[Laser] laser pointer

[C. Turner]

As mentioned on the web page to which I linked, there are several 
cautions about using a green laser pointer:  None are insurmountable, 
but they do need to be considered.

- Many green laser pointers are modulated.  I'm not sure if this is 
intentional or not, but many are switched in some way - possibly as part 
of their regulator or just some inherent instability in their 
circuitry.  I believe that Sam Goldwasser's Laser FAQ site makes some 
mention of this.  If the laser is already modulated - or if there is 
already some circuitry "in the way" then modulating them with your very 
own signal is made somewhat more complicated.

- Increased attenuation/scintillation.  This has to do with the shorter 
wavelength, but over very short distances, this is less of an issue.

- Reduced sensitivity using silicon detectors.  You'll take a 10-15dB 
hit at this shorter wavelength in sensitivity if you are using a 
solid-state detector of this sort.  If city lights are in the field of 
view of the detector, theatrical gels such as "Kelly Green" offer some 
broad bandpass characteristics - although the 546nm mercury line is 
still prevalent.  Again, for shorter paths, this isn't a big deal.

- Attention-getting.  The green beam being more visible may or may not 
be an advantage.  It does help to aim the beam as it can be more easily 
seen at the transmit end, but as mentioned before, it's still easy for 
the "other" end to spot a red laser pointer - even at a distance of >100km.

As far as a "sighting guide" a green laser could be useful if precisely 
coupled with a red one so that they beams were precisely parallel.  From 
a practical standpoint, it is unlikely that you could exactly point the 
laser using such a sighting guide alone - even if you have a direct 
visual reference of the far end (spotlight, headlights, etc.) although 
being "close" does make it far easier:  The simple expedient of 
scattering dirt in the red laser's beam (especially at some distance in 
front of the laser to minimize parallax) is also helpful.

Ultimately, you will require some sort of feedback from the far end:  In 
our tests, this is greatly simplified by having already set up the 
receiver (for prior LED communications) and using the "Audible S-Meter" 
system.  With that, it took only a few minutes to set up 2-way 
laser-pointer communications on a 172km path.  Barring this, we could 
have used the "observer" method by which the flashes are sighted 
visually - but as mentioned on the web page, this is much more awkward 
and takes longer.

73,

Clint
KA7OEI