[Laser] IR laser Array???

[n9jim-6]

1) A few years ago we had an debate about  power addition.
In terms of pure power present at a non-correlated receiver, this will 
increase the received power in areas where the beams overlap. However, 
there will be phase differences in each  tx-rx path. So in a correlated 
receiver, they do not add.
(this is what I can remember)

A simple experiment that I just ran...

Using a silicon detector (power meter with a diffuser) I measured 2 
different laser pointers individually:
Laser A - 5.6 mW
Laser B - 2.4 mW

applying both lasers on the same detector at the same time resulted in:
8.0 mW

Conclusion: A+B works for non-correlated detectors (Si PIN diodes)

2) Safety
a) aircraft
We've been advised to never point any laser of any power at aircraft, 
even class 1 (eye safe).  Why? because IF it is seen by a pilot, it can 
distract him (dazzle).  Its not for eye safety but flying safety.  Do 
you believe this?  If you have ever set up a long distance (10+ miles) 
thru air experiment with a simple laser pointer, you know this is real! 
 A couple years ago, a friend and I set up a MCW (tone on off keying) 
experiment.  One end was up on a hill outside town (VERY DARK).  The 
other was in the middle of  a 1 million+ metro area, among the lights. 
 This was a distance of 10-12 miles. When he finally got "on target" 
that <5mW RED laser was the brightest thing anywhere in the metro area! 
No other light source could compare. A good approximation is that a 5mW 
laser is roughly equivalent to a 5KW bulb.
b) IR
IR has the advantage of not being "seen". So dazzle effects are 
eliminated.  However, eye damage is not.  So if  power levels exceed 
class 1 damage may occur, and no eye blink response exists for class 3a 
protection. So power density level at the aircraft must be "eye safe".
Some detectors can see IR: night vision goggles, most B&W cameras, some 
military aircraft sensors.  
So there is the potential for an issue should an aircraft pass through 
the signal path.
BOTTOM LINE: if an aircraft appears to be coming close to the signal 
path... stop transmitting until it is clear.

Jim


Richard Burrows wrote:

>Hi
>Has anyone done any experiments using several CD type IR lasers to increase power levels without collimation or large lens.
>I have seen LED arrays being talked about and used by Hams, but have found only complex University type schemes for laser arrays using complex wavefront feedback methods.
>I have a lot of old CD laser diodes  circa 2-5mw that I thought I could use on a straight forward :) array ! Anyone got any thoughts on this please.Or am I wasting my time?
>I want to use such an array for cloud/troposcatter over the horizon BPSK, that's the aim anyway.
>Another question re Safety and Aircraft/helicopters. If low power IR were used the dazzle effect on pilots eyes is not there, but are there any instrumentation on aircraft that might be effected? 
>Obviously Police TV spy in the sky cameras could be a problem.
>
>                          regards   
>                                 Dick G8BYI
>
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