Multiphoton microscope intensity modulation

[P. Muller]

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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 Smith]

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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 <
[hidden email]> wrote:

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> *****
>
> 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  [phone]
Fax [phone]
e-mail: [hidden email]
https://vision.berkeley.edu/faculty/core-grants-nei/core-grant-microscopic-imaging/

[maiti]

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Dear Peter,

I concur with Ben Smith's beautifully detailed reply. I think a half waveplate and
polarizer combo is the best and the simplest option for intensity control, and we
use it regularly. However, even if you have the half waveplate mounted on a
stepper motor, it is not going to be a fast modulation option.

I have other worries about AOM's, in addition to the GVD. Since they are
diffractive elements, they would introduce spatial chirp if used with a
femtosecond beam with a large spectral content ( if we are not using the zero
order). However, I have not properly tested them to see if this  really matters.
May be the spectral chirp is properly compensated by the AOM manufacturers. Has
anyone in this group tested it for the spatial chirp?

Sudipta

 
  On Tue, 24 Sep 2019 06:03:38 +0000, Peter Müller wrote

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> 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

Prof. Sudipta Maiti
Dept. of Chemical Sciences
Tata Institute of Fundamental Research
Homi Bhabha Road, Colaba
Mumbai 400005, India
Ph. [phone]
Alternate e-mail: [hidden email]
webpage: biophotonics.co.in

[P. Muller]

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Sudipta, Ben,

thanks for your really nice answers.

We do want fast modulation (>1MHz).

In fact we have seen some spatial chirp from the AOM, mostly in form of an slightly elliptical beam profile. The blue wavelength components of the laser get less diffracted than the red components and will spatially separate, as you mention. This gets worse with an increased AOM carrier frequency (typ. 80MHz). But if go through the math, you'll notice that you need more than 5 meters of free space propagation for a 100 fs beam at 800nm to significantly separate the beam's components.

Best,
Peter
    Am Dienstag, 24. September 2019, 19:57:18 MESZ hat maiti <[hidden email]> Folgendes geschrieben:  
 
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Dear Peter,

I concur with Ben Smith's beautifully detailed reply. I think a half waveplate and
polarizer combo is the best and the simplest option for intensity control, and we
use it regularly. However, even if you have the half waveplate mounted on a
stepper motor, it is not going to be a fast modulation option.

I have other worries about AOM's, in addition to the GVD. Since they are
diffractive elements, they would introduce spatial chirp if used with a
femtosecond beam with a large spectral content ( if we are not using the zero
order). However, I have not properly tested them to see if this  really matters.
May be the spectral chirp is properly compensated by the AOM manufacturers. Has
anyone in this group tested it for the spatial chirp?

Sudipta

 
  On Tue, 24 Sep 2019 06:03:38 +0000, Peter Müller 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

Prof. Sudipta Maiti
Dept. of Chemical Sciences
Tata Institute of Fundamental Research
Homi Bhabha Road, Colaba
Mumbai 400005, India
Ph. [phone]
Alternate e-mail: [hidden email]
webpage: biophotonics.co.in
 

[Mark Cannell-2]

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Hi All

When we used an AOM for 2P intensity modulation we used the zero order beam and with 95% diffraction efficiency the 2P excitation rate was so low when the AOM was fully energized that the zero order beam was effectively shut off. For safety there was also a 2ms response time mechanical shutter that would block the beam when not scanning. That  said, we did not expect the AOM to provide full beam intensity control, but had a separate attenuator (reflective ND filter) to set the maximum power desired.

HTH Mark

Mark Cannell Ph.D. FRSNZ, FISHR
University of Bristol, UK

On 9/26/19, 7:12 AM, "Confocal Microscopy List on behalf of Peter Müller" <[hidden email] on behalf of [hidden email]> wrote:

    *****
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    http://lists.umn.edu/cgi-bin/wa?A0=confocalmicroscopy
    Post images on http://www.imgur.com and include the link in your posting.
    *****
   
    Sudipta, Ben,
   
    thanks for your really nice answers.
   
    We do want fast modulation (>1MHz).
   
