[Laser] Light Communications Questions.

[Chuck Hast]

On Tue, Mar 3, 2009 at 9:29 AM, Stan <stan at capecod.com> wrote:
> Hello The Net:
>
> I have accumulated some questions:
>
> 1. Looks like power LEDs are advancing nicely, with respect to actual
> light power output (lumens).
> Any thoughts as to the applicability to light communications ?
> I just got in a 5W ( 3 x AAA batteries) flashlight and it looks like the
> single LED is fed DC power
> directly from the battery bank. No driver module. No idea how many
> lumens it puts out.
> Colored filters (band pass filters) are not commonly available for it yet.
>
> Ebay also has listing for a 12W, single LED, flashlight, using 2 each
> CR123A ( 3.7V @ 1000 mAh)  batteries.
>
>
> 2. Any idea what the band pass of the Kodak red wratten sheet filter
> material is ?
>  Yes red, but 627 or 635 nM ?  or 600 or 7000 nM,  half power bandwidth ?
>
> 3. Any know issues with modulating the higher power LED sources ?
> Can they handle 10 MHz modulation or is there severely limited
> frequency response due to the actual (large) die construction ?
>
I am not sure on the white ones but I have worked with the 5w Lumileds
in red and I think I was able to get them to switch up to 10Mhz but not
much above. The capacitance is the issue.


I am not sure why you would use the white ones, they are the solid
state version of a fluorescent lamp and as such the phosphor has delays
as it changes state. I would just use one of the monochromatic leds
as they will switch a lot faster and give you what you are probably looking
for.

> I have seen current sourcing "dimmable"  drivers, but I expect the
> frequency to be in the low audio range.
>
> 4. I am looking for a source of larger acrylic fresnel lenses.
> At one time this reflector pointed me to a Asian source, but I have lost
> the reference.
>
> I am still in the thinking stage.
>

Phillips LumiLEDs is a great source, I suspect that the 1 watt LED will work
just fine, and it has a smaller die so should have less capacitance. I did
some work with the burn lasers out of DVD R/W, these diodes will work
quite well up at those speeds. You might gather a few DVD machines from
your local computer repair shop and try them. They range from 150mw
8X write speed) to 300mw (24X write speed) you will have plenty of optical
power there. Also I found that the spread from lasing to full power is quite
a bit more than dealing with the 5mw devices which are operating on the
edge of disaster from the start. The DVD laser will lase at about 95ma,
and is at full power at about 150ma, so you have a lot of spread from one
end to the other. I used 74HC05's ganged to modulate the laser, did a great
job and gave me nice square waves at 10Mhz.

-- 
Chuck Hast  -- KP4DJT --
To paraphrase my flight instructor;
"the only dumb question is the one you DID NOT ask resulting in my going
out and having to identify your bits and pieces in the midst of torn
and twisted metal."