[Laser] Strange effect

[Tim Toast]

Hi Dave an Yves

They must have some good detectors for 10 microns if they can 
make laser radar work well with CO2. At least as good as any 
silicon or germanium maybe. I wasn't sure if any of the exotic
materials they use as detectors were diodes or not. They may all
be bulk or crystal piezo or pyro types and none diodes.

The 3 Hz oscillating peltier junction just blows my mind... 
It would be wierd and funny to be able to feel it getting hotter 
and colder just touching it? I imagine it's too slight a change though to feel directly. I bet spectrumlab can see it good though.

If ordinary filament lamps could be used as sources then that 
would be great, not to mention cheap. I think the glass envelope
would block any far IR directly from the filament but the envelope
itself would be hot and bright. Small bulbs, like a flashlight or 
penlight, would have a lower thermal interia and better higher 
modulation rates. 
Also, I wonder if you could underdrive a lamp and get the 10 
micron emission directly from the filament?? (without the 
visible light). It's well known this works for near IR. Maybe 
this shift to longer wavelengths goes on far enough to generate 
mainly 10 micron?? At such low drive levels the glass enveloped 
might then be removed so it wouldn't block the radiation. But 
maybe a homemade copper wire coil filament would work better than 
a modified lamp - with no glasswork to worry about. A homemade 
low frequency modulated 10 micron "lamp".

I'm going to our Radio Shack store this weekend to pick up a pack 
of those SFH203 detectors i've been hearing about lately. I've 
been using some unknown types in the PGP and OEIv3 receivers i 
have. They might even have some actual pyro detectors in stock 
at RS. I wouldn't mind having a few to experiment with. I'm also 
trying to finish a low voltage laser transceiver "kit" i've been
working on.

-toast


> 
> > Date: Sun, 18 Mar 2012 14:34:34 -0700 (PDT)
> > From: Tim Toast<toasty256 at yahoo.com>
> > Subject: Re: [Laser] Strange effect
> > To: laser at mailman.qth.net
> > Message-ID:
> >     <1332106474.81544.YahooMailClassic at web37903.mail.mud.yahoo.com>
> > Content-Type: text/plain; charset=us-ascii
> >
> > Thanks for all the info Yves.
> > Maybe this means there's a new band we can use here.
> For use
> > with short light pulses at less than about 100 Hz rates
> and maybe
> > linear modulation with low frequencies under 100 Hz
> (PSK etc)
> > 10 microns is in a good atmospheric window for long
> range use.
> > Eye-safe too at the lower powers (I THINK).
> 
> Hmm, interesting.  I wonder if there are any
> low-band-gap energy 
> semiconductors
> that would respond directly to such radiation?  I know
> that Lead Sulfide 
> is a
> fairly low-band-gap energy semiconductor (band-gap energy of
> about .37 eV,
> compared to 1.107 eV for Silicon, and .67 eV for
> Germanium).  There are
> even a few others that are even lower.
> 
> If I'm running the numbers correctly (E=.37 eV, c=3E8 m/s, 
> h=4.135667516E-15 eV*s),
> then a band gap energy of .37 eV corresponds to a wavelength
> of about 3.353
> micrometers (33530 Angstroms).  Thus, using a Lead
> Sulfide detector as a 
> photodiode
> (or, even as a PIN diode?) should give you sensitivity down
> to 3.353 
> microns.  That's
> not quite down to 10 microns, although it's getting pretty
> close.
> 
> 10 microns would be a band-gap energy of .124 eV, which is
> getting quite 
> low.
> The only thing I show with a band-gap energy that low is the
> alpha phase 
> of Tin (.08 eV),
> and I don't know if anyone has tried making a semiconductor
> out of that 
> (Note that
> there very well may be other materials with band-gap
> energies that low; 
> it's just
> that I haven't come across them in the table I've
> compiled.).  There are 
> a couple
> of candidates that come close, though, such as Copper Iron
> Selenide 
> (CuFeSe2) at
> .16 eV, Cadmium Arsenide (Cd3As2) at .14 eV, Cadmium Tin
> Arsenide 
> (CdSnAs2) at
> .26 eV, Indium Antimonide (InSb) at .17-.18 eV, Mercury
> Selenide (HgSe) 
> at 0-.061 eV!,
> Mercury Telluride (HgTe) at .2-.3025 eV (at 4K), and Lead
> Selenide 
> (PbSe) at .26 eV.
> Of course, I don't know if there's any work being done on
> producing 
> semiconductors/
> photodiodes/PIN diodes/etc., out of such materials. 
> But, there's some 
> information
> to get someone started if they want to do some
> research.  :-)
> 
> Anyway, using a low-band-gap semiconductor for such
> wavelengths would allow
> you to run full data rates, although it very well may
> require cryogenic 
> cooling to
> minimize leakage to an acceptable level.  Maybe.
> 
> > As for transmitters in this band - CO2 stands out.
> People have
> > been home building CO2 lasers for quite a while now.
> There are
> > several kinds including CW and TEA pulse types. Lots of
> room
> > for experimentation and not out of the question for
> Home building
> > of all but the detector itself.
> 
> And, perhaps even the detector itself!  :-)
> 
> > I don't know of any natural sources of 10 micron pulses
> except
> > for maybe lightning or astronomical sources. This band
> should
> > be nearly completely free of manmade light pollution.
> There would
> > be lots of heat sources but mostly CW from heated
> objects and the
> > Sun.
> 
> I think you're mostly correct on that, although you may get
> quite a
> bit of 120 Hz hum from incandescent lights, which can emit
> at quite
> low frequencies.
> 
> Dave
> 
> 
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> End of Laser Digest, Vol 87, Issue 8
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