*****
To join, leave or search the confocal microscopy listserv, go to:
http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
Post images on http://www.imgur.com and include the link in your posting.
*****

For linear scanning 2P microscopes, an achromatic half-wave plate (in a
motorized rotating mount) paired with a glan-laser polarizer is a clear
winner.  Our measured contrast ratio with this attentuator setup is almost
10,000:1, which is pretty impressive when you consider that the spec sheet
for the laser states a polarization ratio of only >500:1 - guess we got a
good one.  Additionally, since the half wave plate is achromatic, the
attenuation is more or less flat across the entire tuning spectrum of the
laser. The reason this setup works specifically for linear scanning
microscopes is that the angular velocity of a linear scanning mirror is
very constant.  Therefore, the laser intensity will be uniform across the
scan field, negating the need for flyback blanking unless it is absolutely
critical not to have the laser shine outside of your scan field.  On our
systems, the flyback over-scan is about 50% the width of the imaging scan
field (25% on each side).  In comparison, a Pockel's cell on a linear
scanning system adds more cost, more operational complexity, and less
performance, for no real gain.

That said, if you have a resonant scanner, than your only real choice is a
Pockel's cell.  This is because since a resonant mirror is under constant
angular acceleration, the mirror stalls out at the ends of the scan for an
appreciable amount of time.  The end result being that if the 2P laser was
left on for the full resonant sweep, you would see that the laser is much
brighter at the edges of the scan field due to the increased dwell time.
Therefore a Pockel's cell is needed to rapidly attenuate the laser at the
end of each sweep of the mirror to keep the laser from burning the tissue.
One thing that surprised me, the GVD for the KDP crystals in a Pockel's
cell is a measly 5.28 fs2/mm at 950 nm which is very low, so even though
the path length of a Pockel's cell looks long, the GVD is still very low
compared to most optical glasses.

You also inspired me to look up the GVD of TeO2 at 950 nm, which is 388 fs2/mm,
that's higher than most flint glasses!  By my math, a 140 fs pulse going
through 2 cm of the stuff gets broadened to 208 fs, and a 70 fs pulse gets
broadened to 315 fs.  While bad, keep in mind that this is still comparable
to the measured GVD in objectives (and likely the scan lens and tube lens,
but couldn't find any papers or specs on those):
http://users.ox.ac.uk/~atdgroup/publications/Tullis,%20I.D.C.,%20Proc.%20SPIE,%206089,%202006.pdf

In summary: Achromatic half wave plate in motorized mount  vs. Pockel's cell
Pros:
Relatively cheap
Isn't damaged if left on for long periods of time
Spectrally flat attenuation =  Very broad spectral bandwidth
High contrast ratio
Easy to calibrate and use

Cons:
No fly-back blanking - Cannot be used for resonant 2P scanning microscopes
without risk of burning sample outside of scan field

One final note, if you do get a Pockel's cell, definitely get a model with
an integrated glan-laser polarizer and beam dump.  This makes them much
simpler to install and safer to align.

Cheers,
   Ben Smith



On Mon, Sep 23, 2019 at 11:28 PM Peter Müller <
[log in to unmask]> wrote:

> *****
> To join, leave or search the confocal microscopy listserv, go to:
> http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
> Post images on http://www.imgur.com and include the link in your posting.
> *****
>
> Dear Microscopists,
> for our multiphoton scope we use an acousto-optical modulator for
> intensity modulation. Those AOM have a nice overall throughput (>85%) and a
> high bandwidth (>1MHz), however they add a lot of group delay dispersion
> (GDD) to our setup - typically they use a 1cm - 2cm TeO2 crystal for the
> NIR wavelengths from 700nm to 1000nm. For our multiphoton setup, the AOM is
> the main contributor of GDD right now. I was wondering: what is your
> preferred choice for intensity modulation and what are the respective
> advantages/disadvantages?
>
> Thanks,Peter
>


-- 
Benjamin E. Smith, Ph. D.
Imaging Specialist, Vision Science
University of California, Berkeley
195 Life Sciences Addition
Berkeley, CA  94720-3200
Tel  (510) 642-9712
Fax (510) 643-6791
e-mail: [log in to unmask]
https://vision.berkeley.edu/faculty/core-grants-nei/core-grant-microscopic-imaging/