How Well Can the Point Source Microscope (PSM) Locate the Source of a Single Mode Fiber?

I needed to measure the height of a free space fiber termination above an optical bench the other day and in the process remembered a question I had been asked in passing. Since I had the pieces of such an experiment in front of me, all I had to do was save a couple of images.

In this case I was looking at the end of a single mode fiber patch cord to set it a particular distance above an optical breadboard.

With the laser turned on at a minimum intensity the light from the fiber looks like this zoomed in image of about 400 x 300 pixels of the whole 1.6 M pixel frame.

The laser spot (white dot) was purposely decentered from the magenta cross that is the origin of the PSM coordinate system so the shape of the spot could be clearly seen to get best focus. The centroid of the spot was measured to 0.2 μm in x and y using a 4x objective on the PSM.

Without touching anything other than turning off the laser source and shining a light past the objective to illuminate the front of the fiber I got this image of the 2.5 mm diameter fiber ferrule with the 125 μm core embedded in its center.

If you squint you can still see the magenta cross and the red scale bar to give a feel for the relative scale of the 2 pictures. The faint darker cross at 45 degrees gives a hint as to how the ceramic ferrule extrusion was made.

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