[Laser] Laser Digest, Vol 87, Issue 13

[Dave]

> Date: Sun, 25 Mar 2012 15:03:31 +0100 (BST)
> From: Garnier Yves<f1avy at yahoo.fr>
> Subject: [Laser] Re :  radiation from a photo diode
> To: Free Space LASER Communications<laser at mailman.qth.net>
> Message-ID:
> 	<1332684211.93875.YahooMailNeo at web29504.mail.ird.yahoo.com>
> Content-Type: text/plain; charset=utf-8
>
> Hi Stuart
>
> Thank you for the very good experiment reporting.
> The difference with mine is the photodiode max current value.
> In my test the current was well over 20 mA.
> It seems to me a BPW34 can be pulsed to 100 mA direct way with a 1/2 duty cycle ?
> The infrared yield convertion seems rather poor...?

That's not too surprising.  Silicon is an indirect band-gap material, so 
the efficiency
is quite low.  Along the same lines, Silicon Carbide is also an indirect 
band-gap
material, which is one of the reasons why Silicon Carbide LEDs aren't 
used much
anymore, despite being the earliest material reported to exhibit the LED 
effect (GaN
is usually used now for blue LEDs.).

> I continue to test with some TECs to get a modulated thermal radiation.
> The thermal inertia effect could be well lower it seems first if the direct and reverse currents was pulsed.
> Because the good thermal "in a plane" production, the TEC provides hot waves followed by a cold waves through the ceramic layer.
> The thermal waves propagation laws (speed, back diffusing effects, puzzle me for the moment)...
> Some duty cycle and frequencies values seem to give a strong modulation enhancement...
> The sinusoidal supplying seems not to be the good strategy but very high current short pulses could be well better.

I wonder if there's some type of thermal resonance effect happening? 
I'm not even
sure how to characterize that, though.

It may also be possible to use the individual TEC diodes, rather than the
entire array.  I think most of those devices are built with the diodes 
between
two heat-sinks.  The heat-sinks are good for distributing heat, but they add
thermal inertia.  I think that quite a few of the TEC modules are made from
diodes consisting of Bismuth Telluride, although they very well may be
made from other materials:

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

> 73 ?
>
> Yves F1AVY
> http://f1avyopto.wifeo.com

Dave