[Laser] Re : radiation from a photo diode

[Garnier Yves]

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...?

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.
73  

Yves F1AVY
http://f1avyopto.wifeo.com

________________________________
 De : "stuart.wisher at talktalk.net" <stuart.wisher at talktalk.net>
À : laser at mailman.qth.net 
Envoyé le : Dimanche 25 mars 2012 14h16
Objet : [Laser] radiation from a photo diode
 

First of all, apologies for jumping in with the comment about the use of PIR-type sensors, I had not seen the earlier messages since I have been away on holiday. I have read through the archive now and I have seen the suggestion has already been made.


I have had time now to repeat Yves’ work and I can confirm that some radiation is emitted by a forward biased photo diode. I used the SFH203 diode in series  with a 1k resistor, connected to my function generator which can add a DC offset to the output signal. The receiver was another SFH203 photo diode in a KA7OEI derived optical receiver circuit connected to an audio amp and speaker. The generator frequency duly appeared in the speaker with a few volts of output from the generator. 


Lenses were used to extend the range of the system and I satisfied myself that it is some sort of radiation, not inductive or capacitative coupling. I could not detect any near IR from the diode (using my digital camera that detects radiation from 850nm and 940nm LEDs so easily, nothing was visible on the screen). Modulation of the diode output signal is strong at 500Hz and audible right up to 5kHz, the cut-off frequency of the audio amplifier, intended for voice signals, so it should be possible to use this for voice communications. 


The power level used in the photo diode and resistor (a few mW) makes it unlikely that the radiation is simply heat, if it were, it would have doubled the signal frequency, like a filament lamp. (I did try a filament lamp, rated at 6V, 40mA and found that copious amounts of IR were produced, plus with a DC bias only, it is microphonic, tap the glass bulb and it becomes obvious that the filament is a spring!) The lamp doubles the signal frequency without the application of a suitable DC bias. A very much stronger signal is produced from the lamp than the photo diode, even with the lamp brightness so low that only a dull red glow is evident, but I cannot tell (except from theory) which part of its radiation is being received.


It was interesting to note that the receive photo diode used responded better than the alternative type (SFH203F) that has a black case intended for use at 800 - 1100nm, the pigment may not pass long wave IR as well as the water-clear cased type. I also tried various long-pass filters in the signal path and found that a 950nm long-pass filter allows the signal straight through. So all I can conclude at present is the signal is of longer wavelength that that detectable using a digital camera.


Incidentally, the SFH203 is available from RS Components Ltd in the UK.


I now have a E700STO from Murata, with a specified response from 5um to 14um which I am going to try next as a receiver. A look at the data sheet (link on my previous message) tells me that there are actually 2 sensors behind the window, connected in series opposing, so illuminate both equally and you get no signal out! You could always focus a signal on to one side or another, or as I am also going to try, mask one side off. Using this device will limit the wavelength range to which the receiver will respond and hopefully tell me more about the radiation emitted by the photo diode.


Stuart


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