[Laser] Re : Strange effect

[Garnier Yves]

Hi Tim

For the moment my far infrared source is a lampblack covered small thermoelectric cooler biased with direct and reverse currents on 3 Hz.

With a well equilibrated cycle, the TEC seems to accept staying without heatsink and the ceramic layers seems well oscillate around the ambiant temperature (?).

A simple symetric multivibrator with a H switch with power transistors seems to work rather well but many improvements could be done may be with high level pulsed currents ...

A 30 cm x 30 cm acrylic fresnel lense gives a good signal to 50 m when the TEC is at the far infrared focal point (about 5 cm back the visible focal point because a very big "chromatic" aberration).

For today the signal is detected only by the fingers touching the loud speaker cone of the receiver :o))

Lot of work to do to improve the system.

73

Yves F1AVY
http://f1avyopto.wifeo.com

________________________________
 De : Tim Toast <toasty256 at yahoo.com>
À : laser at mailman.qth.net 
Envoyé le : Dimanche 18 mars 2012 22h34
Objet : Re: [Laser] Strange effect
 
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).

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.

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. 



-------------------------------------------------------
Hello Tim
The pyro has two limiting speed response parameters
Its own cristal thermal inertia and the electrostatic pyroelectric loading and unloading speed.
The cristal
thermal inertia can be from 1 second to 1/100 second duration range versus its size and its thickness.
The electrostatic pyroelectric voltage changing speed depends highly the cristal loading resistor value and the cristal capacitance. 
The lowest this resistor the highest the speed but the lowest the sensitivity and the NEP...
In normal use as a motion detector, the internal FET is mounted as a source follower design that give gigantic impedance but a very slow response
With a charge amplifier design, the cristal sees an artificial grounded input via the feedback capacitor and has this way a low dynamic load.
The charge is high speed transfered in the feed back capacitor but the price to pay after an optical pulse is a slow recovery slope from this capacitor.
The K3PGP RX gives the same effect with a pyro or a photodiode.
A very short optical pulse is transfered in the Miller drain-gate capacitance of the FET and the output signal is followed by a slow recovery
slope.
The pulse can  be recovered in the following stage by a signal differentiator to remove the slow slope and to keep the high speed transiant rising step.
In my schematic I use a 100 K adjustable resistor with 10 MF decoupling for the source of the internal FET and a 100 K resistor to the drain.
The lowest the current in the FET integrated into the pyroelectric detector case, the lowest its spurious thermal effect.  
Have a look on these datas :
http://www.lasercomponents.com/de/?embedded=1&file=fileadmin/user_upload/home/Datasheets/pyreos/py-ftir-01.pdf&no_cache=1
http://eltecinstruments.com/PDF/Dn/ELTECdata%20100.pdf
73

Yves F1AVY
http://f1avyopto.wifeo.com


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