    In fact we have seen some spatial chirp from the AOM, mostly in form of an slightly elliptical beam profile. The blue wavelength components of the laser get less diffracted than the red components and will spatially separate, as you mention. This gets worse with an increased AOM carrier frequency (typ. 80MHz). But if go through the math, you'll notice that you need more than 5 meters of free space propagation for a 100 fs beam at 800nm to significantly separate the beam's components.
   
    Best,
    Peter
        Am Dienstag, 24. September 2019, 19:57:18 MESZ hat maiti <[hidden email]> Folgendes geschrieben:  
     
     *****
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    *****
   
    Dear Peter,
   
    I concur with Ben Smith's beautifully detailed reply. I think a half waveplate and
    polarizer combo is the best and the simplest option for intensity control, and we
    use it regularly. However, even if you have the half waveplate mounted on a
    stepper motor, it is not going to be a fast modulation option.
   
    I have other worries about AOM's, in addition to the GVD. Since they are
    diffractive elements, they would introduce spatial chirp if used with a
    femtosecond beam with a large spectral content ( if we are not using the zero
    order). However, I have not properly tested them to see if this  really matters.
    May be the spectral chirp is properly compensated by the AOM manufacturers. Has
    anyone in this group tested it for the spatial chirp?
   
    Sudipta
   
     
      On Tue, 24 Sep 2019 06:03:38 +0000, Peter Müller 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
   
   
    Prof. Sudipta Maiti
    Dept. of Chemical Sciences
    Tata Institute of Fundamental Research
    Homi Bhabha Road, Colaba
    Mumbai 400005, India
    Ph. [phone]
    Alternate e-mail: [hidden email]
    webpage: biophotonics.co.in
     
   

[Zdenek Svindrych-2]

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Hi Mark,
brilliant idea! Now just shoot the beam through the crystal three times and
get 8000:1 dynamic range :-). I think I got inspired by some of Stefan
Hell's papers here... But you'll need a bit more GDD pre-compensation.

On the other hand, with the first order beam (let's call it "the old way")
different spectral components will diffract under slightly different angles
(but I would not call this a spatially variable chirp, rather very poor
temporal focusing). So if all spectral components recombine in the focus,
that is, the volume of the crystal is roughly conjugate to the focal plane,
there should be no problem. This effect is only slight (depending on your
laser bandwidth), but you can still observe it on some microscopes: when
you unscrew the lens, the laser beam should be centered in the BFP, when
you switch to a different wavelength the beam drifts away as the laser is
being tuned, and then jumps back as the crystal driving frequency is
adjusted to a new value (this is what I've seen with LSM-780 and Chameleon
Vision, but now when I'm thinking about it, it might be confounded by some
effects of the internal GDD compensation and beam stabilization elements
inside the laser).

So even if the "grating effect" of the crystal is small, it should be
possible to eliminate it by proper optical design.

