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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 <[log in to unmask]> 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 <[log in to unmask]>
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 <
> [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 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
>>
>