Strange Ti:Sa pulse shape after a 2P microscope

[Peter Rupprecht-2]

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Dear list,
I have a question about a near IR pulsed laser and why the laser pulse looks very strange after going through a multiphoton microscope.
I'm using a custom-built two-photon microscope, and I realized that the fluorescence yield is lower than I would have expected by comparison with a very similar microscope. I also have the impression that it was better about a year ago. To figure out the reason for this, I tried out many things, but I also measured the pulse shape using a Carpe autocorrelator (https://www.ape-berlin.de/en/autocorrelator/carpe/), both before and after the microscope (after the microscope means: below the objective).
Before the microscope, it looks okay (https://i.imgur.com/vm7r5pN.jpg; ~160 fs pulse width), whereas it is not only broadened but also strangely reshaped after the microscope (https://i.imgur.com/lFukA9n.jpg), as if there was a second pulse coming with a delay of few 100 fs. This double-pulse is very strange and probably reduces two-photon excitation drastically for a given average power. The shape of this pulse varies strongly with wavelength but is bad over the whole range of wavelength that I'm interested in (900-940 nm).
Does anybody have an idea where this could come from?Could this be a measurement artifact from using the autocorrelator? Or does it rather stem from a component in the microscope?If the latter is more likely, do you have any ideas what I could do to get rid of it?
The microscope itself is a Sutter two-photon microscope with a custom-built remote z-scanning module and is also described here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4871072/. As far as I can see, there are no fancy components that are not also used by many other multiphoton microscopes.
I would be happy about any form of input on this question!
Best,Peter

[Craig Brideau]

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The 'pulse on a pedestal' is classic of higher order dispersion caused by
large amounts of glass, or possibly phase distortion by a coating. Possible
candidates include a lens with high index glass, dielectric coatings
causing phase steps, or some issue with the way your mirror actuates the
beam. There is also some chance that the input to the autocorrelator could
be messy in some way due to back reflections. If it is not an instrument
error, chirped mirrors or a prism compressor may be able to compensate for
some of the higher order distortions.

Craig

On Wed, May 30, 2018 at 10:19 AM Peter Rupprecht <
[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.
> *****
>
> Dear list,
> I have a question about a near IR pulsed laser and why the laser pulse
> looks very strange after going through a multiphoton microscope.
> I'm using a custom-built two-photon microscope, and I realized that the
> fluorescence yield is lower than I would have expected by comparison with a
> very similar microscope. I also have the impression that it was better
> about a year ago. To figure out the reason for this, I tried out many
> things, but I also measured the pulse shape using a Carpe autocorrelator (
> https://www.ape-berlin.de/en/autocorrelator/carpe/), both before and
> after the microscope (after the microscope means: below the objective).
> Before the microscope, it looks okay (https://i.imgur.com/vm7r5pN.jpg;
> ~160 fs pulse width), whereas it is not only broadened but also strangely
> reshaped after the microscope (https://i.imgur.com/lFukA9n.jpg), as if
> there was a second pulse coming with a delay of few 100 fs. This
> double-pulse is very strange and probably reduces two-photon excitation
> drastically for a given average power. The shape of this pulse varies
> strongly with wavelength but is bad over the whole range of wavelength that
> I'm interested in (900-940 nm).
> Does anybody have an idea where this could come from?Could this be a
> measurement artifact from using the autocorrelator? Or does it rather stem
> from a component in the microscope?If the latter is more likely, do you
> have any ideas what I could do to get rid of it?
> The microscope itself is a Sutter two-photon microscope with a
> custom-built remote z-scanning module and is also described here:
> https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4871072/. As far as I can
> see, there are no fancy components that are not also used by many other
> multiphoton microscopes.
> I would be happy about any form of input on this question!
> Best,Peter
>

[Craig Brideau]

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Expanding on the autocorrelator error, a 'double tap' pulse pair where two
pulses are closely following each other due to some reflection could also
possibly cause this result, but double-tapping can also be caused by third
order and higher nonlinear phase distortions.

