  A generic fiber positioning strategy and a fabrication path are presented for
microlens-fiber-coupled integral field units. It is assumed that
microlens-produced micro-images are carried to the spectrograph input through
step-index,multi-mode fiber, but our results apply to micro-pupil reimaging
applications as well. Considered are the performance trades between the filling
percentage of the fiber core with the micro-image versus throughput and
observing efficiency.A merit function is defined as the product of the
transmission efficiency and the etendue loss. For a hexagonal packing of
spatial elements, the merit function has been found to be maximized to 94% of
an ideal fiber IFU merit value (which has zero transmission loss and does not
increase the etendue) with a microlens-fiber alignment (centering) tolerance of
1 um RMS. The maximum acceptable relative tilt between the fiber and the
microlens face has been analyzed through optical modeling and found to be 0.3
degree RMS for input f-ratio slower than f/3.5 but it is much more relaxed for
faster beams. Several options of fabricating fiber holders have been compared
to identify cost-effective solutions that deliver the desired fiber positioning
accuracy. Femto-second laser-drilling methods deliver holes arrayed on plates
with a position and diameter accuracy of 1.5 um RMS, and with an aspect ratio
of 1:10. A commercial vendor produces plates with thickness of 5 mm, but with
similar (1 um RMS) positioning accuracy. Both of these techniques are found to
be moderately expensive. A purely photo-lithographic technique performed at
WCAM (a facility at the University of Wisconsin, Madison), in tandem with deep
reactive ion etching, has been used to produce a repeatable recipe with 100%
yield. Photo-lithography is more precise (0.5 um RMS) in terms of hole
positioning and similar diameter accuracy (1 um RMS).
