial-and-celebration-of-life-for-eric-lee/

https://today.slac.stanford.edu/feature/2007/profile-eric-lee.asp

https://www.researchgate.net/scientific-contributions/8490440_Eric_R_Lee