Craig

On Wed, May 30, 2018 at 11:58 AM Craig Brideau <[hidden email]>
wrote:

> The 'pulse on a pedestal' is classic of higher order dispersion caused by
> large amounts of glass, or possibly phase distortion by a coating. Possible
> candidates include a lens with high index glass, dielectric coatings
> causing phase steps, or some issue with the way your mirror actuates the
> beam. There is also some chance that the input to the autocorrelator could
> be messy in some way due to back reflections. If it is not an instrument
> error, chirped mirrors or a prism compressor may be able to compensate for
> some of the higher order distortions.
>
> Craig
>
> On Wed, May 30, 2018 at 10:19 AM Peter Rupprecht <
> [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.
>> *****
>>
>> Dear list,
>> I have a question about a near IR pulsed laser and why the laser pulse
>> looks very strange after going through a multiphoton microscope.
>> I'm using a custom-built two-photon microscope, and I realized that the
>> fluorescence yield is lower than I would have expected by comparison with a
>> very similar microscope. I also have the impression that it was better
>> about a year ago. To figure out the reason for this, I tried out many
>> things, but I also measured the pulse shape using a Carpe autocorrelator (
>> https://www.ape-berlin.de/en/autocorrelator/carpe/), both before and
>> after the microscope (after the microscope means: below the objective).
>> Before the microscope, it looks okay (https://i.imgur.com/vm7r5pN.jpg;
>> ~160 fs pulse width), whereas it is not only broadened but also strangely
>> reshaped after the microscope (https://i.imgur.com/lFukA9n.jpg), as if
>> there was a second pulse coming with a delay of few 100 fs. This
>> double-pulse is very strange and probably reduces two-photon excitation
>> drastically for a given average power. The shape of this pulse varies
>> strongly with wavelength but is bad over the whole range of wavelength that
>> I'm interested in (900-940 nm).
>> Does anybody have an idea where this could come from?Could this be a
>> measurement artifact from using the autocorrelator? Or does it rather stem
>> from a component in the microscope?If the latter is more likely, do you
>> have any ideas what I could do to get rid of it?
>> The microscope itself is a Sutter two-photon microscope with a
>> custom-built remote z-scanning module and is also described here:
>> https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4871072/. As far as I can
>> see, there are no fancy components that are not also used by many other
>> multiphoton microscopes.
>> I would be happy about any form of input on this question!
>> Best,Peter
>>
>

[Peter Rupprecht-2]

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 Hi Craig,
Thank you for your messages!
About your idea that a reflection could cause a pulse pair: the distance in time of the two pulses seems to be around 250 fs, which means a backreflection on a spatial distance of 250e-15*3e8/2 = 38 micrometers. I guess that typical coatings are much less thick than that?
For the dispersion compensation: If it is higher order dispersion, then a prism compressor would not help anyway (since it generates a linear pre-chirping), or am I getting this wrong?
Best,Peter
    Am Mittwoch, 30. Mai 2018, 20:01:32 MESZ hat Craig Brideau <[hidden email]> Folgendes geschrieben:  
 
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Expanding on the autocorrelator error, a 'double tap' pulse pair where two
pulses are closely following each other due to some reflection could also
possibly cause this result, but double-tapping can also be caused by third
order and higher nonlinear phase distortions.

Craig

On Wed, May 30, 2018 at 11:58 AM Craig Brideau <[hidden email]>
wrote:

> The 'pulse on a pedestal' is classic of higher order dispersion caused by
> large amounts of glass, or possibly phase distortion by a coating. Possible
> candidates include a lens with high index glass, dielectric coatings
> causing phase steps, or some issue with the way your mirror actuates the
> beam. There is also some chance that the input to the autocorrelator could
> be messy in some way due to back reflections. If it is not an instrument
> error, chirped mirrors or a prism compressor may be able to compensate for
> some of the higher order distortions.
>
> Craig
>
> On Wed, May 30, 2018 at 10:19 AM Peter Rupprecht <
> [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.
>> *****
>>
>> Dear list,
>> I have a question about a near IR pulsed laser and why the laser pulse
>> looks very strange after going through a multiphoton microscope.
>> I'm using a custom-built two-photon microscope, and I realized that the
>> fluorescence yield is lower than I would have expected by comparison with a
>> very similar microscope. I also have the impression that it was better
>> about a year ago. To figure out the reason for this, I tried out many
>> things, but I also measured the pulse shape using a Carpe autocorrelator (
>> https://www.ape-berlin.de/en/autocorrelator/carpe/), both before and
>> after the microscope (after the microscope means: below the objective).
>> Before the microscope, it looks okay (https://i.imgur.com/vm7r5pN.jpg;
>> ~160 fs pulse width), whereas it is not only broadened but also strangely
>> reshaped after the microscope (https://i.imgur.com/lFukA9n.jpg), as if
>> there was a second pulse coming with a delay of few 100 fs. This
>> double-pulse is very strange and probably reduces two-photon excitation
>> drastically for a given average power. The shape of this pulse varies
>> strongly with wavelength but is bad over the whole range of wavelength that
>> I'm interested in (900-940 nm).
>> Does anybody have an idea where this could come from?Could this be a
>> measurement artifact from using the autocorrelator? Or does it rather stem
>> from a component in the microscope?If the latter is more likely, do you
>> have any ideas what I could do to get rid of it?
>> The microscope itself is a Sutter two-photon microscope with a
>> custom-built remote z-scanning module and is also described here:
>> https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4871072/. As far as I can
>> see, there are no fancy components that are not also used by many other
>> multiphoton microscopes.
>> I would be happy about any form of input on this question!
>> Best,Peter
>>
>

