[Laser] atmospheric scattering question

Mon, 25 Aug 2003 12:29:37 -0400

Art,

Insofar as I understand it, scattering and absorption are two totally
different effects, though they happen simultaneously when we look at an
atmospheric transmission channel. Various types of scattering occur when
the light reflects off or refracts through water droplets, and other
types occur at the molecular level. Getting into the freaky quantum
scattering here - waaaay out of my league, but I could dust off the 'ol
modern physics for engineers textbook. Don't know if I have the stomach
for that right now... :)

Absorption on the other hand is exactly as you describe it - light goes
in and doesn't come out. It undergoes an energy conversion into heat,
either at the molecular or atomic level...being absorption due to water
or due to atmospheric gases, respectively. Maybe someone on the board
has some experience with vapor deposition and could enlighten us as to
how absorption is used on a very different scale to generate plasma for
CVD and similar processes, using much longer wavelengths...again, pretty
far out there for a mere designer like me!

Hope this sheds some light,

73
Dave K2THZ

On Mon, 2003-08-25 at 10:47, Art wrote:
> I read the recent comments by w2cxw and Shyam Sunder Tiwari regarding 
> atmospheric absorption and adsorption relative to the lambda of the light 
> source.
> 
> I have some understanding of Mia and Rayleigh scattering. In discussions of 
> these modes, they are usually referred to as 'scattering' modes, meaning 
> there is little light lost, it's only spread out and 'scattered'.
> 
> Yet, absorption as discussed recently on this list seems to imply that 
> light goes in and DOES NOT COME OUT.
> 
> Is the absorption recently discussed here the same as aerosol effects such 
> as Rayleigh and Mia or are these completely different topics that aren't 
> related at all????
> 
> Thanks,
> 
> Art
> 
> 
> 
> 
> At 03:51 AM 8/24/03 -0400, you wrote:
> 
> >-----Original Message-----
> >From: Andrew T. Flowers, K0SM <[email protected]>
> >To: [email protected] <[email protected]>
> >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
> >
> >
> >
> >_______________________________________________
> >Laser mailing list
> >[email protected]
> >http://mailman.qth.net/mailman/listinfo/laser
> 
> 
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