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

[Brent]

wa4qal at ix.netcom.com wrote:

>>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
>
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>  
>
Speaking of PIN Photodiodes, can they just replace hte phototransistor 
or do I have to Bias it differently?