 

[Craig Brideau]

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38 um is a bit odd, so I agree this seems unlikely to be due to some form
of reflection. The mirror in your focus compensator may possibly have some
effect on this scale but I suspect it is mainly due to extra glass
introduced by the focus system. You most likely have significant higher
order dispersion due to the z-scan lens addressing the mirror.  You can
induce third order with a prism compressor by tilting the prisms, although
chirped mirrors are generally better for this. You can also use a
combination system with both prisms and gratings to give you more
flexibility. Overall though, a chirped mirror pair is the simplest to set
up on an optical table as you simply have to ensure the two mirrors are
parallel to each other and ricochet the beam between them at a shallow
angle. The number of bounces between the mirrors will control the degree of
correction. Note that the wavelength range of chirped mirrors is typically
narrow, so if you tune your laser large distances the chirped mirrors may
not work. There should be a coating set that works for 900-940 though, but
verify this before you buy a pair of mirrors.

Craig

On Wed, May 30, 2018 at 12:34 PM Peter Rupprecht <
[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 Craig,
> Thank you for your messages!
> About your idea that a reflection could cause a pulse pair: the distance
> in time of the two pulses seems to be around 250 fs, which means a
> backreflection on a spatial distance of 250e-15*3e8/2 = 38 micrometers. I
> guess that typical coatings are much less thick than that?
> For the dispersion compensation: If it is higher order dispersion, then a
> prism compressor would not help anyway (since it generates a linear
> pre-chirping), or am I getting this wrong?
> Best,Peter
>     Am Mittwoch, 30. Mai 2018, 20:01:32 MESZ hat Craig Brideau <
> [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.
> *****
>
> Expanding on the autocorrelator error, a 'double tap' pulse pair where two
> pulses are closely following each other due to some reflection could also
> possibly cause this result, but double-tapping can also be caused by third
> order and higher nonlinear phase distortions.
>
> Craig
>
> On Wed, May 30, 2018 at 11:58 AM Craig Brideau <[hidden email]>
> wrote:
>
> > The 'pulse on a pedestal' is classic of higher order dispersion caused by
> > large amounts of glass, or possibly phase distortion by a coating.
> Possible
> > candidates include a lens with high index glass, dielectric coatings
> > causing phase steps, or some issue with the way your mirror actuates the
> > beam. There is also some chance that the input to the autocorrelator
> could
> > be messy in some way due to back reflections. If it is not an instrument
> > error, chirped mirrors or a prism compressor may be able to compensate
> for
> > some of the higher order distortions.
> >
> > Craig
> >
> > On Wed, May 30, 2018 at 10:19 AM Peter Rupprecht <
> > [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.
> >> *****
> >>
> >> Dear list,
> >> I have a question about a near IR pulsed laser and why the laser pulse
> >> looks very strange after going through a multiphoton microscope.
> >> I'm using a custom-built two-photon microscope, and I realized that the
> >> fluorescence yield is lower than I would have expected by comparison
> with a
> >> very similar microscope. I also have the impression that it was better
> >> about a year ago. To figure out the reason for this, I tried out many
> >> things, but I also measured the pulse shape using a Carpe
> autocorrelator (
> >> https://www.ape-berlin.de/en/autocorrelator/carpe/), both before and
> >> after the microscope (after the microscope means: below the objective).
> >> Before the microscope, it looks okay (https://i.imgur.com/vm7r5pN.jpg;
> >> ~160 fs pulse width), whereas it is not only broadened but also
> strangely
> >> reshaped after the microscope (https://i.imgur.com/lFukA9n.jpg), as if
> >> there was a second pulse coming with a delay of few 100 fs. This
> >> double-pulse is very strange and probably reduces two-photon excitation
> >> drastically for a given average power. The shape of this pulse varies
> >> strongly with wavelength but is bad over the whole range of wavelength
> that
> >> I'm interested in (900-940 nm).
> >> Does anybody have an idea where this could come from?Could this be a
> >> measurement artifact from using the autocorrelator? Or does it rather
> stem
> >> from a component in the microscope?If the latter is more likely, do you
> >> have any ideas what I could do to get rid of it?
> >> The microscope itself is a Sutter two-photon microscope with a
> >> custom-built remote z-scanning module and is also described here:
> >> https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4871072/. As far as I can
> >> see, there are no fancy components that are not also used by many other
> >> multiphoton microscopes.
> >> I would be happy about any form of input on this question!
> >> Best,Peter
> >>
> >
>
>

