[NLRS] Laser communications...

[Doug Reed]

At 09:41 AM 12/29/2002 -0600, Ed Marciniak wrote:
>If there is any way you can stay with a HeNe laser or a laser diode emitting
>at the same wavelength it might avoid future problems of incompatible
>frequency.

The easiest entry level systems are going to be using a laser light pen.
They used to be on 630 nm but most of them I've seen now say 650 nm because
the eye is more sensitive there and a slightly lower power diode will have
similar brilliance to the eye as a more powerful 630 nm diode. 

The next step above the <5 mw red laser diodes are the 25-35 mw infrared
laser diodes from CD burners. Since they are infrared they are invisible
which makes them less socially incorrect at the same time they become far
more dangerous to your sight. These absolutely require a beam enlarging
lense or reflector to reduce the optical power density to safe levels. I
like the idea of using one of these in a sky-bounce system since they have
more power and are invisible to the naked eye of your neighbors or pilots.
But you'll also need a cheap CCD video camera so you can watch it on TV
while making adjustments..... 

>There are 632 nm interference filters available which would improve daylight
>performance by reducing background light levels about 20db( 3 nm 1/2 power
>bandwidth) at a reasonable cost ($50 or so new, a few $ surplus)

I've got a couple types of red plastic from 3M Surplus years ago. Some was
for LED watches, the other is a lighter red color and I don't know what it
was for. But I don't think it is an interference filter grade of
material..... Burr-Brown had an interesting optical App Note that suggested
a low pass filter in the bias feed back loop for the optical detector. This
not only compensated for light levels, it also cancelled the major 60-120
Hz components from man-made light sources. 

I really haven't checked any sources for optical sensors recently. I don't
know what Digi-Key has any more...... I do know that the larger the surface
area, the more sensitive the detector. But the larger the surface area, the
lower the frequency response too..... And the most sensitive Burr-Brown
optical detector ICs would not even make it to 1 KHz at full gain.... 

73, Doug Reed, N0NAS.