[Laser] Beacon to test a photodetector sensitivity limit

Sun May 20 13:58:19 EDT 2007

Art,

Not all 1N914 will emit IR. First time I noticed this was looking at IR 
diode through image intensifier. The IR diode was on a circuit board that 
also had a 1N914 in the circuit. The 1n914 was also emitting at a low level. 
I then looked at several more of the circuit boards, some had diodes that 
emitted some did not. I couldn't explain this as Si Diodes do not normally 
emit light. Later I found that contamination of dopping or low level ESD 
event can cause PN junctions in Si to emit.

I agree with you as to non-linear LED operation outside the rated input 
current. Also agree with you on the pin holes and ND filter approch you used 
with success. I know that pin holes and ND filters are not cheap, so out of 
reach of many hams. I have been trying to think of other alternatives.

Considering using B&W film to make a varible density wedge fo use in place 
of the ND filter.

73
Terry W5TDM

>From: Art <KY1K at verizon.net>
>Reply-To: Free Space LASER Communications <laser at mailman.qth.net>
>To: Free Space LASER Communications <laser at mailman.qth.net>
>Subject: RE: [Laser] Beacon to test a photodetector sensitivity limit
>Date: Sun, 20 May 2007 13:29:27 -0400
>
>I couldn't agree more Terry!
>
>I never tried using a 1N914 diode as a photon source though. I have tried 
>only old technology LED's.
>
>But, I always STRONGLY suspected that IR or other wavelength photons were 
>being produced by the LED's at low current levels. I did not try an IR 
>absorbing filter to confirm this, mainly because I didn't have one at the 
>time.
>
>I started out working with red LED's at 10-100 microamps and found they 
>totally swamp any sensitive light receiver-too many photons and had very 
>very poor linearity! It was VERY BAD. Varying the number of photons out by 
>varying the curret to the LED was very un-linear at these lower current 
>levels.
>
>From this point, I decided to use a highly sensitive PM tube for setting 
>the output of the LED to a relative level-then switching to the DUT to take 
>measurements. Even this didn't work! What I found was that the relative 
>output of the photomultiplier didn't correlate to the readings taken by the 
>DUT!
>
>I concluded that the LED must be producing near IR-which the photodiode was 
>sensitive to and the PM tube wasn't....hence the non correlation between 
>the PMT and DUT relative outputs.
>
>The ONLY way I could make this work was to run the LED between 5 and 30 ma 
>and to use a pinhole in combination with a series of absorbers between the 
>pinhole and the PMT/DUT. Using this method, I got excellent correlation 
>between the PMT and the DUT outputs. But, the LED still wasn't very linear. 
>I was happy enough though because I could get a weak signal to evaluate the 
>various photodiode amps I was working with.
>
>
>>>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.
>
>Yves, please BEWARE of evaluating a photodiode without a lens! Without a 
>lens, that photodiode will pick up photons from almost any angle. You need 
>a lens or some method of limiting the angle of admittance of the photons 
>entering your detector. Without it, you will have scattered photons and 
>very much distorted sensitivity data-the detector will seem much more 
>sensitive than it really is.
>
>To test this, go into your dark room and take a reading of the ambient 
>light level with a lens in front of the PD, without a lens over the PD and 
>with the PD in total darkness (covered with a light shield). You will find 
>the ambient light level to be several 10's or hundreds of times higher 
>without the lens-because the few photons that find your way onto the dark 
>room are bouncing around off of every single wall surface in the room and 
>there is allot of reflecting area! Even with the lens in your dark room, 
>you will probably detect a small amount of light compared to the readings 
>obtained with the covered photodiode.
>
>The only way I found around this problem was to mount the photodiode in a 
>black box surrounded by flat black 'flocking' paper and to have a pinhole 
>several inches in front of the photodiode-the only light allowed to reach 
>the PD was through the pin hole. Even then, the electronics had to be 
>totally shielded from light because solder and copper etches reflect stray 
>light that enters the pinhole from extreme angles. For high sensitivity 
>detectors, the detector shielding from RF and EMF also blocks light, so 
>it's not a problem. But, do be aware you have to wrap the metal shielding 
>in flocking paper to prevent it from reflecting stray photons.
>
>It's a very long story, and I won't bore you with ALL the details. The 
>short version is that I eventually migrated to my landlords basement in the 
>apartment building to do optical testing. It was in the lower basement, 20 
>feet below ground level and there were no windows or way for light to enter 
>the room although it was a large open area with lots of junk in the room. I 
>went to this area expecting it to be pitch black so I wouldn't have to 
>worry about stray light. But, using a photodiode without a lens, it was 
>painfully obvious that light was getting in somehow (the ambient light 
>level was 5 to 15 millivolts while the dark level (with a cover over the 
>PD) was well under 1 millivolt. I started using my receiver as a probe to 
>find out where the light was coming from. Basically, light was coming from 
>everywhere, even off my hand placed a few inches in front of the 
>photodiode! There was absolutely no visible light anywhere in the room that 
>I could detect with my eyes. Poking around in pitch black was not easy, but 
>I eventually found a peak-coming from the furnace room door direction. In 
>this room, there was an LED on the furnace. I assumed this to be the source 
>of my light even though it was in the next room with the door closed. Even 
>with this LED completely covered, and the door to the furnace room closed 
>and taped over, I still got residual light levels in the basement area 
>until I put a pinhole over the PD to minimize the admittance angle that of 
>the PD's view.
>
>With the lesson learned here, I eventually was able to build a test setup 
>that allowed me to work in my kitchen (at night with the lights turned 
>off). It was much more convenient this way!
>
>I ended up with a medium sized cardboard box lined with flocking paper with 
>a shoe box lined with flocking paper inside of it. All the optics were 
>inside the shoe box. My light source was an LED covered in rubber cement 
>and painted black except for a small pinhole to let some of the LED's light 
>out.
>
>Between the light source and the PD was anywhere from 1 to 5 pieces of arc 
>welding light absorbing glass with a pin hole on the PD side. In order for 
>it to work, it was necessary to wrap the arc welding absorbers in flocking 
>paper in order to keep the reflections from the glass surface from 
>contaminating the readings (by reflecting the little bit of stray light 
>inside the enclosure).
>
>Eventually, I just left the LED running at 5 ma and varied the number of 
>absorbers between the LED and the PD to get my weak signals. It was all 
>'relative', but it was cheap, and it worked well.
>
>Regards,
>
>Art
>
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