[Michael Giacomelli]

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How are you collimating the light emitted by your objective and did you add
any high index glass or achromats to the microscope when you customized
it?  Many achromats can have extremely high dispersion, especially at short
focal lengths.

Mike

On Wed, May 30, 2018 at 12:14 PM, Peter Rupprecht <
[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.
> *****
>
> Dear list,
> I have a question about a near IR pulsed laser and why the laser pulse
> looks very strange after going through a multiphoton microscope.
> I'm using a custom-built two-photon microscope, and I realized that the
> fluorescence yield is lower than I would have expected by comparison with a
> very similar microscope. I also have the impression that it was better
> about a year ago. To figure out the reason for this, I tried out many
> things, but I also measured the pulse shape using a Carpe autocorrelator (
> https://www.ape-berlin.de/en/autocorrelator/carpe/), both before and
> after the microscope (after the microscope means: below the objective).
> Before the microscope, it looks okay (https://i.imgur.com/vm7r5pN.jpg;
> ~160 fs pulse width), whereas it is not only broadened but also strangely
> reshaped after the microscope (https://i.imgur.com/lFukA9n.jpg), as if
> there was a second pulse coming with a delay of few 100 fs. This
> double-pulse is very strange and probably reduces two-photon excitation
> drastically for a given average power. The shape of this pulse varies
> strongly with wavelength but is bad over the whole range of wavelength that
> I'm interested in (900-940 nm).
> Does anybody have an idea where this could come from?Could this be a
> measurement artifact from using the autocorrelator? Or does it rather stem
> from a component in the microscope?If the latter is more likely, do you
> have any ideas what I could do to get rid of it?
> The microscope itself is a Sutter two-photon microscope with a
> custom-built remote z-scanning module and is also described here:
> https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4871072/. As far as I can
> see, there are no fancy components that are not also used by many other
> multiphoton microscopes.
> I would be happy about any form of input on this question!
> Best,Peter
>

[Peter Rupprecht-2]

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Hi Mike,
Thank you for your suggestions.

> How are you collimating the light emitted by your objective ...
Not at all. For pulse width measurement with the "Carpe" autocorrelator, the light coming from the objective does not need to be collimated. (There is a device, the actual correlator, which needs to be inserted and aligned into the beam path; then, a small power meter-like sensor can be used to measure the pulse width in front of the objective.) The system is illustrated in this pdf on page 23: http://www.ape-berlin.de/content/uploads/2018/05/APE-Ultrafast-Laser-Diagnostics-Rev.-3.2.4.pdf.
> .... and did you add any high index glass or achromats to the microscope when you customized it?  Many achromats can have extremely high dispersion, especially at short focal lengths.
Yes, I did add five rather thick achromatic doublets (Thorlabs), and a 25 mm-thick polarizing beam splitter. Following your email, I removed all of this from the light path and again measured the pulse width/shape. The overall pulse width was a bit shorter, but the strange pulse shape with shoulders remained as it was before.
I'm right now testing all the other components of the microscope (mostly mirrors) to find the culprit.
Best,Peter




 
 
 Am Mittwoch, 30. Mai 2018, 23:52:43 MESZ hat Michael Giacomelli <[hidden email]> Folgendes geschrieben:





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*****

How are you collimating the light emitted by your objective and did you add
any high index glass or achromats to the microscope when you customized
it?  Many achromats can have extremely high dispersion, especially at short
focal lengths.