Best, zdenek

On Thu, Sep 26, 2019 at 5:40 AM Mark Cannell <[hidden email]>
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.
> *****
>
> Hi All
>
> When we used an AOM for 2P intensity modulation we used the zero order
> beam and with 95% diffraction efficiency the 2P excitation rate was so low
> when the AOM was fully energized that the zero order beam was effectively
> shut off. For safety there was also a 2ms response time mechanical shutter
> that would block the beam when not scanning. That  said, we did not expect
> the AOM to provide full beam intensity control, but had a separate
> attenuator (reflective ND filter) to set the maximum power desired.
>
> HTH Mark
>
> Mark Cannell Ph.D. FRSNZ, FISHR
> University of Bristol, UK
>
> On 9/26/19, 7:12 AM, "Confocal Microscopy List on behalf of Peter Müller"
> <[hidden email] on behalf of
> [hidden email]> 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.
>     *****
>
>     Sudipta, Ben,
>
>     thanks for your really nice answers.
>
>     We do want fast modulation (>1MHz).
>
>     In fact we have seen some spatial chirp from the AOM, mostly in form
> of an slightly elliptical beam profile. The blue wavelength components of
> the laser get less diffracted than the red components and will spatially
> separate, as you mention. This gets worse with an increased AOM carrier
> frequency (typ. 80MHz). But if go through the math, you'll notice that you
> need more than 5 meters of free space propagation for a 100 fs beam at
> 800nm to significantly separate the beam's components.
>
>     Best,
>     Peter
>         Am Dienstag, 24. September 2019, 19:57:18 MESZ hat maiti <
> [hidden email]> Folgendes geschrieben:
>
>      *****
>     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 Peter,
>
>     I concur with Ben Smith's beautifully detailed reply. I think a half
> waveplate and
>     polarizer combo is the best and the simplest option for intensity
> control, and we
>     use it regularly. However, even if you have the half waveplate mounted
> on a
>     stepper motor, it is not going to be a fast modulation option.
>
>     I have other worries about AOM's, in addition to the GVD. Since they
> are
>     diffractive elements, they would introduce spatial chirp if used with
> a
>     femtosecond beam with a large spectral content ( if we are not using
> the zero
>     order). However, I have not properly tested them to see if this
> really matters.
>     May be the spectral chirp is properly compensated by the AOM
> manufacturers. Has
>     anyone in this group tested it for the spatial chirp?
>
>     Sudipta
>
>
>       On Tue, 24 Sep 2019 06:03:38 +0000, Peter Müller 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
>
>
>     Prof. Sudipta Maiti
>     Dept. of Chemical Sciences
>     Tata Institute of Fundamental Research
>     Homi Bhabha Road, Colaba
>     Mumbai 400005, India
>     Ph. [phone]
>     Alternate e-mail: [hidden email]
>     webpage: biophotonics.co.in
>
>
>
>

--
--
Zdenek Svindrych, Ph.D.
Research Associate - Imaging Specialist
Department of Biochemistry and Cell Biology
Geisel School of Medicine at Dartmouth

[Craig Brideau]

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GDD compensators can be designed with a retro-reflective trip through all
the glass, but this increases the complexity (and cost) of the design. I've
done this with a manual design that worked fairly well, but still needed
careful initial alignment. Once it was aligned though, tuning the
wavelength and adjusting the main prism wouldn't walk the beam.
For power control I prefer a Glan Laser splitter and an achromatic
zero-order 1/2 waveplate. It gives good extinction and the Glan Laser is
air-spaced so it is very tough. When I needed fast modulation I used an EOM
combined with the Glan Laser-waveplate arrangement. I used the GL to set
the modulation range and let the EOM work within the modulation depth it
was capable of.

