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Dear Greg:

I tend to agree with Prof. Diaspro in that alignment should be considered /
eliminated as the primary cause.  First thing to recognize is that
Ti:sapphire oscillators typically have a divergent beam.  (This is one
reason why it was mentioned that the laser being positioned close to the
microscope is advantageous.)  So, to compensate for this divergence, we
laser manufacturers typically include a focusing lens into the direct
coupling optics. The lens also gives the user some flexibility on the
degree of over(under)filling the objective aperture.  To first order, I
would remove the both this collimating lens and the objective lens and
optimize the beam throughput using the method described by Prof. Diaspro (I
wouldnt' use an average power measurment here since the throughput will be
affected by the larger beam size)  Pay special attention that the beam
height entering the scan head is parallel with the table and traveling
along the optical table's hole pattern.  Basically, the beam needs to be
very straight going into the scan head.  This can be tricky if you have an
upright scope because the periscope adjustments do some funny things if
you're not used to it.  At any rate, once the beam is visible on a piece of
paper, you can properly center it through the scope and perform .  Once
you've done this, you can optimize the throughput using the collimating
lens (Spectra-Physics provides a long focal length lens for this purpose)
Now, be sure that everything is straight entering the scan head.  Keep in
mind that the lens will change the beam position if it's not centered
properly.  Now that you have the beam centered at a single wavelength (use
720nm since you can see it) through the collimating lens and the scope,
replace the objective lens.  Here's where you can run into trouble.  If you
were present during the microscope installation, much effort is placed on
aligning the beam relative to the confocal laser beam.  The first step is
to place a mirror under the objective lens.  This serves to reflect the
visible laser out through the scan head so that the Ti:sapphire beam will
overlap with the confocal beam.  Now, if adjustments to the beam position
are made with the collimating lens in place then you will have problems
because the beam is not passing through the center of the lens anymore, and
the beam will be coming into the objective (potentially) at an angle.  To
avoid this problem, remove the collimating lens while the necessary
adjustments are being made to overlap the two beams.  Once the beams are
overlapped, you can then replace the mirror with a real sample.  Things to
watch for if your alignment is off are: uneven illumination of the sample
and more importantly the image will shift both relative to the confocal
image and as the laser is tuned.  You can minimize these effects by
overfilling the back aperture so that the beam profile entering the
objective is more consistent.  I hope this helps to understand where the
problem may be coming from.  Before you embark on this relatively involved
procedure, I would check with Zeiss as they may feel more comfortable
handling this themselves.  In that case, I hope that you can use this
information as reference for getting the best images possible.

Ian

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Ian Read, Ph.D.
Product Manager
Spectra-Physics
Mountain View, CA 94043
(650) 966-5346    :voice
(650) 969-3546    :fax
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