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

[wa4qal at ix.netcom.com]

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

Well, it sort of depends upon how you've biased the phototransistor, but
it'll most likely drop in as a replacement.  You may find that you have to
change the value of the dropping/load resistor a bit for optimum
performance.

One of the better pages on front-ends is by K3PGP, and is located at:

http://www.qsl.k3pgp.org/frontend1.htm

One of the problems with a static load resistor is that it's prone to 
causing saturation when the ambient light value changes.  One approach
to handling this is to use a transimpedance amplifier.  Another approach
that I've played with is to transformer couple the photodetector so as to
remove any ambient bias.

Dave