[Laser] RE: Beacon to test a photodetector sensitivity limit.

[Des]

Hi to everyone on the list.

Terry (W5TDM) wrote:
use optical methods to control intensity. Best methods
would be either very 
small pin holes or ND filters, or both. Pin holes can
control down to 
decade level, then fine tune level with ND filters.
**********************************

I never seem to get enough time to play with lasers
but I did get as far as building a modest laser beacon
to do some tests with. This was about 4 years ago and
used a 650nm laser pointer diode. For indoor tests
this was way to bright. 

I wanted to do some general sensitivity tests within
the shack (without swamping my PGP receiver front end)
so I used a Polarizing Filter and aranged for it to be
rotatable. I think this appears in "The Laser
Cookbook" in some detail.

The front end was still "swamped" so I added a second
rotating polarizing filter, this reduced the O/P of
the 1mW laser to well below visible limits but still
detectabe with the PGP rx. 

I used the polarizing filters from a couple of
obsolete LCD displays. I mounted the polarizing
filters in the center of a couple of old CD discs
(covering the spindle hole) the discs themselves are
epoxy glued to a couple of PL259 plug shells as
described in the cookbook. These screw onto matching
PL259 sockets (with the center connection drilled out
to let the light through) and this permits the
disc/filter to rotate. The large diameter of the CD
makes for easy rotation of the polarizing filters and
very small angular movements. To "stiffen" the threads
a little I fitted some soft springs between the CD
discs and the PL259 socket body.

The system was not calibrated in any way but it did
give very smooth attenuation over very wide limits. 

I hope this helps someone, if not please excuse the
BW.

73,s to all on the list.


http://www.qrss.thersgb.net/


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