[Laser] PIN photodiodes....

[W2MXW]

-----Original Message-----
From: Andrew T. Flowers, K0SM <flowersa@alltel.net>
To: laser@mailman.qth.net <laser@mailman.qth.net>
Date: Thursday, August 21, 2003 5:40 PM
Subject: Re: [Laser] PIN photodiodes....


>You'll want to be careful about which
>wavelengths you choose--there are a few wavelengths in near IR that have
>high absorbtion due to elements in the atmosphere--Oxygen and Nitrogen
>mostly.  I'm not sure what they are right off hand, but I'm sure someone
>on the list knows (hint hint).

Okay, I can take a hint :-)
The major NIR absorption lines are:
725 nm
759.4 nm
762.1 nm
822.85 nm
940 nm
There's definitely one around 850 as well but I can't find the exact
wavelength in my reference material. Can anyone clarify?
Strongest (most attenuation in terms of dB/km) is the 940 line followed by
823. 940 also has a really ragged 'tail' that extends to either side,
especially toward the shorter wavelength end.
These are all water vapor absorption lines. As such the degree of absorption
is humidity-dependent, just like with millimeter wave. They are mostly due
to modified
harmonic resonances of the OH bond fundamental at 2.7 um. The 2 lines around
760 are less significant than most of the others and should not seriously
impair use
of '780 nm' lasers (which can be off by up to several 10's of nm either way
depending on type and operating condx. thus possibly right on top of these
lines).
Below 1000 nm (1 um) there are lots of absorption lines, some so strong as
to approach zero transmittance, and due, in addition to water, to other
molecules such as CO2, CO, O3, etc. However between these lines there are
many excellent windows, some with greater transmittance than at vis and NIR.

Shyam is right, 1550 nm  is a good window and as a major bonus there is
little or
no solar noise in daytime, unlike at shorter IR wavelengths. It's eyesafe
too (cannot penetrate to retina - could still blast a cornea though, but at
least those can be replaced :-)
I am (slowly) working towards systems for that w/l as well as
for more "conventional" IR & vis wavelengths. The latter are mostly based on
the large-area PDs from All Elec. & the MPJA IR laser printer modules (which
BTW I believe are still on sale for about $3.00 each). I'm also
investigating use of LEDs esp. (in the visible) the new super high power 1
and 5 watt Luxeon Star emitters by LumiLEDs.
1550 nm parts can be had from Digi-Key (both the LD and the PD) among
others but aren't cheap - although still not terrible prices for an
experimenter. Note that the (semi-affordable) 1550 PDs are small-area, if
you want a large area one that is truly BIG bucks. They are a different
chemistry (InGaAs), as Si won't work at this w/l (begins to become
transparent). The
one good thing about the small-area parts is they're blistering fast...if
that matters for your application.

73, Jon W2MXW