Mike

On Wed, May 30, 2018 at 12:14 PM, Peter Rupprecht <
[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.

> *****
>
> Dear list,
> I have a question about a near IR pulsed laser and why the laser pulse
> looks very strange after going through a multiphoton microscope.
> I'm using a custom-built two-photon microscope, and I realized that the
> fluorescence yield is lower than I would have expected by comparison with a
> very similar microscope. I also have the impression that it was better
> about a year ago. To figure out the reason for this, I tried out many
> things, but I also measured the pulse shape using a Carpe autocorrelator (
> https://www.ape-berlin.de/en/autocorrelator/carpe/), both before and
> after the microscope (after the microscope means: below the objective).
> Before the microscope, it looks okay (https://i.imgur.com/vm7r5pN.jpg;
> ~160 fs pulse width), whereas it is not only broadened but also strangely
> reshaped after the microscope (https://i.imgur.com/lFukA9n.jpg), as if
> there was a second pulse coming with a delay of few 100 fs. This
> double-pulse is very strange and probably reduces two-photon excitation
> drastically for a given average power. The shape of this pulse varies
> strongly with wavelength but is bad over the whole range of wavelength that
> I'm interested in (900-940 nm).
> Does anybody have an idea where this could come from?Could this be a
> measurement artifact from using the autocorrelator? Or does it rather stem
> from a component in the microscope?If the latter is more likely, do you
> have any ideas what I could do to get rid of it?
> The microscope itself is a Sutter two-photon microscope with a
> custom-built remote z-scanning module and is also described here:
> https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4871072/. As far as I can
> see, there are no fancy components that are not also used by many other
> multiphoton microscopes.
> I would be happy about any form of input on this question!
> Best,Peter
>

[Peter Rupprecht-2]

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Thanks for the input (on-list and off-list)! It helped me find the most likely culprit:

After eliminating most of the lenses, I still saw the strange pulse shape and realized that it must be somehow due to simple plane mirrors. The mirrors (9 of them, all of them ca. 10 years old and used throughout) that were used for the microscope were EO3 dielectric broadband mirrors (Thorlabs), and one of the mirrors (maybe due to age) contributed a lot to this nonlinear dispersion. I replaced it by another EO3 mirror and saw a nice improvement - at least for some wavelengths. Here is on overview of how the pulse shape looks like (after the objective), and it is very good at 933 nm, while bad at 917 nm : https://i.imgur.com/EdJTH1d.gifv.

Now, the microscope is useable at 933 nm. I have the impression that the dielectric mirrors can also generate a negative dispersion that can - for some wavelengths - reduce the temporal pulse width. Could anybody support this with his experience?

Another option would be to use metallic mirrors to replace all the dielectric ones. Unprotected metallic mirrors seem to be ideal for dispersion-free reflection. Or is it that dispersion is negligeable for 100 fs-pulses also for protected metallic mirrors? On the other hand, unprotected mirrors seem to be difficult to clean and keep scratch-free, which is not a good property for a microscope that is used by many people on a daily basis.

And I would be interested in knowing whether the dispersive effects of dielectric broadband mirrors depend a) on their age and b) on the angle of incidence of the laser beam. I haven't found any data on this topic in the internet, but this would be really interesting to know.

Best,
Peter

[Craig Brideau]

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I don't recommend standard dielectric mirrors for ultrafast applications.
The coating is designed for maximum reflectivity with no regard to phase
implications. The result can be very unpredictable and in your case clearly
led to undesirable higher order dispersion. The best mirrors are protected
silver, or if your budget allows, the slightly more expensive ultrafast
silver mirrors. Ultrafast dielectric mirrors are also available but these
are quite expensive as the coatings are also designed to be nearly phase
neutral.

