Over the 18 meters return trip 65.6ns brings me to within 8% to the actual speed. However I understand this is not a 'measurement' in the sense that I have plenty of variables yet not accounted for. At this stage I'm happy to call it blind luck more then anything.
First of all I saw the two peeks and just arbitraerly decided that was the place to measure. Given the huge difference in vertical scales between the two it's hard say if Df (channel 1) would have risen faster if only it were getting the same amount of laser power as Ds.
What I would like to do is measure at the earliest sign of the signal rising off the floor, but as you can see that's not so easy to spot.
Below is the physical setup, you can see the old camera lens I'm using to focus the faint light from the mirror onto the junction of Df.
What I changed this time around is both detectors are identical now, previously I had three different ones that I was playing around with. The ones used here also differ to that described previously and are now just a photodiode with a 330 ohm load. The reverse bias is still 25 volts.
So I have three detector types
D1: Is the first one I built and described here, in a metal box.
D2 : Is the same as D1 but with C1=100pF and free floating without a box.
D3 : Is the one above that I used in this setup, and also not in a box yet.
The first improvement I want to make next is get some good duplicate cables between the detectors and the scope. Currently I have one stock scope probe, and the second cable is RG85 coax of a different length with reversed protection diodes. I've ordered some RG316 to make two new cables.
The second change is to investigate the how the amount of light reaching Df and Ds affects the response time. To solve that I'm really going to need to collimate my source beam or perhaps use a beamsplitter with a different ratio to the 50/50 one I have now, something more like 90/10 might work.
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