| Simple Mod Turns Diodes Into Photon Counters
The Y axis is probability, the x axis is millivolts. The output ofan avalanche photodiode is current. So what is being measured hereand how does it relate to the article?
If this is the probability of finding particular signal amplitudes(time unspecified), how does it relate to single photon counting?
It seems all back to front. Naively one might expect that multiplephotons would result in a larger initial avalanche current, but thegraph doesn't seem to relate to this at all. It shows a highprobability that the output at no photon will be around 5mV,whatever that is supposed to mean. Is that the dark current of thediode at a given bias, before it triggers?
I suspect that what we are seeing here is a diode biased with somelow voltage from a limited current source, and that the pulseamplitude results from the self-discharge of the diode capacitanceinto the measuring resistor. In which case what is being shown isthat the resting output is around 5mV, and that the pulse heightdepends on the number of simultaneous photons. If so, this couldhave been shown quite clearly on a conventional graph with theoutput pulse height represented on the Y axis, the number ofphotons on the X axis, and the probabilities shown by the shape ofthe distribution.
So, either I don't get it (always a possibility) or someone needsto go and (a) read up statistics again and (b) read Tufte. | 
