[Laser] Re: Laser Digest, Vol 19, Issue 1

[wa4qal at ix.netcom.com]

> Date: Wed, 11 Jan 2006 21:14:34 -0500
> From: Brent <hamfreak2002 at gmail.com>
> Subject: [Laser] Newbie
> To: Laser Mailing List <laser at mailman.qth.net>
> Message-ID: <43C5BB8A.30406 at gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> hey everyone, Just started modding my lase pen to make it into a CW 
> transmitter. But school have picked up for finals and havent had much 
> time to work on it but... got most of the tx done and a good amount of 
> the RX.

Congratulations.  

> Any tips?

Err, have fun.  :-)

> I can use a regualr radio shack phototransistor for the RX even though 
> it says infrared? its broad band right... Its the only one I could find...

That may work, especially if it's one in a clear package.  However, 
phototransistors are rather noisy, which may impact any long range/weak 
signal work, plus some of them have a rather low upper frequency limit,
which may impact you later as you start pumping more data across the
system (voice?  video?  data?).  A better choice would be a PIN photodiode 
with a low noise amplifier.  I picked up a bag of the OSRAM SFH214 Silicon 
PIN photodiodes a while back that I've been pleased with, although they
only have a 1 mm**2 active area.

One of the biggest problems you may encounter is suitably biasing the
photodetector.  There's a huge difference between daylight and night
time operation.  

Another problem you may run into, especially for base-band operation
(as opposed to sending the data on a subcarrier), and especially at night,
is noise from discharge lamps (e.g, Mercury vapour, Sodium vapour, neon, 
fluorescent, etc.).  These produce light which flashes at 120 Hz (100 Hz for 
other parts of the world).  One way of minimizing this is to include a high 
pass (or band pass) filter in the receiver.  

Yet another trick which can help with the daylight biasing issue, as well
as some of the noise source issues is to include an optical filter in the
receiver's optical path.  Such a filter will knock out light from other
portions of the optical spectrum (blue, green, yellow, etc.) other than the 
one your interested in (red).  Of course, the problem is that all filters
have some insertion loss, so it may decrease your signal level for
dark/weak-signal work.

Finally, don't forget that optics can be included to gather more light
on the receiving end.  Even laser beams diverge as they travel through
a distance, and this is especially true of solid state lasers with economy
optical systems on the transmitting end.  If you can gather more light 
on the receiving end, you'll have a better signal to noise ratio.  Also, 
don't forget to consider non-traditional optical systems.  Most people think
that an optical system means a glass lens (e.g., magnifying glass).  
However, fresnel lens can be made quite large and aren't extremely
expensive.  Additionally, concave mirrors will also focus optical energy
and may be cheaper than glass lenses. 

> Well have to go, I will ask more questions when I can start owrking on 
> it again, hihi. 73's

No problem.

Oh, beware that all of this can become addictive.  :-)

Dave