laser collimation

The difference in the amount of light reaching my two detectors was causing huge problems in my ability to take measurements. Ds would be full beam reflected off the beam splitter, while the light getting back to Df was washed out due to divergence in my laser diode over the distance.

My first attempts revolved around using a lens to directly focus the reflected light on to Df. The real solution is collimation of the beam at the source rather then trying to fix the error at the end.

So here it is, the beast. The laser is on the left, in the middle is the microscope objective, and right is my old camera lens. With this setup I am able to get a 10mm beam diameter over the 9.2m distance. Consequently the return beam is more sharp.




The Df detector seen below still receives it's final signal from a tinny little lens I had from another old camera.



Also as mentioned I was going to improve my cables and correctly terminate the lines with 50ohms so that's what I did. The old RG58 cable is gone and replaced with two new identical RG316 ones, t-split with standard 50ohm loads on top.



So far as measurement goes I have found a few things to improve upon the previous method.

The first is to reduce the amount of light hitting the detector Ds to about the same level that Df is seeing, my hope is this would normalize any rise time effects.

The second was to verticaly match the two waveforms after capture and take a measurement from the actual trigger point on Df and then taking the same point from Ds vertically.

Here is 62.0ns over the newly measured distance of 18.2m



And 60.8ns.



To get less light to Ds I am just holding it slightly off axis and hitting the stop button when they are close.

Next I am going to take lots of measurement's and see if I can get a reasonable average.