[Laser] PIN diodes and Noise floor

[laser@mailman.qth.net]

In a message dated 4/18/03 11:14:58 PM GMT Daylight Time, flowersa@alltel.net 
writes:


> I have a question for folks who have played around with PIN diode 
> detectors--
> 
> I've put together a few of the K3PGP front ends now and I'm really 
> looking forward to getting my laser diodes going for a complete 
> tranceiver system.  Anyway, I was wondering if the PIN diode dark 
> current is much of a factor under actual nightime use?  Is the noise 
> floor coming from ambient light or from the diode itself?  I imagine 
> this will depend largely on how big of a lens you are using, and how 
> clear the sky is, but I'm trying to get a ballpark figure.  John 
> mentions that cooling the diode improved the NF, e.g. putting it in the 
> freezer for a while, but does that really matter practice?  Does ambient 
> light drown out the dark current?  I'd test this myself, but it's not 
> easy for me to get away from city lights.
> 
> 

Hi Andy.

The ambient light at night will not have much effect providing it does not 
saturate the detector. The overall Noise Equivalent Power however is the 
limiting factor and the more you can reduce this the better.  My reasoning 
(hopefully) is that the NEP determines the noise per unit bandwidth and in 
general this will not change. However, the noise generated from light sources 
at night generates signals that have 60Hz and harmonics present. Providing 
these do not saturate the detctor they can be removed.
I guess the way I look at this is that total darkness only matters when you 
are looking to measure the light level increase from you lasers contribution 
to ambient. In practice that doesent happen because there is always a 
modulating frequency or sub-carrier which can be recovered and filtered to 
remove unwanted signals.

73

David


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