| 8 | “I don't see a 50 watt (or less) laser fired through 5 miles of atmosphere not dissipating at least an equal amount of energy in heat.” |
I can appreciate where you're coming from, at first blush what you suggest does seem logical, but here is the hole in that theory.
Just for the sake of argument, let's start out presuming that air will absorb that laser light at the same rate, irrespective of the initial power of the beam. For instance, we might find a particular wavelength laser light was absorbed 10% per kilometer. So, when would the beam be at zero power? Answer: never! It would always retain 90% of the power that entered which ever kilometer section you were measuring and would only lose 10% of that value as it transited through that kilometer.
For an eight kilometer path (~five miles) the final power factor would then be calculated like this:
.9 × .9 × .9 × .9 × .9 × .9 × .9 × .9 = ~.430
As you can see, no matter how many times we multiply by .9 (90%), the answer will always be greater than zero.
If your 50 watt laser beam could only travel 5 miles through the air before being entirely absorbed, then no beam, no matter how powerful or weak it was, could travel more than 5 miles through the air. In which case I'd ask you how it is possible for the dim light of stars to shine through 60 miles of air? Surely you don't mean to suggest that a weaker beam can travel farther through the atmosphere than a more powerful one?
Now let's look again at my original presumption that all beams of a given wavelength, no matter their power, will be absorbed at a constant rate as they travel through the atmosphere. Is it possible to employ such a high power beam that you literally "cook" the air and cause a change in its physical properties such that the rate of absorption is changed? The answer is yes, but at 50 watts (an a nominal beam diameter) you aren't even within 6 orders of magnitude of the power density necessary to do that.
I have a 100 watt, 5mm beam diameter, continuous wave, extremely well collimated, Nd-YAG laser in my basement lab and I flat guarantee you that if I ever pointed it at a plane flying 5 miles away, I'd blind the pilot, giving him permanent eye damage, IR absorbing cockpit windshield, notwithstanding.
Kind of hard to get these concepts across in a short post, but I hoped some of what I wrote cleared up the problems of atmospheric absorption of laser beams for you.
--Boot Hill