Craig

On Thu, Sep 26, 2019 at 1:52 PM Zdenek Svindrych <[hidden email]> 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.
> *****
>
> Hi Mark,
> brilliant idea! Now just shoot the beam through the crystal three times and
> get 8000:1 dynamic range :-). I think I got inspired by some of Stefan
> Hell's papers here... But you'll need a bit more GDD pre-compensation.
>
> On the other hand, with the first order beam (let's call it "the old way")
> different spectral components will diffract under slightly different angles
> (but I would not call this a spatially variable chirp, rather very poor
> temporal focusing). So if all spectral components recombine in the focus,
> that is, the volume of the crystal is roughly conjugate to the focal plane,
> there should be no problem. This effect is only slight (depending on your
> laser bandwidth), but you can still observe it on some microscopes: when
> you unscrew the lens, the laser beam should be centered in the BFP, when
> you switch to a different wavelength the beam drifts away as the laser is
> being tuned, and then jumps back as the crystal driving frequency is
> adjusted to a new value (this is what I've seen with LSM-780 and Chameleon
> Vision, but now when I'm thinking about it, it might be confounded by some
> effects of the internal GDD compensation and beam stabilization elements
> inside the laser).
>
> So even if the "grating effect" of the crystal is small, it should be
> possible to eliminate it by proper optical design.
>
> Best, zdenek
>
> On Thu, Sep 26, 2019 at 5:40 AM Mark Cannell <[hidden email]>
> 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.
> > *****
> >
> > Hi All
> >
> > When we used an AOM for 2P intensity modulation we used the zero order
> > beam and with 95% diffraction efficiency the 2P excitation rate was so
> low
> > when the AOM was fully energized that the zero order beam was effectively
> > shut off. For safety there was also a 2ms response time mechanical
> shutter
> > that would block the beam when not scanning. That  said, we did not
> expect
> > the AOM to provide full beam intensity control, but had a separate
> > attenuator (reflective ND filter) to set the maximum power desired.
> >
> > HTH Mark
> >
> > Mark Cannell Ph.D. FRSNZ, FISHR
> > University of Bristol, UK
> >
> > On 9/26/19, 7:12 AM, "Confocal Microscopy List on behalf of Peter
> Müller"
> > <[hidden email] on behalf of
> > [hidden email]> 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.
> >     *****
> >
> >     Sudipta, Ben,
> >
> >     thanks for your really nice answers.
> >
> >     We do want fast modulation (>1MHz).
> >
> >     In fact we have seen some spatial chirp from the AOM, mostly in form
> > of an slightly elliptical beam profile. The blue wavelength components of
> > the laser get less diffracted than the red components and will spatially
> > separate, as you mention. This gets worse with an increased AOM carrier
> > frequency (typ. 80MHz). But if go through the math, you'll notice that
> you
> > need more than 5 meters of free space propagation for a 100 fs beam at
> > 800nm to significantly separate the beam's components.
> >
> >     Best,
> >     Peter
> >         Am Dienstag, 24. September 2019, 19:57:18 MESZ hat maiti <
> > [hidden email]> Folgendes geschrieben:
> >
> >      *****
> >     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 Peter,
> >
> >     I concur with Ben Smith's beautifully detailed reply. I think a half
> > waveplate and
> >     polarizer combo is the best and the simplest option for intensity
> > control, and we
> >     use it regularly. However, even if you have the half waveplate
> mounted
> > on a
> >     stepper motor, it is not going to be a fast modulation option.
> >
> >     I have other worries about AOM's, in addition to the GVD. Since they
> > are
> >     diffractive elements, they would introduce spatial chirp if used with
> > a
> >     femtosecond beam with a large spectral content ( if we are not using
> > the zero
> >     order). However, I have not properly tested them to see if this
> > really matters.
> >     May be the spectral chirp is properly compensated by the AOM
> > manufacturers. Has
> >     anyone in this group tested it for the spatial chirp?
> >
> >     Sudipta
> >
> >
> >       On Tue, 24 Sep 2019 06:03:38 +0000, Peter Müller 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
> >
> >
> >     Prof. Sudipta Maiti
> >     Dept. of Chemical Sciences
> >     Tata Institute of Fundamental Research
> >     Homi Bhabha Road, Colaba
> >     Mumbai 400005, India
> >     Ph. [phone]
> >     Alternate e-mail: [hidden email]
> >     webpage: biophotonics.co.in
> >
> >
> >
> >
>
> --
> --
> Zdenek Svindrych, Ph.D.
> Research Associate - Imaging Specialist
> Department of Biochemistry and Cell Biology
> Geisel School of Medicine at Dartmouth
>

[Mark Cannell-2]

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Hi Zedenek

Another advantage of using the zero order is that the beam doesn't walk when tuning and we found that a 95% beam attenuation was sufficient to effectively eliminate sample damage during the over scan and fly back (400 fold reduction in excitation rate). I think that some heating effects that can appear in the AOM 1st order bean were also reduced. We also had a pre chirp system in that only needed to be adjusted once for all the optical components so alignment was pretty stable.
See https://doi.org/10.1007/s004240050169

Cheers Mark


On 9/26/19, 8:52 PM, "Confocal Microscopy List on behalf of Zdenek Svindrych" <[hidden email] on behalf of [hidden email]> wrote:

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    *****
   
    Hi Mark,
    brilliant idea! Now just shoot the beam through the crystal three times and
    get 8000:1 dynamic range :-). I think I got inspired by some of Stefan
    Hell's papers here... But you'll need a bit more GDD pre-compensation.
   
