AW: [Laser] PIN diodes and Noise floor

Sat, 19 Apr 2003 09:36:45 +0200

Hi all,
keep in mind your system is an OPTICAL SYSTEM. The front lens of your front
end transforms the far image onto your diode. I.e. the far source is also a
lens, not greater then 10 cm diameter, and this far lightsource put a very
small spot on your diode. This takes place only if the system is very well
focused. Assume the source is very far. So you will have a spotsize on your
diode:

                    far diameter * focal length
spotsize on diode = ----------------------------
                           far distance

(spotsize on diode = far diameter times focal length divided by far
distance)

The active area of your diode is 10 or 10thousend times bigger then the
spot, depending on the type of the diode. So you receive many photons from
the surrounding of the source and the ambient light of your own surrounding.
How is the diode and the lens housed? The diode is sensitive up to 1 micron
for Si and not all black is really black for this wavelenght. You get better
results within a small hole as direct as possible in front of the diode.
This hole blocks the unwanted signal, but it is very hard to adjust this
aperture.
For a misaligned system the unwanted signal is bigger then for a well
adjusted.
For common pin-diodes the dark-current is in the order of parts of nanoamp's
up to 10 nano amp's, depending of the active area of the diode. For
laser-communication a diode-diameter of 25 micron is more as needed.
73 de Dieter
dl7udp

-----Urspr�ngliche Nachricht-----
Von: [email protected] [mailto:[email protected]]Im
Auftrag von Andrew T. Flowers, K0SM
Gesendet: Samstag, 19. April 2003 00:13
An: [email protected]
Betreff: [Laser] PIN diodes and Noise floor

> ----------
> From: 	Andrew T. Flowers, K0SM[SMTP:[email protected]]
> Sent: 	Saturday, April 19, 2003 12:12:34 AM
> To: 	[email protected]
> Subject: 	[Laser] PIN diodes and Noise floor
> Auto forwarded by a Rule
>
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.

Andy K0SM/2

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