Craig

On Fri, Jun 1, 2018, 4:55 AM Peter Rupprecht <
[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.
> *****
>
> Thanks for the input (on-list and off-list)! It helped me find the most
> likely culprit:
>
> After eliminating most of the lenses, I still saw the strange pulse shape
> and realized that it must be somehow due to simple plane mirrors. The
> mirrors (9 of them, all of them ca. 10 years old and used throughout) that
> were used for the microscope were EO3 dielectric broadband mirrors
> (Thorlabs), and one of the mirrors (maybe due to age) contributed a lot to
> this nonlinear dispersion. I replaced it by another EO3 mirror and saw a
> nice improvement - at least for some wavelengths. Here is on overview of
> how the pulse shape looks like (after the objective), and it is very good
> at 933 nm, while bad at 917 nm : https://i.imgur.com/EdJTH1d.gifv.
>
> Now, the microscope is useable at 933 nm. I have the impression that the
> dielectric mirrors can also generate a negative dispersion that can - for
> some wavelengths - reduce the temporal pulse width. Could anybody support
> this with his experience?
>
> Another option would be to use metallic mirrors to replace all the
> dielectric ones. Unprotected metallic mirrors seem to be ideal for
> dispersion-free reflection. Or is it that dispersion is negligeable for 100
> fs-pulses also for protected metallic mirrors? On the other hand,
> unprotected mirrors seem to be difficult to clean and keep scratch-free,
> which is not a good property for a microscope that is used by many people
> on a daily basis.
>
> And I would be interested in knowing whether the dispersive effects of
> dielectric broadband mirrors depend a) on their age and b) on the angle of
> incidence of the laser beam. I haven't found any data on this topic in the
> internet, but this would be really interesting to know.
>
> Best,
> Peter
>

[Zdenek Svindrych-2]

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Hi Peter,
thanks for revealing the actual cause!

I've always used protected silver mirrors, because they nicely cover the
range of wavelengths I was mostly interested in (680 - 900 nm). I never
observed any strange effects, but that's most likely due to the fact that I
never used such a fancy autocorrelator :-). Anyway, I doubt you can get
unprotected silver, such mirrors would not last long...

You can ask Thorlabs about the dispersion effects of their dielectric
mirrors, they can easily simulate it in whatever thin film design software
they're using. I'd expect any effects to be weak if you're far from the
wavelength limits. But if the properties of the mirrors change with aging
and (ab)use, Thorlabs might not be able to give you any useful info.

Best, zdenek


---------- Původní e-mail ----------
Od: Peter Rupprecht <[hidden email]>
Komu: [hidden email]
Datum: 1. 6. 2018 6:57:25
Předmět: Re: Strange Ti:Sa pulse shape after a 2P microscope
"*****
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Thanks for the input (on-list and off-list)! It helped me find the most
likely culprit:

After eliminating most of the lenses, I still saw the strange pulse shape
and realized that it must be somehow due to simple plane mirrors. The
mirrors (9 of them, all of them ca. 10 years old and used throughout) that
were used for the microscope were EO3 dielectric broadband mirrors
(Thorlabs), and one of the mirrors (maybe due to age) contributed a lot to
this nonlinear dispersion. I replaced it by another EO3 mirror and saw a
nice improvement - at least for some wavelengths. Here is on overview of how
the pulse shape looks like (after the objective), and it is very good at 933
nm, while bad at 917 nm : https://i.imgur.com/EdJTH1d.gifv.

Now, the microscope is useable at 933 nm. I have the impression that the
dielectric mirrors can also generate a negative dispersion that can - for
some wavelengths - reduce the temporal pulse width. Could anybody support
this with his experience?

Another option would be to use metallic mirrors to replace all the
dielectric ones. Unprotected metallic mirrors seem to be ideal for
dispersion-free reflection. Or is it that dispersion is negligeable for 100
fs-pulses also for protected metallic mirrors? On the other hand,
unprotected mirrors seem to be difficult to clean and keep scratch-free,
which is not a good property for a microscope that is used by many people on
a daily basis.

And I would be interested in knowing whether the dispersive effects of
dielectric broadband mirrors depend a) on their age and b) on the angle of
incidence of the laser beam. I haven't found any data on this topic in the
internet, but this would be really interesting to know.

Best,
Peter
"

[Michael Giacomelli]

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> Now, the microscope is useable at 933 nm. I have the impression that the
dielectric mirrors can also generate a negative dispersion that can - for
some wavelengths - reduce the temporal pulse width. Could anybody support
this with his experience?

Absolutely.  A dielectric mirror is an interference (IIR) filter.  For
normal reflective mirrors, the phase response isn't too important and the
layers (taps) are optimized for reflectivity and cost.  The phase response
is whatever the optimizer picks as cheapest, and can be very strange at
some wavelengths.  They make ultrafast dielectric mirrors that have a
specified phase function, both for dispersion compensation (Thor sells
these too) or just for high reflectivity without (uncontrolled)
dispersion.