    On the other hand, with the first order beam (let's call it "the old way")
    different spectral components will diffract under slightly different angles
    (but I would not call this a spatially variable chirp, rather very poor
    temporal focusing). So if all spectral components recombine in the focus,
    that is, the volume of the crystal is roughly conjugate to the focal plane,
    there should be no problem. This effect is only slight (depending on your
    laser bandwidth), but you can still observe it on some microscopes: when
    you unscrew the lens, the laser beam should be centered in the BFP, when
    you switch to a different wavelength the beam drifts away as the laser is
    being tuned, and then jumps back as the crystal driving frequency is
    adjusted to a new value (this is what I've seen with LSM-780 and Chameleon
    Vision, but now when I'm thinking about it, it might be confounded by some
    effects of the internal GDD compensation and beam stabilization elements
    inside the laser).
   
    So even if the "grating effect" of the crystal is small, it should be
    possible to eliminate it by proper optical design.
   
    Best, zdenek
   
    On Thu, Sep 26, 2019 at 5:40 AM Mark Cannell <[hidden email]>
    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.
    > *****
    >
    > Hi All
    >
    > When we used an AOM for 2P intensity modulation we used the zero order
    > beam and with 95% diffraction efficiency the 2P excitation rate was so low
    > when the AOM was fully energized that the zero order beam was effectively
    > shut off. For safety there was also a 2ms response time mechanical shutter
    > that would block the beam when not scanning. That  said, we did not expect
    > the AOM to provide full beam intensity control, but had a separate
    > attenuator (reflective ND filter) to set the maximum power desired.
    >
    > HTH Mark
    >
    > Mark Cannell Ph.D. FRSNZ, FISHR
    > University of Bristol, UK
    >
    > On 9/26/19, 7:12 AM, "Confocal Microscopy List on behalf of Peter Müller"
    > <[hidden email] on behalf of
    > [hidden email]> 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.
    >     *****
    >
    >     Sudipta, Ben,
    >
    >     thanks for your really nice answers.
    >
    >     We do want fast modulation (>1MHz).
    >
    >     In fact we have seen some spatial chirp from the AOM, mostly in form
    > of an slightly elliptical beam profile. The blue wavelength components of
    > the laser get less diffracted than the red components and will spatially
    > separate, as you mention. This gets worse with an increased AOM carrier
    > frequency (typ. 80MHz). But if go through the math, you'll notice that you
    > need more than 5 meters of free space propagation for a 100 fs beam at
    > 800nm to significantly separate the beam's components.
    >
    >     Best,
    >     Peter
    >         Am Dienstag, 24. September 2019, 19:57:18 MESZ hat maiti <
    > [hidden email]> Folgendes geschrieben:
    >
    >      *****
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    > posting.
    >     *****
    >
    >     Dear Peter,
    >
    >     I concur with Ben Smith's beautifully detailed reply. I think a half
    > waveplate and
    >     polarizer combo is the best and the simplest option for intensity
    > control, and we
    >     use it regularly. However, even if you have the half waveplate mounted
    > on a
    >     stepper motor, it is not going to be a fast modulation option.
    >
    >     I have other worries about AOM's, in addition to the GVD. Since they
    > are
    >     diffractive elements, they would introduce spatial chirp if used with
    > a
    >     femtosecond beam with a large spectral content ( if we are not using
    > the zero
    >     order). However, I have not properly tested them to see if this
    > really matters.
    >     May be the spectral chirp is properly compensated by the AOM
    > manufacturers. Has
    >     anyone in this group tested it for the spatial chirp?
    >
    >     Sudipta
    >
    >
    >       On Tue, 24 Sep 2019 06:03:38 +0000, Peter Müller 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
    >
    >
    >     Prof. Sudipta Maiti
    >     Dept. of Chemical Sciences
    >     Tata Institute of Fundamental Research
    >     Homi Bhabha Road, Colaba
    >     Mumbai 400005, India
    >     Ph. [phone]
    >     Alternate e-mail: [hidden email]
    >     webpage: biophotonics.co.in
    >
    >
    >
    >
   
    --
    --
    Zdenek Svindrych, Ph.D.
    Research Associate - Imaging Specialist
    Department of Biochemistry and Cell Biology
    Geisel School of Medicine at Dartmouth