Congratulations on successfully building the setup. :)
Are you going to investigate something special with it?
Thanks! I actually need to design a FPGA board to measure the photon bunching for mini telescope design, since having an oscilloscope is not always possible. So before I start with FPGA desing, I needed to get the effect in the lab to test FPGA work on.
Yes, I meant the triangle slope of the first graph.
The "triangle" feature you see is an artifact of the correlation between finite signals.
Suppose you have finite signals:
Signal 1: 1 1 1 1 1
Signal 2: 1 1 1 1 1
On the on hand, you can think that since signals are constant, their correlation is constant too, since no matter what the delay between the signals, the correlation is same.
But on the other hand, since signals are finite, each time you shift one with respect to the other, in order to multiply them in the correlation process, the edges of each signal will multiply 0 and therefore will not contribute to the correlation value at given delay point. The more delay you introduce - the more values at edges are multiplied by 0. Therefore you get maximum in the middle, and less and less towards the edges.
For the example above, the correlation in the middle (0 delay between signals) is 1*1+1*1+1*1+1*1+1*1 = 5.
With 1 value shifted you get 1*0+1*1+1*1+1*1+1*1+0*1 = 4
and so forth. So for this case you will have the correlation: 1 2 3 4 3 2 1, which is rectangular pattern.
Therefore, correlation of features in each signal that are more or less constant (mostly the noise), will result in a rectangular feature in the correlation graph. And what's what you see in both graphs (non rotating and rotating diffuser). The feature that is different between the 2 graphs is an additional peak in the rotating diffuser case, which corresponds to the photon bunching.
