[Laser] "retro reflectors"

[Kerry Banke]

Here is a bit of new info from my IR laser diode experiments over the 
weekend.  I read on Sams laser FAQ that in most cases the laser out put 
from pulse lasers will start to level out and start to decrease due to over 
temperature before doing permanent damage. I started out running in pulse 
mode to verify my setup and watched the output on a high speed photodiode 
as well as the  monitor diode internal to the laser diode assembly.  As I 
increased the duty cycle or current beyond a point  I could see the power 
start to level off or drop.  I decided to risk one of the diodes & see how 
it behaved in CW mode by slowly increasing the drive current and monitoring 
the output.  The diode is rated for 2.6A max I believe from the package & 
0.2 A lase threshold. I was able to see steady  power output increase up to 
about .8A where it started to level off so this is where I stopped.  I have 
the diode well heat sinked so the case does not rise in temperature 
significantly.  These diodes have what looks like a single fiber strand 
mounted inside maybe a 3/8 inch long by 1/8" dia tube so  the output 
divergence is many degrees. I used the collimating optics from a dead $2 
laser pointer to collimate the beam while  using an old CCD video camera to 
view it.  I then made up a beam expander to expand the beam up to 4" . I 
had a chance to view the spot on a tree maybe 250' away last night using 
the camera. What an intense 4" spot!   Next stop is seeing how it compares 
to my 1W  input collimated LED for  vertical backscatter off our 1500' smog 
layer. BTW - at this point I'm running a 50% duty cycle square wave drive 
with 0.9A average input current at about 200Hz. I'm probably pushing the 
heck out of the diode but it seems to be surviving my experiments so far. 
The diodes I'm playing with are Laser Diode Diode Inc. units listed on Ebay 
and have 910 nm output. There are a bunch of them  for sale cheap so here's 
an opportunity for those thinking about getting some IR laser  horsepower 
for NLOS experiments. I don't know that they will all survive this type of 
operation so I think each one may need to be tested in a similar manner.
  - Kerry N6IZW -




At 04:55 PM 9/3/2004, you wrote:

>>I plan to heat sink one to see how far it can be pushed in duty cycle.
>
>
>Kerry,
>
>BAD IDEA!
>
>You are certainly free to do as you wish. BUT, the reason why they are 
>rated for pulse only is because the lasing current (threshold current) is 
>above what the diodes can dissipate when run in constant duty or even 
>raised duty cycle. You might run them at 15A peak, but they might require 
>8A to lase. You could run them in LED mode, but it's not likely you can 
>run them at anything other than low duty cycle pulse mode. I don't think 
>you'll be able to run them at any duty cycle above their ratings.