I am a little surprised you saw such an extreme response from that one
filter, but it's possible you happened to hit it at a point where the phase
response flipped very rapidly from the blue end of your pulse to the red
end. Unless you really need high reflectivity, might be worth just using a
silver mirror.

By the way, once you get this sorted out, look at those achromats you added
as well.  Thor gives you the thicknesses and materials, and the dispersions
can be looked up here:

https://refractiveindex.info/?shelf=glass&book=SCHOTT-SF&page=P-SF8

The dispersion of individual achromats varies enormously, with tiny
differences in focal length sometimes giving very large differences in
dispersion.  For example, at 900nm, the Thor's 125mm achromat has double
the dispersion (900 vs 470 fs^2) of its 100mm doublet.  If you can get away
with a singlet, the 100mm LA5817 will give you just 90 fs^2.  For this
reason I usually try to avoid using stock doublets in ultrafast systems, or
if I need them, get ones customized for low dispersion.

Mike

On Fri, Jun 1, 2018 at 9:31 AM, <[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 Peter,
> thanks for revealing the actual cause!
>
> I've always used protected silver mirrors, because they nicely cover the
> range of wavelengths I was mostly interested in (680 - 900 nm). I never
> observed any strange effects, but that's most likely due to the fact that
> I
> never used such a fancy autocorrelator :-). Anyway, I doubt you can get
> unprotected silver, such mirrors would not last long...
>
> You can ask Thorlabs about the dispersion effects of their dielectric
> mirrors, they can easily simulate it in whatever thin film design software
> they're using. I'd expect any effects to be weak if you're far from the
> wavelength limits. But if the properties of the mirrors change with aging
> and (ab)use, Thorlabs might not be able to give you any useful info.
>
> Best, zdenek
>
>
> ---------- Původní e-mail ----------
> Od: Peter Rupprecht <[hidden email]>
> Komu: [hidden email]
> Datum: 1. 6. 2018 6:57:25
> Předmět: Re: Strange Ti:Sa pulse shape after a 2P microscope
> "*****
> 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.
> *****
>
> Thanks for the input (on-list and off-list)! It helped me find the most
> likely culprit:
>
> After eliminating most of the lenses, I still saw the strange pulse shape
> and realized that it must be somehow due to simple plane mirrors. The
> mirrors (9 of them, all of them ca. 10 years old and used throughout) that
> were used for the microscope were EO3 dielectric broadband mirrors
> (Thorlabs), and one of the mirrors (maybe due to age) contributed a lot to
> this nonlinear dispersion. I replaced it by another EO3 mirror and saw a
> nice improvement - at least for some wavelengths. Here is on overview of
> how
> the pulse shape looks like (after the objective), and it is very good at
> 933
> nm, while bad at 917 nm : https://i.imgur.com/EdJTH1d.gifv.
>
> Now, the microscope is useable at 933 nm. I have the impression that the
> dielectric mirrors can also generate a negative dispersion that can - for
> some wavelengths - reduce the temporal pulse width. Could anybody support
> this with his experience?
>
> Another option would be to use metallic mirrors to replace all the
> dielectric ones. Unprotected metallic mirrors seem to be ideal for
> dispersion-free reflection. Or is it that dispersion is negligeable for
> 100
> fs-pulses also for protected metallic mirrors? On the other hand,
> unprotected mirrors seem to be difficult to clean and keep scratch-free,
> which is not a good property for a microscope that is used by many people
> on
> a daily basis.
>
> And I would be interested in knowing whether the dispersive effects of
> dielectric broadband mirrors depend a) on their age and b) on the angle of
> incidence of the laser beam. I haven't found any data on this topic in the
> internet, but this would be really interesting to know.
>
> Best,
> Peter
> "
>

[David C]

*****
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*****

Hi Peter,

If you ask Thorlabs, they will provide you the dispersion plots. For example, here is the plot for their dielectric E03 coating https://imgur.com/nMgg4AZ. You can see that it has large peaks at specific frequencies. In particular, the peak at ~930-950 nm could've shifted around due to aging and might explain your problem. You can also observe that the dispersion can be negative at some wavelengths.

best,

Ph.D. David Chen
Postdoc - Max Planck Institute for Molecular Cell Biology and Genetics