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Photon counting (again)

I measured the 'photon' statistics on the reversed biased GaP led's as photo detectors today.

I stuck an opa134 across the 100k ohm resistor to drive the 50 ohm cable and counter. RC tau = 1.5 us. About 15pF of C. (3pF from opa134 and maybe 1-2pF from layout, 10pF from the LED... is that reasonable?

The time between pulses followed a nice exponential. Except for short times!!! There were more counts than expected for times less than ~50us or so. (Increasing approximately exponentially until hitting the 1 us pulse width. It's like the LED sensitivity increased after a pulse.

I'll post some data tomorrow.

I've been thinking about the size of my detector. Say I'm really detecting single photons, at really terrible sensitivity.

Maybe I'm tickling a single atom! Then the size of my detector, is about the size of the photon.(atoms are small)

And.... I think...HB&T is then like falling off a log. Isn't there some term that goes as, detector-source distance * wavelength / divided by source diameter* detector diameter?

So if my detector is the size of the wavelength, source-detector distance over source diameter is easy..

And there's only the time term,, source bandwidth over detector bandwidth....

OK maybe it's just a boring detector thing. Tomorrow's Friday :^)

George H.

Reply to
George Herold
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Hi, George -

The three that you sent to me measure about 23, 24, and 25 pF at zero bias and 10kHz with an LCR meter, if that helps.

Cheers, John S

Reply to
John S

That's a known problem with SPADs.

Or there are extra photons rattling around - reverse biased LED's emit light when a current flows through them in the wrong direction. Maybe the charge carrier shaken loose by your detected photon is generating more photons as it goes through the LED.

It usually is.

Not any longer.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

om

Ahh great, thanks. I never thought of sticking one on an LCR meter. (Duh) Mine gives the same result.

George H.

Reply to
George Herold

om

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Oh, do you know any good papers that discuss the mechanism?

Here=92s a histogram of the time between pulses. (10 us bin size)

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And the same data but only the short times and a 1 us bin size.

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Yeah, Sigh... I want to look at the same data with a light bulb as source.

Still, A single atom detector would be a pretty cool thing!

Here's a 'scope shot of one pulse (voltage across 100k ohm)

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The pulses are surprisingly uniform. The same with 5 seconds of persistence on the 'scope.

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I was wondering if I could pulse some current into the LED after it fires to 'recharge' it faster.

George H.

Reply to
George Herold

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Sergio Cova has written a few. Some appeared in Applied Optics and some in Rev.Sci.Instrum. You are probably better placed to find them than I am.

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-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

Drool on that one.

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We use tubes of that nature at work.

If you really want to do some photon counting, I think that will do it.

jamie

Reply to
Jamie

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