[Laser] Beacon to test a photodetector sensitivity limit

[Robo]

Hi Yves,

overnight I can measure  emiting infrared emision of redled on low 
current. Tell You later.
I mean, that is possible.

It's a fact, that You have optical coupling between a beacon and rx? 
Not a inductive or capacitance coupling?
Mechanical chopper have a great many advantage.

What You use for detections? PMT? APD?

Robo

btw: I am working on too.



At 03:25 PM 5/20/2007, you wrote:
>Hello to all.
>Is there somebody who knows the properties of LEDs at  very low current (a
>few microamperes range)?
>The constructors datasheets  never give light curves at very low current.
>With the new bright LEDs the  light is not proportional to the current near a
>threshold point (Above a few mA  level).
>The very old red LEDs seem to have a very linear relation between  light and
>current even at ultra low level.
>Our goal is to build a modulated  beacon able to generate a calibrated very
>low flux.
>Is it possible to convert  the electrons flux deducted from the current to
>photons flux with a fixed return  quantum ?
>The very simple diagram of the experimental beacon is  at:
>http://pageperso.aol.fr/F1AVYopto/opto/400+Hz+beacon.jpg
>The peak  current is deducted from the 1K resistor leads voltage with the
>oscilloscope.
>The potentiometer value can be increased to 1 Mohm.
>With our  last photodetectors, we noticed it is possible to hear the signal
>to 1 meter  without lens while it is absolutely not possible to see the LED
>with  eyes.
>With Spectran it is possible to extract signal with a current reduce  again
>by 10 !
>Is the old red LEDs do not conserve their peak frequency with  the current
>decreasing and then go to infrared or are our receivers really 
>well  better the
>eyes sensitivity ?
>Thank you for help.
>73 Yves F1AVY
>
>
>
>
>
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