[Laser] Laser Digest, Vol 87, Issue 9

[MICHAEL COUTURE]

Dave,

If it's the same one I have (laser project) Ramsey Electronics has them. Here's the link to the one I have:
http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=LBC6K

I actually bought 2 of them for 2 way comms. Problem is nobody around here is into electronics.

Mike C. 
sandbar w/fl

--- On Thu, 3/22/12, Dave <wa4qal at ix.netcom.com> wrote:

From: Dave <wa4qal at ix.netcom.com>
Subject: Re: [Laser] Laser Digest, Vol 87, Issue 9
To: laser at mailman.qth.net
Date: Thursday, March 22, 2012, 2:21 PM

> Date: Thu, 22 Mar 2012 00:06:07 -0700 (PDT)
> From: Tim Toast<toasty256 at yahoo.com>
> Subject: Re: [Laser] Strange effect
> To: laser at mailman.qth.net
> Message-ID:
>     <1332399967.88678.YahooMailClassic at web37903.mail.mud.yahoo.com>
> Content-Type: text/plain; charset=iso-8859-1
>
> 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.

That might be an interesting area to investigate.  Typically, pyro-detectors
respond too slowly to allow an application such as radar to work.  Maybe.
I would guess that most systems either use solid state diode detectors,
or vacuum photomultiplier detectors.  Of course, the trick with the vacuum
photomultiplier type detector would be finding a material with a low enough
work function to eject electrons when stimulated with the 10 um radiation.
It's been a while since I've looked at this, but I'm aware that some alloys
have a depressed work function (e.g., Sodium-Potassium alloy), and some
solid state materials have a surprisingly low work function (although it
remains to be seen if it would extend down to 10 um).

It may also be theoretically possible to use a hetrodyne detection system.
Maybe.

> 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.

I've played with Peltier cells before.  3 Hz would be pretty
fast for one.  But, yeah, it's weird to feel one side getting
hot as the other side gets cold.

> 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.

The glass envelope may be a problem, although that depends upon
the type of glass, as well as the specific wavelength involved.  Note
that there are a whole variety of things called glass (although the
light bulb manufacturers probably pick the very cheapest!).

Perhaps a Quartz-Tungsten-Halogen blub would be of use, since the
fused Quartz envelope is rather transparent in the infrared (although
I'm not sure how far down that would extend).

You can get some modulation of small incandescent bulbs up into
the low audio frequency range, but you have to drive them pretty
hard to accomplish that.  And, they definitely do not have a linear
frequency response.

> 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".

The problem is that there's a rather high order exponential
involving the energy emitted with respect to temperature.
As the temperature drops, the amount of energy drops quite
dramatically (4th power?  7th power?  I don't remember exactly.).

In order to make up for that power drop, you'd need a rather
large surface area.  Of course, that's going to add mass and
thermal inertia.  Think of something like the Nichrome ribbon
element in an electric heater.  It might work, but I'm rather
doubting the data rate you could get from it.

Of perhaps more interest may be something along the lines of
a maser.  Those things can be VERY precise in terms of
frequency (e.g., A Hydrogen maser operates at 1,420,405,752 Hertz!).

http://en.wikipedia.org/wiki/Hydrogen_maser

Unfortunately, most of these have a VERY small amount of output
power, but that is somewhat made up for by the frequency stability.
Other materials will produce other frequencies.  So, the question
now becomes, what material will mase at 10 um (well, besides CO2).

Note that most lasers involve electron energy level transitions.  Masers,
though, usually involve rotational or vibrational transitions, although
there are other mechanisms (such as the Hydrogen maser and it's
electron spin reversal).

> 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.

Does RS carry the SFH203s?  I think I have a bag of them packed
away somewhere.  If they're the ones I'm thinking of, they're
decent devices.

I've been wanting to play with a pyro-detector, but just haven't
gotten the time yet.

Is the laser transceiver based on a laser pointer?  Lasers, in free
space and over reasonable distances, usually suffer from
scintillation issues.

> -toast

Dave
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