digital holography problems

Hi!

We are trying to make an inline digital holographic microscope, but we
are quite unexperienced in this field.
The setup is the simplest possible: laser, objective, pinhole (to get a
spherical wave), object, CCD, all these in the same line.

The problem is to make the spherical wave, actually to position the
pinhole to the focused beam. The diameter of the focused beam is at
most 10 microns and the pinhole diameter is 2 microns. We have a 3-axis
positioner, but it is not designed for this precision. We cant position
the pinhole exactly. Sometimes it comes off quite well, but the whole
moves away after a while.

So what do you recommend? I searched for some spatial filters which are
built together with the pinhole and semm to be quite stable but they
are also expensive and they are still adjustable, the best would been a
nonadjustable, compact spatial filter, but I did not found anything
like that.
Another idea is to use an optical fibre instead of the pinhole. The
question is whether a fibre in the diameter range of 2 microns exist
and is it a good idea to use it instead of the pinhole as a spatial
filter?

Any kind of help would been highly appriceated, wishes:

Peter

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